Isotactic Polypropylene Samples Research Articles

Stretching Temperature Dependency of Fibrillation Process in Isotactic Polypropylene.

Isotactic polypropylene samples annealed at three temperatures were used to explore their fibrillation behaviors during tensile deformation at elevated temperatures via in situ synchrotron small and wide-angle X-ray scattering techniques. Fibrillation denotes the transition of the isotropic spherulitic morphology into a highly oriented one during tensile stretching of a semicrystalline polymer. It was found that the fibrillation was accomplished by a stress-induced melting and recrystallization process. Three regions, identified as fibrillation with the formation of only mesophase, fibrillation with the formation of both mesophase and α crystallites, and fibrillation with the formation of complete α modification, were identified in a map of deformation and annealing temperature. Such results are tightly linked to the different molecular mobilities of the samples prepared at different annealing temperatures and deformed at different temperatures. Lower annealing temperature and higher deformation temperature facilitate formation of α modification after stress-induced mechanical melting of the original crystallites. However, thicker lamellar crystallite deformed at lower temperatures presenting limited chain mobility ends up with a large amount of oriented mesophase structure.

Deformation of Stereoirregular Isotactic Polypropylene across Length Scales. Influence of Temperature

The development of a highly oriented fiber morphology by stretching low stereoregular isotactic polypropylene (iPP) samples is studied at two different temperatures. The structural and morphological transformations occurring during deformation are followed in real time by collecting in situ wide (WAXS) and small-angle (SAXS) X-ray scattering data. WAXS analysis reveals that the disordered γ form initially present in the samples gradually transforms by stretching into the oriented mesomorphic form of iPP at room temperature and in well-oriented crystals of α form at 60 °C. SAXS analysis indicates that regardless of the stretching temperature, a cascade of events occurs at mesoscale during deformation, consisting in interlamellar separation and lamellar reorientation at low deformations, followed by formation of chevron-like textures, lamellar fragmentation, and cavitation. Moreover, different fibrillary morphologies develop at the different temperatures. In fact, the fibrillary morphology that develops at ...

Multiscale mechanical characterization of iPP injection molded samples

Mechanical characterizations at different length scales represent a very intriguing possibility for correlating the fine internal morphologies with the macroscopic properties of a polymer material. In this work, molded samples of isotactic polypropylene obtained by injection molding in different processing conditions have been characterized on different lengths scales. The developed morphologies characterized by optical and atomic force microscopy reveal a complex multilayer morphology with globular elements, fibrils and spherulites, from the surface to the sample core. A multiscale mechanical characterization has been performed by Dynamic Mechanical Analysis (DMA), micro indentation and by HarmoniX Atomic Force Microscopy (AFM) tests, from millimeter to nanometer scale. Results show that along sample thickness, well organized and oriented structures (i.e. fibrils) characterized by higher values of mechanical modulus coexist with less organized structures (globules) characterized by mechanical properties closer to the quiescent amorphous phase. Intermediate elasticity is found in the central part of the sample, where well developed spherulitic structures are characterized by both higher crystallinity and lower level of orientation. A good agreement among the relative experimental results has been found, confirming the good complementarity among these techniques for a complete multi-scale sample characterization.

The Influence of Molecular Weight and Tacticity on Thermal, Morphological and Mechanical Properties of Ziegler–Natta Catalyzed Isotactic and Syndiotactic Polypropylene Blends

The thermal, morphological and mechanical properties of polypropylene blends as influenced by the molecular weight and tacticity were investigated. Polypropylene sample blends (50/50 wt.%) were injection moulded into standard disks for morphological and mechanical tests. The thermal properties of the polypropylene blends were measured by differential scanning calorimetry (DSC), while the morphological and mechanical properties of specimens were investigated by means of optical microscope (OM), scanning electron microscopy (SEM), microhardness (MH) and dynamic mechanical analysis (DMA). DSC results of the bulk crystallization of the various isotactic polypropylene blends showed one melting peak, which indicates that cocrystallization of the blends occurred. However, the crystallization behavior of the polymer blends was strongly affected by the configuration (tacticity) and molecular weight of the polypropylene polymers. In addition, the MH and DMA measurements showed that blends of two different isotactic polypropylenes presented MH and storage modulus values between the values of the respective two MH and storage modulus values of the homopolymer samples, which in turn, depended on the type and degree of the crystallinity of the blends. However, the presence of syndiotactic polypropylene in a blend with isotactic polypropylene leads to a decrease in the MH value of the isotactic polypropylene samples. Normal 0 19 false false false LT X-NONE X-NONE DOI: http://dx.doi.org/10.5755/j01.ms.22.3.9056

