iPP Melt Research Articles

Flow-Induced Dendritic β-Form Isotactic Polypropylene Crystals in Thin Films

Flow-induced dendritic β-form isotactic polypropylene (iPP) lamellae in thin films have been investigated using optical microscopy (OM) and atomic force microscopy (AFM). Shear flow in a thin film was induced by scratching supercooled iPP melt with a sharp scalpel at a constant rate. After subsequent isothermal crystallization, an extremely high nucleation density of edge-on α-iPP crystals oriented perpendicular to the flow direction was observed. Increasing shear flow temperature from 130 to 190 °C led to a decrease in both number density of flow-induced edge-on α-iPP lamellae and subsequent β-iPP crystals. Interestingly, two distinct paths for the change in orientation of edge-on crystals could be identified. The most probable route was the transition from edge-on to leaf-shaped α-iPP flat-on crystals. The transition from edge-on α-iPP lamellae to β-iPP flat-on crystals with dendritic shape occurred less frequently. We suggest a stepwise process for flow-induced growth of β-iPP crystals associated with ...

Exploring supramolecular self-assembly of a bisamide nucleating agent in polypropylene melt: The roles of hydrogen bond and molecular conformation

Nucleating agents (NA) is a kind of the most important additives in polymer industry. The phenomena of NA self-assembling/crystallizing into various supramolecular aggregations in the polymer melt and showing different nucleation effects have been widely reported. However, the process of the self-assembly formation, which is essential for understanding the exact nucleating mechanism of NAs, is not well elucidated. In this work, the self-assembly process of a bisamide NA, N,N′-dicyclohexylterephthalamide (DCTH), in polypropylene (iPP) matrix was systematically investigated. The Fourier transform infrared spectroscopy (FTIR) results reveal that the self-assembly of DCTH is related to the dissociation/association of hydrogen bonds and the conformational change of molecules. Upon heating, the hydrogen-bond dissociation generates the “free” DCTH molecules in iPP melt and provides space for the conformational rotation of cyclohexyl groups toward co-plane direction with the aromatic core plane; while upon cooling, the hydrogen-bond association triggers the self-assembly/recrystallization of “free” DCTH molecules into supramolecular aggregations and the conformational rotation makes hydrogen bonds stronger. By exploring the microenvironments of individual chemical groups of DCTH and the interactions between DCTH and iPP matrix, we demonstrate the physical mechanism of how the NA self-assembles into supramolecular crystals at molecular level.

Molecular mechanism leading to memory effect of mesomorphic isotactic polypropylene

ABSTRACTIn this study, memory effect of mesomorphic isotactic polypropylene (iPP) was investigated using polarized optical microscope and small‐angle X‐ray scattering. Differing from classical memory effect, mesomorphic iPP melt had a higher growth rate and a higher memory temperature. The relative growth rate increased with increasing crystallization temperature. Lauritzen–Hoffman plots indicated that the increased growth rate arose from reduced surface nucleation barrier. The highest memory temperature was estimated to be 185 °C, which was close to the equilibrium melting point of iPP crystal. Additionally, Small‐angle X‐ray scattering measurements showed that a liquid crystal layer might exist between lamellar and amorphous layers. Based on above results, a crystallization model was proposed. In the mesomorphic iPP melt, there exist aggregates structurally similar to β phase except α‐phase crystal residuals, which cannot act as nucleation sites or transform to β crystal through surface nucleation. The only way for the aggregate is to transform to α crystal during crystal growth. The aggregate decreases the surface nucleation barrier and promotes the helical growth, leading to higher growth rate. Only when the aggregate relaxes to polymer coils through thickening at a higher temperature, can the memory effect be erased. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 1573–1580

Structural evolution and mechanical properties of iPP melt spun fibers subjected to thermal treatment

