Entropy-driven polymer collapse on increasing the temperature: The case of poly(N-vinylcaprolactam)

Poly(N-vinylcaprolactam), PNVCL, chains undergo a temperature-induced, reversible and endothermic collapse in water at around 32 degrees C. The entropy gain driving PNVCL collapse has to come from water molecules, but it cannot be associated with their passage from the polymer hydration shell to the bulk because there is no difference in their tetrahedral structural order. I have shown that the water entropy gain comes from the decrease in solvent-excluded volume effect associated with chain collapse: there is a marked increase in the spatial configurations accessible to water molecules, leading to a large gain in their translational entropy. This mechanism, even though generic, proves to be effective solely in water and aqueous solutions because the magnitude of the solvent-excluded volume effect is amplified by the large number density of water, stemming from the strength of H-bonds and the small size of water molecules.