IR dichroism

The incorporation of high-resolution optics in a Raman spectrometer allows sampling from areas of less than 1 x 10 m in diameter. The addition of a confocal microscope improves the axial resolution to a couple of microns. Those developments, along with the introduction of notch filters and holographic transmission gratings, allow the reduction of the acquisition time of Raman spectra from minutes to seconds. The fast data collection combined with the high lateral and vertical resolutions makes possible the collection of Raman images with several micron spatial resolutions. 

6 Measuring Polymer Orientation with IR and Raman Spectroscopy 

Orientation can be defined as the preferential alignment (either uniaxial or biaxial) of polymer chains or segments when submitted to an external force. The orientation induces significant changes in the properties both positively and negatively. 

In general, IR absorption is caused by the interaction between the IR electric field vector and the molecular dipole transition moments related to the molecular vibrations. Absorption is at a maximum when the electric field vector and the dipole transition moment are parallel to each other. In the case of perpendicular orientation, the absorption is zero. Directional absorptions are measured using polarised light. The terms parallel and perpendicular refer to the orientation of the polarised beam with respect to a reference axis. For deformation studies, the reference axis corresponds to the stretching direction. 

The IR dichroic ratio may be considered to be characteristic of the directional orientation of the segments of the molecule. For a polymer whose molecular axis is oriented parallel to the spectrometer sampling plane, the dichroic ratio, R, is defined as 

---

In this section we will discuss in some detail the application of X-ray diffraction and IR dichroism for the structure determination and identification of diverse LC phases. The general feature, revealed by X-ray diffraction (XRD), of all smectic phases is the set of sharp (OOn) Bragg peaks due to the periodicity of the layers [43]. The in-plane order is determined from the half-width of the inplane (hkO) peaks and varies from 2 to 3 intermolecular distances in smectics A and C to 6-30 intermolecular distances in the hexatic phase, which is characterized by six-fold symmetry in location of the in-plane diffuse maxima. The lamellar crystalline phases (smectics B, E, G, I) possess sharp in-plane diffraction peaks, indicating long-range periodicity within the layers. 

The IR dichroism measurements allowed a fairly precise determination of the preferential molecular conformations both in the smectic Ai and X phases (see Sect. 2.3). In the smectic Ai phase the biphenyl moiety is parallel on average to the layer normal, while the hydrocarbon and perfluorinated fragments are tilted at angles 18 and 32°, respectively. The phase transition to the smectic X phase is accompanied by a dramatic change in the main molecular conformation - now all the fragments are strongly tilted with respect to the layer normal (especially the biphenyl core which tilts at an angle of around 56°) (Fig. 12). 

IR dichroism has also been particularly helpful in this regard. Of predominant interest is the orientation factor S=( 1/2)(3—1) (see Chapter 8), which can be obtained experimentally from the ratio of absorbances of a chosen peak parallel and perpendicular to the direction in which an elastomer is stretched [5,249]. One representation of such results is the effect of network chain length on the reduced orientation factor [S]=S/(72—2 1), where X is the elongation. A comparison is made among typical theoretical results in which the affine model assumes the chain dimensions to change linearly with the imposed macroscopic strain, and the phantom model allows for junction fluctuations that make the relationship nonlinear. The experimental results were found to be close to the phantom relationship. Combined techniques, such as Fourier-transform infrared (FTIR) spectroscopy combined with rheometry (see Chapter 8), are also of increasing interest [250]. 

The matrix IR spectra of la and several isotopomers (cU-la, l80-la) reveal details of the electronic structure of the carbene.23 In particular the red-shift of the C=0 stretching vibration (compared to p-benzoquinone) below 1500 cm-1 indicates a substantial contribution of the phenoxyl/phenyl resonance structure to the wave function of la. The C2V symmetry of the carbene was experimentally revealed by measuring the IR dichroism of partially oriented samples of matrix-isolated la. The orientation of la in an argon matrix was achieved by irradiation with linearly polarized light. 

The absorption modes of (S)-3-phenyl-2-hydroxypropionic acid, (S)-3-phenyl-2-aminopropionic acid, and (S)-alanine adsorbed on a nickel plate or RNi were studied by Suetaka s group (71, 72). From the measurement of infrared (IR) dichroism in the reflection spectrum, the molecular orientation of the modifying reagent was deduced. Figures 19-21 show molecular orientations of (S)-2-hydroxy-3-phenylpropionic acid on a nickel plate and (R)-alanines on RNis modified at 5° and 100°C, respectively. 

