Giant thermophoresis of poly(N-isopropylacrylamide) microgel particles

Abstract

Thermophoresis is the rectification of Brownian motion induced by the presence of a thermal gradient ∇T, yielding a net drift of colloidal particles along or against the direction of ∇T. The effect is known to depend on the specific interactions between solute and solvent, and quantitative theoretical models are lacking except in a few simple experimental cases. Both the order of magnitude and the temperature dependence of the thermophoretic mobility D_T are known to be very similar for a wide class of aqueous colloidal systems, ranging from latex colloids to polymers, surfactant micelles, proteins, and DNA. Here we show that thermoresponsive microgel particles made of poly(N-isopropylacrylamide) (PNIPAM) do not share, in the temperature range around the ϑ-point, these common features. Instead, D_T displays an unusually strong temperature dependence, maintaining a linear growth across the collapse transition. This behaviour is not shared by linear PNIPAM chains, for which existing data show D_T falling at the transition, with similar values between the expanded coil and collapsed globule states away from the transition point. A possible connection of the observed giant temperature dependence of D_T to microgel hydration is suggested.