Nonisothermal crystallization behavior of isotactic polypropylene/ thermoplastic rubber blends

The morphology and crystallization behavior of blends of polypropylene (PP) and an ethylene-based thermoplastic elastomer (TPO) were investigated by scanning electron microscopy (SEM) and differential scanning calorimetry (DSC). The SEM images showed a two-phase morphology for these blends. As TPO was partially crystalline, two distinct peaks were observed in both heating and cooling scans of DSC. The crystallization temperature of TPO in blends was higher than pure TPO. In contrast, the crystallization temperature of PP in blends was lower than pure PP. The crystallization behavior of blends was modeled by Avrami equation. It was observed that the presence of TPO accelerated the growth rate of crystals of PP in PP/TPO blends.     

Formation of β-crystal from nonisothermal crystallization of compression-molded isotactic polypropylene melt

The crystalline structures of isotactic polypropylene (iPP) subjected to compression-molding were studied by means of differential scanning calorimeter (DSC), optical microscope (OM) and wide-angle X-ray diffraction (WAXD). β-crystal can be formed from nonisothermal crystallization of the sample compression-molded at the molten state and the β-phase content increased with increasing pressure of molding. Thermal treatment of the molten sample at 200 °C could eliminate the effect of compression-molding on crystalline structure. It was suggested that the compression-molding of iPP melt plays an important role in improving the nucleation ability of β-crystal.     

Structure,morphology, and crystallization process of isotactic polypropylene/hydrogenated hydrocarbon resin blends

The structure, morphology, and isothermal and nonisothermal crystallization of isotactic polypropylene/low‐molecular‐mass hydrocarbon resin blends (iPP/HR) (up to 20% in weight of HR) have been studied, using optical and electron microscopy, wide‐ and small‐angle X‐ray and differential scanning calorimetry. New structures and morphologies can be activated, using appropriate preparation and crystallization conditions and blend composition. For every composition and crystallization condition, iPP crystallizes in α‐form, with a spherulitic morphology. The size of iPP spherulites increases with resin content, whereas the long period decreases. In the range of crystallization temperatures investigated, HR modifies the birefringence of iPP spherulites, favoring the formation of radial lamellae and changing the ratio between tangential and radial lamellae. Spherulitic radial growth rates, overall crystallization rates, and melting temperatures are strongly affected by resin, monotonically decreasing with resin content. This confirms miscibility in the melt between the two components of the blends. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3368–3379, 2004     

Shear-induced crystallization in phase-separated blends of isotactic polypropylene with ethylene-propylene-diene terpolymer

The compatibility between isotactic polypropylene(i PP) and ethylene-propylene-diene terpolymer(EPDM) in the blends was studied. SAXS analysis indicates that i PP and EPDM phases in the binary blend are incompatible. Isothermal crystallization behaviors of i PP in phase-separated i PP/EPDM were studied by in situ POM equipped with a Linkam shear hot stage. It was found that typical spherulites of i PP were formed both in neat i PP and in i PP/EPDM blends. The radial growth rate(d R/dt) of spherulites of i PP in the blend was not influenced by EPDM phases. Further investigations on isothermal crystallization of i PP in i PP/EPDM after shear with a fixed shear time showed that the crystallization rate of i PP in the blends increased with increasing shear rates, whereas, the crystallization rate was much lower than that of neat i PP. WAXD results showed that ?-crystal i PP was formed in neat i PP as well as in i PP/EPDM blends after shearing and the percentage of ?-crystal bore a relationship to the applied shear rate. The presence of EPDM resulted in lower percentage of ?-crystal in the blends than that in neat i PP under the same constant shear conditions. SAXS experiments revealed that shear flow could induce formation of oriented lamellae in i PP and i PP in the blends, and the presence of EPDM led to a reduced fraction of oriented lamellae.     

Non-isothermal crystallization behavior of isotactic polypropylene/copper nanocomposites

Journal of Thermal Analysis and Calorimetry - The Non-isothermal crystallization behavior of isotactic polypropylene (iPP)/copper nanocomposites with four different mass percentages (0.5, 1.0, 2.0...     

