Tombari Elpidio, Johari G P
Istituto per i Processi Chimico-Fisici del CNR, via G. Moruzzi 1, 56124 Pisa, Italy.
Department of Materials Science and Engineering, McMaster University, Hamilton, Ontario L8S 4L7, Canada.
J Chem Phys. 2014 Aug 21;141(7):074502. doi: 10.1063/1.4892588.
Change in the entropy, dS, with change in the temperature T, with or without phase transformation, is determined only if the thermal path is reversible. For an irreversible thermal path, dS > dqirrev/T, an expression known as inequality in the Clausius theorem. In the glass formation range, Cp and enthalpy show a time-dependent hysteresis between the cooling and heating paths and the two Cp paths cross each other. We provide new data on Cp of poly(styrene) and use the previous Cp data [E. Tombari, C. Ferrari, G. Salvetti, and G. P. Johari, Phys. Rev. B 78, 144203 (2008)], both data obtained from measurements performed on cooling from melt to glass and heating from glass to melt at unusually slow rates, and show that violation of the Clausius theorem in such cases has insignificant consequences for determining the entropy of glass. We also report Cp of the samples annealed for different times at different temperatures in which the enthalpy spontaneously decreased. These measurements also show that violation of the Clausius theorem is relatively inconsequential for interpreting the entropy of the glassy state.
熵变dS随温度T的变化，无论是否发生相变，只有在热路径是可逆的情况下才能确定。对于不可逆热路径，dS > dqirrev/T，这是一个被称为克劳修斯定理不等式的表达式。在玻璃形成范围内，定压比热容Cp和焓在冷却和加热路径之间呈现出与时间相关的滞后现象，并且两条Cp路径相互交叉。我们提供了聚苯乙烯的Cp新数据，并使用了之前的Cp数据[E. 通巴里、C. 法拉利、G. 萨尔韦蒂和G. P. 乔哈里，《物理评论B》78, 144203 (2008)]，这两个数据均是在以异常缓慢的速率从熔体冷却至玻璃态以及从玻璃态加热至熔体的过程中测量得到的，结果表明在这种情况下违反克劳修斯定理对于确定玻璃的熵影响不大。我们还报告了在不同温度下退火不同时间的样品的Cp，其中焓会自发降低。这些测量结果还表明，违反克劳修斯定理对于解释玻璃态的熵相对来说并不重要。
