Polymer Reviews A Review on Polymeric Membranes

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A Review on Polymeric Membranes and Hydrogels Prepared by Vapor-Induced Phase Separation Process a

Antoine Venault , Yung Chang

a b

, Da-Ming Wang

b c

& Denis Bouyer

d a

Department of Chemical Engineering , Chung Yuan Christian University , Chung-Li , Taiwan b

R&D Center for Membrane Technology , Chung Yuan Christian University , Chung-Li , Taiwan c

Department of Chemical Engineering , National Taiwan University , Taipei , Taiwan d

Institut Européen des Membranes , Université Montpellier 2 , Montpellier , France

To cite this article: Antoine Venault , Yung Chang , Da-Ming Wang & Denis Bouyer (2013) A Review on Polymeric Membranes and Hydrogels Prepared by Vapor-Induced Phase Separation Process, Polymer Reviews, 53:4, 568-626 To link to this article: http://dx.doi.org/10.1080/15583724.2013.828750

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Polymer Reviews, 53:568–626, 2013 Copyright © Taylor & Francis Group, LLC ISSN: 1558-3724 print / 1558-3716 online DOI: 10.1080/15583724.2013.828750

A Review on Polymeric Membranes and Hydrogels Prepared by Vapor-Induced Phase Separation Process ANTOINE VENAULT,1 YUNG CHANG,1,2 DA-MING WANG,2,3 AND DENIS BOUYER4 Downloaded by [Antoine Venault] at 15:25 27 September 2013

1

Department of Chemical Engineering, Chung Yuan Christian University, Chung-Li, Taiwan 2 R&D Center for Membrane Technology, Chung Yuan Christian University, Chung-Li, Taiwan 3 Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan 4 Institut Européen des Membranes, Université Montpellier 2, Montpellier, France In 1918, Zsigmondy and Bachmann presented a new method to induce phase separation of a homogeneous polymeric solution from a vapor phase. The so-called vapor-induced phase separation (VIPS) was born. In a century, the body of knowledge on polymer membranes and hydrogels prepared by VIPS has grown importantly, which suggests the need for a critical review. Slowness of mass transfers involved in VIPS, attributed to the resistance at the gaseous phase/liquid phase interface, permits reaching better control of polymer membrane formation than with the popular wet-immersion process. As a result, a broad variety of morphologies can be obtained and well controlled. The control of testing conditions and formulation parameters also permits tuning and tailoring morphologies, which arises in various membranes properties, and led scientists to investigate the possibility of forecasting mass transfers in VIPS. Therefore, at the end of the twentieth century, first models were developed to describe this process, and validated by comparing simulated data to experimental results. Afterwards, studies demonstrated the possibility of predicting membrane morphologies from the knowledge of operating conditions. This article aims at reviewing the work done so far reporting this process to prepare polymer membranes and hydrogels. The experimental set-ups will be introduced as well as the different polymer/solvent/nonsolvent and polymer/additive(s)/solvent/nonsolvent systems used and the morphologies obtained. The effect of testing conditions and formulation parameters on the structure of the matrices will be subsequently discussed. Close attention will be given to the fundamental theory of VIPS before moving onto the potential applications of such polymer matrices. Keywords VIPS process, polymer membranes, hydrogels, morphology control, testing conditions, VIPS simulation

Received March 25, 2013; accepted May 29, 2013. Address correspondence to Antoine Venault, Department of Chemical Engineering, Chung Yuan Christian University, 200 Chung Pei Rd., Chung-Li 32023, Taiwan. E-mail: [email protected]; Denis Bouyer, Institut Européen des Membranes, ENSC-UMII-CNRS, Université Montpellier 2, Place Eugène Bataillon, Montpellier 34095, France. E-mail: [email protected]

568

Vapor-Induced Phase Separation Process (VIPS)

569


1. Introduction The Vapor-Induced Phase Separation (VIPS) was first presented by Zsigmondy and Bachmann in 19181 and then described by Elford in the 1930s.2 It involves a ternary polymeric system or a quaternary polymeric system when an additive is used. At first, the polymer or polymer mixture is dissolved in a specific solvent. Literature mainly mentions the use of four polymers, including polyvinylidene difluoride (PVDF),3–12 polysulfone (PSf),13–26 poly(ether sulfone) (PES),27–39 and poly(ether imide) (PEI).17,40–50 Indeed, these polymers are of main importance in membrane technology. However, and as revealed by Table 1, trials with many other synthetic polymers have been reported51–80 as well as the use of natural biopolymers, chitin and chitosan, for the elaboration of hydrogels.73–80 After polymer solubilization, the solution is cast on an appropriate substrate and exposed to a non-solvent, which may initially contain solvent to avoid a too fast solvent evaporation from the solution. In VIPS, the nonsolvent phase is a gas. Due to its gaseous state and therefore, technological issues, not many different nonsolvents are reported. Table 1 highlights that only water vapors (in most of the cases), acetone vapors and ammonia vapors, are mentioned. The two first ones are cited in the preparation of membranes while the latter was used in the gelation process of chitosan. Air has been used as well, but it can only be considered as a nonsolvent if it is humid air. Note that for chitosan gelation, the VIPS term is sometimes not mentioned, probably because a chemical reaction is also involved in the liquid phase once ammonia has been transferred from the gaseous phase. The term VIPS-like or reaction-induced phase separation (RIPS) could also be relevant. One should also be reminded at this point of the essential differences between the VIPS process and the controlled solvent evaporation process described earlier.81 In VIPS, nonsolvent penetrates from the vapor phase, whereas in the controlled solvent evaporation process, the nonsolvent is originally contained in the solution, along with the polymer and a more volatile solvent. In VIPS, the main phenomenon responsible for phase separation is a nonsolvent inflow rather than a solvent outflow (the system enters the diphasic region due to nonsolvent intake rather than solvent loss). In controlled solvent evaporation, the volatile solvent evaporation causes the casting solution to be enriched in nonvolatile nonsolvent, so that the polymer precipitates, forming the membrane structure.82 Phenomena governing phase separation are, therefore, different and the VIPS process should be preferentially compared to the wet method, where the initial binary solution (polymer-solvent) is immersed in a non-solvent bath. In this way, the VIPS process is smoother since the mass transfer rates (non-solvent intake and solvent extraction) are considerably reduced compared to those occurring in the wet method. A typical VIPS chamber for polymer membrane elaboration can be a glove box in which the temperature and non-solvent partial pressure (or relative humidity if the non-solvent is water), must be perfectly controlled. In some other circumstances, researchers may use a close thermostated glass chamber, inside which the relative humidity is controlled by a specific salt.80 Finally, to prepare hydrogels from an ammonia gaseous phase, a double wall closed chamber was reported: ammonia solution was placed in the bottom of the reactor and ammonia partial pressure in the vapor phase controlled thanks to a thermostated circulating water bath. The exposure time to non-solvent vapor is another main process parameter that will influence the structuring of matrices. It is usually set depending on the type of morphology one wants to reach, and obviously considering the whole combination of formulation/process parameters. This will be later discussed in section 3. Afterward, the film or gel is usually immersed in deionized water either (i) to complete membrane/hydrogel formation (combination VIPS/LIPS), or (ii) to wash it. In the first case, the newly formed

570

Additive

/

/ /

/

/

/

/

/

Polymer

PVDF

PVDF PVDF

PVDF

PVDF

PVDF

PVDF

PVDF

NMP

DMF/DMSO 1/1

DMF/TEP 1/1

DMF/DMAC 1/1

DMF

DMF DMF

DMF

Solvent

Water

Water

Water

Water

Water

Water Water

Water

Gaseous Nonsolvent

VIPS

VIPS/LIPS

VIPS/LIPS

VIPS/LIPS

VIPS/LIPS

VIPS VIPS

VIPS

Process Simulation study ptRH: 10%, 20%, 40% ptTc : 298 K Simulation study RH: 70% ptTc : 25◦ C ptt: 20 min RH: 30% ± 5% ptTc : 20◦ C ± 1◦ C ptt: 30 s RH: 30% ± 5% ptTc : 20◦ C ± 1◦ C ptt: 30 s RH: 30% ± 5% ptTc : 20◦ C ± 1◦ C ptt: 30 s RH: 30% ± 5% Tc : 20◦ C ± 1◦ C t: 30 s RH: 70% Tc : 25◦ C t: 10 min, 20 min

VIPS conditions

Flat sheet

7

7

Flat sheet

7

Flat sheet

7

7

Flat sheet

Flat sheet

5 6

3,4

Reference

Flat sheet Flat sheet

Flat sheet

Form

Table 1 Systems used to prepare polymeric membranes/hydrogels by VIPS or a combination of VIPS and LIPS processes


571

/

/

/

/

/

/

PVDF

PVDF

PVDF

PVDF

PVDF

Water

Water

Water

Water

Water

Water Water

DMAC/DMSO 1/1 Water

DMAC

DMAC

DMAC

DMAC

DMAC

/ NMP R F108 NMP Pluronic

PVDF

PVDF PVDF

VIPS/LIPS

VIPS

VIPS/LIPS

VIPS

VIPS and VIPS/LIPS

VIPS

VIPS VIPS

Not mentioned RH: 70% ± 2% Tc : 30◦ C ± 2◦ C t: 20 min RH: 70% Tc : 25◦ C t: 20 min RH: 100% Tc : 25◦ C t: 0 to 10 min RH: 100% Tc : 27–75◦ C t: 4,10 min RH: 30% ± 5% Tc : 20◦ C ± 1◦ C t: 30 s RH: 80% Tc : 50◦ C t: 24 h 0.79% DMAC vapor RH: 30% ± 5% Tc : 20◦ C ± 1◦ C t: 30 s


7

Flat sheet

7

(Continued on next page)

Flat sheet

12

11

Flat sheet

Flat sheet

10

7

Flat sheet

Flat sheet

8 9


572

Additive

/

LiCl

Ethanol

PVP

PVP and water

/

Polymer

PVDF

PVDF

PVDF

PVDF

PVDF

PVDF

DMSO

DMAC

DMAC

DMAC

DMAC

DMAC/TEP 1/1

Solvent

Water

Water

Water

Water

Water

Water

Gaseous Nonsolvent

VIPS/LIPS

VIPS

VIPS

VIPS

VIPS

VIPS/LIPS

Process RH: 30% ± 5% Tc : 20◦ C ± 1◦ C t: 30 s RH: 80% Tc : 50◦ C t: 24 h with or without 0.79% DMAC vapor RH: 80% Tc : 50◦ C t: 24 h 0.79% DMAC vapor RH: 30%–90% Tc : 40◦ C–120◦ C t: 24 h 0.79% DMAC vapor RH: 80% Tc : 50◦ C t: 24 h 0.79% DMAC vapor RH: 30% ± 5% Tc : 20◦ C ± 1◦ C t: 30 s

VIPS conditions

12

7

Flat sheet

Flat sheet

12

12

Flat sheet

Flat sheet

7

7

Reference

Flat sheet

Flat sheet

Form

Table 1 Systems used to prepare polymeric membranes/hydrogels by VIPS or a combination of VIPS and LIPS processes (Continued)


573

/

/

/

/

/

/

/ /

/

/

PVDF

PVDF

PSf

PSf

PSf

PSf

PSf PSf

PSf

PSf

NMP

NMP

NMP NMP

NMP

NMP

NMP

DMF

TEP/DMSO 1/1

TEP

Water

Water

Water Water

Water

Water

Water

Water

Water

Water

VIPS and VIPS/LIPS

VIPS

VIPS VIPS

VIPS

VIPS

VIPS/LIPS

VIPS

VIPS/LIPS

VIPS/LIPS

RH: 30% ± 5% Tc : 20◦ C ± 1◦ C t: 30 s RH: 30% ± 5% Tc : 20◦ C ± 1◦ C t: 30 s RH: 50% ± 2.5% Tc : 21◦ C ± 1◦ C t: 3 min RH: 53% Tc : 25◦ C RH: 70%–100% Tc : 20◦ C ± 0.5◦ C t: 3 h RH: 90% Tc : 20◦ C ± 0.5◦ C Simulation study RH: 30% - 90% Tag : 25◦ C Air gap: 0 – 100 cm RH: 53% Tc : 25◦ C t: / RH: 70% Tc : 25◦ C t: 0 – 20 min



20

19

Flat sheet

Flat sheet

17 18

Flat sheet Hollow fiber

16

15

Flat sheet

/

14

13

7

7

Flat sheet

Flat sheet

Flat sheet

Flat sheet

574

DMMSAPS

/

/

PSf

PES

DCM

2P

NMP

Water

Water

Water

Water

R F127 NMP Pluronic

PSf

PSf

Water

Water

R Pluronic F108 NMP

PVP or PANI

PSf

NMP

Water

PSf

PVP

PSf

/

Water

Gaseous Nonsolvent

Water

/

PSf

NMP

Solvent

NMP

/

Additive

PSf

Polymer

VIPS

VIPS/LIPS

VIPS

VIPS

VIPS

VIPS/LIPS

VIPS

VIPS/LIPS

VIPS/LIPS

Process RH: 70% Tc : 25◦ C t: 0 – 30 min RH: 30%, 70%, 90% Air gap: 0 – 30 cm RH: 65% Tc : 25◦ C t: 0 – 20 min RH: 28% ± 1% Tc : 25◦ C ± 1◦ C t: 30 s RH: 95% Tc : 25◦ C t: 5 min RH: 95% Tc : 25◦ C ± 2◦ C t: 5 min RH: 95% Tc : 25◦ C ± 2◦ C t: 5 min RH: 70% Tc : 25◦ C t: 0 – 30 min RH: 50% - 90% Tc : 20◦ C

VIPS conditions

21

27

Flat sheet

26

Flat sheet

Flat sheet

26

Flat sheet

26

24

Flat sheet

25

23

Flat sheet

Flat sheet

22

21

Reference

Hollow fiber

Flat sheet

Form



575

PVP PVP

PVP R , PVP Span

/

R Pluronic

PVP

PVP or PES or R Pluronic PVP

PES PES

PES PES

PES

PES

PES

PES

PES

2-ME

PES

NMP

Caprolactam/ butyrolactone NMP

NMP/TEG

NMP

/ NMP/acetone

NMP NMP

NMP

Water+N2

Water

Water

Air, oxygen, nitrogen, carbon dioxide and argon Water

/ Water

Water Water

Water

VIPS/LIPS

VIPS/LIPS

VIPS/LIPS

VIPS/LIPS

VIPS/LIPS

VIPS/LIPS VIPS/LIPS

VIPS VIPS

VIPS

RH: 50% - 60% t: 1 min Stream of nitrogen saturated with water vapors Tc : 40◦ C t: 3 min

RH: 50% - 60% or 65% - 75% t: 0 - 5 min Not mentioned

RH: 74% t: 2 s–20 s Simulation study 100% water vapor (air free) T < 20 min / Chamber saturated with water vapors Tc : 30◦ C t: 0 – 4 min Tag : 23◦ C Air gap: 28 cm


