Xu Wen-Sheng, Douglas Jack F, Freed Karl F
James Franck Institute, The University of Chicago, Chicago, Illinois 60637, USA.
Materials Science and Engineering Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA.
J Chem Phys. 2016 Dec 21;145(23):234509. doi: 10.1063/1.4972412.
The generalized entropy theory (GET) offers many insights into how molecular parameters influence polymer glass-formation. Given the fact that chain rigidity often plays a critical role in understanding the glass-formation of polymer materials, the GET was originally developed based on models of semiflexible chains. Consequently, all previous calculations within the GET considered polymers with some degree of chain rigidity. Motivated by unexpected results from computer simulations of fully flexible polymer melts concerning the dependence of thermodynamic and dynamic properties on the cohesive interaction strength (ϵ), the present paper employs the GET to explore the influence of ϵ on glass-formation in models of polymer melts with a vanishing bending rigidity, i.e., fully flexible polymer melts. In accord with simulations, the GET for fully flexible polymer melts predicts that basic dimensionless thermodynamic properties (such as the reduced thermal expansion coefficient and isothermal compressibility) are universal functions of the temperature scaled by ϵ in the regime of low pressures. Similar scaling behavior is also found for the configurational entropy density in the GET for fully flexible polymer melts. Moreover, we find that the characteristic temperatures of glass-formation increase linearly with ϵ and that the fragility is independent of ϵ in fully flexible polymer melts, predictions that are again consistent with simulations of glass-forming polymer melts composed of fully flexible chains. Beyond an explanation of these general trends observed in simulations, the GET for fully flexible polymer melts predicts the presence of a positive residual configurational entropy at low temperatures, indicating a return to Arrhenius relaxation in the low temperature glassy state.
广义熵理论（GET）为分子参数如何影响聚合物玻璃化形成提供了许多见解。鉴于链刚性在理解聚合物材料的玻璃化形成过程中常常起着关键作用，GET最初是基于半柔性链模型发展而来的。因此，GET此前的所有计算都考虑了具有一定程度链刚性的聚合物。受完全柔性聚合物熔体的计算机模拟中关于热力学和动力学性质对内聚相互作用强度（ϵ）依赖性的意外结果的启发，本文采用GET来探究ϵ对弯曲刚性消失的聚合物熔体模型（即完全柔性聚合物熔体）中玻璃化形成的影响。与模拟结果一致，完全柔性聚合物熔体的GET预测，在低压范围内，基本的无量纲热力学性质（如折合热膨胀系数和等温压缩率）是由ϵ标度的温度的通用函数。在完全柔性聚合物熔体的GET中，构型熵密度也发现了类似的标度行为。此外，我们发现完全柔性聚合物熔体中玻璃化形成的特征温度随ϵ线性增加，且脆性与ϵ无关，这些预测再次与由完全柔性链组成的玻璃形成聚合物熔体的模拟结果一致。除了解释模拟中观察到的这些一般趋势外，完全柔性聚合物熔体的GET预测在低温下存在正的残余构型熵，这表明在低温玻璃态下回归到阿累尼乌斯弛豫。
