Controlling the polymer molecular weight, changing solvent and Nonsolvent or adding pore formers are the conventional methods which have been used to designing and engineering the pore size of a membrane 24–26.
Designing the pore size of the membranes in the phase inversion of systems containing amphiphilic copolymers, two different diffusions arise. First, solvent molecules diffuse from the membrane matrix to the water bath and substitution of water in the comprised pores. Second, the diffusion of polymer coils through the membrane matrix in order to reduce the surface energy 27,28. Generally, the size of macro-pores during the NIPS method through the liquid–liquid de-mixing process is controlled by polymer concentration and the kinetic of solvent and Nonsolvent replacement. Additionally, self-assembly process cause creating smaller pores which their size depends on the compatibility of copolymer and Nonsolvent blocks 29.
Surface morphology of the fabricated SAN membrane during SNIPS is shown in Fig. 1. As it is illustrated, micropores with identical pore size which are the consequence of NIPS process are scattered on the whole surface of the membrane and their structure is comparable to membranes prepared using other polymers. Two pore-size categories also can be observed in Fig. 1; micropores which are due to the solvent and Nonsolvent replacement and mesoscopic pores which have been created due to the repulsion between dissimilar segments. This result is consistent with the study of Yoo et al 30.
In order to perceive the formation mechanism of this extraordinary structure, the following hypothesis has been developed: in the first step of the film casting on the glass, the block copolymer solution is homogenous, however, evaporation of the solvent from the surface of the film cause the polymer concentration difference across the polymeric film surfaces. Therefore, chemical potential difference drives the solvent molecules to migrate from the bottom surface (in contact with glass) to the top surface (in contact with air) of the polymeric film. As a result, perpendicular orient channels will be created in the membrane matrix due to the fast evaporation of the solvent 31.
After the immersion of swollen polymeric film in the Nonsolvent bath, not only the solvent migrates from the copolymeric matrix to the water bulk, but also, water enters the matrix through the created channels and exchange with the solvent. Throughout these replacements, the copolymer reveals a kind of bilateral effect due to its amphiphilic property and presence of both hydrophobic and hydrophilic chains in its structure. Water has a great tendency to one of the copolymer’s coils and a reluctance to the other one, thereby the hollow channel structure forms which is the result of attaining the thermodynamic stability. Regarding the contrasting water compatibility of copolymer’s coils, the phase inversion begins with the sedimentation of polystyrene which is less soluble in water comparable to PAN and then followed by the deposition of water inclined polyacrylonitrile phase. During the phase inversion, the hydrophilic PAN chain swells more in comparison with the less hydrophilic PS chain due to its stronger attraction to the water; therefore, PAN accumulates on the surface of the membrane while PS place at the bottom. To corroborate on this theory, the EDX analysis of cross-section of the membranes has been performed, which were prepared using DMF as a solvent (Fig. 2). Distribution of nitrogen element in the cross-section of the membrane in the Fig. 2 indicates that the membrane surface contains more nitrogen and its density decreases as going through the depth of the membrane which can prove the aggregation of acrylonitrile chains on the surface.
The results show that, a BWB time of 300 seconds before the water bath can be considered as the maximum level of change of the progress. Obviously, as BWB time decreases, the migration rate of each of the polymer coils decreases. It is completely homogeneous in BWB time of 0 s and there is no difference between PS and PAN concentration on both sides of the membrane and through the cross section. This statement can be proved using the results of membrane contact angle analysis which is indicated in Fig. 3 as an example.
Generally, three parameters affect the self-assembly process; First, it is affected by the solvent evaporation time before the water bath since that the polymeric chains are able to move before the solidification and depends on the film viscosity. Second, self-assembly is affected by movements of polymeric chains in the casting solution, which at the immersion, occur synchronic with the NIPS process. Third, the interaction of polymeric solution with the membrane surface; therefore, the thickness of casted film is an effective parameter. Finally, self-assembly is controlled by the affinity of the polymer blocks to solvent and Nonsolvent that depends on the polarity of segments and the segment fractions. It seems that adjusting these parameters, we can design the final membrane structure and morphology. To attain this goal, thermodynamic and kinetic sciences are required to collaborate on this unrevealed, abstruse and intricate way.
The first step to investigate the behavior of copolymers in the presence of solvent and Nonsolvent is to analyze the thermodynamic behavior of ternary systems containing copolymers. So, a vast variety of issues should be assessed altogether completely; which is not easy, but it's worth it, because the acquired results of thermodynamic modeling would offer potential solutions to describe the structure-property-performance relation in the membranes.
