Shear and tensile relaxation behaviour in oriented linear polyethylene

The dynamic shear behaviour of oriented linear polyethylene has been studied with particular reference to previous studies of the dynamic tensile modulus. First, it has been shown that the increase in the ?50°C plateau shear modulus with draw ratio can be understood on a Takayanagi-type model in terms of an increase in crystal continuity. The crystal continuity is estimated from the longitudinal crystal thickness and the long period on the basis of the random crystalline bridge model. At a similar level of sophistication it is also possible to explain the cross-over in the ranking of samples of increasing draw ratio with change of temperature. The dynamic mechanical behaviour is then considered in terms of a simple extension of this Takayanagi model in which crystalline sequences which span two or more adjacent lamellae are regarded as the fibre phase in a short fibre composite. It can be shown that this model gives a satisfactory prediction of the changes in dynamic tensile modulus and loss with temperature, for a range of samples with different degrees of crystal continuity.     

Sorption and diffusion of solvents in highly oriented polyethylene

Measurements have been made of the sorption and diffusion coefficients of various solvents in oriented linear polyethylene pipe produced by hydrostatic extrusion. In all cases the equilibrium sorption is a factor of about ten less than that for isotropic linear polyethylene, and the diffusion coefficients are a factor of about a hundred less.     

Ultrasonic measurements of the elastic stiffness constants of oriented polyethylene

High density polyethylene (Rigidex 9), a high density copolymer of polyethylene and poly(butene-1) (Rigidex 2000), and low density polyethylene (Alkathene WJG 11) were oriented by hot drawing. The crystalline texture, as determined by wide- and low-angle X-ray diffraction, was a highly oriented chain axis in the draw direction with random orientation transversely and with lamellae surfaces perpendicular to the draw direction. Elastic stiffness constants were measured by a contact-probe ultrasonic pulse technique at 2.5 MHz both before and after annealing in a temperature range which did not significantly alter the crystalline texture. Assuming orthorhombic symmetry the nine stiffness constants of Rigidex 2000 and the three longitudinal and three shear stiffness constants of Rigidex 9 were measured after drawing and after subsequent annealing. Only the longitudinal constants of Alkathene were measured, as shear waves could not be transmitted. The longitudinal stiffness in the draw direction was markedly affected by drawing and by annealing, while the crystalline texture remained substantially unchanged; by comparison the other stiffness constants showed little change. Drawn Rigidex 9 reached a tensile modulus in the draw direction of 69 GPa. The results are compared with ‘static’ and low frequency measurements reported in the literature.     

Eutectic solidification and oriented growth in mixtures of polyethylene and 1,3,5-tribromobenzene

The melting and crystallization behaviour of mixtures of linear polyethylene and 1,3,5-tribromobenzene has been investigated by differential scanning calorimetry, optical and electron microscopy. The phase diagram reveals the existence of a pseudo-binary eutectic (polymer volume fraction of about 58%; T_m=116°C). The experimental observations are compared with the predictions of the Flory-Huggins theory of melting point depression. The polyethylene component of the eutectic assumes a habit in which lamellae are highly oriented with the chains in the [110] contact plane parallel to the tribromobenzene needle axes. This well defined orientation is most probably based on an epitaxial relation between polymer and substrate.     

Ultrasonic absorption and relaxations in ABS composite polymers

We investigated ultrasonic attenuation, dynamic Young's modulus, Izod impact strength, and dielectric relaxation of acrylonitrile-butadiene-styrene composite polymers (ABS) in which styrene-co-butadiene rubber particles are dispersed in styrene-co-acrylonitrile random copolymer. High ultrasonic attenuation was observed around 240 K and the intensity of attenuation increased with increasing rubber content. Temperature dependence curves of dielectric loss in the frequency range from 12 Hz to 200 kHz and temperature range from 80 to 420 K exhibited four relaxation processes designated as the α, β′, β, and γ. From the relaxation map produced for the mechanical and dielectric relaxations, the ultrasonic absorption was attributed to the β and β′ processes. It was deduced that inter-grain thermoelastic process, also contribute to the ultrasonic attenuation. Correlation was found between the ultrasonic absorption and the Izod impact strength indicating that the high impact strength of ABS is partly due to the effective absorption of impact energy through those relaxation processes.     

