Analyse der viskoelastischen Eigenschaften von Poly(tetrafluorethylen) im Bereich des Beta-Übergangs

The ß-relaxation of poly(tetrafluoroethylene) (PTFE) takes place at room temperature. Therefore, an in-depth understanding of the viscoelastic performance is of fundamental importance and of particular interest for technical applications. The ß-relaxation is admittedly complicated by two first order transitions in the crystalline phase at 19°C and 30°C. The basis of the investigations are isothermal creep tests on preconditioned PTFE sheets with a degree of crystallinity of 27%. The creep compliance is converted into relaxation data by an approximation method, because the analysis of relaxation curves by the Two-Component-Model (TC-Model) provides more reliable results. The TC-model takes into account the morphological structure of semicrystalline polymers and, consequently, provides information on the processes in the different morphological phases as well as a consistent description of the mechanical performance of the material. As a result of a first analysis, the exponent m of the TC-Model shows a dependency on temperature. In the ß-transition region the exponent shows a minimum of 0,11. This is inconsistent with the observation of Struik who found a material as well as a temperature invariance of the overall relaxation phenomenon. The dependency of m on temperature can be described by a Gaussian function. Outside the ß-transition region, the threshold value of 0,3 proves Struiks postulated value of m~1/3. In order to exclude that the dependency on temperature is just an artefact resulting from the curve fitting analysis to not less than four parameters or a result of an interaction of two different processes, modified fitting analyses by an modified TC-Model which is extended by another relaxation function are carried out. None of these analyses give a consistent description of the material performance. In this way the dependency of m on temperature needs to be accepted. To minimise the scattering of the fitting process the gauss description of m is used as a specification in a final restricted fitting. The change of the limiting elastic fraction of Young’s modulus shows the influence of the first order transition at 30°C. The relaxation strength confirms the existence of a viscoelastic transition. The change of relaxation times t exhibits an exceptional discontinuity at 30°C, associated with an increase of two decades. Previously a quick sigmoidal decrease in the region of the first order transition at 19°C is observed. This result is confirmed by Thermally Stimulated Creep Recovery experiments with Thermal Sampling. Within the distribution of activation energies, these experiments show two maxima of 278kJ.mol-1 and 214kJ.mol-1 at 15°C and 30°C. This result corresponds with the predicted distribution of activation energies of the TC-model and confirms the cooperative character of the ß-relaxation. In the domain of linear viscoelasticity, dynamic parameters can be calculated from static values. Despite the complex processes at the ß-transition, the dependency of the predicted dynamic values of the TC-Model on temperature and on frequency corresponds to independent dynamical measurements. Both, TC-Model and dynamical measurements show a discontinuous spectrum of tand around 19°C caused by the transition in the crystalline phase and confirm the link of the viscoelastic performance with the first order transitions in the crystalline phase. Consequently the ß-transition can be attributed to the region which behave viscoelastic because of imperfections in the crystalline phase.

Grantor dc:publisher

Publikationsserver der RWTH Aachen University

Year dc:date

2003

Author dc:creator

• Hying, Klaus

Contributors dc:contributor

• Höcker, Hartwig

• info:eu-repo/classification/ddc/540
• Polytetrafluorethylene
• Teilkristalline Struktur
• Viskoelastizität
• Phasenumwandlung
• Temperaturabhängigkeit
• Chemie
• PTFE

Statement dc:rights

• info:eu-repo/semantics/openAccess

Language dc:language

ger

Identifier

https://publications.rwth-aachen.de/search?p=id:%22RWTH-CONV-120977%22

OAI identifier oai:identifier

oai:publications.rwth-aachen.de:59167