Quantitative magnetic resonance imaging study of water uptake by polyamide 4,6

Magnetic resonance imaging (MRI) and NMR relaxation experiments are used to study the water absorption by polyamide 4,6 (PA46) plates. Despite the higher crystallinity, PA46 absorbs more water as compared to the polyamides 6 and 6,6 (PA6 and PA66). Relaxation measurements demonstrate that the volume averaged molecular mobility (T2H) of the absorbed water molecules in PA46 is higher than in PA6 and PA66. Quantitative relaxation results of water saturated PA46 further suggest fast exchange between free water and water molecules bound via hydrogen bonds to amide groups. MRI reveals a gradual decrease in the amount and the T2H relaxation behaviour of water from the surface towards the core part of PA46 plates. This can be explained by the very high crystallisation rate of PA46, which prevents the close coupling of amide groups in the amorphous phase and results in a larger mean distance between the amide groups, especially in the outer part of the PA46 plates. Density measurements (WAXS and gradient column) show an increase of the density of the amorphous phase during annealing, resulting in a lower water uptake and a lower mobility of the absorbed water molecules.