Influence of hydrogen bonding on the structural transition of poly(methacrylic acid) chain in water-ethanol solution by molecular dynamics simulations

The conformational structure of dilute atactic-poly(methacrylic acid) (PMA) solution in binary water-ethanol mixture was investigated by molecular dynamics simulations, over 0-0.9 ethanol (co-solvent) fraction. The radius of gyration < R-g >, torsion angle distribution, intra-chain hydrogen bonds (H-bonds), and H-bonds for PMA-water, PMA-ethanol and water-ethanol, atom-atom and atom-group pair radial distribution functions were analysed. An increase in the ethanol fraction leads to chain expansion. The non-monotonic variation of < R-g >, commensurate with the experimentally observed behaviour of intrinsic viscosity [eta], takes place by H-bonding effects between PMA, water and ethanol, as driven by the differences in the chemical structure of water and ethanol. The PMA repeat units are closer to the CH2 groups as compared with CH3 groups of ethanol, in the nearest coordination shell. Water as compared with ethanol is able to coordinate closer to the PMA repeat unit centre of mass. Intra-chain H-bonding depreciates with an increase in the ethanol content in solution. The changes, across the ethanol fraction range, in chain dimensions and of predicted intrinsic viscosity by the simulations, compare well with experimental results in the literature.