Be careful when using X-ray exposure analysis for polymers

The techniques of X-ray diffraction analysis (XDA) and X-ray photoelectron spectroscopy (XPS) for investigation of the structure and surface constitution of materials respectively are considered to be free from radiation-induced artifacts. But earlier a very strong effect of the X-ray flux on the surface constitution of the polytetrafluoroethylene (PTFE) was observed. Later we studied a crystalline structure of the PTFE by the methods of XDA and differential scanning calorimetry (DSC) simultaneously. Quite unexpectedly the values of crystallinity degree measured by the two methods for as-received samples of the same polymer differed more than two times (26% for DSC and 68% for XDA). It was established that during an analysis by XDA the samples were irradiated by the X-ray doses 0.1-1 kGy depending on exposure. The irradiation of PTFE with such small doses of both X-ray and ⁶⁰Co γ-radiation results in a significant increase of crystallinity that is identified by the DSC method but not by the XDA one. Only after irradiation with doses higher than 10 kGy the measured crystallinity degrees for the two methods become equal. We believe the reason of the observed difference is in the capability of the DSC method to register only thermodynamically stabilized crystallites while the XDA procedure fixes the total response from crystal and paracrystal phases.