An index Xh21 with numerator calculated solely from solubility parameters and denominator measured by on-line viscosity of the fiber precursor in coagulation medium was defined as an indicator of the fiber structure and tensile properties. The Xh 21 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 Xh 21 value of about 0.8 3 10 6 s 21 led to finger-like structure with overall fiber porosity of 82 vol %. By reducing Xh 21 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 Xh 21 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 modulusporosity data verified the solid-liquid phase separation through nuclei growth-resistance as the main governing morphological evolution mechanism.

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The main aim of the present study is to fabricate a high performance chitosan (CS)/polyvinyl alcohol (PVA) electrospun nanofibrous mat having a high content of CS, a desirable morphology (defect‐free structure) and a superfine diameter (approx. 100 nm). As electrospinning of constructions containing CS is known as a complex process, it is necessary to employ systematic control and optimisation of processes. In this regard, the controlling and optimisation of the processes were followed by two subsequent stages. In the first stage, morphology controlling parameters were investigated with respect to CS/PVA solution characteristics including CS concentration, solvent concentration and the content of the partner polymer (PVA). In the second stage, in order to attain the finest possible diameter, process modelling was carried out in terms of processing parameters (applied voltage, nozzle‐collector distance and feed rate) by using response surface methodology (RSM). According to the experimental results of the first stage, the best morphological structure containing the highest content of CS was obtained under 3% (w/v) of CS, concentrated acetic acid (90%) and 20% weight ratio of PVA. The significance of the applied model was confirmed by statistical approaches and the effect of the selected parameters on the diameter was studied. Experimentally, the finest diameter of 104 ± 18 nm was obtained under optimised processing parameters determined from the RSM technique. The experimental value of the nanofibre diameter was in close agreement with the predicted value in which the prediction error of the model was only 1.92% confirming the high reliability of the applied model.

Acrylic fibers are the major precursor of carbon fibers. In this work, the synergistic effect of polymer concentration and jet stretching on porosity, morphology and mechanical properties of wet-spun poly(acrylonitrile) fibers were studied and then the role of void size was elucidated. The results showed that the effect of jet stretching on fiber porosity is negligible below a threshold polymer concentration. Increasing polymer concentration up to 20 vol. % reduced the total porosity significantly but increased the nanometric void population slightly. On the other hand increasing polymer concentration and jet stretching simultaneously, improved mechanical properties as a result of a remarkable reduction in porosity. In addition, stretching shifted nanovoids distribution strongly toward smaller sizes. This was attributed to the effect of stretching and increasing polymer concentration on reducing the diffusion coefficient. Young's modulus of fibers also increased with decreasing overall porosity. Finally, strength-diameter correlation showed a good agreement with the Griffith's theory.