Department of Civil and Environmental Engineering, University of California, Los Angeles, California 90095-1593, USA.
Paris Sorbonne Universités-UPMC, Laboratoire de Physique Théorique de la Matière Condensée, Boite 121, 4, Place Jussieu, 75252 Paris Cedex 05, France.
Nat Commun. 2015 Mar 9;6:6398. doi: 10.1038/ncomms7398.
If crystallization can be avoided during cooling, a liquid will display a substantial increase of its viscosity, and will form a glass that behaves as a solid with a relaxation time that grows exponentially with decreasing temperature. Given this 'off-equilibrium' nature, a hysteresis loop appears when a cooling/heating cycle is performed across the glass transition. Here we report on molecular dynamics simulations of densified glass-forming liquids that follow this kind of cycle. Over a finite pressure interval, minuscule thermal changes are found, revealing glasses of 'thermally reversible' character with optimal volumetric or enthalpic recovery. By analysing the topology of the atomic network structure, we find that corresponding liquids adapt under the pressure-induced increasing stress by experiencing larger bond-angle excursions. The analysis of the dynamic behaviour reveals that the structural relaxation time is substantially reduced in these adaptive liquids, and also drives the reversible character of the glass transition. Ultimately, the results substantiate the notion of stress-free (Maxwell isostatic) rigidity in disordered molecular systems, while also revealing new implications for the topological engineering of complex materials.
如果在冷却过程中可以避免结晶，液体的粘度将显著增加，并形成一种玻璃态，其松弛时间随温度的降低呈指数增长。鉴于这种“非平衡”性质，在玻璃化转变过程中进行冷却/加热循环时会出现滞后环。本文报道了遵循这种循环的致密玻璃形成液体的分子动力学模拟。在有限的压力间隔内，发现微小的热变化，揭示了具有最佳体积或焓恢复的“热可逆”玻璃的特性。通过分析原子网络结构的拓扑结构，我们发现相应的液体通过经历更大的键角偏移来适应压力诱导的不断增加的应力。对动力学行为的分析表明，在这些适应性液体中，结构弛豫时间大大降低，这也推动了玻璃化转变的可逆性。最终，这些结果证实了无序分子系统中无应力（麦克斯韦等静压）刚性的概念，同时也为复杂材料的拓扑工程学揭示了新的意义。
