Sulfur birefringence

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. 

Liquid crystals, due to the presence of the ordered domains, are optically anisotropic, i.e. birefringent. This can be easily verified by observing the liquid crystal, e.g. aramid and sulfuric acid solution, at rest between crossed polarizers. The parallel arrays of polymer chains in liquid crystalline state become even more ordered when these solutions are subjected to shear as, for example, in extruding through a spinneret hole. It is this inherent property of liquid crystal... 

The occurrence of higher concentrations of Soo above 160 °C results in a birefringence of sulfur melts [50] with a maximum between 175 and 185 °C at which temperature the viscosity also attains its maximum value. 

The presence of negatively charged sulfonic groups in CNP obtained by hydrolysis with sulfuric acid significantly affect their properties, such as the thermal stability of the nanoparticles, as well as phase stability, rheological behavior, and birefringence of nanocrystal suspensions. 

Fig. 5. Shear-flow induced birefringence as a function of temperature and shear-rate for an isotropic PpPTA 9% (w/w) solution in sulfuric acid...

Shear flow-induced birefringence measurements of an isotropic solution of PpPTA in concentrated sulfuric acid with a clearing point of 45 °C were also performed by Picken [77]. As shown in Fig. 5 the flow-induced birefringence increases strongly when the isotropic-nematic transition is approached. The results demonstrate that the application of a relatively small shear rate already leads to a degree of orientational order, in the initially isotropic solution, that is comparable with the order in the nematic phase. This points to a strong coupling between the orientation and the external flow field, and to the occurrence of a shear-induced phase transition. 

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