Epitaxial Crystallization of Linear Polyethylene in Blends with Isotactic Polypropylene

The structure and morphology of extrusion-oriented ribbons of polypropylene/polyethylene blends is described. The blends with 20%, 30%, and 40% of oriented isotactic polypropylene fibrils show homo- and heteroepitaxial structures. Partial mutual solubility of the blend components influenced the melting and crystallization behavior.     

Morphology,phase structure,and properties of polyolefin/hydrogenated oligocyclopentadiene blends

The paper discusses the influence of an amorphous oligomer (namely hydrogenated oligocyclopentadiene — HOCP) on the morphology and the phase structure of its blends with several polyolefins as a function of composition and crystallization conditions. In particular the following polyolefins were studied: high-density polyethylene (HDPE), isotactic polypropylene (IPP), poly(l-butene) (PB-1), and poly(4-methyl pentene-1) (P4MP1). The blends under investigation are complex polymer systems. In fact, in dependence on temperature, blend composition, and cooling rate, they assume different morphologies and consequently show different thermal and mechanical behaviors. In the solid state the blends form a generally three-phase system: a crystalline phase of polyolefin and two amorphous phases, one rich in the amorphous polyolefin and the other in HOCP. The crystallization process and the properties are determined by the morphology and the phase structure, as well as by the physical state of the HOCP-rich phase.     

Oriented Crystallization of Polyethylene in Compatible Blends with Polyamide 11

The oriented crystallization of polyethylene (PE) in uniaxially oriented films of compatible blends with polyamide 11 (PA11) was studied. The PE sample used was a random copolymer of PE with methacrylic acid (MAA), poly(ethylene-co-methacrylic acid) (PEMAA), with 4wt% MAA units. Oriented films of PA11/PEMAA blends were prepared by uniaxial drawing of the melt-mixed blends. The drawn films with fixed lengths were heat-treated at 120°C for 3min to melt the PE component, followed by cooling the sample to room temperature at a rate of 2°C/min to recrystallize the PE (designated slowly crystallized sample). The PE component crystallized in elongated domains of PEMAA with diameters of 0.5–2 μ m and lengths of 5–10 μ m for the PA11/PEMAA = 80/20 blend, resulting in the oriented crystallization of PE crystals. The crystal b-axis of PE was highly oriented in the direction perpendicular to drawing, while the crystal a-axis was weakly oriented in the drawing direction in the slowly crystallized sample of the PA11/PEMAA = 80/20 blend. The a-axis orientation of PE crystals in the PA11/PEMAA = 80/20 blend contributes to the improvement of mechanical properties in the direction perpendicular to drawing.     

Application of129Xe NMR to polymer blends

¹²⁹Xe NMR experiments on two different polymer blends are described. The first system, a blend of polypropylene (PP) and the copolymer of polypropylene and polyethylene (EP), shows separate domains of polypropylene and of the copolymer. The latter phase forms rubbery domains in the polypropylene matrix. The second system is the compatible blend of polyvinylidenefluoride and polymethylmethacrylate. For the incompatible blend two Xe resonances are found, one for Xe absorbed in the matrix and one for Xe in the rubbery EP phase. The chemical shift of the Xe absorbed in the rubbery phase can be related to the polyethylene content of the copolymer. The line widths and chemical shifts are affected by the polymer motions as a function of the temperature. Although the system has two very different glass transition temperatures, it is striking to see that upon approaching the polypropylene glass transition temperature the Xe resonance of Xe in the rubbery phase is also affected. Due to the dipolar interaction between Xe spins and polymer proton spins, cross-polarization experiments can be performed. This allows the measurement of correlated NMR spectra. The compatible blend shows only one line with a chemical shift proportional to the composition.     