Blend of high‐density polyethylene and a linear low‐density polyethylene with compositional‐invariant mechanical properties

This article presents the tensile properties and morphological characteristics of binary blends of the high‐density polyethylene (HDPE) and a linear low‐density polyethylene (LLDPE). Two constituents were melt blended in a single‐screw extruder. Injection‐molded specimens were evaluated for their mechanical properties by employing a Universal tensile tester and the morphological characteristics evaluated by using a differential scanning calorimeter and X‐ray diffractometer. It is interesting to observe that the mechanical properties remained invariant in the 10–90% LLDPE content. More specifically, the yield and breaking stresses of these blends are around 80% of the corresponding values of HDPE. The yield elongation and elongation‐at‐break are around 65% to corresponding values of HDPE and the modulus is 50% away. Furthermore, the melting endotherms and the crystallization exotherms of these blends are singlet in nature. They cluster around the corresponding thermal traces of HDPE. This singlet characteristic in thermal traces entails cocrystallization between these two constituting components. The clustering of thermal traces of blends near HDPE meant HDPE‐type of crystallites were formed. Being nearly similar crystallites of blends to that of HDPE indicates nearness in mechanical properties are observed. The X‐ray diffraction data also corroborate these observations. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 2604–2608, 2002     

Dielectric relaxation of polar aromatics in unoriented and oriented linear low‐density polyethylene

The dielectric behavior of some polar aromatics dissolved in nonpolar unoriented and stretched linear low‐density polyethylene was investigated within the temperature region between 150 and 350 K. The measurements were carried out in the frequency range 1 kHz to 10 MHz. The maximum temperatures and the half widths of the loss tangent peaks depend upon the shape and the polar structure of guest molecules. Stretching the samples induced a shift of the loss tangent to higher temperatures, decreased the height, and increased the width of tan δ peak. The activation energy is also influenced by the type of guest molecules and orientation of polymer matrix. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 1278–1282, 2001     

Crystallite thickness measurements on ultrahigh modulus linear polyethylene: effect of draw temperature

The effect of draw temperature on the longitudinal crystal thickness $\text{L}\text{?}{}_{002}$ and long spacing L has been determined for a series of drawn linear polyethylene samples. It was found that both $\text{L}\text{?}{}_{002}$ and L increase almost exactly in proportion with increasing draw temperature. The structural significance of the results is discussed, especially with regard to the interpretation of mechanical stiffness.     

Comparison of High-Pressure Behavior of Physicochemical Properties of the Di- and Triacylglycerols Established by Ultrasonic Methods

Two samples of triacylglycerols i.e., olive oil and triolein, and one sample of diacylglycerol were investigated. In the course of compression, the density of the samples was determined by measurements of the change of piston position in a pressure chamber and volume correction due to chamber expansion under pressure. The speed of sound was evaluated from the time of flight of an ultrasonic impulse between emitting and receiving transducers placed in the high pressure chamber. The adiabatic compressibility, the intermolecular free length, the molar volume, the van der Waals’ constant b and the surface tension were evaluated from the density, the speed of sound and the average molecular mass. All tested liquids undergo a high-pressure phase transition. Discontinuities in the measured isotherms of the physicochemical parameters of the investigated oils indicate the presence of high-pressure phase transitions. Moreover the time dependent change of pressure at constant volume during the phase transition was measured. The fundamental difference in the molecular structure of these acylglycerols influences their behavior significantly under high pressure.     

