Polymorphic structures phase diagram of shear-induced isotactic polypropylene/carbon fiber cylindrites

Publication date: 15 July 2018
Source:Materials & Design, Volume 150
Author(s): Jing Zhao, Chen Lu, Shuo Guo, Ke Wang, Qiang Fu
Depending on the thermal and mechanical conditions of pulling fiber in the supercooled melts of isotactic polypropylene (iPP), three kinds of cylindrite with variable polymorphic structure could be achieved, i.e., pure α-cylindrite, mixed α/β cylindrite, and β-rich cylindrite. However, precisely assessing the synergic action of thermal and mechanical factors on polymorphic structure of cylindrite is still challenging. Thanks to the great convenience of temperature gradient (T-g) technique in surveying structural transitions arisen from temperature variation, a crystallization temperature (Tc)-pulling rate (Rpull) dependent phase diagram of polymorphs in iPP/carbon fiber (CF) cylindrite was achieved readily through adopting different Rpulls. It was intriguing to suggest that (1) the temperature regime of mixed α/β cylindrite became narrower and narrower as increasing Rpull, implying a sudden transition from β-polymorph to α-polymorph upon intensive shearing; and (2) the upper threshold temperature that allowing generation of β fan-shaped domain was independent on Rpull, around 145 °C. Moreover, the structural difference between mixed α/β cylindrite and β-rich cylindrite was revealed in detail by scanning electron microscopy. An approximately regular nucleation line of β-form appeared in β-rich cylindrite, due to prevailing α-to-β bifurcation arisen from the edge-on α-lamellae. Whereas, occasional α/β bifurcation was found among a tremendous number of α-lamellae within the mixed α/β regime. Constructing the polymorphs phase diagram is of significance in manipulation of interfacial crystallization entities and fabrication of fiber-reinforced semi-crystalline polymers with high performances.Brief abstractIn this study, the combined effect of crystallization temperature (Tc) and pulling rate (Rpull) on the polymorphic structure of shear-induced cylindrite in isotactic polypropylene (iPP)/carbon fiber (CF) composite was well ascertained by using the T-g field technique. It was found that reducing Tc or improving Rpull might induce the transformations of polymorphism from pure α-cylindrite to mixed α/β cylindrite up to β-rich cylindrite. A crystallization temperature (Tc)-pulling rate (Rpull) dependent phase diagram of polymorphs in iPP/CF cylindrite was achieved readily. It was intriguing to suggest that (1) the temperature regime of mixed α/β cylindrite became narrower and narrower as increasing Rpull, implying a sudden transition from β-polymorph to α-polymorph upon intensive shearing; and (2) the upper threshold temperature that allowing generation of β fan-shaped domain was independent on Rpull, around 145 °C. The findings of this study are meaningful for construction of appropriate interfacially-crystallized structure, which could efficiently induce interfacial enhancement in polymer/fiber composites.

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