Investigation of Water Diffusion Mechanisms in Relation to Polymer Relaxations in Polyamides

Diffusion in semicrystalline polymers is a complex phenomenon because of the existence of specific interactions (nonpolar or polar), dynamic heterogeneities, and crystalline phases. The diffusion of water in two semicrystalline polyamides (PA6,6 and PA6,10) was investigated in order to determine the diffusion mechanisms and the influence of polymer relaxations on this process. Liquid water diffusion follows a Fickian mechanism in PA6,10 and a non-Fickian or anomalous mechanism in PA6,6. Through a quasi-equilibrium experiment in dynamic vapor sorption, it is shown that this difference results from the dependence of the diffusion coefficients on water concentration. Moreover, the influence of the polymer relaxations was assessed by broadband dielectric spectroscopy. The dielectric characteristic relaxation times of the α relaxation, associated with the glass transition, and of the β relaxation, related to more local dynamics, have been measured. A simple comparison with the time scale of diffusion suggests that diffusion and polyamide α relaxation should not be directly correlated. However, diffusion is correlated to the secondary β relaxation, which encompasses the local chain dynamics of hydrogen-bonded amide groups in the presence of water. A mechanism of diffusion based on the trapping of water molecules between neighboring sorption sites (amide groups) is proposed in these strongly interacting polymers. It is suggested that diffusion is limited by the relaxation time of hydrogen bonds between water molecules and amide groups and the change in conformation of these amide groups present in polyamides.