Synthesis and Self-Assembly of Advanced Bottlebrush Block Copolymers
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Using the 'grafting-from' approach, three novel well-defined bottlebrush block copolymers were synthesized using various controlled polymerization techniques. Resulting macromolecules were characterized by GPC and 1 H NMR and then self-assembled into nanostructured materials for further characterization. Asymmetric polystyrene-polylactide (PS-PLA) bottlebrush block copolymers have been shown to self-assemble into a cylindrical morphology with large domain spacings. PLA cylinders can be selectively etched out of the shear-aligned polymer monoliths to generate nanoporous materials with an average cylindrical pore diameter of 55 nm. The remaining bottlebrush backbone provides a functional, hydrophilic coating inside the nanopores. This methodology significantly expands the range of pore sizes attainable in block copolymer based nanoporous materials. A polystyrene-poly(methyl methacrylate) (PS-PMMA) bottlebrush block copolymer with asymmetric branches was synthesized by grafting from a symmetrical backbone containing a novel dual vinyl initiation system and characterized by atomic force microscopy (AFM) and small-angle x-ray scattering (SAXS). The block copolymer backbone was prepared by sequential reversible addition-fragmentation chain transfer (RAFT) copolymerization of solketal methacrylate and 2-(bromoisobutyryl)ethyl methacrylate (BIEM). From the poly(BIEM) segment, PMMA branches were grafted by atom transfer radical polymerization (ATRP). Solketal groups were then subjected to hydrolysis and functionalized with a RAFT agent. Subsequently, from RAFT sites of the poly(SM) segment, polystyrene branches were grafted to yield the final bottlebrush block copolymer. The resulting polymer was found to have a vertically oriented cylindrical morphology by AFM with an average cylinder diameter of 45 nm; morphology was also confirmed by SAXS analysis. A novel polylactide-poly(methyl methacrylate)-polystyrene (PLA-PMMA-PS) triblock bottlebrush block copolymer was synthesized from a triblock backbone employing ATRP, RAFT and ROP techniques. Novel 5-TMS-4-pentyn-1-ol methacrylate (TPYM) with protected alkyne functionality was developed and characterized as a homopolymer. Click chemistry was used to introduce RAFT groups along the homopolymer backbone. Subsequently, PS branches were grafted from RAFT initiators along the backbone. A well-defined poly(SM- b- BIEM- b- TPYM) backbone was synthesized followed by grafting of PMMA brushes from the BIEM segment by ATRP. Post PMMA grafting, the ATRP end groups were removed and the TMS group was deprotected to allow functionalization of the alkyne with an azido terminated RAFT agent by way of Huisgen 1,3-dipolar cycloaddition. From the RAFT sites, PS was grafted followed by deprotection of the hydroxyl functionality of the SM block. Resulting liberated hydroxyls were used to initiate PLA grafts, yielding the final well-defined triblock bottlebrush block copolymer (PLA-PMMA-PS).