A study of the interphase region in carbon fibre/epoxy composites using dynamic mechanical thermal analysis

We have examined the effect of fibre addition on the glass transition temperature (T-g) of two epoxy resin systems (an amine cured and an anhydride cured epoxy system) using dynamic mechanical thermal analysis (DMTA) and differential scanning calorimetry (DSC). The presence of fibres changes the glass transition temperature (T-g) of an anhydride cured epoxy resin but does not affect that of an amine cured epoxy, The data suggest that two counteracting mechanisms are responsible for these changes: firstly, the presence of fibres causes a restriction of the molecular motion in the resin system, and secondly, the presence of carboxyl and keto-enol groups on the fibre surface inhibit curing of the resin close to the fibre, i.e. in the interphase region. The former increases the T-g and is a long range effect whereas the latter decreases the T-g and is a localised phenomenon. Changes in the dynamic properties of the interphase region are only detected when the samples are loaded in the longitudinal direction and not in the transverse direction where bulk matrix properties dominate, Sizing the fibres before their incorporation into the epoxy resin eliminates the variation in interfacial properties arising from differences in fibre surface chemistry.