Nonisothermal Crystallization Nucleation of In‐Situ Fibrillar and Spherical Inclusions in Poly (Phenylene Sulfide)/Isotactic Polypropylene Blends

Nonisothermal crystallization nucleation and its kinetics of in‐situ fibrillar and spherical dispersed phases in poly (phenylene sulfide) (PPS)/isotactic polypropylene (iPP) blends are discussed. The PPS/iPP in‐situ microfibrillar reinforced blend (MRB) was obtained via a slit‐die extrusion, hot stretching, and quenching process, while PPS/iPP common blend with spherical PPS particles was prepared by extrusion without hot stretching. Morphological observation indicated that the well‐defined PPS microfibrils were in situ generated. The diameter of most microfibrils was surprisingly larger than or equal to the spherical particles in the common blend (15/85 PPS/iPP by weight). The nonisothermal crystallization kinetics of three samples (microfibrillar, common blends, and neat iPP) were investigated with differential scanning calorimetry (DSC). The PPS microfibrils and spherical particles could both act as heterogeneous nucleating agents during the nonisothermal crystallization, thus increasing the onset and maximum crystallization temperature of iPP, but the effect of PPS spherical particles was more evident. For the same material, crystallization peaks became wider and shifted to lower temperature when the cooling rate increased. Applying the theories proposed by Ozawa and Jeziorny to analyze the crystallization kinetics of neat iPP, and microfibrillar and common PPS/iPP blends, both of them could agree with the experimental results.     

Morphology,Nonisothermal Crystallization Behavior and Mechanical Properties of Polypropylene Modified by Ionomers

The morphology and nonisothermal crystallization behavior of polypropylene modified by ionomers based on ethylene copolymers (Surlyn 8920 and 9320) were investigated by using scanning electron microscopy (SEM) and differential scanning calorimetry (DSC). The crystallization rate of polypropylene was accelerated by the ionomers which initiated heterogeneous nucleation of the polypropylene. At low ionomers content (0.25 wt%), Surlyn 8920, neutralized by sodium, was more efficient to enhance the crystallization rate of polypropylene than Surlyn 9320 (neutralized by zinc). The crystallization process of polypropylene modified by the ionomers was analyzed by different kinetics models. The study showed that the Mo approach was applicable for this system, whereas the Avrami, Jeziorny, and Ozawa methods were not. Furthermore, the notched impact strength of polypropylene modified by the ionomers was increased without any reduction of tensile strength and flexural modulus.     

An In situ Experimental Study on Solidification Kinetics of Polyolefins: Effect of Cooling Conditions

The solidification kinetics of polyolefins (PO) under three cooling conditions were investigated using an in situ measurement of the temperature decay within the PO resins. The phase-change temperature range of high-density polyethylene (HDPE) was located between 110 and 120°C, and those of low-density polyethylene (LDPE) and polypropylene (PP) were 90–110°C and 100–120°C, respectively. The cooling rate of the liquid-state stage is larger than that of the crystallization stage, primarily owing to the release of the latent heat of crystallization as well as the reduced temperature difference between the sample and cooling medium; they jointly slow down the cooling rate to an extent. The time with respect to phase transformation and its lasting period have close relations to the materials' molecular characteristics (e.g., Mw, MWD, LCB, etc.). Three empirical equations were proposed, and found to be applicable for the cooling analysis of the PO molten materials at relatively low cooling rates prior to crystallization.     

Unexpected shish-kebab structure in a sheared polyethylene melt

Scanning electron micrographs of a solvent-extracted sheared polyethylene (PE) blend revealed, for the first time, an unexpected shish-kebab structure with multiple shish. The blend contained 2 wt % of crystallizing ultrahigh molecular weight polyethylene (UHMWPE) and 98 wt % of noncrystallizing PE matrix. The formation of multiple shish was attributed to the coil-stretch transition occurring in sections of UHMWPE chains. Synchrotron x-ray data provided clear evidence of the hypothesis that multiple shish originate from stretched chain sections and kebabs originate from coiled chain sections, following a diffusion-controlled crystallization process.     

