Yan-Hui Chen

Simultaneous improvement of strength and toughness in fiber reinforced isotactic polypropylene composites by shear flow and a β-nucleating agent

Inspired by the hierarchical structure of natural plants with a highly oriented and ‘stiff’ outer skin and an isotropic or poorly oriented ‘soft’ inner core, such as bamboo and sugarcane, we demonstrated an analogous hierarchical structure in glass fiber reinforced isotactic polypropylene (GF/iPP) composites with the aid of a β-nucleating agent and an oscillatory shear injection molding technique. The synthetic hierarchical structure consisted of composite assemblies of glass fibers and a semi-crystalline matrix with self-reinforced crystalline shish-kebabs (α-crystals) throughout the molded part. It was found the outer layer was dominated by highly oriented glass fibers and shish-kebabs, while the inner layer contained less oriented glass fibers and shish-kebabs but a large population of β-crystals. As a result, the strength and toughness of the final GF/iPP parts were simultaneously improved (tensile strength was increased by 19.3 MPa and the impact toughness by two folds) compared to the conventional injection-molded GF/iPP parts. The approach exhibited a new pathway to further optimize the properties of GF/iPP composites through structure manipulation.

Shear-Induced Precursor Relaxation-Dependent Growth Dynamics and Lamellar Orientation of β-Crystals in β-Nucleated Isotactic Polypropylene

Although a shear flow field and β-nucleating agents (β-NAs) can separately induce the formation of β-crystals in isotactic polypropylene (iPP) in an efficient manner, we previously encountered difficulty in obtaining abundant β-crystals when these two factors were applied due to the competitive growth of α- and β-crystals. In the current study, to induce the formation of a high fraction of β-crystals, a strategy that introduces a relaxation process after applying a shear flow field but before cooling to crystallize β-nucleated iPP was proposed. Depending on the relaxation state of the shear-induced oriented precursors, abundant β-crystals with a refined orientation morphology were indeed formed. The key to producing these crystals lay in the partially dissolved shear-induced oriented precursors as a result of the relaxation processs ability to generate β-crystals by inducing the formation of needlelike β-NAs. Therefore, the content of β-crystals gradually increased with relaxation time, whereas the overall crystallization kinetics progressively decreased. Moreover, more time was required for the content of the β-phase to increase to the (maximum) value observed in quiescent crystallization than for the effect of flow on crystallization kinetics to be completely eliminated. The c-axis of the oriented β-lamellae was observed to be perpendicular, rather than parallel, to the fiber axis of the needlelike β-NAs, as first evidenced by the unique small-angle X-ray scattering patterns obtained. The significance of the relaxation process was manifested in regulating the content and morphology of oriented β-crystals in sheared, β-nucleated iPP and thus in the structure and property manipulation of iPP.

Hierarchic Structure and Mechanical Property of Short Glass Fiber/Isotactic Polypropylene Composites Containing β-Nucleation Agent

The hierarchic structure of short glass fiber reinforced isotactic polypropylene (GF/iPP) composites with β-nucleating agent were investigated by wide-angle X-ray diffraction, differential scanning calorimetry, polarized light microscopy and scanning electronic microscopy measurements. It was found that the content of β-crystals was deceased due to the α-heterogeneous nucleation of glass fibers, yet its perfection was deteriorated due to the heterogeneous distribution of glass fibers caused by the processing external field. These factors affect the impact strength of GF/iPP composites. Our work paves the way to obtain high-performance GF/iPP composites.

Crystalline structure changes in preoriented metallocene-based isotactic polypropylene upon annealing.

Partially melted metallocene-based isotactic polypropylene (m-iPP), which was preoriented with a high degree of molecular orientation and a shish-kebab structure, was annealed at various temperatures and isothermally crystallized at 130 °C. The melting and crystallization process was examined using synchrotron wide-angle X-ray diffraction, small-angle X-ray scattering, and differential scanning calorimetry. For the m-iPP samples annealed at relatively low temperatures, lamellar thickening, lateral growth, and a decrease in the γ-crystal fraction occurred. Because of parallel evolution of α- and γ-crystal growth in the limited crystallizable melt volume, the fraction of γ-crystals was very low. Furthermore, topological constraints in the melt dominate the chain flux in crystal evolution; the chains are consumed by the thickening lamellae and lateral growth, forming α-crystals with parallel chains in the unit cell. For the m-iPP samples isothermally annealed at medium annealing temperatures, the increase in the amount of crystallizable melt caused the γ-crystal fraction to increase. A shish-kebab (α-crystals) structure with high thermal stability and a newly formed macro-unoriented structure coexisted in the final sample. After annealing at high temperatures, at which no crystals survived, γ-crystal formation was greatly favored; this was attributed to the nature of m-iPP molecules and their dynamic behavior at 130 °C. Because of the lack of oriented nuclei, randomly oriented lamellae were formed. On the basis of the structural cooperative changes at different scales, the morphological features at different annealing temperatures were proposed.