Hamideh Mirbaha

The effect of different elongation conditions on the crystalline structure and physical and mechanical properties of polyacrylonitrile/carbon nanotube (PAN/CNT) microfibers during the wet spinning process was studied. It turns out that the response of polymer chains in PAN/CNT and in PAN fibers to the stretching forces from jet stretching and steam drawing is different. The X-ray diffraction (XRD) results showed that the crystalline domain size in steam-drawn PAN/CNT fibers is 1.5 times larger than in PAN fibers. CNTs alter the optimum stretching conditions, as they improve the crystalline structure of the PAN/CNT fibers at lower steam drawing ratios than PAN fibers, through nucleation of crystals on their surface. Synchrotron-radiation XRD studies revealed that the presence of CNTs improves the crystal orientation of PAN/CNT fibers significantly. In addition, steam drawing is more effective in improving the crystal orientation than jet stretching. The mechanical properties of PAN/CNT fibers have also been affected by steam drawing more than jet stretching. Multiwalled CNTs have the biggest impact on Young’s modulus. The Young’s modulus of PAN/CNT fibers could increase up to 19% higher than PAN fibers at specific stretching conditions, i.e., steam drawing ratio of 2.5. Better orientation of polymers and crystals in the fiber direction is the reason for the enhancement of Young’s modulus. To our knowledge, the differences between the response of PAN/CNT and PAN fibers to stretching forces inside coagulation bath and after fiber coagulation as well as the difference in evolution of crystalline structure at different stretching stages has not been reported elsewhere.

Wet spinning of polyacrylonitrile/carbon nanotubes (PAN/CNT) composite fibers was studied and the effect of spinning conditions on structure and properties of as-spun fibers influenced by the presence of CNTs investigated. Unlike PAN fibers, shear force had a larger effect on crystalline structure and physical and mechanical properties of PAN/CNT composite fibers compared to the elongational force inside a coagulation bath. Under shear force CNTs induced nucleation of new crystals, whereas under elongational force nucleation of new crystals were hindered but the already formed crystals grew bigger. To our knowledge, this key effect has not been reported elsewhere. At different shear rates, strength, Young’s modulus and strain at break of PAN/CNT as-spun fibers were improved up to 20% compared to PAN fibers. Application of jet stretch had less influence on physical and mechanical properties of PAN/CNT fibers compared to PAN fibers. However, the improvement of interphase between polymer chains and CNTs as a result of chain orientation may have contributed to enhancement of Young’s modulus of jet stretched composite fibers.

Thermal stabilization is an important step in production of carbon fiber from polyacrylonitrile (PAN) precursors. During thermal stabilization step different thermochemical reactions take place almost simultaneously. Understanding the onset and temperature range of the stabilization reactions is a key for adjusting processing parameters such as tension, stretching, etc. However, stabilization reactions are very complex and overlap with each other. In order to separate the stabilization reactions, we combined the results of different thermal analysis techniques, namely Differential Scanning Calorimetry (DSC), Thermogravimetry (TGA) and Thermomechanical Analysis (TMA), to study behaviour of PAN precursors during stabilization. By means of combining the results of these techniques, we were able to determine the temperature range and occurrence order of each of stabilization reactions regardless of the composition of initial PAN fibers and history of fiber formation.

Using different indices calculated from Fourier Transform Infrared Spectroscopy (FTIR), X‐ray Diffraction (XRD), and Differential Scanning Calorimetry (DSC) spectra, progress in stabilization reactions of three different commercial grade polyacrylonitrile (PAN) fibers is calculated. From each analysis technique quantitative indices are computed which could assess in some particular reactions. Combination of these indices gives further information about the progress of stabilization reactions which cannot be concluded from single indices. The results show that different indices are not fully consistent with each other, depending on the analysis technique and the changes they assess. The advantages and disadvantages of each index are investigated and practical indices are identified which can be used to design the optimum stabilization process. In addition, by combination of some indices it is possible to separate the temperature ranges in which reactions occur in amorphous or crystalline regions. This approach can be used to design appropriate stretching process during stabilization. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40343.