Mobility enhancement in amorphous polyamide 6,6 induced by water sorption:: A molecular dynamics simulation study

The local dynamics of water and its effect on the segmental mobility in the amorphous phase of polyamide 6,6 have been investigated. At 300 K and below, where the polymer segments undergo no significant motion (displacements below 2 Angstrom, no reorientation) in the time scale of the simulation, water molecules exhibit mostly orientational freedom, and a bimodal distribution of reorientation times is observed. As the temperature is increased, an increasing fraction of the water molecules also experience hopping behavior between polyamide 6,6 cavities, but true diffusion on a nanosecond time scale is observed only above 400 K. The analysis of amide and methylene group reorientation clearly shows lubrication of the intermolecular amide-amide hydrogen bond, detected above 350 K only at the time scale of 2 ns. This phenomenon goes beyond a simple free volume effect, as is shown through the analysis of the local hydration of individual amide groups. The motion of inner methylene segments is also affected by water, but in a more limited extent. The enhancement of their mobility with water content is probably an indirect effect of the lower reorientation times of neighboring amide groups.