Behavior of temperature-responsive copolymer microgels at the oil/water interface

Herein, we investigate the interfacial behavior of temperature-sensitive aqueous microgels on the toluene/water interface. Copolymer microgels based on N-vinylcaprolactam (VCL) and two acrylamides, N-isopropylacrylamide (NIPAm) and N-isopropylmethacrylamide (NIPMAm), with various copolymer compositions were used in this study. It is revealed that these copolymer microgels have the similar internal structure, regardless of the chemical composition. A classic kinetics of interfacial tension with three distinct regimes is found in the dynamic interfacial tension plots of copolymer microgels, which is similar to inorganic nanoparticles and proteins. The influences of the copolymer composition and the temperature on the interfacial behavior of microgels are investigated. The results show that the interfacial behavior of copolymer microgels at the toluene/water interface follows exactly the trend of the volume phase behavior of microgels but, on the other hand, strongly depends upon the chemical compositions of copolymer microgels. In contrast, with respect to the size range of microgels studied here (50-500 nm), the size of the microgel has no influence on the interfacial tension. Below the volume phase transition temperature (VPTT), the equilibrium interfacial tensions of all microgel systems decrease as the temperature increases. Above VPTT, the equilibrium interfacial tension remains at a certain level for poly(N-vinylcaprolactam) (PVCL)- and poly(N-isopropylmethacrylamide) (PNIPMAm)-rich microgel systems and increases slightly for poly(N-isopropylacrylamide) (PNIPAm)-rich microgel systems. The evolution of dynamic interfacial tension for microgel solutions against toluene at T < VPTT is faster than that at T > VPTT, because of the reduced deformability of the microgel with the increase of the temperature. The softer microgels with lower cross-linking degrees exhibit faster kinetics of reduction of interfacial tension compared to those with more cross-linked degrees, which strongly supports the deformation-controlled interfacial behavior of microgels.