Adrjanowicz K, Szklarz G, Koperwas K, Paluch M
Institute of Physics, University of Silesia, ulica Uniwersytecka 4, 40-007 Katowice, Poland.
Phys Chem Chem Phys. 2017 Jun 7;19(22):14366-14375. doi: 10.1039/c7cp01864a.
High pressure and nanoscopic confinement are two different strategies commonly employed to modify the physicochemical properties of various materials. Both strategies act mostly by changing the molecular packing. In this work, we performed a comparative study on the effect of compression and confined geometry on crystallization of a molecular liquid. Dielectric spectroscopy was employed to investigate the crystallization of the van der Waals liquid, dimethyl phthalate, in nanoporous alumina of different pore sizes as well as on increased pressure (up to 200 MPa). The analysis of the crystallization kinetics under varying thermodynamic conditions revealed that both strategies affect the crystallization behavior of the sample in very distinct ways. Compression shifts the maximum crystallization rate towards a higher temperature and broadens it. As a result, it is more challenging to avoid crystallization upon cooling the liquid at high pressure. In contrast, when the same material is incorporated into nanopores, crystallization significantly slows down and the maximum rate shifts towards a lower temperature with decreasing pore size. Finally, we show that crystallization in nanoporous alumina is accompanied by pre-crystallization effects upon which a shift of the α-relaxation peak is observed. An equilibration process prior to the initiation of crystallization was detected for the confined material both above and below the glass transition temperature of the interfacial layer, while not in the bulk.
高压和纳米尺度限制是常用于改变各种材料物理化学性质的两种不同策略。这两种策略大多通过改变分子堆积来起作用。在这项工作中，我们对压缩和受限几何形状对分子液体结晶的影响进行了比较研究。采用介电谱研究了范德华液体邻苯二甲酸二甲酯在不同孔径的纳米多孔氧化铝中以及在压力增加（高达200兆帕）时的结晶情况。对不同热力学条件下的结晶动力学分析表明，这两种策略以非常不同的方式影响样品的结晶行为。压缩使最大结晶速率向更高温度移动并使其变宽。因此，在高压下冷却液体时避免结晶更具挑战性。相比之下，当相同材料被纳入纳米孔时，结晶显著减慢，并且最大速率随着孔径减小向更低温度移动。最后，我们表明纳米多孔氧化铝中的结晶伴随着预结晶效应，在此过程中观察到α-弛豫峰的移动。对于受限材料，在界面层的玻璃化转变温度之上和之下都检测到了结晶开始之前的平衡过程，而在本体中则没有。
