Transcrystallinity in nylon 66 composites and its influence on thermal expansivity

Abstract The crystallization behaviour of nylon 66 in the presence of fibre reinforcement or nucleating agent was investigated under isothermal and non-isothermal conditions. Unidirectional fibre composites were prepared using carbon and aramid fibres. Hot-stage microscopy revealed that a high nucleation density along these fibres produced a uniform transcrystalline layer. Being a nucleation-controlled process, the transcrystallization depended strongly on thermodynamic conditions such as crystallization temperature and cooling rate. With the use of differential scanning calorimetry, the crystallization process was studied and the influence of two main parameters—the presence of the fibre or nucleating agent and the cooling rate—was analysed. The fibres were found to behave like a giant nucleating site and to enhance the rate of crystallization of nylon 66. The hot-stage technique confirmed these facts, showing that under a variety of conditions transcrystallinity was favoured compared with bulk crystallinity. Thermomechanical analysis was carried out to measure the effect of transcrystallinity on the thermal expansivity of aramid fibre-reinforced unidirectional microcomposites. A crystallinity-related strong decrease in both the longitudinal and the transverse coefficients, of which the latter could apparently be assigned to transcrystallinity, was revealed.