Orientation in polymers

Deuteron NMR — A new Tool for Studying Chain Mobility and Orientation in Polymers... 

The aim of this review article is to discuss the type of information which can be obtained concerning orientation in polymers by spectroscopic methods and to select examples which illustrate key problems in the area of polymer physics. 

There are two major aspects to this discussion of orientation in polymers. First, there is the question of defining orientation, and the information which can be obtained in principle by any given spectroscopic technique regarding orientation in a polymer. This leads directly to the problem of relating orientation to deformation mechanisms, because this may permit comparatively limited information to be put to optimum use. Secondly, there is the relationship between orientation and physical properties, especially mechanical properties, where such information has been valuable in stimulating and assessing practical developments such as high modulus polymers. 

Although it would be possible in principle to choose any set of orthogonal axes in the structural unit to define Ox,x2x3, it is implicit in the discussion of orientation in polymers that the structural unit also has at least orthorhombic symmetry (Point group D2) with regard to the development of orientation. As can be appreciated this can lead to an element of awkwardness in dealing with the results of orientation measurements, because the molecular situation is often more complicated. 

In this review recent theoretical developments which enable quantitative measures of molecular orientation in polymers to be obtained from infra-red and Raman spectroscopy and nuclear magnetic resonance have been discussed in some detail. Although this is clearly a subject of some complexity, it has been possible to show that the systematic application of these techniques to polyethylene terephthalate and polytetramethylene terephthalate can provide unique information of considerable value. This information can be used on the one hand to gain an understanding of the mechanisms of deformation, and on the other to provide a structural understanding of physical properties, especially mechanical properties. 

Desper, C. Richard, "Technique for Measuring Orientation in Polymers", AMMRC Report TR 73 50, Army Materials and Mechanics Research Center, Watertown, MA, 1973. 

Birefringence is one of the simplest methods for the characterization of molecular orientation in polymers. The polarizability of a structural unit is usually not equivalent in all directions, leading to three independent refractive indices along its principal axes. In an isotropic sample, a single averaged macroscopic refractive index is observed whereas birefringence or trirefringence is observed... 

Because the determination of the order parameters by Raman spectroscopy is not straightforward, some works have focused on using intensity ratios to evaluate the molecular orientation (see above). Frisk et al. [56], in particular, have shown that the simple parameter R = 1 — Ixx/Izz — 1 — (axx)/(azz) can efficiently, although qualitatively, characterize orientation in polymers. 

Comparison of Rate Constants and Steric Factors of Bimolecular Reactions with Frequency of Reactant Rotation and Orientation in Polymer Matrix (T = 300 K)... 

IM Ward, The measurement of molecular orientation in polymers by spectroscopic techniques, J. Polym. Sci., Polym. Symp., 58 1-21, 1977. 

In favorable situations, segmental orientation in polymer blends can be followed using the technique of IR dichroism. The results can be expressed in terms of the Hermans orientation function shown in Equation 1. 

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