Effect of molecular weight on high pressure crystallization of linear polyethylene. I. Kinetics and gross morphology

Ten linear polyethylenes ranging from M_w = 4.9 × 10⁴ to 4.6 × 10⁶ were crystallized in a dilatometer at 0.51 GPa and 242°C and then cooled slowly. Volume vs time data were used to follow the kinetics of the crystallization. The dilatometer data for the isothermal part of the crystallization were fitted to the Avrami equation. The time exponent was independent of molecular weight and the average was n = 2.2. Electron microscopy of fracture surfaces showed that all of the polyethylenes crystallized in extended chain morphology. The crystalline order and maximum extended chain crystallite thickness decreased with increasing molecular weight. The dominant morphological feature of the crystallized high molecular weight samples was a strand-like network superstructure. Attempts to stabilize the hexagonal structure formed in the isothermal part of the crystallization failed, and all specimens had only the usual orthorhombic crystal structure.     

Molecular weight dependence of melt crystallization behavior and crystal morphology of low molecular weight linear polyethylene fractions

Melt crystallization behavior and corresponding crystal morphology of five low molecular weight (3,900 ≤ MW ≤ 20,800) linear polyethylene (PE) fractions have been investigated. The overall crystallization data indicate that the lower molecular weight (MW) fraction possesses a higher crystallization rate at the same undercooling (ΔT). On the contrary, at the same crystallization temperature (T_c) the rate increases with MW. The Avrami exponent (n) varies from ca. 3 to 4 with decreasing ΔT for the fractions studied, which implies the nucleation process changes from athermal type to thermal type as T_c increases. For the low MW PE’s, the different crystal growth regimes (regime I and II) have been first time identified via linear crystal growth rate (G) measurements. The regime I/II transition temperatures are close to previously reported data, which were obtained through a different method. As reported for intermediate MW PE’s, the transitions occur at an almost constant ΔT of 17.5±1 °C for each fraction studied. Morphological study shows that single crystals could be formed isothermally at low ΔT’s. Typical banded spherulites and axialites, which are MW and ΔT dependent, are also observed. Orthorhombic structure is ascertained to be the dominant crystal structure that exists irrespective of MW and crystal growth regime.     

Effect of molecular weight on the morphology and drawing behaviour of melt crystallized linear polyethylene

The effect of molecular weight on the cold drawing behaviour of melt crystallized linear polyethylene has been studied. It is shown that the draw ratio achieved under comparable conditions rises with decreasing $\text{M}\text{?}{}_{\mathrm{w}}$, very high draw ratios (～36) being possible for optimum morphology of the undrawn polymer. The yield behaviour was also examined, and it is shown that the yield stress is affected in a complex fashion by both crystallization conditions and molecular weight. These results are discussed in terms of the crystallization and morphology of the melt crystallized polymer.     

Effects of high molecular weight component on crystallization of polyethylene under shear flow

Crystallization of polyethylene (PE) blends of low and high molecular weight components under shear flow was studied using time-resolved depolarized light scattering (DPLS), focusing on effects of the high molecular weight component on the shish-kebab structure formation. Anisotropic two-dimensional scattering pattern due to shish-like structure formation was observed above a certain concentration of the high molecular weight PE. The threshold was about 2.5-3 times larger than the chain overlap concentration, suggesting an important role of entanglements of the high molecular weight component. On the basis of these results a gel-spinning-like mechanism for the shish-like structure formation has been proposed. The DPLS results also implied that the shish-like structure was mainly formed from the high molecular weight PE. This was confirmed by small-angle neutron scattering (SANS) and small-angle X-ray scattering (SAXS) measurements on an elongated PE blend of low molecular weight deuterated PE and high molecular weight hydrogenated PE (3 wt%).     

