Orientation, infrared dichroism study

Practical problems associated with infrared dichroism measurements include the requirement of a band absorbance lower than 0.7 in the general case, in order to use the Beer-Lambert law in addition infrared bands should be sufficently well assigned and free of overlap with other bands. The specificity of infrared absorption bands to particular chemical functional groups makes infrared dichroism especially attractive for a detailed study of submolecular orientations of materials such as polymers. For instance, information on the orientation of both crystalline and amorphous phases in semicrystalline polymers may be obtained if absorption bands specific of each phase can be found. Polarized infrared spectroscopy can also yield detailed information on the orientational behavior of each component of a pol3mier blend or of the different chemical sequences of a copoljnner. Infrar dichroism studies do not require any chain labelling but owing to the mass dependence of the vibrational frequency, pronounced shifts result upon isotopic substitution. It is therefore possible to study binary mixtures of deuterated and normal polymers as well as isotopically-labelled block copolymers and thus obtain information simultaneously on the two t3q>es of units. 

By means of both x-ray diffraction and infrared dichroism studies, the degree of orientation in similar drawn PPV films has been further characterized by Gagnon el al. [269]. The Hemians molecular orientation function/is determined in an approximate manner from the angular spread of the (llO) reflection. A maximum value of f— 0.96 is attained at draw ratios of 6 and higher, for which also the lateral crystallite size (lower limit) levels off to 20 A. Whereas the value of / corresponds well, the crystallite size is much smaller than that reported by Bradley [267], which is 270 A. 

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). 

Imahori (1963) prepared an oriented film of y-globulin fibrils by precipitating human y-globulin with antihuman horse-serum y-globulin. Infrared dichroism studies showed a pronounced amide I band near 1635 cm with light polarized parallel to the fibril axis. The appearance of this band implied the presence of intrachain cross-) structure. 

ORIENTATION OF STRETCHED POLY(3-OCTYLTHIOPHENE) FILMS VISIBLE AND INFRARED DICHROISM STUDIES... 

Poly(3-octylthiophene) has been oriented, by stretching a free-standing film and by stretching a sandwich structure, polyethylene/poly(3-octylthiophene). The infrared dichroism studies indicate that P30T is higWy oriented at a rather low draw ratio ... 

Experiments involving relaxation of centrally deuterated polystyrene chains in a higher molecular weight matrix recently provided support for the minor chain reptation model.Using infrared dichroism studies of step-strained PS films, the protonated chain-ends were found to relax faster than the centrally deuterated fraction. This is consistent with the MC model shown in Fig. 1(a) where the chain-ends first lose memory of their initial orientation. 

Observations of infrared dichroism on pol3rmeric systems were first made about 1950 using pols peptides (53,54). Fraser (55,56) developed its use to measure orientation in polymers quantitatively and establish a relationship of dichroic ratio D to the Hermans orientation factor equivalent to equation 2. The first infrared dichroism studies on polyethylene were performed in 1954 by Stein and co-workers (57,58). The article (58) discusses the determination of uniaxial orientation in polyethylene films using wide-angle x-ray (wax) diffraction, birefringence, and infrared dichroism, and explicitly states the interrelation of the former two measurements through... 

Linear viscoelastic measurements using infrared dichroism on the compatible blend polyethylene oxide) and poly(methyl methacrylate) were reported by Zawada et al. [139]. Unlike Monnerie and coworkers [127], who reported seeing only orientation in the PMMA component, and none in the PEO, Zawada et al. observed alignment in the PEO. However, since the PEO was of lower molecular weight (as was the case for Monnerie and coworkers), its relaxation timescales were substantially faster than the PMMA. This may explain the lack of any measurable orientation by Monnerie and coworkers, who studied quenched samples, since their preparation may have allowed the PEO to relax prior to testing. 

Case Study 2 Dynamics of Multicomponent Polymer Melts - Infrared Dichroism 213 tures become oriented in the flow direction. 

