Molecular dynamics study of the effects of calcium ions on the conformational properties of comb-like poly(acrylic acid-co-methyl allyl polyoxyethylene ether)

Abstract

All-atom molecular dynamics simulations were employed to study the microscopic conformational properties of three kinds of comb-like poly(acrylic acid-co-methyl allyl polyoxyethylene ether) (P(AA-co-MAPEG)) in solution. Two different systems, which contained single chain and two chains respectively, were studied, and the binding types of calcium ions (Ca2+) to carboxylate groups (–COO−) were analyzed. For the systems containing a single chain, as the concentration of Ca2+ increased, the value of the radius of gyration, Rg, for P(AA-co-MAPEG) (4:1) (4:1 represented the monomer ratios of acrylic acid to polyoxyethylene methyl allyl ether, the same description below) chain sharply decreased in Ca2+ solution because Ca2+ weakened the repulsive electrostatic interactions between the charged –COO− while hardly interacted with –COO− directly (e.g. coordination). And the values of Rg for the P(AA-co-MAPEG) (6:1) chain and P(AA-co-MAPEG) (9:1) chain changed slightly because Ca2+ easily interacted with the –COO−, and the polymer chains exhibited a more rigid conformation. For the systems containing two chains, due to two competing factors, the solvation of –COO− and steric hindrance induced by side chains, it was favorable for P(AA-co-MAPEG) (6:1) chains to approach each other to form intermolecular Ca2+ bridges. For comb-like polyelectrolyte, intermolecular binding was difficult to form than intramolecular binding. And among all the possible intramolecular binding modes, the type of Ca2+ binding to two oxygen atoms of adjacent –COO− was the most favorable.