J Polym Res (2013) 20:216 DOI 10.1007/s10965-013-0216-3
ORIGINAL PAPER
Effect of solvent type on the morphology and gas permeation properties of polysulfone–silica nanocomposite membranes Mahdi Pourafshari Chenar & Hamed Rajabi & Majid Pakizeh & Morteza Sadeghi & Ali Bolverdi
Received: 29 November 2012 / Accepted: 8 July 2013 # Springer Science+Business Media Dordrecht 2013
Abstract In this study, the effects of various solvents on the structure and permeation properties of polysulfone–silica nanocomposite membranes were investigated. Silica nanoparticles were prepared by the sol–gel method through the hydrolysis and condensation of tetraethyl orthosilicate (TEOS). Polysulfone–silica nanocomposite membranes were prepared by the thermal phase inversion method. N-methyl pyrrolidone (NMP), N,N-dimethyl acetamide (DMAc) and tetrahydrofuran (THF) were used as solvents. Based on the experimental results, it was observed that the CO2/N2 and O2/ N2 selectivities increased in the presence of silica nanoparticles in all cases. However, the permeabilities of the applied gases decreased, except for CO2. Based on the obtained selectivity data, permeability data, and favorable dispersion of silica nanoparticles in the polymer matrix, the results indicate that NMP is the best solvent for polysulfone–silica membrane preparation. The obtained CO2 permeability and CO2/N2 selectivity of the polysulfone–silica (5 wt%) membrane prepared using NMP as the solvent were 7 barrers and 35, respectively. Keywords Polysulfone . Silica . Solvent . Gas permeation . Mixed matrix
Introduction Solvent type is one of the parameters that affect the gas transport properties and morphologies of polymeric M. Pourafshari Chenar (*) : H. Rajabi : M. Pakizeh : A. Bolverdi Chemical Engineering Department, Faculty of Engineering, Ferdowsi University of Mashhad, Mashhad, Iran e-mail:
[email protected] M. Sadeghi Chemical Engineering Department, Isfahan University of Technology, Isfahan 84154, Iran
membranes (Sener et al. [27] and Shao et al. [28]). As stated by Shao et al. [28], solvents have various chemical and physical properties that induce different interactions with polymer chains and also result in different phase inversion processes during the fabrication of membranes. Therefore, membranes may have solvent-dependent morphologies and separation performances. Khulbe et al. [12] studied the effects of various solvents (carbon disulfide, benzene, trichloroethylene (TCE), toluene, chlorobenzene, and bromobenzene) on the gas separation performance of polyphenylene oxide (PPO) membranes. The obtained data revealed that PPO membranes prepared using carbon disulfide as the solvent had an entirely different morphology compared with the other solvents applied, which means that the morphology of the membranes can change as the physical properties of the solvent change. Shao et al. [28] studied the effects of the type of solvent on the morphology and gas separation performance of 6FDA/PMDA-TMMDA copolyimide membranes. The results illustrated that copolyimide membranes cast from CH2Cl2 or N-methyl pyrrolidone (NMP) have an amorphous structure, while films cast from N,N-dimethyl formamide (DMF) have a crystalline structure. The gas transport properties of membranes cast from DMF showed the lowest permeabilities for the gases CO2, N2, and CH4. It should be noted that DMF has a solubility parameter that is closer to that of the applied copolyimide than to those of CH2Cl2 and NMP. Iqbal et al. [8] studied the effects of dichloromethane (DCM) and chloroform on the membrane morphology and CO2/CH4 separation performance of asymmetric polycarbonate membranes. They also investigated the effects of nonsolvent additives on the morphology and gas permeation properties of polycarbonate membranes. The morphological characterization tests showed that DCM-based asymmetric polycarbonate membranes had a less porous substructure than the chloroformbased membranes.