Shape Memory Properties of Melt-Blended Ethylene Vinyl Acetate (Eva)/Metallocene Polyethylene Eco-Blends

EVA(ethylene vinyl acetate) was melt-blended with mPE(metallocene polyethylene elastomer) to form shape memory, eco-based blends with and without vinyl triethoxysilane (VTEOS) modification. The silane crosslinking modification slightly suppressed the crystallization temperatures of the mPE component due to the increased gel content. The crystallinity of mPE also decreased slightly, from 4.1% to 1.8%, due to the silane crosslinking effect, and further, to 1.3%, due to the interference effect of EVA. Despite this negative contribution to thermal crystallization behaviors, the highest tensile strength and Young's modulus were still observed for the EVA/mPE-g-VTEOS blend, attributed to the higher interaction between the silane groups on mPE and the vinyl acetate groups on EVA. In addition, the silane grafting modification on mPE improved the thermal stability and shape fixity of the EVA/mPE-g-VTEOS blend further with respect to their corresponding EVA/mPE blend. The recovery ratio of the EVA/mPE-g-VTEOS system was still high, up to 92.8±0.3%, for the recovery from a temporary deformed shape. In considering both the shape fixity and recovery ratios together, the SMI (shape memory index) of the EVA/mPE-g-VTEOS blend was about 81%, which was higher than that of the EVA/mPE, at about 75%. In particular, the EVA/mPE-g-VTEOS could still be melt processed, in comparison with conventional crosslinked shape memory polymer blends which are hard to be recycled. Herein we justify the merit of the silane modification in the preparation of the EVA/mPE-g-VTEOS shape memory eco-blend to expand its applications, especially in terms of environmental concerns.     

Thermorheology and Crystallization Behaviors of Polyethylenes: Effect of Molecular Attributes

Correlations between polyethylenes of different compositions and branching architectures and the temperature dependence of their viscoelastic behavior as well as the dependence of the nonisothermal crystallization behaviors on the cooling rate were described. To analyze the thermorheological behavior of the various classical polyethylenes, a method proposed by van Gurp and Palmen was utilized and the classical high-pressure low-density polyethylene (LDPE) was found to be thermorheologically complex, while for high-density polyethylene (HDPE) and linear low-density polyethylene (LLDPE), thermorheological simplicity was observed. The Avrami and Mo methods were applied to describe the nonisothermal crystallization kinetics of the different PEs for various cooling rate. The values of the kinetic parameter F(T), kinetic crystallization rate constant (Z_c), and half-time of crystallization (t_1/2) indicated that long-chain branching (LCB) had the role of being a heterogeneous nucleating agent and accelerated the crystallization of polyethylene. Moreover, an HDPE sample of both high molecular weight (M_w) and molecular weight distribution (MWD) had a different crystallization rate dependence from the other samples at various corresponding cooling rates. The crystallization activation energy for nonisothermal crystallization of different PEs was determined using the Kissinger method and showed that the presence of LCB as well as high M_w can increase the crystallization activation energy of polyethylene.     

Raman study of polyethylene-polypropylene blends

The Raman spectra of the blends that result from the melt mixing of polyethylene (PE) and isotactic polypropylene (PP) are studied. The contents of the blend components can be determined using the ratio of the integral intensities of the PE and PP fundamental vibrations.     

The study of compatibility of polyethylene and polypropylene by using irradiated polyethylene wax

The modification of the compatibility between polyethylene (PE) and polypropylene (PP) by using irradiated PE wax (PE wax) is the purpose of this study. In this part, polymer blends based on various ratios of PE and PP were blended with 2.5% PE wax in all the blend ratios to determine the optimum ratio of the blend to be compatabilized. The influence of PE wax as a compatibilizing agent for PE and PP blend was investigated through the measurements of thermal, mechanical and morphological properties. The PP/PE blends modified by this method showed higher mechanical properties than those of the unmodified blends. Also, stress and strain of the modified blend having ratio (60/40) PP/PE blend recorded the maximum mechanical behavior. Scanning electron microscopy (SEM) micrographs of modified blends showed an indication of strong interfacial adhesion and a smooth continuous surface in which giving a support to the effect of irradiated PE wax as a tool for improving the compatibility.     

