Thermo-responsive block copolymers : synthesis, self-assembly and membrane development

Abstract

Block copolymers (BCPs) are remarkable materials because of their self-assembly behavior into nano-sized regular structures and high tunable properties. BCPs are in used various applications such as surfactants, nanolithography, biomedicine and nanoporous membranes. In these thesis, we aimed to fabricate thermo-responsive iso- and nanoporous membranes from BCPs. First, we optimized the synthesis of a thermo-responsive BCP, i.e. polystyrene-poly(N-isopropyl acrylamide) (PS-PNIPAM) with desired properties using controlled/living polymerization methods. We fabricated membranes using self-assembly and non-solvent induced phase separation (SNIPS) method. The membranes were nanoporous, thermo-responsive and exhibited an interconnected worm-like surface. We investigated the self-assembly behavior of BCPs using both theoretical and experimental approaches. The theoretical investigation involves self-consistent field modelling of Scheutjens and Fleer (SF-SCF) which is used for the first time for BCP self-assembly phenomena. Using SF-SCF, first, we found a chain length dependence on the critical point of BCP phase diagram which confirms well with the reported literature. Second, we worked on the stability of the common mesophases (e.g. single and double gyroids, double diamond, hexagonally perforated lamellae) that is observed between hexagonally ordered cylindrical (HEX) and lamellar (LAM) phases; at chain length, =300 and at intermediate segregation regime, =30. Among the mentioned mesophases double gyroid was the only phase dominant over HEX and LAM phases. At strong segregation regime of =120 with the same chain length, double gyroid was found as a metastable phase. The experimental approach of the BCP self-assembly was performed by solvent annealing of BCP thin films. For annealing, common laboratory solvents e.g. methanol, tetrahydrofuran, toluene were used with various ratios to tune the selectivity of the solvent mixtures to the blocks in the copolymer. A lamellar forming triblock copolymer using the solvent mixtures methanol: THF (v:v) 1:2 or methanol: toluene (v:v) 1:1 resulted in HEX phase. In contrast, no sustained long-range order was found when only one type of solvent was used. Next, we optimized the membrane fabrication parameters to obtain membranes with an isoporous surface. We investigated the effect of solvent selectivity, evaporation time and polymer concentration. For PS selective solvents, membranes exhibited a disordered surface whereas PNIPAM selective solvents resulted in membranes with an isoporous surface. For a large parameter space, isoporous membranes were attained which is not common for SNIPS method. Permeability tests at various temperatures proved fully reversible thermo-responsive behavior of these membranes. Finally, we concluded our work with future recommendations to obtain block copolymer membranes that have improved properties and suggested tests that will prove membranes’ suitability for industrial applications.