Chain orientation spectroscopy

IR spectroscopy is not only useful for determining the chemical constitution of polymers. It additionally provides profound information on chain orientation and on the orientation of attached lateral substituents of polymers. In this case, polarized IR radiation is applied which is only absorbed by an IR-active bond if the plane in which the electrical field vector E of the IR beam oscillates is parallel to the transition dipole moment p of the vibration to be excited. If, on the other hand, the transition dipole moment p is perpendicular to the electrical field vector E of the IR beam no absorption is observed. Using this effect, the degree of orientation of a polymer sample (film, fiber) can be estimated by comparing the intensity at maximum /(11) and at minimum I ) absorption, i.e., the dichroic ratio. 

Infrared dichroism has been successfully applied to characterize the orientational relaxation of linear and branched polyst3rrene chains as well as binary blends of long and short chains. By deuterating some chains or parts of chains, infrared spectroscopy provides a method of analyzing the orientational behaviour of the different species and consequently probe the molecular relaxation mechanisms. 

Quite early on, structural investigations showed [17] that the mesogens in the main chain and the side chain orient parallel to one another. This was confirmed by electron microscopy [18], X-ray diffraction [6,7,13, 14, 17, 19, 20] and HNMR spectroscopy [21]. Following the general classification of LC side-chain polymers (Figure 2) [22], these polymers should be classified as type III. In these combined LC polymers, the polymer chains and mesogens orient parallel to one another to define the LC director. 

Polymers may exhibit a biaxial orientation. The segmental orientation function is in this case a function of two angular variables, i.e. /(, v), as shown in Figure 10.7. The in-plane orientation is different in the zx and zy planes (Figure 10.7). There are several methods commonly used to determine chain orientation in-plane birefringence, wide angle X-ray diffraction, small angle X-ray diffraction, infrared spectroscopy and sonic modulus measurements. In the case of uniaxial orientation there is only... 

IR spectroscopy is very useful for the assessment of chain orientation. Recent excellent reviews on the subject have been given by Bower and Maddams [19] and Koenig [20]. The measurement of IR dichroism requires the use of IR radiation with polarization parallel to and perpendicular to a selected reference direction. The fundamental principle underlying the dichroism is that the absorbance A is proportional to the square of the cosine of the angle between the transition moment vector and the electrical field vector. The IR radiation is strongly absorbed when the electric vector of the light and the transition moment vector are parallel. No absorption of the IR radiation occurs however if the two vectors are perpendicular to each other. The dichroic ratio R is defined as... 

Methods used to characterize orientation are wide-angle X-ray scattering. IR spectroscopy, small-angle light scattering, birefringence measurements, polarized fluorescence, and sound-velocity measurements. They refer to effects resulting partly from chain orientation, partly from crystallite orientation, and partly from a combination of both types (combined effect). 

NR-MWNT interface was confirmed using Raman spectroscopy while the composites were under strain, however deconvolution of NR chain orientation from MWNT orientation was obtained when the composites were subjected to uniaxial tension." ... 

This form is obtained by cold-drawing the polymer quenched from the melt. IR spectroscopy and WAXD techniques indicate that in this modification sPP adopts the alTtrans conformation and the crystallographic density is 0.945 g/cm. Upon annealing fibers of this form at about 100°C for a few hours, the more stable helical form results, without losing the preferred chain orientation along the stretching direction. 

The first step in a polymer analysis is to identify the specific type of polymer in a given sample. This may be complicated in a formulated sample by the presence of additives. Infrared spectroscopy will usually provide information on both the base polymer(s) and the additive(s) present. The second step, if possible, is to determine details of the chemical and physical characteristics, which define the quality and properties of the polymer. The chemical properties that can be determined are stereo specificity, any irregularities in the addition of monomer (such as 1,2- versus 1,4-addition and head-to-head versus head-to-tail addition), chain branching, any residual unsaturation, and the relative eoncentration of monomers in the case of copolymers. Other important characteristics include specific additives in a formulated product, and the physical properties, which include molecular weight, molecular-weight dispersion, crystallinity, and chain orientation. Some properties such as molecular weight and molecular-weight dispersion are not determined directly by infrared and Raman spectroscopy, except in some special cases. 

There are several methods which are commonly used for the determination of chain orientation e.g. measurement of in-plane birefringence, wide-angle X-ray diffraction, small-angle X-ray diffraction, infrared spectroscopy and sonic modulus measurements. The first four are briefly presented in this section. 

IR spectroscopy is very useful for the assessment of chain orientation. The measurement of IR dichroism requires the use of IR radiation with parallel and perpendicular polarization to a selected reference direction. Figure 9.13 shows the fundamental principle underlying the dichroism. 

The polymer-related problems which can be solved by spectroscopy are many and varied. They may concern chemical aspects and chain structure, e.g. tacticity, mer sequence distribution, chain branching or structure of radicals. They may concern physical aspects, e.g. chain orientation, crystallinity, crystal thickness, miscibility of polymers, chain conformation or chain dynamics. 

Another important application is infrared dichroism measurements for the assessment of chain (or group) orientation. This topic is treated in Chapter 9, and the discussion is not repeated here. It should be mentioned that Raman spectroscopy can also be used for the determination of chain orientation. Both IR and Raman spectroscopy are very useful for characterization of the physical structure of crystalline polymers. Assessment of the degree of crystallinity can be made by several methods. The preferred and internally consistent methods are X-ray diffraction, density measurements and calorimetry (DSC/DTA). This topic is described in detail in Chapter 7. However, both IR and Raman spectroscopy provide information about the crystallinity, although it is common for the actual crystallinity values obtained by these methods to deviate from values obtained by the three preferred methods WAXS, density and DSC/DTA. 

Chain orientation can be assessed by both spectroscopy and scattering methods. Write a short description of a few of the methods. 

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