The glass transition in cured epoxy thermosets: A comparative molecular dynamics study in coarse-grained and atomistic resolution

摘要

We investigate the volumetric glass transition temperature T-g in epoxy thermosets by means of molecular dynamics simulations. The epoxy thermosets consist of the resin bisphenol A diglycidyl ether and the hardener diethylenetriamine. A structure based coarse-grained (CG) force field has been derived using iterative Boltzmann inversion in order to facilitate simulations of larger length scales. We observe that (T)g increases clearly with the degree of cross-linking for all-atomistic (AA) and CG simulations. The transition T-g in CG simulations of uncured mixtures is much lower than in AA-simulations due to the soft nature of the CG potentials, but increases all the more with the formation of rigid cross-links. Additional simulations of the CG mixtures in contact with a surface show the existence of an interphase region of about 3 nm thickness in which the network properties deviate significantly from the bulk. In accordance to experimental studies, we observe that T-g is reduced in this interphase region and gradually increases to its bulk value with distance from the surface. The present study shows that the glass transition is a local phenomenon that depends on the network structure in the immediate environment. (C) 2015 AIP Publishing LLC.