Axial birefringence

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. 

The rotational diffusion constant in water at 25° and neutral pH as measured by electric birefringence (258) is 230 X 105 sec-1 or 0.73 X HT8 sec as a relaxation time. For a hydrodynamic ellipsoid of dimensions 66 X 22 A and a molecular weight of 14,000, the calculated relaxation tilde is 0.72 X 10-8 sec. However, the apparent asymmetry of the molecule from the X-ray structure corresponds to an axial ratio of no more than 2 1 rather than 3 1. 

The anisotropy of polarizability can be positive (eg, polycarbonate) as well as negative (eg, polystyrene). This offers the possibility of minimizing birefringence by copolymerization or blending of suitable polymers with the right mixture ratio, eg, blends of poly(phenylene ether) (PPE) and polystyrene (PS). The magnitude of birefringence of axial-symmetrically oriented polymers vs their molecule orientation has been described (182). 

Polymeric filaments produced in this way have a diameter of 100 to 300 fim and show anisotropy in their properties, indicating molecular orientation. For instance the birefringence of this fiber is 0.0073. Table 1 compares the modulus measured both in axial and in lateral direction, with the modulus of the same material cured under isotropic condition at 80 C. The increase in modulus in axial direction is obvious. The decrease of the modulus at higher temperatures can be ascribed to both the glass transition and melting of crystalline areas. 

It has been demonstrated that molecular orientation can be achieved starting with a low molecular weight species which is oriented in an elongational flow and subsequently cured under UV-irradiation. The orientation of the monomer is frozen-in by the ultra-fast process of polymerization and crosslinking. Both extrusion and stretching can be carried out at relatively low temperatures and pressures. Polymer filaments produced in this way are definitely anisotropic as is evidenced by their birefringence and by a strong increase of the tensile modulus and a decrease of the thermal expansion coefficient in the axial direction. 

Although both the and 6-functions are too insensitive and uncertain for the determination of axial ratios, the length, L, of the equivalent ellipsoid (prolate) can be calculated with confidence from either the intrinsic viscosity at zero gradient or the more commonly known flow birefringence. This can be shown as follows Eq. (22), (19a and c), and (18a and c) can be rewritten as... 

Proteins and polypeptides Range of axial ratio assumed From [))]o (A) From flow birefringence (A) From light scattering (A)... 

As mentioned earlier, ideally the best answer should come from the determination of the 5-function. Unfortunately at present the rotary diffusion coefficient is usually the least reliable quantity in all hydrodynamic measurements because of errors inherent in the physical methods of flow birefringence and perhaps also non-Newtonian viscosity (see Section IV). (Electric birefringence also may not give the same rotary diffusion coefficient as the other two methods, since the equivalent ellipsoids can be different under shearing stress and under electrical field.) Edsall (1954) has also illustrated the impossibility of evaluating the axial ratio from the 5-function. The latter was about 0.80 for fibrinogen which corresponded to a prolate ellipsoid with an axial ratio of more than 300. If the rotary diffusion coefficient were only about 15% greater than that listed in Table V the calculated axial ratio would decrease to between ten and twenty. 

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