Stimuli-Responsive Behavior of PNiPAm Microgels under Interfacial Confinement

Abstract

The volume phase transition of microgels is one of the most paradigmatic examples of stimuliresponsiveness, 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.

Microgels at Interfaces Behave as 2D Elastic Particles Featuring Reentrant Dynamics

“…Instead, there is no reported evidence of deswelling when microgels are compressed at the interface. This is again due to the dominant role of the surface tension which makes adsorbed microgels much less responsive to external stimuli [32,35]. In this way, their compression is simply associated with a smooth and monotonic decrease of their interparticle distance, as described experimentally in Ref.…”

“…So far, the effects that an interfacial confinement induces on such particles have been mainly limited to the study of their characteristic shape, and hence, to their structural arrangement at oil-water and air-water interfaces. These aspects have been widely investigated experimentally [30,31] and, more recently, also numerically [29,32,33].…”

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

“…Correspondingly, the volume phase transition is strongly modified due to the presence of the interface. Recent experiments [50][51][52] have analyzed the volume phase transition at the interface and showed that the volume phase transition in the lateral direction is strongly hindered by the presence of the liquid interface. In addition, a hysteresis behaviour upon swelling and deswelling of microgels dependent on whether the microgels were deposited in the swollen or collapsed state was found.…”

Stimuli Responsive Hydrogels: NIPAM/AAm/Carboxylic Acid Polymers

“…They are also termed 'smart-', 'intelligent-', or 'environmentally sensitive' polymers. 2,[10][11][12][13][14] One important feature of this type of material is reversibility, i.e. the ability of the hydrogel to return to its initial state upon application of a counter-trigger.…”