Uwe Geißler

Changes in the chemical structure of polytetrafluoroethylene induced by electron beam irradiation in the molten state

Abstract Polytetrafluoroethylene (PTFE) was exposed to electron beam radiation at elevated temperature above the melting point under nitrogen atmosphere and in vacuum for comparison. Fourier-transform infrared (FTIR) spectroscopy was used to study the changes in the chemical structure. The irradiation under nitrogen atmosphere leads to the same structures as described recently for PTFE irradiated in vacuum. Trifluoromethyl branches and double bond structures were detected. The concentrations of terminal and internal double bonds are higher after irradiation under nitrogen than in vacuum. Annealing experiments have shown that the thermal oxidative stability of the radiation-modified PTFE is reduced compared to unirradiated PTFE. The reason are the formation of unstable structures such as double bonds.

Radiation-induced branching and crosslinking of poly(tetrafluoroethylene) (PTFE)

Abstract The effect of electron beams on poly(tetrafluoroethylene) (PTFE) at elevated temperatures above the melting point on oxygen-free conditions has been studied using differential scanning calorimetry (DSC), wide-angle X-ray scattering (WAXS), Fourier-transform infrared (FTIR) spectroscopy, thermo-gravimetric analysis (TGA) and tensile test. The investigations have shown that the chemical structure and several properties of PTFE are greatly altered by the irradiation. DSC and WAXS indicate that the crystallinity of the PTFE irradiated with high doses is reduced. CF3 side groups and branched structures are assumed to hinder the crystallization. TGA has shown that the thermal stability of the radiation-modified PTFE is considerably lower than that of unirradiated PTFE.

Modification of polytetrafluoroethylene by electron beam irradiation in various atmospheres

Abstract The changes in the chemical structure of polytetrafluoroethylene (PTFE) induced by electron beam irradiation in vacuum and in ammonia gas atmosphere have been investigated by means of Fourier-transform infrared (FTIR) spectroscopy. Double bonds and trifluoromethyl branches are formed by irradiation in vacuum. The CF3 branching is enhanced distinctly by irradiation at elevated temperature above the melting point of PTFE. After irradiation in ammonia gas atmosphere ammonium fluoride salt was detected in the irradiated fluoropolymers.

Identification of new chemical structures in poly(tetrafluoroethylene-co-perfluoropropyl vinyl ether) irradiated in vacuum at different temperatures

Abstract Electron beam irradiation of perfluorinated copolymer poly(tetrafluoroethylene-co-perfluoropropyl vinyl ether) (PFA) was carried out in vacuum at different temperatures between room temperature and temperature above the melting point of PFA. Changes of chemical structure were studied by 19F solid-state NMR and IR spectroscopy. Chain scissions and branching reactions occur simultaneously, which are indicated by formation of CF3, COF and COOH end groups, CF branching points and CF3 side groups. Branching density based on concentration of CF groups increases with increasing temperature. Formation of cross-links is concluded for PFA irradiated in molten state. Perfluoropropyl vinyl ether (PPVE) units are found to be more sensitive to radiation than tetrafluoroethylene units. COF and COOH groups are assumed to result from PPVE degradation.

Apparent dissociation constants of polycarboxylic acids in presence of polycations

The dissociation behaviour of poly(acrylic acid) and poly(ethylene-alt-maleic acid) has been investigated by both classical potentiometric titration and colloid titration as an alternative way. In the case of colloid titration, the monovalent counterions are assumed to be replaced by the oppositely charged polycation in the course of titration, so that the total concentration of anionic groups of the polycarboxylic acid is determined. As a consequence the apparent dissociation constants determined by polyelectrolyte complex formation do not depend on the degree of dissociation. In contrast to this finding, potentiometric titration is known to result in apparent dissociation constants which decrease with increasing degree of dissociation. The release of protons from the acid groups of the polycarboxylic acids is promoted by the complex formation with the strong cationic polyelectrolyte.

Potentiometrische Titration von oberflächennahen Carboxygruppen in bestrahlten Fluorpolymeren

Bei der Elektronenbestrahlung von Fluorpolymeren in Anwesenheit von Sauerstoff werden Carboxygruppen in den Polymeren gebildet. Deren Konzentration kann durch potentiometrische Titration bestimmt werden. Da sich auch strahlenmodifizierte Fluorpolymerpulver von Wasser schlecht benetzen lassen, wurden verschiedene wasrige Dispersionsmittelgemische untersucht. Die Qualitat der Dispersionen wurde mittels Partikelgrosenanalyse gepruft. Es wurde festgestellt, das sich hierfur ein Gemisch aus 50 Vol.-% Ethanol und 50 Vol.-% Wasser besonders gut eignet. Die wesentlichen Vorteile gegenuber der bisher eingesetzten N,N-Dimethylacetamid-Losung bestehen darin, das die Herstellung dieses Ethanol/Wasser-Gemisches mit einem geringen Zeitaufwand verbunden ist und die Blindwerte auch bei langfristiger Lagerung niedrig und stabil bleiben.