Vacuum Radiolysis

The G-values for radical formation on vacuum radiolysis of some polyimides at 77 K are given in Table I. These radical yields provide an assessment of the relative radiation sensitivities of the polyimides. The results show that Kapton is the most radiation resistant (lowest G-value) of the polyimides, but that the stabilities of the other transparent polyimides are also... 

A study of the products produced upon vacuum radiolysis of ethylene homopolymers and copolymers is another means of obtaining information on branching in these polymers [58]. If a correction is applied, to take into account the fragments arising from scission at chain ends, the remaining products can be quantitatively accounted... 

Enhancement of the total butene yield is observed when various additives whose ionization potential falls below about 9.4 e.v. are present during ethylene radiolysis (35). This is consistent with the above interpretation (Figure 2). In the vacuum ultraviolet photolysis of cyclobutane the yield of butenes varies with the ionization potential of the additives in the same way as observed here (12). The maximum enhancement corresponds closely to the yield of C4H8+, as expected from our mechanism. 

On the other hand, the formation of ethylene was ascribed mainly to the unimolecular decomposition of a neutral excited propane molecule. These interpretations were later confirmed (4) by examining the effect of an applied electrical field on the neutral products in the radiolysis of propane. The yields of those products which were originally ascribed to ion-molecule reactions remained unchanged when the field strength was increased in the saturation current region while the yields of hydrocarbon products, which were ascribed to the decomposition of neutral excited propane molecules, increased several fold because of increased excitation by electron impact. In various recent radiolysis 14,17,18,34) and photoionization studies 26) of hydrocarbons, the origins of products from ion-molecule reactions or neutral excited molecule decompositions have been determined using the applied field technique. However, because of recent advances in vacuum ultraviolet photolysis and ion-molecule reaction kinetics, the technique used in the above studies has become somewhat superfluous. 

A third advantage that matrix isolation has over frozen solvents is that the reactive intermediates must not necessarily be generated in situ, but can be made by flash vacuum pyrolysis or in plasma processes prior to their quenching with an excess of the host gas on the cold surface. Of course, this considerably widens the range of reactive intermediates that can be investigated, beyond those that require photolysis or some form of radiolysis for their formation. 

We have studied the alkane and alkene yields from the radiolysis of copolymers of ethylene with small amounts of propylene, butene and hexene. These are examples of linear low density polyethenes (LLDPE) and models for LDPE. Alkanes from Ct to C6 are readily observed after irradiation of all the polymers in vacuum. The distribution of alkanes shows a maximum corresponding to elimination of the short-chain branch. This is illustrated in Figure 8 for the irradiation of poly (ethylene-co-1-butene) containing 0.5 branches per 1,000 carbon atoms at 20 C. 

Cobalt-60 y-Radiolysis Studies. Samples of PIPTBK powder were sealed under vacuum and exposed to 60Co 7-radiation at the National Bureau of Standards in Washington D.C. The samples received 0, 6, 12, and 20 Mrads and the number-average and weight-average molecular weight of each polymer, relative to polystyrene, were determined in THF by GPC. The scission efficiency and the cross-linking efficiency were determined from... 

Fig. 6. Scheme of the laser-driven RF electron accelerator of pulse radiolysis facility ELYSE. IP ion vacuum pump, CPC cathode preparation chamber, W vacuum valve, SOL solenoid, D dipole, TRl and 2 triplets, Q quadrupole, WCM wall current monitor, PC Faraday cup, T translator for Cerenkov light emitter and visualization screen LME laser entrance mirror, LMEx laser exit mirror, VC virtual cathode FIS horizontal slit, VS vertical slit. (Reproduced with permission from Ref 28.)... 

In the absence of oxygen, the mechanism of radiolysis of aqueous hydroxy acids is modified somewhat. During the 7-irradiation of glycolic acid solutions under vacuum, the products are glyoxylic, oxalic, and tartaric acids. These acids may be formed as follows. 

Radiolysis of liquid cresols under vacuum was found to produce H2 as the main gaseous product the radiolytic yield varied from 0.019 for m-cresol to 0.031 (rmol for the o-cresol. Small amounts of CH4 were also detected. Radiolysis of cyanophenols produced less H2, only ca 0.003 ttmol various yields of CO and CO2, mainly from the ortho isomer, and minute amounts of N2. The difference in the yield of H2 may be due to reaction of hydrogen atoms with the methyl group of the cresols to form H2 as compared with addition to the CN group and to the ring, which do not produce H2. No mechanistic details were derived from these studies. 

Figure 10 shows the isomerization kinetics for a —20/1 mixture of 2-methyl-1-pentene with S02 after a dose of 0.8 Mrad. No significant isomerization occurred upon irradiation of pure 2-methyl-1-pentene, even after 40 Mrad of absorbed dose. However, after the small dose of 0.8 Mrad 2M1P (in the presence of S02) underwent isomerization to 2M2P as seen in Figure 10. The kinetics are similar to that observed following PMPS radiolysis (compare Figure 3). The equi-molar mixtures prepared under vacuum also showed accelerated isomerization after irradiation, see Figure 11. During preparation of the 1 1 mixtures upto 20% isomerization occurred prior to any irradiation. This effect was not observed for the 20/1 mixture (Figure 10) and its cause is not known at present.

Radical formation in the polyimides on y-radiolysis under vacuum has been investigated by ESR spectroscopy. At 77 K the radical spectra of the irradiated polyimides are composed principally of two partially overlapping singlets (8,9), one of which decays when the temperature is raised to 200 K. This component has been assigned to anion radicals formed at 77 K by trapping of thermalized electrons in the polymer matrix. The other singlet has been assigned to neutral radicals with the free electron delocalized over several units of the polymer chain. These radicals are thus similar to the radicals formed on UV photolysis. Other minor radical components are also sometimes evident in the spectra, for example in that of Ultem that has been irradiated at 77 K (9). 

Table I. G-values for radical formation on y-radiolysis at 77 K under vacuum.

The radiolysis of polycarbonate presents similarities to that of aromatic polysulphones [383, 384]. It undergoes main-chain scission with a G value of 0.09 under vacuum and 0.14 in an oxygen atmosphere. The gases evolved are carbon monoxide, carbon dioxide, hydrogen and methane. The G values are, respectively, 3.6 x 10 1,1.9 x 10 1, 1.3 x 10 2 and 1.3 x 10-3. Since G(CO + C02) is larger than the G value for chain scission, cage recombination of the macroradicals is supposed to occur. 

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