Molecular cooperativity in large deformation and subsequent structural relaxation for epoxy glass

Abstract

In the present article, results from our recent studies on effects of chemical crosslinks in epoxy glass subjected to large deformation and subsequent aging under strain are reviewed. Bis-A type epoxy monomer was cured with DDM to form a sample with a crosslinked network structure. The monomer was also polymerized with aniline to form another sample with an uncrosslinked linear molecular structure. First, change in birefringence in the course of elongation was measured, and the modified stress-optical rule was applied to the experimental results. For the crosslinked sample, both internal potential and entropy changed in a small strain range. On the other hand, for the linear molecular sample only internal potential increased in a small strain range. Another series of results was obtained through thermomechanical experiments with largely deformed specimens, and the obtained results indicated that chemical crosslinks forced more rubbery strain to recover at temperatures below T g. From these studies it was proposed that chemical crosslinks in epoxy glass enhance cooperativity between molecular motions taking place in deformation as well as in subsequent retardation process. Such an effect of chemical crosslinks was also observed in the crosslinked sample under the state of aging under strain. © 2007 The Society of Rheology.