37

36

35

34

33

31 32

29 30

28


Polymeric microsieves

Flat sheet

Flat sheet

Flat sheet

Hollow fiber

Flat sheet Polymeric microsieves

Flat sheet Bistructured membrane

Flat sheet

576

DEG

/

/ /

/

/

/

/

/

PES

PEI PEI

PEI

PEI

PEI

PEI

PEI

Additive

PES

Polymer

NMP

NMP

NMP

NMP

NMP

NMP NMP

DMF

DMAC

Solvent VIPS/LIPS

Process

Water

Water

Water

Water

Water

Water Water

VIPS

VIPS or drycast

VIPS or drycast

VIPS or drycast

VIPS or drycast

VIPS VIPS

Water (containing VIPS/LIPS DMF)

Water

Gaseous Nonsolvent RH: 60%, 90% Air gap: 0 – 30 cm Chamber saturated with water vapors Tc : 295 K t: 0 – 57,500 s Simulation study Simulation study RH: 43%, 75% Tc : 40◦ C ± 0.5◦ C RH: 0% - 70% Tc : 40◦ C ± 0.5◦ C t: 5 min RH: 0% - 50% Tc : 40◦ C ± 0.5◦ C t: 0 - 12 h RH: 0% - 70% Tc : 40◦ C ± 0.5◦ C t: 0 - 50 h RH: 0% - 70% Tc : 40◦ C ± 0.5◦ C t: 0 - 50 h RH: 43%, 75% Tc : 40◦ C ± 0.5◦ C

VIPS conditions

40

44

Flat sheet

Flat sheet

43

42

Flat sheet

Flat sheet

41

Flat sheet

17 40

39

Flat sheet


38

Reference

Hollow fiber

Form



577

DGDE, AA

Additives∗

PEI

PEI

TPX

PEI/copolyimide TPX

NMP/DMSO/γ GBL

NMP/Water/ Ethanol NMP

NMP

Water

Water

Water

Water

Water

Water

AA or NMP or Cyclohexane Water Propionic acid or Acetone or 1-Butanol AA Methylcyclohexane Water

Isocyanate DMF/acetone (cross-linker), Glycerol (nonsolvent additive) NMP

/

PEI

PEI/CA blend

/

PEI

VIPS

VIPS

VIPS/LIPS

VIPS/LIPS

VIPS/LIPS

VIPS/LIPS

VIPS/LIPS

VIPS

RH: 15 - 95% Tc : 28◦ C, 31◦ C t: 30 s – 1h

RH: 65% Air gap: 1 cm RH: 15 - 95% Tc : 28◦ C, 31◦ C t: 30 s – 1h

RH: 50% Tc : 40◦ C ± 0.5◦ C t: 30 min Tag : 25◦ C ± 1◦ C Air gap: 0 – 10 cm RH: 65% ± 5% Tc : 25◦ C ± 1◦ C t: 30 s RH: 35% ± 5% Tag : 20◦ C ± 2◦ C Air gap: 0.9 m RH: > 95% Tc : 50◦ C t: 0.5 - 30 min


51

51


Flat sheet

Flat sheet

50

49

Flat sheet

Hollow fiber

48

47

Flat sheet

Hollow fiber

46

45

Hollow fiber

Flat sheet

578

Dioxane/DMF

DMSO Ether/alcohol

acetone Acetone/water

/

/

/ /

/

/ /

/ Naproxen

PC

PC PC

PC/PAN

PAN Nitrocellulose CA CA

NMP

DMF NMP

DMF

DMF

/

PS-bPDMS PC

DMF

Cyclohexane or Toluene

Solvent

/

AA

Additive

PS-bPDMS

PS

Polymer

Water Water

Water Water

Water

Acetone Water

Water

Water

Water

Water

Water

Gaseous Nonsolvent VIPS conditions

RH: 15–95% Tc : 28◦ C, 31◦ C t: 30 s – 1h VIPS RH: 10%, 60% ± 5% Tc : room T VIPS RH: 25%–80% Tc : room T VIPS/LIPS Air saturated with water vapors Tc : 283, 293, 313 K t: 600, 900, 1200 s VIPS RH: 20–75% ± 5% Tc : room T VIPS Tc : room T VIPS and RH: 65 - 75% VIPS/LIPS Tc : 28◦ C, 31◦ C VIPS RH: 70% Tc : 25◦ C t: 30 min VIPS/LIPS Air gap: 1–20 cm VIPS and solvent / evaporation VIPS Simulation study VIPS RH: 10–95% or dry cast

VIPS

Process

57

Flat sheet


5 59

58 2

55 56


Hollow fiber Flat sheet

55

Flat sheet

53

Flat sheet

54

52

Flat sheet

Flat sheet

51

Reference

Flat sheet

Form



579

THF/n-BA

Water

Water

Water

Water

Water

Propionated lignin

AA

CTA

EVA

Cyclohexane

CH2 Cl2

Water

Water

Modified lignin Methylene chloride Water

PVP

CPVC

CTA

PVP

CPVC

THF/n-BA

Methanol, CaCl2 DMF

PVC

PVC

Methanol, NMP Ethanol, 2Propanol, Mg (ClO4 )2 Methanol, CaCl2 THF/water

CA/PAI

VIPS

VIPS

VIPS

VIPS

VIPS

VIPS/LIPS

VIPS

VIPS/LIPS

RH: 65% Tc : 20◦ C t: 1h RH: 65% t: 10 s–20 min RH: 35% - 88% ± 2% Tc : 25◦ C ± 2% t: 2h RH: 45%–80% ± 2% Tc : 25◦ C ± 0.5◦ C t: 2h RH:10%, 30%, 70% Tc : 35◦ C, 45◦ C, 55◦ C t: 2h RH:10%, 30%, 70% Tc : 35◦ C, 45◦ C, 55◦ C t: 2h RH: 65% Tc : 28◦ C t: 30 s–1h

Air gap: 2.5 cm


51

Flat sheet


66

65

Flat sheet

Flat sheet

64

62–64

Flat sheet

Flat sheet

61

61

60

Flat sheet

Flat sheet

Hollow fiber

580

R Matrimid PBI/PSf Chitosan

Water Ammonia

Water + AA

/

Water

Water

Water

Water

Water

Water

Water

Gaseous Nonsolvent

DMAC/NMP

Chlorobenzene, NMP

acetone

2P

DCM/HFIP

dioxane

DMA/DMF

DMSO

Solvent

/

PEG-400

BPPO + TEOA

/

PMMA

Glycerol

/

PLLA

CN

/

PLLA

PVP

/

EVAL

PEEKWC

Additive

Polymer

VIPS

VIPS/LIPS

VIPS/LIPS

VIPS

VIPS/LIPS

VIPS/electrospinning

VIPS

VIPS/LIPS

VIPS/LIPS

Process

t: 48 h

RH: 75% Tc : room T t: 1, 3, 5 min RH: 10%–100% Tag : 20◦ C ± 2◦ C Air gap: 50 cm 100% water vapor (air free) T < 20 min RH: 55% ± 5% Tc : 23◦ C ± 2◦ C Air gap: 100 mm RH: 70% Tc : 25◦ C t: 0 – 30 min RH: 70%, 80%, 90% Tc : 25◦ C t: 0.5–20 min RH: 20%–90% Tc : 25◦ C t: 0–10 min Air gap: 0–3 cm

VIPS conditions

Hydrogel

Hollow fiber

Flat sheet

73

72

71

70

21

Flat sheet

Flat sheet

69

30

68

67

Reference

Fibers

Bistructured membrane

Hollow fiber

Flat sheet

Form



581

Ammonia Ammonia

Activated Carbon Water + AA


Chitosan

Chitosan

/

Chitin

LiCl/NMP

Water

VIPS

VIPS

VIPS

VIPS

VIPS

VIPS

VIPS

Simulation study PNS : 1346 – 4342 Pa Tc : 293, 301, 309 K t: 1500 - 3500 min Simulation study PNS : 1368 Pa, 3342 Pa Tc : 293.15, 303.15, 303.15 K ± 0.5 K t: 4 h – 24 h Simulation study Tc : 303 ± 0.5 K t: 100,000 s Tc : 10◦ C-50◦ C t: 0.5 h – 24 h PNS : 1680 Pa Tc : 30◦ C ± 1◦ C t: 12 h RH: 75% Tc : 40◦ C t: 24 h Simulation study RH: 43%, 75% Tc : 20◦ C, 40◦ C t: 24 h 80

79

Hydrogel

Hydrogel

78

77

76

75

74

Hydrogel

Hydrogel

Hydrogel

Hydrogel

Hydrogel

–RH: relative humidity, PNS : partial pressure of nonsolvent (if not water), t: exposure time to nonsolvent vapors, Tc : temperature of chamber, Tag : temperature of air in the air gap. (when up to 3 values for a same testing condition are tested, they are all mentioned in the table, otherwise, the range is given). –In Simulation studies, simulated conditions tried are too numerous to be mentioned herein. Readers are invited to see the reference for more details. Only conditions related to experimental validation are provided, if relevant.

/

Chitin

Water

Ammonia


Chitosan

LiCl/NMP

Ammonia


Chitosan

/

Ammonia

Chitosan

Water + AA


582

A. Venault et al.

matrix will be subsequently immersed in another water bath to wash it and ensure that no solvent trace remains. The washing step is not critical, since membrane formation has already occurred, and may last a few hours to a few days. Finally, the film is removed out of the water bath and dried. This step can be performed either at ambient temperature or in an oven.


2. Polymer Membranes and Hydrogels Prepared by VIPS Process The VIPS process alone, or combined with another phase separation process—LIPS process most of the time (also named wet-immersion process)—is suitable for preparing membranes or hydrogels. In this paper, we review the flat-sheet polymer membranes and hydrogels prepared by VIPS. We also consider the flat-sheet membranes prepared by a combination of VIPS and LIPS process. For this specific class, the VIPS step is too short to allow the phase separation of polymer solution and solidification of the matrices over their whole thickness, but long enough to initiate membrane formation. Finally, the formation of polymer hollow fibers is mentioned in this review. Usually, the term used to describe the preparation process of such specific membranes is the “dry/wet spinning process.” Dry refers to the fact that from the moment the polymeric mixture is extruded out of the spinneret, it often enters an air gap region, in which the humidity can be controlled. This is a VIPS step which can strongly influence the properties of the final membranes. Finally and even if it is beyond the scope of this paper, focusing on polymer flat-sheet membranes, hollow-fiber membranes, and hydrogels totally or partially prepared by VIPS, it should be mentioned, to be complete, that VIPS is an important mechanism controlling the morphology of fibers electrospun in a humid environment. However, other potential important process parameters than those found in the VIPS process (see section 3.2.1) influence their final structure such as the electrical conductivity of the solution or the polymer flow rate. In this respect, electrical phenomena do play a major role as well on fiber formation, which is not discussed herein. 2.1 Flat-Sheet Polymer Membranes prepared by VIPS Process Once the polymeric solution is homogeneous, it is cast on a plate (glass plate most of the time) positioned inside the VIPS chamber at a desired initial thickness, using a casting knife. The testing conditions (especially exposure time to non-solvent vapors and relative humidity) are set in order to ensure a complete phase separation over the whole thickness. Then, the glass plate is immersed in a bath of water and the flat-sheet newly formed detaches from the glass. Herein, the immersion step is a washing step only, permitting to remove all the traces of solvent, and does not influence the membrane formation and the final physical properties of the membranes. The water bath may be changed to improve the cleaning of the membrane and the removal of the solvent. Subsequently, a drying step is performed. Typical polymers used for such geometries are PVDF,3–121 PSf,13–15,17,19–21,23–26 PES,27–29,31,34–36,39 or PEI.17,40–45,47,49 2.2 Polymer Hydrogels Prepared by VIPS-like Gelation Process Similarly to the flat-sheet membranes, polymer hydrogels may be prepared by a VIPS process. At least, the term “VIPS” is employed in literature. Gelation might be more thought of as a modification of the rheological state of the system rather than as a thermodynamic destabilization as that occurring in membrane preparation. Hence, several terms such as



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VIPS-like, VIG (vapor-induced gelation), or RIPS (reaction induced phase separation, when a reaction is also involved) would be more accurate for naming the process. To the best of our knowledge, a VIPS-like process has only been reported to prepare chitin79,80 or chitosan hydrogels.73–78 In this process, the solution is usually poured in a glass Petri dish. The initial thickness of the hydrogel is also controlled via the diameter of the dish and the amount of solution that was cast. Afterwards, the glass Petri dish is positioned inside a glass vessel in which the gaseous atmosphere has been previously saturated with water vapors (chitin hydrogel) or ammonia vapors (chitosan hydrogels) (Table 1). The water and ammonia partial pressure can be controlled74–78 or not.73 Water or ammonia is then transferred from the gaseous phase to the polymeric system. In the preparation of chitosan hydrogels, once ammonia is transferred in the liquid phase, an increase of pH occurs. The hydroxide ions generated react with the soluble form of chitosan (Chit-NH3 +) resulting in the formation of the insoluble form of chitosan (Chit-NH2 ). A gel is formed over the whole thickness after a certain time. Water is entrapped within the polymeric chains so that the term hydrogel is employed. The newly formed hydrogel is subsequently washed in water to remove the excess of solvent (for chitin hydrogel) or ammonia (chitosan hydrogel), and stored in water until use. 2.3 Flat-Sheet Polymer Membranes Prepared by VIPS/LIPS Process Sometimes, the term VIPS/LIPS process would be more suitable to describe the formation of flat-sheet polymeric membranes. The solution is exposed for a certain time to non-solvent vapors, but the exposure step does not last long enough to allow phase separation over the whole thickness. Therefore, the subsequent immersion step ensures a total phase separation and the morphology and other physical properties arise from a combination of the two NIPS processes. Polymeric systems are then immersed in another water bath to be rinsed. 2.4 Polymer Hollow-Fiber Membranes Prepared by the Dry/Wet Spinning Process, Role of the VIPS Step Last but not least, the hollow-fiber membranes do present a VIPS step in their preparation process.22 This step influences the membrane morphology so that a technique has been reported to minimize its influence on final morphology.48 The process used to prepare hollow fibers, that is, spinning of hollow fiber, is as follows: First, the polymer, with or without additives, is dissolved in an appropriate solvent (or combination of solvents). The solution obtained is called the dope. A so-called bore-fluid is also prepared, which serves as an internal coagulant and whose composition is usually a mixture of solvent/non-solvent. Afterwards, the spinning dope and the bore liquid are driven to a hollow-fiber spinneret. As the dope has been contacted with the bore fluid, phase separation has been initiated by a process similar to LIPS process. Later, the nascent hollow-fiber passes through an airgap region before being immersed in a non-solvent bath (coagulant bath). The reason why hollow-fibers are included in this review is the existence of a VIPS step in the air gap region, whose humidity and temperature have to be controlled. In the air-gap region the solvent evaporates, so that the concentration of the polymer at the polymer solution/air interface tends to be higher, leading to denser structures near the top interface, while away from this surface, a more porous substructure is formed. But nonsolvent (water) also penetrates the polymer solution in the air gap, unless the humidity is set to 0, so that a VIPS steps occurs. The structure of final hollow-fibers depends on how long it is going to take for the fibers to travel from the extremity of the spinneret, to the external coagulant ( = exposure time

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to non-solvent vapors). Therefore, the air-gap region is crucial in the polymer hollow-fiber membrane formation, which finally arises from a combination of the two NIPS processes.

