Crystallization of Flow-Induced Precursors in Polyamides at Low and High Supercooling

摘要

Flow induced crystallization (FIC) occurs in polymers after an interval of shear flow at rates which exceed the reciprocal of the Rouse time of the longest chains. Under these flow conditions, the longest chains orient and nucleate crystallization processes which occur after the cessation of flow. The flow-induced precursors are stable to temperatures greater than the melting temperature of the crystal lamellar phase, and their nucleating efficiency has been explored using fast scanning calorimetry (FSC), polarized light microscospy, and X-Ray diffraction. Carefully sheared polyamide samples were created using parallel plate and capillary rheometery techniques, and after rheological characterization the samples were removed and then used for FSC investigation. For flexible polymers such as polyamide 66, the flow-induced precursors strongly accelerate crystal nucleation at temperatures where heterogeneous nucleation is dominant. The magnitude of acceleration scales with the shear work used to create the flow-induced precursors in the me increasing work yields increasing rate of crystallization until a saturation in work is reached. The structure and lifetime of the flow-induced precursors in the melt will also be discussed.