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

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., pu re alpha-cylindrite, mixed alpha/beta cylindrite, and beta-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 (T-c)-pulling rate (R-pull) dependent phase diagram of polymorphs in iPP/carbon fiber (CF) cylindrite was achieved readily through adopting different R(pull)s. It was intriguing to suggest that (1) the temperature regime of mixed alpha/beta cylindrite became narrower and narrower as increasing Rpull, implying a sudden transition from beta-polymorph to alpha-polymorph upon intensive shearing; and (2) the upper threshold temperature that allowing generation of beta fan-shaped domain was independent on Rpull, around 145 degrees C. Moreover, the structural difference between mixed alpha/beta cylindrite and beta-rich cylindrite was revealed in detail by scanning electron microscopy. An approximately regular nucleation line of beta-form appeared in beta-rich cylindrite, due to prevailing alpha-to-beta bifurcation arisen from the edge-on alpha-lamellae. Whereas, occasional alpha/beta bifurcation was found among a tremendous number of alpha-lamellae within the mixed alpha/beta 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 abstract: In this study, the combined effect of crystallization temperature (T-c) and pulling rate (R-pull) 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 T-c or improving R-pull might induce the transformations of polymorphismfrompure alpha-cylindrite to mixed alpha/beta cylindrite up to beta-rich cylindrite. A crystallization temperature (T-c)-pulling rate (R-pull) dependent phase diagram of polymorphs in iPP/CF cylindrite was achieved readily. It was intriguing to suggest that (1) the temperature regime of mixed alpha/beta cylindrite became narrower and narrower as increasing R-pull, implying a sudden transition from beta-polymorph to alpha-polymorph upon intensive shearing; and (2) the upper threshold temperature that allowing generation of beta fan-shaped domain was independent on Rpull, around 145 degrees 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. (C) 2018 Elsevier Ltd. All rights reserved.