The effect of annealing of linear and branched polyamide 46 on the phase composition, molecular mobility and water absorption as studied by DSC, 1H and 2H solid-state NMR

The effect of annealing on the phase composition, molecular mobility and water absorption in linear and branched polyamide 46, poly(tetramethylene adipamide) - PA46, was studied by DSC, H-1 and H-2 solid-state NMR. A series of samples with varying amount of long-chain branches was synthesized for this study. The branches as well as a higher molecular weight of branched PA46 cause a decrease in crystallization rate as it follows from a lower crystallization temperature. 1H NMR transverse magnetization relaxation (T-2 relaxation) experiments show that, at temperatures well above T-g, the amount of low mobile chain segments (rigid fraction) in all injection-molded samples equals 60-63%. This value is close to the crystallinity of linear PA46 as measured by WAXD. Although chain branches hardly affect the amount of the rigid fraction in injection-molded samples, molecular mobility in the crystalline phase of branched PA46 is lower than that in linear PA46, suggesting a more perfect crystalline structure in branched PA46 due to slower crystallization. 12.5 h annealing at 260 degrees C causes a large increase in the amount of the rigid fraction in linear PA46, whereas WAXD crystallinity only slightly increases upon annealing. The increase in the rigid fraction is largely caused by immobilization of the amorphous phase due to chains rearrangements resulting in stronger hydrogen bonds between amide groups in the amorphous phase. The amount of the rigid fraction in annealed samples is smaller in samples with higher amount of branches, and molecular mobility in the rigid and soft fractions is larger. This suggests that chain branches hamper chain rearrangements both in the crystalline and the amorphous phases during annealing of PA46. This results in a less ordered structure in the crystalline and the amorphous phases in annealed branched PA46. The amount of absorbed water decreases upon increasing amount of branches in injection-molded samples and upon annealing. H-2 NMR experiments for PA46 saturated with D2O show that the mobility of absorbed water is strongly hindered as compared to pure water. Below ambient temperatures, mobility of water in PA46 is characterized by a very broad distribution of molecular motions. The mobility of water molecules gradually decreases upon sample cooling from ambient temperature to -40 degrees C. The majority of water molecules is immobilized below -40 degrees C. The results of the present study suggest that water uptake by PA46 is mainly determined by the strength of hydrogen bonds between amide groups in the amorphous phase, which is largely affected by the crystallization rate and annealing at elevated temperatures. (C) 2014 Elsevier Ltd. All rights reserved.