Thermooxidative degradation of polyolefins in the solid state .2. Homogeneous and heterogeneous aspects of thermal oxidation

A semi-crystalline polymer such as polyethylene is expected to oxidize in a highly heterogeneous way. However, it is found that oxidation reveals numerous aspects which are easily explained by concepts coming from solution chemistry. Formal homogeneous kinetics can explain to a large extent the variation of the concentration of different functional groups with oxidation time. Oxidation of relatively large film samples does not show considerable variation in the different portions of the sample. It is only the comparison of the oxidation of films of different thickness which shows aspects that cannot be explained by homogeneous chemistry. It pertains to preferential initiation of oxidation in or through the superficial film layers. This is already clearly visible in PE-LD at 80 degrees C and becomes dominant if the oxidation temperature is decreased further. (C) 1996 Elsevier Science Limited.