Niss Kristine, Dalle-Ferrier Cécile, Giordano Valentina M, Monaco Giulio, Frick Bernhard, Alba-Simionesco Christiane
Laboratoire de Chimie Physique, Universite Paris-Sud, 91405 Orsay, France.
J Chem Phys. 2008 Nov 21;129(19):194513. doi: 10.1063/1.3005646.
We present an extensive analysis of the proposed relationship [T. Scopigno et al., Science 302, 849 (2003)] between the fragility of glass-forming liquids and the nonergodicity factor as measured by inelastic x-ray scattering. We test the robustness of the correlation through the investigation of the relative change under pressure of the speed of sound, nonergodicity factor, and broadening of the acoustic exitations of a molecular glass former, cumene, and of a polymer, polyisobutylene. For polyisobutylene, we also perform a similar study by varying its molecular weight. Moreover, we have included new results on liquids presenting an exceptionally high fragility index m under ambient conditions. We show that the linear relation, proposed by Scopigno et al. [Science 302, 849 (2003)] between fragility, measured in the liquid state, and the slope alpha of the inverse nonergodicity factor as a function of T/T(g), measured in the glassy state, is not verified when increasing the data base. In particular, while there is still a trend in the suggested direction at atmospheric pressure, its consistency is not maintained by introducing pressure as an extra control parameter modifying the fragility: whatever is the variation in the isobaric fragility, the inverse nonergodicity factor increases or remains constant within the error bars, and one observes a systematic increase in the slope alpha when the temperature is scaled by T(g)(P). To avoid any particular aspects that might cause the relation to fail, we have replaced the fragility by other related properties often evoked, e.g., thermodynamic fragility, for the understanding of its concept. Moreover, we find, as previously proposed by two of us [K. Niss and C. Alba-Simionesco, Phys. Rev. B 74, 024205 (2006)], that the nonergodicity factor evaluated at the glass transition qualitatively reflects the effect of density on the relaxation time even though in this case no clear quantitative correlations appear.
我们对通过非弹性X射线散射测量的玻璃形成液体的脆性与非遍历性因子之间的拟议关系[T. Scopigno等人，《科学》302, 849 (2003)]进行了广泛分析。我们通过研究分子玻璃形成剂异丙苯和聚合物聚异丁烯的声速、非遍历性因子以及声学激发展宽在压力下的相对变化，来检验这种相关性的稳健性。对于聚异丁烯，我们还通过改变其分子量进行了类似的研究。此外，我们纳入了在环境条件下具有异常高脆性指数m的液体的新结果。我们表明，当增加数据库时，Scopigno等人[《科学》302, 849 (2003)]提出的液态下测量的脆性与玻璃态下作为T/T(g)函数的非遍历性因子倒数的斜率α之间的线性关系未得到验证。特别是，虽然在大气压下仍存在建议方向上的趋势，但通过引入压力作为改变脆性的额外控制参数，其一致性并未得到维持：无论等压脆性如何变化，非遍历性因子倒数在误差范围内增加或保持不变，并且当温度按T(g)(P)缩放时，观察到斜率α有系统地增加。为了避免可能导致该关系失效的任何特殊方面，我们用其他经常提及的相关性质（例如热力学脆性）取代了脆性，以理解其概念。此外，正如我们两人之前所提出的[K. Niss和C. Alba-Simionesco，《物理评论B》74, 024205 (2006)]，我们发现即使在这种情况下没有明显的定量相关性出现，在玻璃化转变时评估的非遍历性因子定性地反映了密度对弛豫时间的影响。
