Shish Formation and Relaxation in Sheared Isotactic Polypropylene Containing Nucleating Particles

Morphology of Shear-Induced Polymer Cylindrites Revealed by 3D Optical Imaging

Two-photon confocal laser microscopy was used to obtain three-dimensional (3D) images of the morphology of poly(lactic acid) after shear-induced crystallization. The necessary fluorescence contrast was achieved by doping the polymer with Nile Red. The dye gets partially rejected from the growing crystalline aggregates during their formation, thus creating a renderable high-low fluorescence boundary outlining the shape of the aggregates. Parallel-plate melt-shearing and pulling a glass fiber through the melt were used as the two methods to achieve shear-induced crystallization. This study focuses on the shape of the resulting cylindrites, i.e., large-diameter shish-kebabs. The first 3D images of polymer cylindrites show that, if far from boundaries, they are circular cylinders, highly regular after fiber pull, but less so after parallel-plate shear. In the latter case, the cylindrite reveals the trajectory of the foreign particle that had nucleated its growth. Interestingly, lateral growth of the cylindrites was found to accelerate toward the sample surface when approaching it, giving the cylindrite an elliptical cross section. Furthermore and surprisingly, in the case of fiber pull, a row of spherulites is nucleated at the polymer-substrate interface nearest to the fiber, aligned along the fiber axis and appearing ahead of the rest of the space-filling spherulites. Both the phenomena, elliptical cylindrites and row of spherulites, are attributed to negative pressure buildup peaking at the cylindrite growth front and at the nearby film surface caused by crystallization-induced volume contraction. The pressure and flow distribution in the system is confirmed by numerical simulation. The results illustrate the value of 3D imaging of crystalline morphology in polymer science and polymer processing industry.

In-Situ Synchrotron X-ray Study on the Structure Variation of Morphology-Identified Injection-Molded β-Nucleated iPP under Tensile Deformation

The deformation behavior of semi-crystalline polymers is strongly dependent on the morphology formed during processing. In this study, in-situ synchrotron X-ray was firstly used to identify the morphological distributions of injection-molded isotactic polypropylene (iPP) with different concentrations of β-nucleating agent. It was found that under relatively high concentration of β-nucleating agent (i.e., ≥0.03 wt.%), the outer region (skin and shear region) of the iPP was dominated by mainly highly oriented α-phase as well as certain amount γ-phase, while the core region was rich in β-crystals with little if any orientation. The addition of the β-nucleating agent was beneficial for the formation of lamellae with large lamellar stacking distance in the shear layer. Then the synchrotron X-ray was applied to study the structure variation of those morphology-identified samples under tensile deformation. It was found that voids and cavities along the stretching direction existed in the deformed iPP samples and their volume increased with increasing concentration of β-nucleating agent. The increased volume of void and cavity was associated with the β to α phase transition, which mainly occurred at the core region. In addition, upon stretching crystalline fragmentation and rearrangement took place following the formation of thinner lamellae.

Fundamental studies on shear-induced nucleation and beta-phase formation in the isotactic polypropylene—effect of the temperature

The complex and incompletely understood phenomenon of shear-induced crystallization of polymers may be nowadays analysed via the in situ POM-shear stage methodology. In this research, the two main issues were investigated with the use of the Linkam CCS450 shear stage connected with POM microscope. It was found that the secondary nucleation in the tree well-known temperature regimes plays the greater role in the overall crystallization kinetics than the shear induced primary nucleation. Furthermore, it was found that the tendency towards β-phase formation in shear conditions is dependent on the temperature value during shear treatment. It may be concluded that the temperature is the key parameter in the primary and secondary nucleation process and beta-phase formation in the iPP melts.

Flow-Induced Crystallization in Polyethylene: Effect of Flow Time on Development of Shish-Kebab

The flow-induced formation and relaxation of the representative oriented shish-kebab structure were studied with synchrotron small-angle X-ray scattering (SAXS) method. The flow duration was varied from 2 to 6 s at an identical strain rate to reveal the effect of flow time on stability and dimension of formed shish. It was found that the short flow time of 2 s was able to generate shish during flow, which, however, relaxed during the isothermal process after cessation of flow. An increase in flow time can improve the shish stability and the long flow time of 6 s can generate the stable shish that nucleate the growth of kebab lamellae. In addition, the quantitative analysis of SAXS results showed that with increasing flow time from 2 to 6 s, the shish length increased from 242 to 574 nm, while the shish diameter remained around 34 nm. This detailed information of the formed shish-kebab structure can be used to shed light on their evolution that occurred during flow from 2 to 6 s, where shish grew at a longitudinal speed of around 80 nm/s, and there was an improvement in the stability and nucleation capability for kebab lamellae.

