Cellulose birefringence

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. 

Bipolar plates, MCFC, 12 223 Bipolar transistors, silicon based semiconductors in, 22 246-249 Bipolymers, 20 533, 534 Bipropellants, 10 727 Bipyridines, uses for, 21 127 Bipyridinium herbicides, 13 315 Bipyridium, 24 51 Bipyridyl trimers, 24 50 Biquinolines, 21 200 Birefringence, 14 675, 680 19 745 in ferroelectric crystals, 11 94 polycarbonate, 19 822 of regenerated cellulose fibers,... 

Chanzy and Peguy (13) were the first to report that cellulose forms a lyotropic mesophase. They used a mixture of N-methyl-morpholine-N-oxide (MMNO) and water as the solvent. Solution birefringence occurred at concentrations greater than 20% (w/w) cellulose. The concentration at which an ordered phase formed increased as the cellulose D.P. decreased. The persistence length of cellulose in MMNO-H2O is not known but presumably it has an extended chain configuration in this solvent. Again the question arises as to what is the relevant axial ratio to be used for cellulose. This will be discussed further below. 

Fig. 5.8. Cotangent of the doubled extinction angle (corrected for solvent birefringence) vs. reduced shear rate f s for a series of cellulose tricarbanilate fractions in benzophenone at 55° C (772). Broken and dotted lines are explained in the text. Hatched area indicates location of experimental results on anionic polystyrenes. Molecular weights of cellulose tricarbanilate fractions () 27000, (f) 38000, (A) 57000, (a) 90000, (o) 152,000, ( ) 280,000, (o) 500,000, ( ) 720,000 and...

FIG. 13.93 Birefringence An as a function of the dynamic compliance E 1 = 5 for various regenerated cellulose fibres. 

Nematic phase birefringence, which was observed in the cellulose solutions made from the 24.5/75.5 solvent, appeared immediately following dissolution and persisted throughout a two week period. The uniformly dispersed birefringent patterns were the most prevalent ones in the nematic solutions at all DPs studied, but Schlieren and thread-like patterns indicating nematic phases were readily observed in solutions of DP 450 cellulose. 

Table II. Specific Layer Birefringence Cellulose/NH3/NH4SCN+ of ...

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. 

More recently, Hawkinson (8) observed birefringence between crossed polars for solutions of cellulose in TPA-CH2C12 (70 30 /v) at concentrations as low as 4% (w/w). 

Cellulose Mesophases. An anistopropic phase is formed in a 6% (w/w) solution of cellulose in TFA-CH2Q2 (60 40 sN) and remains anisotropic for at least 16 days (Table I). In TFA-CH2CI2 (70 30 l ) solutions birefringence was observed as... 

B/C = Birefringent throughout undissolved cellulose powder of fiber visible... 

The microform effect can also substantially affect flow birefringence is induced by a change in the refractive index of the solvent and may influence not only the value but also the sign of the observed anisotropy. According to the data in Table 6 this occurs, for example, for cellulose butyrate and nitrate Milutions. 

This suggestion is in agreement with the well-known fact that the equilibrium rigidity of cellulose ether and ester molecules changes greatly with solvent composition and is also confirmed by the stroi n ative temperature dependence of the statistical size of their chains. The latter property is manifested in a decrease of the intrinsic viscosity, translational friction - and the flow birefringence ) with increasing temperature (Fig. 32) ... 

Fig. 19. Plot of birefringence as measured by the compensator technique against the orientation function as measured by the sonic method. Values in parenthesis represent theoretical values for Nylon 66, cellulose, and polypropylene from top to bottom respectively...

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