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
