Tacticity effects on conformational structure and hydration of poly-(methacrylic acid) in aqueous solutions-a molecular dynamics simulation study

2015-09-30T00:00:00Z (GMT) by Abhishek K. Gupta Upendra Natarajan
<p>The influence of tacticity on chain dimensions, backbone and side-group conformational states and relaxation dynamics, intermolecular hydrogen bonding and its relaxation dynamics was investigated for 30 repeat unit poly(methacrylic acid) (PMA) as a function of the degree-of-neutralisation, <i>f</i> (i.e. charge density) [0 < <i>f</i> < 1 range] in explicit water with Na<sup>+</sup> neutralising counter-ions, using molecular dynamics simulations. Chain expansion with increase in charge density is observed. Simulation results, where applicable, compare well with experimental results in the literature. <i>Isotactic</i> (<i>i</i>-PMA) chain exhibits the largest expansion (89% change in <i>R</i><sub><i>g</i></sub>), and <i>syndiotactic</i> (<i>s</i>-PMA-RR-0.7) chain shows 31%. For fully neutralised chain (high charge density), the probability for <i>trans</i> conformation at backbone bonds follows the trend <i>i</i>-PMA > <i>a</i>-PMA > <i>s</i>-PMA. At high charge density, a higher probability of <i>trans</i> state is obtained at <i>meso dyads</i> as compared to <i>racemic dyads</i> and the side group in <i>i</i>-PMA relative to other tacticity is conformationally more extended. RR <i>triads</i> impart better hydrogen bonding with water. In the COOH side group, greater rotational mobility is observed for <i>i</i>-PMA as compared to <i>s</i>-PMA. Water coordination to PMA is invariant with tacticity at low <i>f</i>. For <i>f</i> = 1, <i>s</i>-PMA exhibits the best level of H-bonding, due to its 3D chemical structural configuration. Binding distance between water and PMA atoms and their coordination number remain unchanged with respect to ionisation of COOH groups. <i>Isotactic</i> (<i>i</i>-PMA) exhibits the fastest relaxation of polymer-water H-bonds. While counter-ion coordination on to PMA is unaffected by tacticity, the intensity of ion condensation as well as hydrogen bond relaxation time at <i>f</i> = 1 varies in the order <i>i</i>-PMA > <i>a</i>-PMA > <i>s</i>-PMA.</p>