3. Control of Polymer Membrane Formation by Vapor-Induced Phase Separation


3.1 The Different Morphologies Obtainable by Vapor-Induced Phase Separation Taking a close look at the literature related to VIPS, one will realize that a wide range of morphologies can be obtained, from asymmetric structures with a dense top-layer to porous symmetric polymeric matrices. This variety makes this process of great interest in membrane science. Indeed, it permits modifying and tailoring membrane structure. The four main different morphologies obtainable by the pure VIPS process, that is not combined to LIPS, are displayed by Fig. 1. They result from a complex combination of parameters, both formulation and process parameters, so that a change of one single parameter may lead to a totally different morphology. Mechanisms for polymer membrane formation given by the

Figure 1. The most common morphologies obtainable by the VIPS process. (a) Symmetric cellularlike structure. Reproduced from Park et al.15 with permission from Elsevier. (b) Asymmetric cellularlike structure. Reproduced from Bouyer et al.80 with permission from Elsevier. (c) Symmetric Nodular-like structure. Reproduced from Venault et al.9 with permission from Elsevier. (d) Symmetric bi-continuous structure. Reproduced from Tsai et al.21 with permission from Elsevier (Color figure available online).


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authors of various studies will be provided in this section. However, one should also refer to Strathmann et al.’s study, published more than 35 years ago, who discussed the formation of sponge-structured membranes and finger-structured membranes by dividing the casting solution into three layers (casting solution layer close to the substrate, fluid polymer layer, and solid polymer layer contacted with the nonsolvent).83 They also insisted on the fact that by changing a single preparation variable, many intermediate structures lay between the finger-like morphology and the sponge-structured membrane. Although they rationalized membrane formation in the case of LIPS process and even if finger-like structures cannot be obtained by the VIPS process alone, their study may be applied to the rationalization of VIPS, especially when high non-solvent partial pressures are used, resulting in a more rapid transfer of the nonsolvent to the liquid polymer system. Many intermediate polymeric structures may be found, and the most common are presented herein. The effect of testing conditions will be subsequently investigated. 3.1.1 Symmetric Cellular Structures across the Membrane’s Cross-Section. Typical symmetric cellular morphologies are depicted in Fig. 2 and extensively reported.15,16,18,43,48,54,84 Park et al. raised the question of the origin of such morphology.15 They explained that since the activity of water in a vapor phase is similar to that in a liquid phase, the water vapor induced phase inversion membrane should have shown a finger-like morphology similar with one membrane coagulated by liquid water because the thermodynamic status of all the other components are the same. They subsequently assumed that the difference in two cases seems not to originate from the differences in thermodynamic terms, but from the differences in kinetic terms. Therefore, the vapor/liquid interface playing a key role on mass transfer kinetics directly affects the final structures of polymer membranes. Their conclusion actually followed that of Bodzek and Bohdziewicz, who explained that the process of gelation of polycarbonate had to be rather slow to yield to symmetric cellularlike structures.54 If the diffusion of non-solvent through the polymer system is slow, then the changes in its concentration over the whole cross-section of the matrix are negligible, resulting in typical symmetric structures. 3.1.2 Asymmetric Cellular Structures across the Membrane’s Cross-Section. Asymmetric cellular structures across the membrane’s cross sections are also commonly reported20,40,42,44 and some of them are displayed in Fig. 3. Among these studies, most

Figure 2. Membrane presenting a symmetric cellular-like cross-section morphology. (a) Reproduced from Park et al.15 with permission from Elsevier. (b) Reproduced from Lee et al.16 with permission from Elsevier; (c) Reproduced from Tsai et al.18 with permission from Elsevier.

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Figure 3. Membrane presenting an asymmetric cellular-like cross-section morphology. (a) Reproduced from Menut et al.42 with permission from Elsevier. (b) Reproduced from Su et al.20 with permission from Elsevier. (c) Reproduced from Bouyer et al.80 with permission from Elsevier.

of them described a decrease of cellules size when going from the top layer (vapor/system interface) to the bottom one (system/substrate interface). Menut et al. came out with an explanation for the obtaining of such structures.42 They evidenced the existence of a surface liquid layer. According to their work, this liquid layer generates a solvent gradient across the system. Consequently, a polymer gradient is also observed with a lower concentration near the top surface. Coarsening of droplets then becomes easier near this surface as the viscosity of the solution is lower than in the deeper layers. Finally, cell growth near the top surface is delayed, compared to that in the bulk of the membrane, eventually resulting in a cellular size gradient. 3.1.3 Symmetric Nodular Structures Across the Membrane’s Cross-Section. A third common morphology encountered in membranes formed by VIPS is the nodular morphology that may also be termed granular or packed sphere structure as those displayed in Fig. 4.6,9,10,34,52,55,67 According to Kimmerle and Strathmann in their analysis of the structure-determining process of phase inversion membranes, these morphologies are mostly found in top layers of membranes prepared from strong nonsolvent, in which the phase separation occurs quickly.85 The polymer-rich phase tends to minimize its surface in such an environment, originating in solid sphere. Some recent studies evidenced a nodular-like morphology over the whole cross-section of the membrane made from a semi-crystalline or crystalline polymer.6,9 Morphologies would result from the nucleation and growth of crystalline domains.

Figure 4. Membrane presenting a nodular morphology. (a) Reproduced from Young et al.67 with permission from Elsevier. (b) Reproduced from Zhao et al.55 with permission from Wiley. (c) Reproduced from Zhao et al.52 with permission from the American Chemical Society.


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Figure 5. Membrane presenting a bi-continuous morphology. (a) Reproduced from Su et al.20 with permission from Elsevier. (b) Reproduced from Li et al.6 with permission from Elsevier. (c) Reproduced from Tsai et al.21 with permission from Elsevier.

3.1.4 Sponge-Like/Bicontinuous Structures Across the Membrane’s Cross-Section. To be applied in processes requiring high fluxes, the best membrane structure obtainable by VIPS is undoubtedly the sponge-like or bi-continuous structure (Fig. 5). It is made of open pores, thereby offering little resistance to permeating species.6,12,20,21,62,63 This morphology is also not the easiest structure to prepare so that cellular-like or nodular structures are more common. Moderately slow phase separation kinetics should be reached to lead to such structures.85 Also, the control of some specific parameters, which in a first approach were not thought of as predominant, such as the temperature of dissolution of the polymer or the viscosity of the solvent, may be very helpful in yielding to bicontinuous morphology.6,21 3.1.5 Asymmetric Finger-Like Structures across the Membrane’s Cross-Section. Asymmetric finger-like structures are commonly found when polymer solutions freshly cast are immersed in non-solvent (LIPS process or wet-immersion process). They result from fast phase separation rates. When the exposure time to vapors is short and as explained later in this manuscript, the typical morphologies of the membranes presented in sections 3.1.1 to 3.1.4 are not obtained and a finger-like structure may be formed instead.10,20,34,71 Also, the VIPS step may be too short to solidify the whole matrix, but long enough to initiate phase separation on a significant thickness. Then, part of the structure (upper layers) is typical of VIPS whereas the other layers (deeper) arise from LIPS, but the role of each process taken individually on the polymer membrane formation is unclear. In other words, the actual knowledge on membrane formation cannot permit to state which process dominates in the final structure of the polymer membrane. 3.1.6 Dense Surfaces vs. Porous Surfaces. Both dense surfaces, suitable for gas separation, or porous surfaces, that can be applied in water treatment, are obtainable by VIPS or the combination VIPS/LIPS. From what has been mentioned earlier, one will figure that membranes presenting a cellular-like or a finger-like cross-section usually display a dense layer also termed as “skin.” On the contrary, nodular-like structures and, of course, bicontinuous structure made of open pores, present a very porous top-layer. We will go through the details later in this review to explain the effect of testing conditions on the structuring of polymer membrane. However, it can already be mentioned that the polymer concentration51 as well as the growth of the polymer lean phase during phase separation will drastically affect the nature of the top layer. In addition, nodules may be found on dense surfaces42 whose formation may be correlated with the spinodal decomposition.13

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3.2 Effect of Testing Conditions on Polymer Membranes’ Morphology


The testing conditions can be broadly classified into two main categories—the process parameters and the formulation parameters. The process parameters correspond to the setting of the conditions of the VIPS chamber or to the physical parameters associated to the preparation of the polymeric solutions. The formulation parameters are related to the chemical parameters involved in the preparation of the polymeric solution. Some, such as the relative humidity or the exposure time to nonsolvent vapors, have been known for a long time to significantly influence the morphology of the final matrices. On the other hand, recent work evidenced that a change of previously not studied parameter, especially the temperature of dissolution of the matrix polymer, could dramatically influence the final structure of polymer membranes.6 3.2.1 Effect of Process Parameters on Polymer Membranes’ Morphology. 3.2.1.1 Effect of Exposure Time to Non-Solvent Vapors on Polymer Membranes’ Morphology. Considering all the other potentially influencing parameters as constant, the exposure time to vapors affects the porosity and interconnectivity of pores. For too short exposure times to vapors, the VIPS step often barely influences membrane formation, so that the morphology tends to be close to finger-like, typical from immersion in nonsolvent bath (LIPS process). Extending the exposure time to vapors gives rise to VIPS typical structures, as presented in Fig. 6. Generally speaking, the exposure time affects the droplet size of the polymer-lean phases and the extent of the coarsening.15 Chen et al. studied the effect of the time of phase inversion induced by water vapor on the permeability of cellulose acetate/polyethyleneimine blend microfiltration membranes.49 After casting the solution, it was allowed to evaporate for 0 to 30 s, before being exposed to water vapors (RH > 95% and T = 50◦ C) from 0.5 to 30 minutes and subsequently immersed in distilled water. Their results indicated that a maximum for permeability was observed for an exposure time to water vapors of 2 minutes. This result highlighted a change in membrane morphology with exposure time to vapors. For short times, the morphology was close to that of a membrane prepared by direct immersion in water, that is, an asymmetrical structure with a dense layer and finger-like macrovoids. When the time was increased, a symmetrical structure with sponge-like pores was formed. As the time was further increased, coarsening of the polymer-lean phase at the late stage of phase separation was responsible for (i) an increase of the size of cells and a (ii) decrease of pores connectivity. In other words,

Figure 6. Schematic representation of morphology changes with exposure time to non-solvent vapors. It is considered that the polymer is amorphous and that the solvent has an average viscosity (e.g, NMP, DMAC, DMF, etc.). The system is exposed to nonsolvent vapors and then immersed in a water bath. The morphology goes from finger-like, typical from LIPS, for too short exposure times, to cellular-like, while the bicontinuous structure is an intermediate state (Color figure available online).



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the morphology tended to a cellular structure with a dense top-layer as the exposure time reached 30 minutes. During membrane formation, nucleation and growth of the polymerlean phase occurs. For low exposure times to water vapors, nucleation prevails over growth, and the structure is highly porous with open pores, that is, the structure is bi-continuous. On the other hand, as the exposure time to water vapors is increased, growth of the polymerlean phase increases the sizes of the voids (pores) but is concomitant to the coalescence of the polymer-rich phase. This later phenomena reduces the interconnectivity of pores. Even though they are bigger, they are isolated and the structure is no longer an openporous structure but a cellular-like one. The consequence is a drastic decrease of membrane permeability. A similar change of membrane structure was reported by Shin et al. in their study carried out on the preparation of polyethersulfone microfiltration membranes using a 2methoxyethanol additive.28 However, the characteristic times for which a change of membrane structure was observed were different from those reported by Chen et al.,49 due to a change of testing conditions (polymer, temperature, relative humidity, etc.). Also, the additive delayed the liquid-liquid demixing and still permitted to obtain a structure with a porous top-layer as the exposure time to water vapors was increased. But their results still evidenced the growth of the polymer-lean phase and the formation of a thicker polymer-rich phase with long exposure time to water vapors. Sun and coworkers focused on the formation of cellulose nitrate membrane by VIPS.70 Like Chen and coworkers earlier,49 they studied the permeability to water as a function of the exposure time to water vapors during the formation step, and also measured an increase of permeability with exposure time to non-solvent vapors, followed by its quick decrease. Their morphological observations also evidenced that as the induced time increased, the structure of the membranes changed from an asymmetrical structure to symmetrical. The final cellular structure observed for longer times was thought of as the result of the coarsening of the polymer-lean phase at the late stage of phase separation. The work of Su et al. confirmed previous studies and the evolution of the morphology from a finger-like structure to a cellular-like structure, with a bi-continuous intermediary state.20 They investigated the formation of polysulfone membranes at 70% RH and the exposure time to nonsolvent vapors varied from 0 to 20 min. The evolution was also shown to be strongly dependent on polymer concentration, which affected the viscosity of the system and therefore, the resistance to non-solvent diffusion. The evolution aforementioned was found for low polymer concentration along the whole thickness of the film whereas for higher concentrations, it was limited to a small region near the interface. 3.2.1.2 Effect of Non-Solvent Partial Pressure on Polymer Membranes’ Morphology. The effect of the non-solvent partial pressure on the morphology of membranes prepared by VIPS is among the most studied and reported.3,4,15,44,62–64,70 As water is almost always the nonsolvent as far as membranes are concerned, the term relative humidity (RH) will be used instead. Matsuyama and his coworkers presented their results on the formation on PVDF membranes by VIPS in the late 1990s. They first published their presentation of a model, immediately followed by an experimental validation in which the effect of RH on the structure of membranes was investigated.3,4 All the other parameters being constant, they studied membrane formation for three different RH: 10%, 20%, and 40% and observed for these values a dense, cellular-like, and lacy-like structure, respectively. Their modeling work allowed explaining these observations. For RH = 10%, the composition path did not cross the bimodal, so that no liquid-liquid demixing was possible and a dense structure was


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obtained. Further increasing RH to 20%, the composition path got closer to the binodal line and tended to cross it, so that the mechanism responsible for membrane formation was the nucleation and growth phenomenon. It resulted in a more porous membrane. Finally, for RH = 40%, the composition path went across both the binodal and spinodal lines, evidencing that the spinodal decomposition was responsible for membrane formation. These results were among the first ones highlighting the importance of the difference of water chemical potential between the air side and the system on the membrane formation. Also, one should note that Matsuyama and his coworkers did not choose the easiest system since they considered a semi-crystalline polymer (PVDF), for which crystallization phenomena also influences membrane formation. The same year, Park et al. focused on the PSf/NMP/water system.15 They changed the RH from 70% to 100% and observed the effect on morphologies. In all cases the same structure was obtained as far as the shape was concerned, but the size of cellules varied: it actually decreased with an increase of RH. The authors proposed the following mechanism to explain it. For lower RH, the driving force for water transfer is lower. As a result, phase separation is slower, therefore offering more time for the coarsening process of the polymer-lean phase to occur, yielding in larger cellules. Nevertheless, a reverse result was reported in the preparation of bi-continuous microporous chlorinated poly(vinyl chloride) membranes:62–64 when the RH increased, the pore size onto the surface of the membrane increased and the pore size distribution was broadened. In these studies, a combination of solvents, THF and n-BA, was used and the authors observed the formation of crumples (at RH lower than 30%) appearing in the solution. These formations were suspected to arise from an increase of the ratio n-BA/THF, due to a higher volatility of THF. What is more, it was shown that when increasing RH, crumples appeared more rapidly. The authors concluded that a higher RH led to a faster phase separation and to an increase of pore size onto surfaces. Clearly, both the change in RH and solvent composition influenced membrane formation, so that the role of RH, independently from that of solvent composition, remains unclear. Caquineau et al. focused on the formation of poly(etherimide) membrane and reported for the first time the effect of RH on such systems.44 From SEM observations, they studied the number of cells per unit area (NCPUA) and evidenced an increase of NCPUA with RH concomitant to a decrease of cell size. Anisotropy along the cross-sections was reported—the cells were smaller and smaller from the top-layer to the bottom one. Observations reported in this study were in accordance with those made in Park et al.’s work a few years before. For high RH, Caquineau stated that the important amount of water penetrating into the system rapidly induced the occurrence of a large amount of nuclei. Consequently, the polymer concentration in the polymer-rich phase continuously increased. In the meantime, the viscosity of this phase was enhanced, reducing the cell-growth rate and hindering the coarsening phenomena. The conclusion was that higher RH favored nucleation to the detriment of the cell-growth rate and coalescence. In their aforementioned study, Sun et al. also investigated the effect of RH on the formation of cellulose acetate membranes by VIPS processing.70 They insisted on the fact that the pore size was strongly affected by the relative humidity but also underlined the role of solvent evaporation in membrane formation. Their study, in which the solvent was acetone, suggested that at low RH (70% to 50%), the solvent evaporation was dominant in the course of membrane formation, so that structures obtained tended to denser morphologies, close to those observed in the dry cast process. On the other hand, when the RH was in the range (70%-90%), the influence of solvent evaporation on membrane formation was minor and morphologies were controlled by the RH. At 70%, the driving force for non-solvent



