Effect of crosslink density on molecular relaxations in diepoxide‐diamine network polymers. Part 2. The rubbery plateau region

Abstract

AbstractA comparison of the prediction of the theory of rubber elasticity with the experimentally observed variation of the shear storage modulus, G, as a function of crosslink concentration shows that deviations occur when the network strand concentration in diepoxide‐diamine polymers exceeds approximately 1.5 mole kg−1. The rapid rise in G above this level is accounted for in terms of the increasing importance of non‐Gaussian chain statistics and steric interactions. It is also established that the contribution from entanglements is significant and the behavior over the entire crosslink density range can be described by the following equation where v and ϵTe are the concentrations of elastically active strands which orginate from fixed points and entanglements respectively, ψ is an empirical constant related to the importance of the non‐ideal behavior, and ϕ is the so‐called “front factor”. This latter constant is found to depend on the functionality of the network junctions, varying from 0.9, for a system with tetrafunctional junctions, to an average of 0.53 for those networks with trifunctional junctions.