Solvation free-energy (ΔG) is calculated for the first time to understand the role of tacticity in vinyl polyacids for synthetic polyelectrolytes in aqueous solution using explicit molecular dynamics simulations. Solvation free energy of atactic (a-PMA) and isotactic poly(methacrylic acid) i-PMA, at different values of degree-of-ionization (i.e. charge density f) in water was obtained via thermodynamic integration. Polymer-water interaction was systematically varied using coupling parameter λ in going from vacuum (λ = 0) to the fully solvated (λ = 1). Simulated values of ΔG show that a-PMA chain is soluble and i-PMA is insoluble in their acidic form (i.e. charge density f = 0). Solubilization of a-PMA is thermodynamically more favorable with increase in f. Insoluble i-PMA becomes soluble (i.e. at f = 0.3) with increase in f beyond a critical charge (f = 0.2), in excellent agreement with the Wide-Angle X-Ray Scattering measurements in literature. Various RDFs, conformations and molecular thermodynamic contributions along the λ curves were analysed. The intermolecular distance between the relatively hydrophobic methyl groups and the water molecules is greater for soluble chain as compared to insoluble chain. The contribution to solvation from PMA-water interactions shows that electrostatic interaction is significantly favorable for soluble chains; uncharged a-PMA and ionized i-PMA at f = 0.3. The shift in solubilization due to change in polarity by a change in charge density is the effect of PMA-water hydrogen bonding. Solubilization of the i-PMA occurs by chain expansion due to electrostatic repulsion between neighboring COO- groups as well as by increase in the number of PMA-water h-bond and decrease in PMA-PMA hydrogen bonds. A greater number of water molecules are accommodated in the hydration shell of solvated PMA as compared to non-solvated state. PMA-water electrostatic interaction is significant and favorable in the case of solvated chains (a-PMA and ionized i-PMA). The correlation function between PMA repeat units and water oxygens show that a decrease in λ causes a decrease in number of water molecules in the vicinity of the soluble chains. Water molecules are highly populated in the hydration shell of PMA for the electrostatic coupling interaction regime as compared to van der Waals coupling as observed from PMA-water pair correlation functions.

Effect of tacticity and charge on solubilization free energy of synthetic anionic polyelectrolyte in water by molecular dynamics simulations

2019 Fall National Meeting

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