Swelling of amphiphilic block copolymers (BCPs) by selective solvents has emerged as an extremely simple strategy to producing mesoporous materials with well-controlled porosities. When exposed to selective solvents preferential to the more polar blocks of BCPs, (e. g. the P2VP block in polystyrene-block-poly (2-vinyl pyridine), PS-b-P2VP), P2VP domains will be swollen selectively by ethanol and mesopores form upon ethanol removal, while the major, nonpolar PS block holds the infrastructure of the BCP material mainly unchanged. Due to the migration of the P2VP blocks onto the pore wall, the resulting porous materials possess an intrinsically responsive and functionalizable surface. By annealing the PS-b-P2VP films to align the P2VP cylinders either normal or parallel to the substrate surface followed by swelling, we are able to obtain two different types of homoporous structures: “standing” pores and “sleeping” pores. Based on this mechanism we developed a number of mesoporous materials with various dimensions including one-dimensional nanofibers. PS-b-P2VP nanofibers were prepared either by templating from anodic aluminum oxide (AAO) or by electrospinning. After swelling treatment in hot ethanol, the original solid nanofibers took a mesoporous morphology with interconnected porosity which was tunable by changing the swelling conditions. Further increase in the interaction between the selective solvents and the copolymer resulted in a morphology transition from mesoporous nanofibers to bunches of partially interconnected nanostrands, to strings of spheres, and finally to isolated spherical micelles. Webs of the electrospinning nanofibers after swelling treatment were used as separation membranes, and they showed fast water permeation and tight retention because of their hierarchical porosity. Interestingly, the nanofiber membranes exhibited an inherent pH-sensitive water flux as a result of the selective enrichment of P2VP chains on the fiber surface.

Mesoporous block copolymer nanofibers by selective swelling

253rd National Meeting (2017)

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