The effect of initial morphology on the mechanical properties of ultra-high molecular weight polyethylene

Studies of the physical and mechanical properties of melt crystallized ultra-high molecular weight polyethylene morphologies from melts with different thermal histories indicate that their properties depend on the degree of fusion of the powder particles during their processing and can be enhanced by heating the polymer above 220°C. The degree of cohesion of the powder particles and their initial morphology also have a significant effect on the deformability of the polymer in the solid state forming methods used to prepare high modulus and strength products.     

The effect of mixing shear rate on the properties of liquid crystalline polymer/polyethylene terephthalate blends

The effect of mixing speed of a batch mixer on the properties of liquid crystalline polymer/polyethylene terepthalate (LCP/PET) blends is investigated through two techniques: scanning electron micrographs to examine morphological changes, and tensile testing to determine the mechanical property dependence of the degree of mixing. The results of the two methodologies are well correlated, indicating that the increased degree of mixing of the blend, which is a function of the mixing speed, can be related directly to improved mechanical properties. The results are discussed in the light of existing theories on polymer mixing. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 75: 1783–1787, 2000     

Polymer modulus and morphology: The tensile properties of polyethylene

The general properties of a novel process for producing high modulus polyolefins are discussed. The technique is an extrusion drawing involving a crystal-crystal transformation. The principal tests have been made on polyethylene and the guidelines have been established for extending the technique to other polyolefins. The characterization of such materials is extensively discussed, particularly in the light of the concept of continuous crystals.     

The morphology and mechanical properties of phenoxy/liquid crystalline polymer blends and the effect of transesterification

Morphological, rheological, and mechanical properties of poly(hydroxy ether of bisphenol A) (phenoxy) and a Vectra liquid crystalline copolyester (LCP) blends were investigated. Scanning electron micrographs of fracture surfaces of injection-molded samples show that the LCP forms an elongated fibrous dispersion in the phenoxy matrix. As the mixing time increases, the tensile strength and modulus increase while the elongation at break remains almost constant. These improvements are attributed to the formation of LCP-grafted phenoxy by the interchange reaction between phenoxy and LCP. The interchange reaction was identified by DSC, a rheometer, and a FTIR spectrometer. The graft copolymer gives better adhesion between the two phases and thus improves mechanical properties of the blends. © 1995 John Wiley & Sons, Inc.     

The effect of mixing history on the morphology of immiscible polymer blends

A laboratory prototype tester was used to systematically study the effects of geometrical and operational variables on the mixing of two immiscible polymer melts. Results verify that the number of passages is a dominant variable in dispersive mixing. The development of micromorphology in a controlled fashion was studied extensively and backed up with a finite element simulation of the flow in the tester geometry. Complex deformational fields in the laboratory mixer are evident from the highly deformed dispersed phase morphologies.     

Mechanical properties and morphology of high-density polyethylene/linear low-density polyethylene blend

The binary blend of high-density polyethylene (HDPE) and linear low-density polyethylene (LLDPE) in the range of composition from 100% HDPE to 100% LLDPE has been investigated for tensile and flexural properties and the morphology in the deformed state on tensile fracture. Tensile properties (initial modulus, yield stress, and elongation-at-yield, ultimate tensile strength and elongation-at-break, and work of yield and work of rupture) and flexural properties (flexural modulus and flexural yield stress) are studied as a function of blend composition. Behavior, in terms of these properties, is distinguishable in three zones of blend composition, viz. (i) HDPE-rich blend, (ii) LLDPE-rich blend, and (iii) the middle zone. In zones (i) and (ii), the variations of these properties are more or less linear, whereas in the middle region [i.e., zone (iii)], there is a reversal of trends in variation or sometimes a behavior opposite to the expected one. The results are explained on the basis of the effects of cocrystallization and the presence of octene-containing segments in the amorphous phase. Scanning electron micrographs of the tensile fracture surfaces are presented to illustrate the occurrence of transverse bands interconnecting the fibrils.     

The effect of deformation history on the morphology and properties of blends of polycarbonate and a thermotropic liquid crystalline polymer

The effect of deformation history on the morphology and properties of liquid crystalline polymers (LCP) blended with polycarbonate resin was assessed. The addition of an immiscible LCP phase was found to improve the melt processability of the host thermoplastic polymer. In addition, by employing a suitable deformation history, the LCP phase may be elongated and oriented such that a microfibrillar morphology can be retained in the solid state.     

