Change in entropy in thermal hysteresis of liquid-glass-liquid transition and consequences of violating the Clausius theorem
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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.
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