Transition in Crystal Morphology for Flow-Induced Crystallization of Isotactic Polypropylene

Deformation applied to a semicrystalline polymer melt prior to quenching can dramatically increase the concentration of nuclei and even transform the final crystalline morphology, phenomena collectively known as flow-induced crystallization (FIC). Using polarized optical microscopy and atomic force microscopy, we image the changes in morphology of an isotactic polypropylene (iPP) sample previously sheared in a rotational rheometer. Sufficiently large deformations promote the formation of “rice grain” shaped crystalline domains. These anisotropic structures are randomly oriented and are about 1.5–3.0 μm long (depending on applied work), with an aspect ratio of about 2:1. Well below the critical specific work threshold Wc, the crystalline morphology of our iPP sample is predominately composed of spherulites, with a fraction of rice grains that increases as the applied work increases. Above Wc, rice grains predominate, with no visible spherulites. The size of spherulites appears to be independent of applied work. In contrast, the size of rice grains decreases with increasing applied work, up to a saturation specific work Wsat, remaining constant thereafter. Similarly, the isothermal crystallization time decreases with increasing applied work up to Wsat and is constant thereafter. The effects of flow-induced precursors persist after repeated melting and recrystallizations, observable by elevated freezing temperatures in DSC experiments. Here, we probe directly the robustness of these precursors in optical microscopy, in which we repeatedly melt, anneal, and recrystallize a thin slice of an FIC sample. We observe rice grain domains appearing over and over, with the first domains to crystallize tending to reappear near the same locations.

A study of propylene polymerization over bimetallic titanium–vanadium catalysts

New bimetallic catalysts based on microspherical TiCl3 modified with vanadium trichloride to obtain its different concentrations on the TiCl3 surface were synthesized. The presence of a VCl3 crystalline phase in the catalyst was confirmed by X-ray analysis. The synthesized catalysts were applied to studying propylene polymerization in a liquid monomer medium. The kinetics of the polymerization process was determined, and the structure and properties of the synthesized isotactic polypropylene (IPP) were established. It was demonstrated that the PPI samples synthesized over the developed titanium–vanadium catalysts has enhanced freeze resistance as compared to IPP sample obtained over conventional titanium catalysts.

Effects of stereo-defect distribution on the crystalline morphology and tensile behavior of isotactic polypropylene prepared by compression molding process

ABSTRACTIn this study, the aggregation morphology, tensile behavior, and morphology evolution during the tensile test of two isotactic polypropylene (iPP) samples with similar molecular weight and average isotacticity but different uniformities of stereo-defect distribution are investigated by differential scanning calorimetry (DSC), two-dimensional wide angle X-ray diffraction (2D-WAXD), and scanning electronic microscopy (SEM). The results revealed that the uniformity of stereo-defect distribution of iPP determines the crystalline structure and aggregation morphology, and further influences the tensile behavior and morphology evolution during the tensile test. For PP-A with less uniform stereo-defect distribution, its ability of crystallization is stronger compared with PP-B, resulting in smaller spherulite sizes, higher melting point and degree of crystallinity, and narrower distribution of lamellar thickness of the compression molding specimens. During the tensile test, mainly the inter-spherulite deformation takes place at the early stage for deformation, which further results in drastic deformation of lamellar and high degree of reorientation at the strain increases, exhibiting higher yield strength and elastic modulus, and lower elongation at break compared with PP-B; for PP-B with more uniform stereo-defect distribution, larger spherulite sizes, lower melting point and degree of crystallinity in its compression molding sample are observed. During the tensile test, intra-spherulite deformation mainly takes place, which can disperse the tensile stress more uniformly. As the strain increases, lower degree of crystalline destruction and reorientation of the crystallites take place. The yield strength and elastic modulus of PP-B is lower than PP-A, and its elongation at break is higher.

