Sheared Isotactic Polypropylene Research Articles

Synergies among the self-assembled β-nucleating agent and the sheared isotactic polypropylene matrix

The impacts of both shear temperature and shear rate on both the self-assembled β-nucleating agent (β-NA) in the hosting isotactic polypropylene (iPP) matrix and the subsequent crystallization of the hosting iPP matrix were investigated by polarized optical microscopy (POM) and wide-angle X-ray diffraction (WAXD), and small-angle X-ray scattering (SAXS) measurements, respectively. Under static condition, β-NA could self-assemble into snowflake-like aggregate. Once shear was applied, three kinds of self-assembled aggregates could be developed prior to the crystallization of matrix depending on the applied shear temperatures, that is, snowflake-like self-assembled aggregate at 200 °C, deformed snowflake-like one with smaller branches at 180 °C, tiny and dense needle-like one if shear temperature was further decreased to 160 °C. Since β-NA with various self-assembled aggregates can act as nucleating templates for the subsequent nucleation of iPP matrix, it is conceivable that the crystallization behavior of the iPP matrix can be altered. As expected, β-phase crystallinity, long period and lamellar thickness are steadily dependent on the shear temperature and shear rate. That is, when shear was applied at 200 °C, β-phase crystallinity, long period and lamellar thickness keeps almost constant regardless of the shear rate. If melt was sheared at 180 °C, β-phase crystallinity was decreased while long period and lamellar thickness remained unchanged with elevating shear rate. At lower shear temperature (i.e., 160 °C), β-phase crystallinity, long period and lamellar thickness were further decreased with increasing the shear rate. These results are of practical significance to understand the long-term ignored issue: apart from the loading of β-NA, different self-assembled aggregates formed in flow field are another crucial factor in determining the microstructure development of matrix.

Polymorphism in Sheared Isotactic Polypropylene Containing Nucleant Particles

AbstractThis study explored the combined influence of shear flow and nucleating agents on isotactic polypropylene (iPP) crystallisation. The study showed that oriented α‐iPP crystals were not necessary to form β‐iPP crystals, contrary to the accepted view that the surface of oriented α‐iPP crystals provides nucleation sites for an α‐iPP to β‐iPP growth transition. Instead it was proposed that flow‐induced mesomorphic point‐like nuclei preferentially nucleates β‐iPP. The combined effect of shear flow and nucleant was found to promote γ‐iPP crystals. The surprising γ‐iPP nucleation effect was explained by the high density of flow‐induced and heterogeneous nuclei present at the start of crystallisation. magnified image

Shish Formation and Relaxation in Sheared Isotactic Polypropylene Containing Nucleating Particles

The formation and relaxation of shish in molten isotactic polypropylene (iPP) and two samples containing different commercial nucleating particles was studied during and after continuous shear flow. In-situ measurements of the shish were made by simultaneous wide- and small-angle X-ray scattering (rheo-SAXS/WAXS) using synchrotron radiation. In the SAXS patterns, sharp meridional reflections were observed in all three samples during the initial stages of shear flow. Their presence indicates that shish form in discrete steps, by the aggregation of pointlike nuclei into oriented threads. Compared to pure iPP, the addition of both nucleants increased the amount and the dimensions of shish formed. However, the shish also relaxed faster in the presence of both nucleated materials. This behavior was attributed to the particles inducing lower molecular weight chains to participate in shish formation. The particle aspect ratio was observed to greatly influence shish formation. This is explained by modification of...

Effect of additives on distributions of lamellar structures in sheared polymer: a study of synchrotron small-angle x-ray scattering

The effects of additives on the distributions of lamellar morphology and orientation in sheared isotactic polypropylene were investigated using the small beam of synchrotron small-angle x-ray scattering. The Cu-phthalocyanine can template the lamellar orientation even under low shear rates, whereas the ultramarine blue cannot. The surface contact is suggested to play a role in stabilizing the formation of oriented nuclei which subsequently direct the growth of oriented lamellae. The additives have no notable effects on the long spacing in the shear region. However, at high shear rates, they decrease the thickness of crystalline lamellae or increase the thickness of amorphous lamellae. Since the additives increase the degree of volume crystalline in the shear region, the number of crystalline lamellae should be increased. The results are helpful in designing and selecting suitable additives for controlling lamellar morphology and orientation.