216, Page 2 of 9
Mousavi et al. [22] also studied the effect of the type of solvent on the gas permeation properties of an ethylene vinyl acetate (EVA) copolymer for several pure gases, and concluded that the solvent acts as a transient template and controls the packing density of the final product by covering the polymer molecules with a layer of solvent in the nascent membranes. Chen et al. [3] investigated the effects of polarity through the addition of a co-solvent during the dehydration of ethanol/ water mixtures using polysulfone (PSf) membranes. Increasing the amount of chloroform (a nonpolar solvent) supplied to the PSf/NMP/water ternary system caused the skin-layer thickness to increase because the demixing of the casting solution was delayed. Recently, Aroon et al. [2] investigated the effects of co-solvent additives on the morphology and gas permeation properties of polysulfone membranes. The data showed that the addition of tetrahydrofuran (THF, a volatile solvent) to the PSf/NMP solution increased the gas selectivity of the membrane, which was attributed to the elimination of macrovoid formations during the instantaneous demixing process and the selective loss of highly volatile solvents from the top surface of the membrane. It should be noted that THF is miscible with water and that polysulfone is highly soluble in THF. Increasing the efficiency of polymeric membranes, e.g., through the development of modern membranes with sufficient permeability and selectivity, is a primary concern of membrane science and technology researchers (Lee et al. [17], Panndey and Chauhan [25], and Stern [29]). One of the most important methods of improving the performance of polymeric membranes used in gas separation, especially CO2 separation, is the incorporation of inorganic materials, such as silica particles, into the polymer membrane chains (Ahn et al. [1], Cornelius and Marand [4], Hu et al. [7], Joly et al. [9], Joly et al. [10], Kim and Lee [13], Kim et al. [14], Kusakabe et al. [16], Moaddeb and Koros [21], Nunes et al. [24], Sadeghi et al. [26], Wang et al. [31], and Zhou et al. [32]). Silica particles favorably distributed throughout the polymer matrix lead to an enhancement of the mechanical strength and thermal stability of the polymer. Control of the morphology and the phase separation phenomenon are key factors for achieving a homogeneous structure in composite membranes. The required homogeneity is most often provided by physically bonding the organic and inorganic phases (for example, through the use of compatibilizers). To our knowledge, there are few investigations on the effects of solvent type on the morphology of mixed-matrix membranes and the distribution of nanoparticles in the crosssection of the membrane. Sener et al. [27] investigated the effects of different solvents (dimethyl sulfoxide (DMSO), DMF, N,N-dimethyl acetamide (DMAc), and THF) on the performance of polyacrylonitrile (PAN)/zeolite composite membranes used for pervaporation. They showed that solvents with a high boiling point (e.g., DMSO) and a similar solubility parameter to the polymer exhibited a favorable
J Polym Res (2013) 20:216
distribution of particles, higher selectivity, and intermediate permeation flux. In the present study, polysulfone has been chosen as the membrane material because it is commercially available, easy to process, and has favorable selectivity–permeability characteristics for CO2/N2 and O2/N2 separations. Polysulfone belongs to a class of highly mechanical, thermally and chemically resistant materials. Consequently, it is a glassy polymer and exhibits poor interactions with inorganic fillers, such as zeolites and silica (Mahajan et al. [18]). Published articles investigating mixed-matrix membranes include polysulfone mixed-matrix membranes fabricated using zeolite (Wang et al. [31]), carbon nanotubes (Kim et al. [14]), silica (Ahn et al. [1]), and magnetite particles (nano or micro) (Ficai et al. [5]). However, a systematic investigation of the effects of solvent type on the morphology and gas permeation properties of such membranes has not yet been reported. In all of the abovementioned polysulfone mixed-matrix studies, chloroform and NMP were used as solvents, but none of them directly compared the effects of solvent type. In the study described in the present article, the effect of solvent type on the gas permeation performances of polysulfone–silica nanocomposites was investigated using NMP, THF, and DMAc.
Experimental Materials Polysulfone (Udel-P1700), a glassy polymer with a glass transition temperature (Tg) of 185 °C and specific gravity of 1.24 kg/L, was supplied by Amoco Chemicals (Chicago, IL, USA). The chemical structure of PSf is shown in Fig. 1 [33]. Tetrahydrofuran (THF) and N-methyl pyrrolidone (NMP) were supplied by Merck (Darmstadt, Germany) and used as solvents. The chemical structures of the solvents are shown in Table 1. Tetraethyl orthosilicate (TEOS), 3-glycidyloxypropyl trimethoxysilane (GOTMS) as a coupling agent, hydrochloric acid (HCl), and ethanol (required for the preparation of the silica sol) were also purchased from Merck. The chemical structures of TEOS and GOTMS are presented in Table 1. Table 2 shows the Hansen solubility parameters of polysulfone and the applied solvents (Mark [19]). As presented in Table 2, among the solvents (NMP, DMAc, and THF), the solubility parameter of NMP is the most similar to that of polysulfone. The gases CO2, O2, and N2 (purity 99.99 %) were purchased from the Iran Polymer and Petrochemical Institute (Tehran, Iran). Polysulfone was dried in an oven at 80 °C overnight before being used, while all other organic chemicals were of reagent grade and used as received.
