Irradiation-induced cross-linking

In the same manner as blends of thermoplastics or thermoplastic/ elastomer, irradiated blends of elastomers can undergo chain scission due to degradation or cross-linking, depending especially on the dose range. For example, Zurina et al. i measured tanS versus temperature for 50/50 epoxidized natural rubber (ENR-50)-EVA blends by DMA. At 60 kGy the irradiation-induced cross-link enhanced the Tg of the blend, whereas at higher dose (100 kGy), the Tg decreased due to the occurrence of oxidative degradation that broke the cross-link structure. 

Dubrovic et al. have studied the NR films crosslinked by y-irradiation under both relaxed and uniaxial deformation state by ESR. Figure 25.11 shows the ESR spectra of the spin probe dilfused non-irradiated and irradiated NR matrix at relaxed state. The intensity of the broad component is more pronounced and the narrow component is strongly reduced in case of the 200 kGy irradiation dose applied crosslinked NR films in comparison with the non-crosslinked NR films at unstretched condition. The ratio of intensity for narrow (7n) to broad (4) spectral lines reduces with application of irradiation, which is due to the decrease in sol components by irradiation induced cross-linking process. The amount of broad component is not pronounced at higher irradiation doses, which may be attributed to the heterogeneous distribution of... 

Besides chemicai staining, contrast enhancement can aiso be achieved through physical effects. This is of particular interest if the polymers do not possess the double bonds or reactive groups needed for chemical staining to work. One effect is based on irradiation-induced cross-linking processes. This mechanism, called irradiation-induced fixation and contrast enhancement in semicrystalline poiymers, is illustrated in Fig. 1.48. 

Procedure. The radiation-induced cross-linking was carried out as follows. About 0.1 g of polyethylene film was placed in a glass ampoule of 30 mm diameter and 200 mm long. Gaseous CTFE and the mixture of CTFE/butadiene was introduced into the ampoule under the gas pressure of 1 atm. after evacuation of the ampoule. The ampoule was irradiated by Y-ray with a cobalt-60 at the dose rate of 0.05Mrad/hr at room temperature. 

Radiation-Induced Cross-Linking in the Presence of CTFE/Butadiene Mixture. On the basis of the results mentioned in the previous section, it is concluded that in the irradiation of polyethylenes in the presence of CTFE the polyethylenes are mainly cross-linked through the addition reaction of the unsaturated groups contained in the main and the side chains of the polymers to the propagating graft chain radical of CTFE. Therefore, the radiation-induced cross-linking of polyethylene is expected to be accelerated by the presence of the mixture of CTFE and a diene monomer effectively than the presence of pure CTFE. 

An advantage of radiochemical induced cross-linking is the possibility to combine polymers with different properties. For that purpose, the polymers were separately dissolved in water. The solutions were mixed and irradiated. (Gottlieb et al. 2005) describes the synthesis of temperature-sensitive hydrogel blends of PVME (as thermo-sensitive polymer) and the radiation-cross-linkable polymer PVP (a polymer that is applied in pharmaceutics). The experiments show that the gelation dose of the blend is between the gelation doses of the two pure polymers. 

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. 

---