The effect of mixing time on the morphology of immiscible polymer blends

The effect of mixing time on the morphology, with the viscosity ratio and composition as parameters in the mixing process, was studied for two immiscible binary polyblend systems, polyamide/polyethersulfone (PA/PES) and poly(butylene terephthalate)/polystyrene (PBT/PS), by selective dissolution followed by macroscopic and microscopic observations. At a short mixing time, the morphology of each phase depends not only on the composition, but also on the viscosity difference of two phases, shown by the results of PA/PES blends with a viscosity ratio of 0.03. The lower viscous phase (PA) forms particles, fibrils, and layers successively with its increasing content and becomes a continuous one at low concentrations as the minor phase, while the high viscous phase (PES) appears mainly in the form of particles and directly becomes a continuous one at high concentrations. With increasing mixing time, the effect of the viscosity ratio becomes less and the morphology is determined mainly by the volume fraction of each phase. Particles are the final morphology of the minor phase. Only at a viscosity ratio of unity is the morphological development of two phases (PBT and PS) with mixing time the same, and any one of these two components is in the form of particles when it is the minor phase. At the composition near 50/50, fibrillar or continuous structure may coexist for both phases. The composition range of co-phase continuity is decided not only by the viscosity ratio but also by the mixing time. With increasing mixing time, this range becomes narrower and finally occurs at volume fraction of 50/50, no longer affected by the viscosity ratio. © 1996 John Wiley & Sons, Inc.     

The effect of matrix morphology on nanocomposite properties

In order to understand the effect of bulk matrix morphology on polymer nanocomposite properties, nanocomposites containing chemically similar but morphologically different polyamide matrices and carbon nanofibers were processed and characterized. Two polyamide matrices were used, one amorphous and one semi-crystalline. Experimental results indicated that the reinforcing efficacy of the amorphous matrix was higher than the semi-crystalline matrix at temperatures below the glass transition. At a carbon nanofiber loading of 0.5 wt.%, the experimentally measured modulus with the amorphous matrix exceeded the Halpin-Tsai prediction for an isotropic material. Overall, these results provided distinct evidence that the underlying bulk matrix morphology plays an important role in polymer nanocomposite mechanical design.     

The effect of colorants on the properties of rotomolded polyethylene parts

An experimental study is reported on the effect of colorants on the warpage, shrinkage, and mechanical properties of rotomolded polyethylene (PE) parts. Five pigments were investigated (titanium dioxide white, cadmium oxide yellow, iron oxide red, carbon black, and phthalocyanine blue). Their concentration was in the range of 0.11 to 0.36 wt%. The pigments were added to the linear low-density polyethylene (LLDPE) by various techniques, dry blended in low intensity or high intensity mixer or compounded with a single-screw extruder. Some blends also contained 0.077 wt% zinc stearate as surfactant. It was found that zinc stearate surfactant in the dry blend increases warpage by about a factor of 2 and shrinkage by ˜ 30%, but color quality and powder handling properties are improved. The dry-blended pigments concentrate along the fusion lines of the PE particles in rotomolded parts, which results in >50% decrease of impact strength with some pigment-surfactant combinations. Mold pressurization minimizes warpage and shrinkage, but causes no significant improvement in impact resistance. Impact resistance can be improved by using compounded pigments or making two-layer parts with a pigmented outer and unpigmented inner layer.     

The influence of the volatility of reaction products on thermogravimetric results of polymer degradation

Summary It is evidenced that the volatility of reaction products may alter the results of thermogravimetric measurements of polymers. Not only kinetic computation is influenced, but also the evaluation of thermal stability if larger fragments are formed during degradation.To Prof. C.I. Simionescu's 60th birthday     

The effect of confined spherulite morphology of high-density polyethylene and polypropylene on their gas barrier properties in multilayered film systems

