Self-diffusion and spatially heterogeneous dynamics in supercooled liquids near Tg

威斯康星大学麦迪逊分校的马克·埃迪格由实验物理化学计划资助，开展将测量低分子质量过冷液体中平移运动的基本方面的实验。在利用热程序脱附和正反冲光谱的实验中，将在玻璃化转变温度附近测量四种不同脆性的过冷液体的自扩散系数，在这个范围内目前很少有实验结果。选择进行研究的体系是甘油、沙洛尔、邻三苯基和十氢萘，范围从文献中确定的最不脆弱的单组分有机玻璃成形之一到最脆弱的单组分有机玻璃成形之一。这些体系的自扩散结果将与PI最近关于三萘苯(TNB)的数据结合在一起。对于TNB，中子反射率将被用来确定分子在平移运动变得扩散之前必须移动的距离。小角中子散射技术将被用来直接探测非均匀动力学区域并确定其大小。此外，还将使用中子反射率来确定附近界面对玻璃化转变温度附近动力学的影响。所有这些实验都将使用气相沉积来制备由正常玻璃和氢化玻璃组成的薄双层。实验结果将对玻璃化转变温度附近的分子运动模型施加强烈的限制，并将用于测试这些模型。了解玻璃化转变温度附近的输运性质对于无序或无定形的聚合物和其他具有重要技术意义的材料是很重要的。预计结果将提高操纵聚合物和金属玻璃性质的能力，为通过光刻制备的小聚合物结构的崩溃提供基本见解，并对开发涉及非晶态材料的实用纳米结构和设备至关重要。同样，蛋白质、泡沫和颗粒材料都表现出玻璃化动力学，这些研究将通过对过冷液体的研究来加强对它们的理解。

Mark Ediger of the University of Wisconsin Madison is supported by the Experimental Physical Chemistry Program to carry out experiments that will measure fundamental aspects of translational motion in low molecular weight supercooled liquids. In experiments using thermally programmed desorption and forward recoil spectrometry, the self-diffusion coefficients of four supercooled liquids of varying fragility will be measured very near the glass transition temperature, a range in which there are currently very few experimental results. The systems selected for study are glycerol, salol, ortho-terphenyl, and decalin, ranging from one of the least fragile to one of the most fragile single component organic glass formers identified in the literature. Self-diffusion outcomes on these systems will be combined with the PI's recent data for tris-naphthylbenzene (TNB). For TNB, neutron reflectivity will be used to determine the distance over which molecules must move before translational motion becomes diffusive. The technique of small angle neutron scattering will be used to directly detect regions of heterogeneous dynamics and to determine their size. As well, neutron reflectivity will be used to determine the influence of nearby interfaces on dynamics near the glass transition temperature. All of these experiments will be performed using vapor deposition to prepare thin bilayers of normal and deuterated glass formers. Experimental outcomes will place strong constraints on models of molecular motion near the glass transition temperature, and will be used to test these models. The results will also increase understanding of the origin of spatial heterogeneities in dynamics.Understanding transport properties near the glass transition temperature is important for polymers and other technologically important materials that are disordered, or amorphous. Outcomes are expected to advance the ability to manipulate properties of polymers and metallic glasses, to provide fundamental insights into the collapse of small polymer structures prepared by lithography, and to be essential for the development of practical nanoscale structures and devices involving amorphous materials. As well, proteins, foams, and granular materials all exhibit glassy dynamics whose understanding will be enhanced by these studies on supercooled liquids.