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The effects of crosslink functionality (fc), molecular weight between crosslinks (Mc), and chain stiffness display on the thermal and mechanical behavior of epoxy networks are determined. Both fc and Mc are controlled by blending different functionality amines with a difunctional epoxy resin. Chain stiffness is controlled by changing the chemical structure of the various amines. In agreement with rubber elasticity theory, the rubbery moduli are dependent on fc and Mc, but independent of chain stiffness. The glassy moduli and secondary relaxations of these networks are relatively independent of fc, Mc, and chain stiffness. However, the glass transition temperatures (Tg) of these networks are dependent on all three structural variables. This trend is consistent with free volume theory and entropic theories of Tg. fc, Mc, and chain stiffness control the yield strength of these networks in a manner similar to that of Tg and is the result that both properties involve flow or relaxation processes. Fracture toughness, as measured by the critical stress intensity factor (KIc), revealed that fc and Mc are both critical parameters. The fracture behavior is the result of the fracture toughness being controlled by the ability of the network to yield in front of the crack tip. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1371–1382, 1998
Crawford et al. (Mon,) studied this question.