Refractive light microscopy

Microscopy (qv) plays a key role in examining trace evidence owing to the small size of the evidence and a desire to use nondestmctive testing (qv) techniques whenever possible. Polarizing light microscopy (43,44) is a method of choice for crystalline materials. Microscopy and microchemical analysis techniques (45,46) work well on small samples, are relatively nondestmctive, and are fast. Evidence such as sod, minerals, synthetic fibers, explosive debris, foodstuff, cosmetics (qv), and the like, lend themselves to this technique as do comparison microscopy, refractive index, and density comparisons with known specimens. Other microscopic procedures involving infrared, visible, and ultraviolet spectroscopy (qv) also are used to examine many types of trace evidence. 

Optical properties of fibers are measured by light microscopy methods. ASTM D276 describes the procedure for fiber identification using refractive indexes and birefringence. Other methods for determining fiber optical properties have been discussed (3,38—44). However, different methods of determining optical properties may give different results (42). 

The existence of crystal lamellae in melt-crystallised polyethylene was independently shown by Fischer [28] and Kobayashi [39]. They observed stacks of almost parallel crystal lamellae with amorphous material sandwiched between adjacent crystals. At the time, another structure was well known, the spherulite (from Greek meaning small sphere ). Spherulites are readily observed by polarised light microscopy and they were first recognised for polymers in the study of Bunn and Alcock [40] on branched polyethylene. They found that the polyethylene spherulites had a lower refractive index along the spherulite radius than along the tangential direction. Polyethylene also shows other superstructures, e.g. structures which lack the full spherical symmetry referred to as axialites, a term coined by Basset et al. [41]. 

Phase contrast light microscopy has been applied extensively to the analyses of unfilled binary elastomer combinations. This method is based on differences in the refractive indices of the polymers and has been reviewed by Kruse (1973). CaUan et al. (1965,1971a), and Scott et al. (1969) have shown the versatility of... 

Low- and high-powered microscopes are used to examine the morphological features of the fibres and the initial determination of whether the fibre is natural or man-made. FTIR microscopy can be used on a synthetic fibre to provide information in relation to the functional groups present this can be used to pinpoint which synthetic fibre it is. Polarising light microscopy is used with synthetic fibres plane-polarised Ught interacts with the fibres in order to provide refractive index values (many of these fibres have two refractive indices due to the chemical structure of the fibre and are said to be birefringent). This helps in the identification of the synthetic fibre. 

Phase contrast light microscopy has been applied extensively to the analyses of unfilled binary elastomer combinations. Ihis method is based on differences in the refractive indices of the polymers and has been reviewed by Kruse [27a]. Callan et al. [27b-d] have shown the versatility of the method for a wide range of binary blends containing NR, SBR, BR, CR, NBR, EPDM, HR, and CIIR. The results of these experiments are shown in Table III, which lists the measured areas of the disperse phase in more than 50 combinations of Banbury-mixed 75/25 binary blends containing eight different elastomers. Blends of IIR-CIIR and SBR-BR are excluded since the contrast was low. It can be seen that NBR produced the greatest heterogeneity in all blends except those with CR. 

In order to increase the contrast in light microscopy, staining with selective dye molecules that absorb light or have a dilferent refractive index is often used. This process is not widely used in the synthetic polymer field, but there are three specific instances that are worth noting ... 

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