Crosslinking formation during irradiation

As shown in Table II, in the presence of polymer, the enclosed nitrous oxide is completely consumed during irradiation. In the place of nitrous oxide, nitrogen and water are formed. The yield of nitrogen or water corresponds stoichiometrically to the loss of nitrous oxide. A large G value, about 2000, is given for the disappearance of nitrous oxide. Estimation of the G value is based on the assumption that the available energy for the consumption is only that absorbed directly by the gas dissolved in the polymer solid. The G values for the formation of water and nitrogen should be equal to 2000. Moreover, the summation of the amount of the excess formation of crosslinks and unsaturation becomes stoichiometrically almost equal to the loss of nitrous oxide, as shown in Table III. The equation of material balance of nitrous oxide, therefore, should be written as follows ... 

UV irradiation, while they were disturbed at a later stage [305,306], The acceleration during the early stage is due to the lower molecular weight of PLLA caused by the chain cleavage of UV irradiation, whereas the disturbed degradation at a later stage is ascribed to the formation of double bonds and/or crosslinks by UV irradiation. 

The irradiation environment plays an important role in the evolution of polymer stability. While unsaturated hydrocarbon like acetylene [61] or divinyl benzene [62] is present in the material surrounding and provides radicals for the formation of intermolecular bridges, oxidative atmosphere, oxygen or air, promotes oxidation as the result of diffusion inside the polymer matrix. The distribution profile for carbonyl products that generated during irradiation takes a parabolic form [63]. The source of radicals may be one of the components of blends, which presents a lower stability. This case can be illustrated by various blends, EPDM/PP [64], EPDN-NR [65]. These polymer mixture show the maximum level of crosslinking at about 120-150 kGy. 

The crosslink structure study showed that during irradiation of the sample containing sulfur, but without crosslinking accelerator (sample 2/0), the participation of polysulfide crosslinks in the total crosslink density is approx. 40%. The difference between the number of polysulfide crosslinks formed upon irradiation with 122 and 198 kGy is very little. In sample 2/1.5 in which both sulfur and crosslinking accelerator are present, the number of polysulfide crosslinks is lower than in sample 2/0, and it slightly increases with irradiation dose (from 28% for 122 kGy up to 32% for 198 kGy). The presence of complex of crosslinking accelerator with sulfur promoted thereby formation of shorter crosslinks. 

FIGURE 13.1 During irradiation, carbon-hydrogen bonds are broken, forming free radicals along the backbone of the UHMWPE molecule. The reaction of two free radicals in two separate molecules results in the formation of a crosslink. 

To reduce these effects, LDPE samples were cross-linked by e-beam irradiation and then subjected to the photosulfonation process. Compared to standard LDPE, crosslinked LDPE displayed a higher content of — SO3H groups and higher surface polarity after photosulfonation. This was evidenced both by zeta potential and contact angle measurements. It is thus demonstrated that sample pretreatment by crosslinking provides more stable surfaces which maintain their polar properties during water contact. This is explained by a lower amount of extractable components as a result of radiation-induced network formation. 

It is noted (Carswell et al., 1996) that the concentration of radicals almost reaches millimolar levels in the crosslinking system, i.e. 40 times that for the uncrosslinked polymer. Species other than those responsible for network formation may also be observed, if they are stable, and then used to monitor crosslinking. The oxidation of amine-containing epoxy resins may occur during cure or on UV irradiation, and ESR has been used to identify the cation radical species formed (Figure 3.10) (St John, 1993, Fulton et al., 1998). 

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