Let’s describe this phenomenon; during phase inversion, both kinetic and thermodynamic aspects play dominant roles that make the process to be sophisticated. Furthermore, the process would be terminated in a few milliseconds that cause the investigations to face a great challenge 32,33. In the thermodynamic point of view, the self-assembly structure of di-block copolymer in a solution, depends on the equilibrium of three portions which are involved in the amount of free energy:
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Interactions of two coils and the level of their repulsion interaction
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Solvent type and its propensity to each of the coils
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Interactions of identical segments in each of the coils
Evidently, the variation of these interactions or the copolymers segment ratio can change the equilibrium condition and cause various structures to be created during the self-assembly procedure. As this phenomenon occurs aligned with the Gibbs free energy minimization, thus the selectivity of polymeric coils (PS or PAN) would directly affect it.
We have applied a simple model to describe this process thermodynamically which is the combination of NIPS and self-assembly of amphiphilic di-block copolymers in Supplementary information (S1).
Related Hansen solubility parameters for the studied systems are listed in Table S2-1. It can be concluded from Table S2-1 that NMP, in comparison with DMF, is a more appropriate solvent for both PAN and PS by easily compare the solubility parameter values. Since that the viscosity of the solutions prepared using both of these solvents is almost equal, different generated morphologies could be assigned to the solubility parameters of the components and therefore to their interactions. According to the tendency of solvent-copolymer chains and increasing the solvent-water tendency can shift the pores structure from spherical to cylindrical. In other words, by enhancing the solvent affinity toward the polymeric chains, cylindrical pores convert to the spherical pores which is depicted in Fig. 4 (Fig. 4a and 4b correspond to the NMP and DMF solvents, respectively).
Likewise, this phenomenon is expected to be observed when increasing the segment fraction of PAN in the copolymer chain. Therefore, by increasing the segment fraction of acrylonitrile from 25 to 30 in a special solvent, it is predicted that the structure tends from cylindrical to the spherical since that solvent-polymer tendency improves when the solvent-water tendency is kept constant. This behaviour could be attributed to the decrease of solvent and block copolymer interface in order to reduce the surface energy 34,35. To confirm this statement, the SAN 30 membrane was fabricated using NMP and DMF. The SEM cross-sectional images of the membranes are depicted in Fig. 4c and 4d; although the membrane has a cylindrical structure, the solvent has shown more tendency to acculmulate in the polymeric matrix and the strucure has grown upon spherical in comparison with Fig. 4a and 4b. However slight variations could be observed due to little changes (5%) in the content of polymeric chains which is in line with the study of Zhang et al. 36.
The kinetic prospect of phase-inversion process is concerned with the rate of solvent and Nonsolvent substitution which demands the transport phenomena investigations during the coagulation process. The maximum chemical potential difference emerges at the first moment when the polymeric film touches the coagulation bath which leads to the greatest exchange rate. Over the time, the exchange rate declines which is ascribed to the reduction of chemical potential difference in the two phases. Moreover, as the solvent get out of the polymeric film, the permeability of the top layer decreases which would intensify the mass transfer restriction. Two different ways could be followed in the phase inversion process with respect to the exchange rate. Concluding the presented investigations, it can be apprehended that when the initial solution immerses in the water bath right away after casting, a non-equilibrium structure apperas in which the hydrophobic polystyrene chain had sedimented immediately. In addition, PS comprises a major part of the copolymer and thus forms a continuous phase, which covers the acrylonitrile core like a shell and causes the acrylonitrile phase to trap in pores. In contrary, when the casting solution experience enough BWB time before the water bath, an equilibrium structure would form in which the acrylonitrile chains move to the surface of the pores in order to minimize the surface energy. Table 1 compares the porosity and thickness of SAN 25 (containing 25% acrylonitrile and 75% styrene) membranes prepared using DMF as solvent, at different temperatures of water bath and BWB times before immersion to water bath.
Table 1
The porosity and thickness of SAN 25 and SAN 35 at different temperatures of water bath and BWB times before immersion to water bath using DMF as solvent, polymer concentration= 8%; Casting knife gap= 250 µm.
