Yuefang Zhang

Modification of polytetrafluoroethylene by radiation—1. Improvement in high temperature properties and radiation stability

Polytetrafluoroethylene (PTFE) has never been reported to form a network structure when subjected to high energy radiation. Results obtained in this work indicates that when irradiation is performed under 330-340-degrees-C in vacuo PTFE can be crosslinked

Radiation crosslinking of polytetrafluoroethylene

Abstract Polytetrafluoroethylene (PTFE) is considered to be a typical radiation degradative polymer. In this work the polymer was crosslinked by means of radiation at 335 ± 5°C under vacuum. Crosslinked PTFE showed a great improvement in high temperature properties and radiation stability.

The compatibility and morphology of chitosan-poly (ethylene oxide) blends

Abstract Thermal behavior and morphology of blends prepared by solution casting of mixtures of chitosan and poly(ethylene oxide) were studied by means of differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). The preliminary results indicate that both melting point and crystallinity depend on the composition of the blends, and that they exhibit minimum values when the blend contains 50% chitosan. From the prediction of melting point depression analysis, the compatibility of the blends shows a transition at this specific composition. This conclusion was further confirmed by observation of the morphology.

Radiation-induced crystal defects in PTFE

Changes induced in the crystal structure of PTFE by irradiation at different temperatures have been investigated. In the dose and temperature range examined, the density of PTFE was observed to increase continuously with increasing dose due to the radiation-induced increase in crystallinity, while after post-irradiation annealing at 380-degrees-C, the density was observed to increase for samples irradiated at 20-degrees-C, and to begin to decrease after a certain dose for samples irradiated at 150 and 200-degrees-C. On the basis of the observation of radiation-induced separation of the melting peak of PTFE and its stability relative to the change in the rate of heating, the observed decrease in density was explained as being due to the radiation-induced crosslinking and/or branching inhibiting the process of crystallization and existing in the crystalline region as defects.

XPS studies of radiation-induced structural changes in polyvinylidene fluoride

Abstract In this work the radiation-induced structural changes in PVDF were studied using XPS. It was found that for PVDF irradiated at 150°C, double bonds were formed mainly through the further dehydrofluorination of crosslinked and/or branched molecules, whereas for samples irradiated at 20°C the dehydrofluorination of macromolecular radicals to form stable polyene radicals is the main source of unsaturated structures.

Radiation stability of PTFE irradiated under various conditions

As a typical example of a polymer degraded by radiation, the radiation stability of PTFE was observed to depend upon irradiation conditions. Increases in irradiation temperature and crystallinity were found to increase its radiation stability whereas increase in the concentration of oxygen in the system over a certain range was observed to have little effect on radiation-induced reactions of PTFE as measured by changes in number-average molecular weight, melting temperature and crystallinity.

Radiation stability of perfluoroethylene-propylene copolymer

The gel content of crosslinked perfluoroethylene-propylene copolymer was obtained by extraction with fluoro-chloro oil. It was then treated by the Charlesby-Pinner equation. The results obtained revealed that both the gelation dose and the ratio of fracture to crosslinking density decreased with increasing irradiation temperature.

Studies on radiation effects of PTFE. I. Changes in crystallizing behavior and number average molecular weight of PTFE irradiated at different temperatures

Abstract The present work deals with changes in the number average molecular weight (M n) of PTFE irradiated at different temperatures. A decrease in M n was observed and attributed to the scission of polymer chains. On the other hand, it was also observed that M n decreased to a minimum value with increasing irradiation dose and then increased; the doses at which the minimum value of M n occurs are temperature dependent. The increase is explained as being due to crosslinking reactions occurring during the irradiation of PTFE.

Estimation of the radiation induced damage in PTFE by depression of the melting and crystallization temperatures

Abstract The radiation induced depression of the melting and crystallization temperatures of PTFE irradiated at various temperatures followed by heat treatment at 380°C, and their relationship to structural changes, were investigated. The G(-units) values obtained in this work are different from those of samples which have not undergone heat treatment and seem to be more closely associated with radiation induced branched structures.

Studies on radiation effects of PTFE. II. Radiation–induced structural changes and the temperature effect

Abstract In this work PTFE sheets irradiated with γ rays at 150°C and 200°C were studied using x-ray photoelectron spectroscopy (XPS). The main structural changes in PTFE due to radiation are the formation of CF3 and CF groups. An irradiation temperature dependence of the relative content of the three kinds of groups in irradiated PTFE was observed. The CF3 groups, especially when irradiation is carried out a lower temperatures, can defluorinate in the same manner as previosly reported for CF2 groups. The CF groups, on the other hand, are observed to increase with increasing irradiation dose and irradiation temperature; the latter was explained as due to an increase in branching structures.