Radiation, thermo-oxidative and storage induced changes in microstructure, crystallinity and dielectric properties of (un)oriented isotactic polypropylene

摘要

Orientation and radiation processing of polymeric materials are the main steps in certain modern technologies and have extensive applications. The properties of such treated polymers can change significantly in time and under various exploitation conditions and therefore, investigation of some specific effects of aging and storage of these systems is of great importance, although the overall degradation behavior of iPP is well known. In this work, isotactic polypropylene (iPP) samples were oriented via solid-state stretching at elevated temperature (at 100°C, up to draw ratio λ=20). This was followed by gamma irradiation in the air (up to 700 kGy) and/or thermo-oxidative (TO) aging (in oxygen, at 100°C, up to 8 days) and/or storage (at room temperature, up to 3 years). As the iPP is very sensitive to orientation and degradation, microstructures were analyzed by optical (OM) and scanning electron microscopy (SEM). Wide-angle X-ray diffraction (WAXD) and differential scanning calorimetry (DSC) were used to monitor changes in structure and recrystallization. Considering the fact that at higher irradiation doses (＞200 kGy), iPP starts to gel under the ambient conditions, gel measurements were employed to determine the yield of network formation induced by irradiation of stretched structures. Irradiation, TO degradation and storage, can induce substantial alternations of (di)electric properties which can be properly connected to the changes in microstructure and crystallinity; therefore, dielectric loss analysis was performed in a wide temperature range and polar groups that were introduced in apolar iPP by oxidation were considered as tracer groups.