We have done most of our studies on granular PTFE by compacting it into a preform and sintering to make a shape matching ASTM D-1708-66 for tensile testing. Orientation measurements by IR dichroism require thinner specimens, and for this purpose we used dispersion-grade material, commercially cast and processed into film (Dilectrix Corp., Farmingdale, NY). These films were used as received without further heat treatment. ... 

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... 

Studies have been conducted on poly (tetramethylene oxide )-poly-(tetramethylene terephthalate) -segmented copolymers that are identical in all respects except for their crystalline superstructure (66,67,68). Four types of structures—type I, II, and III spherulites (with their major optical axis at an angle of 45°, 90°, and 0° to the radial direction, respectively), and no spherulitic structure—were produced in one segmented polymer by varying the sample-preparation method. Figures 10 and 11 show the stress-strain and IR dichroism results for these samples, respec-... 

The compatibility, mechanical properties, and segmental orientation characteristics of poly-e-caprolactone (PCL) blended with poly (vinyl chloride) (PVC) and nitrocellulose (NC) are described in this study. In PVC blends, the amorphous components were compatible from 0-100% PCL concentration, while in the NC system compatibility teas achieved in the range 50-100% PCL. Above 50% PCL concentration, PCL crystallinity was present in both blend systems. Differential IR dichroism was used to follow the dynamic strain-induced orientation of the constituent chains in the blends. It was found for amorphous compatible blends that the PCL oriented in essentially the same manner as NC and the isotactic segments of PVC. Syndio-tactic PVC segments showed higher orientation functions, implying a microcrystalline PVC phase. 

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. 

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. 

Figure 4. Carbonyl and N02 orientation function vs. elongation curves for 50% NC in a cyclic IR dichroism experiment...

Differential IR dichroism was used to follow the dynamic strain-induced orientation of the constituent chains in PVC/PCL and NC/PCL blends. It was found for amorphous compatible blends that PCL oriented in essentially the same manner as NC and the isotactic segments of PVC. Syndiotactic PVC segments showed much higher orientation functions, which implied the existence of a microcrystalline PVC phase. 

The molecular orientation on a conventionally extruded PVC pipe, a uniaxially oriented PVC pipe and a biaxially oriented PVC pipe was studied by IR dichroism. The... 

Additional evidence for the presence of alpha-helical fibrous protein in stratum corneum is provided by IR dichroism studies (42). The transmission IR of newborn rat corneum (Figure 7) is diffuse because of a large variety of side-chain bands, but it is characteristically protein. 

X-ray diffraction and IR dichroism studies suggest that the long-range elasticity of wool is related to a reversible molecular transformation of the alpha-keratin to an extended beta form (66). No convincing evidence supports this mechanism in stratum corneum viscoelasticity. In fact, the available evidence suggests that the elastic behavior of corneum is primarily entropic in origin. At low deformations, the mechanical properties of hydrated stratum corneum is best described as the behavior of a lightly-crosslinked rubber. 

The structural parameters, energies, and vibrational frequencies of 2-methylthiobenzothiazole were obtained by ab initio RHF calculations using the 6-31G basis set for various conformations. A detailed assignment of most of the observed bands was proposed from the IR dichroism, Raman polarization data, and frequency calculations <2000SPL535>. 

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). 

TABLE 3-XI H Bonding Crystals for which IR Dichroism Studies Have Aided Vibrational Assignments... 

Another type of IR measurement which should be mentioned is the study of IR dichroism. Such data are valuable in determining gross molecular conformation and, more important, they contain information about electron distributions and mobilities as interesting as that obtainable from intensities. 

Abbott and A. Elliott. Proc, Roy. Soc. London) 234A, 247-68 (1956). IR dichroism of crystalline acetanilide (and deu-tero-). 

IR dichroism of globular proteins, a-fibrous proteins, insulin. 

Soc. (London) 206A, 192-206 (1951). IR dichroism and bond directions, diketopiper-azine, adipic acid. 

Discussions Faraday Soc. 1950, No. 9, 251-60. IR dichroism, x-ray chitin, chitosan, chitin nitrate. 

---