Thermoresponsive microgels at the air–water interface: the impact of the swelling state on interfacial conformation

Abstract

Poly(N-vinylcaprolactam) (PVCL) is a new temperature-responsive type of polymer microgel with improved biocompatibility as compared to more commonly used poly(N-isopropylacrylamide) (PNIPAM). Both polymers swell at low temperatures and collapse at high ones, showing a volume phase transition temperature (VPTT) around the physiological temperature. Exploring the interfacial characteristics of thermoresponsive microgels is important due to their potential application in emulsion based systems with tailored stabilities and controlled degradation profiles. In this work, we study the properties of charged PVCL particles at the air-water interface by a combination of adsorption, dilatational rheology and Langmuir monolayers. Although PVCL particles adsorb spontaneously at the air-water interface in both, swollen and collapsed conformations, the interfacial properties show significant differences depending on the swelling state. In particular, the total amount of adsorbed microgels and the rigidity of the monolayer increase as the temperature increases above the VPTT, which is connected to the more compact morphology of the microgels in this regime. Dilatational rheology data show the formation of a very loose adsorbed layer with low cohesivity. In addition, collapsed microgels yield a continuous increase of the surface pressure, whereas swollen microgels show a phase transition at intermediate compressions caused by the deformation of the loose external polymer shell of the particles. We also provide a qualitative interpretation for the surface pressure behavior in terms of microgel-microgel effective pair potentials, and correlate our experimental findings to recent rescaling models that take into account the importance of the internal polymer degrees of freedom in the rearrangement of the conformation of the microgel particles at the interface.

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

“…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].…”

Kinetics of spontaneous microgels adsorption and stabilization of emulsions produced using microfluidics

“…The most studied microgels are prepared by batch polymerization. It has been earlier demonstrated that such microgels are able to adsorb spontaneously at any temperature [13][14][15][16] giving rise to a sensible decrease of the interfacial tension. It is also known that such microgels are able to stabilize emulsions [17][18][19][20].…”

Deswelling and deformation of microgels in concentrated packings

“…To achieve effective packing densities of well above the random close packing limit for hard spheres, the reduction in available volume must be accommodated by either shape or volume changes in the constituent particles. This can occur either by forming facets at the contact points with the surrounding particles 8 , 14 , 15 and by the expulsion of solvent from the particle, leading to homogeneous deswelling and volume reduction 16 , 17 . Recent work has highlighted how the latter can have pronounced effects on the interpretation of experiments on microgels, since osmotic deswelling can lead to substantial deviations between the apparent and real particle volume fraction 18 .…”