SCISSION AND CROSSLINKING RATES DURING PHOTO-OXIDATIONOF THICK-SECTIONED POLYOLEFINS: DEPTHPROFILES FOR DIFFERENT EXPOSURES

摘要

The numbers of macromolecule scission and crosslinking events occurring duringrnphoto-oxidation are good indicators of the progress of degradation and are expected torncorrelate well with deterioration of mechanical properties. They can be derived fromrnmolecular weight distributions (MWDs) measured before and after UV exposure usingrna computer-aided procedure. MWDs of samples extracted at different depths from thernexposed surface of samples after UV exposures of various periods of time have beenrnmeasured using gel permeation chromatography and processed to yield correspondingrnscission and crosslinking rates. A high density polyethylene (HDPE), a low densityrnpolyethylene (LDPE), a linear low density polyethylene (LLDPE), a polypropylenernhomopolymer (PPHO) and a polypropylene copolymer (PPCO) showed differentrndegradation characteristics. This is partly explained by the detailed oxidationrnmechanisms displayed by the different molecules but is also partly the consequence ofrnthe different crystallinities of the different polymers. Oxidation occurs almost exclusivelyrnin the amorphous phase and, in addition, diffusion rates of reactants such as oxygenrnand radicals (that are produced by oxidation) depend on crystallinity. Oxygen-diffusion-limitedrnconditions often restrict reaction in the interior of thick samples. The scissionrnand crosslinking depth profiles obtained here provide valuable insights into thernprogress of degradation, especially when used in conjunction with other measuredrncharacteristics such as fractional crystallinity. The scission/crosslinking ratio varied withrnboth depth and time and the results indicate that the different polymers follow quiterndifferent degradation paths. Inclusion of photo-stabilizer or pigment caused significantrnchanges in the degradation depth-profile and the scission/crosslinking ratio.