Orientation of the Samples

Studies of polymer films, oriented by extension toward a certain direction, have also been widely employed, since essential information about the structure of the corresponding high-molecular compounds can be obtained by employing this procedure [2,9]. 

The technique for orientation in a polymer matrix introduced initially for studying electron transitions [5] has been widely onployed for the characterization of the IR-LD spectra of organic compounds [60-64]. The disadvantages of the method are the relatively long time necessary for the preparation of samples and diffusion phenomena associated with the compound studied, which result in changes in its concentration in the matrix during the course of time. 

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In describing a particular surface, the first important parameter is the Miller index that corresponds to the orientation of the sample. Miller indices are used to describe directions with respect to the tluee-dimensional bulk unit cell [2]. The Miller index indicating a particular surface orientation is the one that points m the direction of the surface nonual. For example, a Ni crystal cut perpendicular to the [100] direction would be labelled Ni(lOO). 

The transition into the oriented state is accompanied by the formation of a neck , a sharp and abrupt local constriction of the sample, in which the extent of orientation and the degree of extension are mudh higher than in the rest of the polymer. After the neck has been formed, further orientation of the sample occurs by spreading of the neck to the entire length of the polymer. When the sample is extended after passing into the oriented state, it undergoes further deformation and at some critical extension it breaks. 

A more complex but faster and more sensitive approach is polarization modulation (PM) IRLD. For such experiments, a photoelastic modulator is used to modulate the polarization state of the incident radiation at about 100 kHz. The detected signal is the sum of the low-frequency intensity modulation with a high-frequency modulation that depends on the orientation of the sample. After appropriate signal filtering, demodulation, and calibration [41], a dichroic difference spectrum can be directly obtained in a single scan. This improves the time resolution to 400 ms, prevents artifacts due to relaxation between measurements, and improves sensitivity for weakly oriented samples. However, structural information can be lost since individual polarized spectra are not recorded. Pezolet and coworkers have used this approach to study the deformation and relaxation in various homopolymers, copolymers, and polymer blends [15,42,43]. For instance, Figure 7 shows the relaxation curves determined in situ for miscible blends of PS and PVME [42]. The (P2) values were determined... 

For USAXS and SAXS data the tangent-plane approximation is valid and the relation between scattering angle and the units of reciprocal space are given by Eq. (2.7). If the scattering pattern is properly aligned with the vertical direction identical to a fiber axis or the polymer chain direction, then sy = 53. In similar manner the. vx-axis of the detector is related to the actual orientation of the sample with respect to the beam. 

An initial experiment involved determination of Arapahoe Smoke Chamber results for samples with and without the zinc coating present. Data are presented in Table II. Depending upon orientation of the sample, an increase in char occurred for some samples with zinc present, while no change in smoke formation was seen. Initial pyrolysis GC/mass spectroscopy results at 90CPC in helium showed no difference in volatiles formed with or without zinc. These results suggested enhanced char formation as the origin of the Radiant Panel results for zinc on modified-polyphenylene oxide (m-PPO). Zinc oxide is a known, effective thermal stabilizer in the alloy. The next work then focused on DSC/TGA studies. 

The coefficients /, g, and h are unique for each second-harmonic signal and depend on the three susceptibility tensors. We normalize the relative values of the tensor components to = 1- The task is then to determine the complex values of the other 14 tensor components (see Table 9.2). A sufficient number of 8 independent measurements is provided by the p- and s--polarized components of the reflected and transmitted second-harmonic signals for the two orientations of the sample shown in Figure 9.17. The change in sample orientation corresponds to a coordinate transformation that reverses the... 

Monochromatic x-rays are used and the diffracted x-rays are detected by their action on photographic flues or plates. They can also be detected by means of a radiation counter and electronic equipment feeding data to a computer. Since x-rays of a given wavelength are diffracted only for a certain specific orientations of the sample so if the sample is a single crystal it should be placed in all possible orientations during experiment. For this the sample is rotated or oscillated about one of its axes. This can also be achieved by using a sample made as a powder of a very small crystal, in such a powder the minute particles are randomly oriented and all possible orientations are included. 

The Charpy test and the Izodtest are both pendulum-type impact tests. The difference between these two tests is essentially the orientation of the sample. In the Charpy test, or simple-beam method, the sample is supported at both ends, but is not held down, as shown on the left in Figure 15.30. In the Izod test, or cantilever beam method, the sample is supported on one end in a vice, as shown on the right in Figure 15.30. In both tests the sample is at the bottom of the pendulum arc and the amount of energy absorbed by the sample is measured. 