双向拉伸聚乙烯的微观结构及热学表征

采用红外光谱、核磁共振波谱和高温凝胶色谱对2个双向拉伸聚乙烯试样(分别记作M1,M2)的链结构进行了表征,确定了试样的共聚单体类型、支化度、相对分子质量及其分布。利用差示扫描量热法(DSC)研究了试样的熔融和结晶行为。通过连续自成核退火热分级法测定了不同共聚物中亚甲基序列长度(MSL)的分布。结果表明:M1以短链支化为主,M2则以长链支化为主;M1的总支化度略高于M2,与红外光谱测试结果一致;MSL分布随MSL的增加呈单调递增趋势,M2的MSL集中在198,其峰面积占总峰面积的52.4%;M1的MSL分布呈现2个峰值,在MSL为61处有小峰,峰面积占总峰面积的8.3%,MSL为273处的峰面积占总峰面积的35.1%,与DSC测试的双熔点相对应。     

Molecular association complex of urea with polyethylene: 3. Structural studies of the complex

The crystal structure of urea-polyethylene complex in the hexagonal form, obtained by one of our preparation methods, is analysed by X-ray power diffractometry. In this analysis, the fixed molecular parameters of urea are used, and three models with respect to rotational disorder of the guest polyethylene molecule are assumed. It is shown that the host lattice structure constructed by urea molecules is essentially the same as that of urea-n-paraffin complex. Effects of the disorder structure of the guest on X-ray scattering of the complex are presented. However, there is a difficulty in identifying the disorder structure by X-ray analysis. With the aid of a potential energy calculation between the hexagonal urea tunnel and the guest molecule, a rotational disorder model is preferred.     

Wetting of oriented and etched ultrahigh molecular weight polyethylene

Highly oriented gel‐spun ultrahigh molecular weight polyethylene (UHMWPE) fibers possess many outstanding properties desirable for composite materials but their adhesion to such matrices as epoxy is poor. This article describes the combined effects of drawing and surface modification on the bulk and surface properties of gel‐cast UHMWPE films emphasizing the effects of etching on both undrawn and drawn films. Drawing the films yields a fibrillar structural hierarchy similar to UHMWPE fibers and a significant increase in orientation, melting point, modulus, and strength. The effects of drawing on bulk properties were more significant than those of etching. The poor adhesion of epoxy to the smooth, fibrillar, and relatively nonpolar drawn film surface improves significantly with oxidization and roughening on etching. The interlaminar shear failure occurred cohesively in the UHMWPE, and thus the interlaminar shear failure strength was greater for the drawn UHMWPE with its greater tensile strength. Nitrogen plasma etching yielded the best results, both removing any low molecular weight surface layer and etching the UHMWPE beneath. Oxygen plasma etching enhanced wetting but was too harsh, causing extensive surface degradation and a significant reduction in mechanical properties. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 72: 405–418, 1999     

Heat capacities of polyethylene and linear aliphatic polyoxides

Heat capacities at constant pressure and volume are calculated from full and partially approximated normal mode frequency spectra for crystalline polyethylene, poly(oxymethylene), poly(oxyethylene), poly(oxytrimethylene), poly(oxytetramethylene), poly(oxyoctamethylene), poly(oxymethyleneoxyethylene), and poly(oxymethyleneoxytetramethylene). A calculation scheme using a Tarasov-function for 2N skeletal modes and approximation of the residual normal modes from known data on polyethylene and poly(oxymethylene) is developed for all homologous, linear, aliphatic polyoxides. N is the number of CH₂-groups in the repeating unit. Calculations can be carried out over the whole temperature range OK to melting. For θ-temperatures and constant A_o for the C_v? to ?C_p conversion, oxygenconcentration dependent curves are given. Recommended experimental data bank heat capacities agree to ±5% or better.     

