Sampling orientation

An interesting point concerns polarisation effects in the Raman spectra, which are commonly observed in low-dimensional materials. Since CNTs are onedimensional (ID) materials, the use of light polarised parallel or perpendicular to the tube axis will give information about the low dimensionality of the CNTs. The availability of purified samples of aligned CNTs would allow us to obtain the symmetry of a mode directly from the measured Raman intensity by changing the experimental geometry, such as the polarisation of the light and the sample orientation, as discussed in this chapter. 

Fig. 3. Raman intensities as a function of the sample orientation for the (10, 10) armchair CNT. As shown on the right, 0 and 62 are angles of the CNT axis from the z axis to the x axis and the y axis, respectively. 63 is the angle of the CNT axis around the z axis from the x axis to the y axis. The left and right hand figures correspond to the VV and VH polarisations [12].

Ordered dialkoxy PPV derivative has been prepared by Yoshino et al. [491. oly(2 -nonoyloxy-1,4-phenylene vinylene) 27a forms a nematic liquid-crystalline phase upon melting. The material retains its order upon cooling to room temperature, and its band gap (2.08 eV) is measurably smaller than in an unoricnted sample. Oriented electroluminescence may be achieved by rubbing a thin fdin of the material to induce molecular orientation [50],... 

According to Hosemann-Bonart s model8), an oriented polymeric material consists of plate-like more or less curved folded lamellae extended mostly in the direction normal to that of the sample orientation so that the chain orientation in these crystalline formations coincides with the stretching direction. These lamellae are connected with each other by some amount of tie chains, but most chains emerge from the crystal bend and return to the same crystal-forming folds. If this model adequately describes the structure of oriented systems, the mechanical properties in the longitudinal direction are expected to be mainly determined by the number and properties of tie chains in the amorphous regions that are the weak spots of the oriented system (as compared to the crystallite)9). 

Complex Anisotropy is studied in texture goniometers (p. 193) as a function of sample orientation. If the study is aiming at quantitative analysis of scattering data, the absorption correction may become an issue. Conversely, by choosing a special kind of scanning modus (e.g., symmetrical reflection SAXS SRSAXS), the absorption correction problem can be simplified. 

Sample Orientation If possible orient the sample in such a way that the beam-stop holder does not cut through an important region (peak). If you expect that the sample exhibits fiber symmetry, check it rotating the sample about the assumed fiber axis and take some patterns. 

In general, it is not recommended to study WAXS crystallinity of anisotropic materials. The recording of the corresponding data is laborious, because the WAXS must be recorded as a function of both scattering angle and sample orientation in a texture goniometer (cf. p. 193, Fig. 9.3) before the data can be isotropized. 

Orientation Determination. While polarized edge studies, together with a known sample orientation, can provide information about the electronic structure of the absorber, one can also use polarized edges to probe ordered systems of unknown orientation. This sort of approach was used in a study of B adsorbed on graphite (26,27). In this case, the orientational dependence of an edge transition was used to calculate the degree of orientational purity of the graphite surface. 

For systems where the polarization dependence of the edge structure is already known, polarized measurements could be used to determine the sample orientation. For example, the intense pre-edge transition described for (acac) V 0 is also found in the isotropic absorption spectra of M 0(porphyrin) (M Ti,V,Cr) systems (33). Strongly polarized transitions like these could be used, for example, to determine the orientation of the porphyrin moiety within an ordered system such as a biological membrane or fiber (24). 

It follows from (12) that Bplt(QP/) = Bp 0 is independent of the sample orientation. 

Measures sample ignitability, with a conical combustion module. Normal sample orientation horizontal. 

This contains two ignitability tests one uses a glowing wire and one a needle flame. Setchkin ignition apparatus. Measures flash ignition and spontaneous ignition temperatures. Normal sample orientation horizontal. 

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... 

Ito, A. and Kashiwagi, T., Characterization of flame spread over PMMA using holographic interferometry, sample orientation effects, Combustion and Flame, 1988, 71, 189. 

Fig.3 -NMR spectra at different sample orientations. The oriented sample of Fig. 2B was manually tilted in the static magnetic field Bq to acquire spectra at different angles, where 0 = 0° corresponds to the usual alignment of the sample normal parallel to the direction of Bq. The arrows indicate the shift of the anisotropic signals with respect to the isotropic value (dotted line), according to the factor 3(cos - l)/2 as expected for a molecule undergoing fast long-axial rotation about the membrane normal...

The theory of nematic liquid crystal deformation, forced by an electric field is well developed and permits to establish the relationship between the threshold voltage U, causing sample orientation, with Ae and elasticity constants of a liquid crystal (Kn). For the main S and B types of deformation the equation is the following27 ... 

For a quenched lamellar phase it has been observed that G = G"scales as a>m for tv < tvQ. where tvc is defined operationally as being approximately equal to 0.1t and r is a single-chain relaxation time defined as the frequency where G and G" cross (Bates et al. 1990 Rosedale and Bates 1990). This behaviour has been accounted for theoretically by Kawasaki and Onuki (1990). For a PEP-PEE diblock that was presheared to create two distinct orientations (see Fig. 2.7(c)), Koppi et al. (1992) observed a substantial difference in G for quenched samples and parallel and perpendicular lamellae. In particular, G[ and the viscosity rjj for a perpendicular lamellar phase sheared in the plane of the lamellae were observed to exhibit near-terminal behaviour (G tv2, tj a/), which is consistent with the behaviour of an oriented lamellar phase which flows in two dimensions. These results highlight the fact that the linear viscoelastic behaviour of the lamellar phase is sensitive to the state of sample orientation. 

Air distribution Air velocity is never completely uniform across the sample surface Though surface air velocity does vary in emission cells as in chambers, cells do preclude the variability of sample orientation... 

---