Insight into understanding the evolution of the epitaxy crystallization in isotactic polypropylene and polyethylene blends

In this article, epitaxial structures have been successfully obtained in the isotactic polypropylene (iPP)/polyethylene (PE) blends by an accessible injection molding methods. By studying a series of iPP/PE blends, the evolution of the epitaxial growth of PE lamellae on the oriented iPP lamellae has been detailedly discussed via wide‐angle X‐ray diffraction, small‐angle X‐ray scattering, scanning electron microscopy and differential scanning calorimetry. Unexpectedly, the exactly epitaxial angles between peculiarly arranged PE lamellae and oriented PP lamellae are all larger than the classical epitaxy theory value of 50°, and it even increases gradually with increasing PP content. It is inferred that the special crystallization of PE is the consequence of joint construction of the oriented PP crystals and the continuous intense shear field provided by pressure vibration injection molding. The epitaxial structures play a positive role in the interfacial connection between two components; thus, the mechanical properties of the blends are improved. This work provides an insight understanding on the formation mechanism of the epitaxy crystallization under shear field. Copyright © 2017 John Wiley & Sons, Ltd.     

Morphologies of long chain branched isotactic polypropylene crystallized from melt

The crystallization behavior and resulting crystalline morphologies of long chain branched isotactic polypropylene (LCBPP) under different conditions were studied by means of differential scanning calorimetry and transmission electron microscopy combined with electron diffraction. The results indicate that the crystallization of LCBPP during fast cooling process, or at lower crystallization temperature, leads to the formation of mainly edge-on lamellar structures. The LCBPP exhibits also the wide angle lamellar branching frequently observed for linear isotactic polypropylene. Crystallizing LCBPP in temperature range of 110∼140 °C results in the formation of both edge-on and flat-on crystals, which coexist side by side with the content of flat-on crystals increases with increasing crystallization temperature. At high crystallization temperature, e.g. 145 °C, flat-on crystals with chain axis aligned perpendicular to the film plane are the only observed morphology. Moreover, the crystals of LCBPP grow slower than its linear counterpart and the crystal growth rates of both linear and long branched PPs are temperature dependent.     

Modeling of the crystallization of isotactic polypropylene chains

The mechanical properties of polypropylene depend critically on the crystallizability of the chains, which depends in turn on their stereochemical structures. These dependences were investigated using Monte Carlo methods to generate chains having various stereochemical sequences and then scrutinizing parallel arrays of these chains to look for matches in isotactic stereochemistry that could lead to the formation of crystallites. The fraction of such units in matches, for example, gives a direct measure of the degree of crystallinity expected for the specified degree of isotacticity. Other quantities of interest obtainable in this way were the natures of the sequence distributions themselves, melting points, free energies of fusion, interfacial free energies, and moduli (calculated on the basis of the crystallites acting as crosslinks within an elastomeric network structure). © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 2757–2764, 1997     

Shear-induced conformational ordering, relaxation, and crystallization of isotactic polypropylene

The shear-induced coil-helix transition of isotactic polypropylene (iPP) has been studied with time-resolved Fourier transform infrared spectroscopy at various temperatures. The effects of temperature, shear rate, and strain on the coil-helix transition were studied systematically. The induced conformational order increases with the shear rate and strain. A threshold of shear strain is required to induce conformational ordering. High temperature reduces the effect of shear on the conformational order, though a simple correlation was not found. Following the shear-induced conformational ordering, relaxation of helices occurs, which follows the first-order exponential decay at temperatures well above the normal melting point of iPP. The relaxation time versus temperature is fitted with an Arrhenius law, which generates an activation energy of 135 kJ/mol for the helix-coil transition of iPP. At temperatures around the normal melting point, two exponential decays are needed to fit well on the relaxation kinetic of helices. This suggests that two different states of helices are induced by shear: (i) isolated single helices far away from each other without interactions, which have a fast relaxation kinetic; (ii) aggregations of helices or helical bundles with strong interactions among each other, which have a much slower relaxation process. The helical bundles are assumed to be the precursors of nuclei for crystallization. The different helix concentrations and distributions are the origin of the three different processes of crystallization after shear. The correlation between the shear-induced conformational order and crystallization is discussed.     

