Giant Casimir Nonequilibrium Forces Drive Coil to Globule Transition in Polymers

We develop a theory to probe the effect of nonequilibrium fluctuation-induced forces on the size of a polymer confined between two horizontal, thermally conductive plates subject to a constant temperature gradient, ∇T. We assume that (a) the solvent is good and (b) the distance between the plates is large so that in the absence of a thermal gradient the polymer is a coil, whose size scales with the number of monomers as Nν, with ν ≈ 0.6. We find that above a critical temperature gradient, ∇Tc ≈ N–5/4, a favorable attractive monomer–monomer interaction due to the giant Casimir force (GCF) overcomes the chain conformational entropy, resulting in a coil–globule transition. Our predictions can be verified using light-scattering experiments with polymers, such as polystyrene or polyisoprene in organic solvents in which the GCF is attractive.