Role of Melt Plasticizing Temperature in Morphology and Properties of PE100 Pipes Prepared by a Rotational Shear System

To increase the maximum internal pressure that a polyethylene (PE) pipe can withstand, a novel rotational shear system (RSS) was constructed in this study to fabricate PE pipes with enhanced hoop strength by applying hoop shear on the pipes using a rotational mandrel. The microstructure and morphology with the influences of melt plasticizing temperature on PE pipes processing under rotational shear were investigated indirectly using small-angle X-ray scattering and wide-angle X-ray diffraction (SAXS/WAXD) measurements. In the SAXS patterns, equatorial streaks and meridional scattering peaks were clearly observed in all three samples prepared at different melt plasticizing temperatures, 215, 235, and 255 °C. Their presence indicated that shish-kebab crystals form in rotational shear. Compared to those at the low melt temperature, the increase in the melt temperature enhanced the amount and the dimensions of shish formed. However, the shish also relaxed faster at the high melt temperature. This behavior was attributed to the enhancement of the molecular chain's athletic ability. The hoop tensile strength and the heat resistance of the pipes peaked at the melt plasticizing temperature of 235 °C, 75.2 MPa, 102.4 °C, up 1 MPa, 0.2 °C (compared to the 215 °C) and 7.8 MPa, 3.2 °C (compared to the 255 °C). The axial strength increased with an increase of melt plasticizing temperature. However, the increase of melt plasticizing temperature worsens the inherent good tensile toughness of PE100 pipes as the axial elongation at break decreases.

Poly(Acrylonitrile-Butadiene-Styrene) as a Special β-Nucleating Agent on the Toughness of Isotactic Polypropylene

The influence of poly(acrylonitrile–butadiene–styrene) (ABS) as a special β-nucleating agent on the impact and tensile properties of isotactic polypropylene (iPP) were investigated by dynamic rheological measurements, wide-angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC), scanning electron microscopy (SEM), and mechanical properties tests. It is found that the β nucleation efficiency of ABS is closely related to its concentration, dispersibility, and molding method for the iPP/ABS blends. The content of β-crystal (K_β) rises with the incorporation of ABS and shows a maximum with the introduction of 1% ABS for compression-molded blends and 2% ABS for injection-molded blends, respectively, which is followed by a decrease in K_β. The addition of a small amount of ABS has a significant reinforcing and toughening effect on iPP. Compared with the compression-molded samples, the ABS dispersed phase in injection-molded samples has a smaller particle size and a larger specific surface area, which are favorable for stress transmission and higher β nucleation efficiency, and therefore, better tensile and impact properties can be expected.

Influence of Organo-Sepiolite on the Morphological, Mechanical, and Rheological Properties of PP/ABS Blends

To improve the poor impact toughness of polypropylene (PP), organo-sepiolite (O-Sep) filled 80/20 (w/w) polypropylene/poly(acrylonitrile-butadiene-styrene) (PP/ABS) nanocomposites were fabricated. The contents of O-Sep were correlated with the morphological, mechanical, and rheological behavior of PP/ABS/O-Sep blends. Scanning electron microscopy (SEM) was applied to study the morphology and thermogravimetric analysis (TGA) was applied to study the thermal stability. Differential scanning calorimetry (DSC) and X-ray diffraction (XRD) were applied to study the crystallinity. The obtained results show that O-Sep enhanced the dispersion of ABS in the PP matrix and increased the crystallinity of blends. The rheological results show that O-Sep could increase the viscosity, storage modulus, and loss modulus of blends. Moreover, the mechanical behavior shows that O-Sep (at proper content) simultaneously increased the tensile modulus, flexural modulus, and impact strength of PP/ABS/O-Sep blends.

