Smectic Ordering in Main-Chain Siloxane Polymers and Elastomers Containing p-Phenylene Terephthalate Mesogens

Main-chain liquid crystalline polymers that form low-temperature smectic mesophases are prepared 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 S_CA ordering in both polymers. Smectic elastomers are prepared by nonlinear polymerization of PPT-H or PPT-CH₃ mesogens with F3 spacers and a tetrafunctional cross-linker, tetrakis(dimethylsiloxy)silane (A₄). The dynamic mechanical response of unoriented, polydomain elastomers is characterized in small-strain, oscillatory shear. A prominent peak in tan δ (≡ G‘ ‘(ω)/G‘(ω)) associated with the smectic−isotropic clearing transition dominates the mechanical loss spectrum. Mesogen ring substitution is a simple synthetic approach to tuning the dynamic mechanical response of smectic elastomers for possible applications in vibration isolation or impact absorption.