Tailored Phase Transitions via Mixed-Mesogen Liquid Crystalline Polymers with Silicon-Based Spacers

Control over thermotropic phase behavior in low-T_g main-chain liquid crystalline polymers (LCPs) is desired for a variety of applications, including soft actuation when cross-linked. Here, we describe the synthesis of new silicon-based main-chain LCPs, including homopolymers, blends, and copolymers, with tunable clearing temperatures as governed by their chemical composition. Two mesogenic groups, namely, 1,4-bis[4-(4-pentenyloxy)benzoyl]hydroquinone (M₁) and 2-tert-butyl-1,4-bis[4-(4-pentenyloxy)benzoyl]hydroquinone (M₂), were polymerized with various silicon-based flexible spacers, specifically, 1,4-bis(dimethylsilyl)benzene (S₁), 1,1,3,3,5,5-hexamethyltrisiloxane (S₂), and hydride-terminated poly(dimethylsiloxane) (DP = 8) (S₃) spacers, following routine hydrosilation reaction techniques. These mesogens and flexible spacers were chosen so that both copolymerization and blending of homopolymers would allow for potential tailoring of phase behavior. Indeed, despite their similar chemical structure, the clearing transition temperatures of M₁ and M₂ differ dramatically (ΔT_NI = 140 °C), while the silicon-based spacers offer accessibility to a large range of molecular flexibility. High-molecular-weight LCPs were successfully prepared using Pt-catalyzed addition polymerization. Interestingly, the polymers exhibited wide liquid crystalline windows with relatively high degree of order (smectic phases) except for the S₁-based blends, which, in addition to a smectic phase, also displayed a narrow nematic phase. As expected, a drastic decrease of the glass transition temperature arose on polymerizing with longer, more flexible spacers, from about 56 to −17 °C. Finally, in comparing the two approaches to phase behavior tailoring, namely, blending vs copolymerization, the former led to apparently immiscible systems with constant isotropization temperatures, while the latter yielded homogeneous, single-phased materials with tunable isotropization temperatures dictated by the M₁/M₂ ratio of the copolymers.