Melting curve, dilatometric

Figure 5. Dilatometric melting curve of 164A. Two inflections suggest two different melting temperatures.

The melting transition in polymers is conveniently observed by dilatometric measurements of the volume as a function of the temperature. Results thus obtained for poly-(N,N -sebacoyl piperazine) are shown in Fig. 129, and for poly-(decamethylene adipate) by the lowest curve in Fig. 130. Most of the melting, as evidenced by the abnormal increase in volume, occurs within a range of 10°, and it terminates abruptly at a temperature which may be defined within 0.5°. The data shown were obtained by raising the temperature... 

Solids absorb heat on melting and, with the notable exception of ice, expand. They evolve heat when they undergo polymorphic transformation to a more stable polymorphic and contract. Consequently, dilatometric (specific volume-temperature) curves bear a close resemblance to calorimetric (enthalpy-temperature) curves. The melting dilation corresponds to the heat of fusion, and the coefficient of cubical expansion, a, corresponds to the specific heat capacity, c. The ratio cja is virtually a constant independent of temperature. 

Results from extensive dilatometric measurements of the crystallization of polyethylene from the melt are shown in Fig. 3.100. Without normalization, the data for different molar masses are first matched to Avrami curves of different exponents. Higher molar mas ses seem to fit better to lower exponents, but deviate earUer from the calculated curves. At later times the crystallization is not described by the Avrami expression. It is then governed by secondary crystallization within the spherulites. 

An example of a dilatometric curve which illustrates the successive melting of butter fat is given in Fig. 8.57. Owing to the differences in dilation between the solid and liquid state, it is possible to estimate solid/liquid ratios at various temperatures from dilatometric curves. 

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