Effect of small amount of ultra high molecular weight component on the crystallization behaviors of bimodal high density polyethylene

In order to clarify the effect of high molecular weight component on the crystallization of bimodal high density polyethylene (HDPE), a commercial PE-100 pipe resin was blended with small loading of ultra high molecular weight polyethylene (UHMWPE). The isothermal crystallization kinetics and crystal morphology of HDPE/UHMWPE composites were studied by differential scanning calorimetry (DSC) and polarized optical microscopy (POM), respectively. The presence of UHMWPE results in elevated initial crystallization temperature of HDPE and an accelerating effect on isothermal crystallization. Analysis of growth rate using Lauritzen-Hoffman model shows that the fold surface free energy (σ_e) of polymer chains in HDPE/UHMWPE composites was lower than that in neat HDPE. Morphological development during isothermal crystallization shows that UHMWPE can obviously promote the nucleation rate of HDPE. It should be reasonable to conclude that UHMWPE appeared as an effective nucleating agent in HDPE matrix. Rheological measurements were also performed and it is shown that HDPE/UHMWPE composites are easy to process and own higher melt viscosity at low shear rate. Combining with their faster solidification, gravity-induced sag in practical pipe production is expected to be effectively avoided.     

Change of morphology in high molecular weight polyethylene during die drawing

Deformed high molecular weight polyethylene (HMWPE) rod, produced by die drawing at 115°C, was cleaved longitudinally at liquid nitrogen temperature and the cleaved surface was etched using a permanganic etching technique. A series of etched surfaces of HMWPE sections with different draw ratios (from 1 to 13) were analyzed employing scanning electron microscopy (SEM). The change of crystalline structure in HMWPE during die drawing was observed. In undrawn HMWPE. the spherulites were made up of sheaflike lamellae and scattered within the amorphous phase. During die drawing in the first instance, the microscopically inhomogeneous deformation occurred and the spherulites aligned along the drawing direction. At a draw ratio of about 7, local melting occurred, the spherulites disintegrated, and the sheaflike lamellae oriented, followed by straininduced recrystallization and the growth of the lamellae. Finally, at a draw ratio of about 12, the plastic deformation of the lamellae occurred and microfibrils were formed. © 1993 John Wiley & Sons, Inc.     

Uniaxial tensile creep behaviour of ultra high molecular weight linear polyethylene

The present paper examines the room temperature constant-load uniaxial tensile creep response of two ultra high molecular weight linear polyethylene (UHMW LPE) materials and compares it with that of a normal molecular weight linear polyethylene (NMW LPE). It was found that at all stress levels examined, the magnitude of creep deformation is significantly higher in UHMW LPE than in NMW LPE. Possible reasons for this behaviour are explored. Potential techniques for improving the tensile creep behaviour (i.e. decreasing the creep deformation) of LPE are discussed.     

The effects of orientation and crystallinity on the solvent-induced crystallization of poly(ethylene terephthalate). II. Physical structure and morphology

This series of papers summarizes the results of an experimental research program designed to establish the effects of preexisting orientation and crystallinity on the solvent-induced crystallization of poly(ethylene terephthalate) films. Dimethylformamide was used as a model for a strongly interacting solvent. This paper deals with the morphological and structural modifications induced by the solvent crystallization process. The effects on the structure and morphology of the solvent crystallization were dominated by surface cavitation, creation of voids, and by changes in the degree of crystallinity. The surface structure varied from a spherulitic cavitated structure for films of low orientations (low draw ratios) to a smooth surface at the higher draw ratios. In addition to the change in the surface morphology, an internal void structure was formed due to crystallization taking place in a swollen state. The formation of voids is dependent on the treatment temperature, draw ratio, and the method of solvent removal. The voids collapse upon annealing at high temperatures. Increases in the degree of crystallinity and changes in orientation were dependent on the treatment temperature. The crystallite size goes through a maximum at an intermediate degree of orientation. The crystalline orientation decreases with increasing treatment temperature. The structures formed during solvent treatments did not show any characteristic premelting endotherms at the low draw ratios. In the case of higher draw ratios, a small premelting endotherm was noticeable at a temperature 70°C greater than that of the solvent treatment.     