Liquid-liquid phase separation in blends of a linear low-density polyethylene with a low-density polyethylene

A linear low-density butene copolymer, of overall branch content 3 mol %, has been blended with a low-density polyethylene. The low-density polyethylene has an overall branch content of 5 mol %, including both long and short branches. The two materials were blended in a wide range of compositions and the phase behavior investigated using indirect experimental methods, the examination of quenched blends by differential scanning calorimetry, and transmission electron microscopy. After quenching from temperatures up to 170°C, blends, of almost all compositions, show two crystal populations, separated on a micron scale. It is argued that this implies that the blends were phase separated in the melt before quenching. This behavior shows good agreement with predictions based on previous extensive studies of binary and ternary blends of linear with lightly branched polyethylenes. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 65: 1921–1931, 1997     

N.m.r. study of molecular dynamics in chemically crosslinked polyethylene

Results of measurements of the proton spin-lattice relaxation time T₁ in chemically crosslinked low-pressure polyethylene are presented. The dicumyl peroxide used for crosslinking changed the α-process dynamics without disturbing the β -process or the motion of the methyl groups. Classical relaxation processes (which are analogous to the case of pure polyethylene) have been observed for samples with 0.5–2.5 wt% crosslinking.     

Blends of nylon with polyethylene: Effect of compatibilization on mechanical and dynamic mechanical properties

Blends of Nylon 6 with very low density polyethylene (VLDPE) have been studied. The blends exhibit two phase morphology wherein VLDPE is dispersed in the form of spherical domains in Nylon 6 matrix. The water absorption of the blends decreased with increasing VLDPE content. The mechanical properties of these blends show loss in tensile and impact strength due to poor adhesion at the interface. Addition of a compatibilizer containing MAH groups was found to result in improved properties. The tensile strength increased significantly whereas impact testing showed no break confirming better stress transfer across the interface. The dynamic mechanical analysis showed presence of microheterogeneity resulting into merging of tan delta peaks as a result of compatibilization. The observed results are ascribed to the possible reaction between reactive groups in Nylon 6 and the compatibilizer leading to compatibilization through copolymer formation. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 69: 161–168, 1998     

Sorption and transport of linear and branched ketones in biaxially oriented polyethylene terephthalate

Equilibrium sorption and kinetics of acetone, methyl ethyl ketone (MEK), methyl n-propyl ketone (MnPK), and methyl i-propyl ketone (MiPK) uptake in uniform, biaxially oriented, semicrystalline polyethylene terephthalate films were determined at 35 °C and low penetrant activity. Sorption isotherms for all penetrants were well described by the dual-mode sorption model. Sorption and desorption kinetics were described either by Fickian diffusion or a two-stage model incorporating Fickian diffusion at short times and protracted polymer structural relaxation at long times. Diffusion coefficients and equilibrium solubility at fixed relative pressure decreased in the following order: acetone>MEK>MnPK>MiPK. Diffusion coefficients for each penetrant increased with increasing penetrant concentration.     

LLDPE与甲基丙烯酸接枝共聚

采用溶液聚合和悬浮聚合法研究了线型低密度聚乙烯对甲基丙烯酸的接枝行为,发现溶液接枝法的产物有较高的接枝率,对铝箔的粘合力很强。     

Melt strength of blends of linear low density polyethylene and comb polymers

The melt strength of a metallocene linear low-density polyethylene (m-LLDPE) can be enhanced significantly by blending in less than 10 wt% of long chain branched comb polymer. The extent of the enhancement could be ten-fold and depends on the architectural details of the comb polymer. Comb polymers primarily affect melt strength and have little effect on other properties such as shear thinning, melt index, melt index ratio, and intrinsic tear.Balancing melt strength properties against shear-thinning properties is important in LLDPE fabrication processes. One approach would be to augment the effect of comb polymer by blending in another component, namely an easy processing (also known as sparsely long chain branched) LLDPE. In the examples given here, the enhancements in melt strength and shear thinning properties of the base polymer were found to be additive, i.e. a simple weighted sum of component properties matched the blend properties within 10%.