A Novel Method for Investigation of Entropic Stress in Prestabilization of PAN‐Based Precursor Fibers

Applying stress in the fiber axis direction during the prestabilization process is the best method to control entropic relaxation. Evaluation of entropic stress is necessary for exact determination of required imposed stress. Accordingly, in this paper three types of special-grade polyacrylonitrile (PAN) fibers with different physical-mechanical properties and morphological characteristic are studied. To determine thermal-induced entropic stresses, we use the thermomechanical analysis method from room temperature to beginning of chemical reactions under certain stresses. It is shown that three thermophysical behaviors including the first return, second return, and sans return occurs under certain stresses during the thermal process. There are distinct temperatures for the first and second return behaviors, in which fibers return to primary length after shrinkage or elongation. These temperatures depend on imposed stress during prestabilization. We found that if no elongation or fiber shrinkage occurs in a specific stress and temperature, it shows that imposed stress is equivalent to thermal entropic stress. Changes in shrinkage as a function of temperature for various external imposed stresses are also observed. According to our findings, the onset of entropic stress strongly depends on the orientation index of the paracrystalline region. The maximum entropic stress occurs in the rubbery flow region in amorphous phases and can be obtained from a derivative of the second-order polynomial trend line of the entropic stress-temperature diagram.