Birefringence of polymer solutions

Flow birefringence of polymer solutions is, in general, measured with the aid of an apparatus of the Couette type, containing two coaxial cylinders. One of these cylinders is rotated at constant speed, the other is kept in a fixed position. The light beam for the birefringence measurement is directed through the annular gap between these cylinders, in a direction parallel with the axis of the apparatus. In this way, the difference of principal refractive indices An is measured just in the above defined plane of flow (1—2 plane). 

K. Osaki, N. Bessho, T. Kojimoto, and M. Kurata, Flow birefringence of polymer solutions in time-dependent field, J. Rheol., 23,457 (1979). 

M. Copic, Streaming birefringence of polymer solutions anisotropy of internal field, J. Chem. Phys. 26,1382 (1957). 

Gotlib YuYa, Svetlov YuE (1964a) To the theory of anomalous angles in dynamic birefringence of polymer solutions. Vysokomolek Soedin 6(5) 771-776 (in Russian)... 

From data on flow birefringence of polymer solutions, values of the segmental anisotropy, Aa = ai — a2, can be calculated by means of Kuhn s equation on flexible rubber chains ... 

Many polymer scientists have used homogenous analysis for the determination of molecular size via limiting viscosity numbers and translational and rotary diffusion constants but few recognize that these techniques and investigations of birefringence of polymer solutions were pioneered by Charles Sadron. 

Thurston, G.B., Peterlin,A, Influence of finite numbers of chain segments, hydro-dynamic interaction, and internal viscosity on intrinsic birefringence and viscosity of polymer solutions in an oscillating laminar flow field. J. Chem. Phys. 46, 4881-4885 (1967). 

Philippoff, W., Gaskins,F.H., Brodnyan, J.G. Flow birefringence and stress. V. Correlation of recoverable shear strains with other rheological properties of polymer solutions. J. Appl. Phys. 28,1118-1123 (1957). 

If the present author would be asked why he did not first try to investigate the flow birefringence of polymer melts with the aid of a concentric cylinder apparatus, he could only answer because of the inconveniences experienced with such a type of an apparatus, when the viscosity of polymer melts is measured. As a matter of fact, the apparatus for polymer solutions, as described in the previous sections, would not.be suitable because of the impossibility to fill it with a polymer melt. As a consequence, a new type of apparatus had to be designed in any case. 

W.H. Talbott and J.D. Goddard, Streaming birefringence in extensional flow of polymer solutions, Rheol. Acta, 18, 507 (1979). 

Noordermeer JaWM, Ferry JD, Nemoto N (1975) Viscoelastic properties of polymer solutions in high-viscosity solvents and limiting high-frequency behaviour. Ill Poly (2-substituted methyl acrylates). Macromolecules 8(5) 672-677 Okamoto H, Inoue T, Osaki K (1995) Viscoelasticity and birefringence of polyisobutylene. J Polym Sci B Polym Phys 33 1409-1416... 

In this review of work at the Institute for Polymers and Organic Solids at UCSB, we summarize the results obtained from spectroscopic studies, quasi-elastic light scattering and field induced birefringence of dilute solutions. We then turn to more recent studies which focus on gels made up of networks of the rodlike molecules. 

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. 

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

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)). 

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. 

Zimm, BH, Dynamics of Polymer Molecules in Dilute Solution Viscoelasticity, Flow Birefringence and Dielectric Loss, Journal of Chemical Physics 24, 269, 1956. 

If one follows the solution viscosity in concentrated sulfuric acid with increasing polymer concentration, then one observes first a rise, afterwards, however, an abrupt decrease (about 5 to 15%, depending on the type of polymers and the experimental conditions). This transition is identical with the transformation of an optical isotropic to an optical anisotropic liquid crystalline solution with nematic behavior. Such solutions in the state of rest are weakly clouded and become opalescent when they are stirred they show birefringence, i.e., they depolarize linear polarized light. The two phases, formed at the critical concentration, can be separated by centrifugation to an isotropic and an anisotropic phase. A high amount of anisotropic phase is desirable for the fiber properties. This can be obtained by variation of the molecular weight, the solvent, the temperature, and the polymer concentration. 

Lodge, A. S. A network theory of flow birefringence and stress in concentrated polymer solutions. Trans. Faraday Soc. 52,120-130 (1956). 

At this point it should be noted that the conclusion drawn from flow birefringence measurements, viz. that p22 — p33 of polymer systems is very small compared with pn — pn is not always supported by other types of measurement. With the aid of pressure measurements in the walls of various rheometers (e.g. cone-and-plate apparatus) results have been obtained by a number of authors (refs. 26, 43, 44), showing that p23 — p33 should be positive and can have values up to 20 per cent of Pn Pta- 1-7 suggests for the investigated polyisobutylene solution... 

On the other hand, for dilute solutions one must take care of the refractive indices of polymer and solvent. The refractive index increment dnjdc of the polymer for the chosen solvent must be as small as possible, since otherwise the influence of the shape of the coil (form birefringence) would be noticeable. A more detailed discussion of this problem, however, is postponed to Section 5.1.1. Solvents with a sufficiently small refractive index increment have been called matching solvents. 

The most evident reason is that dilute solution measurements can preferably be compared directly with the unmodified dilute solution theory as reviewed in Chapter 3. As has already been pointed out in Section 2.6.1, the form birefringence in dilute solution can effectively be suppressed by the choice of a solvent of practically the same refractive index as the polymer. In such a "matching solvent the contrast between the coil of the macromolecule and its surrounding practically disappears. This means that, at the same time, the influence of the shape (form) of the coil disappears. Also the comparison with measurements on con-... 

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