Glass, entropy

Figure 18.5. Gibbs-DiMarzio configurational entropy glass transition model representation of T2. Reproduced from DiMarzio (1981), by permission of John Wiley Sons, Ltd.

The heat capacity of thiazole was determined by adiabatic calorimetry from 5 to 340 K by Goursot and Westrum (295,296). A glass-type transition occurs between 145 and 175°K. Melting occurs at 239.53°K (-33-62°C) with an enthalpy increment of 2292 cal mole and an entropy increment of 9-57 cal mole °K . Table 1-44 summarizes the variations as a function of temperature of the most important thermodynamic properties of thiazole molar heat capacity Cp, standard entropy S°, and Gibbs function - G°-H" )IT. 

A. Milchev, I. Gutzow. Temperature dependence of the configurational entropy of undercooled melts and the nature of the glass transition. J Macromol Sci B 22 583-615, 1982. 

Supercooled liquids (glasses111) also have residual entropy at 0 Kelvin. As an example, glycerol supercools badly so that a glass is usually obtained at a low temperature. A crystalline solid can also be obtained if the liquid is cooled in a certain manner to initiate crystalization. Gibson and Giauque9 started with... 

G. E. Gibson and W. F. Giauque. "The Third Law of Thermodynamics. Evidence from the Specific Heats of Glycerol that the Entropy of a Glass Exceeds that of a Crystal at the Absolute Zero". J. Am. Chem. Soc.. 45. 93-104 (1923). 

The expression derived for the wet glass transition, Tgw, assuming the configurational entropy as the temperature dependent function30) ist ... 

We may now use Equations 7 and 12 for the vapor pressure of crystal and glass to calculate the change in entropy when we pass from crystal to glass at temperatures near the absolute zero. 

Continuing the process we may now change the pressure on the gas to the vapor pressure of the glass as given by (12) and then carry out a reversible condensation. These steps will be seen to involve the entropy change... 

In the foregoing article we have applied the methods of statistical mechanics to a determination of the entropy of crystals and supercooled glasses, and have reached the following conclusions. 

The difference per mole in the entropy of a given substance at the absolute zero in the form of a glass and crystal can be given by an equation of the form SgisLSS — 5crystai = R log a, where a is a small number increasing with the complexity of the molecule. 

The increase in the length of the side chain results normally in an internal plasticization effect caused by a lower polarity of the main chain and an increase in the configurational entropy. Both effects result in a lower activation energy of segmental motion and consequently a lower glass transition temperature. The modification of PPO with myristoyl chloride offers the best example. No side chain crystallization was detected by DSC for these polymers. 

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