J Polym Res (2013) 20:216
Page 3 of 9, 216
Fig. 1 Chemical structure of polysulfone (Udel-P1700)
Preparation of silica nanoparticles Silica nanoparticles were prepared by the hydrolysis and condensation of TEOS in ethanol and deionized water in the presence of hydrochloric acid as a catalyst. TEOS (25 g), GOTMS (4 g), and dry ethanol (30 ml) were stirred at 70 °C for approximately 1 h. Subsequently, a mixture of ethanol (30 ml), deionized water (7.5 g), and hydrochloric acid (0.83 g) was added dropwise to the reaction mixture. After 1.5 h of vigorous stirring, a clear silica sol was prepared. To calculate the silica content of the prepared silica sol and its density, a specific volume of silica sol was dried in the oven (at 60 °C) for 1 h to evaporate the ethanol, and then to completely remove the ethanol it was placed in a vacuum oven (at 60 °C) for 3 h. The remaining solid was then weighed. The density of silica was measured as 2.2 g/cm3. Preparation of polysulfone and polysulfone–silica composite membranes To prepare the polysulfone membrane, polysulfone (5 g) was dissolved in DMAc (45 g) at room temperature (25 °C). The obtained solution (10 wt% of polysulfone) was cast on a flat sheet and dried at 70 °C for 1 day. Residual solvent was removed from the prepared films using a vacuum oven at 70 °C for 5 h.
Polysulfone–silica nanocomposite membranes were prepared by adding the silica sol at a specific weight fraction to the polymer solutions to attain composite membranes containing 5 wt% of silica. After degassing the solution for 2 h, it was cast on a flat sheet and dried using the same method employed for the polysulfone membrane, except for THF-PSf-silica, which was dried at room temperature for 1 day before placing it in a vacuum oven. The membrane thickness was measured as 70–90 μm in all cases. Characterization The morphological aspects (i.e., the distribution of the silica particles in the polymer matrix) were investigated using a scanning electron microscope (SEM) (TESCAN-VEGA, Oxford Instruments, Abingdon, UK). The membrane samples were fractured in liquid nitrogen to expose a cross-section and subsequently sputtered with Au under a vacuum. The formation and presence of silica nanoparticles in nanocomposite membranes were investigated using Fourier transform infrared spectroscopy (FT-IR) using a Bruker (Ettlingen, Germany) Equinox 55 instrument in the range 600– 4,000 cm−1. SEM mapping was also applied to observe the distributions of silica nanoparticles in the nanocomposite membranes.
Table 1 Chemical structures of the applied solvents, TEOS and GOTMS
DMAc
T E OS
T HF
NMP
GOTMS
216, Page 4 of 9
J Polym Res (2013) 20:216
Table 2 Hansen solubility parameters of the applied solvents and polysulfone (Mark [19]) Material
δD
δp
δh
δt
PSf NMP DMAc THF
19.7 18 16.8 16.8
8.3 12.3 11.5 5.7
8.3 7.2 10.2 8
22.93 22.96 22.77 19.46
D¼
ℓ
2
6θ
;
ð4Þ
|δt,PSf −δt,Solvent|
where θ is the time lag (s), i.e., the intercept obtained from extrapolating the linear region of the pressure versus time plot to the time axis. The solubility coefficient (S) was then calculated as follows:
0.03 0.16 3.47
δD is the dispersion cohesion (solubility) parameter, δh is the hydrogen bonding cohesion (solubility) parameter, δp is the polar cohesion (solubility) parameter, and δt is the total cohesion (solubility) parameter, which is calculated as follows:
S¼
P : D
ð5Þ
δt2 =δD2 +δp2 +δh2
Gas permeation tests
Results and discussion
Gas permeation tests were performed with a constant volume/ variable pressure setup using the gases O2, CO2, and N2. The setup applied was the same as that described by Tabe Mohammadi et al. [30]. Circular membrane discs with an effective permeation area of 9 cm2 were used in this study. The permeation rate in the constant volume system was determined from the rate of the rise in pressure (dp/dt) using a known permeate side volume (Vp) with the following equation:
In this study, polysulfone and polysulfone–silica nanocomposite membranes were prepared using polymer solutions from various solvents: DMAc, NMP, and THF, which differ in their abilities to dissolve polysulfone (Table 2).
