Factors that Influence Tg

In general, the glass transition temperature depends on five factors. 

The free volume of the polymer Vf, which is the volume of the polymer mass not actually occupied by the molecules themselves (think of a kitchen sponge, but on a molecular level), that is, ry = v v, where v is the specific volume of the polymer mass and is the volume of the solidly packed molecules. The higher Vf, the more room the molecules will have to move aroimd, resulting in a lower Tg. It has been estimated that for aU polymers Vf/v 0.025 at Tg. 

Example 6.1 Glass transition temperatures have been observed to increase at pressures of several thousand psi. Why  

Solution. High pressures compress polymers, reducing v. Since does not change appreciably, Vf is reduced. 

FIGURE 6.4 Rotation about a bond in a polymer chain backbone, viewed along the bond. The dotted substituents are on the rear carbon atom. 

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