Dilute-regime polymer thermophoresis from coil to globule via molecular dynamics simulations

Particles (molecules, colloids, polymers, etc.) in solution migrate along a temperature gradient in a process called thermophoresis, thermodiffusion, or the Soret effect. Experiments have revealed many striking behaviors in dilute polymer solutions, including sign changes and extrema in the Soret coefficient ST as a function of solvent quality. We investigate the phenomenology of dilute-regime polymer thermophoresis using a minimal particle-based model that interpolates between a swollen coil and a compact globule, simulating various chain lengths. Our model exhibits sign changes in ST as a function of solvent quality, from thermophilic (ST < 0) to thermophobic (ST > 0) behavior, in both the good-solvent and poor-solvent regimes. To our knowledge, the latter has not been previously reported and occurs when increasing the monomer-monomer attraction. We correlate the increase/decrease of ST with that of the negative solvation entropy. Furthermore, our results highlight that the difference between the thermophoretic and drift velocities can be large, even for relatively large polymers approaching the long-polymer limit. We observe the counter-intuitive situation of a net drift against the direction of particle accumulation for portions of the good-, Θ-, and poor-solvent regimes.