Dichroism chain orientation

IR dichroism is another useful technique for assessing chain orientation." In a uniaxial ly deformed polymer sample, there is assumed to be rotational symmetry about the draw direction, in which case the orientation of a chain molecule is... 

What we have shown in this brief review is that a combination of denaturation experiments with circular dichroism, air oxidation, and redox equilibrium experiments can lead to an understanding of the specificity of coiled-coil formation (hetero- versus homodimer formation or parallel versus antiparallel chain orientation). 

IR Dichroism. Two types of IR-dichroism experiments were used in this study to follow segmental orientation. First, dynamic differential dichroism was used to follow chain orientation while the sample was elongated at a constant strain rate. This experiment was performed with different IR peaks which allowed a comparison of the molecular orientations for each blend constituent. Second, a cyclic experiment was used where the film was strained to a predetermined elongation, relaxed at the same strain rate until the stress was reduced to zero, and then elongated to a higher level of strain, and so forth. 

Rheo-optical techniques (46 8) afford information on the strain dependence not only of stress but also of optical quantities associated directly with the structure or molecular morphology. The techniques were developed extensively for crystalline polymers to investigate the molecular deformation mechanism underlying the tensile elongation. In this part, the chain orientation behavior is characterized by infrared dichroism measured simultaneously with tensile deformation at a constant rate of elongation. 

The degree of microcrystallite orientation in crystalline films can be determined from careful analysis of the polarized Raman scattering line intensities. Since the induced polarization (P) and incident electric field (E) of the probe laser are three-dimensional vectors related by the polarizability tensor (o), individual elements of the tensor ( ij) be determined from the line intensities establishing molecular orientation in materials. Molecular chain orientation in polymer glasses has been demonstrated by Raman and infrared dichroism studies (12). 

Several sources of error may influence the calculations of chain orientation carried with the use of data from IR dichroism experiments. Three specific problems are the improper treatment of chain conformation, the imperfect polariser behaviour, and the inaccurate assumptions regarding the value of the angle between a particular IR-active transition moment and the polymer chain axis (309). 

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... 

To correlate the macroscopic properties and the structural changes occurring dtuing deformation the orientation functions evaluated from Eqs. (20 a—c) have been plotted in dependence of strain in Fig. 14. Onc and Asada have discussed in detail the expected changes of infrared dichroism and orientation functions due to the different molecular processes of lamellar orientation and chain unfolding. 

Figure 9.14 Change in chain orientation parameter with the speed of rotation of the cyiindricai target. The chain orientation parameter from dichroism measurements (fiiied symbois) and from the 5.3 A equatoriai peak in WAXD measurements (open symbois) are shown in the figure. Redrawn from Fennessey and Farris (2004).

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. 

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. 

In anisotropic materials, if vibrators are directed toward a certain direction, the absorption will depend on the respective orientation of the dipoles and the polarization of the incident beam. The dichroism ratio (Rzy) is defined as the ratio of the absorbances along two directions (z,y), from which the average chain orientation can be deduced. In the case of randomly orientated vibrators, there is no preferential orientation for absorption at the macroscopic level. 

Infrared dichroism is a powerful technique to study the evolution of the chain orientation in films, particularly when coupled with photoelastic modulation [95]. FT-IR is a rapid technique that is very efficient in industrial applications, such as weathering [96] or analysis of blends [97]. 

As mentioned frequently the mechanical and optical response of molecules — and of their crystallites — is highly anisotropic. Depending on the property under consideration the carriers of the molecular anisotropy are the bond vectors (infrared dichroism), chain segments (optical and mechanical anisotropy), or the end-to-end vectors of chains (rubber elastic properties). For the representation of the ensuing macroscopic anisotropies one has to recognize, therefore, the molecular anisotropy and the orientation distribution of the anisotropic molecular units (Fig. 1.9.). Since these are essentially one-dimensional elements their distribution and orientation behavior can be treated as that of rods such a model had been used successfully to explain the optical anisotropy [78], and the anisotropies of thermal conductivity [79], thermal expansion or linear compressibility [80], and Young s modulus [59,... 

Zhurkov et al. [42] derived the degree of chain orientation from the dichroism of deformed-IR bands of PETP (cf. Chapter SIB and Fig. 8.5) they observed that for highly stressed segments an initial orientation with cos d = 0.75 had turned into practically complete orientation (cos — 1.0). The problems posed by an application of this method are discussed in detail by Read et al. [43]. Bouriot [44] deduced from IR measurements a reversible cis-trans transformation of the ethylene glycol segment in PETP fibers under the effect of tensile stresses. In polyamide 66 he observed a reversible increase of free (non-assodated) NH groups and an increase in the average chain orientation with chain tension. 

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