Polymer-Silica Nanocomposite Membranes for CO2

Accepted Manuscript Original article Polymer-Silica Nanocomposite Membranes for CO2 capturing Nezar H. Khdary, Mamdouh E. Abdelsalam PII: DOI: Reference:

S1878-5352(17)30119-3 http://dx.doi.org/10.1016/j.arabjc.2017.06.001 ARABJC 2110

To appear in:

Arabian Journal of Chemistry

Received Date: Accepted Date:

16 March 2017 1 June 2017

Please cite this article as: N.H. Khdary, M.E. Abdelsalam, Polymer-Silica Nanocomposite Membranes for CO2 capturing, Arabian Journal of Chemistry (2017), doi: http://dx.doi.org/10.1016/j.arabjc.2017.06.001

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Polymer-Silica Nanocomposite Membranes for CO2 capturing Nezar H. Khdary1, * and Mamdouh E. Abdelsalam3 *1

King Abdulaziz City for Science and Technology, P O Box 6086 Riyadh 11442, Kingdom of Saudi Arabia; Tel: +966555515619. Email: nkhdary-kacst.edu.sa

3

Department of Chemistry, Faculty of science, King Faisal University, Al-Ahsa 31982, Saudi Arabia.

Abstract Reducing carbon dioxide (CO2) is an area of great interest in current international efforts geared towards lowering emissions and combating global warming. In this work, amino-silica composite membranes were prepared and used to capture carbon dioxide. The surface of silica particles was chemically modified with amine to efficiently capture carbon dioxide. The phase separation technique was used to prepare the membranes from a composite containing polyvinylidene-fluoride-hexafluoropropylene (PVDF-HFP), amino-silica particles, acetone and water. SEM images revealed that the membranes composed of multi layers of porous polymer uniformly impregnated with silica particles. Both XRD and FTIR results have validated the perfect integration of silica particles within the polymeric network. The mechanical properties of the membrane is improved by the presence of silica particles as proved by the high tensile strength value (1.5 N/cm2) obtained for the PVDF-HFP/SiO2 membrane compared to (0.9 N/cm2) obtained for bare PVDF-HFP membrane. Also, we succeeded in recording SEM images to show that the plastic deformation of the film associated with the formation of macro-holes. To the best of our knowledge this is the first time for such results to be monitored with SEM to observe the macroscopic evolution of the structure.

Additionally, the surface area was significantly

increased from (3.8 m2/g) for bare PVDF-HFP membrane to (116.4 m2/g) for PVDF-HFP impregnated with silica particles. Moreover, the CO2 separation efficiency depends on both surface area and the quantity of amino-SiO2 added to the membrane. The addition of aminosilica particles leads to a significant uptake of carbon dioxide compared to non-modified polymer membrane. The results obtained indicated that combing the phase separation with amino silica particles provided a cost effective route to scaling up the synthesis of membranes that were mechanically stable and highly efficient at CO2 capture.

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Key words: Nanocomposite membrane; phase separation; SiO2 nanoparticle; PVDF-HFP membrane; CO2 capturing.

1. Introduction Carbon dioxide (CO2) has been nominated as the main anthropogenic greenhouse gas and also the primary felon in climate variation1,2. Decreasing anthropogenic CO2 release and depressing the greenhouse gas concentration in the earth’s atmosphere is currently utmost high priority environmental issue. More recently, there has been increased interest in synthesizing new materials to solve CO2 issue. Different types of membranes including polymeric, inorganic and composite membranes have been employed for gas separation over last few decades3,4,5,6. Inorganic membranes e.g. alumina and zeolite, have exceptional separation efficiency compared to polymer membranes including high chemical and thermal stability. However, their brittleness, poor mechanical properties, difficult processing and high cost make them less attractive. On the other hand, the organic membranes have inherent advantages such as high permeability, low cost, easy processing and reasonable gas separation properties7,8,9. Nevertheless, they suffer from poor selectivity consequently low separation efficiency. Incorporating inorganic nanoparticles into polymer membranes produces nanocomposites. These hybrid materials lead to an emerging field of research which incorporates the possibility of combining characteristics of several materials and significantly contributing towards resolving some of the associated challenges. Materials which efficiently adsorb CO2 elect themselves to be

