Effect of cross-linker length on the thermal and volumetric properties of cross-linked epoxy networks: A molecular simulation study

Volumetric and thermal properties of cross-linked epoxy systems consisting of diglycidyl ether of bisphenol A (DGEBA) and poly(oxypropylene) (POP) diamines of four different lengths ranging from 3 to 68 units were investigated by molecular dynamics (MD) simulations. The cross-linked structures were built by using the simulated annealing polymerization approach. The density, coefficients of volume thermal expansion and glass transition temperature (T-g) of each of the four cross-linked epoxy systems were obtained from their volume temperature behavior. The density obtained in the simulations agreed well with the experimental value, whereas the coefficients of volume thermal expansion were at least 30% lower than their corresponding experimental results. The predicted T-g values were higher than the experimental values due to the considerably faster cooling rates that are used in the simulations. It was observed that an increase in the chain length of the cross-linker POP-diamines led to a larger difference between the predicted and experimental values of T-g. Three different approaches were used to estimate the expected shift in the experimental T-g to higher values had these measurements been made at cooling rates comparable to those used in MD simulations. It is shown that, in general, the T-g values obtained in MD simulations are consistent with such shifted T-g values that account for the difference in the cooling rates, although no one particular shift approach worked well for all four epoxy systems studied. (C) 2012 Elsevier Ltd. All rights reserved.