pubs.acs.org/cm Published on Web 08/10/2009 r 2009 American Chemical Society Chem. Mater. 2009, 21, 4071–4079 4071 DOI:10.1021/cm901560e Thermoresponsive Dual-Phase Transition and 3D Self-Assembly of Poly(N-Isopropylacrylamide) Tethered to Silicate Platelets Yu-Min Chen, † Hsiao-Chu Lin, † Ru-Siou Hsu, † Bi-Zen Hsieh, † Yu-An Su, † Yu-Jane Sheng,* ,†,‡ and Jiang-Jen Lin* ,† † Institute of Polymer Science and Engineering, and ‡ Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan Received June 6, 2009. Revised Manuscript Received July 28, 2009 Thermoresponsive poly(N-isopropylacrylamide) (PNiPAAm) was covalently tethered to nanosi- licate platelets (NSP) to generate a new class of organic-inorganic hybrid that exhibits self-assembly and phase transformation properties under applied stimuli. Hybrids of two grafting densities were prepared and the PNiPAAm length was precisely controlled to yield a degree of polymerization of 350-1890 and a narrow molecular weight distribution (1.21-1.50 polydispersity or M w /M n ). Two distinctive second-order transitions were observed during differential scanning calorimetry analysis, indicating the existence of dual-segment density zones. The difference between the two transition temperatures gradually vanished with increasing chain length, and a single endothermic first-order transition emerged. The hybrid also underwent a heat-induced phase transformation after treatment with several heating and cooling cycles. It is believed that fixation of PNiPAAm onto NSP greatly inhibited chain relaxation movements and hindered reversible coil-globule transitions. Further- more, thermally induced self-assembly behavior was directly observed by transmission electronic microscopy of the hybrid coating as a thin film on a silicon wafer surface. The formation of a 3D network of nanostructures was directed by the platelet shape at temperatures higher than the critical solution temperature of the PNiPAAm chains. The temperature-controllable phase separation for formation of an ordered domain network of 100-500 nm in dimension has potential for the fabrication of new smart nanomaterials. Introduction Over the past decade, poly(N-isopropylacrylamide) (PNiPAAm) has received a great deal of attention as a thermoresponsive polymer. The polymer exhibits lower critical solution temperature (LCST) behavior at 32 °C and undergoes a coil-to-globule phase transition induced by expulsion of water from the chain. 1 Stimulus-respon- sive polymers have been widely reported for hybrid forms attached to nanoparticles or solid substrates including wafers, 2 SiO 2 , 3,4 Au, 5,6 carbon nanotubes, 7 Fe 3 O 4 , 8 and layered silicates 9 for use as materials that respond to external triggers. 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