MATRIX RELAXATION AFTER FRACTURE IN A TOUGHENED EPOXY
Aaron M. Forster, Cyril Clerici, Donald L. Hunston
An efficient method to increase the Mode I fracture energy of a polymer adhesive is to incorporate a rubbery phase within the glassy matrix. In general, the failure surface of a toughened polymer is characterized by cavitated rubber particles and plastic flow of the matrix. This region has commonly been referred to as the stress-whitened zone. The textured surface of the whitened zone is in contrast to the smooth fracture surface characteristic of fast-fracture. We have shown previously that the plastic deformation within the whitened zone may be quantified with atomic force microscopy (AFM) and laser scanning confocal microscopy (LSCM) by analyzing the root-mean-square roughness at measurement length scales (scan size or magnification) before and after heating the epoxy near the glass transition temperature, Tg. In this presentation, we further our previous work by investigating the overall matrix relaxation as a function of heating. The fracture toughness of a widely studied model epoxy system was measured (DER-332 epoxy toughened with a butyl-nitrile rubber) at one loading rate. The microstructure in and outside the stress-whitened zone was characterized with LSCM before and after heating. The position of individual cavitated particles are tracked in the precrack zone, the stress-whitened zone, and the fast-fracture zone, before and after heating above the epoxy Tg. It will be shown that the position of these particles shifts towards the pre-crack in the epoxy matrix with heating. The magnitude of contraction is dependant on the location of the cavitated particle relative to the crack tip and the residual stress of the original epoxy adhesive. It is not found dependent on the local microstructure within this epoxy system.