Specific Volume and Morphology

The polymer density p is a function of pressure, temperature, and cooling rate. Specific volume V is the reciprocal of density, V = 1/p. The general P, V, T diagram of an amorphous polymer is shown in Fig. 6.24. 

If the material is cooled very slowly, the specific volume will reach a lower value than at a relatively high cooling rate. In simple terms, at a low cooling rate the polymer molecules, because of their thermal motion, have more opportunity to position themselves closer together. This reduces the free volume of the polymer, i.e., the volume fraction not occupied by polymer molecules. Below the glass transition temperature, the thermal motion of the polymer molecules is drastically reduced and the free volume remains approximately constant. Therefore, the change in specific volume with temperature is much larger above Tg than below Tg. The reduction in specific volume below Tg is primarily due to the reduced thermal motion of the polymer molecules. 

Increasing the molecular weight increases the glass transition temperature as 

A general P, V, T diagram of a semi-crystalline polymer is shown in Fig. 6.25. 

The behavior in the liquid state is essentially the same as amorphous polymers. In the transition region, an abrupt change in slope occurs as crystallization begins to take place. This is the crystallization temperature T, . If the material is cooled very rapidly, the crystallization rate can be depressed, depending on the crystallization kinetics. In fact, in some materials with sufficientiy slow crystallization kinetics, the crystallization can be almost completely suppressed by rapid cooling. A well-known example is polyethylene terephthalate (PET). If PET is rapidly quenched, it is almost completely amorphous with a density of about 1.33 g/-cm. If it is cooled slowly, it will crystallize with a resulting higher density of about 1.40 gZ-cm.  

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