Effects of different colorants on morphological development of sheared isotactic polypropylene: A study using synchrotron wide-angle x-ray scattering

Sheared isotactic polypropylene (iPP) forms a special crystalline assembly called branched shish kebab. The effects of two colorants, Cu-phthalocyanine and ultramarine blue, on the branched shish-kebab structure have been investigated by using synchrotron wide-angle x-ray scattering. The effects of the colorants on the morphological distributions of iPP are well constructed through the shear field or temperature gradient, including crystallinity, molecular orientation, fraction of lamellae, and thickness of crystalline lamellae. It is found that the Cu-phthalocyanine can significantly affect the morphological development of sheared iPP but the ultramarine blue cannot. In the presence of Cu-phthalocyanine, the degree of crystallinity is increased, the molecular orientation is enhanced, the more a∗-axis-oriented components form, and the crystalline lamellae are thickened.

Morphological Development of Oriented Isotactic Polypropylene in the Presence of a Nucleating Agent

The morphological development of sheared isotactic polypropylene (iPP) containing different amounts of sodium benzoate (SB) has been investigated using wide- and small-angle X-ray scattering techniques. The microbeam of synchrotron radiation allows us to gain, precisely and efficiently, the local morphology at a given position with a good spatial resolution. The morphological distributions such as crystallinity, fractions of α-form and β-form crystals, crystal size, and orientation functions can therefore be well constructed through the shear field or temperature gradient. The distribution of crystallinity is changed from a U-shape without SB to an almost flat with SB, in company with a minimum fraction of α-form crystal and a maximum fraction of β-form crystal. The epitaxial growth of branched lamellae in the presence of SB is confirmed from the orientation functions of two axes using wide-angle diffraction. The effect of SB on the molecular orientation is understood from the orientation function of c-axis and the azimuthal patterns of small-angle scattering. It is found that both the molecular orientation and the lamellar orientation of the α-form crystal are increased with increasing SB. The thickness of crystalline lamellae with respect to flow direction is found to increase with increasing SB in the whole range from shear zone to core center. A slight declination of crystalline lamellae has been observed in the presence of SB.

Effects of sodium benzoate on distributions of lamellar structures of sheared isotactic polypropylene: A study using small angle synchrotron X-ray scattering

Small-angle X-ray scattering (SAXS) studies were carried out to investigate effects of sodium benzoate (SB) on the shear-induced distributions of lamellar structures through the depth direction of injection-moulded isotactic polypropylene (iPP) plates, using the micro beam available from a synchrotron to gain good spatial resolution. It was found that in the presence of SB, the shish–kebab structure emerges over a broader range to include regions in which a random arrangement of crystalline lamellae is found without SB. The oriented crystalline lamellae are even found in positions that are very close to the core center. The SB-induced shish–kebab structure under low levels of shear flow is attributed to lower free energy barriers for the coil-to-stretch transition and shorter times to start the crystallization. The effect of SB on the lamellar dimensions is significant only in the core region, and only when the concentration of SB is sufficiently high. A down-U distribution of the long spacing is found, which is mainly ascribed to the variation of thickness of crystalline lamellae with the depth. The results show that a higher level of lamellar orientation is not necessarily accompanied by a higher degree of linear crystallinity.

