Poly(N-isopropylacrylamide-co-sodium acrylate) copolymer brushes grafted on SiO2-coated quartz crystal surface were prepared with a surface-immobilized initiator. The collapse and swelling of the thermally sensitive copolymer brushes in water were studied with quartz crystal microbalance in situ. The frequency and dissipation changes with the temperature increasing in the range 20-38 degrees C indicate that the brushes undergo a continuous collapse transition. Our results show that the copolymer brushes collapse to a state where the brushes were prepared. A hysteresis was observed in the cooling process. Fourier transform infrared (FTIR) results revealed that the formation of some additional hydrogen bonds within the copolymer chains at their collapsed state is responsible for the hysteresis. (c) 2006 Wiley Periodicals, Inc.