Molecular Aspects of Flow-Induced Crystallization of Polymers

Abstract

Like teeth, bone and sea shells, semicrystalline polymers combine strength with toughness by forming a nano-scale composite with platelet-like crystals stacked with noncrystalline material between them. The morphology and orientation distribution of the nanostructure dictate the material properties. Dynamics of polymer chains in the melt play an important role in controlling the morphology, especially under the influence of flow. Using bimodal isotactic polypropylenes to reveal the effects of small concentrations of very long chains, in collaboration with Mitsubishi Chemical, we show that long chains have a profound effect when they are so long that they can undergo chain stretching, particularly when the long chain concentration is at or above their overlap concentration. When subjected to identical stress at identical subcooling, the blends containing long chains undergo dramatically faster crystallization with very strong orientation. The "long chains" enhance formation of highly oriented crystallization precursors ("shish") on which oriented lamellae ("kebabs") subsequently grow. In collaboration with Sumitomo Chemical and The University of Tokyo, we test the hypothesis that the kebabs are actually composed of long chains using isotopic labelling of selected fractions and small angle neutron scattering (SANS). The results show that long chains in the shish are at the same concentration as they are everywhere else: there is no neutron scattering contrast when the long chains are the deuterium labelled ones! The long chains are essential for the formation of shish and they play their role by "recruiting" adjacent chains into formation of the shish. Placing molecular defects on the longest chains inhibits their ability to serve this role, providing a molecular tool to independently control the melt elasticity (by choice of the length and concentration of the long chains) and the flow-induced crystallization behavior (by selecting the comonomer content, for example, of the long chains).

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