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good candidate to formulate nanocomposite membranes. Examples of these materials are highly porous metal–organic frameworks (MOFs)10,11,12

activated carbon13, zeolites14 and amine

modified mesoporous silica15,16. On the other hand, organic polymer membranes such as polyvinylidene fluoride (PVDF) have been extensively applied to scientific research and industrial process due to its outstanding properties. The membrane exhibits excellent chemical resistance and thermal stability with high mechanical strength. Also it has excellent aging resistance which is very important for the actual application of separation membranes17,18. In addition, the porous PVDF membranes could be readily prepared by the non-solvent induced phase separation process from a polymer solution in two miscible solvents19,20,21 . First, PVDF copolymer is dissolved in a solvent which has low boiling point, e.g., acetone. Afterwards, a second solvent, e.g., water is added to the polymer solution. The second solvent is selected in such a way that it has higher boiling point than the first solvent and does not dissolve the PVDF copolymer. Once a thin polymer film is formed on a substrate, the preferential loss of the low boiling point solvent will produce meta-unstable complex system, and phase separation takes place. Porous membrane was thereby obtained22,23. The physical and surface properties of organic membranes also gets enhanced due to the inorganic fillers24,25,26,27. Inorganic materials mixed with PVDF have been included in Al2O328, ZrO229, TiO230, and SiO231. Silica is the most convenient and widely used among all these materials because of its mild reactivity and wellknown chemical properties. A study has been carried out on ultrafiltration membranes by composite materials (e.g. PVDF with Al2O3) using the phase inversion method This investigation based on the characteristics like porosity, membrane hydrophilicity, surface morphologies and protein retention. The addition of Al2O3 nanoparticles promoted the surface hydrophilic character which resulted in a significant increase in the permeation flux of the membrane32..

3

In this work, we fabricate nanocomposite membranes, comprised of porous PVDF-HFP film impregnated with SiO2 particles using phase separation process which contains the solution of PVDF-HFP, amino-SiO2 particles, acetone and water. The structure and morphology of the films were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) coupled with Energy Dispersive X-ray spectroscopy (EDXS), N2 adsorption isotherm and Fourier Transform Infrared Spectroscopy (FTIR). The CO2 adsorption–desorption behavior of PVDFHFP and PVDF-HFP/SiO2 films were assessed by thermal gravimetric analysis (TGA).

2. Experimental 2.1 Materials The reagent grade solvents and chemicals were used directly without further purification. Poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP) (MW 400,000 g mol-1), tetraethyl orthosilicate (98%) and hydrochloric acid (37%) were obtained from Sigma-Aldrich (Steinheim, Germany). High surface area silica ultra-pure (SiO2-FS) which has a particle size of 40-60 µm, pore size of 60 A° and BET surface area of (411.6 m2/g) was obtained from Acros (New Jersey, USA). N-(3-(Trimethoxysilyl propyl) ethylenediamine) (PEDA) was also obtained from the same supplier. Acetone, methanol, ammonia (32 %, w/w) were obtained from Scharlau (Barcelona, Spain). The solutions were prepared freshly using deionized water (18 MΩ cm-1), integral systems from Millipore (USA) were used. Ultra-high purity helium gas and CO2 gas with purity of 99.995 % were used in this study.

2.2 Synthesis of silica nanoparticles

4

In this work two different types of silica have been investigated. The first type was the high surface area silica ultra-pure (SiO2-FS) which was purchased from Acros. The second type was low surface area silica nanoparticles (SiO2-NS). The latter was prepared by adding 5 ml of tetraethoxysilane, as a precursor, to a mixture of 250 ml methanol and 100 ml of ammonia. The reaction was performed at 20 °C for 6 hours with stirring. The silica particles were collected by centrifuge at 4000 r min-1 for 1 hour. The particles were washed and dried at 60 °C under vacuum overnight33. The surface area of SiO2-NS was (36.0 m2/g) calculated using BrunauerEmmett-Teller (BET) method with multipoint adsorption data from the linear section of the N2 adsorption isotherm. The particle size was measured at 300 nm by The NanoSight nanoparticles characterization system (NS-500).

