Assessing Fast Structure Formation Processes in Isotactic Polypropylene with a Combination of Nanofocus X-ray Diffraction and In Situ Nanocalorimetryстатья
Статья опубликована в высокорейтинговом журнале
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Статья опубликована в журнале из списка Web of Science и/или Scopus
Дата последнего поиска статьи во внешних источниках: 17 ноября 2021 г.
Аннотация:A combination of in situ nanocalorimetry with simultaneous nanofocus 2D Wide-Angle XrayScattering (WAXS) was used to study polymorphic behaviour and structure formation in a singlemicro-drop of isotactic polypropylene (iPP) with defined thermal history. We were able to generate,detect, and characterize a number of different iPP morphologies using our custom-built ultrafast chipbasednanocalorimetry instrument designed for use with the European Synchrotron Radiation Facility(ESRF) high intensity nanofocus X-ray beamline facility. The detected iPP morphologies includedmonoclinic alpha-phase crystals, mesophase, and mixed morphologies with different mesophase/crystallinecompositional ratios. Monoclinic crystals formed from the mesophase became unstable at heatingrates above 40 K s−1 and showed melting temperatures as low as ~30 K below those measured foriPP crystals formed by slow cooling. We also studied the real-time melt crystallization of nanogramsizediPP samples. Our analysis revealed a mesophase nucleation time of around 1 s and the co-existenceof mesophase and growing disordered crystals at high supercooling ≤ 328 K. The further increaseof the iPP crystallization temperature to 338 K changed nucleation from homogeneous to heterogeneous.No mesophase was detected above 348 K. Low supercooling (≥ 378 K) led to the continuousgrowth of the alpha-phase crystals. In conclusion, we have, for the first time, measured the mesophasenucleation time of supercooled iPP melted under isothermal crystallization conditions using a dedicatedexperimental setup designed to allow simultaneous ultrafast chip-based nanocalorimetry andnanofocus X-ray diffraction analyses. We also provided experimental evidence that upon heating, themesophase converts directly into thermodynamically stable monoclinic alpha-phase crystals via perfectionand reorganization and not via partial melting. The complex phase behaviour of iPP and itsdependence on both crystallization temperature and time is presented here using a time–temperature–transformation (TTT) diagram.