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transfer was necessarily lower than at 90%, giving more time to the coarsening mechanism to occur and therefore, yielding to larger pores. From these various studies, it can be concluded that dense to porous membranes can be formed by increasing the activity of the nonsolvent in the vapor phase. From a certain value, also depending on the system, further increasing the RH will prevent the polymer-lean phase to grow, resulting in smaller pores. 3.2.1.3 Effect of Temperature of VIPS Chamber on Polymer Membranes’ Morphology. Even though the temperature of the VIPS chamber may play a key role on mass transfers, the consequence on the type of morphology does not seem as important as that of exposure time to non-solvent vapors. For instance, Gironès et al. studied the formation of polymeric microsieves by a combination of phase separation techniques including VIPS.32 The influence of vapors’ temperature on the perforation level of their microsieves was minor. Similarly, a more recent study also highlighted that some little differences might be encountered when varying the temperature.40 Indeed Bouyer et al.’s study revealed that when decreasing the temperature from 40◦ C to 25◦ C, the cell size gradients of cellular-like structures were slightly reduced from the top to the bottom of the membrane. Hence, cells near the upper region observed at 25◦ C were smaller than those formed at 40◦ C. The study was further completed using a model and more specifically results from simulation of volume fraction profiles found throughout the VIPS process. At low temperature, the gradient of polymer volume fraction was reduced at 25◦ C prior to phase separation, due to a decrease of mass transfer rates subsequently leading to an increase of demixing time, compared to results obtained at 40◦ C. This gradient that we observed was believed to explain differences of cell sizes evidenced by scanning electronic microscopy observations. However, a very recent work pointed a significant effect of air temperature on surface morphology of PVDF membranes but for very low polymer concentration (4 wt%).11 Actually, a dense skin was gradually replaced by a bi-continuous network when the air temperature was increased in the range 27◦ C–75◦ C. This transformation resulted from spinodal decomposition. 3.2.1.4 Effect of Dissolution Temperature of the Polymer on Polymer Membranes’ Morphology. The dissolution temperature was not known to be a major parameter influencing membrane formation until very recently. Indeed, Li and coworkers highlighted a drastic effect of this parameter on the formation of PVDF membrane.6,86 Depending on the dissolution temperature range the membrane formation was mainly driven by crystallization or liquid-liquid demixing. PVDF membranes morphologies made of polymer nodules were obtained with dissolution temperature higher than 40◦ C when NMP was the solvent used. Lowering this value from 150◦ C to 45◦ C permitted decreasing the nodule size from 10 μm to 1.4 μm. In addition, the connectivity among the nodules was affected as well; it increased when reducing the dissolution temperature. When it was further reduced to 32◦ C, no nodule could be observed anymore and the structure had become bi-continuous, that is the pores were inter-connected and so were the polymer domains. Therefore, the PVDF membrane morphology could be tuned by adjusting the dissolution temperature, without modifying any other testing condition. As PVDF is a semi-crystalline polymer, the membrane can be formed by crystallization and/or liquid-liquid demixing, but whatever the dissolution temperature used, Li et al. reported that crystallization happened first, followed by liquid-liquid demixing. Nevertheless, gelation was strongly affected by the dissolution temperature: it was faster for low temperatures. In addition, FT-IR analysis permitted to detect the occurrence of an ordered conformation, appearing more quickly at lower dissolution temperature. At low dissolution temperature, the gelation mechanism happened before the occurrence of the ordered conformation, while this ordered conformation was responsible for the gelation mechanism at higher temperature. In other words, different

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mechanisms induced membrane formation whether the dissolution temperature was low (32◦ C) or higher (> 40◦ C). Also, different types of ordered conformations were detected at low or high temperature. To sum it up, the two solutions started gelation with different mechanisms, favored different ordered chain conformations, and resulted in membranes with different structures. 3.2.2 Effect of Formulation Conditions on Polymer Membranes’ Morphology. 3.2.2.1 Effect of Polymer Concentration on Membranes’ Morphology. Before explaining the important role of the polymer concentration on morphology of membranes prepared by VIPS, as reported in several papers and reports,4,15,46,70,87 one should be reminded of its important effect on kinetics on phase separation, which was studied by Di Luccio and coworkers.56 Their work concerned the formation of membranes by liquid-induced phase separation. However, some of their results are valid for the VIPS process as well. They used a light transmission method and noted that the precipitation onset was dependent on the polymer concentration. They stated that the most concentrated solutions precipitate first, as they need less water to become unstable. Nevertheless, they pointed that changes in this behavior might occur, owing to the occurrence of other effects such as the decrease of the precipitation rate due to the formation of an interfacial resistance between the non-solvent and the polymer solution. Actually, as the polymer concentration goes higher, the viscosity of the polymer solution gets higher too. Lee and coworkers studied the phase separation of polymer casting solution by nonsolvent vapor and reported more specifically the effect of the concentration.16 When increased, the self-diffusion coefficient depending on the friction and entanglements decrease because they are dependent on the viscosity of polymer solutions over an overlap concentration. Therefore, the kinetic of phase separation is drastically affected as well. As a result, the mobility of the polymer chains will be reduced, and the non-solvent diffusion will be lowered as well. Consequently, a higher resistance is applied against the coarsening of the polymer-lean phase. In other words, smaller pores are expected, and this was also observed by Park et al. in their study on polysulfone membrane formation15 or by Zhao et al.53 The effect of polymer concentration was previously explained by Mulder88 in the case of the LIPS process and their explanation can be applied herein as well: increasing the polymer concentration at the interface implies that the volume fraction of polymer increases, which results in a lower porosity. Hence, Matsuyama et al. studied the phase separation of PVDF/DMF solutions by water vapors and noted that when the polymeric concentration was increased from 10 wt% to 20 wt%, with a relative humidity fixed to 20%, fewer pores were found on the surfaces, which is strongly in accordance with Mulder’s rationalization.3,4 3.2.2.2 Effect of Solvent on Polymer Membranes’ Morphology. The effect of the solvent on phase separation rates and arising morphology of polymer membranes prepared by LIPS was previously reported.88 As the choice of solvent concerns first (i) the initial step, that is, the solution preparation common to VIPS and LIPS, and (ii) the affinity solvent-non-solvent—independent of the choice of the NIPS process, we may refer as well to LIPS. The solvency power is often considered as a key parameter influencing the kinetics of phase separation. If it is low, the solvent is termed as a weak solvent for the polymer, so that when the casting solution is contacted with a small amount of nonsolvent, phase separation occurs quickly. As a result, a porous morphology such as a finger-like structure is often obtained. On the other hand, kinetics will be lowered when a stronger solvent is used owing to the fact that a greater amount of nonsolvent will be necessary to induce phase separation. In this case, more dense morphologies may be obtained. The choice of solvent


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can have subsequent dramatic effect on membrane morphology and consequently, on other physical properties such as, for instance, the mechanical properties. If it seems appropriate to investigate the effect of the solvent on the morphologies of membranes prepared by VIPS, one has to remember that in VIPS, kinetics are necessarily slower than in LIPS owing to the gas/liquid interface, so very porous finger-like structures are rarely obtained using VIPS alone, even with a very weak solvent for the polymer. As a general rule valid for any nonsolvent induced phase separation process, the polymer has to be soluble in the solvent and the solvent must be miscible with the nonsolvent. From a theoretical point of view, the polymer solubility may be described by the so-called solubility parameter δ. According to the Hansen theory89–91 this parameter can actually be divided into three contributions due to dispersion forces (δ d ), polar forces (δ p ), and hydrogen bonding (δ h ) so that the expression of the solubility parameter is: δ=

δd2 + δp2 + δh2

(1)

Mulder explained that the three components could be considered as three vectors in a three-dimensional space and that the solubility parameter could be given at the end-point of the radius.88 Similarly, the solvent can be located in the (δ d , δ p , δ h ) space. In the end, the affinity between the polymer and the solvent is represented by the distance between the endpoints of the two vectors. The lower it is, the greater the affinity. Theoretically, it is then possible to figure the best solvent for one given polymer, by taking a look at the solubility parameters. These parameters for very common polymers and solvents used in VIPS process are given in Table 2. We may wonder as to what extent these solubility parameters and solubility, more generally speaking, will affect the morphology of polymer membranes prepared by VIPS. Li et al. studied the dependence of minimum and critical dissolution temperatures on solvent solvency for PVDF.86 They used three different solvents: NMP, DMAC, and DMF and showed that the better the solvent is, the lower the minimum and critical dissolution temperature are. Also, by selecting a dissolution temperature between the minimum and Table 2 Solubility parameters for common polymers and solvents used to prepare polymer membranes by VIPS process Species PSf PES PEI PVDF CA PC NMP DMF DMAC DMSO THF Dichloromethane

δd

δp

δh

δ

Reference

19.7 18.7 17.3 17.2 16.9 18.6 18.0 17.4 16.8 18.4 19.0 18.2

8.3 10.3 5.4 12.5 16.3 8.4 12.3 13.7 11.5 16.4 10.2 6.3

8.3 7.7 6.3 9.2 3.7 6.0 7.21 11.3 10.2 10.2 3.7 7.8

22.94 22.70 19.19 23.20 23.78 21.28 22.96 24.86 22.77 26.68 21.88 20.78

92 91 91 6 92 91 6 6 6 93 92 92


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critical dissolution temperature, one can obtain a porous membrane suitable for practical applications. In other words, the mechanical properties will be good enough, arising from a structure with high connectivity (bi-continuous), if the dissolution temperature is chosen in this appropriate range. Nevertheless, if the solvent solvency affects the maximum polymer concentration that can be obtained, Li and coworkers did not report a significant effect of the solvent on the morphology of the membranes. Using either DMAC or DMF as a solvent led to a bi-continuous structure when the dissolution temperature was close to the minimum dissolution temperature, while it was nodular when it was above the critical dissolution temperature. If Li and coworkers did not notice a significant change of morphology when changing the solvent for the polymer, a study published the same year reported that the solvent could actually have a drastic influence on the structures.21 Systems concerned this time were PSf/NMP/water and PSf/2P/water. With 2P, the PSf membranes presented a uniform bi-continuous structure throughout the whole cross-section. On the other hand, when NMP was the solvent for PSf, it was observed that the nascent lacy structure transformed to a cellular-like one and the membrane pore connectivity was lost; the main difference with the work of Li et al. is the viscosity range of the solvents used. While DMF, DMAc, and NMP, all have viscosities in the range [1–2 mPa.s] at 20◦ C, that of 2P is larger than 13 mPa.s at the same temperature. The viscosity of the solvent is, therefore, the key point explaining the effect or the lack of effect on the final morphologies. Tsai et al. explained that the employment of 2P increased dramatically the viscosity and elasticity of the polymer-rich phase formed after VIPS.21 They further confirmed their observations of the lacy structure by using another polymer, PMMA. Also, they elucidated the role of 2P on the membrane formation. According to their study, the increase in viscosity slowed down the coarsening of the phase-separated domains, and the increase in elasticity made the polymer-rich domains easier to gel, reducing the time allowed for the domains to coarsen. This study highlighted that more than the solvent solvency, its viscosity may be of major importance to control the morphology of matrices prepared by the VIPS process. Last but not least, the solvent volatility is of major importance. Especially, when rather low nonsolvent partial pressures are used, phase separation will be mainly controlled by solvent evaporation.70 Before a significant amount of nonsolvent penetrates the polymer system, destabilization has already occurred owing to a large solvent outflow. In this case, denser structures should be obtained, the mechanism involved in phase separation being close to that involved in the dry cast process. In order to make the nonsolvent inflow mainly responsible for demixing of the polymer solution compared to the solvent outflow, solvents with low volatility are usually employed in the VIPS process. 3.2.2.3 Effect of Additives on Polymer Membranes’ Morphology. Generally, the use of additive aims at increasing the viscosity of the casting solution. It then reduces the miscibility of the solution with the nonsolvent and therefore, hinders the phase-separation kinetics but greatly enhances the thermodynamics for phase separation. These additives may in the end influence the final nature of the morphology, modify the porosity, improve the pore connectivity, or change other properties (increase hydrophilicity, improve mechanical properties). There are quite a lot of additives that have been mentioned in the preparation of polymer membranes by the VIPS process as shown in Table 1. Many of them are polymer R ). In other cases, they can be additives with hydrophilic groups (PVP, PEG, Pluronic solvent or nonsolvent additives which will modify the quality of the solvent, subsequently affecting the phase separation rate and hence the morphology. But in almost all cases, the additives enhance the hydrophilicity of the polymer system, by either modifying the



