Glass transition temperature conformational factors

In particular, blends of PVDF with a series of different polymers (polymethylmethacrylate [100-102], polyethylmethacrylate [101], polyvinyl acetate [101]), for suitable compositions, if quenched from the melt and then annealed above the glass transition temperature, yield the piezoelectric [3 form, rather than the normally obtained a form. The change in the location of the glass transition temperature due to the blending, which would produce changes in the nucleation rates, has been suggested as responsible for this behavior. A second factor which was identified as controlling this behavior is the increase of local /rans-planar conformations in the mixed amorphous phase, due to specific interactions between the polymers [102]. 

It was noted that, above the glass-transition temperature, the polymer is able to have considerable conformational freedom involving concerted motion over 50 atoms, which results in a decrease in modulus by a factor of about 10". The polymer is still far from the melt and the chain entanglements result in a material with viscoelastic properties. The rubbery nature may be seen from the tendency of the polymer to recover when a stress is applied. This recovery is a consequence of the higher order conferred on the chains when they are distorted so that when the stress is released there will be an entropic drive to return to the coiled state (Queslel and Mark, 1989). It is this entropic recovery that results in the shrinking of a loaded crosslinked elastomer (shown schematically in Figure 1.14) when it is heated. 

Polyphosphazenes are among the most flexible polymers known. This is reflected in low glass transition temperatures (Tg). Only below the glass transition temperatures all conformational mobility is frozen and polymer becomes a glass. The skeletal flexibility of polyphosphazenes seems to arise from many factors. 

Reactions which can be considered to be associated with caged radicals, such as the photo-Fries, will require very little free volume and can be expected to be quite efficient in solid polymers below the glass transition, whereas photochemical processes such as the Norrish Type II process are substantially reduced in glassy polymers below the glass transition temperature (Tg), unless the geometry of the cyclic six-membered ring is particularly favoured by steric factors in the chain, so that the most stable conformation corresponds to that required for reaction [874]. 

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