Interfacial\nAssembly of Anisotropic Core–Shell\nand Hollow Microgels

Abstract

Microgels,\ncross-linked polymers with submicrometer size, are ideal\nsoft model systems. While spherical microgels have been studied extensively,\nanisotropic microgels have hardly been investigated. In this study,\nwe compare the interfacial deformation and assembly of anisotropic\ncore–shell and hollow microgels. The core–shell microgel\nconsists of an elliptical core of hematite covered with a thin silica\nlayer and a thin shell made of poly­(<i>N</i>-isopropylacrylamide).\nThe hollow microgels were obtained after a two-step etching procedure\nof the inorganic core. The behavior of these microgels at the oil–water\ninterface was investigated in a Langmuir–Blodgett trough combined\nwith ex situ atomic force microscopy. First, the influence of the\narchitecture of anisotropic microgels on their spreading at the interface\nwas investigated experimentally and by dissipative particle dynamic\nsimulations. Hereby, the importance of the local shell thickness on\nthe lateral and longitudinal interfacial deformation was highlighted\nas well as the differences between the core–shell and hollow\narchitectures. The shape of the compression isotherms as well as the\ndimensions, ordering, and orientation of the microgels at the different\ncompressions were analyzed. Due to their anisotropic shape and stiffness,\nboth anisotropic microgels were found to exhibit significant capillary\ninteractions with a preferential side-to-side assembly leading to\nstable microgel clusters at low interfacial coverage. Such capillary\ninteractions were found to decrease in the case of the more deformable\nhollow anisotropic microgels. Consequently, anisotropic hollow microgels\nwere found to distribute more evenly at high surface pressure compared\nto stiffer core–shell microgels. Our findings emphasize the\ncomplex interplay between the colloid design, anisotropy, and softness\non the interfacial assembly and the opportunities it therefore offers\nto create more complex ordered interfaces.