Influence of architectural imperfections on the dynamic mechanical response of main-chain smectic elastomers.

摘要

Main-chain liquid crystalline polymers that form low-temperature smectic mesophases are synthesized by linking terephthalic acid, bis-(4-allyloxyphenyl) ester (PPT) mesogens with 1,1,3,3,5,5 hexamethyltrisiloxane (F3) spacers via Pt-catalyzed hydrosilylation. Significant differences in thermal behavior and mesomorphic ordering are found between the polymer having unsubstituted PPT mesogens (F3-PPT-H) and the polymer having methyl substituents on the terminal rings of the mesogens (F3-PPT-CH3). Combined evidence from polarized light optical microscopy, differential scanning calorimetry, and X-ray diffraction reveals SCA ordering in both polymers. To our knowledge, the SCA phase has been reported for the first time in the siloxane containing main-chain liquid crystalline polymers.;Smectic elastomers are synthesized by non-linear polymerization of PPT-H or PPT-CH3 mesogens with F3 spacers and a tetrafunctional crosslinker, tetrakis(dimethylsiloxy)silane (A4). The influence of structural imperfections on mechanical damping in polydomain smectic main-chain liquid crystalline elastomers (MCLCE) subjected to small strain oscillatory shear is examined. The mechanical loss factor tan delta = G"(o)/G'(o) exhibits a strong maximum (tan delta ≈ 1.0) near the smectic-isotropic (clearing) transition. "Optimal" elastomers that exhibit minimal equilibrium swelling in a good solvent are compared to highly swelling "imperfect elastomers" that contain higher concentrations of structural imperfections such as pendant chains. For the imperfect elastomers, tan delta is markedly enhanced in the isotropic state due to relaxation of pendant chains and other imperfections. However, within the smectic state, the magnitude of tan delta and its temperature dependence are similar for optimal and imperfect elastomers at o = 1 Hz. The prominent loss peak near the clearing transition arises from segment-level relaxations that are insensitive to the details of chain connectivity. Smectic MCLCE can be tailored for applications as vibration-damping materials by manipulating the clearing transition temperature through the backbone structure or by deliberate introduction of structural imperfections such as pendant chains.;The stress-strain behavior of polydomain smectic main-chain liquid crystalline elastomers (MCLCE) subjected to uniaxial deformation is examined. The stress-strain curves of the MCLCE exhibit three regions. In region I, at low strain, excluded amorphous material and crosslink junctions in between smectic domains deform with minimal disordering of smectic domains. In Region II, characterized by a "plateau" stress, deformation of microdomains takes place by hairpin unfolding, followed by reformation of smectic domains in a globally oriented "monodomain" state. In Region III, at higher strain, hairpins are depleted except for "trapped" hairpins, and as elastic chains can not elongate further, deformation of the "monodomain" takes place by layer buckling.