Dario Cavallo

Polymer crystallization studies under processing-relevant conditions at the SAXS/WAXS DUBBLE beamline at the ESRF

Recent developments on the experimental infrastructure and the acquisition of new detectors on the Dutch–Belgian beamline BM26B at the ESRF offer novel and promising possibilities for synchrotron X-ray experiments in the field of polymer crystallization under processing-relevant conditions. In this contribution, some of the most recent experiments mimicking conditions similar to those relevant for the plastics processing industry are discussed. Simultaneous thermal analysis and wide-angle X-ray scattering (WAXS) experiments, at the millisecond time-frame level, on β-nucleated isotactic polypropylene (i-PP) samples subjected to ballistic cooling up to 230 K s−1, show that the efficiency of the nucleating agent can be suppressed when quenched cooling rates higher than 130 K s−1 are used. In situ WAXS experiments using small-scale industrial equipment during a real film blowing process reveal the dependence of the onset of crystallinity (the so-called freeze line) and the crystal orientation as a function of different take-up and blow-up ratios. In situ small-angle X-ray scattering (SAXS) experiments during high-flow fields reveal the formation of shish and kebab structures in i-PP as a function of the imposed stress. Quantitative analysis of i-PP flow-induced structures is presented. The beamline specifications required to obtain high quality and industrially relevant results are also briefly reported

Exploration of the effect of 2,6-(t-Bu)2-4-Me-C6H2OH (BHT) in chain shuttling polymerization

The interactions of a sterically hindered phenol [2,6-(t-Bu)2-4-Me-C6H2OH] (BHT) with the scavenger MAO (AlR3) and ZnR2 during Hf/Zr-based chain shuttling polymerization in a semi-batch reactor have been investigated. NMR model studies indicated preferential binding of BHT to aluminum under these conditions. Subsequently, reproducible polymerization runs gave rise to copolymers that were thoroughly characterized by HT-SEC, HT-HPLC, DSC, thermal fractionation (SSA), 13C NMR, density measurements, CRYSTAF and optical microscopy to unravel their complex microstructures. The obtained materials differ from a simple solution blend of materials, separately produced by single catalysts, but also from multi-block copolymers as obtained by DOWs continuous process, although a blocky structure can be rationalized.

Dynamics of fibrillar precursors of shishes as a function of stress

. Shishes are fibrillar crystallites that can be created by deforming a polymer melt. The formation of shishes takes place when flow is strong enough to stretch molecules. In the early stages, bundles of stretched molecules with pre-crystalline order form metastable precursors whose stability depends on their size and, hence, on the stress level. We find that for a specific isotactic polypropylene, close to the nominal melting point, a stress larger than 0.10 MPa leads to stable fibrillar precursors that are partially crystalline immediately after flow. On the other hand, below 0.10 MPa, the aspect ratio of precursors tends to unity and the lack of crystallinity makes these structures prone to dissolution.

The morphology and polymorphism of self-nucleated trigonal isotactic poly(1-butene) studied by synchrotron IR microspectroscopy

The self-nucleation behaviour of isotactic poly(1-butene) in the trigonal modification was investigated. Large and isolated Form I spherulites, successfully generated via a purposely designed thermal history protocol, were self-nucleated at different temperatures and subsequently crystallized at 90 °C. The morphology that developed was analysed using polarized optical microscopy, and the polymorphic content and distribution were monitored using synchrotron IR microspectroscopy. Images of the polymorphic distribution were generated using both univariate (band ratio) and multivariate (chemometric) analyses of the spectral data. From the morphological point of view, polarised optical microscopy showed that low seeding temperatures (<130 °C) enabled preservation of the original large spherulite that served as a seed for the growth of Form II at its periphery through cross-nucleation. However, with increasing seeding temperature, more complex morphologies arise, through progressive fading of the original morphology. Firstly, re-crystallization results in a defective large spherulite, and then at temperatures above 132 °C, it results in numerous highly-nucleated Form II spherulites. The IR microspectroscopy images generated either by univariate (band ratio) or multivariate (hierarchical cluster analysis) mappings provided concise information on the spatial distribution of the two polymorphs in the re-crystallized specimens, with a resolution in the range of several microns. It was shown that the content of Form II in the region of the self-nucleated Form I spherulite increases with increasing seeding temperature, demonstrating that the morphological changes accompany the gradual erosion of the memory of the original crystalline structure.

Combining Fast Scanning Chip Calorimetry with Structural and Morphological Characterization Techniques

Thanks to the development of fast-scanning (chip-based) calorimeters (FSC) it is nowadays possible to achieve very high cooling rates, which enabled the study of polymer crystallization at large supercoolings, in conditions similar to what is experienced in real industrial processes. In such extreme conditions formation of structures very different from those commonly obtained under relatively slow cooling can occur.

Flow induced crystallization of LDPE nanocomposites: A rheological and morphological characterization

Abstract The flow induced crystallization behaviour of a LDPE:PE-g-MA:D72T 90:9:1 nanocomposite has been investigated by in-situ Rheo-SALS technique and data have been compared with those obtained from a reference LDPE:PE-g-MA 90:9 sample. Rheo SALS results, confirming thermal analysis findings, indicate that under mild shear flow fields the organoclay exhibits a negligible nucleating effect. Both nucleation density and, as a consequence, crystallization rate, are not appreciably affected by the application of external flow field for both the examined systems, revealing that no evident synergic effects between the organoclay and the shear flow are present. On the other hand, Rheo SALS analysis indicates that the nanocomposite submitted to flow exhibits a higher level of crystal orientation. TEM morphological analyses support this observation suggesting that the orientation of the nanofiller along the flow direction templates the growth of oriented crystals.

Cross-Nucleation between Polymorphs: Quantitative Modeling of Kinetics and Morphology

Cross-nucleation is defined as the nucleation of one polymorph on the surface of another polymorph of the same substance. Although the description of this particular form of heterogeneous nucleation is mainly phenomenological, recently dedicated quantitative studies are performed on several systems. In this work we propose a model framework that captures the phenomenon of cross-nucleation for a spherulitic seed-surface geometry, as well as the kinetic competition between the seed growth and the cross-nucleus formation, by the introduction of a tangential growth rate of the daughter polymorph. Regardless of the growth rate of the parent spherulite, this model describes the experimental data up to and including the final amount of cross-nuclei on its periphery, solely based on one parameter, the cross-nucleation rate. Furthermore, a strong temperature dependency of the kinetic competition between concomitantly growing α- and β-phase isotactic polypropylene is observed and related to the previously reported anomalous behavior of this cross-nucleating system.

Probing polymer crystallization at processing-relevant cooling rates with synchrotron radiation

Processing of polymeric materials to produce any kind of goods, from films to complex objects, involves application of flow fields on the polymer melt, accompanied or followed by its rapid cooling. Typically, polymers solidify at cooling rates which span over a wide range, from a few to hundreds of °C/s. A novel method to probe polymer crystallization at processing-relevant cooling rates is proposed. Using a custom-built quenching device, thin polymer films are ballistically cooled from the melt at rates between approximately 10 and 200 °C/s. Thanks to highly brilliant synchrotron radiation and to state-of-the-art X-ray detectors, the crystallization process is followed in real-time, recording about 20 wide angle X-ray diffraction patterns per second while monitoring the instantaneous sample temperature. The method is applied to a series of industrially relevant polymers, such as isotactic polypropylene, its copolymers and virgin and nucleated polyamide-6. Their crystallization behaviour during rapid cooling is discussed, with particular attention to the occurrence of polymorphism, which deeply impact material’s properties.