Orientation distribution of sheared isotactic polypropylene plates through thickness in the presence of sodium benzoate

The orientation distribution of sheared isotactic polypropylene (iPP) containing different amount of sodium benzoate (SB) has been investigated through the gradient of shear flow field using microbeam of synchrotron wide-angle x-ray techniques. The degree of the overall orientation of α-phase crystal is found to increase with increasing concentration of SB. Compared with the sheared iPP in the absence of SB, the orientation of α-phase crystal is found to distribute over a broader range of shear flow field in the presence of SB. The overall orientation of α-phase crystal is explained in terms of a parent-daughter model or lamella-branched shish-kebab structure. As the concentration of SB increases, the contribution from the c-axis orientation of parent lamellae decreases in the flow direction. The contribution from the a*-axis orientation of daughter lamellae is developed to be dominant in the flow direction when the concentration of SB exceeds a critical value.

Synchrotron Infrared Microscopy Study of the Crystalline Morphology of the Interphase in Polypropylene/LCP‐Fiber Model Composites

The interphase of quiescent and sheared isotactic polypropylene–liquid crystalline polymer (iPP–LCP) fiber model composites has been examined with synchrotron infrared microscopy down to a lateral spatial resolution of 3 µm. By using the polarization of the beam, it is possible to distinguish between α and β transcrystalline (TC) phases. Structural evidence is presented for the presence of an α layer that nucleates the β TC phase in agreement with previous morphological studies.

Nature of Shear-Induced Primary Nuclei in iPP Melt

Although observations of molecular processes in the formation of primary nuclei prior to actual crystallization are beyond the detection limits of current instrumentation, we attempted to probe the nature of primary nuclei in sheared isotactic polypropylene (iPP) polymer melt. In situ rheo-SAXS (small-angle X-ray scattering) and -WAXD (wide angle X-ray diffraction) experiments using synchrotron radiation were carried out to evaluate the effects of an addition of a high molecular weight atactic polypropylene (aPP) (5 wt%), which is compatible with the iPP matrix but does not crystallize, on the evolution of oriented structures in the sheared iPP melt and its crystallization kinetics. It is unlikely that the aPP chain segments can be incorporated into iPP nuclei or crystal; hence, its addition effects, if any, would be seen only in the amorphous melt prior to crystallization. The results showed stonger orientation and improved crystallization kinetics in the iPP/aPP blend compared to pure iPP. Observations that the presence of long chains of an amorphous polymer aid in nucleation and crystallization kinetics of iPP, combined with our previous synchrotron results of sheared iPP melts at high temperature (165°C), lead us to conclude that primary nuclei in iPP most likely consist of liquid-crystalline or mesomorphic bundles of aligned chain segments prior to the formation of crystals.

Precursors of primary nucleation induced by flow in isotactic polypropylene

Small-angle X-ray scattering (SAXS) patterns of the sheared isotactic polypropylene (i-PP) melt at 175°C, above the nominal melting point of 162°C, showed development of oriented structures or aggregates of polymer molecules that did not disappear even after a long time relaxation (up to 2 h ). However, the corresponding wide-angle X-ray diffraction (WAXD) patterns did not show any visible Bragg reflections, suggesting that the induced structures are probably non-crystalline. The results suggest that metastable aggregates of polymer molecules are generated in the melt by assembly of bundle of oriented chain segments (especially in long chains). We speculate that the oriented aggregates of chain segments are precursors of primary nuclei. The spatial arrangement of these precursors showed a layer-like superstructure, evident by the appearance of the meridional maxima in SAXS. The spacing between the ‘layers’ was estimated to be 430 A ̊ . A microstructural model of the orientation-induced precursors for primary nucleation is presented. An Avrami model was used to fit the evolution of the oriented structures in i-PP melt, which provided insights into the early stages of polymer crystallization.

The role of metastability in the micromorphologic features of sheared isotactic polypropylene melts

The micromorphologic features of sheared isotactic polypropylene (iPP) in the vicinity of fibre were investigated through polarized light microscopy (PLM), phase contrast optical microscopy (PCLM), scanning electron microscopy (SEM), and atomic force microscopy (AFM). The results indicate that the polymorphic structures and their boundary lines can be explained from a modified model by combining the concept of metastability and the theory of shear-induced crystallization without assuming epitaxial growth.