Molecular dynamics simulation of shish-kebab crystallization of polyethylene: Unraveling the effects of molecular weight distribution

By means of molecular dynamics simulations, extensional flow was performed on five polyethylene models with different molecular weight distributions (MWDs) precisely designed in view of Grubbs, metallocene, Ziegler-Natta, and chromium-based catalysts, while ignoring the sequence distributions of short branches to shed light on the molecular mechanism of MWD on shish-kebab formation. The formation of shish-kebab crystallites can be divided into three stages: the emergence of precursors, evolution from precursors to shish nuclei, and the formation of lamellar crystallites. The results demonstrated that the precursors initiated from trans-rich segments with local order and minor crystallinity grew into large shish nuclei and eventually evolved into lamellae. There were more inconsecutively trans-state bonds occurring in long chains rather than in short chains, which promoted an easier transformation from precursors to shish nuclei. Therefore, broader MWDs make positive contributions to the formation of shish nuclei, increase the crystallization speed, and the generation of a more regular, compact, and thicker lamella with less tie molecule fractions, while the final crystallinity is independent of MWD.

Effect of Thermal History and Shear on the Viscoelastic Response of iPP Containing an Oxalamide-Based Organic Compound

We report on the role of temperature and shear on the melt behavior of iPP in the presence of the organic compound N1,N1′-(propane-1,3-diyl)bis(N2-hexyloxalamide) (OXA3,6). It is demonstrated that OXA3,6 facilitates a viscosity suppression when it resides in the molten state. The viscosity suppression is attributed to the interaction of iPP chains/subchains with molten OXA3,6 nanoclusters. The exact molecular mechanism has not been identified; nevertheless, a tentative explanation is proposed. The observed viscosity suppression appears similar to that encountered in polymer melts filled with solid nanoparticles, with the difference that the OXA3,6 compound reported in this study facilitates the viscosity suppression in the molten state. Upon cooling, as crystal growth of OXA3,6 progresses, the decrease in viscosity is suppressed. Retrospectively, segmental absorption of iPP chains on the surface of micrometer-sized OXA3,6 crystallites favors the formation of dangling arms, yielding OXA3,6 crystallites decorated with partially absorbed iPP chains. In other words, the resulting OXA3,6 particle morphology resembles that of a hairy particle or a starlike polymer chain. Such hairy particles effectively facilitate a viscosity enhancement, similar to branched polymer chains. This hypothesis and its implications for the shear behavior of iPP are discussed and supported using plate–plate rheometry and slit-flow experiments combined with small-angle X-ray scattering analysis.

Effect of Self-Assembly of Oxalamide Based Organic Compounds on Melt Behavior, Nucleation, and Crystallization of Isotactic Polypropylene

We report on the effect of an aliphatic oxalamide based nucleating agent (OXA3,6) on the melt and crystallization behavior of isotactic polypropylene (iPP) under defined shear conditions. Through polarized optical microscopy, we demonstrate that OXA3,6 self-assembles from the iPP melt into rhombic crystals whereas their size and distribution proved highly dependent on the employed cooling rates. The presence of 0.5 wt % of OXA3,6 in iPP results in a significant suppression in iPP melt viscosity, which could not be explained via molecular modeling. A possible cause for the drop in viscosity in the presence of OXA3,6 is attributed to the interaction (absorption) of high molecular weight iPP chains with the nucleating agent, thereby suppressing their contribution to the viscoelastic response of the melt. This proposed mechanism for the suppression in melt viscosity appears similar to that encountered by the homogeneous distribution of nanoparticles such as CNTs, graphene, and silica. Shear experiments, performed using a slit flow device combined with small-angle X-ray diffraction measurements, indicate that crystallization is significantly enhanced in the presence of OXA3,6 at relatively low shear rates despite its lowered sensitivity to shear. This enhancement in crystallization is attributed to the shear alignment of the rhombic OXA3,6 crystals that provide surface for iPP kebab growth upon cooling. Overall, the suppression in melt viscosity in combination with enhanced nucleation efficiency at low as well as high shear rates makes this self-assembling oxalamide based nucleating agent a promising candidate for fast processing.