2.3 Surface modification of Silica Modifications were made with PEDA by pouring 100 ml of dry toluene into the 250 ml flask connected with a reflux condenser to both high surface area (SiO2-FS) and low surface area (SiO2-NS) silica. Overnight oven dried (120 °C) SiO2 (1g) was dispersed completely in toluene and the temperature was increased up to 90 °C. During the undergoing stirring, after the temperature has stabilized, PEDA (2 ml) was added slowly in the flask and stirred for 6 hours under nitrogen environment, then solution temperature reduced to room temperature. Further, this modified silica (amino–SiO2) was collected through centrifugation and washed with toluene thoroughly before drying in the vacuum oven34. The abbreviation of SiO2-AFS for amine modified high surface area silica will be used while SiO2-ANS abbreviation will be used for amine modified low surface area nano-silica. 2.4 Synthesis of polymer-silica nanocomposite membranes. 5

The phase separation method was employed to fabricate the porous polymer membranes using a PVDF-HFP in both acetone and water35. Typically, PVDF-HFP was dissolved in acetone and stirred for 2 hours, until a clear polymer solution was obtained. In a separate container, the required weight of amine modified SiO2 particles were mixed thoroughly with acetone. Then the SiO2-AFS or SiO2-ANS suspension was added to the polymer solution. The PVDF-HFP/SiO2 mixture was stirred for 4 hours to achieve uniform and homogenous dispersion of SiO2 particles in the composite. The amount of acetone which was used to prepare the PVDF-HFP/SiO2 solution was controlled to produce 1 weight percentage concentration of PVDF-HFP in the final mixture. The water was added subsequently in the mixer to get 1:1 water to acetone weight ratio. Adding water to the polymer solution leads to increase the viscosity. Further increase of water to acetone weight ratio over 1:1 develops the polymer to precipitate. After water addition step, the mixture was thoroughly stirred for 6 hours to achieve good dispersion of inorganic particles in the membrane structure. The dip coating method was executed to develop a thin layer of polymer film on a glass microscope slide under ambient conditions. As Acetone is more volatile compared to water, acetone evaporates first and leads to rapid mutual diffusion between the nonsolvent and solvent and creates a highly porous polymer matrix impeded with amine modified silica. The dip coating techniques was chosen because it produces uniform films with controllable thickness.

2.5 Instrumentations The shape and morphology of the SiO2 particles, and membrane were investigated by Field Emission SEM (JSM-7800F, Jeol) coupled with Energy Dispersive X-ray spectroscopy (EDXS). Fourier transform infrared spectroscopy (FTIR) spectra were recorded at room temperature by a 6

Vertex 70 V spectrophotometer with a resolution of 4 cm-1 and number of scans was 64. The BET surface area analysis was achieved by the Micromeritics ASAP 2020 instrument. The samples were degassed at 90 oC for 4 hours prior to the analysis. The X-Ray Powder Diffraction was utilized to identify the possessions of the composite membrane using the Bruker D8 Advance, the source of x-ray was a 2.2 kW Cu anode and the running conditions were 40 kV and 40 mA. TGA-DSCA (SDTQ600 V 20.9) was used to monitor the weight change of samples with the temperature. In this experiment, a known sample weight was placed in the platinum pan. Then the sample was pre-activated for 40 min. under a flow of 10 ml/min of ultra-pure helium at 50 o

C. Subsequently, the sample was exposed to CO2 gas. The adsorption capacities and heats of

adsorption were calculated based on the DSC–TGA weight gain versus time. Dip coater (ZR4200) was used to produce thin film of membranes on top of glass microscope slides. The sample was taken out of the coating solution at a controlled speed of 0.2 cm/min and the traveling distance was 5 cm. The membranes tensile strength was measured by Gatan micro tensile stage model MTEST-200, stage module fitted with high sensitive 200N load cell (1 % accuracy) and an integrated extensometer (3 µm resolution & 10 µm accuracy) to provide precise measure of sample elongation. Membrane samples of fixed size (6.0 mm long and 2.0 mm wide) were used to perform all tensile strength measurements. The tensile stage was installed inside the SEM chamber after the sample settled on the stage, then the elongation was observed in the SEM.

