Abstract
The flow-induced microstructure of a mesophase pitch was studied within custom-made dies for changing wall shear rates from 20 to 1,100 s − 1, a flow scenario that is typically encountered during fiber spinning. The apparent viscosity values, measured at the nominal wall shear rates ranging from 500 to 2,500 s − 1 using these dies, remain fairly constant. The microstructure was studied in two orthogonal sections: r–θ (cross section) and r–z (longitudinal mid plane). In these dies, the size of the microstructure gradually decreases toward the wall (to as low as a few micrometers), where shear rate is highest. Furthermore, as observed in the r–θ plane of the capillary, for a significant fraction of the cross section, discotic mesophase has a radial orientation. Thus, the directors of disc-like molecules were aligned in the vorticity (θ) direction. As confirmed from the microstructure in the r–z plane, most of the discotic molecules remain nominally in the flow plane. Orientation of the pitch molecules in the shear flow conditions is consistent with that observed in controlled low-shear rheometric experiments reported earlier. Microstructral investigation suggests that the radial orientation of carbon fibers obtained from a mesophase pitch originates during flow of pitch through the die.
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Acknowledgement
This material is based upon work partly supported by The Engineering Research Centers Program of the National Science Foundation under NSF award number EEC-9731680 and the Department of Energy Experimental Program to Stimulate Competitive Research program under grant number DE-FG02-07ER46364. Any opinions, findings, conclusions, or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the Department of Energy or the National Science Foundation.
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Kundu, S., Ogale, A.A. Rheostructural studies of a discotic mesophase pitch at processing flow conditions. Rheol Acta 49, 845–854 (2010). https://doi.org/10.1007/s00397-010-0448-7
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DOI: https://doi.org/10.1007/s00397-010-0448-7