Molecular orientation in polymers

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. 

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... 

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

Nishijima, Y., Onogi, Y., Asai, T. Fluorescence method for studying molecular orientation in polymer solids. J. Polymer Sci. C, 15, 237 (1966). 

Fluorescence method for studying molecular orientation in polymer... 

Molecular orientations in polymer solids studied by the fluorescence... 

The results of measurements of intrinsic birefringence permit estimation of chain segment stifi iess in an oriented polymer chain. Birefringence is among the techniques used to evaluate molecular orientation in polymers. Figure 2.27 shows changes in birefrin-genee of PVC film on its extension at 90"C. Data in the paper show that the... 

A prime consideration in sample presentation has to be the purpose of the study. For instance in solids, one must decide whether the chemical structure of the solid is of singular importance, or whether its physical characteristics are of interest. If, for example, the requirement is to study polymorphism in drugs or minerals, or perhaps crystallinity or molecular orientation in polymers, then the integrity of these properties will need to be maintained throughout sample preparation, presentation, and measurement. Sample presentation methods that provide simple, rapid generic identification may prove inadequate for more detailed substructure investigations, such as compositional or conformational analyses. 

Raman spectroscopy is one of the optical molecular spectroscopic techniques capable of giving quantitative information about molecular orientation in polymers. A resonant Raman-active agent and/or highly anisotropic rigid rod polymeric substance incorporated into polymers can be easily detected at low concentration levels and used as an indicator of the molecular orientation of the processed polymer itself [425]. It is possible to apply resonance Raman spectroscopy to many more problems. The combined application of UV/VIS and near-IR Raman excitation may be advantageous. UV excitation selects for a small number of resonance-enhanced bands of an analyte and the measurements are made with high sensitivity and selectivity. In contrast, with non-resonance visible and near-IR excitation, numerous Raman bands occur with similar intensities for all components in the sample in proportion to their concentrations. Consequently, one obtains both average and specific information on sample composition [419]. 

This Chapter describes the use of both point mapping and global imaging techniques to study subtle spatial variations in polymer chemistry and morphology. Mapping and imaging of additives and crystallinity/molecular orientation in polymer articles will be illustrated [384]. Quantitative acoustic microscopy was reviewed [385] as well as scanning acoustic microscopy [386-388]. Laser ablation mi-croanalytical techniques are discussed in Chp. 3. 

J. A. Gailey, Determination of Molecular Orientation in Polymer Films by Infrared Spectrophotometry, Anal. Chem. 33, 1831, 1961. 

The three spectroscopic techniques for studying molecular orientation in polymers described in detail use polarised electromagnetic radiation. The principles of the techniques are outlined, relevant theory and experimental details described, and a selection of results presented. 79 refs. 

There are significant differences in the behavior of polymeric fluids in these two types of deformation, and each type of deformation has a different effect on the orientation of macromolecules. For example, uniaxial and planar extensional flows impart significant molecular orientation in polymers during flow compared to shear flows. On the other hand, biaxial extensional flow is a weak flow and does not lead to a strong degree of molecular orientation. Furthermore, the rheological response can be significantly different for a polymer in extensional flow versus shear flow. We demonstrate these differences later in this chapter. 

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