  It is well known that poly(N-isopropylacrylamide) (PNIPAm) exhibits an
interesting, yet puzzling, phenomenon of co-non-solvency. Co-non-solvency
occurs when two competing good solvents for PNIPAm, such as water and alcohol,
are mixed together. As a result, the same PNIPAm collapses within intermediate
mixing ratios. This complex conformational transition is driven by preferential
binding of methanol with PNIPAm. Interestingly, co-non-solvency can be
destroyed when applying high hydrostatic pressures. In this work, using a large
scale molecular dynamics simulation employing high pressures, we propose a
microscopic picture behind the suppression of the co-non-solvency phenomenon.
Based on thermodynamic and structural analysis, our results suggest that the
preferential binding of methanol with PNIPAm gets partially lost at high
pressures, making the background fluid reasonably homogeneous for the polymer.
This is consistent with the hypothesis that the co-non-solvency phenomenon is
driven by preferential binding and is not based on depletion effects.