Q¼
V p V stp ðdp=dt;Þ RT
ð1Þ
where R is the universal gas constant, T is the absolute temperature, and Q is the gas permeation rate under standard temperature and pressure (STP) conditions (273.15 K and 101.33 kPa, respectively). The molar volume of any gas under STP conditions (Vstp) is 22,415 cm3 (Kruczek and Matsuura [15]). Therefore, the gas permeability of each membrane was determined using the following equation: P¼
Qℓ Aðp1 −p2 Þ
ð2Þ
where P is the permeability, expressed in barrer (1 barrer=10−10 cm3(STP) cm/(cm2 s cmHg)), ℓ is the membrane thickness (cm), p1 is the absolute feed pressure (cmHg), p2 is the absolute permeate side pressure (cmHg), and A is the effective membrane area (cm 2 ). The ideal selectivity (permselectivity) of the membrane was determined using the following equation: αij ¼
pi : pj
ð3Þ
The diffusion coefficient (D) was determined by the time lag method, which is represented by
Characterization of membranes The formation and presence of silica nanoparticles in nanocomposite membranes were investigated using FT-IR analysis. The spectra of pure silica, polysulfone, and polysulfone– silica nanocomposite membranes are shown in Figs. 2 and 3. The polysulfone consists of a backbone composed of diaryl sulfone (Ar–SO2–Ar) and diaryl ether (Ar–O–Ar) groups (as shown in Fig. 1), which resulted in strong absorption bands at 1147 and 1,235 cm−1, respectively. The bands at 1,485 and 1,583 cm−1 can be attributed to the vibration of the aromatic C=C in the polysulfone molecule, while the band at 1,235 cm−1 can most likely be attributed to the vibration of the C–O–C ether linkage. Two additional strong absorption bands were observed at 1,294 cm−1 and 1,324 cm−1, which is the region associated with the vibration of a sulfone group (Ng et al. [23]). In the case of silica nanoparticles, the observation of Si–O–Si symmetric stretching at 800 cm−1 suggested that the sol–gel reaction of TEOS to SiO2 was successfully performed. The strong band at 1,075 cm−1 represents an asymmetric Si–O–Si vibration, which is observed in the spectra of nanocomposite membranes. The broad band observed at approximately 3,500 cm−1 in the silica sample indicates that there was a significant amount of OH groups in the silica sol. The bands consistently observed at approximately 1,011 cm−1 were used to normalize the FT-IR spectra. Figure 2 shows three spectra: silica, polysulfone prepared from DMAc as the solvent, and a composite membrane containing 5 wt% of silica also prepared from DMAc. There are two regions in the spectrum of the composite membrane, 850–950 cm−1 and 950–1,350 cm−1 (Fig. 3), that are affected by silica bands, which indicates that silica particles are present in the composite films.
J Polym Res (2013) 20:216
Page 5 of 9, 216
Fig. 2 FT-IR spectra of the silica, polysulfone, and polysulfone– silica composites
Silica
Silica
The morphologies of the prepared pure and composite membranes were investigated using SEM. As can be observed in Fig. 4 at a magnification of 100,000, the silica particles are in the range 50–100 nm, but agglomerations are also present, as also stated by Ahn et al. [1]. Another significant observation is that the membranes are dense, but there is heterogeneity in the membrane structure, particularly between the agglomerated particles and the polymer, which might affect the membrane selectivity and permeability (as shown in Fig. 4 for PSf–5 wt% silica cast from DMAc). SEM mapping, the results of which are displayed in Fig. 5, shows the distribution of silica nanoparticles in the cross-section of the nanocomposite membrane and confirms the presence of agglomerated silica particles in localized areas. As shown in the SEM images and SEM mapping, a better distribution and less agglomeration of the nanoparticles was observed when NMP was used. Gas permeation
Fig. 3 FT-IR spectra of the silica, polysulfone, and polysulfone–silica composites in the range 850–1,350 cm−1