An investigation of the structure and mechanical behavior of melt-spun isotactic polypropylene (iPP) fibers subjected to thermal treatment in an inert atmosphere is described. Two iPP formulations, Basell Pro-fax PH835 and ExxonMobil Achieve 3854, synthesized by the Ziegler-Natta and metallocene catalysts respectively, and spun at take-up velocities of 1000 to 3000 m/min are considered. The evolution of the structure is monitored with WAXS, SAXS, Raman spectroscopy and birefringence measurements. The fibers spun at 1000 m/min are predominantly mesomorphic, while those spun at 3000 m/min are semi-crystalline in the as-spun state. Thermal treatment for 20 min at 145 °C erases the processing history and increases the crystallinity of all samples. It is shown that thermal treatment leads to the formation of a secondary set of kebab lamellae which are thinner than the original ones, separated by thicker lamellae. The spatial variability of the lamellar thickness and of interlamellar spacings is estimated from the SAXS data and it is concluded that the variability is rather pronounced in all samples. Both annealed and non-annealed fibers are subjected to monotonic and cyclic mechanical testing. Large differences are seen in the behavior of non-annealed fibers processed in different conditions. The monotonic mechanical behavior of the annealed fibers is not very much different from that of the corresponding non-annealed fibers. The central difference between annealed and non-annealed samples is observed in the cyclic behavior; annealed samples containing lamellae with bimodal distribution of thickness exhibit bimodal hysteresis curves while this feature is not observed in non-annealed samples.

Mixing Assisted Direct Formation of Isotactic Poly(1-butene) Form I′ Crystals from Blend Melt of Isotactic Poly(1-butene)/Polypropylene

The influence of mixing of iPB-1/iPP blend on the polymorphism of iPB-1 under processing-relevant conditions is studied with emphasis on the competition between the thermodynamically stable form I′ crystal and the kinetically favored form II. In situ optical microscopy measurements reveal that the upper critical solution temperature (UCST) of iPB-1/iPP blend locates in the range of 180–200 °C. Unexpectedly, by quenching mixed iPB-1/iPP melt down to temperatures below UCST and melting point, form I/I′ can be produced directly which is further identified as form I′ by temperature-dependent WAXS and DSC. The formation of form I′ is promoted by increasing the annealing time above UCST, while is suppresses by raising the quenching temperature. In addition, the crystallization of iPP also displays a similar trend as iPB-1 does. The correlated crystallization of each constituent with dependence on the initial mixing degree suggests that the crystallization behavior of the binary blends is determined by the interplay between simultaneous processes concomitant with the liquid–solid transition. The experimental results reveal the possibility to modify the crystallization pathway of iPB-1 in iPB-1/iPP blend through the mixing degree which is initially controlled by annealing but is subject to evolve during the subsequent thermal treatment. Possible mechanisms are discussed including the roles of phase separation and concentration fluctuation in crystallization.

Simultaneously improving tensile strength and toughness of melt‐spun β‐nucleated isotactic polypropylene fibers

ABSTRACTPolymer processing methods generally play a crucial role in determining the development of microstructure in the fabricated product. In this study, isotactic polypropylene (iPP) melt containing 0.05 wt % β‐nucleating agent (β‐NA) was extruded via a melt flow rate indicator. The molten extrudate was stretched into a fiber upon various take‐up velocities (TVs). The microstructures of the fiber were investigated by differential scanning calorimeter, two‐dimensional wide‐angle X‐ray diffraction, and small‐angle X‐ray scattering. Also, its tensile properties (including tensile strength, modulus, elongation at break, and toughness) were measured by tensile test. Interestingly, the tensile strength (135.0 MPa) of a melt‐spun β‐nucleated iPP fiber fabricated at 400 cm/min was enhanced by 115.2%, compared with that (62.7 MPa) prepared at 100 cm/min, with a considerable increment in toughness (from 661 to 853 MJ/m3). The enhancement mechanism for tensile properties was discussed based on the microstructures. This work offers a simple approach to prepare β‐nucleated iPP fibers with excellent strength and toughness. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 43454.

