Spatially Resolved Flow-Induced Crystallization Precursors in Isotactic Polystyrene by Simultaneous Small- and Wide-Angle X-ray Microdiffraction
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Abstract
The structural and morphological evolution of shear-induced crystallization precursors generated by pulling a single fiber through a thin layer of molten isotactic polystyrene (i-PS) has been spatially resolved by simultaneous small- and wide-angle X-ray microdiffraction in samples with different thermomechanical histories. Frozen precursor structures with shish-kebab morphology have been detected at different levels of evolution during early stages of their formation, the first step being the development of bundles of few parallel chains oriented along the flow direction (shish) followed by the growth of layered lamellar assemblies (kebabs) simultaneously to the appearance of a very weak wide-angle X-ray diffraction (WAXS) signal corresponding to a crystallinity index lower than 1%. A high degree of orientation of the ordered precursors has been found in the polymer matrix at very close distance from the fiber surface when the sample is quenched to room temperature immediately after cessation of flow; much lower crystal orientation has been detected when the sample was left to crystallize during some time at 180 °C before quenching to below Tg. Competition between relaxation and crystallization leads to destruction of the smallest and less oriented precursors, while new crystallites with a slightly different orientation are generated. The experimental evidences are consistent with the existence of quasi-ordered clusters in the melt whose size and orientation distributions are dictated by characteristics of the flow field.
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