Polymers birefringence

The theoretical descriptions of biaxial orientation and the relationships to physical properties are not as well developed as those of uniaxial orientation. In amorphous polymers, birefringence can measure total molecular orientation and can be used to predict properties. For semicrystalline polymers, a more... 

It is useful at this juncture to note the capabilities of the other major methods for assessing orientation in polymers. Birefringence measurements yield values for and broad line NMR provides and , together with diffraction measurements define orientation uniquely, and so give values for , where n takes on all even values. Infrared spectroscopy and birefringence measurements yield the... 

See, for example, Marshall, K. L., E. Kimball, S. Mcnamara, T. Z. Kosc, A. Trajkovska-Petkoska, and S. D. Jacobs. 2004. Electro-optical behaviour of polymer cholesteric liquid crystal flake/fluid suspensions in a microencapsulated matrix, in Liquid Crystals VIII, edited by 1. C. Khoo, SPIE Proceedings Vol. 5518 (SPIE Bellingham, WA 2004) pp. 170-181 and references therein see also, Marshall, K. L., T. Z. Kosc, S. D. Jacobs, S. M. Paris, and L. Li. 2003. Electrically switchable polymer liquid crystal and polymer birefringent flake/fluid host systems and optical devices utilizing same, U.S. Patent No. 6,665,042 B1 (Dec 2003). 

The index of refraction of most polymers is greater parallel to the chain than normal to the molecular axis. Substances showing this anisotropy of refractive index are said to be birefringent. 

Noncrystalline domains in fibers are not stmctureless, but the stmctural organization of the polymer chains or chain segments is difficult to evaluate, just as it is difficult to evaluate the stmcture of Hquids. No direct methods are available, but various combinations of physicochemical methods such as x-ray diffraction, birefringence, density, mechanical response, and thermal behavior, have been used to deduce physical quantities that can be used to describe the stmcture of the noncrystalline domains. Among these quantities are the amorphous orientation function and the amorphous density, which can be related to some of the important physical properties of fibers. 

Many ceUulosic derivatives form anisotropic, ie, Hquid crystalline, solutions, and cellulose acetate and triacetate are no exception. Various cellulose acetate anisotropic solutions have been made using a variety of solvents (56,57). The nature of the polymer—solvent interaction determines the concentration at which hquid crystalline behavior is initiated. The better the interaction, the lower the concentration needed to form the anisotropic, birefringent polymer solution. Strong organic acids, eg, trifluoroacetic acid are most effective and can produce an anisotropic phase with concentrations as low as 28% (58). Trifluoroacetic acid has been studied with cellulose triacetate alone or in combination with other solvents (59—64) concentrations of 30—42% (wt vol) triacetate were common. 

Biaxial Orientation. Many polymer films require orientation to achieve commercially acceptable performance (10). Orientation may be uniaxial (generally in the machine direction [MD]) or biaxial where the web is stretched or oriented in the two perpendicular planar axes. The biaxial orientation may be balanced or unbalanced depending on use, but most preferably is balanced. Further, this balance of properties may relate particularly to tensile properties, tear properties, optical birefringence, thermal shrinkage, or a combination of properties. A balanced film should be anisotropic, although this is difficult to achieve across the web of a flat oriented film. 

For substrates of WORM and EOD(PCR) disks the industry in the future wants polymers that have a markedly improved resistance to heat softening compared to BPA-PC and, if possible, a lower water absorption and lower birefringence, but otherwise maintain the good characteristics in toughness, production, and cost (194). This goal is being approached in different ways further modification of BPA-PC, newly developed polymers, improvement of the processing characteristics of uv-curable cross-linked polymers, and development of special copolymers and polymer blends, eg,... 

Reduction or even complete compensation of birefringence by mixing polymers with positive birefringence (PC, PVC, PETP, PPE, PVDF, etc) with polymers with negative birefringence (PMMA, PS, PAN, etc) has been the consistent strategy. 

Fig. 26. Qualitative compatison of substrate materials for optical disks (187) An = birefringence IS = impact strength BM = bending modulus HDT = heat distortion temperature Met = metallizability WA = water absorption Proc = processibility. The materials are bisphenol A—polycarbonate (BPA-PC), copolymer (20 80) of BPA-PC and trimethylcyclohexane—polycarbonate (TMC-PC), poly(methyl methacrylate) (PMMA), uv-curable cross-linked polymer (uv-DM), cycHc polyolefins (CPO), and, for comparison, glass.

The birefringent (BR) modulator makes use of polarized light and tensorial nature of the electrooptic coefficient. For example, poled organic polymer films are characterized by two nonzero components for the electrooptic tensor and parallel and orthogonal to the poling direction,... 

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. 

B. Zimm. Dynamics of polymer molecules in dilute solutions viscoelasticity, low birefringence and dielectric loss. J Chem Phys 24 269-278, 1956. 

Tsvetkov, V. and Andreeva, L. Flow and Electric Birefringence in Rigid-Chain Polymer Solutions. Vol. 39, pp. 95-207. 

Janeschitz-Kriegl, H. Flow Birefringence of Elastico-Viscous Polymer Systems. Vol. 6, pp. 170-318. 

Many papers deal with the crystallization of polymer melts and solutions under the conditions of molecular orientation achieved by the methods described above. Various physical methods have been used in these investigations electron microscopy, X-ray diffraction, birefringence, differential scanning calorimetry, etc. As a result, the properties of these systems have been described in detail and definite conclusions concerning their structure have been drawn (e.g.4 13 19,39,52)). 

Fig. 9. Plot of amorphous orientation average fa, obtained from birefringence, etc., versus f, obtained from im etc- Triangles, draw temperature 80 °C circles draw temperature 85 °C open symbols, single-stage draw full symbols, two-stage draw. Reproduced from Polymer by permission of the publishers, Butterworth Co (Publishers) Ltd. (C)...

Isihara, A. and Guth, E. Theory of Dilute Macromolecular Solutions. Vol. 5, pp. 233-260. Janeschitz-Kriegl, H. Flow Birefringence of Elastico-Viscous Polymer Systems. Vol. 6, pp. 170-318. 

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