Synthesis and Characterisation of Photoresponsive Thermoplastic Liquid Crystal Elastomers

Abstract

Liquid crystal elastomers (LCEs) are polymeric materials capable of rapid, reversible shape change on application of an external stimulus. However, their application as soft actuators has, to date, been limited by the requirement to induce a liquid crystal monodomain, with common alignment methods restricting possible structures to thin films. Thermoplastic LCEs have the potential for processing through large-scale techniques such as extrusion or fibre spinning but have been relatively unexplored, with low-yielding synthetic methods preventing further investigation into their suitability for processing into alternative geometries to films.
For this reason, photoresponsive thermoplastic side-chain LCEs based on a polystyrene-b-poly(methylvinylsiloxane)-b-polystyrene block copolymer backbone have been synthesised for processing into responsive fibres. A two-step post-polymerisation modification was used to first attach a linking group, followed by the mesogen. A yield of 85% was achieved in the final coupling stage, resulting in polymers with 65-85% of linking groups functionalised with mesogen. Through the choice of linking group or mesogen it was possible to tune the thermal properties of the LCEs to obtain useful transition temperatures for use as room temperature actuators. The presence of a nematic mesophase was shown to have a strong effect on the physical properties of the polymer, resulting in a microphase segregated morphology at temperatures far above the order-disorder transition temperature of the unfunctionalised triblock copolymer. This, combined with the relatively high molecular weight of 150,000 g mol-1, was demonstrated to limit the ability to process the LCE by melt extrusion, with gel-like behaviour observed at temperatures up to 200 °C. However, the high molecular weight of the polymer made it suitable for processing from solution, and preliminary experiments demonstrated for the first time that fibres of a thermoplastic LCE could be produced by electrospinning, with elongation of the jet resulting in the formation of an aligned nematic LC mesophase.

Date of Award	23 Jan 2019
Original language	English
Awarding Institution	The University of Bristol
Supervisor	Ian Manners (Supervisor), Valeska Ting (Supervisor) & Annela M Seddon (Supervisor)