Glass transition temperature measurement with modulated

The thermal and thermomechanical properties of the polymer/HAp composites (glass transition temperature, melting and crystallization behaviour, thermal stability, crosslinking effects, phase composition, modulus, etc.) can be evaluated by thermal analysis methods, like TG, DSC and DMA. Recently, a modulated temperature DSC (MTDSC) technique has been developed that offers extended temperature profile capabilities by, for example, a sinusoidal wave superimposed on the normal linear temperature ramp [326]. The new capabilities of the MTDSC method in comparison with conventional DSC include separation of reversible and non-reversible thermal events, improved resolution of closely occurring and overlapping transitions, and increased sensitivity ofheat capacity measurements [92,327]. 

Dealing with simple deconvolution first, it can be seen from Equation 4.3 that the heat flow signal is composed of two sets of terms. The first, Cp(b + Bto costow)), is dependent on the magnitudes of the terms b, B, and to (all experimental rather than sample parameters) but has no sample dependence other than the value of Cp. Consequently, this first term is a measure of the sample s heat capacity (as defined previously) and, except in the case of the glass transition (see later discussion), can be considered, for the purposes of this discussion, to be an effectively instantaneous sample response. If we focus on the modulation, we see that this is a cosine wave as opposed to the sine wave in the temperature. This means that, when there is no transition, the modulation in heat flow will, in principle, follow this cosine wave (the modulation in heating rate) with zero phase lag when the convention is adopted that endothermic is up. It will be 180° out of phase when the convention is adopted that endothermic is down. 

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. 

Fig. 11. Glass transitions of poly(ethylene terephthalate) with different degrees of crystallinity produced by the indicated thermal histories. The curves were measured with quasi-isothermal TMDSC at the temperatures marked by the filled circles, with modulation amplitudes extrapolated to zero and periods of 60 s (67).

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