Cholesteric mesophase in side-chain liquid crystalline polymers: influence of mesogen interdigitation and motional decoupling

Abstract

In this paper, the thermal, optical and mesomorphic properties of side-chain liquid crystalline (SCLC) homopolymers (PNBCh-n), in which cholesteryl mesogens are linked to a polynorbornene backbone by different lengths of methylene spacer (n = 4, 5, 9, 10 and 15), are investigated. By doing so, the impact of flexible spacer length on the formation of different types of LC mesophase is explained. Because of comb-shaped SCLC polymer architecture, smectic mesophases are primarily found in PNBCh-n. Interestingly, cholesteric mesophases are identified only in PNBCh-9 and 10, which are proved by (1) the selective light reflection in UV-Vis analysis, (2) the characteristic oily streak texture under polarized optical microscope (POM) and (3) a long range periodicity in the cross-sectioned films from transmission electron microscope (TEM) investigation. Moreover, temperature-controlled X-ray scattering measurements are performed to examine mesomorphic structure evolution. By comparing the mesomorphic structures in this series of SCLC homopolymers, both extent of mesogen interdigitation and motional decoupling between backbone and mesogenic side-chains are found to play a critical role in the development of cholesteric mesophase. This structure–property study of PNBCh-n will help elucidate the importance of molecular structure, particularly the length of spacers, on the formation of cholesteric mesophase and its importance in the design of thermochromic devices.