Patterning of microgel particles on polymer surfaces controlled by autophobicity and interfacial tension

We investigate the thermal response of microgel particles ($\mu $GPs) composed of a cross-linked divinylbenzene core and poly(methyl methacrylate) (PMMA) arms as they segregate from PMMA homopolymer due to autophobicity. When in contact with a free surface, the particles migrate to the PMMA surface but remain inside the PMMA. When a thin film of polystyrene (PS) is placed on top of a PMMA/$\mu $GP film, the $\mu $GPs segregate to and thus roughen the PMMA/PS interface, as evidenced by AFM analysis. We attribute this behavior to a change in surface vs. interfacial energetics. Specifically, while the high surface energy of the native PMMA film keeps the particles inside the bulk PMMA, placing a thin PS layer on top of the PMMA/$\mu $GP film decreases the PMMA/PS interfacial tension by about an order of magnitude, which consequently permits segregation of the $\mu $GPs to the PMMA/PS interface. We follow the segregation kinetics of core-shell $\mu $GPs with and without fluorescent tagging, and we demonstrate the possibility of patterning the segregated $\mu $GPs by contacting a corrugated poly(dimethylsiloxane) (PDMS) layer to PMMA/$\mu $GP films. Regions of the PMMA/$\mu $GP film touching the PDMS layer exhibit $\mu $GP segregation, while non-contacted regions appear featureless.