Structure and property relationships in model diglycidyl ether of bisphenol-A and diglycidyl ether of tetramethyl bisphenol-A epoxy systems. I. Mechanical property characterizations

Model epoxy networks, with variations in crosslink density and in epoxy mon omer rigidity, were prepared to study how the network structure affects mod ulus, T-g, and toughness/toughenability of epoxy resins. Diglycidyl ether o f bisphenol-A and diglycidyl ether of tetramethyl-bisphenol-A, along with t he corresponding chain extenders, were chosen to study how monomer backbone rigidity and crosslink density affect physical and mechanical properties o f epoxies. The present study indicates that, as expected, the backbone rigi dity of the epoxy network, not the crosslink density alone, will strongly i nfluence modulus and T-g of epoxy resins. Upon rubber toughening, it is fou nd that the rigidity of the epoxy backbone and/or the nature of the crossli nking agent utilized are most critical to the toughenability of the epoxy. That is, the well-known correlation between toughenability and the average molecular weight between crosslinks (M-c) does not necessarily hold true wh en the nature of epoxy backbone molecular mobility is altered. The potentia l significance of the present findings for a better design of toughened the rmosets for structural applications is discussed. (C) 1999 John Wiley & Son s, Inc.