Water effect on the rheologic behavior of PAN solution during thermal‐induced gelation process

Abstract

The thermal‐induced gelation process in a polyacrylonitrile (PAN) solution containing different amounts of water was investigated through both dynamic and steady‐state rheologic measurements. During the cooling process, the storage modulus G′ was found to intersect with the loss modulus G″ at a temperature which can be considered as the incipient gel point Tig. This crossover point increased with the water content in the PAN solution. On the basis of the power law $G'\tilde{G}\tilde{\omega} {}^{n} $ and the relation ${\rm tan}\ \delta = G(\omega )/G^{'}(\omega ) = {\rm tan}(n\pi /2) = {\rm const}$, the loss tangent tan δ was traced with decreased temperature at various oscillation frequencies, and a crossover point was observed at the temperature a little higher than the Tig above for all PAN solutions. The water content might affect the value of the relaxation exponent n via two different mechanisms, that is, hydration decoupling of the pendant nitrile groups in PAN at low water content (2 wt% water) and molecule aggregation at high water content (4 and 6 wt%). Nevertheless, the slight difference in the n values reflects the unique structure of the critical gels irrespective of the water level. The apparent viscosity ηa of either the PAN solution or the PAN gel was found to rise with increased water content and both showed pronounced shear‐thinning behavior. Compared with temperature, the water effect on the apparent viscosity ηa is more significant due to the water‐induced aggregation of the molecular chains. Copyright © 2010 John Wiley & Sons, Ltd.

Viscoelastic properties and structure of poly(acrylonitrile‐co‐methacrylic acid) polymer solutions for gel spinning at long aging times

“…Several rheological studies have been conducted on PAN solutions to understand gelation in this system because of its relevance to carbon fiber manufacture, and the results presented here are largely consistent with those published previously. However, few efforts have included characterization of aging behavior at long times, which was the motivation of this study.…”

“…As many of the methods used to produce precursor fibers are solution‐based, rheological studies are used to more fully understand the link between polymer solution attributes such as solvent, polymer concentration, polymer molecular weight, homogeneity, and the precursor fiber structure. In particular, rheological characterization has been used to characterize gelation and gel structure in precursor materials for gel spinning . Gelation of PAN‐based polymers occurs via thermoreversible and irreversible pathways depending on the system temperature with irreversible gelation occurring at temperatures high enough to liberate HCN from the polymer and induce chemical crosslinking between polymer chains .…”

“…In particular, rheological characterization has been used to characterize gelation and gel structure in precursor materials for gel spinning. [14][15][16][17] Gelation of PAN-based polymers occurs via thermoreversible and irreversible pathways depending on the system temperature with irreversible gelation occurring at temperatures high enough to liberate HCN from the polymer and induce chemical crosslinking between polymer chains. 18 Thermoreversible gelation is studied more frequently as it is more relevant to the production of precursor carbon fiber.…”

Viscosity of polyacrylonitrile solutions: The effect of the molecular weight

“…Recent studies are devoted to the investiga tion of gelation in solutions [12][13][14][15] and of the effects of the polymer-solvent interaction [16,17], increased molecular mass [18,19], and chain microstructure [20] on the viscosity and viscoelasticity of solutions.…”

Designing Materials and Processes for Strong Polyacrylonitrile Precursor Fibers

“…The mechanical properties of the gel-spun fibers are expected to be excellent owing to their high orientation [73]. Indeed, the pregelled gel-spun fibers exhibited high crystallinity (e.g., 70.48% [74]) with a desirable cross-section (i.e., circular, low pore density, and little core/shell difference) (Figure 7) [71,75,76]. Gel-spun PAN fibers with a high molecular…”