Pieruccini M, Ezquerra T A, Lanza M
CNR, Instituto per i Processi Chimico-Fisici, Messina, Salita Sperone Contrada Papardo, sn 98158 Faro Superiore, Messina, Italy.
J Chem Phys. 2007 Sep 14;127(10):104903. doi: 10.1063/1.2771166.
The segmental relaxation in poly(ethylene terephthalate), crystallized from either an isotropic or a cold-drawn glass, is investigated by means of dielectric spectroscopy. It is shown that there exist two distinct alpha relaxation modes: a slow one, characterized by a rather wide frequency interval, and a faster, much narrower one. A simple phenomenological model is developed in order to analyze the polarization autocorrelation functions phi(t)'s associated with these relaxation modes. The model is based on the idea that the growth of crystalline domains causes a progressive confinement of the amorphous regions where, eventually, the observed alpha processes take place. The mechanism of confinement is accounted for by applying to the case of constrained density fluctuations, well known concepts introduced by Adam and Gibbs [J. Chem. Phys. 43, 139 (1965)] concerning the relaxation dynamics in liquids close to the glass transition. Randomness on confining conditions is then introduced, leading to the derivation of analytical expressions which are used afterwards to fit the asymptotic behavior of the phi's for long-time tails. It is found that the slow, broad alpha process takes place in regions where the confining effect of crystals is strong, whereas the amorphous domains relaxing via the fast mode are those where the confinement effect of crystals is weak. The analysis of the phi's by means of this model allows us to relate the fitting exponents to the dispersion in the free energy associated with structural rearrangement.
采用介电谱法研究了由各向同性或冷拉玻璃态结晶的聚对苯二甲酸乙二酯中的链段弛豫。结果表明，存在两种不同的α弛豫模式：一种较慢，其特征是频率区间较宽；另一种较快，频率区间窄得多。为了分析与这些弛豫模式相关的极化自相关函数φ(t)，建立了一个简单的唯象模型。该模型基于这样的观点：晶畴的生长导致非晶区逐渐受限，最终在这些非晶区发生观测到的α过程。通过将适用于受限密度涨落情况的、由亚当和吉布斯[《化学物理杂志》43, 139 (1965)]引入的关于接近玻璃化转变的液体中弛豫动力学的熟知概念应用于此，来解释受限机制。然后引入受限条件的随机性，从而推导出解析表达式，之后用这些表达式来拟合φ对于长时间尾部的渐近行为。结果发现，缓慢、宽泛的α过程发生在晶体受限效应较强的区域，而通过快速模式弛豫的非晶畴是晶体受限效应较弱的区域。通过该模型对φ的分析使我们能够将拟合指数与结构重排相关的自由能色散联系起来。
