Endothermic Effects on Heating Physically-Aged Sucrose Glasses, and the Clausius Theorem Violation in Glass Thermodynamics.
Endothermic Effects on Heating Physically-Aged Sucrose Glasses, and the Clausius Theorem Violation in Glass Thermodynamics.
复制标题
加热物理老化蔗糖玻璃的吸热效应，以及玻璃热力学中克劳修斯定理的违反。
DOI：
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发表时间：
2020
影响因子：
3.3
通讯作者：
G. P. Johari
中科院分区：
文献类型：
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作者：
E. Tombari;G. P. Johari
某些物理老化聚合物的实验热容，Cp,app，在材料仍处于玻璃态时，在加热时表现出吸热峰或肩部。在本文的第一部分中，我们报道了在蔗糖分子玻璃中观察到的类似特征，表明分子运动在固态下的动态解冻会增加焓和熵。老化降低玻璃的Cp，app。老化时间age或老化温度age的增加会干扰Cp的起始温度，使其向液态值增加。当吸热特征在玻璃态的Cp，app -T图中不明显时，它的存在可以在dCp，app /dT对t的图中辨别出来。产生这种特征的分子运动对老化玻璃的势能图中的状态点有影响。在本文的第二部分，我们使用Cp，app数据来检查我们对克劳修斯定理的违反对Cp， applnt积分确定的熵的影响程度。除了计算（不可逆）冷却和加热路径闭合循环的积分外，我们还建议使用仅在加热路径上测量的<s:1> Cp， applnt积分（<s:1> Cp，app是老化玻璃和未老化玻璃的Cp，app之间的差值）进行分析。闭合循环Cp、applnt的积分值小得可以忽略不计。其积分值随年龄的增长而增大。它等于老化时损失的焓除以年龄。克劳修斯定理的违反对由陈化玻璃的Cp、applnt积分确定熵无显著影响。
Experimental heat capacity, Cp,app, of some physically aged polymers had shown an endothermic peak or a shoulder on heating when the material was still in the glassy state. In the first part of this paper, we report observation of a similar feature in a molecular glass, sucrose, indicating increase in the enthalpy and entropy from kinetic unfreezing of molecular motions in the solid state. Aging decreases Cp,app of glass. Increase in the aging time, tage, or aging temperature, Tage, interferes with the onset temperature of Cp,app increase toward the liquid state value. When the endothermic feature is not obvious in the Cp,app -T plots of the glassy state, its presence may be discerned in a plot of dCp,app /dT against T. Molecular motions producing this feature have implications for the state point in a potential energy landscape of an aging glass. In the second part of the paper, we use the Cp,app data to examine how much our violation of the Clausius theorem affects the entropy determined from the Cp,appdlnT integral. In addition to calculating this integral for a closed cycle of (irreversible) cooling and heating paths, we suggest an analysis which uses the Cp,appdlnT integrals (Cp,app is the difference between the Cp,app of the aged and the un-aged glass) measured only on the heating paths. The closed cycle Cp,appdlnT integral value is negligibly small. The Cp,appdlnT integral value increases with tage. It is equal to the enthalpy lost on aging divided by Tage. Clausius theorem violation has no significant effect on determination of the entropy from Cp,appdlnT integral of an aged glass.