Department of Chemistry and The James Franck Institute, University of Chicago, Chicago, IL 60637, USA.
J Chem Phys. 2010 Feb 28;132(8):084504. doi: 10.1063/1.3304738.
An analysis of glass formation for polymer melts that are diluted by structured molecular additives is derived by using the generalized entropy theory, which involves a combination of the Adam-Gibbs model and the direct computation of the configurational entropy based on a lattice model of polymer melts that includes monomer structural effects. Our computations indicate that the plasticization and antiplasticization of polymer melts depend on the molecular properties of the additive. Antiplasticization is accompanied by a "toughening" of the glass mixture relative to the pure polymer, and this effect is found to occur when the diluents are small species with strongly attractive interactions with the polymer matrix. Plasticization leads to a decreased glass transition temperature T(g) and a "softening" of the fragile host polymer in the glass state. Plasticization is prompted by small additives with weakly attractive interactions with the polymer matrix. However, the latter situation can lead to phase separation if the attractive interactions are sufficiently strong. The shifts in T(g) of polystyrene diluted by fully flexible short oligomers (up to 20% mass of diluent) are evaluated from the computations, along with the relative changes in the isothermal compressibility at T(g) (a softening or toughening effect) to characterize the extent to which the additives act as antiplasticizers or plasticizers. The theory predicts that a decreased fragility can accompany both antiplasticization and plasticization of the glass by molecular additives. The general reduction in the T(g) of polymers by molecular additives is rationalized by analyzing the influence of the diluent's properties (cohesive energy, chain length, and stiffness) on glass formation in fluid mixtures and the variation of fragility is discussed in relation to changes in the molecular packing in diluted polymer melts. Our description of constant temperature glass formation upon increasing the diluent concentration directly leads to the Angell equation (tau(alpha) approximately A exp{B/(phi(0,p)-phi(p))}) for the structural relaxation time as function of the polymer concentration, where the extrapolated "zero mobility concentration" phi(0,p) calculated from the theory scales linearly with the inverse polymerization index N.
用广义熵理论推导了由结构分子添加剂稀释的聚合物熔体的玻璃形成分析，该理论涉及到亚当-吉布斯模型的组合和基于包含单体结构效应的聚合物熔体的晶格模型的构象熵的直接计算。我们的计算表明，聚合物熔体的增塑和反增塑取决于添加剂的分子性质。反增塑伴随着相对于纯聚合物的“增韧”，并且当稀释剂是与聚合物基体具有强相互作用的小物种时，会发现这种效应。增塑会导致玻璃化转变温度 T(g)降低，并使玻璃态下的脆弱主链聚合物“软化”。弱相互作用的小添加剂会促使增塑。然而，如果吸引力相互作用足够强，这种情况可能会导致相分离。从计算中评估了完全柔性短寡聚物稀释的聚苯乙烯的 T(g)偏移（高达 20%的稀释剂质量），以及 T(g)下等温压缩率的相对变化（软化或增韧效应），以表征添加剂作为反增塑剂或增塑剂的作用程度。该理论预测，分子添加剂对玻璃的反增塑和增塑都可以伴随脆性降低。通过分析稀释剂性质（内聚能、链长和刚度）对流体混合物中玻璃形成的影响以及稀释聚合物熔体中分子堆积变化与脆性变化的关系，合理地解释了分子添加剂对聚合物 T(g)的普遍降低。我们描述的随着稀释剂浓度的增加而在恒温下形成玻璃的方法直接导致了结构弛豫时间作为聚合物浓度函数的 Angell 方程（tau(alpha)近似等于 A exp{B/(phi(0,p)-phi(p))}），其中从理论计算出的“零迁移浓度”phi(0,p)与聚合指数 N 的倒数呈线性关系。