In 1958, Beer et al. re-examined the infrared dichroism situation and remeasured the dichroic ratio for several of the peptide bands in collagen. On the basis of studies on model compounds, they calculated the directions of the transition moments for the principal bands in the peptide link and, using the atomic coordinates for various proposed collagen models, the inclination of these transition moment vectors to the fiber axis. Since Beer (1956) had shown that the dichroism of a partially oriented polymer may be considered equivalent to that of a sample containing fully aligned and completely random portions, a disorientation parameter (/) characteristic of the degree of order of the sample could be calculated for each band. The spread of the values obtained for this parameter, which is characteristic of the sample only and not of the bands used to determine it,... 

Spectra of Crystals Infrared Dichroism. The spectrum of a single crystal observed in polarized light depends upon the orientation of the crystal axes relative to the plane of polarization. A similar situation exists for macroscopically oriented high polymers, obtained by extruding or stretching polymeric materials such as nylon, polyvinyl alcohol, and polyethylene. Ellis and co-workers were the first to make systematic studies of IR dichroism in H bonded systems (604, 779, 780). Further attention was drawn to the method by Mann and Thompson (1334), Crooks (463), and Ambrose, Elliott, and Temple (595, 38, 589, 4). 

A number of infrared absorption studies on the purified epidermal protein are described. In combination with the X-ray diffraction data, they entirely confirm the recent advances in the definition of absorption spectra, which are characteristic of a- and /3-type proteins and polypeptides, in terms of the kinds of dichroism and the frequency changes associated with the (3 form. It is pointed out that as the dichroism changes with the a — change it is not possible to read off the relative amounts of a and from the absorption spectra because of the differences in the orientation of the absorbing bonds with reference to the plane of the film. In the absorption spectra of highly stretched epidermin, where... 

No. 19, 9th Sept. 1996, p.6256-61 RELAXATION OF SEGMENTAL ORIENTATION AND CHAIN EXTENSION IN POLYCARBONATE STUDIED BY INFRARED DICHROISM AND SHRINKAGE Lundberg L Stenberg B Jansson J F Stockholm,Royal Institute of Technology... 

The applicability of infrared dichroism to the study of PUs has been determined by the availability of at least one weU-characterized absorption band in each type of domain. It has been shown [67, 140, 364] that the soft domains are comprised of prepolymer (polyether or polyester) segments, while the hard domains contain urethane segments. The N-H stretching which is located in the urethane linkage characterizes the orientation of the higher modulus hard domains. For the DBDI polymers, we have shown that it appears at 3 320 cm [67]. Similarly, the asymmetric C-H stretching absorption was determined. It appears at 2 940 cm and was used to describe the orientation of prepolymer segments which comprise the rubbery soft domains. 

There is a totally different manner in which infrared spectroscopy is applied to study macromolecular conformations. Here, researchers make use of what is known as infrared dlchroism. In the next chapter we will be discussing the phenomenon called circular dichroism. Circular dichroism means the difference in the absorption of right- and left-handed circularly polarized light by an optically active compoimd. The phenomenon used in infrared dichroism is not circular dichroism. One can refer to this as linear dichroism. This phenomenon means that molecules oriented in one direction will absorb plane polarized light differently when the electric vector of the light is parallel to the orientation of the molecule and when it is perpendicular to this orientation. Whether the compound is optically active or not is of no consequence to this phenomenon even optically inactive compounds will show linear dichroism. 

The conformation of fibrous proteins has been studied by Infrared dichroism. Thus rat tall tendons, porcupine quills, silk fibre etc. have been studied. In these proteins the absorptions at 1640 cm" and 3300 cm" are maximum when the electric vector of Infrared li t is perpendicular to the fibre axis. These two frequencies are for C=0 and N—H stretching respectively. This observation can only be interpreted one way - these two bonds in the peptide backbone must be oriented perpendicular to the fibre axis (Figiu 8.25). This inference is backed up by other studies on fibrous protein structure as well. 

Another widely used method for studying molecular orientation in polycrystalline polymers b the infrared dichroism technique. In the infrared spectra it b practically... 

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]. 

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