Straining-induced contrast enhancement

An additional problem associated with the direct investigation of polymeric materials by electron microscopy is in the usually low contrast between structural details. In connection with deformation, the effect of straining-induced contrast enhancement is of particular importance. Structural elements, such as particles, which possess a smaller Young s modulus than the surroimding material, show a preferred extensibility or deformation during stretching of the sample. In the direction of the electron beam, the thickness of the sample decreases at these places, thus causing a difference in the intensity of the transmitted electron beam and an enhanced contrast (2,10). 

Rgure 1.49 Scheme showing the effect of strain-induced contrast enhancement ... 

Figure 3.23 SAN copolymer with small domains, which are visible due to a straining-induced contrast enhancement area in front of a crack tip, deformation direction horizontal, semi-thin section in HEM.

The last experiment was repeated in part using hr/hr Oslo strain mice and extended by the use of SEN mice. The data were mainly presented as summary statistics. In contrast to the earlier results, the gel without benzoyl peroxide did not enhance DMBA carcinogenesis in hr/hr mice and Panoxyl did not enhance DMBA carcinogenesis in SEN mice. Groups of 32 hr/hr mice were also treated with ultraviolet radiation (UV) from new Phillips HP 3114 sunlamps, either alone twice a week or 5-30 min before treatment with Panoxyl or the gel without benzoyl peroxide. The numbers of mice with skin carcinomas (and the total numbers of skin tumours in each group of mice) were UV alone, 26/32 (103 papillomas, 45 carcinomas) UV + Panoxyl, 22/32 (94 papillomas, 30 carcinomas) UV + gel, 23/32 (126 papillomas, 29 carcinomas). Thus, neither Panoxyl nor the gel without benzoyl peroxide had any significant effect upon the multiplicity of UV-induced skin tumours (Iversen, 1988). 

BALB/c animals display IL-12 p40 mRNA, in apparent contrast to PMN from the genetically susceptible C57BL/6 mouse strain [73]. Neutrophil depletion of BALB/c mice exacerbates infection. This is accompanied by decreased Thl cytokines (IL-12 p40, IFN-7, TNF-a) and increases in the IL-10 response in spleens of infected animals, suggesting a Thl to Th2 switch [73]. In contrast, during T. cruzi infection of susceptible C57BL/6 mice, neutrophil depletion induces resistance to disease and enhances Thl cytokine production. Neutrophils have also been implicated in induction of the CD4-I- Th2 response in L. major-infected BALB/c mice [85]. Thus, the early burst of IL-4 that instructs Th2 differentiation in this model is prevented by neutrophil depletion, and draining lymph node T cells remain IL-12-responsive, unlike cells from nondepleted mice. 

The change induced in the stress-strain behavior by incorporation of elastomeric particles into a PS matrix has been reported [15]. On toughening, PS brittle behavior changes into ductile with flow. The rubber-modifled polymer absorbs considerably more energy thus, higher elongation to break can be achieved. By contrast, an addition of rigid resin to a ductile polymer enhances the modulus and the heat deflection temperature. 

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