Heat capacity data bank

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

In this paper our efforts are directed towards the establishment of a heat capacity data bank which are detailed. Of necessity, the initial steps are directed towards the equilibrium properties. Extensions to include glass transitions,... 

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. 

Out of this block the user would enter the Data Bank proper. Ue envision four interacting data banks of which we have chosen the Heat Capacity Data Bank for... 

The recommended data, as developed above, form subblock A of the Heat Capacity Data Bank. They can be retrieved as such, or they can be searched for their base (i.e., detailed references and data tables) and also for additional, non-used data (detailed references and data tables as weil as reasons for exclusion - see, for example Refs. 9 and 10). If there are no recommended data, or only insufficient recommended data are available or if information on prediction schemes is requested, block B of the Heat Capacity Data Bank is entered. 

This subblock of the Heat Capacity Data Bank contains empirical as well as theoretical prediction calculations of the heat capacity which are continuously updated. Based on the chemical structure (back-bone, side-chain) as well as on the physical state (glass, crystal, mesophase, liquid, semicrystalline, equilibrium, history), a heat capacity is retrieved. The Prediction Scheme subblock can also be searched for its base, i.e., the precise assumptions which go into the prediction, with documentation to the 1iterature. 

The final subblock of the Heat Capacity Data Bank involves programs for needed calculations in the thermal analysis field. The simple stages involve data treatment for input and output, calculation of derived functions as given, for example, in egs. 1 to 3-Further stages include the data analysis in form of Debye and Tarasov 0-temperatures and group vibration frequencies, a stage already completed (VI). Self-... 

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). 

The application of the ATHAS has produced a large volume of critically reviewed and interpreted heat capacity data on solid and liquid homopolymers. This knowledge is helpful in the determination of the integral thermodynamic functions which are also part of the data bank. Even of greater importance is the help these basic data give in the separation of nonequilibrium enthalpy and heat capacity effects as will be illustrated in a number of examples. 

The description of the theory of heat capacity and the application of heat capacity measurements have been given by Wunderlich and other researchers [2,3-17]. The most comprehensive and updated heat capacity data are collected in the ATHAS data bank (Advanced THermal Analysis) which has been developed over the last 25 years by Wunderlich (Chemistry Department, The University of Tetmessee), and coworkers. 

A DNA sequence data bank has been established, to supplement the long-established protein sequence data bank, " information from which has been used in several of the above mentioned studies. Heat-capacity data on non-biological molecules has also been entered into a data bank, and from this various thermodynamic functions of the substances may be computed. " Several... 

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... 

Varna-Nair M and Wunderlich B, "Heat Capacity and other Thermodynamic Properties of Linear Macromolecules", X-Update of the ATHAS (i.e. Advanced Thermal Analysis) Data Bank, a computerized version of the data bank of heat capacities. 

Data banks for the pure species may be viewed as a collection of data records, each containing the constants and parameters for a single chemical [e.g., the critical properties Tc, Pc-, Vc), the normal boiling point vapor pressure coefficients, heat capacity... 

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. 

Similar analyses were accomplished for more than 150 macromolecules. The data on N, 0, and 3 together with the ranges of experimental Cp data, are collected in the ATH AS Data Bank and summarized in Appendix 1. The precision of these computed heat capacities is in general better than 5%, close to the experimental accuracy. 

The reversing heat capacity and the total heat-flow rate of an initially amorphous poly(3-hydroxybutyrate), PHB, are illustrated in Fig. 6.18 [21]. The quasi-isothermal study of the development of the crystallinity was made at 296 K, within the cold-crystallization range. The reversing specific heat capacity gives a measure of the crystallization kinetics by showing the drop of the heat capacity from the supercooled melt to the value of the solid as a function of time, while the total heat-Uow rate is a direct measure of the evolution of the latent heat of crystallization. From the heat of fusion, one expects a crystallinity of 64%, the total amount of solid material, however, when estimated from the specific heat capacity of PHB using the ATHAS Data Bank of Appendix 1, is 88%, an indication of a rigid-amorphous fraction of 24%. 

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... 

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