Polyoxymethylene heat capacity

Fig. 4. The heat capacity of hexagonal polyoxymethylene from simulation [6] (open circles) compared with experiment [14] (fiU ch-cles). The dadied line is for orthorhombic polyoxymethylene from simulation...

The heat capacity of polyoxymethylene, determined as above [6], is shown in Fig. 4 along with experimental values [14]. It is worth noting that the heat capacity experiments must be performed on specimens that are semicrystalline. Extrapolations are made vs crystallinity to arrive at the values tabulated and shown as experimental. Thus there can be uncertainties due to the extrapolation process and therefore the calculated values form a valuable check on the experimental ones. 

The analysis of heat capacity of a given homopolymer thus starts with the evaluation of the experimental crystalline and amorphous heat capacities over as wide a temperature range as possible. For amorphous polymers, the glassy and liquid heat capacities are directly measurable. For crystallizing polymers, the crystalline and amorphous heat capacities may have to be extrapolated, as illustrated in the polyethylene example in Fig. 5.17. Only in rare cases are almost completely crystalline polymers samples available (as for example, for polyethylene, polytetrafluoroethylene, polymeric selenium, and polyoxymethylene). 

Since group vibrations are not affected much by their chemical environment, it becomes possible from the table in Fig. 5.19 to compute the heat capacity not only of polyoxymethylene, but also of all other aliphatic polyoxides, as shown in the bottom figure. The abbreviations are to be translated as follows ... 

The heat capacities of liquids are much more difficult to understand. The motion now also involves large-amplitude rotations and translations. Since, however, in the liquid state polymers are usually in equilibrium, measurements are more reproducible, as is shown using the example of polyoxymeth-ylene in Fig. 5.22. The graph is a direct copy of 36 runs of differently treated polyoxymethylenes. The almost vertical approach of some of the curves to the liquid heat capacity is caused by the end of melting of crystallized samples. 

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