Single glass transition criterion

Qeveral criteria have been used to examine the miscibility or compati- bility of two or more polymers in the bulk phase (2). These include the presence in the mixture of mechanical integrity, optical transparency, a single glass transition temperature (I, 12), and homogeneity on a sub-microscopic level as revealed by electron microscopy. In each case the presence of the property listed has been taken as evidence for compatibility. However, it is by no means clear that if a system satisfied any given criterion, it would satisfy one or more of the other criteria, nor, in most cases, is it obvious to what extent the mixing of two polymers would have to be carried to satisfy any of these criteria. One may visualize a spectrum of miscibility ranging from the most intimate, at the polymer... 

The most common and reliable criterion for establishing that two polymers are compatible is the existence of a single glass transition and many of the papers cited above have used this criterion. The alternative criteria such as optical clarity can often be misleading. However, it is not always possible to detect a glass transition, especially with crystalline polymers. In this case it is more common to use information such as the melting point depression of one component. 

The generally accepted criterion of polymer misdbility is the detection of a single glass transition temperature (T, the value of which is intermediate between those corresponding to two-component polymers. The variable temperature ellipsometric technique has been used widely to determine Tg in thin polymer films by... 

The criterion for miscibihty here is based on the experimental observation of a single, very hroad glass transition from DSC. 

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