Tailored Phase Transitions via Mixed-Mesogen Liquid Crystalline Polymers with Silicon-Based Spacers Ingrid A. Rousseau, †,‡ Haihu Qin, †,§ and Patrick T. Mather* ,†,| Institute of Materials Science and Chemical Engineering Department, University of Connecticut, Storrs, Connecticut 06269 Received August 13, 2004; Revised Manuscript Received February 22, 2005 ABSTRACT: 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 (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 (ΔTNI ) 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 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. Introduction Glassy side-chain liquid crystalline polymers (SC- LCPs) have been widely studied in the past 1 as a materials approach that uniquely combines the me- chanical 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 cross- linked SC-LCPs because of the interesting thermome- chanical properties they exhibit as liquid crystalline elastomers (LCEs). 3 Indeed, side-chain nematic LCEs have been shown to display spontaneously large strain- reversible actuation and soft elasticity when exposed to specific stimuli. 4,5 Here, a thermally stimulated actua- tion behavior, shrinking on heating through a clearing transition and expanding on cooling through the same, has been explained by a coupling between liquid crys- talline order and rubber elasticity resulting from the underlying cross-linked structure. 6 Yet higher actuator performance has been anticipated 7 and recently dem- onstrated for main-chain liquid crystalline elastomers (MC-LCEs) because of an enhanced coupling between their intrinsically high, yet labile, orientational order and network strain compared to their side-chain ana- logues. 8 Challenges exist, however, for main-chain liquid crystalline polymers, particularly regarding their syn- thesis and subsequent processing into elastomers. More specifically, main-chain LCPs (MC-LCPs) usually ex- hibit comparatively high transition temperatures rela- tive to room temperature, so that, to date, they have not received adequate attention. In addition, aside from some studies examining network architecture variation, only limited attention has been given to other important synthetic variables when dealing with cross-linked structures, particularly the influence of mesophase type, nematic, cholesteric, or various smectic on the resulting thermomechanical behavior. A general lack of knowledge regarding the structure- property relationships in these materials, coupled with their high potential to yield improved thermomechanical properties (i.e., soft actuation), has led us to study new MC-LCPs for eventual incorporation into elastomeric structures by chemical or physical cross-linking. We are particularly interested in understanding the influence of mesogen structure and flexible spacer length as well as the various polymer architectures, homopolymers, blends, and copolymers, on liquid crystalline phase behavior and glass transition temperature. Such an understanding will allow tailoring of associated ther- momechanical (actuation) behavior in related cross- linked structures so that their utility in strongly re- strictive environments may be facilitated. An example category of such restrictive applications include bio- medical devices, where both biocompatibility and low temperature activation are required. In the area of low temperature activation, we recently reported the shape- memory behavior of a new siloxane-based main-chain smectic-LC elastomer that was shown to allow the fixing of large strains for subsequent recovery on heating through the smectic-C-to-isotropic transition. 9 Here, we report on a parallel effort in developing structure-property relationships in analogous linear * Author to whom correspondence should be addressed. E- mail: patrick.mather@case.edu. † Institute of Materials Science. ‡ ingrid@mail.ims.uconn.edu. § haihu.qin@case.edu. | Chemical Engineering Department. 4103 Macromolecules 2005, 38, 4103-4113 10.1021/ma048327y CCC: $30.25 © 2005 American Chemical Society Published on Web 04/22/2005