Chemiluminescence studies on the photooxidation of isotactic polypropylene

Abstract

Photooxidation of polypropylene (PP) was investigated by chemiluminescence (CL) after UV irradiation of main wavelengths of 254 nm and 365 nm in air at an ambient temperature. From the kinetic analysis of CL intensity change just after irradiation, CL intensity at the time of the cessation of UV irradiation, (I(t=0)), was obtained. The irradiation time dependence of I(t=0) gave a time-course, where the CL intensity increased with time in the first stage and reached a steady state, and after maintaining the same level for a while, it largely increased in the later stage. The steady state intensity, which is considered the rate of peroxy radical formation, was found to be dependent on the crystallinity of PP. The lower the crystallinity is, the higher the steady state intensity is, indicating the preferential photoinitiation reaction to occur in the amorphous phase. The isothermal CL measurement at above the melting point of the samples, which were stored at 25 °C for various periods after UV irradiation, suggests that there are two types of CL emitting sources of different lifetimes; one is quenched in about one day and the other survives for more than 10 days. The temperature rising measurement also indicates the existence of two types of CL emitting sources, too. Further, thermal oxidation before UV irradiation was found to clearly promote the photooxidation. In contrast, thermal oxidation in oxygen was increasingly accelerated by the pre-photooxidation.