The permeation of the gases N2, O2, and CO2 through the PSf and PSf–silica nanocomposite membranes under a feed pressure of 10 bar and at a temperature of 25 °C was investigated using a constant volume/variable pressure system and Eq. 3. The permeation properties of the polysulfone membrane cast from DMAc and the permeation properties of the nanocomposite membranes cast from various solvents are compared in Tables 3, 4, and 5. The permeability data obtained for pure polysulfone are very similar to those reported by Ahn et al. [1], Ghosal et al. [6], and McHattie et al. [20], as shown in Table 5. Since permeability, by definition, is the product of solubility and diffusivity, in order to understand the permeability changes caused by the addition of silica nanoparticles to the polymer matrix, it is better to compare the diffusivity and
216, Page 6 of 9
J Polym Res (2013) 20:216
PSf–5 wt% silica (Solvent: NMP)
PSf–5wt% (Solvent: THF)
PSf–5wt% (Solvent: DMAc)
Fig. 4 SEM micrographs of nanocomposite membranes prepared with different solvents
solubility changes of the pure PSf membrane and the PSf– silica (5 wt%) nanocomposite membrane prepared with the same solvent (DMAc), as shown in Tables 3, 4, and 5. The permeation of gases through polymeric membranes is usually described by a solution-diffusion mechanism. In the solutiondiffusion mechanism, the permeation of a gas is determined by the solubility and diffusivity of a gas molecule in the membrane. The permeability of a gas through a membrane is significantly affected by changes in its morphology, which occur during the preparation of nanocomposite membranes from different solvents. As shown in Table 3, the diffusivity coefficient of each gas through the nanocomposite membrane prepared with DMAc was lower than its diffusivity through a pure PSf membrane prepared using the same solvent (DMAc). This phenomenon can be explained by the presence of nonpermeable silica particles, which reduce the mobility of the polymer chains and act as barriers in the membrane, resulting in an increase in the tortuosity of the polymer pathways. As shown by FT-IR characterization, the presence of OH groups in the silica nanoparticles allowed them to interact with polymer chains. The silica nanoparticles have less
PSf-5wt% silica (NMP)
mobility than the polymer chains and are generally more rigid than PSf, so the polymer chain mobility was reduced by the interactions of the nanoparticles with the polymer molecules. Silica nanoparticles act as weights (beads) that can form hydrogen bonds with the polymer chains, reducing the chain mobility. They can occupy the free volume of the polymer and reduce the number of access routes for gas molecules diffusing through the polymer chains. Therefore, the gas molecules had less space and a tortuous diffusion route through the polymer chains. Gas diffusivity through the prepared nanocomposite membranes was found to decrease in the following order: NMP PSf silicað5wt%Þ > DMAc PSf silica ð5wt%Þ > THF PSf silica ð5wt%Þ
The boiling points of the applied solvents increase as the molecular size of the solvent increases. The molecular sizes and boiling points of the solvents decrease in the following order:
PSf-5wt% silica (THF)
Fig. 5 SEM mapping of PSf–silica nanocomposite membranes
PSf-5wt% silica (DMAc)
J Polym Res (2013) 20:216
Page 7 of 9, 216
Table 3 Diffusivity coefficients of pure CO2, N2, and O2 through PSf and PSf–silica (5 wt%) membranes at 25 °C Solvent type–polymer (silica content)
DMAc-PSf DMAc-PSf-silica (5 wt%) NMP-PSf-silica (5 wt%) THF-PSf-silica (5 wt%)
Diffusivity selectivity
Diffusivity (×10−8 cm2/s) CO2
N2
O2
CO2/N2
O2/N2
1.80 1.40 1.50 1.32
1.23 1.14 1.18 1.00
4.20 3.75 3.90 3.50
1.46 1.23 1.27 1.32
3.41 3.29 3.31 3.50
NMP > DMAc > THF:
This correlation shows that increasing the molecular size of the solvent in the membrane will increase the gas diffusivity. This phenomenon is known as a “transient vehicle” in membrane preparation (Kesting and Fritzsche [11]). After leaching out the solvent from the membrane, the loss of larger solvent molecules leads to more free volume in the membrane than the loss of smaller solvent molecules does. Therefore, the penetration of a gas through a membrane with a high free volume occurs more easily and the gas diffusivity increases. Some residual OH groups are incorporated into the composite membranes with the introduction of the silica particles, thus increasing the solubility of polar gases, such as CO2, as shown in Table 4. Upon introducing the silica nanoparticles, more OH