Investigation of chemo-mechanical actuation in polymer gels

Author

Salvador Enrique Lobato Larios, University of Wollongong

Year

2009

Degree Name

Doctor of Philosophy

Department

School of Mechanical, Materials & Mechatronic Engineering - Department of Engineering

Recommended Citation

Larios, Salvador Enrique Lobato, Investigation of chemo-mechanical actuation in polymer gels, PhD thesis, School of Mechanical, Materials & Mechatronic Engineering, University of Wollongong,2009. http://ro.uow.edu.au/theses/3069

Abstract

The objective of this thesis is to investigate chemically – driven gel actuators and their usefulness to perform mechanical work. The thesis work has mainly concentrated on chitosan as the gel actuator material. Chitosan is a widely used biopolymer that shows a large volume transition through changing pH. The pH induced actuation behaviour of polymer gels makes these materials analogous to a biological muscle. From a mechanics viewpoint, polymers gels are very compliant, unlike more common engineering material such as metals or ceramics or even most polymers. The mechanical work output of an actuation system is determined from a combination of the displacement and force generated. A complication arises with gel actuators since prior work has shown that the application of an external force can change the gel volume transition. Very little information is available regarding the specific effect of an external load on the displacement generated by a gel actuator. The thesis included experiments on a homogeneous solvent swollen rubber as a model system. Actuation of the rubber was effected through solvent exchange. The mechanical actuation was investigated by three different approaches: Linear Elastic, Thermodynamic and Linear Elastic generalized to 3D (finite element model). These models are to account for both the mechanical properties and the actuation behaviour of these materials. Uniaxial mechanical testing has been conducted on natural rubber and chitosan gel in both the fully expanded and fully contracted states, and the material parameters from these mechanical tests were used in the various models. The model and material parameters are then used to predict the rubber/chitosan actuation behaviour under isotonic loading. Actuation tests on chitosan fibres showed unexpected results. Chitosan fibres showed a considerable strain difference between length and diameter during free actuation. An applied load during actuation process gave completely unexpected results: contraction in the length direction when expansion was expected and vice versa.

The 3 - dimensional elastic approach based on the Finite Element Model was modified from the isotropic material (natural rubber) to account for the anisotropic chitosan fibres. This model enables a reasonable estimate of the Young’s modulus and Poisson’s ratio. The results suggest that the materials have different Young’s modulus in the longitudinal and transversal directions which produces a high expansion in the transversal direction in comparison with the longitudinal, affecting directly the actuation process.

Moreover the changes in Poisson’s ratio and modulus due to a change in the chemical environment (pH or solvent) were also shown to account for the unexpected actuation results. Useful dynamic information on the molecular scale was obtained on the structure of the fibre using a real-time small angle X-ray scattering (SAXS) technique. Parameters (particularly correlation lengths and intensity) fitted to the experimental data clearly indicated structural differences in the directions perpendicular and parallel to the fibre axis, however detailed description of the molecular organisation requires further investigation.