Scission, radiation-induced irradiated

PMMA has been known, since the 1950s, to show the ESR spectrum with five intense and four weak hyperfine lines (see the insert of Fig. 9), when it is irradiated with y-rays at room temperature. After a long history of the study on this anomalous ESR spectrum [32-34], the interpretation is now almost settled that it is due to the propagating-type radical -CH2-C(CH3)COOCH3 (the radical expected to form during the radical polymerization of methyl methacrylate) [35, 36], Although the formation of this radical is a definite proof of the radiation-induced scission of the PMMA main-chain, the previous ESR studies have failed to elucidate the mechanism for the formation of this radical. 

This work shows the application of quantitative pyrolysis-gas chromatography coupled with infrared spectroscopy and electron impact mass spectrometry in the study of radiation-induced scission of bisphenol-A polycarbonate (PC). PC under vacuum was gamma-irradiated using a 60Co source in the dose range from 0.125 to 1.0 MGy. This was followed by flash pyrolysis under an inert atmosphere observed by GC-FTIR-MS. Pyrolysis of the irradiated PC gave different products depending on the dose. Yields of carbon dioxide and methane decreased with dose whereas those of phenol and 4-methylphenol increased. The yields of benzene and toluene were unaffected by irradiation. Analysis of the products in this study helped to infer two main pathways for the radiation-induced scission of PC that involve carbonate bond rupture or aliphatic-aromatic bond rupture. 30 refs. 

Crystallinity has been studied by x-ray irradiation (85). An initial increase caused by chain scission in the amorphous phase was followed (above 3 kGy or 3 X 10 rad) by a gradual decrease associated with a disordering of the crystallites. The amorphous component showed a maximum of radiation-induced broadening in the nmr at 7 kGy (7 x 10 rad). 

Creep rates of three glassy polymers are much greater during electron irradiation than before or after. Radiation heating is eliminated as a possible cause. Essentially the same concentration of unpaired electrons and ratio of cross-linking to scission were found in polystyrene samples in the presence or absence of stress. The effects of radiation intensity, stress, and temperature on creep during irradiation are examined. The accelerated creep under stress is directly related to a radiation-induced expansion in the absence of stress. This radiation expansion is decreased by increase in temperature or plasticizer content and decrease in sample thickness. It is concluded that gas accumulation within the sample during irradiation causes both the expansion under no stress and the acceleration of creep under stress. 

The mechanism of y-ray irradiation-induced scission of polyisobutylene was studied, based on the structural characterisation of end-groups by 13C-NMR as well as GC, GC/ MS, and SEC [77], The assignments of signals were made by comparison with those from model compounds and predictions based on empirical rules. Quantitative 13C-NMR measurements of chain-ends allowed the determination of radiation yield of products and of chain scission. 

Radiation-induced crosslinking of acetylene impregnated polymers — Enhanced crosslinking and reduction in chain scission are found in the amorphous regions of polycrystalline polyesters, when they are irradiated in the presence of acetylene [7], Similar effects have been observed in the crosslinking of some biopolymers which are otherwise radiation degradable. 

Deterioration of polymers, PO in particular, resulting from y- or other high-energy irradiation, takes place in diverse areas of their application. This includes controlled modification of polymers for industrial purposes, use of polymers in some radiation exposed parts of nuclear reactors and radiation sterilization of food packaging materials or of equipment and materials for medical uses. Radiation-induced primary free radicals result from either the scission of the main chain... 

The blends were irradiated with electrons, then the radiation-induced chain scission (PMMA) and crosslinking (PS) were measured. The results show that in the freeze-dried blends, PS protects PMMA against radiation-induced chain scission, and PMMA inhibits the radiation-induced cross-linking of PS. The authors concluded from these results that there was a considerable amount of intermixing of these two immiscible polymers, in their freeze-dried blends. 

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