Designing index of void structure and tensile properties in wet-spun polyacrylonitrile (PAN) fiber. I. Effect of dope polymer or nonsolvent concentration

An index X eta(-1) with numerator calculated solely from solubility parameters and denominator measured by on-tine viscosity of the fiber precursor in coagulation medium was defined as an indicator of the fiber structure and tensile properties. The X eta(-1) values of wet-spun and wound polyacrylonitrile fibers from their dimethylformamide solutions with different polymer concentrations (series A) or nonsolvent concentrations in 10 vol% polymer solutions (series B) into water with draw ratio of one were determined and compared with the corresponding fiber structure and tensile properties. The X eta(-1) value of about 0.8 x 10(6) s(-1) led to finger-like structure with overall fiber porosity of 82 vol%. By reducing X eta(-1) through dope polymer concentration enhancement to 20 vol%, overall fiber porosity decreased to 62 vol % via substitution of some micrometer voids with dense polymer ligament. Accordingly, strong fiber modulus and elongation at break enhancement were observed due to structural defect reduction and cohesive energy density increment. On the other hand, dope nonsolvent concentration increment from 0 to 5 vol % at 10 vol %, polymer concentration showed minute overall fiber porosity decrement via X eta(-1) increment through micrometer void substitution with nanometer ones (nuclei). Therefore, mild fiber modulus and elongation at break improvements were detected due to defect size reduction which magnifies mechanical properties improvements. Curve fitting of the Wang's second order modulus-porosity correlation to the as-spun fibers modulus-porosity data verified the solid-liquid phase separation through nuclei growth-resistance as the main governing morphological evolution mechanism. (c) 2008 Wiley Periodicals, Inc.