Scherillo Giuseppe, Loianno Valerio, Pierleoni Davide, Esposito Rosario, Brasiello Antonio, Minelli Matteo, Doghieri Ferruccio, Mensitieri Giuseppe
Department of Chemical, Materials and Production Engineering (DICMAPI) , University of Naples Federico II , Naples , Italy.
Department of Civil, Chemical, Environmental and Materials Engineering (DICAM) , Alma Mater Studiorum University of Bologna , Bologna , Italy.
J Phys Chem B. 2018 Mar 22;122(11):3015-3022. doi: 10.1021/acs.jpcb.8b01766. Epub 2018 Mar 9.
Atactic polystyrene, as reported in a recent contribution by our group, displays a marked change in glass transition when exposed to toluene vapor due to plasticization associated with vapor sorption within the polymer. The dependence of the glass transition temperature of the polymer-penetrant mixture on the pressure of toluene vapor is characterized by the so-called "retrograde vitrification" phenomenon, in that, at a constant pressure, a rubber to glass transition occurs by increasing the temperature. In this contribution, we have used a theoretical approach, based on the nonrandom lattice fluid thermodynamic model for the polymer-toluene mixture, to predict the state of this system, i.e., rubbery or glassy, as a function of fluid pressure and system temperature. The experimentally detectable glass transition is assumed to be a kinetically affected evidence of an underlying II order thermodynamic transition of the polymer mixture. On the basis of this hypothesis, the Gibbs-Di Marzio criterion, stating that equilibrium configurational entropy is zeroed at the glass transition, has been applied to locate the transition. The working set of equations consists of the expression of configurational entropy obtained from the adopted lattice fluid model equated to zero, coupled with the equation expressing the phase equilibrium between the polymer phase and the pure toluene vapor phase in contact and with the equations of state for the two phases. Theoretical predictions are in good qualitative and quantitative agreement with the experimental results previously obtained gravimetrically performing "dynamic" sorption experiments, which represent a neat example of the occurrence of so-called "type IV" glass transition temperature vs pressure behavior. The peculiar retrograde vitrification phenomenon and the glass transition temperature vs pressure envelope determined experimentally are well described by the proposed theoretical approach.
正如我们小组最近的一篇论文所报道的，无规聚苯乙烯在暴露于甲苯蒸气时，由于聚合物内部的蒸气吸附导致增塑作用，其玻璃化转变表现出明显变化。聚合物 - 渗透剂混合物的玻璃化转变温度对甲苯蒸气压的依赖性以所谓的“逆行玻璃化”现象为特征，即在恒定压力下，通过升高温度会发生从橡胶态到玻璃态的转变。在本论文中，我们采用了一种基于聚合物 - 甲苯混合物的非随机晶格流体热力学模型的理论方法，来预测该系统的状态，即橡胶态或玻璃态，作为流体压力和系统温度的函数。实验可检测到的玻璃化转变被假定为聚合物混合物潜在的二级热力学转变的动力学影响证据。基于这一假设，吉布斯 - 迪马尔齐奥准则（即平衡构型熵在玻璃化转变时为零）已被用于确定转变位置。方程组的工作集包括将从所采用的晶格流体模型获得的构型熵表达式设为零，再加上表示聚合物相与接触的纯甲苯蒸气相之间相平衡的方程以及两相的状态方程。理论预测与先前通过重量法进行“动态”吸附实验获得的实验结果在定性和定量上都有很好的一致性，这些实验结果是所谓“IV型”玻璃化转变温度与压力行为出现的一个清晰例子。所提出的理论方法很好地描述了独特的逆行玻璃化现象以及实验确定的玻璃化转变温度与压力曲线。
