Thickness Changes in Temperature-Responsive Poly(N-isopropylacrylamide) Ultrathin Films under AmbientConditions

摘要

In this paper, we report detailed experimental observations of unusual changes in the thickness of solid poly(N-isopropylacrylamide) (PNIPAM) ultrathin films, which are well known to have temperature-responsive hydrophilic–hydrophobic switching properties. To date, a number of studies have been carried out on the bulk and the brush forms of PNIPAM in contact with liquid water, as well as in highly humid environments, and, recently, these ultrathin films have been preliminarily shown to exhibit temperature responses even under low-humidity, ambient conditions. In this work, the thicknesses of ultrathin PNIPAM films in a temperature/moisture-controlled sample stage were monitored continuously using multichannel X-ray reflectometry. At room temperature, the sample thickness showed an unexpected increase after thermal treatment at 70 °C for 3 h. In the temperature cycle between 15 and 60 °C, heating and cooling resulted in some clear differences. During cooling, initially, the thickness was almost constant but began to increase when the temperature exceeded 33 °C, which corresponds to thelower critical solution temperature (LCST). This observation indicatesthat the PNIPAM ultrathin film is sensitive to the small amounts ofwater contained in the air, even under ambient, low-humidity conditions.On the other hand, during heating run from 15 to 60 °C, the humiditydependence was monotonic, and no specific changes in the PNIPAM filmswere observed at around the LCST. By studying the humidity dependence,we found that the hydrophilic and hydrophobic states of the PNIPAMultrathin film exhibit different temperature dependence behaviors.In addition, we found that swelling takes place even under low-moistureconditions. To understand the difference in the thickness changesobserved on cooling and heating further, some models considering theeffect of the boundary conditions in the polymer ultrathin film systemwere considered. In the case of the ultrathin film, the hydrophilic/hydrophobicswitching property occurred only in the surface layer, which dominatedthe absorption of water molecules from air. In contrast, the interfacelayer was time-stable and provided an escape route for water moleculesduring heating.