Silicone hydrogels are receiving considerable interest due to their important biomedical application areas such as contact lenses and wound dressings. The applications of such materials are usually in the hydrated state, as hydrogels. However, manufacturing and molding processes are mostly carried out in the organically solvated state, as organogels. This thesis investigates the effects of some of the manufacturing parameters such as curing time and thermal processing on thermal, mechanical, viscoelastic and adhesive/cohesive fracture properties of silicone semi-interpenetrating polymer network organogels. Curing time may affect the extent of reaction and the crosslink density of a gel network. In order to investigate the effect of this parameter, materials were photo-cured for different times within the range of 150s to 1800s. Gel content, uniaxial tensile, dynamic mechanical, adhesive fracture and cohesive fracture properties were obtained as a function of photo-curing time and results were correlated with each other in order to have a better understanding of the effects on the material properties. Additionally, thermal properties of the gels were studied in detail. Crystallization and melting behavior of one of the solvents in the organogel were investigated by differential scanning calorimetry and thermal optical microscopy. Correlation between the thermal properties of the solvent and the gel network structure was shown. Dynamic mechanical analysis experiments were performed to investigate the effect of solvent crystallization on the mechanical properties. Finally, the effect of thermal processing parameters such as the heating  rate and the minimum cooling temperatures on the crystallization and the thermo-mechanical properties were studied. / Master of Science

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. / Master of Science