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structure (physical effect) or by changing the nature of the membrane’s interface with air (chemical effect). Polyvinylpyrrolidone (PVP) is the most reported additive polymer in studies of membrane preparation by VIPS.12,23,24,29–32,35–37,62–64,68 Kang et al. studied the formation of microporous chlorinated poly(vinyl chloride) membranes.62–64 Among the different operating conditions tested, the use of PVP in the casting solution was one of them. They studied the surface topologies using field emission scanning electron microscopy, which revealed that the pore size of the membrane’s top surface became large and the pore-size distribution was broadened with increasing PVP concentration. According to them, the use of PVP, a rather hydrophilic polymer, accelerated the water transfer from the gaseous atmosphere to the polymeric solution, which arose in an increase of the surface pore size. They used the term of “spontaneous phase separation” but one should keep in mind that a spontaneous phase separation usually leads to finger-like structures. As they obtained lacy structures with high interconnectivity, the phase separation rate was not as fast as in LIPS and the role of the other influencing parameters such as the RH or the exposure time to vapors was crucial too. It highlights the complex role of PVP on membrane formation as well as the fact that other factors might influence its action. This is also reported by Han and Nam who incorporated PVP in the casting solution to prepare polysulfone membranes.23 They noticed that the membrane prepared from a low PVP content (5 wt%) presented enlarged macropores while for a 20 wt% content, these macropores tended to be suppressed. They explained the morphology evolution considering both the thermodynamic instability and the rheological behavior of systems. According to them, the trade-off between the two factors is essential. At low PVP concentration, the viscosity of the solution is barely affected by the additive and the phase separation kinetics is enhanced. On the contrary, at high PVP content, phase separation is delayed through diffusive hindrance. This arises in a skin region whose morphology is that of a finger-like structure at low PVP content and of a cellular-like structure at higher additive concentration. Also, a too high PVP content may be responsible for shrinkage, tearing or similar defects in membrane formation, as reported by Gironès et al.32 The conclusion of the role of PVP is the following: At low content, it improves the occurrence of porous structures while at high content (>10 wt%), denser structures are obtained, resulting in lower permeability (section 5.1).36 Another class of additives used in membrane formation by NIPS in general, and VIPS in particular is the polyethylene glycol (PEG) and polyethylene oxide (PEO) systems.9,25,26,36,71 These systems are mainly used to increase the hydrophilicity of the memR branes. In particular, Pluronic refers to triblock poly(ethylene oxide) - poly(propylene oxide) - poly(ethylene oxide) (PEO-PPO-PEO) copolymers. These are amphiphilic molecules: hydrophilicity and hydrophobicity are provided by the PEO and PPO groups, respectively. R can be prepared by changing the Mw or the PEO groups to A large range of Pluronic PPO groups ratio. Their role on membrane morphology is not clear up to date. For instance, R with PES but did not evidence any significant Susanto and Ulbricht blended Pluronic influence of the additive on membrane structuring.36 Also, their use was reported in the formation of PEGylated PSf and PEGgylated PVDF membranes.9,25,26 The PSf membranes obtained presented a dense surface with the occurrence of nodules on top, which were R in the initial solution. As for the PVDF membranes, morsmaller when using Pluronic phologies obtained were nodular whatever the formulation of the initial solutions. The R on membrane formation may come from the fact that the small influence of Pluronic concentrations tested were rather small (0 to 5 wt%). Some additives are sometimes referred to as non-solvent additives, and their influence on membrane formation is important. This is the case of 2-methoxyethanol which can


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permit to change a PES membrane morphology from a finger-like structure to a spongelike one, using the VIPS/LIPS combination.28 According to Shin et al. the non-solvent can delay the kinetics of liquid-liquid demixing.28 Referring to Smolders et al.’s work,94 from a certain concentration of non-solvent additive in the casting solution (“the minimum value”) membranes present a sponge-like structure. Young and Chen proposed that the presence of large amounts of non-solvent were responsible for the formation of nuclei.95 These nuclei were believed to inhibit the growth of the finger-like structure by limiting the non-solvent drainage from the sublayer. This was somewhat reported by Wang et al. also, who highlighted that when a hydrophobic casting solution containing hydrophilic additives (AA, propionic acid, ethanol, etc.) was exposed to air containing water vapor, the water vapor could be drawn onto the casting solution, resulting in nucleation of emulsion drops spontaneously.51 This would end up with cellular surface pores, whose size could be determined by the growth of the nuclei of emulsion drops. As this phenomenon can be controlled by both the amount of the hydrophilic additives and the time period allowing for the nuclei to grow, the results suggested that the pore size could be tailored.

4. Fundamentals of Phase Separation during Polymer Membrane /Hydrogel Formation by VIPS Process 4.1 Introduction During the last two decades, different numerical models have been developed for describing the mass transfer phenomena occurring during the polymer membrane formation, prior to the phase separation (whatever the phase inversion process, that is, dry cast, LIPS, TIPS). The complexity of these models increased little by little, including for instance the coupling between the mass and heat transfer phenomena for better describing the boundary conditions at the interface. If the first models involved only two components (polymer/solvent), ternary systems have been included more recently to take into account the cross-diffusion phenomena involved during the non-solvent induced phase separation. Some rare models also exist for describing the mass transfer modeling during the VIPS process (Matsuyama et al.,3 Yip and McHugh,5 Khare et al.,29 and Bouyer et al.40). Whatever the phase inversion process, the thermodynamics have been always described using the Flory-Huggins theory, assuming that the interaction parameters were known. Modeling the transfer phenomena in the polymer system surely permits to improve the understanding of mechanisms during polymer matrix formation, and provides a useful predictive tool,3–5,17,29,40,74–77,80,96–98 but cannot be used alone, without a parallel experimental study permitting to validate the model. In the specific case of modeling of polymeric film elaboration, Khare et al.,29 Yip and McHugh,5 and later, Werapun97 and Bouyer et al.,40 reminded that a scientific barrier exists—the understanding of diffusion phenomena within polymeric matrices. But other phenomena must be taken into account with accuracy, regardless of diffusion formalism when simulating the membrane formation. These phenomena represented in Fig. 7 include: • Thermodynamics in the polymer matrix (to get the chemical potential of each specie) and therefore at the interface between the external environment and the polymeric system (equilibrium assumption); • Chemical reactions between species (for chitosan gelation induced by basic vapors); • Mass transfers by natural or forced convection, between the external environment and the polymeric matrix, that is, solvent or non-solvent evaporation and/or absorption;



597

Figure 7. Mechanisms associated to the phase separation of a polymeric solution induced by a gaseous non-solvent (Color figure available online).

• Heat transfers between the polymeric matrix and the external environment. The lumped parameter approach is assumed in most of cases for VIPS process because of the small thickness of the casting films; • The variation of the spatial position of the interface. It will be seen in this section that after appropriately describing the system, a thermodynamic formalism must be expressed, classically the Flory-Huggins model. Once transferred from the gas phase, diffusion of non-solvent within the initial polymer/solvent system (or polymer/additive/solvent system) occurs, which corresponds to the onset of phase separation. In the meantime, a solvent outflow may occur. Transport at the interface, solvent non-solvent inflow and solvent outflow, are controlled by the chemical potential differences between the gaseous phase and the system. Diffusion of non-solvent can be described using various empirical or semi-empirical correlations. 4.2 Preliminary Assumptions and Geometry Adopted to Simulate Phase Separation Each model for VIPS process is based on a set of preliminary assumptions that permits to simplify and shorten calculations. These assumptions are as follows: • Solvent and non-solvent fluxes occur in a direction perpendicular to the plan of the interface; • The characteristic length of the casting film (diameter for instance) is much higher than the thickness, therefore, the wall effects can be neglected. Based on both the two latest assumptions, the system of equations can be written in only one

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A. Venault et al.

• • • • •


•

dimension (one-dimension geometry is frequently assumed). Also, the thickness of the polymeric system may vary over time, from an initial value H(t = 0) = H0 ; There is no evaporation of the polymer or of the potential additive or copolymer. Therefore, external mass transfers only concern solvent and non-solvent; Specific volumes of solvent and non-solvent remain constant throughout the elaboration; At the vapor phase/polymeric system interface, gas behavior is assumed to follow that of perfect gases; Thermodynamic equilibrium is ensured during the whole elaboration time at the air/solution interface. There is no influence of the polymer and of the potential additive on the activity of other species. The system is bounded by a substrate at the bottom and the non-solvent interface on top.

4.3 Thermodynamic Formalism of the Polymer System The thermodynamics of the system must be integrated for two main reasons: (i) the phase diagram of the polymeric system must be plotted to identify the homogeneous, gel, and diphasic regions, depending on its composition and temperature. (ii) The external mass transfers are controlled not only by the external hydrodynamics but also by the gradient of chemical potential of each transferring component (solvent, non-solvent). The knowledge of the chemical potential must be calculated at the air/solution interface. 4.3.1 The Gibbs Free Energy Formalism: Preparation of Membrane from a Water Vapor Phase. The Gibbs free energy of mixing (Gm) formalism constitutes the thermodynamic formalism mainly adopted. Actually, it is also that adopted in modeling of LIPS process. Equations (2) and (3) are then used in the case of a ternary system or a quaternary one, respectively: Gm = n1 ln ϕ1 + n2 ln ϕ2 + n3 ln ϕ3 + g12 n1 ln ϕ2 + g13 n1 ln ϕ3 + g23 n2 ln ϕ3 (2) RT Gm = n1 ln ϕ1 + n2 ln ϕ2 + n3 ln ϕ3 + n4 ln ϕ4 + g12 n1 ln ϕ2 + g13 n1 ln ϕ3 RT +g14 n1 ln ϕ4 + g23 n2 ln ϕ3 + g24 n2 ln ϕ4 + g34 n3 ln ϕ4 (3) where subscripts 1, 2, 3, and 4 stand for the non-solvent, solvent, polymer, and additive, respectively. T is the temperature and R the constant of perfect gases, while ϕ i and ni are the volume fraction and the number of moles of component i, respectively. The parameters gij are the interaction parameters, and may be concentration-dependent, as detailed later in this paper. From the Gibbs free energy of mixing, it is then possible to obtain the expression of chemical potential of species (μi ), by applying the following equation: Gm ∂ μi = (4) RT ∂ni RT |P ,T ,ni=j 4.3.2 Phase Diagram Calculation. Based on Gibbs free energy equations, the phase diagram of the binary or ternary system can be calculated. The calculus is based on the fact


599

that in the diphasic region, two phases exist which are in thermodynamic equilibrium: the polymer-lean phase α and the polymer-rich phase β. Hence, the chemical potential μi of component i in both phases are equal: β

μαi = μi

(5)

For a ternary system, Eq. (5) leads to three equalities and the mass balance equations must be added in each phase ( i ϕiα,β = 1). The whole system can be solved using optimization routines to plot the binodal curve at given temperature. The spinodal curve can be calculated if the following system is solved: G22 G33 = (G23 )2

(6)


with Gij = Vi

∂ 2 Gtm ∂ϕi ∂ϕj

(7)

In the last expression, Gtm is the Gibbs free energy of mixing per unit volume. Whether for the binodal or the spinodal, it is critical to know the interaction parameters gij . In the case of a ternary polymer/solvent additive system, as those studied by Matsuyama et al. or Yip and McHugh,3,5 g12 is concentration-dependent. On the other hand, the nonsolvent/polymer interaction parameter g13 is considered constant. Finally, the solvent/polymer interaction parameter may be concentration-dependent or not, but to the best of our knowledge, no unanimous expression for the dependence of g23 on polymer concentration has been reported yet. The different expressions and values of interaction parameters reported in the study of ternary and pseudo-ternary phase diagrams and compositions paths associated to VIPS process are gathered in Table 3. 4.4 Mass Transport Model in Polymeric System For all phase inversion processes, the Bearman statistical mechanical theory has been used to calculate the mass transfer by diffusion in a polymeric system.99 Bearman claimed that the frictional force in an n-components system is equal (but opposite in sign) to the chemical potential gradient: n (8) cj ξij νi − νj ∇μi = − j =1

where ∇μi is the chemical potential gradient of component i, cj is the molar concentration of j, ξij is the friction coefficient between molecules i and j, and vi is the velocity of the ith component. Bearman exhibited that the self-diffusion coefficients Di derive form the friction factors as follows: n Di = RT ci ξij (9) j =1

The determination of the friction factors remains difficult, since they may depend on both the composition and the temperature. Moreover, the lack of experimental data make impossible to give analytical expression in order to describe ξij . Therefore, preliminary assumptions must be made to calculate the friction factors. They can be derived from the

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A. Venault et al. Table 3 Interaction parameters reported in studies on VIPS process

System PVDF/DMF/Water


PES/PVP/NMP/water

CA/acetone/water

PVDF/DMF/water

PSf/NMP/water

PEI/NMP/water

PEI/NMP/water

PSf/2P/water

Interaction parameter g12 g23 g13 g12 g23 g13 g14 g24 g34 g12 g23 g13 g12 g23 g13 g12 g23 g13 g12 g23 g13 g12 g23 g13 g12 g23 g13

Expression

Reference

0.5 + 0.04 × u2 + 0.8 × u22 − 1.2 × u32 + 0.8 × u42 0.43 2.09 1.0 0.5 1.5 0.5 0.5 -1.0 1.3 0.5 1.4 0.5 + 0.04 × u2 + 0.8 × u22 − 1.2 × u32 + 0.8 × u42 0.43 2.09 0.785 + 0.665 × u2 0.24 2.7 0.785 + 0.665 × u2 0.507 2.1 / 0.507 2.1 0.47 0.32 + 1−0.50×u 2 0.50 2.50

3

29

5

5

5

5

40

21

u2 is defined as: u2 = φ 2 /(φ 1 +φ 2 ).

mutual diffusion coefficients for miscible fluids in the limit of quasi-binary diffusion.29,40 For solvent/polymer systems, Vrentas and Duda proposed to estimate the self-diffusion coefficients by the free-volume theory.100 Then the free-volume theory has been extended for ternary systems assuming some assumptions such as the independence of the friction coefficients with the solution composition.29,101,102 Thus, two ways can be considered to write the mass transfer model: (i) the mass flux can be directly derived from the Bearman theory (Khare et al.29), or (ii) the mass balance equations involving mutual-diffusion coefficients (Matsuyama et al.,3 Yip and McHugh,5 and Bouyer et al.40) expressed using specific diffusion formalisms (see also section 4.5.1). For more details, one can refer to these specific studies on diffusion whose references are also mentioned in Table 4.


601

Table 4 Diffusion formalism adopted in VIPS simulation VIPS model reference Matusyama et al.3

D11 D12

Diffusion formalism

∂μ2 1 = − NV2 1E E22 ∂μ − E 12 ∂ϕ1 ∂ϕ1 A 0

∂μ1 ∂μ2 V1 = − N 2 E E22 ∂ϕ2 − E12 ∂ϕ2 A

0

Khare et al. Yip and McHugh5

D22 = D2 (1 − ϕ2 ) (1 − 2g23 ϕ2 ) Constants Dik =

3 ∂μj ρi ∂μi D i, k = ϕ − D j i j RT ∂ρk ∂ρk

Bouyer et al.80

1, 2 Dij =

Bouyer et al.80 Bouyer and PochatBohatier98

Dik = (1 − ρi Vi ) ρi Di

2 1 ρj Vj ρi Dj RT

Venault et al.76

D12 = 7.4 × 1012 ×

29

2

Originally presented by Vrentas et al.103 Vrentas et al.103 Vrentas and Duda100 Alsoy and Duda104


j =1j =i

kT 6πaμ

j =1j =i

1 ∂μi RT ∂ρk

∂μj ∂ρk

Einstein;105 Stokes106 −

Price and Romdhane107

i, k = 1, 2

(φ×M2 )0.5 ×T μ2 ×V10.6

Wilke and Chang108

Di : self-diffusion coefficient of i, NA : Avogadro number, E0 and Eij : variable defined in 2, k: Boltzmann constant, a: molecular radius, μ’i : dynamic viscosity of i, Mi : molar mass of i, ϕ:association factor.

