Characteristics of High-Density Poly(N-isopropylacrylamide) (PNIPA) Brushes on Silicon Surface by Atom Transfer Radical Polymerization

Abstract

High-density poly(N-isopropylacrylamide) (PNIPA) brushes were synthesized on silicon surfaces by surface initiated ATRP at various polymerization conditions. Polymerization was achieved using CuCl/tris(2-(dimethylamino)ethyl)amine (Me₆TREN) as a catalytic system in DMSO at 20℃. The linear evolution of number average molecular weight (M_n) versus monomer conversion, the increase in layer thickness with polymerization time and relatively low molecular weight distribution (～ 1.2) indicate a well-controlled manner of polymerization. The average value of grafting density of PNIPA brushes was around 0.48 chain/nm²: We obtained high-density PNIPA brushes. During the measurement of air bubble contact angle under the surface of the PNIPA brushes in water, the surface property of PNIPA brushes shows an interesting phenomenon, which is antithetic to that of typical PNIPA gel. With the increase of temperature from 10℃, the surfaces of the PNIPA brushes gradually change to more hydrophobic natures. But as temperature approaches the LCST, the brush surfaces turned back to hydrophilic state. This might be the effect of the change in the surface morphology of the polymer brushes and/or the change in physical state of the terminal end groups of the polymer, depending on temperature.