Second Order Glass Transition Point

At this point one is tempted to anticipate the results of the quantitative analysis and suppose that the liquid phase has / >p , the glass phase has pglass transition temperature. If so, the transition would be second order because the infinite cluster is formed sharply." Calculations based on the model show that it cannot be second order in most circumstances, but is first order, with a range of values of p around p excluded. Elimination of the simplifications we have introduced wipes out the second-order phase transition, but the first-order phase transition persists in the circumstances we believe to hold experimentally, as we shall discuss after presenting the calculations. 

Several aspects of a genuine second-order thermodynamic transition can be observed with what is known as the glass transition temperature (also often called the glass temperature), i.e., discontinuities in Cp, a, and k. But the glass transition is not a genuine thermodynamic transition since there is no equilibrium between both sides of the glass transition point. The position of... 

In DSC, the glass transition is defined as a change in heat capacity as the respective domain changes from the glass state to the viscoelastic state. This second order endothermic transition appears as a step transition. In contrast a peak is observed for first order transition (e.g. melting transition). The inflection point of the step transition indicates the Tg, while the is represented by the peak maximum of the melting temperature range. Usually a Tg can be better identified by DMTA measurements since this is typically less clear in DSC. 

Although the glass transition temperature is not a thermodynamic transition point, it bears resemblance to second-order Ehrenfest transition ... 

From a thermodynamic viev point the glass transition has the appearance of a second-order phase transition. The slope of the temperature dependence of quantities such as the specific volume or enthalpy change, although the quantities themselves vary in a continuous manner v ith temperature. The gradient of specific volume Avith respect to temperature determines the volume thermal expansion coefficient a, whilst the variation of enthalpy with temperature defines the heat capacity ... 

In contrast to a change in slope at the glass transition, a thermodynamic property such as specific volume exhibits a discontinuity with temperature at the crystalline melting point in polymers as in other materials (Figure 6.2). The glass transition is therefore known as a second-order thermodynamic transition (where v versus Tis continuous and dv/dT versus T is discontinuous) in contrast to a first-order transition such as the melting point (where v versus T is discontinuous). 

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