Length effect of short base resin on thermomechanical properties of crosslinked epoxy resin via molecular dynamics simulation

Epoxy resin is commonly used as the matrix in carbon fiber-reinforced polymer (CFRP), fabricated through the curing reaction of the base resin and curing agent, whose properties have a deep relationship with the crosslinking degree of the polymer. However, under certain circumstances, the desired conversion rate cannot be achieved in the experiments. To shed light on this phenomenon and the underlying mechanisms, the curing process of diglycidyl ether of bisphenol A (DGEBA) with different chain lengths and 4,4 '-diamino diphenyl sulfone (4,4 '-DDS) was simulated as per our previously developed crosslinking algorithm. The longer chain length postpones but does not change the reaction progress (like the gel point), leading to a lower conversion rate, and ultimately a decrease in the diffusion coefficient. In addition, the influence on the glass transition temperature is the result of the competing effects of the conversion rate and monomer diffusivity. Moreover, the negligible effect on thermal conductivity is closely linked with a slight change in the monomeric degree index (MDI), reflecting the whole internal intrinsic structure. Furthermore, the effect on Young's modulus and yield stress has a deep relationship with the ring degree index (RDI), i.e., higher RDI demands more non-bonded energy for the collective movement of monomer chains. Our systematic investigation unifies the mechanism of the impact of chain length on various thermomechanical properties, which is instructive for the improved performance of epoxy resins.