Scission backbone

However, the important considerations are sensitivity and tone (positive or negative). When covalent bonds are formed, the increase in molecular weight results in negative-acting resists. A molecular weight decrease because of polymeric backbone scissions produces positive-acting resists. 

Fig. 1 Mechanism of carbonyl formation from peptide backbone scission...

For PP, PET and PmPiPA, the primary photoinitiation steps all involve backbone scission, yet none of these polymers deteriorates appreciably in mechanical properties during irradiation with UV >290 nm in the absence of O2. In fact both PET (I7. 

The presence of extensive backbone scission during photo-oxidative degradation has been demonstrated by molecular weight (Mn) determination, based on solution viscosities. PET with an initial f n of -20,000 decreased to ll+,000 after 600 hr xenon arc irradiation (2 ). PmPiPA with an initial Mn of -1+0,000 decreased to -10,000 after 1+0 hr of xenon arc irradiation ( ). The actual extent of backbone scission is even more startling when it is remembered that degradation has occurred only in the irradiated surface layer (surface layer will be of the order of 1000, i.e., oligomer in each case. 

The surface oxidation and associated backbone scission discussed above can be expected to result in a drastic reduction in surface toughness and resistance to stress. In fact electron microscopy of the PP surface shows that the film becomes covered in... 

Distinct increases in bonds sensitive to the helical conformer of the PP backbone are also visible (for example at 997 and 841 cm-1). These changes have previously been observed in photo-oxidized samples and correlate with the backbone-scission-induced restructuring of the polymer, y-irradiation of PP film also caused a sharp drop in the elongation at break of the films (Table 1). Tensile strength was little effected. For the same total dose, both the oxidation product quantities and relative proportions as well as the elongation at break were markedly dependent on dose rate (Table 1). 

Hydroperoxide photolysis by sunlight in the presence of air is considered to be the major source of free radicals and backbone scission during the photodegradation of polypropylene [37]. 

The alkoxy radicals formed after the decomposition of hydroxy-peroxides can split by the 3 scission process which plays an important role in backbone scission and in alkyl radical formation at the end of chain ((28) and (29)) [7,125, 405]. 

The uniform oxidation of the cold-drawn filament results in a high overall degree of oxidation and associated backbone scission before surface deterioration reaches the level at which spontaneous restructuring and crack formation can occur. Lateral cohesion of these fibers was much greater than for the highly oriented filaments. For example, the latter could be easily peeled, whereas the cold-drawn fiber stretched rather than fibrillated after notching. 

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