Microstructure and Mechanical Properties of Isotactic Polypropylene Films Fabricated via Melt-Extrusion and Uniaxial-Stretching

ABSTARCTIn this work, isotactic polypropylene (iPP) melt was slowly extruded through a slit die of a single-screw extruder. Once the iPP melt left the die, it was uniaxially stretched at different stretching rates (SRs). Via this process its microstructure can be manipulated, it was subsequently investigated by wide-angle X-ray diffraction (WAXD), small-angle X-ray scattering (SAXS), differential scanning calorimetry (DSC), polarized optical microscopy (POM), and Fourier transform infrared spectroscopy (FTIR). Furthermore, the mechanical properties (including tensile strength, modulus, toughness, and strain-hardening) were investigated. The results showed that the tensile strength and modulus of the melt-stretched iPP films gradually increased with increasing SRs. In addition, the toughness and elongation at break showed maximum values for iPP films melt-stretched at 30 cm/min. Moreover, compared with other melt-stretched films, the iPP films melt-stretched at 90 cm/min exhibited an obvious strain-hardening behavior at lower strain.

Shear-induced interfacial sheath structure in isotactic polypropylene/glass fiber composites

Shear-induced interfacial sheath structure was generated by pulling a single glass fiber (GF) in the supercooled isotactic polypropylene (iPP) melt. Its morphological evolution and microstructure were investigated by means of polarized optical microscopy (POM), ex-situ polarized Fourier transform infrared spectroscopy (FTIR), ex-situ two-dimension wide-angle X-ray diffraction (2D-WAXD) and scanning electron microscopy (SEM). The results revealed that higher fiber-pulling speed was crucial to promote the formation of sheath structure. Such sheath structure was proved to be the most thermodynamically stable α-crystal that could effectively induce the generation of β-cylindrite. Meanwhile, the thickness and orientation level of the sheath structure were increased with the increasing fiber-pulling speed. More interestingly, the sheath structure was composed of aligned shish-kebabs, and the lamellar-branched β-iPP crystals directly “rooted” in these kebabs, viz., β-iPP crystals grew at the growth front of oriented fibrillar lamellae. This study on the interfacial sheath structure provides another direct evidence for the formation mechanism of β-iPP crystals upon shear field.

Understanding the effects of nucleating agent concentration on the polymorphic behavior of β-nucleated isotactic polypropylene with different melt structures

To elucidate the synergetic effect in enhancing β- crystallization between β-nucleating agent (β-NA) and or- dered structures of isotactic polypropylene (iPP), i.e., ordered structure effect (OSE), in this study, the role of β-NA concen- tration (Cβ-NA) was studied by differential scanning calorim- etry (DSC) and in-situ small-angle X-ray scattering (in-situ SAXS). Results showed that Cβ-NA was a determining factor of OSE: Cβ-NA<0.01 wt% was too low to induce β- crystallization while Cβ-NA≥0.09 wt% was saturated β-NA concentration, and none of them can induce OSE; only when Cβ-NA=0.01-0.08 wt% (i.e., unsaturated concentration) can OSE take place. There existed a competitive relationship in the enhancement of β-phase between OSE and conventional β-nucleation effect of β-NA. Moreover, in-situ SAXS re- vealed that at Tf=190 °C (around upper limiting temperature for occurrence of OSE), when Cβ-NA was different, the geo- metric shapes of ordered structures in iPP melts were quite different from each other. The involved mechanism was proposed.