3. Results and discussion 3.1 Fabrication and characterization of nanocomposite membranes 7

Nanocomposite membranes were prepared by the phase separation technique from a blend composed of PVDF-HFP/SiO2, acetone and water. The steps involved in the preparation process

Scheme 1. Polymer undergoes phase separation in non-solvent to produce porous PVDF-HFP membrane impregnated with silica particles are illustrated in Scheme 1.

Acetone was carefully chosen as it is a good solvent for the PVDF-HFP polymer with low flash point. Accordingly, water was chosen as it is a non-solvent for PVDF-HFP, miscible with acetone and has a higher flash point than acetone. After thorough mixing of the composite components, a thin polymer film was applied on a glass microscope slide substrate using the dip coating method with well-controlled speed of 0.2 cm min-1. It was found that, the thickness of

8

the film could be increased by increasing the withdrawal speed of the substrate out of the coating vessel. Thickness is determined by observing the force balance at stagnation point on the liquid surface. With an increment of withdrawal speed, excess amount of fluid is pulled up onto the substrate surface before it settles back down into the solution. After completing the coating process, the ambient conditions were used to dry the substrate. Acetone evaporation leads to a rapid rate of mutual diffusion between the non-solvent and solvent. Thus, developing a well oriented porous structure in the polymer matrix. The differing diffusion roots to evaporate the solvent from the thin film, while the non-solvent will enter the film. Additionally, when the PVDF-HFP polymer is exposed to these two types of diffusion, it will undergo a micro phase separation to form a diverse array of periodic nanostructures embedded with SiO 2 nanoparticles36. The weight loading of SiO2 can be adjusted by controlling the quantity of SiO2 added to the polymer solution. The produced membranes after complete evaporation of acetone and water have poor adhesion to the glass substrate and were easy to lift from the surface to form free standing films. These films are mechanically very stable and can be shaped to the required size to fit the target capturing applications. Figure 1a-d shows a series of SEM images of the PVDF-HFP thin films developed on top of the glass substrate. According to the formation conditions, the polymer films have different pore sizes with various morphologies. Figure 1a and 1e shows a porous membrane produced from a polymer solution in an acetone/water mixture. The created pores have irregular shapes with an average pore size of 3.37±0.9 µm. additionally; the polymer film is relatively thick with multiple layers of misaligned and connected spherical macro cavities. When SiO2 particles were added during synthesis, a porous film was produced in which SiO2 were uniformly distributed into the polymer network. Figures 1b and 1c illustrate the morphology of the PVDF-HFP membrane 9

loaded with 20 and 40 percent by weight of SiO2-AFS, respectively. It is very clear that the SiO2 did not block the pores of the polymer film. However, when the weight percentage of the SiO2 was increased from 20 percentage to 40 percentage the pore size decreased and the film become less porous. Figure 1d and 1f show the structure of membrane loaded with 20 weight percent of SiO2-ANS.

Figure 1. SEM images of porous PVDF-HFP films produced from a solution containing (a) 2 wt% of PVDF-HFP in acetone/water mixture, (b-d). Similar to (a) 10 %, (c) at 40 wt % while (d) contains after adding SiO2 -AFS particles at (b) 20 wt

SiO2-ANS at 20 wt %, (e) 2 wt% of PVDF-HFP in acetone/water mixture at high magnification and (f) contains SiO2-ANS at 40 wt % at high magnification

