Structure and mobility of nanoconfined polyamide-6,6 oligomers: application of a molecular dynamics technique with constant temperature, surface area, and parallel pressure.

A molecular dynamics simulation method with coupling to an external bath is used to simulate polyamide-6,6 trimers confined between graphite surfaces. In this simulation method, the temperature and the parallel component of pressure are kept fixed, and the distance between the confining graphite surfaces is changed to achieve equilibrium. The simulation results on the oscillatory behavior of solvation force, the number density of confined oligomers, and stepwise variation of the oligomer numbers as a function of distance between the confining graphite surfaces are reported and discussed. The hydrogen bonding in confined oligomers has also been studied in detail, and it is shown that hydrogen-bond formation depends on the layering effect and on the geometrical restrictions and reveals an oscillatory behavior like the solvation force oscillations. The autocorrelation functions for NH, CO, and end-to-end vectors are also studied, and it is concluded that the confined fluid has a considerably lower relaxation time than that of the bulk fluid, and the relaxation times for confined fluid show an oscillatory behavior with maxima corresponding to well-formed structures parallel to the surfaces. The study of local dynamics via calculating the autocorrelation functions for bins parallel to the surfaces reveals that the fluid layers close to the surfaces have higher relaxation times than the fluid in the central region. Our calculated center-of-mass diffusion coefficients also show oscillatory behavior with out-of-phase oscillations with respect to the solvation force oscillations.