Molecular Dynamics Simulation Study of Water in Amorphous Kapton

Abstract

Molecular dynamics (MD) simulations of more than 40 model systems of dry and hydrated poly(pyromellitimide-1,4-diphenyl ether) (PMDA-ODA) polyimide, also known as Kapton, were undertaken in order to study the specific interactions between water and a glassy polymer matrix. Dry amorphous Kapton models were generated using a hybrid pivot Monte Carlo-MD single-chain sampling technique and were found to reproduce many features found experimentally. Water was subsequently introduced into the polyimide systems with weight percentages ranging from 1.4 to 10 wt % at 373 K. At the lower concentrations, water is found to occupy the pre-existing void space in the polymer matrix. On the other hand, there are significant changes in the volumetric, energetic, and conformational properties of the chains for water contents above ∼3%. The static and dynamic properties of the hydrogen bonds have been analyzed. In all cases under study, water is preferentially hydrogen-bonded to two sites, thus forming bridges. These water bridges, whose occurrence depends on the weight percentage of the penetrant, can link either two polymer sites, one polymer site and another water, or two other waters. Water clusters have been analyzed and found to be in agreement with findings from NMR, dielectric, and infrared attenuated total reflection (ATR) measurements. Their geometries are more chainlike and at high water contents approach a cocontinuous network. Such a characterization at the molecular level calls into question the commonly used interpretation of solubility curves based on the Zimm−Lundberg theory.

Atomistic models of three fluorinated polyimides in the amorphous state

“…The average intermolecular potential energies of the systems 〈 U 〉 were very much comparable to those for other long‐chain polyimide reported in the literature,35, 71, 95 with the major part of the potential energy (>90%) coming from the van der Waals component 〈 U 〉 (refer Table 3). Within the statistical errors, the van der Waals energies for both isomers are fairly similar whereas a slightly more important electrostatic contribution 〈 U 〉 is seen for 6FDA‐6FpDA.…”

“…Figure 5 compares the results obtained for the chains before the introduction of excluded volume with the same ones following relaxation by MD. Differences are typically less than 0.5%, i.e., of much the same order as found for some nonfluorinated polyimides 58, 93, 95. These relatively small differences are attributed to the highly rigid nature of the polyimides and confirm that the effect of introducing the full potential remains very limited in such systems.…”

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.…”

Water clustering in polychloroprene

“…But in fact, PWn clusters can be linear, eventually branched, as found by molecular dynamics simulation [9] but also by dielectric or NMR measurements [15,16], and can reach relatively large sizes without phase separation, which could solve the problem of full reversibility.…”

“…Here, clusters can be chains or networks [8,9] of hydrogen bonded water molecules rather than quasi-spherical aggregates and it is not unreasonable to imagine relatively large clusters not phase separated from the polymer matrix contrary to quasi-spherical clusters.…”