Finely tuning the self-assembled architectures of liquid crystal polymers by molecular engineering: phase transitions derived from terminal group variations

Abstract

Liquid crystal polymers (LCPs) have gained tremendous scientific attention due to their potentials in fabrication of intelligent soft robots and responsive devices. Modulating the superstructures by molecular engineering via a bottom-up way and gaining insight into the relationships between molecular structures and self-assembled architectures are of significance in understand the self-organization mechanism of mesogens as well as the fabrication of advanced functional materials, given that the properties and stimuli-responsiveness of LCPs are mainly determined by the self-organized superstructures. In this study, a series of innovative homopolymers featuring distinct terminal groups in the side chains were synthesized via thiol–ene click reactions between poly[3-mercaptopropylmethylsiloxane] (PMMS) and a range of monomers with a nearly identical rigid core but varying terminal substituents (4-methylcyclohexyl, p-methylbenzene, and p-biphenyl groups). The thermal behaviour, phase structure, and molecular packing mode were systemically investigated. Experiment results indicate the formation of a smectic E-smectic A-isotropic phase sequence for most polymers. Intriguingly, an additional loosely stacked hexagonal smectic B phase is observed in P-4, which bears biphenyl mesogenic tails, positioned between smectic E and smectic A phases as the temperature increases. This work presents an effective regulatory strategy for achieving diverse phase transition behaviors and hierarchical constructions through subtle tuning of the molecular structure in the side chains of liquid crystalline polysiloxanes.