Thermally Stable Photocuring Chemistry for Selective Morphological Trapping in Block Copolymer Melt Systems

Abstract

In this work, we report the novel application of thermally stable photocuring chemistry toward high fidelity translation of block copolymer based melt-state morphologies into their equivalent solid analogues. The thermal stability of the cationic photcuring chemistry allows for both extended thermal processing prior to cure, as well as precise trapping of selected morphologies afforded by the temperature independent initiation mechanism. We demonstrate this powerful approach using a model polyisoprene-b-poly(ethylene oxide) (PI−PEO, fPEO = 0.39, Mn = 10 120 g mol−1) block copolymer, prepared by two step anionic polymerization and subsequent partial epoxidation (7.3−16.8 mol % relative to diene repeat units) with 3-chloroperoxybenzoic acid. Small angle X-ray scattering (SAXS) and dynamic rheology were used to determine morphological behavior of the block copolymers synthesized. The targeted PI−PEO parent block copolymer exhibited multiple melt-state morphologies including crystalline lamellae (Lc), hexagonally packed cylinders (C), bicontinuous gyroid (G), and a final isotropic disordered state (Dis). The partial epoxidation and the addition of the UV activated cationic photoinitiator (4-iodophenyl)diphenylsulfonium triflate (1.0−1.25 mol %) acted only to shift transition temperatures between phases, without disturbing the overall morphological sequence present in the neat, unmodified PI−PEO block copolymer. Exposure of the photacid/block copolymer blends to UV radiation at selected temperatures permitted successful permanent trapping of both the cylinder and gyroid morphologies from a single block copolymer sample, as verified by pre- and postcure SAXS measurements. Importantly, this approach should be applicable to any block copolymer in which cationically polymerizable functional groups can be incorporated.