SAN 25 (containing 25% acrylonitrile and 75% styrene)
|
|
30 seconds
|
60 seconds
|
|
porosity
|
Thickness (µm)
|
porosity
|
Thickness (µm)
|
0 oC
|
0.87
|
71
|
0.81
|
76
|
10 oC
|
0.90
|
83
|
0.84
|
79
|
30 oC
|
0.93
|
95
|
0.88
|
79
|
40 oC
|
0.93
|
92
|
0.91
|
81
|
SAN 30 (containing 30% acrylonitrile and 70% styrene)
|
|
30 seconds
|
60 seconds
|
|
porosity
|
Thickness (µm)
|
porosity
|
Thickness (µm)
|
0 oC
|
0.87
|
70
|
0.8
|
76
|
10 oC
|
0.91
|
85
|
0.83
|
83
|
30 oC
|
0.94
|
97
|
0.89
|
86
|
40 oC
|
0.96
|
103
|
0.91
|
86
|
Comparing the data presented in Table 1, it can be concluded that increasing the temperature leads to an increase in the size of the membrane pores and increasing the BWB time before the water bath slightly reduces the size of the membrane pores. This can be attributed to the evaporation of the solvent from the surface of the casted polymer film and consequently the increase in the polymer solution viscosity.
According to the hypothesis it can be predicted that with increasing the amount of acrylonitrile using SAN 30, the same behavior will be seen, but the amount of thickness and the porosity will increase because the amount of hydrophilicity of the polymer increases. The porosity and thickness values of SAN 30 membranes at different water bath temperatures and BWB time before water bath are also shown in Table 1. As can be seen, the values of thickness and porosity have increased with increasing temperature, and in all cases, the corresponding values of thickness and porosity in the SAN 30 membrane are greater than in the SAN 25. The porosity has remained almost constant and has not changed, which can be attributed to the low temperature of the water bath and, consequently, the low rate of phase separation.
The results of EDX analysis of the cross-section of the fabricated membranes (Fig. S2-1) show that, as the thickness of the polymer film increases, the PS coil has more time to migrate to the bottom of the membrane, resulting in less nitrogen accumulation at the bottom of the membrane. As it is mentioned before, in copolymer solutions, depending on the system conditions and type of solvent interaction with each coil, various morphologies and configurations can be created in the solution which is affects the final membrane morphology 37. Therefore, having a powerful tool that can detect the type of configuration according to the concentration of the solution and predict the structure of the surface, can lead to faster development of research in the field of fabrication of copolymer membranes which can be quantified using the small-angle X-ray scattering (SAXS) analysis 38. In other words, the microphase separation of copolymers in stock solution was performed using SAXS analysis.
The SAXS patterns for the bulk SAN 25 copolymer solution using DMF and NMP as solvent was represented in Fig. 5 (a and b) using SAXS analysis.
The SAXS pattern for concentration of 25 wt.% of SAN 25 in DMF (top curve of Fig. 5a) shows a locally microphase-separated nanostructure because of the no sharp well-developed higher first order peak. Furthermore, as can be seen from Fig. 5a, there is no discernible underlying lattices at low copolymer concentration of 10 wt.%; therefore, the order-disorder boundary is around 10 wt.% of copolymer concentrations, where the obtained micelles begin to pack into the lattices. Moreover, a Hex lattice emerged at polymer concentration of 20 wt.%; but at higher concentrations of 25 wt.% no discernible lattices were detected.
In the case of using NMP (Fig. 5b), the SC lattices were found for 20 wt.% of copolymer, whereas, BCC lattice emerged for higher copolymer concentration (25 wt.%); which can be distinguished by peak position of q/q*=\(\sqrt 7\).
In order to investigation of the hydrophilic segment fraction effect of copolymer on lattice formation in the solutions, the scattering intensity as a function of scattering vector magnitude in a logarithmic plot for the bulk SAN 30 copolymer solution using DMF and NMP as solvent was represented in Fig. 5 (c and d).
As can be seen from Fig. 5 (a and d), a similar behavior was observed using different segment fractions of the copolymer; so, it can be concluded that manipulation of the segment fraction will not have much effect on the copolymer solution structure, by comparing Fig. 5 (a and b) with (c and d), respectively. It also be found that, as the concentration of SAN 30 copolymer solution increases from 20 wt.% to 25 wt.% the SAXS pattern changes from SC to BCC, Comparing the presented data in Fig. 5d.
Generally, the presented scattering patterns in Fig. 5, were consistent with one of the hexagonal morphologies (Hex), micelles packed in a body-centered cubic lattice (BCC), micelles packed in a simple cubic lattice (SC) or disordered (DO) micelle structures. The expected peak positions for these structures in SAXS measurements are represented in Table S2-2. It should be noted that the only difference between BCC lattice and the SC lattice is the peak at a scattering vector of 7.
The results show that the proposed hypothesis to predict the morphology of the copolymer membrane, can be expanded according to the temperature of the water bath, the concentration of the polymer solution, the fraction of the copolymer segment (hydrophilicity or hydrophobicity of the copolymer) and the interaction between components.