Correlation between chain microstructural changes and embrittlement of LLDPE-based films during photo- and thermo-oxidative degradation

The changes to the polymer microstructure during thermo- and photo-oxidative aging of LLDPE-based films have been investigated by thermal fractionation. During thermo-oxidative aging there was an exponential increase in the rate of the reduction in the fraction of polymer segments with a methylene sequence length (MSL) of approximately 249. All films embrittled only after the fraction of these segments decreased to below 7.5%, suggesting that the thermal fractionation data can be correlated to embrittlement time. In contrast, during photo-oxidative aging there was an initial rapid reduction followed by a gradual increase in the relative fraction of segments with an average MSL of 249. The initial decrease in the fraction of segments with an average MSL of 249 was similar to what was observed during thermo-oxidative aging and was consequently attributed to chain scission and/or oxidation of chains including tie-molecules. The latter increase in the fraction of chains with this average MSL was attributed to alkyl radical recombination which would result in an overall loss of branches and the formation of chain segments with high MSLs.