Crosslinked liquid crystal polymers from liquid crystal monomers: Synthesis and mechanical properties

Abstract

AbstractDifunctional acrylates and methacrylate monomers have been made which are high order smectic liquid crystal (or crystalline) at room temperature. This report discusses materials with the following structure: F–S–M–S–F, where F is a functional group, acrylate or methacrylate (A or M); S is a spacer (CH2)n(n), and M is a mesogen—in this case 4,4′‐dioxybiphenyl (B). They are codified as BnA or BnM where n is the number of methylenes in the spacer. High conversion with high Tg can be obtained when polymerizing in the smectic state because the reactive end groups are concentrated in a small volume and can react well with little or no diffusion. B2A, B3A, B6A, B11A, and B3M were polymerized in the smectic state and compared to polymers made at temperatures where the monomers were isotropic. High conversion was obtained below final Tg—even then, probably because the polymers were ordered. All the polymers were studied by WAXD and dynamic mechanical spectroscopy. Solid‐state NMR on B3A showed that there was very high conversion of the double bonds at all temperatures. B3A photopolymerized in the smectic state (60–76°C) produced a crystalline polymer with Tg = 185°C (1 Hz). When photopolymerized at 85°C, above the isotropization temperature (Ti), a poorly organized polymer was obtained with a Tg of 155°C (1 Hz). Monomers with an odd number of methylene groups as spacers were crystalline after polymerization. With an even number of methylene groups, they lost most of their crystallinity on polymerization below Ti, but retained a low order smectic structure. Similar structures were obtained with all the monomers when they were polymerized above Ti. There was little effect of polymerization temperature on Tg when the spacers had an even number of methylene groups. © 1993 John Wiley & Sons, Inc.