Semicrystalline polymers heat capacity

Schick C, Wurm A, Mohammed A (2001) Vitrification and Devitrification of the Rigid Amorphous Fraction of Semicrystalline Polymers Revealed from Frequency-dependent Heat Capacity. Colloid Polymer Sci 279 800-806. 

Menczel, J. and Wunderlich, B. Heat capacity hysteresis of semicrystalline macromolecular glasses. J. Polymer Sci., Polymer Letters Ed. 19, 261 (1981)... 

The term QAP is the power required to pressurize the melt (pumping power) and represents less than 5% of the total power required for the process (9). Hence, as can be seen from Equation 3, the power requirement is essentially determined by the product CQAT. Typical processing temperatures are listed in Table III (10, 11. 12). Thus, it is found that high-density polyethylene requires the most power per pound of product while polystyrene requires the least. It should be noted that the average heat capacity values in Table II include the heat of fusion for the semicrystalline polymers such as high- and low-density polyethylene. 

The first step in the analysis must thus be to establish the crystallinity dependence of the heat capacity. In Fig. 2.44 the heat capacity of polyethylene, the most analyzed polymer, is plotted as a function of crystallinity at 250 K, close to the glass transition temperature (T = 237 K). The fact that polyethylene, [(CH2-)xl, is semicrystalline implies that the sample is metastable, i.e., it is not in equilibrium. Thermodynamics... 

Finally, the left curves of Fig. 2.45 show that above about 260 K, melting of small, metastable crystals causes abnormal, nonlinear deviations in the heat capacity versus crystallinity plots. The measured data are indicated by the heavy lines in the figure. The thin lines indicate the continued additivity. The points for the amorphous polyethylene at the left ordinate represent the extrapolation of the measured heat capacities from the melt. All heat capacity contributions above the thin lines must thus be assigned to latent heats. Details of these apparent heat capacities yield information on the defect structure of semicrystalline polymers as is discussed in Chaps. 4-7. 

The reversing specific heat capacity in the glass transition region is illustrated in Fig. 6.52 [21 ]. The analysis in terms of the ATHAS Data Bank heat capacities shows that there is no low-temperature contribution due to conformational motion below the glass transition. The glass transition of the semicrystalline sample is broadened to higher temperature relative to the amorphous sample, as found in all polymers. Of... 

This table includes all data collected, measured, and updated as of November 1994. Please correspond with us about improvements, new data, errors, etc. In the column of the table labeled, (a) represents the amorphous sample, and (c) represents the 100% crystalline sample the mark represents heat capacities for semicrystalline polymers the mark next to the reference numbers, given in italics, indicates that an update is available only in the ATHAS Data Bank. The last line for each entry lists the abbreviation under which data can be retrieved in the computer version of the data bank, available in our web-site, and also listed the reference number to the last update on the given entry. At this reference, information on the source of the experimental data can be found. 

The apparent reversing heat capacity shows two distinct kinetic processes of almost equal magnitude, but with time-scales differing by a factor of about 10. The data fit a double-logarithmic function and can be extrapolated to infinite time. Figure 4.75 reveals that this extrapolation reduces the apparent heat capacity to the reversible heat capacity of a semicrystalline polymer of about 15% crystallinity, , but not the 35% crystallinity that was still present after the experiment. This difference in heat capacity must be a reversible latent heat. 

In semicrystalline polymers, the heat capacity of the amorphous phase is larger than the heat capacity in the crystalUne phase. This implies that the heat capacity values depend on the percentage of the polymer s crystallinity. 

The symbol Lg derives from latent heat, ie, the recoverable heat in a reversible process. Thermolytic cleavage of primary chemical bonds in the polymer backbone to produce volatile fuel and char is obviously not a reversible process, but the symbol Lg will be used throughout to conform with the literature in the fire sciences. Table 2 illustrates the magnitude of these enthalpic terms for amorphous PMMA, polystyrene (PS), and semicrystalline polyethylene (PE). The stored heat Ahs was obtained by numerical integration of heat capacity versus temperature from ambient to the dissociation temperature as per equation 24. The dissociation... 

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