Self-Assembly and Temperature-Driven Chirality Inversion of Cholesteryl-Based Block Copolymers

Velmurugan Krishnasamy,Feng Liu,Velraj Gothandapani,Changlong Chen,Wentao Qu,Qilu Zhang,Karthick Ramanagul,Georg H Mehl,Haohui Huo

doi:10.1021/acs.macromol.9b02264

Abstract

Block copolymers (BCPs), comprising a poly(methyl methacrylate) (PMMA) block and a poly(cholesteryloxyhexyl methacrylate) (PChMA) block, were synthesized via reversible addition–fragmentation chain transfer polymerization. The self-assembly of the liquid crystalline BCPs was characterized by differential scanning calorimetry, polarized optical microscopy, and synchrotron-based small/wide-angle X-ray scattering. The results indicate the formation of both tilted and nontilted chiral smectic (SmC* and SmA*) phases. A phase transition from the SmA* to SmC* phase on cooling was observed for BCPs but not for PChMA homopolymers. The layer spacing (5.00 ± 0.18 nm) between those can be controlled to maintain the number of ChMA units while varying the lengths of the PMMA block, thus introducing systematically the SmC* phase. Furthermore, BCPs with the short PMMA block showed inversion of chirality at specific temperatures; however, for PChMA attached with the long PMMA block, no chirality inversion was observed. This mode of chirality switching, investigated by circular dichroism, NMR, and theoretical studies, is associated with the methyl substituents in the backbone affecting the packing of the polymers. The basic rules, described here, have the potential to be implemented for the design of a wide range of functional materials, where helix–helix conversion is of use.

Highlights

Synthesis of cholesterol based liquid crystalline (LC) polymers A series of cholesterol derived LC polymers were prepared by polymerization of tailor-made enantiomerically pure cholesterol monomer, via reversible addition-fragmentation chain transfer (RAFT) polymerization,[27,28] which allows the preparation of block copolymers with well-defined structures (Scheme 1)
The obtained PChMA homopolymers were further utilized as a macromolecular chain transfer agent (CTA) to mediate the polymerization of MMA, which lead to cholesterol-based block copolymers with defined degree of polymerization (DP), PChMA-b-PMMA. 1H NMR spectra of the polymerization solutions were used to determine the conversions of the monomers, which were further used to calculate the DP’s and absolute molar mass (Mn,NMR) of the copolymers
Characteristic resonance signals for both cholesterol groups from cholesteryloxyhexyl methacrylate (ChMA) and methoxy (–OCH3) groups from MMA are observed by 1H NMR, in combination with the shift of size exclusion chromatography (SEC) traces the data indicate the success of extending of PMMA on PChMA macro-CTAs

Summary

Introduction

The research on polymers with liquid crystalline (LC) pendants has emerged as one of the major topics in materials science.[1,2,3,4,5,6,7,8] The combination of BCP topology and LCs is expected to allow for the design of advanced features in functional materials, allowing miniaturization of domain spacing in nanoscopic materials, high processability and the designing of novel morphologies.[9,10,11,12] In addition, the introduction of chiral functionality to LCPs has become one of the most important topics in LC research as it may permit the introduction of chiral nanostructures due to chiral amplification. Upon cooling from the isotropic state, all homopolymers and short-chain BCPs (PChMA24-b-PMMA27, PChMA40-b-PMMA32 and PChMA40-b-PMMA46) showed the formations of batonnêt textures upon annealing at various temperatures (Figure 2), which indicates the emergence of a layered phase with the molecules on average positioned perpendicular to the layer normal (SmA), and, as the molecules are chiral, identified as a SmA* phase.

Results

Conclusion