3.2 Energy Dispersive Spectroscopy (EDX) EDX was also utilized to confirm the elemental analysis of the membrane. Two peaks have been identified for the PVDF-HFP film without SiO2 as shown in figure 2a. The first peak at Kα = 0.277 KeV is attributed to carbon while the second peak at Kα = 0.677 KeV is attributed to fluorine. When the PVDF-HFP film was impregnated with SiO2, two more peaks were observed. The peak at Kα = 0.52 KeV represents oxygen and the peak at Kα = 1.74 KeV represents silicon as shown in fig 2b, c, d. The intensity of the Si and oxygen depends on the weight loading of the SiO2 in the composite. This is clearly indicated by the higher intensity of Si and of oxygen peaks for the film containing 40 percent by weight of the high surface area SiO 2-AFS than the same film containing 20 percent by weight of the SiO2-AFS composite. Moreover, the intensity of the Si and oxygen peaks is independent of the type of SiO2 at the same weight loading. This can be seen from the comparable intensities of the Si and oxygen peaks for the membrane comprised 20 percent by weight of high (SiO2-AFS) and low surface area SiO2-ANS as shown in figure 2b and 2d respectively.

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Figure 2. Energy Dispersive X-ray spectroscopy (EDXS) of (a) PVDF-HFP membrane, (b) PVDF-HFP membrane impregnated with 20 wt % of SiO2-AFS, (c) PVDF-HFP membrane impregnated with 40 wt % of SiO2-AFS, and (d) PVDF-HFP membrane impregnated with 20 wt % of SiO2-ANS.

3.3

BET Surface area.

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Figure 3 shows the N2-adsorption-desorption isotherms curves for PVDF-HFP film without SiO2, SiO2-AFS powder and for PVDF-HFP films embedded with SiO2-AFS and SiO2-ANS respectively. Curve 3a, which was recorded for PVDV-HFP membrane alone, matches a type III isotherm demonstrating the presence of a weak adsorptive adsorbent interaction between nitrogen molecules and the polymer membrane37. On the other hand, samples containing high surface area SiO2-AFS, produce a type IV isotherm, indicating capillary condensation and monolayer-multilayer adsorption. Table 1 reports the specific surface area for all investigated samples. The values in the table were calculated by the BET method with multipoint adsorption data from the linear section of the N2 adsorption isotherm. It is evident from the table that, the attachment of NH2 to the SiO2-FS surface drastically reduces the N2 uptake. The BET value calculated for the modified SiO2-AFS was (198 m2/g), which is lower than the value of (411.6 m2/g) calculated for unmodified silica SiO2-FS. Similarly, the BET value for SiO2-NS decreased from 36 to 19 m2/g following modification with NH2. The reduction in the surface area of amine modified SiO2 is consistent with the published literature38,39. Apparently, the presence of NH2 grafted groups limits the ease of access and adsorption of nitrogen gas to the surface of SiO2 particles. Another trend identified, was that the surface area of the PVDF-HFP film was increased following embedding of SiO2. It was found that the surface area significantly improved from (3.8 m2/g) for the PVDF-HFP film alone to (116.4 m2/g) for the PVDF-HFP/SiO2 nanocomposite. On the other hand, impregnating the PVDF-HFP film with SiO2-ANS only slightly increased the surface area from (3.8 m2/g) to

13

(12.0 m2/g). This enhancement in the surface area will improve the separation properties of the nanocomposite membrane towards CO2.

(b)

(a)

(c)

(d)

Relative pressure (P/Po) Figure 3. N2 adsorption – desorption isotherms for (a) porous PVDF-HFP (b) SiO2-AFS (c) PVDF-HFP impregnated with 20 wt % of SiO2-AFS (d) PVDF-HFP impregnated with 20 wt % of SiO2-ANS. Table 1: BET for SiO2 particles and PVDF-HFP membranes with and without SiO2 particles Sample

BET(m2/g)

Pore volume cm3g-1

Average pore diameter (Å)

14

PVDF-HFP membrane

3.80

0.0102

108.12

SiO2-FS

411.6

0.336

32.68

SiO2-AFS

198.0

0.183

47.53

PVDF-HFP-SiO2-AFS

116.4

0.1826

62.79

SiO2-NS

36.0

0.1092

82.23

SiO2-ANS

19.0

0.0289

112.36

PVDF-HFP-SiO2-ANS

12.0

0.0189

180.36

3.4 Fourier Transform Infrared Spectroscopy (FTIR) FTIR was used to analyze the membranes and SiO2 particles. The Spectrum of amino-silica is shown in Fig. 4a. The bands at around 1106 cm-1 and a shoulder at 1283 cm-1 are attributed to anti-symmetrical stretching40 of Si-O-Si. The band at 795 cm-1 is ascribed to Si-O-Si vibration41. The band at 918 cm-1 corresponds to Si-OH stretching42, the two bands at 2919 and 2838 cm-1 are due to stretching of C-H in the silane coupling agent. The N-H stretches band at 3360 and 3216 cm-1 are characterized by weak intensity; therefore they are not observed in the spectrum under the current experimental conditions.

