Stimuli-responsive behavior of PNiPAm microgels under interfacial confinement

TL;DR

This study reveals that the temperature-induced volume phase transition of PNiPAm microgels is suppressed at air/liquid interfaces, altering their stimuli-responsive behavior due to interfacial confinement effects, supported by experiments and molecular dynamics simulations.

Contribution

It demonstrates that interfacial confinement significantly suppresses microgel phase transitions, challenging the assumption that bulk properties directly translate to interfacial environments.

Findings

Phase transition is suppressed at interfaces.

Hysteresis observed in morphology and pressure.

Molecular simulations support experimental results.

Abstract

The volume phase transition of microgels is one of the most paradigmatic examples of stimuli-responsiveness, enabling a collapse from a highly swollen microgel state into a densely coiled state by an external stimulus. Although well characterized in bulk, it remains unclear how the phase transition is affected by the presence of a confining interface. Here, we demonstrate that the temperature-induced volume phase transition of poly(N-isopropylacrylamide) microgels, conventionally considered an intrinsic molecular property of the polymer, is in fact largely suppressed when the microgel is adsorbed to an air/liquid interface. We further observe a hysteresis in core morphology and interfacial pressure between heating and cooling cycles. Our results, supported by molecular dynamics simulations, reveal that the dangling polymer chains of microgel particles, spread at the interface under the influence of surface tension, do not undergo any volume phase transition, demonstrating that the balance in free energy responsible for the volume phase transition is fundamentally altered by interfacial confinement. These results imply that important technological properties of such systems, including the temperature-induced destabilization of emulsions does not occur via a decrease in interfacial coverage of the microgels.