groups can adsorb more CO2 gas onto the membrane, increasing the amount of this gas solvated in the membrane matrix. As shown in Table 4, the solubility coefficients of the gases are highest for the solvent NMP. This is because better solvents cause the polymer chains to extend into the polymer solution. As the coiled chains expand, additional space is created for particles to insert themselves between polymer chains. This leads to a better distribution of particles in the polymer, as shown by SEM analysis. It should be noted that a smaller particle size leads to increased particle surface area in the membrane, and thus to an increased area of the Table 4 Solubilities of pure CO2, N2, and O2 through PSf and PSf–silica (5 wt%) membranes at 25 °C Solvent type–polymer (silica content)
DMAc-PSf DMAc-PSf-silica (5 wt%) NMP-PSf-silica (5 wt%) THF-PSf-silica (5 wt%)
Solubility (×10−2 cm3(STP)/ cm3 cmHg)
Solubility selectivity
CO2
N2
O2
CO2/N2
O2/N2
3.11 4.64 4.67 4.55
0.19 0.16 0.17 0.17
0.31 0.30 0.31 0.29
16.37 29.00 27.47 26.76
1.63 1.88 1.82 1.71
Table 5 Permeabilities of CO2, N2, and O2 through PSf and PSf–silica (5 wt%) membranes at 25 °C Solvent type– polymer (conditions or silica content)
Permeability (barrer)
PSf (4.4 atm and 35 °C) PSf (10 atm and 35 °C) PSf (10 atm and 35 °C) DMAc-PSf (10 bar and 25 °C) DMAc-PSf-silica (5 wt%) NMP-PSf-silica (5 wt%) THF-PSf-silica (5 wt%)
6.3
0.24 1.4
5.5
0.22 1.29 25
5.7
5.6
0.25 1.4
5.6
5.6
0.23 1.31 24.35
5.70
Ahn et al. [1] Ghosal et al. [6] McHattie et al. [20] This work
6.5
0.19 1.11 34.21
5.84
This work
7
0.20 1.20 35.00
6.00
This work
6
0.17 1.00 35.29
5.88
This work
CO2 N2
Selectivity
O2
Reference
CO2/N2 O2/N2 26.25
22.4
5.6
interfacial region exposed to gas molecules, which in turn leads to more adsorption sites in the membrane. The increased number of adsorption sites in the membrane results in higher gas solubility. Inspection of the solubilities of the gases in the nanocomposite membranes shows a reduction in the solubilities of other gases in these membranes. The resulting permeabilities are shown in Table 5. As expected, the permeability of CO2 through the nanocomposite membranes was higher than that through the pure polymer. The presence of silica nanoparticles in the nanocomposite membranes introduced OH groups, which enhanced the interactions with and the solubility of CO2 molecules in the membrane and led to higher CO2 permeability in the nanocomposite membranes. The presence of silica nanoparticles in the polymer matrix may cause morphological changes at the interface of the silica and polymer, which could increase the area of the amorphous region in the nanocomposite membrane that provides good sites for solvating CO2 gas in the membrane. Despite a reduction in the diffusion coefficient of CO2 through the membrane, the enhancement in the solubility coefficient led to higher permeability of CO2 through the nanocomposite membranes. The permeabilities of all of the gases through the nanocomposite membrane prepared using NMP as the solvent were greater than those for the nanocomposite membranes prepared using DMAc and THF. This can be explained by analyzing the results for the diffusivity and solubility coefficients. The highest diffusivity coefficient was acquired for the nanocomposite membrane prepared using NMP. This may be due to NMP being a better solvent, which causes greater extension of the polymer chains in the solvated state and increased free volume after solidification. As shown in Tables 3 and 4, both diffusivity and solubility coefficients of all gases
216, Page 8 of 9
for nanocomposite membrane prepared using NMP as solvent are the highest amongst all nanocomposite membranes prepared in this study, that corroborate the presence of higher free volume in the nanocomposite membranes prepared using this solvent. On the other hand, as mentioned regarding SEM mapping (Fig. 5), using a poor solvent leads to more agglomeration of the silica particles, which can lead to a greater number of gas molecule blockages, and thus smaller diffusivity coefficients of gases through the membrane would be expected (Table 3). As shown in Table 5, the CO2/N2 and O2/N2 selectivities of all of the nanocomposite membranes increased as the silica