Glass rubber thermodynamic

Staverman,A.J. Thermodynamic aspects of the glass-rubber transition. Rheol. Acta 5,283 (1966). 

The glass-rubber transition temperature, commonly known as glass transition temperature (Tg), is a phase change reminiscent of a thermodynamic second-order transition. In the case of a second-order transition a plot of a primary quantity shows an abrupt change in slope, while a plot of a secondary quantity (such as expansion coefficient and specific heat) then shows a sudden jump. 

First, the glass-rubber transition is not a real thermodynamic phase transition, neither a first- nor a second-order transition, as was proved by Staverman (1966) and Rehage et al. (1967, 1980,1984), (see Scheme 6.1) the glassy state is not thermodynamically stable and thus not defined by the normal state variables also its history and its age play a part. At the very best the Tg-transition may be seen as a quasi-second-order transition but certainly not as a first-order one. 

In contradiction to the melting point, the glass-rubber transition temperature is not a thermodynamic transition point. It shows some resemblance, however, to a second order transition. For a second-order transition, the following relationships derived by Ehrenfest (1933) hold ... 

Staverman (1966) and Breuer and Rehage (1967) extensively discussed the thermodynamics of the glass-rubber transition. They concluded that it is not a real second-order transition, mainly because the glassy state is not completely defined by the normal state variables p, V and T. 

The liquid crystal melt, which comes into being at the glass-rubber transition or at the crystal-melt transition, may have several phase states (Mesophases) one or more smectic melt phases, a nematic phase and sometimes a chiral or cholesteric phase the final phase will be the isotropic liquid phase, if no previous decomposition takes place. All mesophase transitions are thermodynamically real first order effects, in contradistinction to the glass-rubber transition. A schematic representation of some characteristic liquid crystal phase structures is shown in Fig. 6.13, where also so-called columnar phases formed from disclike molecules is given. 

While the melting of crystallites is known as a thermodynamic first-order transition (since it involves a discontinuity, at a characteristic temperature, in a property, such as volume, that is a first derivative of free energy), the glass-rubber transition is often considered as a second-order transition (since it involves a discontinuity at a characteristic temperature in a property, such as coefficient of expansion, that is a second derivative of energy). ... 

The view that the glass-rubber transition is truly thermodynamic in nature, and not just a kinetic anomaly, is not universally accepted, though a strong case for a thermodynamic origin can be made (Meares, 1965). 

In the following sections the different thermodynamic models presented in the preceding text will be applied in the calculation of the penetrant solubility for both the cases of rubbery and glassy polymeric systems. Binary as well as ternary systems will be considered to show the ability of the models to represent observed isotherms in rubbers as well as in glasses, based on their equilibrium versions and non-equilibrium extensions, respectively. 

In connection with the facts that the peak at 213 K corresponds exactly to the glass transtion of the SKD-KTRA rubber and that system softening at 328 K coincides with the glass transition of the epoxy matrix, one can affirm the incompatibility of the rubber and epoxy components of the above EEC. Electron-microscopic data also indicate the existence of the EEC two-phase structure only in the case of SKD-KTEA as a modifier, which is thermodynamically incompatible at the stage of blending with the epoxy oligomer. 

The increase of the glass-transition temperature and the decrease of creep of epo3 polymers with 10% SKD-KTRA demonstrate the effect of the rubber upon the epoxy matrix itself. Such abnormal behavior of EEC can be explained thermodynamically. An SKD-KTRA content of 10% in ERG ensures thermodynamic conditions for separation of the... 

---