Water absorption in aliphatic polyamide mixtures

Essential properties of plastics, such as longevity and mechanical strength, depend on their water absorption capacity. In particular, the water uptake of polycondensates, like polyamides, strongly depends on their composition. To enhance physical characteristics, formulations of polyamide mixtures can be devised. Here, we present dynamic vapor sorption measurements, propose a theoretical approach to predict moisture uptake by aliphatic polyamide mixtures, and compare theoretical and experimental results. Experimental moisture uptake is determined for a wide range of relative humidities, using the weight difference of 10-15 mg samples measured with a specificity of $0.1\mu g$. Our experimental results show that moisture uptake depends on composition and environmental humidity. Our theoretical model considers these dependencies in an extension of Flory–Huggins theory, and demonstrates semi-quantitative predictions of moisture absorption in various mixtures of PA 46, PA 48, PA 412, PA 418, and PA 436. The largest deviations are observed for the PA 46/PA 418 mixture, in which the measured moisture uptake is higher than the computed wt%. With a newly defined effective amide-concentration, the moisture uptake as a function of concentration can be predicted using a single master curve for different polyamide combinations. The absorption of water in a homogeneous polyamide mixture is found to be lower than in mixtures in which (micro-)phase separated domains are formed. The theory predicts up to 43% less moisture uptake in a homogeneous, 50/50 PA 46/PA 436 mixture compared to a phase separated mixture. Additionally, moisture uptake is marginally influenced by the volume of these domains and interfacial areas. The predictions regarding total miscibility and water absorption closely align with the experimental results obtained through dynamic vapor sorption measurements of moisture uptake. The proposed theoretical framework holds the potential to significantly contribute to the development of polyamides with optimized and more desirable properties.