Dynamic Investigations of Polymeric Systems in Viscoelastic State with Polymer Concentration Above Critical
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Fibre Chemistry, Vol. 52, No. 2, July, 2020 (Russian Original No. 2, March-April, 2020)
CHEMISTRY AND TECHNOLOGY OF CHEMICAL FIBERS DYNAMIC INVESTIGATIONS OF POLYMERIC SYSTEMS IN VISCOELASTIC STATE WITH POLYMER CONCENTRATION ABOVE CRITICAL Yu. A. Vinogradov
UDC 541.6:539:538
The dynamic deformation of polymeric systems (spinning solutions of polymers) with a polymer concentration above the critical value, characterized by an unstable deformation regime, was investigated. The unified theory of deformation of polymeric systems in a viscoelastic state, by means of which it is possible to undertake a rheological analysis of the viscoelastic characteristics of polymeric systems from two measurement points over a wide range of variation of velocity gradient, was used for the investigation. It was shown that the intersection of the classical loss modulus and elasticity modulus curves is due to transition of the polymeric system to an enforced elastic state with the appearance of an effect from negative complex viscosity and its negative elastic component. Good agreement is observed during comparison of the data of unified deformation theory and experimental data for the stable deformation region.
The theory of static deformation of polymeric systems (melts and solutions of polymers) in the viscoelastic and solid states was presented earlier [1]. More recently [2, 3] the theory was examined as applied to dynamic deformation. The results from experimental verification for the case of dynamic deformation of a polymeric system of the first type with a polymer concentration somewhat below the critical value were presented. For this type of curve in a wide range of variation of cyclic frequency (velocity gradient) a stable deformation regime, characterized by constant complex viscosity, was observed. This makes it possible to compare experimental and theoretical data on the complex viscosity and its viscous and elastic components with each other. Good agreement between them was observed. The present work gives the results from examination of the more complex process of dynamic deformation of polymeric systems of the second type with a polymer concentration above the critical value in stable and unstable regimes. A characteristic feature of such a deformation regime is intersection of the curves of the viscous component of the tangential force for the first measurement point and the curve for the elastic component of the tangential force for the second measurement point, which corresponds to intersection of the classical curves of the elastic modulus G′(ω1o) and the shear modulus G″(ω1o). A 14% spinning solution of polyacrylonitrile (PAN) and dimethyl sulfoxide (DMSO) at 298 K was used for the investigation. The viscosity-average molecular mass was calculated by means of the equation [ η] = 1,75 ⋅ 10 −3 Mυ0 ,66 [4] where η is the characteristic viscosity. The molecular mass of the polymer amounted to 290 kDa. The viscoelastic characteristics of the polymeric systems were investigated on a Haake RheoStress-1 rheometer (German
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