Molecular Dynamics Simulation of Poly(methacrylic Acid) Chains in Water†
Boštjan Jerman,* Črtomir Podlipnik and Ksenija Kogej
Department of Chemistry and Biochemistry, Faculty of Chemistry and Chemical Technology,
University of Ljubljana, Aškerčeva 5, P. O. Box 537, SI-1000 Ljubljana, Slovenia
Molecular dynamics simulation studies were performed on the isotactic and syndiotactic poly(methacrylic acid), i-PMA and s-PMA, respectively, in water at 25 °C in dependence on degree of neutralization, α, of carboxyl groups. The simulations were performed with a single chain built of 64 monomer units. For both polyacids results have shown that the compact coil conformation of i-PMA is stabilized by hydrogen bonds at low α values. Conformation of the i-PMA appears to be stiffer because of a large number of neighboring hydrogen bonds, whereas the s-PMA conformation has fewer neighboring hydrogen bonds and adopts a more curve-like conformation. Average polyion dimensions increase with increasing ionization. However, shorter distance between charges and shorter counterion contact distance for isotactic stereoisomer have been calculated using modified radial distribution function. This leads to a greater charge density and stronger counterion binding in the i-PMA case. In addition, tendency for local helix-like conformation of i-PMA, especially at low α values, has been recognized. We have shown that s-PMA is more extensively stabilized by hydration at α = 0 than i-PMA. This demonstrates a larger hydrophobic character of i-PMA compared to s-PMA.
Keywords: Stereoregular poly(methacrylic acid), isotactic, syndiotactic, molecular dynamics, conformation.