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

The physical and mechanical properties of microdroplets can differ signific antly from those of the bulk material, This is especially true for epoxyde droplets, owing to (i) diffusion and vaporization of the hardener during th e first step of the cure, (ii) surface oxidation and (iii) possible hydroly sis of the hardener during the first step of the cure schedule. The glass t ransition temperature of microdroplets was found to be 20-50 degreesC lower than that of the bulk material. It is: shown that this leads to microdropl ets exhibiting a higher Young: a modulus and a lower yield point than the b ulk network. Those discrepancies influence the adhesion between fibre and m atrix by changing the stress distribution at the interface and can lead to biased results when comparing different matrices with the microbond test. T he general idea that microbond specimens behave like ideal elastic componen ts 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 funda mental question regarding the use of microcomposites as a model of macrosco pic-scale specimens. (C) 2001 Elsevier Science Ltd. All rights reserved.