White Ronald P, Buculei Dragos, Beale Alexia M J M, Goovaerts Ilias, Keddie Joseph L, Lipson Jane E G
Department of Chemistry, Dartmouth College, Hanover, New Hampshire, 03755, USA.
Department of Physics, University of Surrey, Guildford, Surrey, GU2 7XH, UK.
Soft Matter. 2022 Sep 14;18(35):6660-6673. doi: 10.1039/d2sm00959e.
Strategies for synthesizing molecularly designed materials are expanding, but methods for their thermodynamic characterization are not. This shortfall presents a challenge to the goal of connecting local molecular structure with material properties and response. Fundamental thermodynamic quantities, including the thermal expansion coefficient, , can serve as powerful inputs to models, yielding insight and predictive power for phenomena ranging from miscibility to dynamic relaxation. However, the usual routes for thermodynamic characterization often require a significant sample size ( one gram), or challenging experimental set-ups ( mercury as a confining fluid), or both. Here, we apply spectroscopic ellipsometry, which is an optical technique for thin film analysis, to obtain thermodynamic data. We clarify issues in the scientific literature concerning the connection between ellipsometric and volumetric thermal expansion coefficients for substances in both the glass and melt states. We analyze temperature-dependent data derived using both ellipsometry and macro-scale dilatometric techniques for ten different polymers. We find superb correlation between the values obtained the two techniques, after considering the effects of mechanical confinement by the substrate for a glassy thin film. We show how the ellipsometric can serve as input to the locally correlated lattice theory to yield predictions for the percent free volume in each polymer as a function of temperature. We find that the ellipsometric at the glass transition temperature, , is not only material dependent, but it is linearly correlated with itself. Spectroscopic ellipsometry, which requires only very small quantities of sample and is straightforward to perform, will significantly expand the range of systems for which thermodynamic properties can be characterized. It will thus advance our ability to use theory and modeling to predict the miscibility and dynamic relaxation of new materials. As such, ellipsometry will be able to underpin materials synthesis and property design.
合成分子设计材料的策略正在不断扩展，但对其进行热力学表征的方法却没有相应发展。这一不足给将局部分子结构与材料性能及响应联系起来的目标带来了挑战。包括热膨胀系数在内的基本热力学量可以作为强大的模型输入，为从混溶到动态弛豫等各种现象提供深入理解和预测能力。然而，通常的热力学表征途径往往需要大量样品（一克），或者需要具有挑战性的实验装置（以汞作为限制流体），或者两者都需要。在这里，我们应用光谱椭偏仪（一种用于薄膜分析的光学技术）来获取热力学数据。我们澄清了科学文献中关于玻璃态和熔融态物质的椭偏热膨胀系数与体积热膨胀系数之间联系的问题。我们分析了使用椭偏仪和宏观尺度膨胀技术获得的十种不同聚合物的温度相关数据。在考虑了玻璃态薄膜中基底的机械限制效应后，我们发现两种技术获得的 值之间具有极好的相关性。我们展示了椭偏 如何作为局部相关晶格理论的输入，以预测每种聚合物中自由体积百分比随温度的变化。我们发现，在玻璃化转变温度 时的椭偏 不仅取决于材料，而且与 本身呈线性相关。光谱椭偏仪只需要极少量的样品，且操作简便，将显著扩大能够表征热力学性质的系统范围。因此，它将提高我们运用理论和模型来预测新材料的混溶性和动态弛豫的能力。这样，椭偏仪将能够为材料合成和性能设计提供支持。