4.4.1 The Model of Khare et al. to Simulate Vapor-Induced Phase Separation of Quaternary Systems. Khare et al. proposed simulating the VIPS in the case of a quaternary system made of a polymer (PES), a polymer additive (PVP), a solvent (NMP) and a nonsolvent (water).29 They developed an isotherm mass transfer model (no coupling with heat transfer). Khare and coworkers expressed the relationship between the thermodynamic driving forces and fluxes: 4 ∂μi =− cj ξij Wi − Wj , i = 1, 2, 3, 4 ∂z j =1

(10)

Here, ξ ij is the friction coefficient between components i and j, and Wi the ith component velocity with respect to the stationary coordinates. The species velocity difference between the polymer and the polymer additive is negligible compared to the other species velocity differences (W3 = W4 ). Moreover, the very low polymer vapor pressures permits to assume that the polymer and polymer additive fractions in the vapor phase is zero, so that the local equilibrium condition is satisfied. The mass transfer of nonsolvent, solvent, and polymer in the quaternary system can then be described by both the continuity equation and those related to the species-conservation: ∂ (ρW ) ∂ρ =− ∂t ∂z ∂Jiz ∂ρi =− , i = 1, 2, 3 ∂t ∂z

(11) (12)

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where ρ is the mass density of the liquid phase and Jiz is the z component of the mass flux of i. Further assuming a low Peclet number, the final form of the mass transport equations were written as:

1 ∂ ∂ω1 ∂ω2 ∂ω3 ∂ω1 =− f1 + g1 + h1 (13) ∂t ρ ∂z ∂z ∂z ∂z

1 ∂ ∂ω2 ∂ω1 ∂ω2 ∂ω3 =− f2 + g2 + h2 (14) ∂t ρ ∂z ∂z ∂z ∂z

ω3 ∂ω3 ∂ω1 ∂ω2 ∂ω3 1 ∂ (f1 + f2 ) =− + (g1 + g2 ) + (h1 + h2 ) ∂t ρ ∂z 1 − ω1 − ω2 ∂z ∂z ∂z


(15) where ωi is the mass fraction of component i, fi , gi , and hi , are variable depending on friction coefficients, derivatives of chemical potentials with respect to mass fraction and molecular weights. In order to have the expressions of the three latter parameters, one may refer to the work of Khare et al.29 Next, initial and boundary conditions were written. An impermeable boundary was considered at the system/substrate interface while the mass balance yielded to the condition at the upper interface. Khare et al. considered that both the outflows of solvent and polymer were negligible, even though complementary studies showed the important influence of solvent evaporation on membrane formation by VIPS.70,74,79 The expressions of fi , gi , and hi involve the friction factors, which depend on the mutual diffusion coefficients between species Dij , that is, solvent/polymer, solvent/non-solvent, non-solvent/polymer, solvent/additive, non-solvent/additive, and polymer/additive: ξij =

Mj ωiB ρijB Dij

dμBi dωiB

(16)

where ωi B, ρ ij B, and μi B are the binary mass fraction, mass concentration, and chemical potential of component i, respectively. Khare and coworkers considered that the mutual diffusion coefficients were constant and they had to make additional assumption to calculate the friction factors. Their model gave reliable results and helped to understand why asymmetric polymer membranes were prepared by VIPS process using low volatile solvent.29 4.4.2 Mass Transport Model Based on Diffusion Formalisms. Matsuyama and coworkers were the first ones to propose a model for VIPS.3,4 Yip and McHugh developed a similar model including the coupling between the mass and heat transfers.5 Bouyer et al. derived the same model but paid special attention to the diffusion formalism and its influence on the final concentration gradients.40 The mass transfer models first expressed the mass fluxes using a Fickian diffusion approach: ⎛ ⎞ 2 ∂ρj ⎠ ∂ ⎝ ∂ρi = Dij − , i = 1, 2 ∂t ∂z ∂z j =1

(17)


603

where the subscripts refer to non-solvent (1) and solvent (2). Dij is the mutual diffusion coefficient and ρ j the mass concentration of i defined as: ρi =

ϕi Vi

(18)


ϕi and Vi are respectively the volume fraction and the partial specific volume of component i. Due to mass exchanges, the upper boundary of the domain moves so that it can be convenient to make a coordinate transform to fix the boundaries between 0 and 1: z (19) η= H (t) where H(t) is the thickness of the system at time t. Equation (17) is also rewritten in the new coordinate system: ⎛ ⎞ 2 ∂ρj ⎠ ∂ρi η ∂H (t) ∂ρi 1 ∂ ⎝ = + Dij − , i = 1, 2 (20) ∂t H (t) ∂t ∂η H (t)2 ∂η dη j =1 This system can be solved considering the initial conditions for the concentrations ρ i0 , temperature T0 and thickness H0 of the system: ρ1 (0, η) = 0

(21)

ρ2 (0, η) = ρ20

(22)

T (0) = T0

(23)

H (0) = H0

(24)

As for the boundary conditions, at the substrate/system interface, a symmetry condition applies (Eq. 25), while at the air/solution interface, conditions of fluxes for each component are assumed (Eq. 26, 27). Boundary conditions are then written as follows: η = 0, −

∂ρ2 ∂ρ1 =− =0 ∂η ∂η

(25)

For a ternary system, and considering a one-dimensional diffusion, diffusion fluxes of nonsolvent J1 and solvent J2 are then obtained by: i g D12 ∂ρ2 D11 ∂ρ1 dH (t) ∞ (T ) − ρ1g − − ρ1 = k1 ρ1g η=1 − T (26) H (t) ∂η H (t) ∂η dt i g D21 ∂ρ1 D22 ∂ρ2 dH (t) ∞ (T ) − ρ2g − − − ρ2 = k2 ρ2g T (27) H (t) ∂η H (t) ∂η dt i i ∞ where ρ1g and ρ2g are the concentrations of component i at the air/solution interface. ρ1g ∞ and ρ2g are the concentrations of component i in the gas phase at the temperature T g . k1 and k2 are the mass transfer coefficients of non-solvent and solvent, respectively. The concentration and activity of component i can be derived as follows:

∞ ρ1g =

∞ ρ2g =0

(28)

(T g ) .M1 RT

(29)

RH.P10

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A. Venault et al. ai .Pi0 (T ) , i = 1, 2 Vˆ ig P

μi , i = 1, 2 ai = exp RT i ρig =

(30) (31)

Due to mass exchanges, the film/air boundary moves and its position can be calculated using the following expression: 2


dH (t) = dt

i=1

Jig

ρp0

(32)

where Jig is the flux of i in the gas phase and ρp0 is the initial density of the polymer solution. These equations constitute the starting point of any model aimed at simulating transport of a solvent and a non-solvent during vapor-induced phase separation. Some important parameters need to be evaluated, which is further detailed in section 4.5. 4.4.3 Modeling Mass Transport during Gelation of Chitosan. Modeling of chitosan gelation is a bit different from those reported in sections 4.3.1 to 4.3.3 because the mass balance equation contains one supplementary term related to chemical reactions.73–76 Indeed, not only the rate of increase of mass of i per unit volume of the system and the rate of addition of mass of i per unit volume by diffusion had to be considered, but also the rate of production of mass of i per unit volume by reaction. Also, the system involved is chitosan/water + acetic acid/water + ammonia. But some species are created or consumed, so that the model presented two years ago actually involved seven different species i including chitosan (Chit-NH2 ), the protonated form of chitosan (Chit-NH3 +), water, ammonia, ammonium ion, acetic acid, and acetate ion. Therefore, the following mass balance equation, in which the reaction term Ri took into account reversible reactions, had to be solved for the seven species: ∂ρi + ∇ −Di/m ∇ρi = Ri ∂t

(33)

where Di/m is the mutual diffusion coefficient of i within the matrix. Ri can be expressed as a linear function of reaction rates rj and stoichiometric coefficients ν ij : νij rj (34) Ri = j

Like for the other models aforementioned, initial conditions were written. As for the boundary conditions, a symmetry condition was applied at the substrate system interface for all species while a flux condition for water and ammonia described the upper interface. These conditions can be written in a similar way as in Eqs. (21), (22), and (25–27). 4.5. Heat Transfer Model in Polymeric System Since adsorption and evaporation phenomena occur during VIPS, a coupling between mass and heat transfer improves the accuracy of the whole model. A lumped parameter approach is sufficient and the heat equation does not have to be solved in each case. Indeed, due to the low casting film thickness, heat transfer resistance is located in the gas phase, that


605

is, no temperature gradients are expected along the film thickness. In this case, uniform temperature is assumed and the variation of the film temperature is described by the following expression:5,40,75 hup T − Tg + hdown T − Tg + i Jig Hvi dT =− (35) dt ρs Cps H + ρp Cpp Hp


where ρ, Cp , and H are the density, heat capacity, and the thickness, respectively. The subscripts s and p refers to the substrate and the polymer solution, respectively. Jig and Hvi are the mass flux in gas phase and the vaporization enthalpy, respectively. T is the temperature of the solution and Tg is the bulk temperature in gas phase, above and below the solution. hup and hdown are the heat transfer coefficients above the polymer solution and below the substrate, respectively. 4.6. Determination of Model Parameters 4.6.1 Diffusion Coefficients in Polymeric Matrices. Diffusion formalism is essential to describe the transport of the solvent and non-solvent within the matrix. It is probably the key point to the obtaining of an accurate model. Different formalisms have been adopted for the simulation of VIPS, as discussed in section 4.3, and expressions of diffusion coefficients are summarized in Table 4. 4.6.2 Convective Mass Transfer Coefficients. Convective mass (ki ) and heat (he ) transfer coefficients can be obtained from experiments28 or from empirical correlations.5,40 In the latter case, expression for both free convection (Eqs. 36 and 38) and forced convection (Eqs. 37and 39) are available: ki =

0.27 × Dig (Gr × Sci )0.25 Lc × yair,lm

(36)

0.27 × Dig 0.5 0.33 Re Sci Lc × yair,lm

(37)

h e Lc = 0.27 (GrPr)0.25 λg

(38)

ki =

0.33 h e Lc = 0.664Re0.5 Pr λg

(39)

where Dig is the mutual diffusion coefficient of i in the gas phase, yair,lm is the log mean mole fraction difference of air, Lc is a characteristic length of the film surface, λg the gas thermal conductivity, he the heat transfer coefficient, Re is the Reynolds number, Sci is the Schmidt number, Pr is the Prandtl number, and Gr the Grashof number. 4.7 Simulation Results Although several numerical models have been developed, their validation using experimental results is still tricky. A complete model that aims at being fully predictive integrates lots of independent parameters, in the diffusion equations or in the empirical correlations, which should be determined using independent experiments. Consequently, the numerical


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Figure 8. Four different approaches to validate a VIPS model. (a) Morphological study, reproduced from Bouyer et al.40 with permission from Elsevier. (b) Gravimetric measurements. (c) Gelation front tests. (d) NIRS experiments (Color figure available online).

results given by the model depend on various parameters so that its validation is quite difficult and requires dedicated and localized experiments. The approaches that have been used to validate simulated data will be presented in this section. They include global and local validation methods, as shown in Fig. 8 and listed as follows: • The comparison of numerical data with morphological studies of polymer matrices.3,4,40 The concentration profiles along the casting film thickness prior to phase inversion are compared to the spatial distribution of the pore size in the membrane cross section; • Gravimetric measurements, that is, the variation of the global casting polymeric film weight during the process.3,74,80,96 This method is global and gets insight into both nonsolvent and solvent fluxes but cannot permit to distinguish the contribution of each to the weight variation; • Gelation front experiments during the process.74 Such a method involves a local validation of the transfers, but is still not a direct method in the sense that the composition of the system at each point is not measured. Also, the length scale in the experiments is expanded in this method; • Near-infrared spectroscopy applied to the quantitative determination of mass transfers.98,109 This method uses in-situ and in-line analysis that permits to quantify


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the solvent and nonsolvent concentration change in a polymer solution, separately. For this method, the length scale has been expanded as well to conduct the local experiments. Only after this validation step, the model can be used as a predictive tool and help better understand the mechanism responsible for film elaboration. 4.7.1 Validation of Models.


4.7.1.1 Global Validation. Comparison with polymer membrane morphology. A first method consists of comparing the polymer concentration profile of the polymer along the membrane thickness to the membrane morphology of the membrane.3,4,40 From an experimental point of view, this is easy to achieve since only scanning electronic microscopy (SEM) observations are required. Matsuyama et al. used their model to compute the volume fraction profiles of the different components.3,4 They evidenced that at initial stage of VIPS, the polymer volume fraction near the upper interface increased, whatever the testing conditions, while that close to the substrate interface remained constant. Subsequently, as the film thickness decreases during the elaboration, their model evidenced an increase of polymer concentration near the bottom surface, until a flat concentration profile was eventually obtained. They managed to correlate these data and compositions paths obtained from the thermodynamic model to morphologies in order to successively predict isotropic structures. Similarly, Yip and McHugh explained that the membranes fabricated by the VIPS process from PEI/NMP/water system were classically symmetric since the polymer volume fraction just before the phase inversion was flat. They expected opposite trends for other systems including solvents with higher volatility.5 Nevertheless, using the same system for membranes elaborated by the VIPS process, Bouyer et al. have given more details about the link between the mass transfers and membrane morphology.40 They compared their numerical data to SEM observations of the cross-sections of the membranes at increasing polymer concentrations. At 12 wt% of PEI, they exhibited that asymmetric membranes could be obtained with higher cell sizes near the air/solution interface. Furthermore, when the polymer content in the casting solution was increased from 16 wt% to 25 wt%, the cell-size distribution tended to be homogeneous along the cross-section. Actually, increasing the polymer concentration induces an increase of the solution viscosity and a corresponding reduction of the solvent and non-solvent fluxes since the resistance to mass transfer was enhanced within the solution. Consequently, concentration gradients were reduced, explaining the reduction of the cell size gradients in the membrane cross-section at higher polymer concentrations. In their work, they also presented the polymer concentration profiles obtained via numerical simulation. Their data clearly evidenced flatter polymer concentration profiles along the membrane’s thickness for higher polymer contents, which was strongly in accordance with SEM observations. Furthermore, their model could be further correlated to morphological studies by taking a closer look at the upper region, just beneath the air/system interface. Simulation exhibited that the polymer volume fraction reached a minimum and then increased near the upper interface (from η = 0.9 to η = 1), a trend that was supported by the SEM data exhibiting a reduction of the cell size in this region.


