ISSN:
0098-1273
Schlagwort(e):
Physics
;
Polymer and Materials Science
Quelle:
Wiley InterScience Backfile Collection 1832-2000
Thema:
Chemie und Pharmazie
,
Physik
Notizen:
The heat of fusion of virgin and melt-processed polytetrafluoroethylene (PTFE) was determined using the Clapeyron equation. Experimental data were obtained from PVT experiments and high-temperature x-ray diffraction measurements. For virgin, as-polymerized PTFE, the melting temperature is given by \documentclass{article}\pagestyle{empty}\begin{document}$$ T_{\rm m} = A + BP $$\end{document} where, for Tm in degrees Celsius, A = 346.3±1.2, B = 0.095±0.003, and P is the pressure in kilograms per square centimeter. At the end of the atmospheric-pressure melting interval, the amorphous and crystalline specific volumes V1 and Vc are 0.6517 and 0.492 cm3/g, respectively. Thus the heat of fusion is 24.4 cal/g, or nearly twice the value reported previously. The increases in enthalpy and volume at the melting point both indicate a degree of crystallinity of about 75-80% although infrared, x-ray, and NMR data give much higher levels. Data from calorimetry, NMR, and dynamic mechanical measurements indicate that in virgin PTFE some of the crystals continue to experience torsional oscillations at temperatures below the room-temperature transitions. This indicates that there are at least two kinds of crystalline regions. For previously melted PTFE, Tm is determined by A = 328.5±0.7 and B = 0.095±0.002, the volumes are Vam = 0.6349 and Vcr = 0.4855 cm3/g, and the heat of fusion is 22.2 cal/g. The entropy of fusion for PTFE is much closer to that of polyethylene than was previously believed.
Zusätzliches Material:
8 Ill.
Materialart:
Digitale Medien
URL:
http://dx.doi.org/10.1002/pol.1982.180200416
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