Thermal characterization of the as-received polymer sample using fast scanning calorimetry

An experimental approach to analyze the melting behavior of externally crystallized polymeric material is introduced. Because of the large reorganization effects at the low heating rates in conventional DSC fast scanning calorimetry (FSC) is used with a heating rate of 1000K/s. Isotactic polypropylene (iPP) samples crystallized differently are analyzed in the first test. Because of the undefined heat contact during the first heating run, oil was used as a contact medium. Due to the experimental uncertainty of single measurements, multiple measurements were performed for a range of sample size. The melting temperature of the initial material was determined from this analysis, which was also used to validate a simple procedure for the preparation of polymer films crystallizing at a pre-selected cooling rate.

Critical role of depressurization and effects of saturation conditions in the formation of β‐Crystal during isotactic polypropylene foaming with supercritical CO2

The melting behavior and crystalline forms of isotactic polypropylene (iPP) samples crystallized under different conditions of pressure and temperature were investigated using differential scanning calorimeter (DSC) and wide‐angle X‐ray diffraction (WAXD), respectively. When treated with dynamic supercritical CO2(Sc‐CO2), iPP samples undergo the formation of β‐crystal that does not occur on the treatment with atmospheric pressure and static supercritical CO2(Sc‐CO2) pressure. In addition, the relative content of β‐crystal has deep dependence on melt state and depressurization rate. Depressurization plays very critical role in the formation of β‐crystal by means of imposing three‐dimensional tensile field during cell growth. The tensile field induced α‐row nuclei where the formation of β‐crystal occurred. This finding will provide one new method to induce β‐crystal in iPP parts. POLYM. ENG. SCI. 56:980–986, 2016. © 2016 Society of Plastics Engineers

The banded spherulites of iPP induced by pressure vibration injection molding

Isotactic polypropylene (iPP) samples obtained by pressure vibration injection molding (PVIM) and conventional injection molding (CIM) were studied by polarized-light microscopy (PLM), respectively. It was found that the alternating bright and dark banded spherulites were generated in the transitional region of PVIM parts. It is the first time that the banded spherulites of isotactic polypropylene were observed in polymer processing. What’s more, the banded spherulites were proved to be constituted of α-form crystal by hot stage polarized-light microscopy (HT-PLM) and wide angle X-ray diffraction (WAXD). Morphology of the banded spherulites was also studied by scaning electronical microscopy (SEM).

Cavitation in Isotactic Polypropylene at Large Strains during Tensile Deformation at Elevated Temperatures

Two quenched isotactic polypropylene samples with different molecular weight were used to explore the sudden stress whitening activated at large strains during stretching at elevated temperatures via the ultrasmall-angle X-ray scattering technique. This kind of whitening was confirmed to be caused by the creation of cavities within the highly oriented samples. The critical strain for initiating the stress whitening increases with the increase of stretching temperature and molecular weight whereas the critical stress for the stress whitening depends only on molecular weight irrespective of stretching temperature. Thus, this peculiar stress whitening behavior under large strain deformation can be understood as a consequence of disentanglement of the highly oriented amorphous network initiated by the breaking of interfibrillar tie chains. During large strain deformation, two independent processes can occur. Besides the disentanglement induced cavitation, the stacked lamellar structure within microfibrils can...

Effects of ultrasound on the conformational and crystallization behavior of isotactic polypropylene polymerized with different Ziegler—Natta catalyst

Using different Ziegler—Natta catalysts, we have prepared two samples of isotactic polypropylene with similar molecular weight and average isotacticity, but different stereo-defect distributions. The influence of ultrasonic irradiation on the conformation and crystallization behavior of those samples were investigated by differential scanning calorimetry, wide-angle X-ray diffraction and Fourier transform infrared spectroscopy. The ultrasonic treatment induced a decrease in the degree of crystalline perfection and a wider distribution range of the crystalline perfection of polypropylene. Meanwhile, the crystal particles pack looser and the crystallite size decreased under the influence of ultrasonic irradiation. On the conformation aspect, the molecular movement in polypropylene became more intensive, the molecular entanglement was reduced when ultrasound was applied. Moreover, it was found that the uniformity of stereo-defect distribution also played important role in determining the conformation and crystallization behavior of polypropylene samples irradiated by ultrasound.

