Glass-rubber transition thin films

The thin film technique described in Sect. 2 has been applied to PMMA [37, 38] from room temperature to the glass-rubber transition temperature. 

In unsaturated rubbers, the dominant chemical process is cross-linking, due to radical formation in the presence of high concentrations of double bonds. Increased cross-link density reduces the average molecular weight between cross links and causes an increase in modulus until the glass transition temperature rises sufficiently above the use temperature that the polymer becomes brittle. If the sample is thin enough, crack propagation under load then causes mechanical failure. Embrittlement failure of this type is typical of rubbers in film or sheet form. In extreme cases, the rubber may deteriorate to a friable powder. 

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