43

. The spectrum for the PVDF-HFP membrane is

shown in Fig 4b, which confirmed the presence of α and β PVDF crystal phases44. The bands at 3022 and 2980 cm-1 are attributed to the CH2 asymmetric and symmetric vibration of PVDF17. The band at 1403 cm-1 corresponds to CH2-CH2 wagging vibration and the C-C band of PVDF is appearing at 1185 cm-1, the bands at 878 and 840 cm-1 are related to C-C-C asymmetrical stretching vibration and C-F stretching vibration of PVDF respectively32. Figures 4 c, d and e are for the silica embedded in the PVDF-HFP membrane. It was found that the characteristic absorption peaks of the PVDF-HFP were retained in the spectrum of the composite membrane. For instance, the peaks of PVDF-HFP at 3022 and 2980 cm-1 were related to the CH2 asymmetric and symmetric vibration of PVDF. The peak at 1403 cm-1 which represented CH2 wagging vibration was also retained. Additionally, the peak at 840 cm-1 which corresponded to C-F 15

stretching vibration of the PVDF was also observed in the composites.

(a)

(b)

(c)

(d)

(e)

Wavenumber (cm-1) Figure 4. FTIR spectra of (a) amino-silica, (b) PVDF-HFP membrane, (c) PVDF-HFP membrane impregnated with, (c) 20 wt % of SiO 2-AFS, (d) 40 wt % of SiO2-AFS and (e) 20 wt % of SiO2-ANS.

3.5

X-Ray Diffraction Analysis

The XRD diffractogram in Fig 5 shows the amorphous phase for silica and amino-silica (Fig 5a and b) which confirm that, the silica particles maintain their amorphous phase after chemical 16

modification. The PVDF-HFP membrane shows four main peaks at 2θ = 18.2(020), 19.6(100), 26.8(021) and 39.5(002) which confirm the crystalline α phase for PVDF-HFP. When the silica particles were impregnated into the membrane; the intensity of these peaks decreased. The reduction in the intensity of the peak depends on the weight percentage of the silica (Figures 5d, e and f). However, broad peaks for amorphous phase of silica were diminished after incorporating the silica particles into the membrane. This is due to the low amorphous silica content present in the membrane relative to the pure sample used to record 5a and 5b spectra45,46.

(a)

(100)

(b)

(020) (110) (002)

X-ray Intensity

(021)

(c)

(d)

(e) (f)

17

2θ(degree)

Figure 5. XRD diffractogram of (a) amorphous silica, (b) amino-silica, (c) PVDF-HFP membrane, (d) PVDF-HFP membranes impregnated with 20 wt % of SiO2-AFS, (e) 40 wt % of SiO2-AFS and (f) 20 wt % of SiO2-ANS

3.6 Mechanical properties of nanocomposite membranes Mechanical properties of the nanocomposite membranes have been investigated by measuring the tensile strength. Figure 6 a-d shows the stress versus elongation curves for PVDF-HFP films with and without silica particles of different types and weight loadings. Alongside these curves, SEM images were recorded simultaneously by placing the stress testing device inside the SEM chamber and monitoring the film integrity while applying the loading force. It is very clear from the curves and coupled images that for each film the structure was maintained up to the yield point which is defined as the local maximum point in the stress-elongation curve36. Beyond this point the plastic deformation occurs and the film deforms permanently. The SEM images show that the plastic deformation of the film is associated with the formation of macro-holes. To the best of our knowledge this is the first time for such results to be monitored with SEM to observe the macroscopic evolution of the structure. The size of the hole increased as the loading force was increased until the complete rupture of the film. Table 2 reports the tensile strength, which was obtained by measuring the maximum stress of the film withstands before rupturing. Table 2 also includes the calculated strain, which is the ratio of the elongation to the original length of the investigated membranes. The tensile strength recorded for the PVDF-HFP alone was 0.91 N/cm2 and had a strain of 0.52. Adding 20 wt % SiO2 to the PVDF-HFP increased the tensile strength to 1.15 N/cm2 and strain to 0.53. Further increasing the 18

