On-Off of Co-non-solvency for Poly(N-vinylcaprolactam) in Alcohol-Water Mixtures: A Molecular Dynamics Study

Poly(N-vinylcaprolactam) (PVCL) exhibits co-non-solvency in aqueous solutions of 2-propanol but not in methanol. What distinguishes the impact of these two cosolvents on the polymer conformational stability? We report a molecular dynamics simulation study on PVCL 50-mer and monomers dissolved in methanol-water and 2-propanol-water mixtures. We show that the alcohol-concentration dependence of the effective attraction between a pair of PVCL monomers closely resembles the conformational changes in a single PVCL 50-mer as well as the experimentally observed behavior for PVCL chains. We also found that, at the co-non-solvency maximum, the monomer-monomer attraction works over a long-range beyond the solvent-separated distance. Then, we correlate the long-range attraction to the appearance of a dense alcohol concentration accumulated between the monomers. Furthermore, we distinctly demonstrate that the co-non-solvency of PVCL monomers can be switched on/off by artificially tuning the alcohol size while keeping the energetic parameters. Thus, we conclude that the magnitude of the excluded volume effect in alcohol accompanying the gain in translational entropy of the solvent is crucial to the occurrence of PVCL polymer co-non-solvency.