Atomistic Simulation of the Water Influence on the Local Structure of Polyamide 6,6

Abstract

To understand, at a molecular scale, the effect of water on the structure of the amorphous region of polyamide 6,6 (PA6,6), atomistic molecular dynamics simulations have been carried out. Our results concerning the very local water organization relative to PA moieties agree qualitatively with a two-step sorption model. The first sorption mode seems to be saturated well below the lowest water concentration studied (5% relative to the amorphous phase). Moreover, above this saturation, the overall water organization displays at 300 K larger clusters than the 2−3 molecules usually assumed in the literature. The temperature dependence of free volume, hole size, and hydrogen bonding has also been investigated. It shows a transition between plasticized and antiplasticized behavior.

Hydrogen Bonding and Dynamic Crossover in Polyamide-66: A Molecular Dynamics Simulation Study

“…The parameters for best fits along with the amplitude-weighted average τ R for four temperatures are listed in Table. 1. As we can see in Table 1, we have an abrupt increase in the structural relaxation time as the glass transition temperature T g (330 K) as calculated from our atomistic model 26 approaches.…”

“…The distance between the hydrogen of the donor group and the acceptor O has to be <0.297 nm 3 and the donor-hydrogen-acceptor angle >130°. 26 To calculate the enthalpy (∆H) of HB breaking, we adopted the model proposed by Schroeder and Cooper 6 which relates ∆H to the bonded amide fraction (X b ). The equilibrium between the hydrogen-bonded and nonbonded amides can be represented by (NH···OdC) bonded T (NH) free + (OdC) free (1) with an associated equilibrium constant K given by…”

“…Each chain had a molecular weight of around 4540 g/mol, which is just below the experimental entanglement molecular weight (4700 g/mol). 25 The atomistic force field and the initial coordinates of PA-66 were taken from Goudeau et al; 26 all details concerning the simulations can be found therein. Here, we summarize only the main parameters which define the MD simulations.…”

On the structure and rejection of ions by a polyamide membrane in pressure-driven molecular dynamics simulations

“…Molecular dynamics (MD) is an attractive technique for gaining insights into RO membrane features such as polymer configuration, free volume distribution, and transport phenomena [8][9][10][11]. Indeed, it offers unique opportunities to connect some macroscopic properties to a microscopic description of the physical phenomena involved in the separation.…”

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

“…,26,27 Atomistic molecular dynamic simulations that were performed for PA66 have shown that the fraction of "unbound" water molecules that do not share hydrogen bonds with any amide group at a given time is only 20% for 5% absorbed water and increases to 40% for 10% absorbed water 27. Thus, a very broad distribution of the frequency of molecular motions of D 2 O in PA46, which is observed below 0 °C, is due to a variety of hydrogen bonds formed, a slow exchange between neighboring D 2 O molecules, and heterogeneous mobility of PA46 chains in the amorphous phase.Fig.…”