Mobility Enhancement in Amorphous Polyamide 6,6 Induced by Water Sorption:  A Molecular Dynamics Simulation Study

Abstract

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 Å, 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.

Interfacial thermal transport and structural preferences in carbon nanotube–polyamide-6,6 nanocomposites: how important are chemical functionalization effects?

“…For the grafted PA chains only 10 monomers have been investigated. 65, 66 We have used an atomic Verlet neighbor list with a cutoff radius of 1.1 nm updated at an interval of 15 time steps. The force field parameters of Li and Chou 58 have been chosen for the CNT unit.…”

Grand canonical ensemble molecular dynamics simulation of water solubility in polyamide-6,6

“…46 All nonbonded interactions were truncated at 0.95 nm with a reaction field correction for the Coulombic interactions. 45 The reaction-field dielectric of PA-6,6+water mixtures was approximated as the sum of weight fractions of components multiplied by their corresponding reaction-field dielectrics; the reaction field dielectric of PA-6,6 was taken equal to the experimental value, 5.5, 39,40 and that of water, in polymer and gas phases, was taken equal to 72.0. 47 An atomic Verlet neighbor list was used, which was updated every 15 time steps, as well as every time steps at which insertions or deletions were performed, and the neighbors were included if they were closer than 1.0 nm.…”

Multiscale modeling of thermal conductivity of polymer/carbon nanocomposites

“…The pressure was reduced from 100 to 1 atm over 10 ps, followed by a MD run at 1 atm and 300 K for 100 ps. Such equilibration procedures are typical for the simulation of polymers [15,16]. Running MD at high temperatures usually allows the polymers to relax out of any nonphysical configurations.…”