Interface-induced hysteretic volume phase transition of microgels: simulation and experiment

TL;DR

This study combines simulations and experiments to investigate how interface effects induce hysteresis in the volume phase transition of thermo-responsive microgels, revealing that interfacial morphology causes hysteresis absent in bulk dispersion.

Contribution

It introduces a monomer-resolved simulation model that reproduces experimental hysteresis in microgels at interfaces, highlighting the role of interfacial morphology in phase transition behavior.

Findings

Hysteresis occurs only at the interface, not in bulk dispersion.

Simulations match experimental hysteresis behavior.

Interfacial morphology causes kinetically trapped states.

Abstract

Thermo-responsive microgel particles can exhibit a drastic volume shrinkage upon increasing the solvent temperature. Recently we found that the spreading of poly(N-isopropylacrylamide)(PNiPAm) microgels at a liquid interface under the influence of surface tension hinders the temperature-induced volume phase transition. In addition, we observed a hysteresis behavior upon temperature cycling, i.e. a different evolution in microgel size and shape depending on whether the microgel was initially adsorbed to the interface in expanded or collapsed state. Here, we model the volume phase transition of such microgels at an air/water interface by monomer-resolved Brownian dynamics simulations and compare the observed behavior with experiments. We reproduce the experimentally observed hysteresis in the microgel dimensions upon temperature variation. Our simulations did not observe any hysteresis for microgels dispersed in the bulk liquid, suggesting that it results from the distinct interfacial morphology of the microgel adsorbed at the liquid interface. An initially collapsed microgel brought to the interface and subjected to subsequent swelling and collapsing (resp. cooling and heating) will end up in a larger size than it had in the original collapsed state. Further temperature cycling, however, only shows a much reduced hysteresis, in agreement with our experimental observations. We attribute the hysteretic behavior to a kinetically trapped initial collapsed configuration, which relaxes upon expanding in the swollen state. We find a similar behavior for linear PNiPAm chains adsorbed to an interface. Our combined experimental - simulation investigation provides new insights into the volume phase transition of PNiPAm materials adsorbed to liquid interfaces.