weight loading from 20 to 40 wt % resulted in additional enhancement of these values, as seen in the tensile strength of 1.51 N/cm2 and strain of 0.67. This indicates that, mechanical stability of the PVDF-HFP depends upon the weight loading. Another important parameter is the size of particles; this can be seen when comparing the same weight loading for SiO2-AFS and SiO2ANS particles, the membrane impregnated with SiO2-ANS has higher tensile strength (1.4 N/cm2) than the membrane which impregnated with SiO2-AFS 1.15 N/cm2. The enhanced tensile strength for the PVDF-HFP/SiO2-ANS membrane could be ascribed to the good dispersion of SiO2 nanoparticles and the strong interaction between SiO2 nano-particles and PVDF matrix. Both mechanical strength and toughness of the membrane has significantly improved after impregnation with SiO2. Additionally, the mechanical integrity of the PVDF-HFP/SiO2 has been monitored by in-situ recording of SEM images while stretching the membrane. Fig. 7 shows SEM images recorded using two detector mode both lower electron detector (LED) and back scatter detector (BSD). The figures confirm the finding that adding SiO2 to the PVDF-HFP improves the mechanically stability and toughness.

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Table 2. Tensile strength and elongation at break for PVDF-HFP films with and without SiO2

Sample

Wt% SiO2

Tensile strength N/cm2 Strain

PVDF-HFP membrane

0

0.91

0.52

PVDF-HFP-AFS

20

1.15

0.53

PVDF-HFP-AFS 40 1.51 0.67 Figure 6. Stress versus elongation curve with real-time SEM images to show the evolution of PVDF-HFP-ANS 20 1.40 0.75 the film stability versus stretching. (a) PVDF-HFP membrane, (b) PVDF-HFP/SiO2-AFS at 20 wt %, (c) PVDF-HFP/SiO2-AFS at 40 wt%, and (d) PVDF-HFP/SiO2-ANS at 20 wt%. The top SEM images were recorded simultaneously with the stress testing device inside the SEM chamber.

(a)

(b)

(c)

(d)

20

Figure 7: Using two detectors, LED & BSD in SEM to study the effect of stress on the membrane for (a) PVDF-HFP, (b) PVDF-HFP/SiO2-AFS at 20 wt%, (c) PVDF-HFP/SiO2-AFS at 40 wt% and (d) PVDF-HFP/SiO2-ANS at 20 wt %. The arrows point to the stress area where the membrane starts to tear

3.7 CO2 Adsorption The TGA technique was used to measure the CO2 adsorption–desorption behavior of PVDF-HFP and PVDF-HFP/SiO2 films. All measurements were recorded at 50 oC and 1.0 atm. Prior to the experiment; the films were activated for 120 min at 40 oC under vacuum. The CO2 uptake was measured by selecting the isotherm at 50 oC. Thereafter, CO2 was passed through the film until no further weight gain was noticed. The complete adsorption–desorption cycle lasted for 60 min for all the samples. The PVDF-HFP film alone did not adsorb CO2 indicating a lack of affinity. While PVDF-HFP/SiO2-AFS and PVDF-HFP/SiO2-ANS films were found to gradually adsorb CO2 over the first 24 min. Additional adsorption was observed at a slower rate until equilibrium was attained. The maximum CO2 adsorption capacity and heat of adsorption are reported in Table 3. The results demonstrate that, the CO2 adsorption capacity depends on the surface area of the SiO2 at the same weight loading. This is clearly seen when comparing the CO 2 uptake of PVDF-HFP/SiO2-AFS at 26.27 mg/g with that of PVDF-HFP/SiO2-ANS at only 12.36 mg/g, both films were at 20 weight percent SiO2 loading. Also, the CO2 uptake was found to be dependent on the weight percent of the SiO2 in the composite. An increase in the CO2 uptake from 26.27 mg/g to 33.75 was obtained for PVDF-HFP/SiO2-AFS membrane when weight loading was increased from 20 to 40 weight percent. This is in good agreement with the number reported for amine modified SiO2 powder47. CO2 uptake values of 17.7 mg/g, 26.84 mg/g and 21

