THE EFFECT OF NETWORK ARCHITECTURE ON THE THERMAL AND MECHANICAL-BEHAVIOR OF EPOXY-RESINS

The effects of crosslink functionality (f(c)), molecular weight betwee n crosslinks (M-c), and chain stiffness display on the thermal and mec hanical behavior of epoxy networks are determined. Both f(c) and M-c a re controlled by blending different functionality amines with a difunc tional epoxy resin. Chain stiffness is controlled by changing the chem ical structure of the various amines. In agreement with rubber elastic ity theory, the rubbery moduli are dependent on f(c) and M-c, but inde pendent of chain stiffness. The glassy moduli and secondary relaxation s of these networks are relatively independent of f(c), M-c, and chain stiffness. However, the glass transition temperatures (T-g) of these networks are dependent on all three structural variables. This trend i s consistent with free volume theory and entropic theories of T-g. f(c ), M-c, and chain stiffness control the yield strength of these networ ks in a manner similar to that of T-g, and is the result that both pro perties involve flow or relaxation processes. Fracture toughness, as m easured by the critical stress intensity factor (K-Ic), revealed that f(c) and M-c are both critical parameters. The fracture behavior is th e result of the fracture toughness being controlled by the ability of the network to yield in front of the crack tip. (C) 1998 John Wiley & Sons, Inc.