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Thermal properties and influence of orientation on crystalline morphologies in stereoblock polypropylene and related elastomers

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Abstract

Atomic force microscopy (AFM), small angle X-ray scattering (SAXS), temperature modulated differential scanning calorimetry (TMDSC), variable heating rate DSC, an independent rapid heating rate method for melting points, and cyclic mechanical testing were used to study semicrystalline thermoplastic elastomeric polypropylenes (ELPPs) and related semicrystalline polyolefins including ethylene copolymers. Low crystallinity (ca., 9 and 15%) ELPP samples were studied by AFM in the nonoriented and melt-oriented states. AFM images taken as a function of time after quenching of a melt-drawn and highly nucleated film resolved details of secondary crystallization involving lateral growth on the ordered row-nucleated structures. For nonoriented films, isothermal melt crystallization at high temperatures (110 °C) led to similar features for the two ELPPs. The dominant crystalline morphology studied by AFM consisted of small (several nm in width) granular crystallites organized into immature but large spherulites spanning tens of microns. A striking cross-hatch morphology was detected in regions of the surface in 110 °C crystallized samples, which is contrasted with melt-drawn films where row nucleated structures dominated the morphology in the film under no external stress. AFM was also used to monitor the morphological changes that occurred as the films were stretched at 25 °C. Break-down of lamellae was observed, resulting in oriented narrow fibrils. Cyclic stress-strain curves showed the expected result where lower crystallinity ELPPs had higher recoverable levels of set after both 100 and 500% elongation. TMDSC was used to resolve the broad melting and recrystallization regions in these low to medium crystallinity ELPP systems, and to contrast the results with ethylene copolymers. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011

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