Title

Development of soft polymeric networks showing actuation behavior: From hydrogels to liquid crystalline elastomers

Date of Completion

January 2004

Keywords

Chemistry, Polymer

Degree

Ph.D.

Abstract

The development of polymer-based or ‘soft’ actuators has witnessed tremendous interest prompted by potential applications in biomedical engineering that replace passive, hard materials with smart, compliant, and biocompatible polymers. Such materials should exhibit great sensitivity to various applied stimuli allowing for fast, possibly reversible, and high strain-amplitude response. Their intrinsically low density calls for low power needs compared to their ‘hard’ equivalent, while their mechanical properties are appealing, with a close matching to mechanical properties of biological tissues. In this dissertation, two distinct approaches for soft actuation are pursued, with many new phenomena discovered. In particular, we report on the design, synthesis, characterization, and actuation behavior of smectic liquid crystalline elastomers and swollen polyelectrolyte gels. ^ We have designed, synthesized, and investigated the actuation behavior of smectic main-chain liquid crystalline elastomers (MC-LCEs), which combine composition-dependent phase behavior with low modulus. Importantly, we have found that smectic-C LCEs exhibit both shape memory properties triggered by the glass transition and spontaneously reversible actuation at the clearing transition. By adequately choosing the composition, the latter could be switched “on” or “off”, leading pure shape memory response. Our elastomers revealed excellent and tailorable performances with shape fixing and shape recovery above 95% and reversible strains up to 250%. We explain our results on the basis of the underlying microstructural changes and classic rubber elasticity. ^ We additionally studied the electrically-stimulated actuation behavior of poly(acrylic acid) hydrogels, considering the influence of both internal (crosslink density, neutralization degree, synthesis water content) and external (ionic concentration of the testing medium) parameters. Unexpectedly, the actuation behavior occurs in three stages, each active over a distinct time-scale: an ‘early bending’ stage consisting of a curvature toward the anode, a ‘late bending’ stage that reverses the bending direction toward the cathode, followed by shrinkage of the gels in all directions. Actuation was found to mainly depend on the mechanical properties of the hydrogels: a compromise of low modulus, large pore size, and high concentration of counterions in the testing medium was found to yield faster actuation. ^