Structure of interpolymer complex between poly(acrylic acid) and poly(ethylene oxide) in aqueous salt solution: a molecular dynamics simulation study

Inter-macromolecular interaction between uncharged polymers in aqueous solutions can be influenced by concentration of polymer, solvent quality, molecular weight, solution pH, temperature, salt concentration and type of salt. Atomistic molecular dynamics simulations of aqueous NaCl solution containing poly(acrylic acid) and poly(ethylene oxide) are presented. The size of the polymer chains decreases with an increase in salt concentration resulting from water becoming a poor solvent which enhances compositing of polymers. A close pair is formed between carboxylic hydrogen (-COOH) and ether oxygen (-O-) of the polymer at high salt concentration. The change in structure of the PAA/PEO complex is analysed using simulated SAXS and SANS profiles. The solubility of chains decreases and attains the minimum energy conformation at a particular salt concentration. The solvent accessible surface area (SASA) of the complex at higher salt concentration is less as compared to the lower. Increase in salt concentration enhances the cooperative interaction via hydrogen bonding between PAA and PEO and weakens the polymer-water-hydrogen bonding. The net result is a decrease in solubility of these polymers and thermodynamically stable polymer-polymer complex is formed at higher salt concentrations. Simulation results are in agreement with available experimental data in literature.