Irradiation-induced contrast enhancement

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... 

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. 

For photon Irradiation these effects are generally very pronounced. In contrast, with increasing LET the fractionation effect decreases and eventually completely disappears in some cases even an enhanced effectiveness is reported [85]. This indicates, that no repair of radation damage is possible in the Interfractlonatlon interval, supporting the view of more complex damage Induced by particle radiation. 

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