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.
Langmuir. 2018 Sep 25;34(38):11272-11280. doi: 10.1021/acs.langmuir.8b01607. Epub 2018 Sep 11.
The present paper reports some experimental observations of reproducible uniaxial negative thermal expansion (u-NTE) in an amorphous polyvinyl acetate (PVAc) ultrathin film. It has been found that the mechanism of the phenomena is different from latest reports on so-called NTE in crystal or other topological materials. It is known that PVAc exhibits glass transition at around 31 °C. During cooling from the high-temperature side, one can observe the decrease of the thickness by monitoring interference fringes in the X-ray reflectivity curve as a function of temperature. Across the glass transition, however, the thickness starts to increase, instead of reducing. In the heating process, the thickness decreases as long as the temperature is lower than that for glass transition ( T). In the present research, such changes in thickness during repeated heating/cooling cycles have been studied systematically. To discuss the mechanism, dependence on film thickness has been investigated as well. It has been found that the present phenomena are well explained as u-NTE, which induces reduction and increase of thickness ( z-direction) just by thermal expansion and shrinking in x- y directions, respectively. This would be caused and enhanced by the growth of a mechanically hard, high-density layer near the interface to the surface of hydrophilic silicon dioxide. The structural change during heating/cooling cycles is discussed in detail.
本文报道了在非晶态聚醋酸乙烯酯（PVAc）超薄薄膜中观察到的可重复单轴负热膨胀（u-NTE）的一些实验观察结果。研究发现，这种现象的机制与最近关于晶体或其他拓扑材料中所谓的 NTE 的报告不同。众所周知，PVAc 在 31°C 左右表现出玻璃化转变。在从高温侧冷却的过程中，通过监测 X 射线反射率曲线随温度变化的干涉条纹，可以观察到厚度的减小。然而，在玻璃化转变处，厚度开始增加，而不是减小。在加热过程中，只要温度低于玻璃化转变温度（T），厚度就会减小。在本研究中，系统地研究了在重复加热/冷却循环过程中厚度的这种变化。为了讨论这种机制，还研究了薄膜厚度的依赖性。研究发现，目前的现象可以很好地解释为 u-NTE，它通过在 x-y 方向上的热膨胀和收缩分别导致厚度（z 方向）的减少和增加。这是由亲水二氧化硅表面附近界面处机械坚硬、高密度层的生长引起和增强的。详细讨论了加热/冷却循环过程中的结构变化。