Confined crystallization of high-density polyethylene (HDPE) in multilayer films is studied in this paper. A new cyclic olefin copolymer (COC), HP030, is co-extruded with HDPE by a layer multiplying technique. The number of layers and layer compositions are changed to study the effect of layer thickness on the crystalline morphology of the HDPE layers under confinement. Atomic force microscopy (AFM) is used to investigate the crystalline morphology of the HDPE layers. MOCON (Minneapolis, MN, commercial instrument) units are employed to measure both oxygen permeability and water vapor transport rate (WVTR) of these co-extruded HDPE/HP030 multilayer films. We report that when the HDPE layer nominal thickness is about 290 nm in the HDPE/HP030 multilayer films, the HDPE layer effective gas barrier property is improved approximately 2 times for oxygen and 5 times for water vapor. This is the result of confined spherulite morphology of HDPE, which increases the tortuosity for gas to diffuse through the films. Similar phenomenon is found for polypropylene (PP), when PP is co-extruded against polycarbonate (PC). The same experiments as for HDPE are conducted to confirm that PP spherulites have been confined by PC in PP/PC multilayer films. We discover that the confined spherulites of PP improve its gas barrier properties as well.     

Influence of polymer matrix crystallinity on nanocomposite morphology and properties

In order to understand more fully how polymer matrix attributes influence polymer nanocomposite properties, nanocomposites containing hydroxyapatite nanoparticles and a poly(3-hydroxybutyrate) matrix were prepared and compared to results for a chemically-similar nanocomposite system with a lesser degree of matrix crystallinity. Experimental results indicated that the higher degree of matrix crystallinity hinders nanoparticle dispersion at loadings above 0.5 wt.% and together these structural factors, high matrix crystallinity and nanoparticle aggregation, produced different mechanical reinforcement behavior below and above the glass transition temperature than has been seen previously in amorphous matrices or matrices with moderate crystallinity levels. Overall, these results suggested that the amorphous character of the polymer does not govern the properties at all crystallinity levels in polymer nanocomposite matrices.     

Tensile properties and morphology of blends of polyethylene and polypropylene

The tensile behavior of blends of linear polyethylene (PE) and isotactic polypropylene (PP) was examined in relation to their morphology. Yield stress increases monotonically with increasing PP content, while true ultimate strength is much lower in all blends than in the pure polymers as a result of early fracture. The blends fail at low elongation because of their two-phase structure, consisting of interpenetrating networks or of islands of PE in a PP matrix, as shown by scanning electron microscopy of fracture surfaces and transmission electron microscopy of thin films. While spherulites in PP are very large (～100 μm in diameter), addition of 10% or more of PE drastically reduces their average size. This, together with the profusion of intercrystalline links introduced by PE, may be associated with maximization of tensile modulus in blends containing ～80% PP. Introduction of special nucleating agents to PP reduces average spherulite size and is accompanied by slight improvements in modulus. Thin films of blends strained in the electron microscope neck and fibrillate in their PE regions, but fracture cleanly with little fibrillation in areas of PP.     

The effect of molecular weight distribution on polyethylene film properties

Polymer molecular weight heterogeneity affects the rheological properties of polymer melts such as melt viscosity, fracture and die swell. These rheological properties affect the conversion of the polymer from the bulk resin state to its final usable form. In this particular study, the effect of molecular weight distribution on polyethylene blown film characteristics was studied. The effect of the molecular weight heterogeneity on the rheological characteristics of the polymer in the molten state and its effect on the film properties is presented. The properties studied included film gloss, haze, tear resistance and film impact strength. This study shows that broadening the molecular weight distribution increases haze and reduces film gloss. Further, it was shown that a linear relationship exists between film gloss and external haze. Both values are measures of surface irregularities in the film which are affected by the drawing characteristics of the polymer. A broader molecular weight distribution results in increased impact strength as measured by the Dart Drop Impact Test. This is, it is believed, a result of the increase in long chain branching of the higher molecular weight fractions of the polymer which cause a higher degree of molecular weight entanglement at the branch sites. In contrast the tear strength is reduced as the molecular weight distribution broadens because of the low molecular weight fraction in the broad spectrum material which tend to decrease resistance to tear.     

