  We study the lower critical solution temperature (LCST) in thermoresponsive
polymer solutions by means of a coarse grained single polymer chain simulation
and a theoretical approach. The simulation model includes solvent explicitly
and thus accounts for solvent interactions and entropy directly. The
theoretical model consists of a single chain polymer in an implicit solvent
where the effect of solvent is included through the intra-polymer solvophobic
potential proposed by Kolomeisky and Widom. Our results indicate that the LCST
behavior is determined by the competition between the mean energy difference
between the bulk and bound solvent, and the entropy loss due to the bound
solvent. At low temperatures, solvent molecules are bound to the polymer and
the solvophobicity of the polymer is screened, resulting in a coiled state. At
high temperatures the entropy loss due to bound solvent offsets the energy gain
due to binding which causes the solvent molecules to unbind, leading to the
collapse of the polymer chain to a globular state. Furthermore, the coarse
grained nature of these models indicates that mean interaction energies are
sufficient to explain LCST in comparison to specific solvent structural
arrangements.