Investigation on the crystallization behavior and polymorphic composition of isotactic polypropylene/multi-walled carbon nanotube composites nucleated with β-nucleating agent

The melt structure of isotactic polypropylene (iPP) nucleated with 1.0 mass% multi-walled carbon nanotubes (CNTs) and 0.05 mass% rare earth β-phase nucleating agent (WBG-II) was tuned by changing the fusion temperature T f. In this way, the role of melt structure in the crystallization behavior and polymorphic composition of the sample was studied by calorimetry, X-ray diffraction, and scanning electronic microscopy. In this system, the sample cannot form any β-phase when T f = 200 °C, since the α-nucleation efficiency (α-NE) of CNTs was obviously higher than the β-NE of WBG-II. By tuning T f, the content of ordered structures in iPP melt was controlled and it was found that the polymorphic composition of the sample can be efficiently controlled. When T f was in the range of 166–174 °C, an amount of ordered structures survived in iPP melt, resulting in a significant increase of β-phase proportion; moreover, study on the crystallization behaviors of single CNTs-filled iPP and WBG-II-nucleated iPP (iPP/CNTs and iPP/WBG-II, respectively) revealed that the α-NE of iPP/CNTs stayed almost constant as T f varies from 200 to 166 °C; meanwhile, the β-NE of iPP/WBG-II increased gradually. Once T f decreased to 174 °C and lower, the NE of iPP/WBG-II became higher than that of iPP/CNTs; meanwhile, WBG-II began to play determining role in the polymorphic composition and enhanced the β-phase proportion of iPP nucleated with CNTs/WBG-II. The findings of this study brought a new pathway to control the polymorphic composition and mechanical properties of iPP/CNTs composites.

Morphology, crystallization behavior and tensile properties of β-nucleated isotactic polypropylene fibrous membranes prepared by melt electrospinning

β-nucleated isotactic polypropylene (iPP) fibers with diameters less than 5 μm were prepared through melt electrospinning. The effects of electrospinning process and rare earth β-nucleating agent (WBG) on the crystal structure of iPP fibers were investigated. The results indicate that the addition of WBG can improve the fluidity of iPP melt remarkably and help the formation of fine fibers with thinner diameter, while the electrostatic force applied on the iPP melt is not favorable for the formation of β-crystal in iPP fibers. In addition, the morphology and crystalline structure of WBG/iPP electrospun fibers depended on the content of WBG. Both the crystallinity and the percentage of β-crystal form of WBG/iPP electrospun fibers increase with the rise of the content of nucleating agent, which endows the prepared electrospun fibers excellent mechanical properties. The β-nucleated iPP electrospun fibrous membranes prepared in this study can be used for protective clothing material, filtration media, reinforcement for composites and tissue engineering scaffolds.

New understanding on the memory effect of crystallized iPP

In this study, recovery processes of isotactic polypropylene (iPP) melted spherulites at 135 °C after melting at higher temperatures (170 °C–176 °C) were investigated with polarized optical microscopy and Fourier transform infrared spectroscopy. The recovery temperature was fixed to exclude the interference from heterogeneous nuclei. After melting at temperatures between 170 °C and 174 °C, the melted spherulite could recover back to the origin spherulite at low temperatures. Interestingly, a distinct infrared spectrum from iPP melt and crystal was observed in the early stage of recovery process after melting at low temperatures, where only IR bands resulting from short helices with 12 monomers or less can be seen, which indicates that the presence of crystal residues is not the necessary condition for the polymer memory effect. Avrami analysis further indicated that crystallization mainly took place in melted lamellae. After melting at higher temperatures, melted spherulite cannot recover. Based on above findings, it is proposed that the memory effect can be mainly ascribed to melted lamellae, during which crystalline order is lost but conformational order still exists. These conformational ordered segments formed aggregates, which can play as nucleation precursors at low temperatures.

Effect of water‐absorbing nanospheres on antistatic property of isotactic polypropylene fibers

ABSTRACTIn this article, novel antistatic isotactic polypropylene (iPP) fibers were prepared through adding ternary polymer water‐absorbing nanospheres (TPWANs) into iPP melt in melt spinning process. The TPWANs were synthesized through emulsion copolymerization of styrene (St), butyl acrylate (BA), and sodium allylsulfonate (SAS). The characterization results of Fourier transform infrared spectrometer (FT‐IR) and 1H nuclear magnetic resonance (1H NMR) confirmed the ternary components in this copolymer. Transmission electron microscopy (TEM) images indicated the nanospherical shape of TPWANs with the characteristics of 80–90 nm size range of diameter and good dispersion in iPP fibers. The volume resistance of iPP fibers decreased by three orders from 1012 to 109 Ω cm when the 6 wt % TPWANs was added into iPP fibers, indicating that the blend fibers have good antistatic property. Most importantly, the nanospherical structure of TPWANs imparts both the compatibility and antistatic property into the blend iPP fibers. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40568.

