Irradiation-induced contrast

Figure 1.48 Scheme showing the effect of irradiation-induced contrast enhancement in semicrystaiiine poiymers due to y or eiectron irradiation ... 

On the other hand, an electron beam can initiate processes in the polymeric material that yield an increase in material contrast. Spherulites with a radial arrangement of lamellae have been made clearly visible in an HEM micrograph in Fig. 1.65 through the use of irradiation-induced contrast enhancement. Similar effects of contrast enhancement can be used in polyethylenes (see Fig. 1.9 and Figs. 2.69, 2.70) [1,5]. 

UDS without staining, contrast deveiopment during eiectron microscopic inspection (irradiation-induced contrast enhancement, see Fig 1.48), TEM... 

Unfortunately, most investigators did not search for steam sterilization- or irradiation-induced chemical degradation. It should be kept in mind that degradation does not always cause increased particle sizes. In contrast, the formation of species like lysophosphatides or free fatty acids might even preserve small particle sizes, but might also cause toxicological problems (e.g., hemolytic activity). Detailed studies that involve the aspects of chemical stability are clearly necessary to permit valid statements of the possibilities of SLN sterilization. 

The gel fraction and melt viscosities of the irradiated samples indicated that in the blends LLDPE was crosslinked (Table 11.20). The tensile modulus of the LLDPE-rich blends was found to be improved by irradiation. In contrast, the PA-6 underwent predominantiy chain branching. As a result, impact strength of the PA-6-rich blends was found to be ameliorated [Valenza et al, 1994] (Table 11.20). For the neat LLDPE, the gel fraction varied from 84% at 50 kGy to 95% at 400 kGy. The gel fraction of the LLDPE decreased only marginally as the LLDPE component was reduced in the blends, e.g., from 95% at 100% LLDPE to 92% at 25% LLDPE at 400 kGy. However, at 10% LLDPE the gel fraction dropped sharply to 19%, indicating that at very low concentrations the radiation-induced crosslinking may be affected by partial solubilization of the LLDPE in PA-6 [Spadaro et al., 1993]. 

Metalloproteinase- gelatinolytic activity was expressed in A549 cells, and exposure to ionising radiation (16 Gy) increased this activity (Araya et al. 2001). In contrast to treatment with p53 sense oligonucleotide or with lipofectin alone, treatment with p53 antisense oligonucleotide almost perfectly abrogated the irradiation-induced accumulation of p53 protein in A549 cells 3 h after radiation exposure. 

The method of differential radiation induced contrast depends on enhancement of contrast in multicomponent polymers where the components have different electron beam-polymer interactions [173]. Contrast has been observed in sections of styrene-acrylonitrile/poly(methyl methacrylate) (SAN/PMMA) polymers where the PMMA exhibits a high rate of mass loss compared to SAN, creating contrast between the phases. It is well known that electron irradiation results in chain scission and crosslinking, loss of mass and crystallinity [75]. Polystyrene, polyacrylonitrile and SAN crosslink and thus are stable in the electron beam whereas polymers exhibiting chain scission, PMMA and poly(vinyl methyl ether), degrade in the beam. It is suggested that experiments be conducted on the homopolymers to determine the expected irradiation damage mechanism in the multi-component system [173]. 

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. 

The presence of irradiation-induced copper precipitates of clusters in the irradiated RPV steel (A 302B) was confirmed in more detail by Solt et al. [63], and Curich defect clusters were detected. The mean particle radius was 5 A (0.43-5.1 X 10 n/cm ), and increased to 7-9 A as the neutron fluence was raised. The clusters were considered to be composed of two components, Cu and vacancies or Cu and Mn atoms. The Cu content in the particles was determined from their magnetic vs. nuclear contrasts. 

The irradiation-induced loops in ferritic alloys, including FM steels, are generally of interstitial type as demonstrated for instance by Schaublin and Victoria [31] using the inside—outside contrast technique in TEM. 

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