Molecules uniaxially oriented

Any extended part of a linear polymer molecule exhibits a strong anisotropy of many properties since its atoms and atomic groups are oriented and the macromolecule is actually a one-dimensional crystal . The parallel packing of these parts during the formation of a uniaxially oriented polymer substance imparts the anisotropie properties of a single molecule to the whole polymeric material. 

In this review the definition of orientation and orientation functions or orientation averages will be considered in detail. This will be followed by a comprehensive account of the information which can be obtained by three spectroscopic techniques, infra-red and Raman spectroscopy and broad line nuclear magnetic resonance. The use of polarized fluorescence will not be discussed here, but is the subject of a contemporary review article by the author and J. H. Nobbs 1. The present review will be completed by consideration of the information which has been obtained on the development of molecular orientation in polyethylene terephthalate and poly(tetramethylene terephthalate) where there are also clearly defined changes in the conformation of the molecule. In this paper, particular attention will be given to the characterization of biaxially oriented films. Previous reviews of this subject have been given by the author and his colleagues, but have been concerned with discussion of results for uniaxially oriented systems only2,3). 

LINEARLY POLARIZED PHOTO- AND ELECTROLUMINESCENCE FROM UNIAXIALLY ORIENTED MOLECULES... 

Another technique used for obtaining macroscopically polar films involves mechanical extension of the material. Uniaxial plastic deformation induces a destruction of the original spherulitic structure into an array of crystallites in which the molecules are oriented in the deformation direction. In case of PVF2 when such deformation takes place below 90 °C the original tg+ tg chains are forced into their most extended possible conformation which is all-trans [32]. 

Figure 5. Diffraction from an uniaxially oriented specimen of stretched calf thymus DNA (courtesy M. H. F. Wilkins). The molecules are approximately 14, helices with c = 10.6 nm. The Bragg reflections on the equator can be indexed on the basis of a rhombic net with a = b = 2.30 nm.

Fig 6. Schematic diagram of polarization measurements of a. Completely uniaxially oriented molecules, b. Two-dimensional system with partially oriented molecules, and c. Three-dimensional system with randomly oriented molecules. Axis of chromophoric group of the molecule lies along the double-headed arrows. The intensities of the incident exciting light and the fluorescence emission are represented by and //, respectively. The vertical and horizontal components of // are represented by J and /j respectively... 

Actually in the case of uniaxially oriented dye molecules unpolarized exciting light could also be used to give the same result since the emission axis of the molecule coincides with the axis of the fiber. This technique... 

The fibers spun from nematic phase systems and solvent composition 25.0/75.0 exhibited twice the tenacity and modulus of those extruded from the system of the solvent composition 29.3/70.7. The two fiber types showed significant differences in birefringence, and in the appearance of their external surfaces and fracture surfaces. The comparable moduli of the fiber spun from a nematic solution with those of Fortisan may result from high uniaxial orientation of cellulose molecules in the former fiber. 

Anisotropy of the Spin-Spin Coupling Tensor. - In NMR experiments performed in anisotropic liquid crystal (LC) phases or in the solid state, the anisotric part of an indirect nuclear spin-spin coupling tensor J appears combined with the direct dipolar coupling D. The NMR spin Hamiltonian appropriate for spin 1/2 nuclei in molecules partially oriented in uniaxial LC solvents can be written in the high field approximation as... 

In the following, an organic film is denoted as highly ordered, if there is at least one perfect monolayer with the molecules oriented in the same direction, i.e. having the same tilt angle against the surface normal ( uniaxial orientation ). Crystalline films have a three-dimensional order with sharp reflexes in diffraction measurements, whereas epitaxial films are crystalline with a respective orientation to a single-crystalline substrate surface. 

Another attempt is to prepare films with electronically isolated oligothiophene molecules in uniaxially oriented matrices. The matrix can consist of cyclic siloxanes [ 108] or dextrines which both possess cavities of defined diameter and length. Oligothiophenes can diffuse into the anisotropic matrix. Substitution with certain polar head-groups allows for a strong bonding to the metal substrate, i.e. nitriles or carboxyles couple to Pt, thiols to Au. 

The angular distribution of polarized components of fluorescence intensity of electrically or numerically oriented films of polypeptides show the pattern similar to those typical of the uniaxially oriented film. The degree of orientation of the films as expected from the contour of the pattern, is not so good despite the excellent orientation of the films, which may indicate that most of the fluorescent molecules are introduced into the gap regions that may possibly form amorphous regions. 

The theory of the method is rather complicated, because the amplitude of Raman scattering is described by a second-rank tensor, so it will not be discussed here. Just like for fluorescence, P2 cosff)) and (P4(cosd)), or cos 6) and (cos" 6), can, at least in principle, be obtained for the simplest type of uniaxial orientation distribution and these values now refer directly to the molecules of the polymer itself. In practice it is often necessary to make various simplifying assumptions. For biaxially oriented samples several other averages can be obtained. 

The birefringence An for a uniaxially oriented sample with draw ratio X = 3.5 was found to be 8.1 x 10 . Assuming that the orientation of the polymer molecules is adequately described by equation (11.6) using only the first term on the RHS, deduce An for a similarly drawn sample with X = 2.0. 

Reprinted with permission of Elsevier Science from Nobbs, J. H., Bower, D.I., Ward, I. M. and Patterson, D. A study of the orientation of fluorescent molecules incorporated in uniaxially oriented polyfethylene terephthalate) tapes . Polymer 15, 287-300. Copyright 1974. 

Uniaxially Oriented A state of material such as polymeric film or composite characterized by the permanent orientation of its components such as polymer molecules or reinforcing fibers in one direction. The orientation is achieved by a number of different processes, e.g., stretching, and is intended to improve the mechanical properties of the material. 

The choice of fluorescent probe depends on a variety of factors. It has already been pointed out that what is determined directly is information which characterises the distribution of orientations of the fluorescent molecules. The ideal experiment would be one in which the polymer molecules themselves contained fluorescent groups. Stein has considered the theory of the fluorescence method specifically for a uniaxially oriented fluorescent rubber but no experiments to study orientation have been reported for such a system. Nishijima et al have, however, made some qualitative observations on the polarisation of the fluorescent light from polyvinylchloride films which had been first stretched and then irradiated with light of wavelength 185 nm to produce fluorescent polyene segments. 

Fig. 4. CorO for the unique axes of fluorescent VPBO molecules dispersed in uniaxially oriented tapes of polyethylene terephthalate plotted against the birefringence — of the tapes. O, samples drawn at S(PC to draw ratios between 1 and 6 . samples drawn to draw ratio 2 7 at temperatures between 65° and 90°C. (Reproduced by permission from Ref. 18. Copyright I PC Business Press Ltd.)...

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