Calculated Interfacial Free Energies and Hetrogeneous Nucleation of Isotactic Polypropylene

The surface free energy of crystalline isotactic polypropylene (iPP) and the interfacial free energy of crystalline iPP with a featureless Hamaker constant matched surface are determined by an all-atom lattice dynamics method including contributions from phonons treated quantum-mechanically and analytical “tail corrections” for long-ranged dispersion forces. The relative magnitudes of quantum effects, tail corrections, and finite system size effects on the surface free energy are assessed. The surface tension of iPP melt and the interfacial tension of iPP melt with a featureless substrate are determined from all-atom molecular dynamics simulations. We apply these results to a heterogeneous “cylindrical cap” nucleus on a featureless substrate, for which a contact angle of 79° ± 3° is found. This value is in reasonable agreement with analysis of quiescent crystallization of commercial iPP samples, and thus lends support to the cylindrical cap model for heterogeneous nucleation.

Comparison Study on the Heterogeneous Nucleation of Isotactic Polypropylene by Its Own Fiber and α Nucleating Agents

The crystallization of isotactic polypropylene (iPP) in the presence of its own fiber or/and α nucleating agents, i.e., DBS and NA11, was studied. The results show that both nucleating agents exhibit nucleation ability toward iPP and accelerate iPP melt crystallization. This is manifested by an upward shift of crystallization temperature by up to 10 °C compared with its bulk crystallization. The iPP fiber exhibits also high nucleation ability toward the iPP matrix as demonstrated by the formation of transcrystalline layers. The crystallization of iPP in the nucleated iPP matrix/fiber system shows that the nucleating agents do not affect the transcrystallization of iPP surrounding its fiber. This indicates that the fiber exhibits higher nucleation ability toward iPP than the nucleating agents. On the basis of the exact same chemical and crystallographic structures of the iPP fiber and matrix, the results demonstrate the importance of structure similarity and matching between the crystallographic structures...

Slippage and shear inhomogeneity in shear-induced crystallization of isotactic polypropylene on metal substrates

Substrate effect, slippage, and shear inhomogeneity in the shear-induced crystallization of an isotactic polypropylene are studied by rheological and optical experiments. Significant wall slip that reflects the polymer chain desorption from the wall was observed for the supercooled isotactic polypropylene (iPP) melt at 131 \(^{\circ }\)C in both the pre-shear and the subsequent small-amplitude oscillatory shear that monitor the crystallization of iPP. Crystallization of the iPP melt on aluminum substrate is faster than that on stainless-steel substrate with the same pre-shearing condition. Because the surface energy of aluminum plate is higher than that of stainless-steel plate, when using the aluminum plates, the slip during the pre-shearing is smaller; thus, the real shear rate (or shear strain, shear work) exerted to the melt is higher and so is the shear-induced nucleation density. By using the observed nucleation density and the estimated nucleus growth rate, the Kolmogoroff equation can yield correct orders of magnitudes of crystallization rates of iPP. Assuming that higher shear rate induces higher nucleation density in the iPP melt, the shear inhomogeneity during the pre-shearing can be inferred based on the optical observation on the crystallized iPP samples.