The effect of short-chain branch structure on the properties of low-density polyethylene

In order to determine the effect of the molecular structure of short side branches in lowdensity polyethylene upon the physical properties of the resin, a study was carried out in which small amounts of various comonomers were added to an otherwise relatively unbranched polymer. It was found that linear short side branches have about the same effect in decreasing stiffness and increasing toughness as the natural short-chain branches of polyethylene have. However, branches containing a tert-butyl group increased resin toughness more than linear branches while decreasing stiffness by about the same amount. Thus, by adding a small amount of branched, short side branches, it is possible to obtain an optimum balance of physical properties not obtainable from low-density ethylene homopolymer.     

The effect of temperature on the elastic properties of polymer melts

A correlation is developed between the melt elasticity and temperature, the melt elasticity being defined by the normal stress difference. The correlation follows the form of the Arrhenius equation, for the temperature range tested, for high density polyethylene and polypropylene melts. Measurements were taken using the capillary rheometer which is described in detail in the author's earlier publications. The author further presents a correlation between the pressure drop at the entrance of a capillary and the residual pressure at the exit of the capillary (exit pressure). The correlations presented in the paper support the author's earlier contention that the exit pressure is indeed a manifestation of elastic behavior.     

Effect of nano inclusions on the structural and physical properties of polyethylene polymer matrix

The reinforcement mechanism of nano inclusions on the mechanical properties of nanocomposites is still a controversial issue. In this work, the polyethylene (PE) polymer nanocomposites were simulated by infusing different kinds of nano inclusions (bucky-ball, graphene, single-walled-carbon-nanotube (SWNT), X-shaped SWNT junction and Y-shaped SWNT junction) into PE matrices. The effects of these nano inclusions on structural and physical properties of PE matrices were considered through molecular dynamics (MD) simulations. The packaging densities of PE matrices were highly different from each other. The graphene inclusion had larger interaction with the PE matrix than the others. However, Y-shaped SWNT junction applied the strongest restriction on diffusion behaviors of PE matrices. Therefore, mean squared displacement of the PE matrix infused with Y-shaped SWNT junction is much smaller than that of pure PE. The effect of nano inclusions on primitive chain network of PE matrices was also considered. However, these nano inclusions did not significantly alter the underlying mesh of primitive chain network of PE matrices. The change of mechanical properties associated with these nano inclusions should be directly attributed to the interaction between polymer matrices and nano inclusions.     

Heterogeneous polymer–polymer composites. II. Preparation and properties of model systems

A two-stage emulsion polymerization procedure has been developed and used to prepare relatively uniform populations of heterogeneous acrylic latex particles (HLP). One class of particles (HLP1) can be described as composite materials comprising a glassy continuous phase and a rubbery discrete phase. Another class (HLP2) can be described (at high rubber content) as composite materials comprising a rubbery continuous phase and a glassy discrete phase. The phase structure of the HLP1 is sufficiently stable to allow fabrication of composites having a uniform spatial distribution of inclusions by direct compression molding. Although the observed particle structure of the HLP2 does not depend markedly on crosslinking, the phase structure and mechanical properties of compression moldings do. Crosslinking of the glassy stage appears to stabilize HLP2 phase structure during molding, while crosslinking of the rubbery stage favors phase inversion. The observed HLP2 particle structures and the morphology of molded HLP1 specimens are consistent with a shell-core model. It is found that the modulus and thermal expansion coefficient of many of these materials can be adequately described in terms of a simple theoretical model for the elastic and thermoelastic properties of particulate composites, provided that an interaction parameter interpreted as a maximum packing fraction is introduced.     

The effect of ultraviolet light on the mechanical properties of polyethylene and polypropylene films

Measurements were made of dynamic mechanical response spectra and stress–strain properties at room temperature on films of isotactic polypropylene and low-density polyethylene prior and after ultraviolet irradiation in a Xenotest 450 apparatus. The period of irradiation that caused a deep deterioration of ultimate mechanical properties influenced the dynamic mechanical properties only insignificantly. This is attributed to the heterogeneous nature of the photo-oxidative degradation process which is concentrated in a finite number of sites, thus forming crack precursors rather than changing the material properties in bulk. For a biaxially oriented tubular film of low-density polyethylene, anisotropic embrittlement after exposure in Xenotest 450 was observed. This even reversed the order of strain-at-break values in the two main directions of the film. This is remarkably similar to the effect of artificial incisions introduced into the specimens.