  Smart polymers are a modern class of polymeric materials that often exhibit
unpredictable behavior in mixtures of solvents. One such phenomenon is
co-non-solvency. Co-non-solvency occurs when two (perfectly) miscible and
competing good solvents, for a given polymer, are mixed together. As a result,
the same polymer collapses into a compact globule within intermediate mixing
ratios. More interestingly, polymer collapses when the solvent quality remains
good and even gets increasingly better by the addition of the better cosolvent.
This is a puzzling phenomenon that is driven by strong local concentration
fluctuations. Because of the discrete particle based nature of the
interactions, Flory-Huggins type mean field arguments become unsuitable. In
this work, we extend the analysis of the co-non-solvency effect presented
earlier [Nature Communications 5, 4882 (2014)]. We explain why co-non-solvency
is a generic phenomenon that can be understood by the thermodynamic treatment
of the competitive displacement of (co)solvent components. This competition can
result in a polymer collapse upon improvement of the solvent quality. Specific
chemical details are not required to understand these complex conformational
transitions. Therefore, a broad range of polymers are expected to exhibit
similar reentrant coil-globule-coil transitions in competing good solvents.
