Liu Yuwei, Sakurai Kenji
University of Tsukuba, 1-1-1, Tennodai, Tsukuba, Ibaraki 305-0006, Japan.
National Institute for Materials Science, 1-2-1, Sengen, Tsukuba, Ibaraki 305-0047, Japan.
ACS Omega. 2019 Jul 15;4(7):12194-12203. doi: 10.1021/acsomega.9b01350. eCollection 2019 Jul 31.
In this paper, we report detailed experimental observations of unusual changes in the thickness of solid poly(-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 the lower critical solution temperature (LCST). This observation indicates that the PNIPAM ultrathin film is sensitive to the small amounts of water contained in the air, even under ambient, low-humidity conditions. On the other hand, during heating run from 15 to 60 °C, the humidity dependence was monotonic, and no specific changes in the PNIPAM films were observed at around the LCST. By studying the humidity dependence, we found that the hydrophilic and hydrophobic states of the PNIPAM ultrathin film exhibit different temperature dependence behaviors. In addition, we found that swelling takes place even under low-moisture conditions. To understand the difference in the thickness changes observed on cooling and heating further, some models considering the effect of the boundary conditions in the polymer ultrathin film system were considered. In the case of the ultrathin film, the hydrophilic/hydrophobic switching property occurred only in the surface layer, which dominated the absorption of water molecules from air. In contrast, the interface layer was time-stable and provided an escape route for water molecules during heating.
在本文中，我们报告了对固态聚（N-异丙基丙烯酰胺）（PNIPAM）超薄膜厚度异常变化的详细实验观察结果，PNIPAM超薄膜以具有温度响应性亲水-疏水切换特性而闻名。迄今为止，已经对与液态水接触的本体和刷状形式的PNIPAM进行了大量研究，以及在高湿度环境中进行了研究，并且最近已经初步表明，即使在低湿度、环境条件下，这些超薄膜也表现出温度响应。在这项工作中，使用多通道X射线反射仪连续监测温度/湿度控制的样品台上的PNIPAM超薄膜的厚度。在室温下，样品在70°C热处理3小时后厚度出现意外增加。在15至60°C的温度循环中，加热和冷却导致了一些明显的差异。在冷却过程中，最初厚度几乎恒定，但当温度超过33°C（对应于较低临界溶液温度（LCST））时开始增加。这一观察结果表明，即使在环境低湿度条件下，PNIPAM超薄膜对空气中所含的少量水也很敏感。另一方面，在从15至60°C的加热过程中，湿度依赖性是单调的，并且在LCST附近未观察到PNIPAM薄膜有特定变化。通过研究湿度依赖性，我们发现PNIPAM超薄膜的亲水和疏水状态表现出不同的温度依赖性行为。此外，我们发现即使在低湿度条件下也会发生溶胀。为了进一步理解在冷却和加热时观察到的厚度变化差异，考虑了一些考虑聚合物超薄膜系统中边界条件影响的模型。对于超薄膜，亲水/疏水切换特性仅发生在表面层，这主导了从空气中吸收水分子。相比之下，界面层是时间稳定的，并在加热过程中为水分子提供了逃逸途径。
