Characterization of the Viscoelastic Fracture of Solvated Semi-Interpenetrating Polymer Network Silicone Hydrogels

The unique compressive, optical, and biocompatible properties of silicone hydrogels allow them to be used in a wide variety of applications in the biomedical field. However, the relatively weak mechanical behavior, as well as the highly deformable nature of these elastomeric materials, presents a myriad of challenges when attempting to understand their constitutive and fracture properties in order to improve hydrogel manufacturing and performance in applications. In this thesis, a series of experimental techniques were developed or adapted from common engineering approaches in order to investigate the effects of rate and temperature on the viscoelastic constitutive and fracture behavior of two solvated semi-interpenetrating polymer network silicone hydrogel systems.

Viscoelastic characterization of these material systems was performed by implementing a series of uniaxial tension and dynamic mechanical analysis shear tests in order to generate relevant master curves and corresponding thermal shift factors of such properties as shear relaxation modulus, dynamic moduli, and the loss factor. Concurrently, the cohesive fracture properties were studied by utilizing a "semi-infinite" strip geometry under constrained tension in which thin pre-cracked sheets of these cured hydrogels were exposed to several different loading conditions. Fracture tests were performed over a relevant range of temperatures and crosshead rates to determine and generate a master curve of the subcritical strain energy release rate. Experimental methods utilizing high-speed camera images and digital image correlation to monitor viscoelastic strain recovery in the wake of a propagating crack were explored. The results from this thesis may prove useful in an investigation of the interfacial fracture of these hydrogel systems on several different polymer substrates associated with an industrial manufacturing problem.