High density polyethylene/ultra high molecular weight polyethylene blend. II. Effect of hydroxyapatite on processing,thermal, and mechanical properties

Hydroxyapatite (HA) is part of bone mineral composition. Several attempts have been made to incorporate HA into high density polyethylene (HDPE) to produce bone replacement biomaterials since neat HDPE is not suitable as bone replacement. The blending of HDPE with ultra high molecular weight polyethylene (UHMWPE) up to 50% by weight was performed with the aim of improving the toughness of composites. Reinforcement of blend with HA of up to 50% by weight was carried out. Methods of characterizing the composites included density, differential scanning calorimetry, thermal gravimetric analysis, ash content, and morphological examination using scanning electron microscope. For the mechanical properties of the composites, tensile, flexural, and impact tests were carried out. Incorporation of HA into HDPE has resulted in the brittleness of the composites. Blending of HDPE with UHMWPE in the presence of HA was found to improve the mechanical properties and promote a ductile failure of the resulting composites. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 3931–3942, 2006     

Studies on nonisothermal crystallization of ultra–high molecular weight polyethylene in liquid paraffin

The nonisothermal crystallization kinetics for ultra–high molecular weight polyethylene (UHMWPE) in liquid paraffin (LP) systems was investigated through differential scanning calorimetry (DSC) measurement. The influence of UHMWPE concentration and cooling rate on crystallization mechanism and spherulitc structure as implied by the modified Avrami equation and Mo's analysis was determined, whereas the Ozawa's approach fails to describe the crystallization behaviors of these UHMWPE‐diluent systems. As a result, in the modified Avrami analysis, it was found that the Avrami exponent is constant around five at various concentrations of UHMWPE and cooling rates. Further, the value of F(T) in the Mo's approach increases with the increasing of relative crystallinity and UHMWPE content in the blends. The nonisothermal crystallization kinetics presented here are the first for UHMWPE‐diluent systems. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci, 2006     

Effect of molecular weight on crystallization isotherms of high molecular weight poly(ethylene oxide) fractions

Dilatometric crystallization isotherms have been determined for a set of poly(ethylene oxide) fractions ranging in molecular weight from 2 × 10⁴ to 1.6 × 10⁶. For a given fraction the isotherms obtained for different crystallization temperatures can be superimposed over most of the crystallization. For a given crystallization temperature the degree of crystallinity obtained in the primary stage of the crystallization varies greatly with molecular weight, and superimposition of the isotherms is not possible. Secondary crystallization processes are pronounced when the molecular weight ($\text{M}\text{?}{}_{\mathrm{v}}$) exceeds 10⁵.     

Ultra-high-modulus linear polyethylene through controlled molecular weight and drawing

Recent work is presented on the plastic deformation of linear polyethylenes. This work demonstrates the importance of molecular weight and initial morphology in determining the drawing behavior, and shows that by appropriate choice of these two factors a substantial increase in draw ratio and hence stiffness can be achieved over conventionally-oriented polyethylenes. Under optimum conditions a modulus of approximately 700 Kbar was obtained at a draw ratio of ～30. These very high modulus materials displayed an extensibility to break of at least 3 percent and a strength of about 4 Kbar. In some cases they also exhibited very high melting points (～139°C) and exceptionally good thermal stability.     

Effects of mixing on morphology,rheology, and mechanical properties of blends of ultra-high molecular weight polyethylene with linear low-density polyethylene

Various blends of ultra-high molecular weight polyethylene (UHMWPE) with linear low-density polyethylene (LLDPE) were prepared in an internal (Banbury type) mixer, a static mixer, and by solvent blending. Two mixing techniques, namely simultaneous and sequential loading methods, were employed with the internal mixer. In the former case, the two polymer components were simultaneously loaded at 180°C and mixed. The latter method allowed the UHMWPE component to diffuse at 250°C and cooled it down to 180°C, then the LLDPE component was added subsequently and mixed. Rheological and mechanical properties of these blends are profoundly affected by the mixing techniques used. Rheological results shows yield characteristics of UHMWPE/LLDPE blends, in particular in blends of high UHMWPE contents. Tensile properties of sequentially loaded blends vary more or less linearly with blend compositions. However, negative or positive deviations are seen in the simultaneously prepared blends. Differential scanning calorimetry (DSC) studies indicate that co-crystallization takes place between UHMWPE and LLDPE components in sequentially mixed blends. DSC and small-angle light scattering (SALS) studies show that separate crystallization takes place in simultaneously blended compounds as a result of poor mixing. It seems that the sequential loading method provides more homogeneous compounds than those of simultaneous blending.     