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Nonetheless and even though it is readily achieved, this validation using morphological studies does not take into account the fact that the morphology of the membrane is not fixed after phase separation. Surely, Khare et al. claimed that there should be a close correspondence between the final membrane structure, and the structure predicted by the VIPS model at the instant of phase separation.29 Yet, and even if the mobility of the polymer (and that of polymer additive) is much less than those of smaller solvent and nonsolvent molecules, it is believed that the morphology can still evolve before polymer vitrification, especially if one considers the ternary diagram, when the composition crosses the binodal line; that is, at the onset of the phase separation, it may remain in the metastable region for a period of time hard to quantify, so that the morphology can still change. This evolution cannot be seen by comparing SEM data to composition paths. Gravimetric measurements during polymer membrane/hydrogel formation. Another global validation consists of performing gravimetric measurements. The change of the casting film weight throughout the phase separation process is recorded on-line. In other words, it permits to follow the global composition changes, which results from solvent and non-solvent fluxes. In most works published presenting this technique applied to the VIPS process, the characteristic gravimetric curve presents a maximum.75,80,96,110 Some exceptions concern the case when the relative humidity is very low so that the system weight decreases from the very beginning of the experiment.4,44 The process is then closer to the dry cast process in the sense that phase separation is mainly driven by solvent evaporation. At intermediate to high RH, the global mass of the system increased from the very beginning of the exposure to non-solvent vapors, due to a fast non-solvent inflow. Once the maximum is reached, the solvent evaporation became the main phenomena responsible for mass changes, so that a decreasing of global mass was recorded. Note that when applied to the preparation of hydrogels, this technique also permitted quantifying and controlling the final water content of the matrix. This latest method is quite easy to carry out, if care has been given to the design of the VIPS chamber, but it does not permit to distinguish the solvent outflow from the nonsolvent inflow. Furthermore, phenomena within the polymer systems are not investigated at all using gravimetric measurements, so that it has a final limited interest and one should switch to more local validation. 4.7.1.2 Local Validation. Gelation (or demixing) front. Local validation requires measuring experimentally the local composition of the solution prior to phase inversion during the process. However, because the film is usually very thin (often in the range 300–500 μm for initial casting solutions aimed at preparing membranes or 1–2 mm for those used to prepare hydrogels), a scale-up of the system has to be performed to make these on-line measurements possible. The first local validation was performed for the elaboration of chitosan hydrogels.74,96 It consisted of recording the position of the gelation front throughout the elaboration time. A specific color indicator whose range of color change contains the pKa of chitosan polymer (6.5) was added to the casting solution prior to the gel preparation. When non-solvent (ammonia) penetrated within the solution, an increase of pH occurred resulting in chitosan gelation and a concomitant change of color of the system could be observed. While the solution was initially yellow, the gel turned blue owed to the diffusion front of ammonium ions, and the gelation kinetics could then be experimentally observed and subsequently plotted. The pH variations within the system and



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through the elaboration time could also be numerically assessed, and simulated data were shown to fit experimental ones, therefore validating the model. Local composition within the solution during the process. A more direct method was recently developed and presented, consisting in following at specific points and increasing depth beneath the top interface the composition of the system and therefore the non-solvent transfer rate by near-infrared spectroscopy (NIRS).98 The local water, polymer, and solvent weight fractions in a polymer solution could be obtained and this method has been proven to be suitable for various systems including PEI/DMAC/Water, PEI/NMP/Water, PES/DMF/Water, and PES/NMP/Water as long as the calibration step is carefully performed.109 In addition, it allows inferring mass transfers until the onset of the phase separation process. In the case of the PEI/DMAC/water system, it has been shown that the model was accurate for low to moderate polymer concentration (12 wt%) by comparing experimental data and numerical predictions. Nonetheless, from a comparison with NIRS data, the model seemed to overestimate the nonsolvent diffusion rate when the polymer concentration was further increased, owing to interactions at a molecular scale leading to polymer aggregation, which strongly affected the mass-transfer mechanisms by modifying the free volumes within the polymer matrix. In this respect, the use of a modified hole-free volume in the computations led to an improvement of the model accuracy, as discussed in the next section. As far as we know, this method was the first one reported that permitted quantifying the solution composition during the process on-line and point by point. However, it was assumed that the scale-up of the thickness had no consequence on the phase separation mechanisms, which is reasonable only if the characteristic time scales of the polymer (phase inversion, gelation, etc.) are short compared to transfer rates. 4.7.2 On the Use of the Models, Helpful Tools in Polymer Membrane/Hydrogel Elaboration. The point of trying to put equations behind a phenomenon is to go further, and use these equations to provide data that could not be reached using experimental methods, unless a lot of time, effort, and money is dedicated to it. This is for instance the case of concentration profiles in thin film geometry.3,5,29,40,74,79,80,96 Another goal is to use the numerical model to evaluate the elaboration feasibility in given conditions or the elaboration time of a solution having a given initial thickness. Moreover, when temperature is changed, it can be useful for predicting as to what extent the kinetics will be affected, owing to the important role of temperature on diffusion formalisms. Also, after having correlated the simulated data (concentration profiles, composition paths) to morphological data, one may predict the morphology of a membrane by inputting the testing parameters in the model, running it, and analyzing the concentration profiles, without having to perform additional SEM characterizations. Finally, numerical models can be used to provide supplementary insights on the phenomenon responsible for membrane formation.29,80,97,98 Some examples of the use of VIPS models as a predictive method are now going to be discussed. Yip and McHugh used their model to study the effect of most important parameters on membrane formation by VIPS, via the simulation of mass and heat transfers, phase diagrams and compositions paths associated to VIPS.5 They presented the effect of relative humidity on the composition paths in the CA/acetone/water ternary system for three different RH—51%, 68%, and 98.5%—and for a same initial temperature (296 K). The mass transfer paths highlighted that for this system it was possible to prepare membranes only for relative humidity higher than about 68%. At lower RH, the composition path never crossed the bimodal line. They could also foresee the precipitation times, which logically decreased with increasing RH. Similarly, their result concerning the PVDF/NMP/water


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system showed that membrane formation was only possible for RH higher than 20%. For the PSf/NMP/water system, their calculation highlighted that more than 24 hours were needed for the system to phase separate at RH below 50% while less than 20 min were necessary at a RH set to 65%. Finally, for the PEI/NMP/system, 3.5 h were needed at 30% RH, while this time fell to 281 s at 70%. Thus this study provided important data concerning the composition paths and demixing kinetics during membrane formation, depending on the VIPS testing conditions. It was completed by investigations on the effect of the type of convection on mass transfer paths in the CA/acetone/water system and for fixed temperature and RH (T = 296 K, RH = 98.5%). It highlighted that increasing the air velocity favored demixing and increased the evaporation rate of acetone (very volatile solvent). In addition, the corresponding polymer volume fraction profiles were plotted and the authors could conclude that membranes with a thinner skin should be obtained in forced convection. Another important result of Yip and McHugh’s study is that evidencing the role of diffusion formalism on mass transfer paths. Indeed, Matsuyama and his coworkers neglected the mutual diffusion coefficient D21 .3 If this parameter is not set to 0 but taken into account in the numerical calculus, then the final composition path obtained is different since the full diffusion model predicts a higher polymer concentration when crossing the binodal and a longer precipitation time. The conclusion was that remarkable differences in the prediction of final film structure could be obtained from different diffusion formalisms, which clearly pointed to the need for an accurate ternary diffusion model in VIPS. VIPS modeling can also be helpful in understanding the effect of a polymer additive in membrane formation. Indeed, Khare et al. used their model to state that the degree of asymmetry within the top-surface region of a membrane prepared form a PES/PVP/NMP ternary solution could be easily altered by changing the dope composition.29 From the numerical predictions, they concluded that smaller values of the PES/PVP ratio lead to greater degrees of asymmetry. Therefore, they manage to highlight the importance of the polymer additive on the structuring of polymer membranes from quaternary systems. As mentioned in the section in this paper dealing with the local validation of the model, Bouyer and Pochat-Bohatier pointed that for high PEI concentrations (16 wt%, 20 wt%), their model overestimated the nonsolvent intake rate, which they explained by a non-negligible influence of the aggregation process on the polymer chain conformation occurring before the end of the experiments.98 Indeed, their NIRS measurements were performed in an expanded scale prior to phase inversion. The strong modification of the polymer matrix observable by viscosity measurements was not initially taken into account in the diffusion model. Still, it strongly affected the mass-transfer rate. In this respect, they modified expressions of the hole-free volume and so, of the self-diffusion coefficients, which arose in more accurate data. The important point herein is that the average holefree volume was shown to be the relevant parameter to adjust for taking into account the modification of the transport ability due to nonsolvent intake in the polymer solution. In the case of the preparation of chitin and chitosan gels by a VIPS-like process, the simulations were performed in order to foresee the time required to prepare a matrix from a solution of a given initial thickness and in specific testing conditions.74–76,80 An example is displayed in Fig. 9 in which the concentration profile of ammonia and corresponding pH profiles are plotted throughout the elaboration of chitosan hydrogels in given testing conditions. It can be seen that in the conditions presented in Fig. 9, gelation over the whole thickness was reached after 2500 s, the time for which ammonia was available (concentration of ammonia > 0) at any position in the film. This is confirmed by the value of the pH. Below 6.5, the system is a solution while above this value, it is a gel. Similar data were presented



611

Figure 9. Ammonia concentration profile (main graph) and pH profile (inset graph) aiming at predicting the gelation time of a 2.55 mm deep chitosan solution for an initial ammonia partial pressure set to 2946 Pa (Color figure available online).

concerning the concentration profiles of the six other species studied.96 For instance, water concentration profiles displayed a maximum gradient at the beginning of the process, owing to an important water outflow from the initial stage of gelation. Eventually, the weak water outflow, assimilated to a drying in smooth condition, resulted in flat concentration profiles. Nevertheless, the concentration profiles in the case of chitosan gelation have not been correlated yet to morphological studies. Indeed, these matrices contain a lot of water (around 96 wt%) since they are termed hydrogels. As a consequence, the analysis of original porous structures is not an easy task to perform. Experiments on these gels have shown that lyophilization or the use of scanning electronic environmental microscopy still led to a partial shrinkage of the structures. Finally, in the case of chitin gels formation, model predictions showed that the nonsolvent (water) transfer from the vapor phase to the polymer solution was mostly controlled by the relative humidity and to a lower extent the temperature in the fabrication chamber.80 Weak concentration gradients were predicted in the whole thickness of the solution for the lowest temperature, owing to higher internal resistance to mass transfer. Increasing temperature, a higher driving force to mass transfer was responsible for larger gradients of water concentration profiles, eventually resulting in a decreasing of gelation time. To conclude, VIPS mathematical models are a powerful tool providing both predictions and important information with regard to the mechanisms responsible for polymer membrane formation. Yet, it can still be improved. To the best of our knowledge, the most accurate local predictions have only been performed by expanding the scale. But the analogy with thin films (polymer membranes and hydrogels) has not been proven yet, so that this validation is based on an assumption. Indeed, the time scale is expanded as well, so that the motion of polymer chains at the bottom of the system in large scale is favored, subsequently leading to potential aggregation and modification of diffusion formalisms, compared to those occurring in thin film geometry.

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5. Performances of Polymer Matrices Prepared by VIPS Membranes prepared by phase separation from the vapor phase present the main advantage to have a controllable structure, as extensively discussed above. Therefore, their range of potential applications is wide so that their method of preparation was patented.111–115 For instance, a combination of LIPS preceded by VIPS was described in which humid air is blown onto a cast homogeneous polymer solution until it phase separates, followed by immersion in water.112 Recently, a patent revealed how the VIPS process could be used to form superhydrophobic surfaces.114 However, owing to practical considerations, the development of commercial membranes by the VIPS process only remains limited, even though it is a very promising process for tailoring the morphologies of membranes. Dense membranes are applied in gas separation processes while porous polymer matrices can be used in water treatment, from nanofiltration to microfiltration applications. Also, some of these polymer matrices prepared by VIPS are aimed at being used in medical (wound dressings) or biological applications. 5.1 Polymer Matrices Prepared by VIPS in Water Filtration Water filtration applications represent the main uses of polymer membranes prepared by VIPS or a combination of VIPS/LIPS processes. Performances of such membranes are reported in Table 5. Whether under the shape of flat sheets or hollow fibers, ultrafiltration and microfiltration applications are extensively reported. Also, some studies mention the range of nanofiltration.46,59 In some very recent studies, an additive is used in the original casting solution. An increase of water permeability24,34 or on the contrary, its decrease is reported.36,68 These different trends may arise from the complex role of additive on membrane formation, as mentioned in section 3.2.2. But each time the hydrophilicity of final membranes is enhanced, in order to improve the resistance to biofouling. Indeed, biofouling is mainly attributed to the interaction of protein and bacteria with membrane materials. It is considered as a main issue, as well as concentration polarization, in membrane technology. Low-biofouling membranes are currently being investigated for water treatment applications. If many strategies already exist, consisting of modifying the interface by various processes (surface coating, surface chemical reaction, grafting, etc.), the VIPS route may be an appropriate process to achieve such a goal.8,24 The advantages of VIPS over other processes are: (i) one single unit operation in the membrane preparation process, (ii) modification of the whole matrix, and (iii) control of membrane structuring. As far as advantages (i) and (ii) are concerned, one can argue that the LIPS process would also lead to similar results. However, because amphiphilic polymers are used, they may be soluble in water and be washed out of the matrix during its formation as it is the case for R or PEG polymers for instance. Therefore, chemical modification is not achieved. Pluronic On the other hand, using VIPS and an exposure time to vapors long enough, hydrophobichydrophobic interactions can be established and the additive remains in the matrix. To ensure a total success of such a method, either a non-water soluble additive or a chemically modified polymer similar to the polymer matrix should be employed. The additive may migrate toward the interface during phase separation, to reach a higher thermodynamic stability. Also, it is expected to turn upside down throughout the elaboration. The reason is that the solution offers a friendly environment to the hydrophilic groups, while the final matrix, very hydrophobic, provides a good milieu to the hydrophobic segments. In the end, the gas/system interface created is hydrophilic and prevents or at least lowers the protein adsorption and bacterial attachment.

613

Geometry

Flat sheet

Flat sheet

Flat sheet

Flat sheet

Flat sheet

Material

PES/2-Me

PC/PAN

CPVC CPVC-g-PNVP

PEI/AA PEI/DGDE PEI/DGDE/AA

CPVC CPVC-PVP

Dead-end

Crossflow

Crossflow 22 cm s−1

Dead-end

Dead-end

Filtration setup

NF

4.6 ton m−2 day−1 at 200 psi 2.75 ton m−2 day−1 at 200 psi 1.6 to 1.27 ton m−2 day−1 at 200 psi (long term test) 2780 L m−2 h−1 at 0.05 MPa 9560 L m−2 h−1 at 0.05 MPa MF

MF

MF

575 L m−2 h−1 at 75100 torr 800 L m−2 h−1 at 75100 torr

atm

−1

/

h

−1

Range mentioned

2300 kg m−2 h−1 at 1 kg cm−2

730 L m

−2

Highest water permeability measured

64

47

62

57

28

Reference

(Continuend on next page)

A sponge structure is superior to a macrovoid structure to reach a high permeate flux. 2-Me draws water vapor into the solution to induce spontaneous emulsification and initiate the formation of a cellular surface structure. VIPS process applied to PC/PAN system suppresses the formation of macrovoids, and results in a skin-free membrane structure, reducing the mass transport resistance. Permeation properties of the modified membrane are superior to those of the unmodified membrane. Grafting hydrophilic NVP monomer on the microporous CPVC improves the performances of membranes. By controlling the ratio of DGDE to AA, the integrally skinned NF membranes exhibit appropriate NF performance characteristics and ideal membrane morphology. High performance NF membrane having moderate pure water flux (1.27 ton m−2 day−1) and high rejection rate (PEG 600, 83%) were designed. The pure water flux increases with RH and PVP content in the polymer solution.

