Preparation of New Amphiphilic Liquid-Crystal Diblock Copolymers Bearing Side-on Cholesteryl Mesogen and Their Self-aggregation

Abstract

Amphiphilic block copolymers bearing rigid hydrophobic liquid crystal mesogen have recently attracted broad interests since they could spontaneously self-assemble into functional objects at the micron and/or nanometer scales, and these soft matters bearing high-order hierarchical structures have potential applications in micro-reactors, advanced catalysts, biosensors, drug delivery, biotechnology and so forth. In this work, we designed and successfully prepared a new series of amphiphilic liquid crystal poly(glyceryl methacrylate)-b-poly(6-cholesteryloxyhexyl methacrylate) (PGMA-b-PMA6Chol) with hydrophobic PMA6Chol block bearing end-on cholesteryl mesogen through sequential controlled reversible addition- fragmentation chain transfer polymerization (RAFT) and successive removal of ketal protection groups. Then, their structures and comonomer composition were examined by nuclear magnetic resonance (NMR) and gel permeation chromatography (GPC). Liquid crystal phase structures and their transition as well as thermal stability of the prepared LC copolymers were characterized by differential scanning calorimeter (DSC), polarized optical microscope (POM), 2D small angle X-ray scat- tering (2D-SAXS) and thermogravimetric analytic instrument (TGA). Moreover, self-aggregation of these amphiphilic LC block copolymers in mixed solution was investigated by means of UV spectrometer, and the aggregate sizes and morpholo- gies were examined by scanning electron microscopy (SEM) and dynamic light scattering (DLS), respectively. As a result, it was revealed that the LC to isotropic phase transition temperature (TLC-I) of as-synthesized LC block copolymers increased with an increase in rigid LC block content, and the presence of bishydroxyl groups of hydrophilic PGMA strongly influenced the self-assembly and LC phase of rigid hydrophobic block in bulk state. In mixed solvent, the synthesized amphiphilic LC copolymers were found to preferably self-aggregate into core-shell spherical objects with average sizes spanning a range from 0.7 to 2.0 μm when the hydrophobic LC block content was less than 93%, in contrast, the LC copolymer with 93 wt% LC block tended to form open hollow spherical aggregates, showing interesting thermal responsive particle sizes in aqueous solution.