Effect of molecular weight on the morphology of melt-crystallized PTFE and the occurrence of row-nucleated crystallization

The crystallites obtained from the crystallization of the melt of as-polymerized, dispersion grade polytetrafluoroethylene (PTFE) samples in the form of thin films (～1 μm) differ in size depending on the molecular weight of the polymer and its thermal history. The production of large crystallites is favoured by slow cooling rates, high maximum melt temperatures and low molecular weight polymer. Subjecting thin, crystallized PTFE films on blade edges to cutting leaves a very thin (～10 nm) oriented film of polymer, which can melt to recrystallize in a row-nucleated form. Reduction of the molecular weight of the PTFE lowers the tendency to adopt this form.     

The forming properties of high molecular weight polyethylene

The formabilities of cold-rolled high molecular weight polyethylene (HMWPE) sheets have been studied by measuring their plastic anisotropy ratio (R value), strain-hardening exponent (n value), strain distribution, and the forming limit diagram (FLD). The deep drawability of the polymer is improved by rolling. After 40% or more reduction in thickness by cold rolling, the HMWPE sheet could be deep-drawn into a cylindrical cup. The results of R value measurement indicate that the R value is responsible for improved drawability. Cold rolling also increases the n value but decreases the strain gradient. Stretch forming tests have also been carried out, and the results show that cold working could also improve the stretchability of this polymer. The results of the FLD are in agreement with the other properties studied. The mechanical properties, environmental stress cracking resistance, and shape, size, and property stability of the deep-drawn HMWPE cups have also been investigated.     

超高分子量聚乙烯溶解均匀性研究

超高分子量聚乙烯纤维生产工艺中,超高分子量聚乙烯溶解设备普遍使用双螺杆挤出机。文章研究了如何使用双螺杆挤出机,具体涉及到螺纹元件的组合、双螺杆挤出机的转速以及各区温度控制等关键技术,制备溶解均匀的超高分子量聚乙烯溶液,以达到纺丝要求。     

超高分子量聚乙烯材料的研究进展

超高分子量聚乙烯(UHMWPE)是高性能聚烯烃材料的典型代表,稳定的线性长链结构使其具有高强度、耐冲击、耐磨损、自润滑、耐化学腐蚀、耐低温等诸多优异性能。近年,UHMWPE加工、改性技术日益扩展、优化,形成了多种多样的UHMWPE制品,广泛应用于军民各项领域。本文综述了UHMWPE在催化聚合、纤维、膜、管材、板材及型材等方面的最新进展,重点介绍在各领域应用、加工、改性等方面的研究成果和发展趋势。     

Die drawing technology of high molecular weight polyethylene

A comprehensive investigation of the die drawing technology of a high molecular weight polythylene (HMWPE) rod has been carried out. The effect of draw temperature, draw speed, nominal draw ratio, and exit diameter of the dies has been studied. The oriented HMWPE products were characterized mainly by the determination of the three-point bend modulus and the tensile strength. The tensile strength and the modulus of the drawn HMWPE rod could reach 700 MPa and 18 GPa, respectively. In addition, it was found that forced cooling at the die exit was essential when drawing billets with large section areas. © 1993 John Wiley & Sons, Inc.     

Rheological behavior of ultrahigh molecular weight polyethylene semidilute solutions. II. Effect of aluminium stearate

The effect of aluminium stearate on the rheological behavior of ultra-high molecular weight polyethylene (UHMWPE) semidilute solutions with paraffin oil as the solvent has been investigated. Adding aluminium stearate to paraffin oil can prevent the spinning solution from adhering to the pipe or screw, greatly improving the flow behavior of UHMWPE solutions. The geometric sizes of spinnerette hole, such as length–diameter ratio L/D and entrance angle of a capillary, also affect the flow behavior of the spinning solution. The calculated first normal stress difference σ₁₁ ? σ₂₂ and the Bagley-end correction e from experimental data show that the elastic effect on spinning solutions in flow is quite large, although the shear rate is below 20 s^?1.