Rodríguez-Tinoco Cristian, Gonzalez-Silveira Marta, Ràfols-Ribé Joan, Lopeandía Aitor F, Rodríguez-Viejo Javier
Grup de Nanomaterials i Microsistemes, Physics Department, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain.
Phys Chem Chem Phys. 2015 Dec 14;17(46):31195-201. doi: 10.1039/c5cp04692k.
While ordinary glasses transform into supercooled liquid via a homogeneous bulk mechanism, thin film glasses of higher stability transform heterogeneously by a front propagating from the surface and/or the interfaces. In this work, we use quasi-adiabatic fast scanning nanocalorimetry to determine the heat capacity of thin glassy layers of indomethacin vapor-deposited in a broad temperature range of 110 K below the glass transition temperature. Their variation in fictive temperature amounts to 40 K. We show that a propagating front is the initial transformation mechanism in all cases. Using an ad hoc surface normalization procedure we determine the corresponding growth front velocity for the whole range of deposition temperatures. Although the transformation rate changes by a factor of 10 between the most and less stable samples, the relation between the mobility of the front and the thermodynamic stability of the glass is not uniquely defined. Glasses grown above 280 K, which are at equilibrium with the supercooled liquid, present a different dependence of the growth front velocity on fictive temperature compared to glasses grown out of equilibrium at Tdep < 250 K. These glasses transform faster with increasing Tf. Our data clarify previous reports and support the evidence that the fictive temperature alone is not an absolute indicator of the properties of the glass, at least when its structure is not completely isotropic. To interpret the data, we propose that the growth front velocity depends on three terms: the mobility of the liquid at a given temperature, the mobility of the glass and the arrangement of the molecules in the glass.
普通玻璃通过均匀的整体机制转变为过冷液体，而稳定性更高的薄膜玻璃则通过从表面和/或界面传播的前沿进行非均匀转变。在这项工作中，我们使用准绝热快速扫描纳米量热法来测定在低于玻璃化转变温度110 K的宽温度范围内气相沉积的吲哚美辛薄玻璃层的热容量。它们的虚构温度变化量达40 K。我们表明，在所有情况下，传播前沿都是初始转变机制。使用特定的表面归一化程序，我们确定了整个沉积温度范围内相应的生长前沿速度。尽管。尽管在最稳定和较不稳定的样品之间转变速率变化了10倍，但前沿迁移率与玻璃热力学稳定性之间的关系并非唯一确定。在280 K以上生长且与过冷液体处于平衡状态的玻璃，与在Tdep < 250 K下非平衡生长的玻璃相比，生长前沿速度对虚构温度呈现出不同的依赖性。这些玻璃随着Tf升高转变更快。我们的数据澄清了先前的报道，并支持了以下证据：至少当玻璃结构不完全各向同性时，仅虚构温度并非玻璃性质的绝对指标。为了解释这些数据，我们提出生长前沿速度取决于三个因素：给定温度下液体的迁移率、玻璃的迁移率以及玻璃中分子的排列。
