Poly large amplitude motion

Glass transitions involve mainly the onset or freezing of cooperative, large-amplitude motion and can be studied using thermal analysis. Temperature-modulated calorimetry, TMC, is a new technique that permits to measure the apparent, fiequency-dependent heat capacity. The method is described and a quasi-isodiermal measurement method is used to derive kinetic parameters of the glass transitions of poly(ethylene terephthalate) and polystyrene. A first-order kinetics expression can describe the approach to equilibrium and points to the limits caused by asymmetry and cooperativity of the kinetics. Activation energies vary from 75 to 350 kJ/mol, dependent on thermal pretreatment. The preexponential factor is, however, correlated with the activation energy. 

The heat capacities of liquids are much more difficult to understand. The motion involves now also large-amplitude rotations and translations. Since, however, in the liquid state polymers are usually in equilibrium, measurements are more reproducible, as is shown in Fig. 2.58 using the example of poly(oxymethylene), POM. The graph is a direct copy of 36 runs of differently treated poly(oxymethylenes). The almost vertical approach of some curves to the liquid Cp is caused by the end of melting of crystallized samples. All curves superimpose when the samples are liquid. 

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