Search

A study has been made of the viscous properties of poly(para-benzamide) (PBA) solutions in dimethyl acetamide, which undergo a transition from an isotopic to an anisotropic (liquid-crystal) state at a definite concentration C*. The polymer solutions behave in many respects (as regards the concentration and temperature dependence of viscosity, etc.) like solutions of low molecular weight compounds forming a liquid crystal phase, although the transitions are less pronounced in the polymer solutions owing to their polydispersity. It is shown that the viscometric method, being extremely sensitive to C*, is convenient for determining phase diagrams of anisotropic polymer solutions. The values of C* as related to the molecular weight of PBA have been determined, and a general criterion for transition from isotropic to anisotropic solutions established; the latter has the form (CM)* ≈ 1.3 × 105 at 20°C. This criterion is in line with the condition for the formation of the liquid-crystal structure in a dispersion of rodlike particles as proposed by Flory. Generalized concentration dependences of viscosity have been plotted by reducing concentration to C* and viscosity, to the maximum viscosity at the phase transition point. In investigating the flow properties of PBA solutions we revealed the existence of a yield point in the range of low shear stresses, and an intersection of the flow curves of solutions of different concentration at high shear stresses, which excludes a generalized representation of the flow curves in reduced ordinary-type coordinates.

The pattern of variation of the content in gel particles of the ripening viscose is of considerable practical significance which explains the large volume of research devoted to the subject /1-3/. In most cases (including those mentioned), however, no correlation was discovered between the number and size of the particles on the one hand and the variation of the principal physico-chemical characteristics of the viscose, e.g. ripeness and viscosity) on the other. The writers showed elsewhere that a close relation exists between the variation of the viscosity of viscose and the chemical processes taking place during ripening. The observed coincidence of minimum viscosity and maximum content in primary xanthate groups (completion of the redistribution process) led to the formulation of a hypothesis to the effect that these phenomena may be interrelated with the changes in the gel-particle spectrum in the solution. To verify this hypothesis a comparison was earried out of the variations of the viscosity of the viscose, the chemical composition of the cellulose xanthate and the gel particle spectrum during ripehing at 20°C. The viscosity of the viscose (a-cellulose 6.8%, NaOH 5.6%, Y2,3 = 65 appr.) was determined on a Heppler viscometer; the relative content in primary Y6 and secondary Y2 3 xanthate groups was determined spectrophotometrically while the number of gel partlcles of varlous slzes was determined on a TuRZG-I particle counter in accordance with a known procedure /5/. The results (Fig.1) demonstrate that in the ripening viscose the minimum viscosity and thus the completion of the redistribution of the xanthate groups coincide with the minimum content in gel particles of all sizes. With further ripening the onset of the