nanoparticles were added to the PSf. A comparison of the differences in gas selectivity between the pure polymer and the nanocomposite membranes shows that CO2/N2 selectivity increased by approximately 44 % and O2/N2 selectivity increased by approximately 4 % for the nanocomposite membranes. The greater increase in CO2/N2 selectivity than O2/N2 selectivity is due to the fact that CO2 permeability is particularly enhanced in comparison to the permeabilities of the other gases. As previously mentioned, the higher gas solubility of CO2 and the introduction of more adsorption sites resulting from the addition of of nanoparticles to the polymer led to an especially high permeability of CO2 in the nanocomposite membranes as compared to the increases in the solubilities of O2 and N2. Finally, the permeabilities of N2 and O2 decreased and the permeability of CO2 increased upon the addition of silica particles. The CO2/N2 selectivity increased drastically in comparison with the O2/N2 selectivity. The rather small increase in O2/N2 selectivity may indicate that the diffusion of nitrogen through the nanocomposite membranes is somewhat restricted due to its relatively large molecular size. As shown in Table 5, no significant trend was obtained for CO2/N2 selectivities of nanocomposite membranes prepared with different solvents. Based on the presented data in Tables 3 and 4; the CO2/N2 diffusivity selectivity of THF-PSf-silica (5 wt%) is the highest amongst all nanocomposite membranes prepared in this study, on the other hand the CO2/N2 solubility selectivity of DMAc-PSfsilica (5 wt%) is the highest. Therefore, CO2/N2 selectivity as the product of diffusivity selectivity (DCO2/DN2) and solubility selectivity (SCO2/SN2) doesn't obey specific trend. It should be noted that the effects of solvent type on the gas diffusivity and solubility coefficients of nanocomposite membranes prepared with different solvents were discussed separately above.
Conclusions In this study, the effects of solvent type on the morphologies and transport properties of PSf–silica nanocomposite membranes were studied. Enhanced nanoparticle dispersion was observed when a better solvent (NMP) for the polymer was
J Polym Res (2013) 20:216
used. This phenomenon was attributed to the long, segmental mobility of the polymer chains and the notable interactions between the silica nanoparticles and the polymer. It was also shown that incorporating silica nanoparticles into the PSf matrix resulted in an increase in the permeabilities of condensable polar gases due to enhanced interactions with the OH groups of the nanocomposite membranes. The CO2/N2 selectivity of the PSf membrane increased upon the incorporation of silica nanoparticles into the PSf, but no significant solvent effect on selectivity was observed. Acknowledgments The authors gratefully acknowledge the Ferdowsi University of Mashhad for its financial support of this research (grant no. 11618). The authors also would like to thank the Iran National Science Foundation (INSF) and Parsian Pooya Polymer Company, Iran.
References 1. Ahn J, Chung WJ, Pinnau I, Guiver MD (2008) Polysulfone/silica nanoparticle mixed-matrix membranes gas separation. J Membr Sci 314:123 2. Aroon MA, Ismail AF, Montazer-Rahmati MM, Matsuura T (2010) Morphology and permeation properties of polysulfone membranes for gas separation: effects of non-solvent additives and co-solvent. Sep Purif Technol 72:194 3. Chen SH, Liou RM, Lai JY, Lai CL (2007) Effect of the polarity of additional solvent on membrane formation in polysulfone/N-methyl2-pyrrolidone/water ternary system. Eur Polym J 43:3997 4. Cornelius CJ, Marand E (2002) Hybrid silica-polyimide composite membranes: gas transport properties. J Membr Sci 202:97 5. Ficai D, Ficai A, Voicu G, Vasile BS, Guran C, Andronescu E (2010) Polysulfone based membranes with desired pores characteristics. Matr Plastice 47:24 6. Ghosal K, Chern RT, Freeman BD, Daly WH, Negulescu II (1996) Effect of basic substituents on gas sorption and permeation in polysulfone. Macromolecules 29:4360. doi:10.1021/MA951310I 7. Hu Q, Marand E, Dhingra S, Fritsch D, Wen J, Wilkes G (1997) Poly(amideimide)/TiO2 nano-composite gas separation membranes; fabrication and characterization. J