Are microcomposites realistic models of the fibre/matrix interface? II. Physico-chemical approach

The physical and mechanical properties of microdroplets can differ significantly from those of the bulk material, This is especially true for epoxyde droplets, owing to (i) diffusion and vaporization of the hardener during the first step of the cure, (ii) surface oxidation and (iii) possible hydrolysis of the hardener during the first step of the cure schedule. The glass transition temperature of microdroplets was found to be 20-50 degreesC lower than that of the bulk material. It is: shown that this leads to microdroplets exhibiting a higher Young: a modulus and a lower yield point than the bulk network. Those discrepancies influence the adhesion between fibre and matrix by changing the stress distribution at the interface and can lead to biased results when comparing different matrices with the microbond test. The general idea that microbond specimens behave like ideal elastic components is now reassessed in view of (i) a plastic flow of the polymeric droplet and (ii) the occurrence of rate-dependent processes. This Leads to a fundamental question regarding the use of microcomposites as a model of macroscopic-scale specimens. (C) 2001 Elsevier Science Ltd. All rights reserved.