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Schematic representation of the process used to fabricate surfaces chemically micropatterned with PS-b-PNIPAAm brushes. (a) Si wafer treated with HMDS in a thermal evaporator. Photoresist spin-coated onto the Si surface presenting Si(CH3)3 groups. Electron-beam lithography used to pattern the photoresist as hole arrays possessing various aspect and duty ratios on the surface. (b) OPT used to chemically modify the exposed regions presenting Si(OCH3)3 groups and to convert the topographic photoresist pattern into a chemically modified surface pattern. (c) The initiator units were assembled selectively onto the bare regions of the bottom Si surface after OPT. (d) Sample grafting, through surface-initiated ATRP of a mixture of styrene and 1,4-divinylbenzene (DVB), from the functionalized areas of the patterned initiator to form pillar arrays of cross-linked PS brushes. (e) Object grafting, through surface-initiated ATRP of NIPAAm from the macroinitiator on the top of the pillars, to generate PS-b-PNIPAAm copolymer brush pillars. (f) Photoresist removed through treatment with a slightly basic aqueous solution. The pillar arrays of the copolymer brushes on the surface exhibited thermally dependent adhesive properties between 25 and 50 °C.
3D (left), 2D (right), and line cross-section (bottom) analysis AFM topographic images (10 × 10 μm) of 200-nm resolution pillar array of PS-PNIPAAm copolymer brush at (a) 25 and (b) 50 °C.
(a) Static WCAs of flat and pillar arrays of Si–PS and Si–PS-b-PNIPAAm surfaces at 25 and 50 °C. (b) Variation of the WCA hysteresis, calculated from the advancing and receding WCAs of droplets, on flat and pillar array of Si–PS and Si–PS-b-PNIPAAm surfaces at 25 and 50 °C.
(a) Changes in adhesive forces of the tethered PS-b-PNIPAAm pillars plotted as a function of resolutions. Insets: Illustrations of an AFM tip alternately attaching to and detaching from the copolymer pillar to observe the transitions of the terminus from brush to globule structures at 25 and 50 °C. (b) Dependence of the friction force under a normal load of 200 nN on 200 nm-resolution copolymer pillar array with temperature. Insets: Illustrations of an AFM tip alternately scraping the copolymer pillar to observe the transitions of the terminus from brush to globule structures at 25and 50 °C.
Atomic ratios of the Si–initiator, Si–PS, and Si–PS-b-PNIPAAm surfaces, calculated through XPS analysis.
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