Non-isothermal crystallization behavior and melting behavior of Ziegler–Natta isotactic polypropylene with different stereo-defect distribution nucleated with bi-component β-nucleation agent

The influence of bi-component β-nucleating agent of pimelic acid/calcium stearate (Pa–Cast) on the non-isothermal crystallization behavior and polymorphic composition of two isotactic polypropylene (iPP) samples with nearly same average isotacticity but different uniformities of stereo-defect distribution were investigated by differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD) and Scanning electronic microscopy (SEM). The results indicated that Pa–Cast has very high β-nucleation efficiency, and greatly increases the crystallization rate of both samples. After the addition of Pa–Cast, the sample with more uniform distribution of stereo-defect and less fraction of high isotacticity (denoted as PCPP-B) is more favorable for formation of large amount of β-phase; meanwhile, its increase of crystallization peak temperature T c is obviously more prominent. Further analysis of melting behavior showed that the β-phase content greatly depends on the cooling rate applied, the stereo-defect distribution of iPP and the end temperature of cooling (T end). When T end = 50 °C, for iPP with less uniform stereo-defect distribution (PCPP-A), the slow cooling rate favors the formation of high β-content, while for PCPP-B, the fast cooling rate favors the formation of high β-content, which should be attributed to the combination effect of the ability for β-crystallization and the thermal stability of the β-phase. After elimination of the confusion of β–α recrystallization during heating (T end = 100 °C), it turns out that for both PCPP-A and PCPP-B, higher cooling rate is more favorable for β-crystallization. PCPP-A is more favorable for the formation of β-phase with higher thermal stability compared with PCPP-B.

Formation of the gamma phase in oriented isotactic polypropylene with varying stereoregularity

Structural transformations have been studied in oriented samples of isotactic polypropylene with various contents of stereodefects. With the use of small- and wide-angle X-ray diffraction, it was found that a stretched film of polypropylene contains small and defective crystallites of the α phase with an axial orienta tion. The γ phase is formed in oriented polypropylene with a significant amount of stereodefects during high-temperature annealing near the melting point owing to epitaxial crystallization of the partial melt on residual oriented crystallites. The clearer this process is expressed, the lower the degree of crystallinity and/or the higher the degree of stretching of the samples. An increase in the amount of stereodefects in the initial material is expressed in the appearance of the second long period, which is associated with the formation of lamellar crystallites of the γ phase.

The Morphology and Mechanical Properties of Isotactic Polypropylene Injection-Molded Samples with the Presence of β-Nucleation Agent and Periodical Shear Field

The pressure vibration injection molding (PVIM) method was used to prepare β-nucleated isotactic polypropylene samples (PVIM β-iPP samples); a relatively low, periodical shear was imposed on the polymer melt in the mold at the filling and packing stages. The crystal structures and crystal orientation of the PVIM β-iPP samples were investigated by polarizing light microscopy (PLM), scanning electron microscopy (SEM), and synchrotron two-dimensional wide-angle X-ray diffraction (2D-WAXD). The PLM observations indicated that a cylindrite layer, rather than the transition layer, was found in PVIM β-iPP samples, which is different from the conventional injection-molded (CIM) samples. In addition, the thickness of the oriented layer of the PVIM samples was obviously greater than that of the CIM samples. The SEM observations demonstrated that a large amount of shish-kebab structures appeared in the shear layer of the PVIM β-iPP samples; at the same time, numerous β-spherulites were formed in the core layer. The 2D-WAXD data indicated that orientation homogeneity, to some degree, could be obtained by the periodical shear during PVIM. As a result, the above-mentioned morphology of the PVIM β-iPP samples leads to potentially useful prominent reinforcement and toughening of the material.

Molecular Weight Dependency of Surface Free Energy of Native and Stabilized Crystallites in Isotactic Polypropylene.

