Irradiation-induced fixation

Figure 1.9 Banded spherulites in LORE with a radiai arrangement of iameiiae, twisted in concentric rings (irradiation-induced fixation of the materiai, UDS, TEM)...

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. 

Studies on the effect of electron beam irradiation on PVC/ENR, EVA/ENR and PVC/NR blends indicated that the blends undergo crosslinking upon irradiation. An acceleration of crosslinking occurs in the blends with the addition of MFA. Thus, akin to any other polymer system, the properties of NR based polymer blends can be improved by irradiation-induced crosslinking. However, control of the amount of scission and crosslinking in either of the two phases, as well as reactions at the interfaces, is of utmost importance for morphology fixation and property enhancement of the blend system. 

Figure 2.4 Scheme for investigating structure formation (crystallization and melting) in semicrystalline polymers through the technique of irradiation-induced cross-iinking and fixation at temperatures in the melting range. After fixation of the amorphous phase in each case, the structure formed can be studied by TEM [6, 7]... 

Photochemical asymmetric fixation, a technique by which the asymmetric element is fixed through a photoreaction induced by CPL irradiation, has not been discussed in the recent literature. However, for the sake of completeness, we will briefly describe the process here. This technique normally requires that the thermal racemization process is fast enough to keep the substrate racemic during the photolysis and that the excited state racemizes much more slowly than the relaxation to the product. It is, of course, essential that there be an appreciable g factor at the irradiation wavelength. Under these conditions, the product s op is governed simply by the ratio of the extinction coefficients at the irradiation wavelength. Only a few reactions have been reported to satisfy these criteria, and none of these more recently than the mid 1970s [11, and refs, therein]. 

Fig. 11 Molecular mechanism of light-induced SME of a grafted polymer network the photosensitive groups open triangles) are covalently linked to the permanent polymer network filled circles, permanent crosslinks), forming photoreversible crosslinks filled diamonds), fixation tmd recovery of the temporary shape are realized by UV light irradiation of suitable wavelengths. Reprinted by permission from Macmillan Pubhshers Ltd Nature, [36], copyright 2005...

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