Refractive indices polymer orientation, measurement

The disposition of phenyl groups in the solid amorphous polymer is of relevance to the photophysical properties. Following refractive index measurements Prest has proposed (1 5) that the phenyl units are oriented in the plane of the film and indeed a thin layer of highly strained polymer can be produced by solvent casting. Since the bulk of excitation radiation is absorbed in the surface of a sample the major component of the emission can occur from a birefringent surface layer, so that the observed polarisation/depolarisation can be dominated by this influence rather than by e.e.m., and therefore by sample preparation. 

The birefringence of textile fibers provides a measure of the molecular orientation of the polymer chain of the fiber. It might also be something of an indicator of the degree of crystallinity of the polymer chain. Birefringence is the difference between the refractive indices of the fiber in a direction parallel to the fiber axis and in a direction at right angles to the fiber axis. Details of how this refractive index is obtained are discussed elsewhere [3], but briefly the fiber is mounted in a liquid whose refractive index is known (or has been previously determined) and the fiber is then examined in plane-polarized monochromatic... 

Polymers are not, however, always randomly oriented and the phenomenon of orientation is discussed in the next two chapters, chapters 10 and 11. One of the ways used to obtain information about the degree of orientation of the polymer is to measure either the refractive indices of the polymer for light polarised in different directions or its birefringences, i.e. the differences in refractive index for light polarised in different directions. In chapter 11 the theory of the method is described in terms of the polarisability of structural units of the polymer. This polarisability is a second-rank tensor like the molecular polarisability referred to in the earlier sections of this chapter and, insofar as the assumption of additivity in section 9.2.3 holds, it is in fact the sum of the polarisability tensors of all the bonds in the unit. Since, however, the whole basis of the method is that the structural units are anisotropic, the tensors must be added correctly, taking account of the relative orientations of the bonds, unlike the treatment used to calculate the refractive index of PVC in example 9.1, where scalar bond refractions are used. 

Donald et al. [2] reported banded structures formed by several thermotropic polymers oriented by shear at temperatures above their softening points. Similar structures were also noted in fibers drawn from polymers with rigid backbones above the softening points. Viney et al. [3] point out that the banded structures observed in shear are due to the variation in the direction of the long molecular axis with respect to the direction of shear. Evidence obtained by both optical microscopy and electron diffraction measurements supports this view. Donald and Windle [4] studied the banded structure by electron microscopy and commented that The near sinusoidal variation in the direction of the principal axis of the refractive index ellipsoid is indeed reflecting the variations in the molecular orientation. Their transmission electron microscopy indicates that the transition from... 

DA Jarvis, IJ Hutchinson, DI Bower, IM Ward. Characterization of biaxial orientation in poly(ethylene terephthalate) by means of refractive index measurements and Raman and infra-red spectroscopies. Polymer 21 41-54, 1980. 

Birefringence bl-ri- frin-j9n(t)s n [ISV] (1898) (1) (double refraction) The difference between any two refractive indices in a single material. When the refractive indices measured along three mutually perpendicular axes are identical, the material is said to be optically isotropic. Orientation of a polymer by drawing may alter the refractive index in the direction of draw so... 

When an amorphous polymer is stretched, the molecules may be preferentially aligned along the stretch direction. In polymethyl methacrylate and polystyrene, such molecular orientation may be detected by optical methods, which measure the small difference between the refractive index in the stretch direction and that in the perpendicular direction. 

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