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

Abstract

Control over thermotropic phase behavior in low-Tg 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 (M 1) 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 M 1 and M2 differ dramatically (∆TNI ) 140°C), while the siliconbased 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 S 1-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 M 1/M2 ratio of the copolymers. IntroductionGlassy side-chain liquid crystalline polymers (SCLCPs) have been widely studied in the past 1 as a materials approach that uniquely combines the mechanical and thermal properties of polymers with the optical properties of small-molecule liquid crystals, with numerous possible applications. 2 It is over the past two decades that researchers have focused more on crosslinked SC-LCPs because of the interesting thermomechanical properties they exhibit as liquid crystalline elastomers (LCEs). 3 Indeed, side-chain nematic LCEs have been shown to display spontaneously large strainreversible actuation and soft elasticity when exposed to specific stimuli. 4,5 Here, a thermally stimulated actuation behavior, shrinking on heating through a clearing transition and expanding on cooling through the same, has been explained by a coupling between liquid crystalline order and rubber elasticity resulting from the underlying cross-linked structure. 6 Yet higher actuator performance has been anticipated 7 and recently ...

Reversible actuation in main-chain liquid crystalline elastomers with varying crosslink densities

“…9(e)) samples. This four point pattern has been observed and reported previously in siloxane-based liquid crystalline polymers [26] and elastomers [27]. The stretch direction (load direction) is horizontal in these patterns.…”

“…Finally, the broader and less intense DebyeeScherrer ring at larger 2q on the 65.7 mm exposure is due to the mesogen packing within the smectic layers, and the smectic ring is lost to the beam stop at this sample-to-detector distance. Identification of the smectic-C phase was possible using the WAXS patterns of stretched LCEs, which showed a four point smectic layer reflection that is typical of a chevron architecture commonly observed in smectic-C systems [26]. Combined with the observation that the smectic dspacing is approximately the same in oriented and unoriented states (shown later in Fig.…”

Reversible Switching of the Shear Modulus of Photoresponsive Liquid‐Crystalline Polymers

“…Liquid crystalline polymeric materials benefit from the rheological integrity that the polymer component provides to the system. Siloxane based LCPs present specific advantages associated with the very low glass transition temperature (T g ) of the siloxane backbone [7,8] , allowing for the materials to retain their properties over a wide temperature range, particularly at room temperature.…”

Main chain photoresponsive liquid crystalline polymer synthesized through hydrosilylation

“…Mather et al described the synthesis of new siloxanebased main-chain LCPs, including homopolymers, blends, and copolymers, with tunable clearing temperatures as governed by their chemical composition [38]. Liquid crystalline polyurethane thermoplastic elastomers with cyano-biphenyl mesogens pendant on the soft segment were synthesized for the first time by Hammond et al [39].…”