Influence of the memory effect of a mesomorphic isotactic polypropylene melt on crystallization behavior

In this study, the memory effect of a polymer is investigated with a mesomorphic isotactic polypropylene (iPP) melt as the initial state. It is found that the nucleation density and crystallization rate at low temperatures are strongly dependent on the initial melting temperature and melting time, indicating a strong memory effect. High melting temperatures decrease the nucleation density, while a short melting time causes a faster crystallization rate than in the melt with its thermal history removed. At 180 °C, it takes about 1 hour for the crystallization rate to be restored to the normal value found in the melt with erasure of its thermal history at 220 °C. Further experiments indicate that the increase in spherulite growth rate is mainly responsible for acceleration of the overall crystallization kinetics. Through comparison between meso crystallization and self-seeding crystallization, it is suggested that some ordered structures with higher thermal stability exist in the mesomorphic iPP melt. The high thermal stability of the ordered structure may be due to the random arrangement of helices of different tacticity. We propose that the ordered structure accelerates spherulite growth, however, long melting times at 180 °C can break down the ordered structure, leading to the formation of an ideal melt and restoration of the spherulite growth rate. This study indicates that a thermodynamically unstable ordered structure can survive in a supercooled melt for a long time and is involved in the crystallization process.

In Situ Observations of Flow-Induced Precursors during Shear Flow

In this work, the formation of flow-induced precursors (FIPs) during and immediately after applying a shear flow in isotactic polypropylene (iPP) melt is studied at high spatial resolution and high time resolution with a high-speed polarized optical microscope (HSPOM) and simultaneous small/wide-angle X-ray scatterings (SAXS/WAXS). A number of studies have reported the formation of FIPs in the early stage of flow-induced crystallization. However, reports on direct observations of the formation of precursors during shearing are scarce. We find that FIPs are observed using HSPOM after a certain incubation time during shearing. After cessation of flow, some FIPs dissolve gradually and others transfer to crystals. This suggests a competition between chain relaxation and crystallization. In SAXS measurements, the formation of FIPs during shearing is indicated by weak equatorial streaks normal to the flow direction after a specific incubation time. The intensity integrated from 2D SAXS images shows a sudden dec...

β-Crystal in the iPP melt encapsulated by transcrystallinity and spherulites: effect of molecular weight

The crystallization in isotactic polypropylene (iPP) melt inclusions, encapsulated by transcrystallinity and spherulites, was investigated by polarized light microscope. Generally, owing to the proceeding of crystallization in melt inclusions, a negative pressure will be built up. Up to a critical value, the negative pressure is released by the formation of cavitation in melt. Unexpectedly, β-crystals with different morphologies emerge at different locations in the melt inclusions. For low molecular weight iPP, only smaller β-crystals around bubbles are discernable. Turn to higher molecular weight iPP, apart from the smaller β-crystals, fan-shaped ones can be developed under suitable conditions (e.g., lower than 131 °C) at the growth front of spherulites and transcrystallinity. Considering the characteristics of molecular weight and morphology, it is proposed that β-crystals are induced by different mechanisms. That is, smaller β-crystals are resulted from the stress caused by the appearance of bubbles while the fan-shaped ones are derived due to the temperature gradient during the release of negative pressure.

Self-Acceleration of Nucleation and Formation of Shish in Extension-Induced Crystallization with Strain Beyond Fracture

Extension flow induced crystallization of isotatic polypropylene (iPP) has been studied with a combination of extension rheological and in situ small-angle X-ray scattering (SAXS) measurements at 140 °C. Rheological data of step extension on iPP melt are divided into before and beyond fracture strain zones in strain–strain rate space, where intermediate strains between them lead to fracture of samples. Coincidently, weak and strong accelerations of nucleation are observed in the before and beyond fracture strain zones respectively, where distinctly different features of crystallization kinetics and nucleation form occur in these two zones. The microrheological model explains the acceleration of nucleation in the “before fracture strain zone” well, while a “ghost nucleation” mechanism is proposed to interpret the strong acceleration of nucleation in the “beyond fracture strain zone”. The “ghost nucleation” is due to the displacement of initial parent point nuclei, where daughter nuclei are induced along the trails. This new mechanism explains well the acceleration of nucleation in orders of magnitude and the formation of shish in iPP melt.