Factors governing Tg

The height of the glass-rubber transition temperature is, in the first instance, governed by the competition between thermal motion and the attraction forces between the chains. 

The thermal motion is dependent on the freedom of the chain to undergo changes in conformation. When this freedom is higher, the chain is subjected to a stronger thermal motion than a chain which, e.g. as a result of hindrance in rotation, is more rigid. The chain stiffness, as discussed in 2.4, plays, therefore, an important role. 

We shall consider some examples of both criteria. In doing so, besides amorphous, also crystalline polymers will be taken into account, since the latter always contain 

Higher chain stiffness results from a smaller number of possible chain conformations this can be caused by  

First a few examples of chain stiffness differences in the main chain the examples are not very exact, because side groups may also play a role as to flexibility or interactions, but they represent a global trend. 

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