Phenylene Ring Motions in Isomeric Glassy Epoxy Networks and Their Contributions to Thermal and Mechanical Properties

Cross-linked epoxies prepared from diglycidyl ether of bisphenol A (DGEBA) exhibit substantial property differences due to isomeric structures of the diamine curatives, 3,3′- and 4,4′-diaminodiphenyl sulfone (DDS). Here, the motions of phenylene rings on both the bisphenol A (BPA) and DDS structures of the networks were investigated through solid-state NMR 2H line shape analysis to trace the molecular origins. Ring deuterated monomers, namely, DGEBA-d 8, 33DDS-d 8, and 44DDS-d 8, were synthesized, and four deuterated networks were prepared. The line shapes of the ring motions were interpreted using a motional model that described both ring π-flips and main-chain fluctuations. The contributions of different ring motions to mechanical relaxations are elucidated, and the molecular origins of property differences are identified.