Double-oriented samples

Fig. 4.6. Doubled extinction angle 2y (closed triangles) and doubled orientation angle 2% (open circles and triangles) as function of shear rate q, and loss angle 6 as a function of angular frequency (closed circles, connected by dashed lines) for the melts of two polypropylene samples. Data of samples are given in Table 3.3. Measurement temperature 210° C (36)...

Infrared linear dichroism measurements may be performed on unidirectionally oriented samples. In the case of DNA, the favored axis of orientation is the axis of the double helix. Films are prepared by unidirectional stroking while the sample is gently dried. Two spectra are recorded with the electric field of the incident light oriented parallelly... 

Quite recently, Finkelmann and co-work-ers [232, 233] showed that an appropriate mechanical deformation of an SmC elastomer 50 yields a permanent macroscopically uniform orientation. This process also unwinds the helicoidal superstructure, and accordingly, frequency doubling is observed where the intensity of the SHG is directly related to the perfection of the uniform smectic layer orientation. The 22 < 23= 34 coefficients for a highly oriented sample were reported to be 0.1 pm/V and 0.15 pm/V, respectively. Taking into account that only 50% of the mesogenic units in the LC elastomer are active groups, these values are of the same order as those reported for low molar mass LCs containing similar chromophores [222]. 

The ease of sample handling makes Raman spectroscopy increasingly preferred. Like infrared spectroscopy, Raman scattering can be used to identify functional groups commonly found in polymers, including aromaticity, double bonds, and C bond H stretches. More commonly, the Raman spectmm is used to characterize the degree of crystallinity or the orientation of the polymer chains in such stmctures as tubes, fibers (qv), sheets, powders, and films... 

Raman microscopy provides a spatial resolution slightly better than IR, and no sample preparation is necessary in many cases. It has advantages with special types of substances (e.g., systems containing conjugated double bonds, oriented systems, amorphous and crystalline carbon, oxides). SNOM techniques (with spatial resolution below 1 pm) have been more popular with Raman than with IR, so far, but as yet are not routinely practiced. 

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