The Effects of Different Processing Methods on the Morphology and Properties of PP/EPDM/Nano-CaCO3 Ternary Blend

The effects of different processing methods (direct extrusion, two-step extrusion or lateral injection extrusion) on the morphology of polypropylene (PP)/ethylene-propylene-diene terpolymer (EPDM)/calcium carbonate nanoparticles (nano-CaCO₃) ternary blend were investigated, including the morphology of the EPDM phase and the distribution of nano-CaCO₃ particles, by means of scanning electron microscopy (SEM). The results showed that the processing methods had a significant influence on the morphology of the EPDM phase and the distribution of nano-CaCO₃ particles. In the lateral injection extruded blends, it was amazingly observed that the EPDM particles encapsulated the PP phase tightly, and the dimension of EPDM particles was remarkably decreased. It was also found that the content of nano-CaCO₃ particles in the matrix of the lateral injection extruded blends was less than that of the two-step extruded blend, and that of the direct extruded blend was most. The properties of the ternary blend, including dynamic mechanical properties, rheological properties, and crystallization, were characterized in order to confirm the variety of morphologies caused by the different processing methods. The differences in the crystallization temperature, elastic modulus, and glass transition temperature of the blends prepared by different methods well agreed with the variation of their morphology.     

红外光谱在光缆保护盒壳体材料组成分析中的应用

本工作利用聚乙烯和聚丙烯材料红外光谱图的差异，提出了一种工程塑料组成的分析方法。并用于光缆接续保护盒壳体材料的研究，取得了良好的结果。     

The Role of Poly(phenylene sulfide) Microfibrils in Isothermal Crystallization of Isotactic Polypropylene under Shear

An optical polarizing microscope with a hot shear stage was used for an in‐situ investigation of the influences of poly(phenylene sulfide) (PPS) microfibrils on isothermal crystallization of isotactic polypropylene (iPP) under shear. As the nucleation sites on the PPS microfibril's surface are not able to induce a transcrystalline layer, there are only spherulites generated in a PPS/iPP in‐situ microfirbillar blend in quiescent condition. Applying shear during isothermal crystallization, the crystalline morphology greatly changes. There are fibrillar nuclei induced after steady shear with a shear rate of 5 and 10 s^–1, and these nuclei formed fibrillar crystals after crystallization completion. Two opposite effects coexist in PPS/iPP in‐situ microfibrillar blends during shear‐induced isothermal crystallization; one is the obstructive effect of PPS microfibrils on the iPP molecular chains orientation; the other is the positive effect provided by stress between fiber and matrix, generated by shear, which reduces the potential barrier of crystallization. The results of wide angle x‐ray diffraction (WAXD) show that there are β‐iPP crystals generated in neat iPP and PPS/iPP blends, but that PPS microfibrils have an inhibiting influence on the formation of β‐iPP.     

ABSENCE OF ISOTHERMAL LAMELLAR THICKENING IN A LINEAR POLYETHYLENE BLEND

Melting and lamellar morphology of a polyethylene blend were studied. Two linear polyethylene (LPE) samples were used. A commercial LPE and a low molecular weight LPE fraction (M _n ≈ 2015) were solution blended. The pure LPE and a blend (30% commercial LPE content) were held in the melt at 126° C for up to 48 h, above the equilibrium melting point of the fraction, but below the crystallization temperature of the commercial LPE. The melting behavior of both materials as a function of storage time was studied using differential scanning calorimetry (DSC), in addition to transmission electron microscopy (TEM) of chlorosulfonated samples. Results showed that, although the LPE lamellae grew at the same temperature, those in the pure LPE were thicker than in the blend. Correspondingly, isothermally grown lamellae in pure LPE melt at higher temperatures.     

