Bi-modal polymer networks: Composition-dependent trends in thermal, volumetric and structural properties from molecular dynamics simulation

Thermal and volumetric properties of mixed epoxy networks were characterized with molecular dynamics simulation. Atomistically detailed models of epoxy networks of diglycidyl ether of bisphenol (DGEBA) cured with stoichiometric binary mixtures of a flexible cross-linker poly(oxypropylene) diamine (POP) and a stiff cross-linker 4,4′-methylenebis(cyclohexylamine) (MCA), having molecular weights of 1987 and 210 g/mol respectively, were prepared. Epoxy networks formed by five different compositions of the cross-linkers ranging from pure POP to pure MCA were constructed, and a network topology analysis was carried out to verify that each network chain was connected to all other chains by a path of bonded molecules. The glass transition temperature (T_g), coefficient of volume thermal expansion (CVTE), heat capacity and thermal conductivity of these network structures were determined as a function of the network composition. The simulated values of these properties are compared with predictions from theories, empirical correlations and experiments from the literature. In general, it is observed that an increase in the mass fraction of MCA leads to an increase in the T_g and a decrease in the CVTE; furthermore, the breadth of the transition as exhibited by the change in the specific volume, CVTE, and heat capacity increases with an increase in the MCA content. The differences in the flexibility of the network components were analyzed using a number of quantitative measures. Using these results, a molecular mechanism is proposed for the observation of the network composition dependence of the breadth of the glass transition in these systems.