Glass transition temperature factors that affect, 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 mechanism of photolysis of poly(ethyl methacrylate) is similar to that of poly(methyl acrylate) [1067,1068]. An important outcome of study of this polymer is that chain mobility is a considerable factor in recombination of macroradicals. The time required to produce an insoluble gel increases when the temperature of irradiation is decreased. Below the glass transition temperature (Tg = — 17°C) no gel was formed. On the other hand, it was observed that chain mobility did not affect the decomposition reaction leading to chain scission. Methane and carbon monoxide were the main volatile products formed, but trace amounts of carbon dioxide and hydrogen were also found. 

It is tempting to relate the temperature at which the ductile-brittle transition takes place to either the glass transition or secondary transitions (Section 5.2.6) occurring within the polymer. In some polymers such as natural rubber or polystyrene Tb and Tg occur at approximately the same temperature. Many other polymers are ductile below the glass transition temperature (i.e. Tb < Tg). In this case it is sometimes possible to relate T to the occurrence of secondary low-temperature relaxations. However, more extensive investigations have shown that there is no general correlation between the brittle-ductile transition and molecular relaxations. This may not be too unexpected since these relaxations are detected at low strains whereas Tb is measured at high strains and depends upon factors such as the presence of notches which do not affect molecular relaxations. 

It can be seen from the considerations outlined above that it is possible to exert a good deal of control upon the melting temperatures of different polymers. It must be borne in mind that, generally, factors which affect the T of a polymer also change the glass transition temperature (Section... 

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