Cooling glass transition

The kinetic nature of the glass transition should be clear from the last chapter, where we first identified this transition by a change in the mechanical properties of a sample in very rapid deformations. In that chapter we concluded that molecular motion could simply not keep up with these high-frequency deformations. The complementarity between time and temperature enters the picture in this way. At lower temperatures the motion of molecules becomes more sluggish and equivalent effects on mechanical properties are produced by cooling as by frequency variations. We shall return to an examination of this time-temperature equivalency in Sec. 4.10. First, however, it will be profitable to consider the possibility of a thermodynamic description of the transition which occurs at Tg. 

Glass-Transition Temperature. When a typical Hquid is cooled, its volume decreases slowly until the melting point, T, where the volume decreases abmpdy as the Hquid is transformed into a crystalline soHd. This phenomenon is illustrated by the line ABCD in Eigure 3. If a glass forming Hquid is cooled below (B in Eig. 3) without the occurrence of crystallization, it is considered to be a supercooled Hquid until the glass-transition temperature, T, is reached. At temperatures below T, the material is a soHd. 

As the temperature is decreased, free-volume is lost. If the molecular shape or cross-linking prevent crystallisation, then the liquid structure is retained, and free-volume is not all lost immediately (Fig. 22.8c). As with the melt, flow can still occur, though naturally it is more difficult, so the viscosity increases. As the polymer is cooled further, more free volume is lost. There comes a point at which the volume, though sufficient to contain the molecules, is too small to allow them to move and rearrange. All the free volume is gone, and the curve of specific volume flattens out (Fig. 22.8c). This is the glass transition temperature, T . Below this temperature the polymer is a glass. 

Two crystalline forms have been observed.One is formed by slow cooling from the melt and the other by slow heating of the amorphous polymer. The properties of the commercial products were therefore to some extent dependent on their heat history. Glass transition temperatures observed range from 7 to 32°C and depend on the time scale of the method of measurement. ... 

With plastics there is a certain temperature, called the glass transition temperature, Tg, below which the material behaves like glass i.e. it is hard and rigid. As can be seen from Table 1.8 the value for Tg for a particular plastic is not necessarily a low temperature. This immediately helps to explain some of the differences which we observe in plastics. For example, at room temperature polystyrene and acrylic are below their respective Tg values and hence we observe these materials in their glassy state. Note, however, that in contrast, at room temperature, polyethylene is above its glass transition temperature and so we observe a very flexible matoial. When cooled below its Tg it then becomes a hard, brittle solid. Plastics can have several transitions. 

Focusing attention on PTEB, it has been found that, similar to the case of PDTMB, the mesophase experiences a very slow transformation into the crystal. Thus, only the isotropization is observed in a sample freshly cooled from the melt [27]. However, after a long time at room temperature, the transformation mesophase-crystal is produced, owing to a glass transition temperature of about 14°C. Moreover, several endotherms were obtained before the final isotropization for a sample of PTEB annealed at 85°C for 12 days, i.e., PTEB shows enantiotropic behavior. The different endotherms may arise from polymorphism or melting-recrystallization phenomena [30]. 

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