Synthesis and self-assembly behaviours of side-chain smectic thiol–ene polymers based on the polysiloxane backbone

Abstract

A series of polysiloxane side chain liquid crystal polymers (PSCLCPs) with chiral and achiral substitutions in the side chains, denoted as PMMS-X_chol-n (n = 0, 0.1, 0.15, 0.2, 0.3, 0.4, 0.5 0.6, 0.7. 0.8, 0.9, and 1.0, respectively, the molar content of the chiral cholesteric unit (X_chol) in a specific polymer), were successfully synthesized via thiol–ene click chemistry. The molecular structures of the polymers were confirmed by ¹H-NMR, FT-IR, gel permeation chromatography (GPC) and thermogravimetric analysis (TGA). Their liquid crystalline (LC) properties and self-assembling behaviors were investigated in detail by a combination of various techniques, such as differential scanning calorimetry (DSC), polarized optical microscopy (POM), and X-ray diffraction. The results demonstrated that the phase transition behaviour and the self-assembly structure of the polymers were significantly influenced by X_chol and temperatures. With increased X_chol, the clearing points increased significantly, their mesogenic temperature ranges greatly widened, and abundant mesophases developed. Generally, two different types of LC phase structures and three different molecular arrangements were observed, depending on the two LC building blocks. Polymers with X_chol below 0.3 could self-assemble into a smectic E (SmE)-like structure and a single layer smectic A (SmA_s) structure upon heating. When X_chol was between 0.4 and 0.7, a single phase structure of a SmA_s or a bilayer smectic A (SmA_d) could be observed. While for polymers with X_chol over 0.8, a SmA_d phase structure was self-organized, further heating led to a SmA_s structure. Moreover, when the molar ratio of the chiral group or achiral group was about 0.1, a microphase-separated smectic morphology could be found, indicating that the introduction of a small amount of any components in the copolymers might destroy the well-ordered structures.