Structure and mechanical anisotropy of injection-molded polypropylene with a plywood structure

Injection-molded isotactic polypropylene (PP) containing a small amount of the β-nucleating agent N,N′-dicyclohexyl-2,6-naphthalenedicarboxamide exhibits a unique molecular orientation that is similar to that of plywood. X-ray diffraction and birefringence distribution measurements revealed that PP chains in β-form crystals are oriented perpendicular to the flow direction through epitaxial crystallization on the needle-shaped nucleating agent, even in the skin layer. Furthermore, PP chains in α-form crystals orient along the flow direction through flow-induced crystallization. Although the skin has a ‘woven-cloth’ structure in which the two crystalline forms are oriented perpendicular to each other, it shows positive birefringence, on average, because of the stronger orientation of α-form crystals. In the core, not only β-form crystals but also α-form crystals are oriented perpendicular to the flow direction. As the average orientation direction in the skin layer is perpendicular to that in the core, the dynamic tensile modulus in the flow direction is almost identical to that in the transversal direction, resulting in a low level of mechanical anisotropy. Moreover, the anisotropy in thermal expansion is greatly reduced by the extraordinary molecular orientation.