Microdeformation and network structure in epoxies

Abstract

Thin films of epoxies with various strand densities, Ν, are strained in tension until localized plastic deformation is observed. The total strand density, a sum of entanglement and crosslinked strand densities, is adjusted by changing the initial resin molecular weight, the weight fraction of added diluent, and the stoichiometric fraction of curing agent. Experiments on uncrosslinked high molecular weight phenoxy are used to investigate the entanglement network. The strand density Ν is computed from measurements of the rubbery plateau modulus using the theory of rubber elasticity and is used to compute the maximum extension ratio of a single network strand, λmax, which varies approximately as Ν−1/2. Transmission electron microscopy is used to quantitatively characterize the plastic deformation. Only plane stress deformation zones (DZs) are observed in the cross-linked epoxies, and the entangled phenoxy resins. The characteristic extension ratio in these DZs, λ, is found to scale as λ−1=0.32 (λmax−1), a relation close to that observed for thermoplastics and cross-linked polystyrene. Rather than promoting a transition from shear deformation to crazing, diluting these networks with unreactive epoxy molecules too short to entangle makes them prone to fracture.