Main observations

Table 5 Water treatment applications and performances of polymer membranes prepared using VIPS or a combination VIPS/LIPS


614

Crossflow

Dead-end

Flat sheet

Hollow fiber

Flat sheet

Flat sheet

PES

PES

BPPO

PES R PES/Pluronic PES/PEG PES/PVP

Crossflow

Dead-end

Dead-end

Flat sheet

CN

Crossflow

Hollow fiber

Geometry

PEEKWC PEKWC-PVP

Material

Filtration setup −1

−1

UF

UF

UF

1236 L m−2 h−1 at 0.2 MPa 120 L m−2 h−1 at 450 kPa 75 L m−2 h−1 at 450 kPa 70 L m−2 h−1 at 450 kPa 45 L m−2 h−1 at 450 kPa

MF

UF

UF

Range mentioned

10–4 m3 m−2 s−1 at 20.7 kPa

6 mL cm−2 min−1 at 0.1 MPa 9 m3 m−2 h−1 atm−1

412 L m h bar 365 L m−2h−1bar−1

−2

Highest water permeability measured Addition of PVP results in a very steep increase of the dextrane rejection (selectivity) with only a limited reduction of the water permeability. Bovine serum albumin adsorption, maximum adsorption capacity: qm = 110.48 μg cm−2. Water permeability increases with exposure time to water vapors owing to an increase of pore sizes. From a certain time (60 to 70 s), shrinkage of structure occurs during VIPS, resulting in lower permeabilities. The pure water permeation and the solute separation depend on the gas type used in the gap (VIPS step). BSA rejection of the membrane depends on exposure time to nonsolvent vapors. 90% rejection for an exposure time set to 2 min. R Addition of Pluronic results in the highest hydraulic permeability. The rejection curves obtained using mixture of dextrans are similar for all modified membranes. R leads to the most hydrophilic Addition of Pluronic membranes. R membranes Antifouling effects of PES/Pluronic are the most efficient at similar protein rejection.

Main observations

36

71

33

39

70

68

Reference

Table 5 Water treatment applications and performances of polymer membranes prepared using VIPS or a combination VIPS/LIPS (Continued)


615

Flat sheet

Hollow fiber

Flat sheet

Flat sheet

PES PES/Pluronic/ TEG

PAI/CA

PSf PSf/PVP PSf/PANI

PES/SPES/ PVP/PEG

Dead-end

Crossflow 0.22 m s−1

Crossflow

Dead-end

A significant increase in resistance toward fouling is obtained (using BSA as a model protein). Lower protein adsorption is observed for the R . membranes prepared with Pluronic Higher fluxes obtained with membranes prepared from the casting solution having a composition of R = 10/30/55/5 (wt%), PES/NMP/TEG/Pluronic with 3 min exposure time to humid air, at 50–60% RH. The pure water permeability may be improved by adding nonsolvent additives (IPA, ethanol, methanol). The addition of both PVP and PANI nanofibers increases membrane pure water flux and antifouling property.

Good wetting with water can be achieved by the addition of a neutral hydrophilic polymer such as PVP and which can be even improved by small quantities of ionic groups by a polymer such as SPES. Additional positive impact of additives on filtration performance with respect to the recovery of permeability after fouling. The dominating effect of pore morphology onto filtration performance is demonstrated.

MF

NF

UF

MF

1200 L m−2 h−1 bar−1 94,960 L m−2 h−1 bar−1

11.93 L m−2 h−1 bar−1 175 L m−2 h−1 at 0.20 MPa 375 L m−2 h−1 at 0.20 MPa 400 L m−2 h−1 at 0.20 MPa 25,000 L m−2 h−1 at 300 kPa


35

24

60

34

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5.2 Polymer Matrices Prepared by VIPS in Gas Separation For gas separation and when hollow fiber membranes are reported, we only refer to the influence of the “dry step”, that is, the VIPS step, on the performances of membranes. An increase of air-gap, and thus of the exposure time to non-solvent vapors, do not have negative influence on selectivity. It either remains steady19 for an initial oxygen/nitrogen mixture or increases for a gas/oxygen mixture.46 When an increase of selectivity is reported, inevitably the decreasing of permeability is measured. The different performances can be correlated to the morphological changes. The adsorbed water initiates a phase separation in the as-prepared membrane and induces a structure change in the cross-section near the outer surface of hollow fibers. The formation of a cellular structure near the outer surface is reported and is responsible for the increase of selectivity. The cellular structure of hollow fibers may also be facilitated by the gravitational forces and elongational stresses as well as slow precipitation rates associated with a small solubility parameter difference.50 But whatever the main phenomenon responsible for the morphology changes during the VIPS step, an increase in the thickness of the closed-cell cellular structure as the air-gap length increases is responsible for the selectivity improvement. 5.3 Medical and Biological Applications of Polymer Matrices Prepared by VIPS Polymer membranes prepared by VIPS can also be used in medical and biological applications. Some applications of membranes prepared by VIPS concern wound dressings,73–80 cell culture,51 drug release,59 or the separation of antibodies.70 Because the use of matrices prepared by VIPS-like for wound dressings applications are well detailed in literature, we will further discuss these applications. Chitin and chitosan are outstanding materials for wound dressing applications because they are biocompatible, bioresorbable, and bioactive. Chitin is a natural biopolymer made of N-acetylglucosamine mostly extracted from the shells of crustaceans. Chitosan is natural as well but mostly obtained from deacetylation of chitin. If the VIPS-like gelation process is not the main route to obtain these gels, the slowness of gelation kinetics may lead to a better control of hydrogels structuring and therefore of their structure-related properties. Therefore, this method was studied by different groups to optimize the preparation of hydrogels suitable for wound dressings.73–80 Gels exhibited a porous structure suitable for an optimal drainage of exudates. Also, their mechanical properties were investigated in details since wound dressings also act as a protective barrier again any possible mechanical stress.77 That study evidenced a significant role of some particular parameters including the polymer concentration, the exposure time to nonsolvent vapors and the temperature of the chamber, on the modulus of elasticity of chitosan hydrogels. By increasing the polymer concentration, more entangled polymer chains in the solution lead to favorable conditions for the formation of physical junctions stabilizing the network. The exposure time to vapors allowed decreasing repulsive electrostatic interactions between chitosan chains. Finally, the temperature of the chamber affected the mobility of chitosan chains, the polymer–polymer interactions, and the mass-transfer kinetics of ammonia. 5.4 Other Applications of Polymer Matrices Prepared by VIPS Among the other possible applications of polymer membranes prepared by VIPS, their use as a metal chelator was reported by Chen et al. They blended PEI and CA and induced the mixture to water vapors.49 The final matrix obtained was shown to be suitable for separating biomolecules, complexing heavy metal or further coupling ligand. One of the


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major highlights mentioned was the use of PEI as metal chelator instead of iminodiacetic acid, in order to prepare immobilized metal ion affinity membrane. Tsai et al. focused on how VIPS affected the formation of macrovoids near the outer surface of hollow fibers and the pervaporation performance of 90 w% aqueous isopropanol solution through PSf hollow fiber membranes.22 They clearly evidenced the strong effect of the VIPS step on the morphology of membranes and consequently, on their performances. The water content in the permeate was shown to increase, then decrease, and increase again while the flux decreased, then increased, and decreased again as the air gap increased. It was correlated to the formation of macrovoids which disappeared, reappeared, and re-disappeared with the increasing air gap. Finally, the use of membranes to adsorb protein adsorption is also reported.12 A very hydrophobic polymer such as PVDF combined to a rough surface provides an effective means to retain proteins including BSA.

6. Conclusions and Outlook This paper aimed at surveying the works done using the vapor-induced phase separation process to prepare polymer membranes, as well as polymer hydrogels. From this review, the following conclusions can be drawn: • Four main synthetic polymers have been used in the preparation of membranes by VIPS process: polyvinylidene difluoride, polysulfone, poly(ether sulfone), and poly(ether imide) owing to their great importance in membrane technology. Moreover, two natural bioactive polymers have been tested, chitin and chitosan, the aim being to use the properties of controlled structures in medical applications. Nevertheless, this study reveals that many other systems, ternary, as well as quaternary, have been tested. • Control of various polymer membrane structures as well as their tunable design can be obtained. Indeed, if one has to remember one single thing, it should be that the VIPS process is an outstanding technique to tailor and tune morphology of membranes prepared by phase separation. This observation arises from slow to very slow phase separation rates owed to a gas/liquid interface. Therefore, the number of testing conditions (process parameters, as well as formulation parameters) that can drastically influence phase separation of polymeric systems is important, making VIPS a process of great interest and potential. • Modeling of VIPS was successfully achieved for the first time in the late 1990s, and improvements have been made throughout the years. One of the main issues probably concerns the diffusion formalism to be embraced. Validation of the VIPS simulation may be experimentally carried out using different techniques involving global mass transfer tests (gravimetric measurements) or more local measurements (gelation kinetics, NIRS tests). The models can be used to predict concentration profiles, elaboration times, and composition paths or to provide further insights in the main mechanisms responsible for phase separation of the polymer system. • Thanks to a large set of structures that can be prepared and controlled by VIPS, gas separation, for which dense structures are required, as well as water treatment applications (mainly ultrafiltration and microfiltration) or bio-medical applications are reported. Yet, some unanswered questions and issues remain. When more than three components are involved, which is now often the case to add some specific properties to the matrices, the additive may modify both the surface/bulk chemistry and the morphology of membranes.


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It is not clear yet as to what extent one or the other effect will be predominant. Especially, for polymer additives, their effect on membrane structuring is not expected to be minor, as the matrix polymer concentration drastically affects the morphology. But this review highlighted that it seemed to depend on the type of polymer additive, so that the role of additive in membrane formation by VIPS remains unclear. Concerning the simulation of VIPS process, the local validation has only been achieved by expanding the scale. Yet, it has to be done in thin polymer film, in order to neglect the potential effect of polymer aggregation on diffusion formalisms. Although many potential applications with outstanding performances have been studied and reported as highlighted by the present paper, the scale-up of membranes prepared by VIPS is still not readily achieved. In this respect, to make the VIPS process more competitive and make it the number one phase separation process, an outstanding control of membranes’ properties must be obtained, and if possible with reduced exposure time to vapors. This control is very good so far, but can still be improved. A great deal of work still needs to be done to control the structuring of polymer matrices at a nano-scale, and reach a perfect tuning of pore sizes and pore size distribution. Also, efforts need to be made to predict membrane morphologies and arising performances when a testing condition is changed. To achieve this goal, both experimental work and simulation studies will be helpful and hopefully lead to the globalization of this promising technology. Finally, when VIPS is combined to LIPS, mechanisms responsible for polymer membrane formation are totally unclear. One fundamental question for which there is still no answer is related to the process that actually dominates membrane structuring.

Abbreviations AA, acetic acid; AC, activated carbon; BPO, brominated polyphenylene oxide; BSA, bovine-serum-albumin; CA, cellulose acetate; CN, cellulose nitrate; Chit-NH2 , insoluble form of chitosan polymer; Chit-NH3 +, soluble form of chitosan polymer; CPVC, chlorinated poly(vinyl chloride); CPVC-g-PNVP, CPVC-g-poly(N-vinyl-2-pyrrolidinone); CTA, cellulose triacetate; DCM, dichloromethane; DEG, diethylene glycol; DGDE, diethylene glycol dimethyl ether; DMA, dimethylamine; DMAC, dimethylacetamide; DMF, dimethylformamide; DMMSAPS, 3-(N,N-Dimethylmyristylammonio)propane-sulfonate; DMSO, dimethyl sulfoxide; EVA, poly(ethylene-co-vinyl acetate); EVAL, poly (ethyleneco-vinyl alcohol); GB, γ -butyrolactone; HFIP, 1,1,1,3,3,3-hexafluoroisopropanol; IPA, isopropanol; LIPS, liquid-induced phase separation; MF, microfiltration; NF, nanofiltration; NIPS, nonsolvent-induced phase separation; NMP, N-Methyl-2-pyrrolidone; NVP, N-vinyl-2-pyrrolidinone; PAI, polyamide-imide; PAN, poly(acrylonitrile); PANI, polyaniline; PBI, poly[2,2-(1,3-phenylene)-5,5-bibenzimidazole]; PC, polycarbonate; PEEK, poly(ether ether ketone); PEEKWC, modified poly(ether ether ketone); PEG, poly(ethylene glycol); PEI, poly(ether imide); PES, poly(ether sulfone); SPES, sulfonated PES; PLLA, poly(L-lactic acid); PMMA, poly(methyl methacrylate); PS, polystyrene; PS-b-PDMS, poly(styrene-b-dimethylsiloxane); PSf, polysulfone; PVDF, polyvinylidene difluoride; PVP, polyvinylpyrrolidone; RIPS, reaction-induced phase separation; TEG, triethylene glycol; TEOA, triethanolamine; TEP, triethyl phosphate; THF, tetrahydrofuran; TPX, poly(4-methyl-1-pentene); UF, ultrafiltration; VIPS, vaporinduced phase separation; VIG, vapor-induced gelation; 2-Me, 2-methoxyethanol; 2P, 2pyrrolidinone.


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Nomenclature


The units are not provided in this table since different authors sometimes use different units for the same parameter. Therefore, for more details, one may refer to the specific works mentioned in Section 4, in which most of the symbols appear. δ δd δh δp Gm Gtm Hvi μαi β μi ξ ij ϕi ϕ η λi μi μi B μ’i νi ν ij ρ ρp0 ρi ρ i0 ρ ij B i ρig ∞ ρig ωi ωi B a ai cj CPp CPs Di Dig Dij Di/m E0 Ei fi gi gij

Solubility parameter Contribution of the solubility parameter due to dispersion forces Contribution of the solubility parameter due to hydrogen bonding Contribution of the solubility parameter due to polar forces Distance between the endpoints of the polymer and the solvent vectors Gibbs free energy of mixing Gibbs free energy of mixing per unit volume Heat of vaporization of component i Chemical potential difference in the polymer-lean phase Chemical potential difference in the polymer-rich phase Friction coefficient between components i and j Volume fraction of component i Association factor Dimensionless position Thermal conductivity of i Chemical potential of component i Chemical potential of component i in a binary system Dynamic viscosity of i Velocity of the ith component Stoichiometric coefficients Mass density of the liquid polymer system Initial mass density of the polymer solution Mass concentration of i Initial mass concentration of i Binary mass concentration of i Mass concentration of i at the gas/system interface Mass concentration of i in the gas phase Mass fraction of component i Binary mass fraction of component i Molecular radius Activity of i Molar concentration of j Specific heat capacity of the polymer solution Specific heat capacity of the substrate Self-diffusion coefficient of i Mutual diffusion coefficient of i in the gas phase Mutual diffusion coefficients Mutual diffusion coefficient of i within the matrix Variable defined in ref. 3 Variable defined in ref. 29 Variable defined in ref. 28 Variable defined in ref. 28 Interaction parameters


620 Gr hi he hup hdown H (t) H0 Jig Jiz k ki L0 Lc Mi NA PNS Pr Pi 0(T g) rj R Ri Re Sci T T0 Tag Tc Tg u2 Vi Wi yair,lm z(t)

A. Venault et al. Grashof number Variable defined in ref. 28 Heat transfer coefficient Heat transfer coefficients above the polymer solution Heat transfer coefficients below the substrate Thickness of polymer system at time t Initial thickness of polymer system Flux of i in the gas phase z component of the mass flux of i Boltzmann constant Gas side mass transfer coefficient of species i Initial thickness of the polymer solution Characteristic length of the film surface Molar mass of i Avogadro number, Nonsolvent partial pressure Prandtl number Saturated pressure of i, function of the gas phase temperature Reaction rate Constant of perfect gases Reaction term Reynolds number Schmidt number Temperature Initial temperature of polymer system Temperature of air gap Temperature of chamber Air temperature Ratio of the volume fraction of nonsolvent to the sum of the volume fractions of solvent and nonsolvent Partial specific volume of component i. The ith component velocity with respect to the stationary coordinates Log mean mole fraction difference of air Position in the film at time t

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