Structure Formation Mechanism of Isoporous Block copolymer Membranes
This thesis deals with the microphase separation of poly(styrene-b-vinylpyridine) block copolymers in highly concentrated solutions induced by evaporation of a binary solvent mixture of a high and low boiling solvent. During self-assembly and non-solvent induced phase separation by immersion in water, asymmetric membranes consisting of a thin top layer with closely packed pore channels with diameters below 100 nm above a much thicker macroporous supporting layer are formed. These membranes can be used for filtration purposes of, e.g., biomolecules. The aim of this thesis was to discover the fundamental principles according to which these materials are developed in order to optimize the preparation conditions for future large-scale membrane productions. The first part of the thesis focused on the experimental and theoretical investigation of the compositional changes of the polymer/binary solvent mixture during evaporation. The calculated polymer/solvent volume fractions were used to construct compositional trajectories in a ternary phase diagram. The calculations revealed a peculiar region of the phase diagram where the PVP-volume fraction is sufficiently high to induce morphological transitions. The block copolymer morphologies resulting from microphase separation in this region were systematically investigated by AFM and SEM as a function of polymer concentration, solvent composition and evaporation times before immersion in water. Several characteristic morphologies during solvent evaporation were identified, such as spinodal networks (SpN), sphere percolation networks (SN), disordered cylinders (DC) and the desired highly symmetric pore network (PN). A general structure growth during film formation was observed and further confirmed by in situ synchrotron SAXS measurements. From these investigations basic conclusions for optimized membrane preparation in terms of solvent and polymer composition and immersion times could be drawn. Porous membrane formation was shown to occur only in semi-dilution (polymer concentration 20wt.% or larger) where polymer-polymer interactions dominate instead of solvent-polymer interactions. Furthermore, the starting amount of THF should be high to follow a trajectory crossing the PN phase in a broad time frame. Using a newly designed automatic film casting apparatus, the kinetics of ordered pore formation in the evaporating polymer film could be monitored in situ by synchrotron SAXS. Effects of molecular weight and relative composition of the block copolymer were investigated and general similarities of the membrane formation process discovered. All kinetic studies revealed an exponential growth of the characteristic domain size until a saturation limit was reached. In a reduced plot using characteristic growth times and domain sizes for each of the block copolymers, all kinetic data as well as a theoretical prediction for two-dimensional domain growth superimposed onto a single curve. This also applied to block copolymer systems with added Cu2+-salts which served to stabilize the pore structure. Only a slight compaction of the domains due to coordinative bonding of the metal cations to the vinylpyridine groups was observed. Also the water immersion process was investigated with the help of an inkjet printer. A rapid increase of the characteristic length scale to the saturation value of the above described evaporation experiments was observed. The expected freezing of the polymer domains did not occur, likely due to an insufficient excess of water which is necessary to completely trap the developing structure. Finally, a route to nanocomposite membranes consisting of PS-PVP and inorganic nanoparticles was investigated with the example of Fe2O3-nanoparticles. To compatibilize the nanoparticles with either the PS- or the PVP-domains, the nanoparticles were coated with end-functionalized PS or PVP using a recently established ligand exchange procedure. Also mixed PS/PVP-compositions were used to investigate possible locations in the PS/PVP-interface. The addition of nanoparticles lead to a surprising stabilization of the desired cylindrical pore membrane structure. TEM-images showed the nanoparticles to be located in the PS- or PVPdomains depending on the type of coating. All particles with mixed shell compositions were located near the domain interface. Interestingly, the PVP-coated nanoparticles were depleted from the thin cylindrical pores in the top layer due to a size exclusion effect. Future studies should focus on the in situ SAXS measurements. It would be interesting to perform experiments in grazing incidence configuration in order to get more detailed information about the top layer morphological changes discussed above. So far only transmission scattering patterns with information from the whole inhomogeneous film cross section were collected. Improved method to mimic the water bath in an in situ measurement should be developed. Concerning the nanocomposite membranes, the next step would be to increase the nanoparticle concentration and try to find a way to place the particles in the pore channels close to the film surface.
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