Polymer Engineering Faculty Research


Atomic force microscopy observations on the structure development during uniaxial stretching of PP from partially molten state: effect of isotacticity

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Our main focus is to investigate the role of isotacticity on the development of morphology in PP films uniaxially deformed from the partially molten state. Polypropylene films with low isotactic index 43 exhibit crosshatched lamella. At the early stages of deformation, this morphology is first broken into small pieces and then microfibrils start to appear. As the deformation proceeds, this morphology transforms from primarily shish kebab morphology to a more fibrillar superstructure with sparse population of laterally grown lamellae along their lengths. The microfibrils 15−20 nm in diameter are observed in isotactic index 43 and isotactic index 64. Thus, a macrofibril is composed of ca. 100 microfibrils along the width of macrofibrils that are more ribbonlike with the width of the ribbons coinciding with the b-axis. A nodular superstructure that contains one or more alternating lamella−amorphous pairs was found to be promoted at lower processing temperatures, lower stretching speed, and higher isotacticities. In low-tacticity PP, upon deformation the formation of a three-dimensional crystalline/entanglement network was found to exert hydrostatic pressure in the amorphous regions that lead to their highly localized blooming out of the surface. Room temperature aging was found to coalesce these amorphous regionsa process that resembles the late stages of phase separation in partially miscible systems.

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