Anisotropic behaviour

Anisotropic behaviour is also exhibited in optical properties and orientation effects can be observed and to some extent measured by birefringence methods. In such oriented materials the molecules are in effect frozen in an unstable state and they will normally endeavour to take up a more coiled conformation due to rotation about the single bonds. If an oriented sample is heated up the molecules will start to coil as soon as they possess sufficient energy and the mass will often distort. Because of this oriented materials usually have a lower heat distortion temperature than non-oriented polymers. 

When we compare the VV with the VH configurations for the polarised light, the Raman intensity shows anisotropic behaviour. Most importantly, the A g mode at 165 cm is suppressed in the VH configuration, while the lower frequeney E g and E2g modes are not suppressed. This anisotropy is due to the... 

The thermal strain measurements described above have the common feature of anisotropic behaviour in a supposed isotropic state (cubic structure). These observations go well beyond the short-range, static strain fields associated with the lattice impurities responsible for Huang scattering. This then raises the question of the temperature at which the lattice symmetry changes and the implications of this for the central mode scattering. 

Glass fibres dominate this field either as long continuous fibres (several centimetres long), which are hand-laid with the thermoset precursors, e.g., phenolics, epoxy, polyester, styrenics, and finally cured (often called fibre glass reinforcement plastic or polymer (FRP)). With thermoplastic polymers, e.g., PP, short fibres (less than 1 mm) are used. During processing with an extruder, these short fibres orient in the extrusion/draw direction giving anisotropic behaviour (properties perpendicular to the fibre direction are weaker). 

Most minerals polarise light that passes through them, i. e. show anisotropic behaviour Birefringence-the maximum difference between the refractive indices of a mineral... 

The performance of most antistatic finishes depends on the kind of fibre and sometimes also on the kind of fabric (anisotropic behaviour, for example, different... 

The material is then extruded through the steel die whose shape is adjusted to that of the required ware. The body being extruded does not have a uniform velocity distribution over its cross section as a result of friction the material at the die wall is retarded with respect to that at the centre. The velocity gradient brings about preferential orientation of the plate-shaped particles of clay minerals, which arc arranged parallel at the outer surface and at random in the centre. The preferential orientation of particles may cause anisotropic behaviour on drying and firing, and may even affect the properties of the final ware. 

In Figs. 15-17 we demonstrate the asymptotic anisotropic behaviour of the 6 lowest 2D PES indicating the correlation of the lowest S-O multiplets of fragments with the lowest adduct states of A",, A", and A symmetry. 

The electrical conductivity of single crystals of Mn Shv has been determined parallel to and perpendicular to the c-axis 770 anisotropic behaviour was observed, particularly at low temperatures. Mn27Si47 was found to be a... 

Dolden et al. [43] successfully synthesised a thermotropic polyetherimide from biphenylene dianhydride and l,12-bis(4-aminophenoxy) dodecane. This polyimide was mobile in the melt at 300 °C and exhibited a highly viscous anisotropic phase. Several LC oligoimides were also synthesised by the same workers who showed that more than two aromatic rings per repeat unit seem to be required in order for anisotropic behaviour to be observed. 

Figure 6.26 shows the chemical structure of PPV. Block letters from A to F show the eight carbon sites in one PPV monomer unit. The principal axes of proton hyperfine coupling of an unpaired tt-electron, defined in Figure 6.18, arc also shown for two proton sites of inequivalent bond orientations. ESR spectra of stretch-oriented undoped PPV films have shown anisotropic lineshapes with an average g value of 2.003 and a spin concentration of about 1 spin/10 PPV monomer units. The observed anisotropic behaviour of the. g value and the linewidth, both larger for the stretch direction, are qualitatively similar to those found in stretch-oriented polyacetylene films mentioned in Section 3.2 and... 

Azaar K., l.D. Rosea, and J.M. Vergnaud. 2002. Anisotropic behaviour of thin EPDM rubber discs towards absorption of toluene. Plastics, Rubber Composites. 31 220-5. 

Thermomechanical measurements can be carried out on a wide range of solid samples. The most usual mode of measurement is either in compression (for self-supporting samples) or tension (for thin films and fibres). Some materials exhibit anisotropic behaviour (particularly films or crystals) in that changes in dimensions will differ depending upon which axis the measurements are performed. 

PrCu2 shows hardly any anomaly while a marked anomaly is observable in the resistivity. The resistivity measurements on a PrCu2 single crystal performed by Andres et al. (1976) show strong anisotropic behaviour for the a, b, and c direction of this orthorhombic structure. Because of the temperature-independent nature of this anisotropic behaviour Andres et al. (1976) conclude that the anisotropy in the resistivity is due to the anisotropy of the Fermi surface and not due to anisotropy in the scattering cross section between conduction electrons and f-electrons. 

Undulating type, where the anisotropic behaviour may originate by more than one anisotropy planes which exist in the rock. 

Even though liquid crystals are fluids, the fact that orientational order exists ensures that all directions in the fluid are not equivalent. This has a profound effect on all the properties of the phase, producing a complex response to external factors such as electric fields and mechanical distortions. Yet it is this combination of factors, namely the flow properties of fluids and the anisotropic behaviour normally absent in fluids, that makes the behaviour of liquid crystals both intrinsically interesting and ripe for technical applications. 

---