Polymer glass formation temperature dependence

The viscosity dependence of intramolecular excimer formation is complex. As in the case of molecular rotors (Section 8.2), most of the experimental observations can be interpreted in terms of free volume. However, compared to molecular rotors, the free volume fraction measured by intramolecular excimers is smaller. The volume swept out during the conformational change required for excimer formation is in fact larger, and consequently these probes do not respond in frozen media or polymers below the glass transition temperature. 

The correlation time tc of the motions involved in intramolecular excimer formation is defined as the reciprocal of the rate constant ki for this process. Its temperature dependence can be interpreted in terms of the WLF equationb) for polymers at temperatures ranging from the glass transition temperature Tg to roughly Tg +100° ... 

Spin-spin relaxation times (T2) in polymer systems range from about 10-5 s for the rigid lattice (glassy polymers) to a value greater than 10-3 s for the rubbery or viscoelastic state. In the temperature region below the glass transition, T2 is temperature independent and not sensitive to the motional processes, because of the static dipolar interactions. The temperature dependence of T2 above Tg and its sensitivity to low-frequency motions, which are strongly affected by the network formation, make spin-spin relaxation studies suitable for polymer network studies. 

For pregelatinised starch or denatured proteins, no cooperative unfolding transitions are observed, and the systems are more similar to synthetic polymers. A similar plastic mass is formed, but in this case its formation is largely determined by the glass transition of the amorphous system and subsequent disruption of any hydrogen-bonded structures. This temperature depends primarily on moisture content, and the... 

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