Role of polymer morphology in photo-oxidation

Polymer morphology (amorphous and semicrystalline regions) plays an evident role in oxidation and/or photo-oxidation reactions. 

It is well known that the permeability of some semicrystalline polymers is reduced by increases in orientation of the amorphous phase, which greatly hinder the diffusion of the penetrant [479]. Similarly, increases in crystallinity are known to decrease the permeability of most semicrystalline polyolefins by reducing the volume available to the penetrant and by limiting the mobility of the amorphous segments nearby [1710]. 

Because polyolefins are semicrystalline, quantitative studies of the role of polymer morphology in photo-oxidation of such hetereogenous polymers are very difficult. On the contrary, ethylene-propylene copolymers are completely amorphous, and can be selected as models for the investigation of the photo-oxidation of polyolefins in general (cf. section 3.1.4). 

The embrittlement time of photoirradiated polyethylene samples increases with decreased polymer density [711, 2124]. These results can be explained by invoking the existence of tie molecules between crystalline lamellae. Oxidation damage in the region of tie molecules plays an important role in 

The photostability of quenched linear low density polyethylene (LDPE) is superior to that of annealed linear low density polyethylene. Quenched polyethylene has crystallites smaller than those of annealed polyethylene. However, the degree of crystallinity does not depend upon the method of film preparation. Consequently, quenched linear low density polyethylene contains a greater number of tie molecules and is able to attain a higher build-up of oxidation products without causing serious damage to the tie molecules [2124]. 

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