Fabrication of hybrid silica nanoparticles densely grafted with thermoresponsive poly(N-isopropylacrylamide) brushes of controlled thickness via surface-initiated atom transfer radical polymerization

This article reports on the fabrication of hybrid silica nanoparticles densely grafted with thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) brushes and their thermal phase transition behavior. Surface-initiated atom transfer radical polymerization (ATRP) of N-isopropylacrylamide (NIPAM) was conducted in 2-propanol at ambient temperature using CuCl/CuCl ₂/tris(2-(dimethylamino)ethyl)amine (Me₆TREN) as the catalytic system, starting from the surface of silica nanoparticles derivatized with ATRP initiators (0.35 nm²/initiator). The surface-initiated ATRP can be conducted in a well-controlled manner, as revealed by the linear kinetic plot, linear evolution of number-average molecular weights (M_n) versus monomer conversions, and the relatively narrow molecular weight distributions (M_w/M_n < 1.25) of the grafted PNIPAM chains. The grafting density of PNIPAM chains at the surface of silica nanoparticls was estimated to be 2.2 nm²/chain based on transmission electron microscopy (TEM) and thermogravimetric analysis (TGA) characterization results. Laser light scattering (LLS) and optical transmittance were then employed to study the thermal phase transitions of PNIPAM brushes at the surface of silica nanoparticles. Both the intensity-average hydrodynamic radius, 〈R_h〉, and average radius of gyration, 〈R _g〉, exhibit a two-stage decrease upon heating over the broad temperature range of 20-37°C, which is in contrast to the fact that free PNIPAM homopolymer in aqueous solution exhibits a phase transition at ca. 32°C within a narrow temperature range. The first phase transition takes place in the temperature range of 20-30°C, which can be tentatively ascribed to the n-cluster-induced collapse of the inner region of PNIPAM brushes close to the silica core; the second phase transition occurs above 30°C, which can be ascribed to the outer region of PNIPAM brushes, possessing much lower chain density compared to that of the inner part. We tentatively expect that the observed unique double phase transition behavior of polymer brushes coated at the surface of inorganic nanoparticle cores can be further utilized to fabricate novel nanostructured devices with more complex functions.