Interfacial Shish-Kebabs Lengthened by Coupling Effect of In Situ Flexible Nanofibrils and Intense Shear Flow: Achieving Hierarchy To Conquer the Conflicts between Strength and Toughness of Polylactide

The challenge of hitherto elaborating a feasible pathway to overcome the conflicts between strength and toughness of polylactide (PLA) still remains among academia and industry. In the current work, a unique hierarchal structure of flexible poly(butylene adipate-co-terephthalate) (PBAT) in situ nanofibrils integrating with abundant PLA shish-kebabs as a strong building block was disclosed and expresses its capability to conquer this dilemma. Substantially simultaneous enhancement on tensile strength, impact strength, and elongation at break could be achieved up to 91.2 MPa, 14.9 KJ/m², and 15.7%, respectively, compared with pure PLA (61.5 MPa, 4.3 KJ/m², and 6.2%). Through investigating the phase (and crystalline) morphology and molecular chain behavior in the PLA/PBAT system, the formation mechanism of this structure facilitated by a coupling effect of PBAT flexible phase and shear flow was definitely elucidated. The dispersed phase of PBAT would be more inclined to existing as a fibrillar form within the PLA matrix benefiting from low interfacial tension. Interestingly, this phase morphology with large specific surface area changes the crystallization behavior of PLA significantly, once introducing an intense shear flow (∼10³ s^-1), in situ shear-formed nanofibrils of PBAT would show strong coupling effect with shear flow on PLA crystallization: they can not only induce abundant shish-kebabs of PLA at its interfaces, which possesses lengthened shish and more densely arranged kebabs, but also further retard the relaxation of PLA chains through hysteretic relaxation of its PBAT phase, which can effectively prevent the collapse of established shish. Of immense significance is this particular hierarchical-architecture composed by flexible nanofibers (PBAT) and rigid shish-kebabs (PLA), which provides significant guidance for the simultaneous reinforcement and toughness of polymer materials.

Relaxation behavior of shear-induced crystallization precursors in isotactic polypropylene containing sorbitol-based nucleating agents with different nucleating abilities

The nature of shear-induced crystallization precursors, especially their relaxation behaviour, is an important issue in polymer chemical physics. In our work, relaxation behavior of shear-induced crystallization precursors in isotactic polypropylene containing various sorbitol-based nucleating agents (NAs) with different nucleating abilities was investigated by using both rheological and in situ small angle X-ray scattering (SAXS) methods. Rheological crystallization kinetics results showed that the amount of shear-induced precursors, calculated separately from the total nuclei, decayed exponentially with relaxation time in both pure and nucleated iPP. By fitting the decay of shear-induced precursors with relaxation time, the relaxation rate of precursors in nucleated iPP was found to be slower than that in pure iPP. Interestingly, it further decreased with the increase in the nucleating ability of sorbitol-based NAs. Meanwhile, the life-time of precursors was prolonged in nucleated iPP with increasing nucleating ability. Similar results were also testified by in situ SAXS measurements. By investigating the life-times at different temperatures, the activation energy for the relaxation of precursors was calculated and found to increase with stronger nucleating abilities. Our results demonstrated that sorbitol-based NAs could stabilize the iPP precursors and the effect of stabilization enhanced with the increase in nucleating ability. We believe that our work can not only help better reveal the relaxation behavior of shear-induced precursors but also provides a new perspective for understanding the role of NAs in real processing.

More dominant shear flow effect assisted by added carbon nanotubes on crystallization kinetics of isotactic polypropylene in nanocomposites

More dominant shear flow effect with different shear rates and shear time with assistance of added carbon nanotubes (CNTs) of low amounts on the crystallization kinetics of isotactic polypropylene (iPP) in CNT/iPP nanocomposites was investigated by applying differential scanning calorimetry (DSC), polarized optical microscopy (POM), and rheometer. CNTs were chemically modified to improve the dispersity in the iPP matrix. CNT/iPP nanocomposites with different CNT contents were prepared by solution blending method. The crystallization kinetics for CNT/iPP nanocomposites under the quiescent condition studied by DSC indicates that the addition of CNTs of low amounts significantly accelerates crystallization of iPP due to heterogeneous nucleating effect of CNTs, whereas a saturation effect exists at above a critical CNT content. The shear-induced crystallization behaviors for CNT/iPP nanocomposites studied by POM and rheometry demonstrate the continuously accelerated crystallization kinetics with assistance from added CNTs, with increasing CNT content, shear rate, and shear time, without any saturation effect. The changes of nucleation density for CNT/iPP nanocomposites under different shear conditions can be quantified by using a space-filling modeling from the rheological measurements, and the results illustrate that the combined effects of added CNTs and shear flow on the acceleration of crystallization kinetics are not additive, but synergetic. The mechanisms for the synergetic effect of added CNTs and shear flow are provided.