Molecular dynamics simulations of structure and dynamics in aqueous solution of neutral and ionized derivatives of poly(F): methyl, n-propyl, and isopropyl substitutions

Chain dimensions, intermolecular structure and hydration of a series of uncharged and cationic poly(vinyl amine) [PVAm] linear polymers having hydrophobic substituent methyl, n-propyl, and isopropyl in the monomer are studied in aqueous solution by molecular dynamics simulations. A conformational transition occurs in the degree of ionization, α, range 0.3 to 0.4. Among the polymers studied, isopropyl substituted PVAm is most hydrophobic and methyl substituted PVAm is the least. The extent of hydrophobicity of the chemical structure is directly correlated to the size of the polymer chain. Conformational dynamics become slower with increase in the degree of charge of the chain and with the size of the substituent side group. The significant hydration of the polymers takes place for 0 ≤ α ≤ 0.5. While the number of H-bonds is not affected significantly by the chemical structure of the chain the relaxation dynamics of polymer-water H-bonds is significantly affected, with the more hydrophobic polymer showing the slowest dynamics. The steric hindrance provided by the hydrophobic substituent groups is responsible for slowing of water orientation dynamics in the vicinity of the polymer. The counter-ion condensation is clearly better and the bound water content is less for the relatively more hydrophobic polymer. The overall behavior of structure and dynamics is in qualitative agreement with that known for other types of polyelectrolytes and solutes in aqueous solution.