Factors Affecting the Glass Transition of Polymers

The influence of factors such as chemical structure, molecular weight, cross-linking and plasticizers in the glass transition of polymers can be related to the changes that they provoke on the free volume fraction, which, as we already know, reaches a critical value at the glass transition temperature. The factors affecting the glass transition can be classified into two types (1) molecular factors, i.e., those related to the chemical structure of the polymer chain, and (2) external or controllable factors. 

The effect of the chemical nature of the main chain of the polymer on the glass transition temperature is similar to the effect that it has on the melting temperature, T. The chemical structure has a determining influence on the flexibility of the chain. For example, polymers such as polyethylene, (—CH2 — CH2—) , and polyoxyethylene, (—CH2—CH2 — O—) , have relatively flexible chains as a result of the ease of rotation around their chain bonds. Thus they have low values of Tg and as can be seen in Table 2.3. The incorporation into the main chain of units that hinder rotation and consequently increase the rigidity of the chain clearly causes a large increase in Tg. For example, the incorporation of a p-phenylene ring (Ph) into the monomeric unit of polyethylene gives poly(p-xylylene), which has a Tg of around 353 K (see Table 2.3). 

In vinyl polymers, (—CH2 — CHR—) , the nature of the side group R has a pronounced effect on Tg as a result of restrictions on the rotation of the macromolecule. Large, inflexible, and bulky side groups cause an increase in rigidity, while flexible side groups have not marked effect. Table 2.4 presents 

The Tg for some n-alkyl ethers of the general formula (—CH2—CH— (OR )—) where R represents an -alkyl group (24), are given in Table 2.5. Lower values of Tg are observed as the length of R increase. The rise in the length of R is associated with an increase in free volume at a given temperature. 

Symmetry also affects Tg. It might be expected that polyisobutylene, (—CH2 — C(CH3)2—) , would have a Tg greater than polypropylene, (—CH2 — CH(CH3)—) , as a consequence of the two —CH3 groups bonded to the backbone, which would tend to increase the chain rigidity. But this is not the case polyisobutylene is a rubber that has a Tg of —71°C, while polypropylene presents Tg = —23°C. Another example of how symmetry plays an important role in the Tg is seen if we compare poly (vinyl chloride) Tg = 81°C) and poly(vinylidene chloride) Tg = — 19°C) (31). As a general rule, it can be said that an increase in symmetry produces a decrease in the glass transition temperature. 

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Factors Affecting the Glass Transition of Polymers Problem Sets... 

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