Influence of the order of polymer melt on the crystallization behavior: II. Crystallization kinetics of isotactic polypropylene

The crystallization behavior of isotactic polypropylene (iPP) melts with a high order has been carefully examined by differential scanning calorimetry (DSC) and polarized light microscopy (PLM). The experimental results show that the helically ordered iPP melt crystallizes by heterogeneous nucleation with two-dimensional diffusion controlled growth and the Avrami exponent is about 2. The data available both from our DSC and PLM experiments and from the literature indicate that the order of a polymer melt can speed up the crystallization process. When some unmelted materials exist in the ordered melt, the crystallization will become more rapid. Received: 16 June 2000 Accepted: 16 October 2000     

Evaluation of the fractionated crystallization of isotactic polypropylene and high density polyethylenes in their blends with Cycloolefin copolymers

Crystallization of semi-crystalline polyolefins (i-polypropylenes and HD-polyethylenes) in their blends with amorphous cycloolefin copolymers (COC) were studied. The thermal behaviour of the blends was characterized by Differential Scanning Calorimetry (DSC) whereas blend morphologies were investigated by Scanning Electronic Microscopy (SEM). In iPP/COC blends, a phenomenon of fractionated crystallization is evidenced when i-PP is finely dispersed in the COC matrix. Such a behavior is generally observed when the number of droplets is much larger than the number of heterogeneities originally present in the bulk polymer. In HDPE/COC blends, complex morphologies are observed which do not fit good correlation with DSC results. The nucleation and crystallization modes seem to be largely influenced by the characteristics of the micro-dispersed phase, largely dependent on the PE molecular weights and polydispersity indices.     

High-temperature hedritic crystallization of the β-modification of isotactic polypropylene

 High-temperature crystallization of the β-modification of polypropylene (β-iPP), induced by an active β-nucleating agent, was studied by polarizing light and scanning electron microscopy (SEM). It was established that, in the early stage of crystallization of β-iPP, hedrites were formed as precursors of the spherulitic crystallization. Hedrites seen flat-on are characteristic hexagonal formations (hexagonites) with low birefringence. The central core of hedrites had positive birefringence. Hedrites seen edge-on were rod-like formations with strong negative birefringence which would transform into a radially symmetric spherulitic form through sheaf-like and then oval (ovalite) arrangements. According to the SEM micrographs, hedrites are clusters of multilayer lamellar crystallites. On the surfaces of lamellae hexagonal etch pits appear referring to screw dislocations. They are responsible for branching and proliferation of lamellae. Hedrites may reach considerable sizes (several hundred micrometres) presumably due to a coordinated cooperative growth of lamellae. There is an unequivocal correlation between the character of birefringence and the morphological structure shown by SEM of hexagonites, rods, ovalites and spherulites. It was demonstrated that, at temperatures of isothermal crystallization higher than the critical T(βα) , mixed polymorphic structures of α- and β-modifications were formed. Received: 15 August 1996 Accepted: 10 January 1997     

Influence of the order of polymer melt on the crystallization behavior: I. Double melting endotherms of isotactic polypropylene

The influence of the order of polymer melt on the subsequent crystallization and melting has been carefully studied. The experimental data show that the order of isotactic polypropylene (iPP) melt decreases with increases in the fusion temperature. For an iPP sample isothermally crystallized at 130 °C for half an hour, the degree of order of melt is higher when the fusion temperature is lower than about 170.5 °C, hence the lamellae formed in a rapid cooling process are perfect. If the fusion temperature is not higher than 167 °C, some thicker lamellae can exist in the melt. The melting of these unmelted lamellae and those lamellae recrystallized in the cooling process result in double endotherms. On the other hand, when the fusion temperature is higher than 170.5 °C, the order of the iPP melt decreases greatly; thus, the lamellae formed in the following cooling process are imperfect. At a lower heating rate, the recrystallization or reorganization of these imperfect lamellae also leads to double melting endotherms. Received: June 16, 2000 Accepted: October 16, 2000     