The complications that arise in solid state NMR spectra as compared to NMR spectra in solution are the consequence of the fixed orientation of the sample relative to the external magnetic field Bq. Mainly three interactions are responsible for the enormous linebroadening that can be observed for solid powder samples. These are (1) the shielding or chemical shift, including the chemical shift or shielding anisotropy CSA or Acr Hqsa) (2) homo- and/or heteronuclear dipole-dipole coupling (ffoo) and (3) in addition, for nuclei with spin >1/2, the quadrupolar interactions (Hq). 

Figures 1 and 2 report the IR spectra of the Na" " montmorillonite (A) and the Li" " modified montmorillonite [Na+Li+ 0.4 (B) Na+Li+ 0.6 (Q] before calcination (1), after calcination at 220°C (2) and orientation of the sample in the IR beam (3). These figures illustrate the OH stretchnig vibration (fig. 1) and the bending (fig. 2) vibrations of the non pillared samples.

The angles a and P define the orientation of the sample relative to the Bo-field cor denotes the rotation frequency. For the REDOR reference experiment, the rotor-synchronised spin-echo experiment for the S nuclei (cf. Figure lA), the dipolar Hamiltonian integrated over one rotor period Tr averages to zero... 

Fig. 17. EPR spectra of the MP -CC14 radical pairs formed upon illumination by polarized light of a vitreous ethanol solution containing 10-4 M zinc meso-tetra-a,a,a,a-o pivalamidophenylpor-phin and 1 M CC14 for two orientations of the sample (cylindrical tube) in the magnetic field, ft, of the EPR spectrometer. The broken line corresponds to the orientation y ft, and the solid line to the orientation ylft. z is the axis of the cylindrical tube y is the axis which is parallel to vector of the polarized light and ft is the direction of the magnetic field of the EPR spectrometer. Data taken from refs. 122 and 123.

Thus, varying the frequency of an applied electric field at a given temperature, one may pass over from homeotropic orientation of the sample (transparent film) to the mode of EHD instability and reverse. This reveals interesting perspectives for controlling the optical properties of polymeric films. 

Of these designs, only the (P/PEM)psg fails to produce a signal that is proportional to the term Sg"S2e" Since it only returns the term S5,c2q , there is insufficient information to simultaneously determine the extinction, 8 , and the orientation angle, 6". For that reason, it can be used only in cases where 0" is known. In this case, the optimal signal is obtained by setting the orientation of the sample to 0" = 45° relative to the initial polarizer of thePSG. It is evident that the coefficients is simplified by setting the modulation amplitude so that JQ (A) =0 when a PEM is used. 

The two factors given previously may be separated experimentally by changing the orientation of the sample compared to the magnetic field B0. For a current spectral resolution of a few Hz, order parameters S (as defined in equation 15.2) as small as 10 4 may be measured, given the large value of the bare interaction constant vQ. Note that only the absolute value of S is measured. 

Two special electrochemical cells are used for XRD and XAS measurements. In one case a polymer membrane is pressed on the specimen surface after its electrochemical treatment to reduce the water layer on top, but still permitting potential control during the measurements. In an other case the beam penetrates an electrolyte layer in front of the electrode, which corresponds to the specimen s dimensions, but which is thick enough to reduce the danger of ohmic drops and crevices. Beam lines often provide the exact orientation of the samples with the cell by a goniometer. For XAS measurements a special low cost refraction stage has been constructed which permits the orientation of the sample within 0.01 degrees and which has been used for the study of several systems [108]. 

Finally, n was determined by spectroscopic ellipsometry. The main drawback with this technique when applied to anisotropic samples is that the measured ellipsometric functions tanlF and cos A are related both to the incidence angle and the anisotropic reflectance coefficient for polarizations parallel and perpendicular to the incidence plane. The parameters thus have to be deconvolved from a set of measurements performed with different orientations of the sample [see (2.15) and (2.16)]. The complex refractive index determined by ellipsometry is reliable only in the spectral region where the sample can be considered as a bulk material. In fact, below the absorption... 

Fig. 18. Dipolar DECODER pulse sequence. The chemical shift term is suppressed during both the evolution and the detection periods by an xy-4 sequence. During the time delay rp, the orientation of the sample is changed and the magnetization is stored with a pair of pulses (3. (Reproduced from Utz et al.259 with permission.) ...

As mentioned, the line intensities are equal exept for the contribution of the Boltzmann factor, the relaxation times and the orientation of the sample crystallites relative to the radio-frequency magnetic field.They thus deserve no further mention. 

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