Molecular dynamics study of the intercalation and conformational transition of poly (N-vinyl caprolactam), a thermosensitive polymer in hydrated Na-montmorillonite

M Camara and HL Liao and JF Xu and J Zhang and R Swai, POLYMER, 179, 121718 (2019).

DOI: 10.1016/j.polymer.2019.121718

In this study, we performed molecular dynamics (MD) simulations of the intercalation of poly (N-vinyl caprolactam) (PNVCL) into Na- montmorillonite (Na-Mt) to evaluate the solvation dynamics, the ordering of H2O molecules and the effect of interlayer attractive forces in inducing conformational transitions in PNVCL from 275 to 335 K. Conformational transformation of PNVCL into Na-Mt and in bulk water were compared and contrasted to understand the effects of interlayer attractive forces on PNVCL. The results showed that the coil-to-globule transition of PNVCL appeared at 305 K when in bulk water and not into Na-Mt due to the reduction of the hydrophobic interaction between PNVCL residues by the electrostatic forces in the interlayer. H2O molecules formed H-bonded cage-like structure around PNVCL, and the amount of these H-bonds and H2O molecules decreased with the thermal perturbation. Our investigations suggest that the solubility, solvation dynamics and the conformation transformation of PNVCL into Na-Mt are determined by the electrostatics forces existing in the interlayer and the structure of proximal H2O molecules when the temperature increased. PNVCL exhibited good inhibition effect on Na-Mt by reducing its interlayer particles mobility. The radial distribution function curves and the solvent accessibility surface area indicated that PNVCL was more exposed to H2O molecules into Na-Mt than in bulk water due to their proximity with the large hydration shells formed around Na cations and the strong H-bonds formed between clay mineral surface and H2O molecules. H-bonds analysis into Na-Mt showed that clay mineral surface was more likely to form H-bond with H2O molecules than PNVCL because of their hydrophilic properties. These results highlight the conformational transition of PNVCL into hydrated Na-Mt and predict its stability against environmental condition changes.

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