Heat treatment of isotactic polypropylene: the effect of free quenching from the melt state

Studying the effect of quenching from the melt state on the structure and impact resistance of Isotactic polypropylene (iPP) was the major aim of this work. Various tests were applied to confirm changes that occur to iPP, namely impact tests, WAXD, FTIR, and the density. The quenching from the melt state to different temperatures decreased the values of Izod impact strength for all the quenching temperatures. The FTIR result showed a decrease in the crystallinity of the polymer at the free quenching temperature of 20?°C. The X-ray diffraction study revealed that the α structure dominates the main morphology of iPP.     

Crystallization and melting of isotactic polypropylene crystallized from quiescent melt and stress‐induced localized melt

Temperature dependency of crystalline lamellar thickness during crystallization and subsequent melting in isotactic polypropylene crystallized from both quiescent molten state and stress‐induced localized melt was investigated using small angle X‐ray scattering technique. Both cases yield well‐defined crystallization lines where inverse lamellar thickness is linearly dependent on crystallization temperature with the stretching‐induced crystallization line shifted slightly to smaller thickness direction than the isothermal crystallization one indicating both crystallization processes being mediated a mesomorphic phase. However, crystallites obtained via different routes (quiescent melt or stress‐induced localized melt) show different melting behaviors. The one from isothermal crystallization melted directly without significant changing in lamellar thickness yielding well‐defined melting line whereas stress‐induced crystallites followed a recrystallization line. Such results can be associated with the different extent of stabilization of crystallites obtained through different crystallization routes. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 957–963     

Monte Carlo simulation study of the effect of chain tacticity on demixing of polyethylene/polypropylene blends

The molecular origin of the demixing behavior for 50: 50 (wt/wt) polyethylene/polypropylene (PE/PP) with different tacticity of PP at the melts (473 K) was investigated by Monte Carlo simulation of coarse-grained polymer model. Isotactic (iPP), atactic (aPP) and syndiotactic (sPP) polypropylenes were used for blending with PE. Coarse-graining polymer chains were represented by 50 beads, corresponding to C₁₀₀H₂₀₂ and C₁₅₀H₃₀₂ for PE and PP, respectively. The simulation was performed on a high coordination lattice incorporating short-range intramolecular interactions from the Rotational Isomeric State (RIS) model and long-range intermolecular interactions Lennard-Jones (LJ) potential function of ethane and propane units. Chain dimensions, the characteristic ratio (C _n ) and self-diffusion coefficient (D) of PE in the blends are sensitive to the stereochemistry of PP chains. Compared with neat PE melts, PE dimension was relatively unchanged in PE/iPP and PE/aPP blends but slightly decreased in PE/sPP blends. PP dimension was increased in PE/iPP and PE/aPP mixture but decreased in PE/sPP blend in comparison with neat PP melts. In addition, diffusion of PE and PP chains in PE/PP mixture was decreased and increased, respectively, compared to the pure melts. Interchain pair correlation functions were used to detect the immiscibility of the blends. The tendency of demixing of PE/aPP and PE/iPP blends were weaker than that of PE/sPP blend.     

Compatibilization of polypropylene/poly (ethylene oxide) blends and crystallization behavior of the blends

The compatibilization of incompatible polypropylene (PP)/poly(ethylene oxide) (PEO) blends was studied. The experimental results showed that the graft copolymer [(PP-MA)-g-PEO] of maleated PP(PP-MA) and mono-hydroxyl PEO (PEO-OH) was a good compatibilizer for the PP/PEO blends in which PP-MA also had some compatibilization. The crystallization of the blends was affected by the compatibility between PP and PEO. The interfacial behavior of the compatibilizers had an important effect on crystallization behavior of the PP/PEO blends. PEO showed fractionated crystallization in the PP/PEO blends. This behavior was studied from the view point of the theory of fractionated crystallization. © 1994 John Wiley & Sons, Inc.     

Molecular mechanism leading to memory effect of mesomorphic isotactic polypropylene

In 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