Polyamides (PAs) are repeatedly exposed to environments of varying humidity throughout their service life. Due to their hygroscopic nature, moisture diffusion can alter the polymer properties, sometimes irreversibly. It has been previously found that the effect of transport of water on the structure, morphology, and physical properties of polymers is not negligibly small. In certain semi-crystalline polyamides, the diffusion coefficient has been shown to be governed by the local chain dynamics (β relaxation). The final molecular weight of PAs achieved after melt processing is a result of the equilibrium between the forward and reverse polycondensation depending on the water concentration. With the growing demand for unreinforced and reinforced polyamides as well as polyamide fibers in high-performance applications, it is critical to understand the physics of the interaction between water molecules and polymer or composite systems. This article reviews the existing literature about polyamide-water interactions with a focus on the governing physical laws of moisture transport within the polyamide matrix, drying kinetics, and dynamics of water in the polymer system. The implications of moisture on the processing and properties of the polyamides class of materials are also discussed, suggesting the need for best practices in instrumentation and control.

Variances in polymers processed by single-screw extrusion are investigated. While vortical flows are well known in the fluids community and fountain flows are well known to be caused by the frozen layers in injection molding, our empirical evidence and process modeling suggests the presence of vortical fountain flows in the melt channels of plasticating screws adjacent to a slower-moving solids bed. The empirical evidence includes screw freezing experiments with cross-sections of processed high-impact polystyrene (HIPS) blended with varying colorants. Non-isothermal, non-Newtonian process simulations indicate that the underlying causality is increased flow conductance in the melt pool caused by higher temperatures and shear rates in the recirculating melt pool. The results indicate the development of persistent, coiled sheet morphologies in both general purpose and barrier screw designs. The behavior differs significantly from prior melting and plastication models with the net effect of broader residence time distributions. The process models guide potential strategies for the remediation of the processing variances as well as potential opportunities to achieve improved dispersion as well as complex micro and nanostructures in polymer processing.