Partially Reacted Substructures Method for Thermoset Epoxies Studied Using Molecular Dynamics Simulations

摘要

We employed the “partially reacted substructure (PRS)” method to enhance thernmaterial properties of the diglycidyl ether of bisphenol A (DGEBA) epoxy thermosets.rnThis method involves first partially curing DGEBA with a small amount ofrnpoly(oxypropylene)diamine (POPDA) and then adding DGEBA andrndiethyltoluenediamine (DETDA) to continue to a fully cured sample. PRS-inducedrnDGEBA-DETDA/POPDA 2:1 epoxy-amine stoichiometric samples have consistentlyrnhigher Tg’s and glassy state densities than samples created using one-step cure of arnblend of DGEBA with both DETDA and POPDA. PRS-induced samples were foundrnto be twice as tough as non-PRS samples. Because the compositions are identical,rndifferences observed must arise from differences in the network isomers formed by therntwo curing protocols and the resulting differences in packing and intermolecularrninteractions. Molecular dynamics (MD) simulations were carried out to betterrnunderstand these phenomena. The density and glass transition temperature in thernsimulations are in quantitative agreement with the experimental results. One reason forrnthis Tg increase in PRS-induced samples may be related to the pendant methyl groupsrnon POPDA, as illustrated using radial distribution functions (RDF’s) computed fromrnthe MD simulation trajectories. Methyl-methyl RDF’s for non-PRS samples did notrndiffer significantly from those of the prepolymerized liquid, but the methyl-methylrnRDF’s for the PRS-samples showed stronger methyl-methyl correlation at allrndistances, signifying better intra- and intermolecular POPDA packing. This results inrnlower POP backbone flexibility in the PRS samples and consequently higher Tg’srnrelative to the non-PRS samples. Further simulations aim to ultimately focus on thernductility enhancement conferred by PRS.