The Effect of Tetraethoxysilane on the Phase State, Rheological Properties, and Coagulation Features of Polyacrylonitrile Solutions

Abstract

The compatibility of tetraethoxysilane (TEOS) with polyacrylonitrile (PAN) solutions in dimethyl sulfoxide, the morphology of the mixed systems, and their rheological behavior have been analyzed. The combination of interferometry, refractometry, and optical microscopy has been employed to study the phase equilibrium realized in mixtures of TEOS with dimethyl sulfoxide and a PAN solution and to plot phase diagrams, which have indicated that TEOS is soluble in the PAN solution up to TEOS concentrations of 10–11%. As the TEOS content is increased within a concentration range of 10–20%, an emulsion with droplet sizes up to 40 µm is formed. At higher TEOS concentrations, the macroscopic separation of the system takes place. It has been shown with the use of rotational rheometry that, in the range of solutions, the presence of TEOS leads to a decrease in their viscosity and elasticity, while the viscosity increases in the range of emulsification. The analysis of the dynamic data has, for the first time, shown the bifurcation of the dependences of the storage modulus on the loss modulus at the point of phase separation. The calculation of the characteristic relaxation time of the ternary system and the dependence of this time on TEOS concentration has indicated a dramatic growth of the relaxation time in the region of emulsification, with this growth being determined by the relaxation properties of interfaces. Considering the obtained systems as a raw material for producing composite fibers, a solution droplet has been used to simulate the effect of TEOS on the kinetics of coagulation of PAN solutions with precipitants having different activities. It has been shown that, depending on TEOS concentration, the precipitation may yield gels with the content of this organosilicon additive either uniformly or nonuniformly distributed over the hypothetic cross section of a fiber.