Effect of stereo-chemistry and hydrophilic nature of synthetic carboxylic acid polymers on conformation, intermolecular structure and hydration at air-water interface

Abstract

Conformation, intermolecular structure and hydration of different stereo-isomers of poly(methacrylic acid) and poly(acrylic acid), specifically isotactic-poly(methacrylic acid) (i-PMA), syndiotactic-poly(methacrylic acid) (s-PMA) and syndiotactic-poly(acrylic acid) (s-PAA) at vacuum-water interface were studied using molecular dynamics simulations. The adsorbed chain conformation was analyzed by radius-of-gyration components parallel and perpendicular to the interface, relative shape anisotropy parameter, distribution and autocorrelation function of dihedral angles, and orientation distribution of different groups relative to interface plane. i-PMA and s-PAA adsorb in planar conformation (i-PMA is curved compared to s-PAA) whereas s-PMA adsorbs in random coil conformation. Different chemical groups of s-PAA and i-PMA show preferential orientation whereas it is absent in the case of s-PMA. The random coil conformation of s-PMA exhibits greater intra-chain hydrogen bonding whereas extended chain of s-PAA exhibits greater polymer-water hydrogen bonding. The overall contribution to polymer-water hydrogen bonds by carbonyl oxygen atoms is greater than that by hydroxyl hydrogen atom. Water molecules hydrogen bonded to s-PAA show faster dynamics compared to those bonded to i-PMA and s-PMA. The slower dynamics of polymer-water hydrogen bonds is correlated to the slower dynamics of polymer conformations. Knowledge of orientation of chemical groups is useful for understanding binding of adsorbed polymer with other polymers and ligands.