Polymer heat capacity data bank

Advanced Thermal Analysis System (ATHAS) Polymer Heat Capacity Data Bank... 

GAUR and WUNDERLICH ATHAS Polymer Heat Capacity Data Bank 359... 

The polymer heat capacity data bank is, as outlined in the Introduction, is only the first step towards the establishment of a comprehensive Thermal Properties Data Bank. Presently we are expanding our efforts to include glass transition temperatures, melting temperatures and heats of fusion. In the planning stage are specific volune, compressibility, and thermal conductivity data banks, as well as the expansion to non-equilibrium properties. 

To address our efforts towards correlating the macroscopic-ally measured heat capacity data to their microscopic origin, the data base started in the 1960 s was updated and computerized about five years ago (.8). The data bank is now incorporated within ATHAS, Advanced Thermal Analysis, a laboratory for research and instruction. The data bank maintains a collection of more than 500 publications on heat capacities of polymers which includes all measurements ever reported. The publication list is updated every six months. From each publication the following information is retrieved ... 

The data bank contains in over 800 tables all information on the heat capacity of polymers. These acceptable heat capacity data have been computer processed, to derive for the first time a comprehensive set of reconmended data. At present, recommended data are available for 98 polymers. These reconmended data are summarized in the table below. They are being discussed in nine successive papers in the Journal of Physical and Chemical Reference Data (I98I/82). The first two papers have already been published (9, 10). 

Although Equation (4) is conceptually correct, the application to experimental data should be undertaken cautiously, especially when an arbitrary baseline is drawn to extract the area under the DSC melting peak. The problems and inaccuracy of the calculated crystallinities associated with arbitrary baselines have been pointed out by Gray [36] and more recently by Mathot et al. [37,64—67]. The most accurate value requires one to obtain experimentally the variation of the heat capacity during melting (Cp(T)) [37]. However, heat flow (d(/) values can yield accurate crystallinities if the primary heat flow data are devoid of instrumental curvature. In addition, the temperature dependence of the heat of fusion of the pure crystalline phase (AHc) and pure amorphous phase (AHa) are required. For many polymers these data can be found via their heat capacity functions (ATHAS data bank [68]). The melt is then linearly extrapolated and its temperature dependence identified with that of AHa. The general expression of the variation of Cp with temperature is... 

An Advanced THermal Analysis scheme, the ATHAS scheme has been developed over the years and is described in Sect. 2.3.7. It was created to improve the quantitative aspects of thermal analysis and includes methods of data collection and evaluation (computation). Furthermore, it provides computer courses for distance learning, and a data bank of critically evaluated heat capacities of linear polymers and related compounds, both in downloadable information from the ATHAS website. An abbreviated data bank, of use in connection with the discussions in this book, is collected in Appendix 1. 

Figure 2.46 illustrates the completed analysis. A number of other polymers are described in the ATHAS Data Bank, described in the next section. Most data are available for polyethylene. The heat capacity of the crystalline polyethylene is characterized by a T dependence to 10 K. This is followed by a change to a linear temperature dependence up to about 200 K. This second temperature dependence of the heat capacity fits a one-dimensional Debye function. Then, one notices a slowing of the increase of the crystalline heat capacity with temperature at about 200 to 250 K, to show a renewed increase above 300 K, to reach values equal to and higher than the heat capacity of melted polyethylene (close to the melting temperature). The heat capacity of the glassy polyethylene shows large deviations from the heat capacity of the crystal below 50 K (see Fig. 2.45). At these temperatures the absolute value of the heat capacity is, however, so small that it does not show up in Fig. 2.46. After...

The quite complicated temperature dependence of the macroscopic heat capacity in Fig. 2.46 must now be explained by a microscopic model of thermal motion, as developed in Sect. 2.3.4. Neither a single Einstein function nor any of the Debye functions have any resemblance to the experimental data for the solid state, while the heat capacity of the liquid seems to be a simple straight line, not only for polyethylene, but also for many other polymers (but not for all ). Based on the ATHAS Data Bank of experimental heat capacities [21], abbreviated as Appendix 1, the analysis system for solids and liquids was derived. 

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 graphs of Fig. 5.17 illustrate the experimental heat capacities for polyethylene. A number of other polymers are described in the ATHAS Data Bank in the Appendix. The curves in the upper left graph show linear crystallinity dependence. For the fiilly crystalline sample (w. = 1.0) there is a temperature dependence of the heat capacity up to 10 K (single point in the graph), as is required for the low-temperature limit of a three-dimensional Deltye function. One concludes that the beginning of the frequency spectrum is, as also documented for diamond and graphite in Fig. 5.16, quadratic in... 

By finding the equilibrium heat capacities of the solid and liquid [Cp(vibration), Cp(liquid)], as well as the equilibrium transition parameters T, A//f(100%), all thermodynamic functions, enthalpy (//), entropy (5), and Gibbs free energy (G), can be calculated as a function of temperature for equilibrium conditions [3]. All recommended results of equilibrium quantities and parameters, for over 200 polymers, have been collected and organized as part of the ATHAS Data Bank, a part of which is available online [20]. 

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