Molecular dynamics simulation of water influence on local structure of nanoconfined polyamide-6,6.

Our recently developed molecular dynamics simulation method for simulating fluids confined in nanometric pores [Eslami et al. J. Chem. Phys.2008, 129, 194702] is employed to simulate nanoconfined polyamide-6,6+water in contact with graphene monolayers. In this work, a number of dry and wet polymer samples, containing 3 and 6 wt % water at 350 K, confined in pores of different sizes as well as the bulk polymer samples are simulated. It is shown that both water and polymer molecules form organized layers beside the surfaces. The effect of nanoconfinement on the hydrogen bonding in the polymer+water system is studied in detail. It is shown that addition of water to the polymer replaces a fraction of looser amide-amide hydrogen bonds with amide-water ones. The distribution of hydrogen bond lengths shows that both the polymer and water form shorter length hydrogen bonds in the confined region compared to the bulk system. The number of hydrogen bonds between polymer and water molecules is shown to depend on the pore size and on the water content. In small pores, water molecules are forced by the surfaces into the polymer layers and form strong hydrogen bonds with the amide groups, a process that increases the water solubility of the polymer in small pores. Increasing the water content in larger pores increases the number of hydrogen bonds between water molecules and causes the formation of big clusters in the central region of the pore. In simulations of to 50 ns, it is shown that such big clusters grow with of time, and hence, phase separate out from the mixture.