Physical and Thermal Properties of High-Density Polyethylene Film Modified with Polypropylene and Linear Low-Density Polyethylene

Abstract

In view of the toughness and processing difficulty of high-density polyethylene (HDPE) film, the HDPE was modified by polypropylene (PP) and linear low density polyethylene (LLDPE), and the melt index, haze, dart impact strength, elongation at break were characterized. In addition the infrared spectra (IR), scanning electron microscopy (SEM), infrared image analysis, and differential scanning calorimetry (DSC) data were obtained. The results showed that the toughening effect of the 10%PP/30%LLDPE/60%HDPE composition was the best; the haze was reduced 6% and its dart impact strength and elongation at break were increased by 27.3% and 47%, respectively, relative to the pure HDPE. The blend of 10%PP/30%LLDPE/60%HDPE had compatibility. The melting point of the 10%PP/30%LLDPE/60%HDPE blend film increased by 5?°C compared with the pure HDPE film, with the results indicating the application fields of HDPE film could be widened.     

MECHANISMS OF TOUGHNESS IMPROVEMENT OF SEMI-CRYSTALLINE POLYMERS

Many normally ductile semi-crystalline polymers, such as polyamide-6 (or 6,6) (Nylon), high density polyethylene (HDPE), and isotactic polypropylene (iPP) are known to be brittle under impact loading.

Some general principles for improvement of toughness of semi-crystalline polymers that rely on the reduction of plastic resistance of the cavitated matrix are presented. As typical cases, the mechanisms of the dramatic toughness jumps achievable in both Nylon and HDPE through the incorporation of either flexible rubbery particles or rigid CaCO₃ particles are discussed. In both cases these result in the establishment of a material component of reduced plastic resistance in the form of a layer of oriented crystallization of well-defined thickness around the particles. The toughness jumps occur when the average interparticle ligament thickness is reduced below a critical value, specific to the particular polymer, and the component of reduced plastic resistance percolates through the structure.     

Phase Diagrams of Semicrystalline Polymer–Crystalline Substances: Polyolefins–1,2,4,5-Tetrachlorobenzene

The full phase diagrams of low-density polyethylene (LDPE), high-density polyethylene (HDPE), and isotactic polypropylene (i-PP) mixtures with 1,2,4,5-tetrachlorobenzene (TeCB), including the solubility curve of TeCB in a solid polymer, were constructed by an optical method. The diagrams contain a eutectic point that corresponds to the situation when the crystallization of TeCB out of its solution in a polyolefin is accompanied by the crystallization of monomer units of the macromolecules. As a result, the polymer acquires a gel structure with crystallites as crosslinks and amorphous regions saturated with TeCB. It is demonstrated that the eutectic point position on the phase diagram can be used for ranking polymers with respect to their thermodynamic affinity to a solvent. For the studied systems, the affinity to TeCB was decreased in the order i-PP, HDPE, and LDPE. Direct experimental evidence was obtained that TeCB crystals can be dissolved in a solid polymer via a vapor phase mechanism, which leads to the polymer amorphization.     

Effect of epitaxial crystallization on packet-like space charge characteristics in low-density polyethylene under multi-field coupling conditions

In order to study the effect of epitaxial crystallization on charge transport in low-density polyethylene (LDPE) under multi-field coupling conditions, three typical epitaxial crystallizations, namely disorder (glass substrate), crossover (isotactic polypropylene substrate), and parallel (polytetrafluoroethylene substrate), were prepared and denoted as LD-G, LD-iPP, and LD-PT, respectively. Packet-like space charge through samples was analyzed by the pulsed electro-acoustic (PEA) method. It is shown that different microscopic surface morphologies appeared in the LDPE samples with different epitaxial crystallizations, which, however, do not change the crystalline structure of the bulk. Packet-like space charge phenomena were observed and the distortion field increased with the temperature which could be attributed to the larger amount of charge injection in a shorter period. The differences of the amount and injection rate of the space charge were explained and verified considering the typical chain alignment of epitaxial crystallization, which, in our opinions, contributes to the pass over of positive charge in LD-iPP samples.