Development and utilization of digital image correlation techniques for the study of structural isomerism effects on strain development in epoxy network glasses
The specific aim of this dissertation is to present the findings regarding the effects of molecular structure on macroscopic mechanical performance and strain development in epoxy networks. Network molecular structure was altered through monomer isomerism and crosslink density/molecular weight between crosslinks. The use of structural isomerism provided a pathway for altering mechanical performance while maintaining identical chemical composition within the network. Isomerism was investigated primarily by the curing of diglycidyl ether of bisphenol A (DGEBA) using either the para- or meta-substituted derivatives of diaminodiphenyl sulfone (DDS). Additional insights into isomerism were gained through the investigation of networks composed of either para- or meta-triglycidyl aminophenol (TGAP) cured with 3,3'- or 4,4'-DDS. Crosslink density of the network was varied through two different methods: (a) increasing the equivalent weight of the linear DGEBA epoxy resin and (b) increasing the functionality of the epoxy resin through the use of TGAP. The effects of molecular structure on mechanical properties and strain development were monitored using a relatively new strain measurement technique known as digital image correlation (DIC). Strain measurement via DIC was particularly useful for the development of strain recovery procedures, which provided key insights to the deformation of epoxy network glasses of varying molecular structure by providing full field analysis of the epoxy specimens. Specific findings of this research revealed that network isomerism plays an important role in the deformation of epoxy network glasses. Networks containing meta-substituted monomers possessed higher modulus and yield stress values and lower yield strains. On the contrary, networks with para-substituted monomers displayed lower modulus and yield stress values, but increased ability to store energy through anelastic strain mechanisms, thereby delaying the onset of yielding. The increased energy storage of these networks was related to sub-Tg molecular motions and the ability for para-substituted phenyl rings to rotate along an axis of symmetry, thus creating more cooperative motion within the network. Insights into post-yield deformation of epoxy glasses were also gained where networks with meta-substitution were able to dissipate more stress post-yield through large segmental rearrangements.