Variables Affecting Tg

The chemical structure of a polymer profoundly affects its glass transition temperature. Some of the most significant factors are the presence of bulky groups, polar groups, and the strength of intermolecular forces. 

Bulky side groups increase Tg by increasing the energy required for bond rotation. For example, the -CH3 group in PP and benzyl group in PS increase T compared 

The presence of double bonds in the main chain creates somewhat of an anomaly. The double bonds themselves are, like ring structures, rigid. There is no rotation around double bonds. However, the presence of a double bond greatly facilitates rotation around the single bonds adjacent to the double bond, in part because of decreased interference with the rotation of groups attached to that next carbon. Therefore, the presence of a double bond actually acts to increase chain flexibility, and hence decreases Tg. Synthetic rubbers such as polybutadiene contain double bonds in the main polymer chain. 

Highly polar groups increase Tg by increasing intermolecular forces, which must be disrupted for rotation to occur. Fig. 3.48 compares the effect of CH3- in PP and Cl- in PVC, showing that Tg not only increases with respect to PE, but that the more polar Cl has a more pronounced effect. 

Hydrogen bonds provide particularly strong intermolecular forces, and therefore have a large effect on Tg. The -OH group is not only polar, but also produces hydrogen bonds that result in an increase of 185°C in the glass transition temperature when PE is compared to PVOH, as shown in Fig. 3.49. 

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