Influence of the side-chain structure and molecular weight on the re-entrant behaviors of mesogen-jacketed liquid crystalline polymers

Abstract

Three series of mesogen-jacketed liquid crystalline polymers (MJLCPs) containing different terminal groups (phenmethyl, diphenylmethyl and triphenylmethyl) in the side chains, abbreviated as Pv-m-Bn, Pv-m-DPM, Pv-m-Tr (m = 2, 4, 6, 8, 10, and 12, which are the number of methylene units between the terephthalate core and terminal groups in the side chains), were designed and successfully synthesized via free-radical polymerization. Molecular characterization of the polymers was performed by ¹H NMR, GPC and TG analysis. The phase structures and transitions of the polymers were investigated by a combination of techniques including DSC, POM and 1D/2D WAXD. The experimental results revealed that all the polymers exhibited excellent thermal stabilities and the re-entrant behaviors of the MJLCPs were found to be strongly dependent on the structure of the side-chain, i.e., the spacer length increased with the volume of the terminal groups when the polymers exhibited the re-entrant isotropic phase. On the other hand, a series of MJLCPs, poly{2,5-bis[(diphenylmethoxy-ethyl)oxycarbonyl]-styrenes} (Pv-2-DPMs), with different molecular weights (M_n) and narrow M_n distributions have been successfully synthesized via ATRP. The results indicated that when the M_n was below 1.73 × 10⁴ g mol⁻¹, only the isotropic phase was observed. When M_n was between 3.40 × 10⁴ g mol⁻¹ and 8.48 × 10⁴ g mol⁻¹, a re-entrant isotropic phase was formed at low temperatures and a columnar nematic phase at high temperatures. By further increasing the M_n to exceed 9.71 × 10⁴ g mol⁻¹, a stable columnar nematic phase was developed. This work provides two effective ways to design and synthesize MJLCPs with re-entrant behaviors; moreover, it is meaningful to deeply understand the structure–property relationships of MJLCPs.