Membr Sci 135:65 8. Iqbal M, Man Z, Mukhtar H, Dutta BK (2008) Solvent effect on morphology and CO2/CH4 separation performance of asymmetric polycarbonate membranes. J Membr Sci 318:167 9. Joly C, Goizet S, Schrotter JC, Sanchez J, Escoubes M (1997) Sol– gel polyimide–silica composite membrane: gas transport properties. J Membr Sci 130:63 10. Joly C, Smaihi M, Porcar L, Noble RD (1999) Polyimide–silica composite materials: how does silica influence their microstructure and gas permeation properties? Chem Mater 11:2331 11. Kesting RE, Fritzsche AK (1993) Polymeric gas separation membranes. Wiley-Interscience, New York 12. Khulbe KC, Matsuura T, Lamarche G, Kim HJ (1997) The morphology characterization and performance of dense PPO membranes for gas separation. J Membr Sci 135:221 13. Kim JH, Lee YM (2001) Gas permeation properties of poly(amide-6b-ethylene oxide)– silica hybrid membranes. J Membr Sci 193:209 14. Kim S, Chen L, Johnson JK, Marand E (2007) Polysulfone and functionalized carbon nanotube mixed matrix membranes for gas separation: theory and experiment. J Membr Sci 294:147
J Polym Res (2013) 20:216 15. Kruczek B, Matsuura T (2000) Effect of metal substitution of high molecular weight sulfonated polyphenylene oxide membranes on their gas separation performance. J Membr Sci 167:203 16. Kusakabe K, Ichiki K, Hayashi J, Maeda H, Morooka S (1996) Preparation and characterization of silica–polyimide composite membranes coated on porous tubes for CO2 separation. J Membr Sci 115:65 17. Lee HJ, Won J, Park HC, Lee H, Kang YS (2000) Effect of poly(amic acid) imidization on solution characteristics and membrane morphology. J Membr Sci 178:35 18. Mahajan R, Burns R, Schaeffer M, Koros WJ (2002) Challenges in forming successful mixed matrix membranes with rigid polymeric materials. J App Poly Sci 4:881 19. Mark JE (2007) Physical properties of polymers handbook, 2nd edn. Springer, Berlin 20. McHattie JS, Koros WJ, Paul DR (1991) Gas transport properties of polysulfones: 1. Role of symmetry of methyl group placement on bisphenol rings. Polymer 32:840 21. Moaddeb M, Koros WJ (1997) Gas transport properties of thin polymeric membranes in the presence of silicon dioxide particles. J Membr Sci 125:143 22. Mousavi SA, Sadeghi M, Motamed-Hashemia MMY, Pourafshari Chenar M, Roosta-Azad R, Sadeghi M (2008) Study of gas separation properties of ethylene vinyl acetate (EVA) copolymer membranes prepared via phase inversion method. Sep Purif Technol 62:642 23. Ng BC, Ismail AF, Abdul Rahman WAW, Hasbullah H, Abdullad MS, Hassan AR (2004) Formation of asymmetric polysulfone flat sheet membrane for gas separation: rheological assessment. J Teknol 41:73
Page 9 of 9, 216 24. Nunes SP, Peinemann KV, Ohlrogge K, Alpers A, Keller M, Pires ATN (1999) Membranes of poly(ether imide) and nanodispersed silica. J Membr Sci 157:219 25. Panndey P, Chauhan RS (2001) Membranes for gas separation. Prog Polym Sci 26:853 26. Sadeghi M, Semsarzadeh MA, Moadel H (2009) Enhancement of the gas separation properties of polybenzimidazole (PBI) membrane by incorporation of silica nano particles. J Membr Sci 331:21 27. Sener T, Okumus E, Gurkan T, Yilmaz L (2010) The effect of different solvents on the performance of zeolite-filled composite pervaporation membranes. Desalination 261:181 28. Shao L, Chung T-S, Wensley G, Goh SH, Pramoda KP (2004) Casting solvent effects on morphologies, gas transport properties of a novel 6FDA/PMDA-TMMDA copolyimide membrane and its derived carbon membranes. J Membr Sci 244:77 29. Stern SA (1994) Polymers for gas separations: the next decade. J Membr Sci 94:1 30. Tabe Mohammadi A, Matsuura T, Sourirajan S (1995) Design and construction of gas permeation system for the measurement of low permeation rates and permeate compositions. J Membr Sci 98:281 31. Wang H, Holmberg BA, Yan Y (2002) Homogeneous polymer– zeolite nanocomposite membranes by incorporating dispersible template-removed zeolite nanocrystals. J Matr Chem 12:3640 32. Zhou H, Chen Y, Fan H, Shi H, Luo Z, Shi B (2008) The polyurethane/ SiO2 nano-hybrid membrane with temperature sensitivity for water vapor permeation. J Membr Sci 318:71 33. Solvay Advanced Polymers (2002) Udel polysulfone design guide. Solvay Advanced Polymers, Alpharetta
