Thermoresponsive Dual-Phase Transition and 3D Self-Assembly of Poly(N-Isopropylacrylamide) Tethered to Silicate Platelets

Thermoresponsive poly( N -isopropylacrylamide) (PNiPAAm) was covalently tethered to nanosilicate 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. Furthermore, 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.