Distribution of higher‐order structures in injection‐molded polypropylenes

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Abstract
Flexural test specimens were injection‐molded from six homoisotactic polypropylenes with MFI = 0.49‐25.1 dg/min under cylinder temperatures of 200‐320°C. Distributions in the flow direction of higher‐order structures such as crystallinity Xc, thickness of skin layer, a*‐axis‐oriented component fraction [A*], and crystalline orientation functions and distributions in the thickness direction of higher‐order structures such as Xc, β‐crystal contents, [A*], and crystalline orientation functions were studied. These higher‐order structures are inhomogeneous in the flow and thickness directions, which strongly influences the product properties such as mechanical and thermal properties. Molecular orientation process in injection molding was theoretically analyzed from a viewpoint of growth of recoverable shear strain at the gate and its relaxation in the cavity, which could considerably well explain the variations in the flow and thickness directions of the quantities such as thickness of the skin layer and crystalline orientation functions which express the degree of molecular orientation.