Two isotactic polypropylene samples were investigated to study the influence of molecular weight on the crystallization and meting behaviors via temperature- dependent small-angle X-ray scattering techniques. In a phase diagram of inverse lamellar thickness and temperature, the crystallization and melting behaviors can be described by two linear dependencies of different slopes and different limiting temperatures at infinite crystalline lamellar thickness. The slope of the crystallization line depends on the surface free energy of the just formed native crystallites, whereas that of the melting line is linked to the surface free energy of stabilized ones. The two polypropylene samples showed different crystallization lines and melting lines, indicating strong changes in surface free energies of the native as well as stabilized crystallites. Such changes are consequences of changes in molecular conformation during crystallization for samples with different molecular weights. Indeed, the low molecular weight sample crystallizes extensively into an extended-chain conformation, whereas the high molecular weight one ends up with normal folded-chain crystallites.

Non-uniform dispersion of toughening agents and its influence on the mechanical properties of polypropylene

To achieve excellent properties of polymer blends and composites, good dispersion and uniform distribution of second component or filler in the matrix are often required. However, more and more evidences reveal that uniform distribution is not always the best. To further prove this idea, in this work, we purposely designed and prepared different samples of isotactic polypropylene (iPP)/elastomer or iPP/!-nucleating agent with uniform and non-uniform distribution of the modifiers via stacking the blending sheets in different sequence. It was found that for a given amount of toughening agent, the impact strength of polymer blends with non-uniform distribution of elastomer or !-nucleating agent could be much higher than its uniformly dispersed counterpart, while the tensile strength and tensile modulus remain relatively constant. The instrumented impact test confirmed that among the samples with different layered structures, the absorbed energy during crack initiation differs little from each other. Whereas absorbed energy during crack propagation process shows the same trend as final impact strength, making it the controlling parameter during the impact process. When cracks are initiated at higher toughening agents content side, the relatively smooth fracture surfaces near the crack edge area proved that they absorb small energy and the adjacent inner part showed obviously plastic deformation, corresponding to higher energy absorption. Our work demonstrates again that design and control of the hierarchical structure of polymer articles is vital for high performance properties and non-uniform distribution of filler could be much better than the uniform distribution.

Influence of heterogeneous nucleation on thermodynamic properties of isotactic polypropylene

Abstract In this paper we have investigated the effect of 1,2,3-trideoxy-4,6:5,7-bis-O-[(4-propylphenyl)methylene]-nonitol sorbitol used in varying amounts (0.01 - 1 wt %) on isotactic polypropylene (iPP) matrix. We have used dynamic mechanical thermal analysis (DMTA) and differential scanning calorimetry (DSC) to study glass transition temperatures and crystallinity as a function of the nucleating agent concentration. Isotactic polypropylene samples showed a strong dependency on amount of α nucleating agent used. An increasing content of sorbitol based nucleating agent led to an increase of crystallization temperature upon cooling from the melt at constant rate and a decrease of the glass transition temperatures.

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−1are used.In situWAXS 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 situsmall-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.

Structure Evolution upon Uniaxial Drawing Skin‐ and Core‐Layers of Injection‐Molded Isotactic Polypropylene by In Situ Synchrotron X‐ray Scattering

ABSTRACTThe structure evolution of the oriented layer (skin) and unoriented layer (core) from injection‐molded isotactic polypropylene samples upon uniaxial drawing is probed by in situ synchrotron X‐ray scattering. The X‐ray data analysis approach, called “halo method”, is used to semiquantitatively identify the transformation process of crystal phase upon uniaxial drawing. The results verify the validation of the stress‐induced crystal fragmentation and recrystallization process in the deformation of the injection‐molded samples under different temperatures. Furthermore, the end of strain softening region in the engineering stress‐strain curves explicitly corresponds to the transition point from the stress‐induced crystal fragmentation to recrystallization process. Basically, the skin and core layers of the injection‐molded parts share the similar deformation mechanism as aforementioned. The stretching temperature which dramatically affects the relative strength between the entanglement‐induced tie chains and the adjacent crystalline lamellae determines the crystal structural evolution upon drawing. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 1618–1631