19.80 mg/g were reported for the SBA-15 grafted with γ-(Aminopropyl)triethoxysilane48, Cu 50

nanoparticles supported on mesoporous silica

and Copper- nanoparticles–ethylenediamine–

silica gel51 respectively. Moreover the heats of adsorption calculated from the DSC heat flow indicate that the heat of adsorption for the PFDV-HFP alone was (0.126 J/mole). Upon adding 20 weight percent of SiO2 the values increased to (0.480 J/mole) for the PVDF-HFP/SiO2-AFS and (0.342 J/mole) for the PVDF-HFP/SiO2-ANS. This increase in the heat of adsorption could be due to the strong interaction between the amine modified SiO 2 and CO2. This interpretation also explains why the high surface area PVDF-HFP/SiO2-AFS has a greater heat of adsorption when compared with the low surface area PVDF-HFP/SiO2-ANS. Additionally, it was found that the heat of adsorption obtained for PVDF-HFP/SiO2-AFS at 40 weight percent (0.607 J mole-1) was significantly higher than the value obtained for the same film at 20 weight percent (0.480 J mol1

). This higher heat adsorption indicates that the CO2 adsorption is also dependent on the weight

loading percentage of amine-SiO2. Table 3: CO2 uptake and heat of adsorption for bare PVDF-HFP membrane and PVDF-HFP membrane impregnated with SiO2. Sample

CO2 uptake mg g-1

Heat of adsorption (J mole-1)

PVDF-HFP Blank

1.70

0.126

PVDF-HFP-20 wt% AFS

26.27

0.480

PVDF-HFP-40 wt% AFS

33.75

0.607

PVDF-HFP-20 wt% ANS

12.36

0.342

4. Conclusion PVDF-HFP/SiO2 nanocomposite porous membrane has been successfully prepared by the phase separation technique. Mixture of acetone and water was used to dissolve the PVDF-HFP 22

polymer. The porosity of the membrane was controlled by controlling the ratio of water to acetone in the mixture. The presence of SiO2 in the composite improved the mechanical properties of the membrane. The highest tensile strength was obtained for PVDF-HFP-AFS membrane which contains 40 wt % of SiO2 particles. The PVDF-HFP/SiO2 composite showed excellent selectivity towards CO2 thanks to the high surface area of the silica particles which was chemically modified with amine to efficiently capture CO2. The maximum CO2 adsorption capacity (33.75 mg/g) was delivered by PVDF-HFP-AFS membrane which contains 40 wt % of SiO2 particles. This value is comparable to the number reported for amine modified SiO2 powder which indicates that incorporating the SiO2 particles in the membrane did not change considerably its adsorption power towards CO2. Ability to produce these composites material with low cost will secure applicability in many diverse areas including air and water purifications.

Acknowledgement The author expresses great gratitude for the financial support of this work from King Abdulaziz City for Science and Technology (KACST) project number 593-32. And for the generous grant from Fulbright Grant program (ID: 68160653).

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Polymer-Silica Nanocomposite Membranes for CO2 capturing

Phase separation technique was used for producing amino-nanocomposite porous organic-inorganic

(PVDF-HFP/SiO2)

membranes for CO2 sequestration. The

mechanical

membrane

are

properties

of

the

improved

by

the

presence of silica particles as proved by the high tensile strength value. We succeeded in recording SEM images to show that the polymer deformation of the film is associated with the formation of macro-holes. To the best of our knowledge this is the first time for such results to be monitored with SEM to observe the macroscopic evolution of the structure. CO2 uptake value as high as 33.75 mg/g was obtained for the PVDF/HFP impregnated with 40 % by weight of SiO2-AFS.