Behavior of temperature responsive microgels at interfaces : deformability and interfacial activity

The interfacial activity is an important property of microgels which consist of intramolecular cross-linked polymer networks, as it is involved in applications like the fabrication of films, emulsions or capsules. To investigate the influence of the chemical composition, cross-linking density and temperature on the interfacial behavior of microgels, thermo-sensitive poly(N-vinylcaprolactam-co-N-isopropylacrylamide) (PVCL/NIPAm), poly(N-vinylcaprolactam-co-N-isopropyl-methacrylamide) (PVCL/NIPMAm) copolymer microgels and homopolymer analogues were synthesized, and their properties were determined. The interfacial activities of PVCL/NIPAm and PVCL/PNIPMAm copolymer microgels were measured with a pendant drop tensiometer. The interfacial behaviors of the copolymer microgels fall between that of homopolymer microgels. It means that the chemical structure contributes to the variation in the interfacial tension. The dynamic interfacial tension of microgels with lower cross-linking densities evolves faster than that of microgels with higher cross-linking densities. This is consistent with the simulation results that loosely cross-linked microgels spread faster at the air/liquid and liquid/liquid interfaces than densely cross-linked ones. Meanwhile, the dimensions of the adsorbed microgels on a solid substrate characterized with atomic force microscopy indicate that the deformability of the microgels at the air/solid interface also increases with the decreasing cross-linking degree. Besides, the evolution of the dynamic interfacial tension for PVCL/NIPAm and PVCL/NIPMAm microgels at TVPTT because of the reducing microgel deformability with increasing temperature. Furthermore, the comparison between the dimensions of microgels on the solid substrate and in solution shows that adsorption of microgels induces higher deformation than the swelling-induced expansion