Mechanism of the short chain branching in low density polyethylene

In this paper the validity of the well-known Roedel's “back-biting” mechanism is questioned. It is shown that number and type of branches depend on the degree of order (entropy) of compressed ethylene molecules. A new mechanism for the short chain branching in low density polyethylene is proposed, based on the effect of the supermolecular species in compressed ethylene in this reaction. An equation for calculation of the number of branches for ethylene polymerization in a wide range of temperature (?5 to + 200°C) and pressure (40 to 7000 atm) is given.     

Infrared spectroscopic studies on polyethylene, 1. The measurement of low levels of chain branching

The problems encountered in the infrared spectroscopic estimation of the low levels of chain branching present in high density polyethylene, using the compensation method, are discussed systematically. The precision to be expected has been determined and it is shown that, if possible, a correction should be applied for methyl groups present at the chain ends. This is feasible with some types of polymer but more difficult with others. The compensating wedge in the reference beam of the spectrometer should be made of polymethylene rather than a high molecular weight polyethylene. The ultimate sensitivity of the manual compensation method is about one methyl branch per two thousand carbon atoms. However, when this is supplemented by computer smoothing and off-line computation of average transients (C.A.T.), a significant improvement is obtained in the signal to noise ratio, and branching levels down to 0,1 methyl groups per thousand carbon atoms are readily measured.     

Modified affine law for the elastic and optical properties of low density polyethylene

A unified study of the anomalous mechanical properties and birefringence of low density polyethylene (LDPE) is attempted within the frame work of the rotating element model. The present paper investigates the validity of the affine transformation rule used in the Ward model and suggests a modified affine law which is based on the consideration that the units are only reoriented and, unlike the affine law, are not deformed due to the uniaxial stretching. The relationship between the angles made by the C-axis of the unit with the drawing direction in the undrawn state (θ) and the drawn state (θ), is deduced as sin θ = f(n) sin θ′. From the birefringence data of different polymers a two parameter relation of the form f (n) = exp [–(n – 1)^α/β] is suggested. Using this new orientation function, the agreement of the predicted values of E₀ and E₉₀ for LDPE for the entire draw ratio range between temperatures of 60°C to ?125°C, is quite satisfactory. An interesting feature is that the abrupt change in the values of the parameters α and β at two different temperatures may be associated with a phase transition of LDPE at an intermediate temperature.     

Solubility parameter and hydrocarbon sorption of low density polyethylene

A new and quick method, namely, the gaschromatographic technique, has been used to determine the solubility parameter of polyethylene. The calculated quantities of sorption of various pure components in polymer bears a logarithmic relationship to the experimental values.     

Long chain branching in ethylene polymerization using constrained geometry metallocene catalyst

We report an experimental investigation on long chain branching (LCB) in ethylene polymerization with the Dow Chemical's constrained geometry catalyst system, CGC-Ti/TPFPB/MMAO, using a continuous stirred-tank reactor (CSTR) at 140°C, 3.45 × 10³ kPa, and a mean residence time (τ) of 4 min. The effects of the catalyst (CGC-Ti) and co-catalyst (TPFPB and MMAO) concentrations on the catalyst activity, polymer molecular weight, and shear thinning were systematically examined. The boron cocatalyst had a great influence on the CGC activity. Increasing the ratio TPFPB/CGC-Ti from 0.66 to 5 gave ethylene propagation rates from 1.65 × 10³ to 1.36 × 10⁴ L · mol⁻¹ · s⁻¹. The addition of MMAO appeared to be essential, most likely acting as an impurity scavenger. The LCB polyethylenes showed enhanced shear thinning properties. The melt flow index ratios I₁₀/I₂ were in the range of 6.96 to 23.4, with the I₂ of 0.172 to 0.681 g/10 min. The weight-average molecular weight M _w was correlated to I₂ using a power equation within narrow I₁₀/I₂ ranges. The exponential factors were in the range of 4.24 to 6.31. The experimental and calculated M _w's were in a good agreement.     

Infrared spectroscopic studies on polyethylene, 5. The measurement of chain branching on powder polyethylene specimens

The scope and limitations of the potassium bromide disc technique for the infra-red determination of chain branching in small samples of polyethylene in powder form have been assessed. Many of the samples encountered have an average particle size, prior to grinding with potassium bromide, of about 100 μm and very satisfactory results have been obtained, but the presence of larger particles leads to poor quality discs and inferior results. The well established double beam technique proves suitable for removing the interfering methylene absorption and compensation with polymethylene in the form of a film rather than as a pressed disc is preferable. Difficulties were encountered in preparing branching standards in a uniform and fine powder form, to avoid variation of the absorption coefficient of the 1 378 cm^?1 methyl band with particle size. Such powders are best obtained by precipitation from solution rather than by low temperature milling. Despite these problems chain branching measurements may be made with a precision of between 5 and 10% and an accuracy of about 15%.     

Thermostimulated creep and depolarization thermocurrent in low density polyethylene

A new method for the study of orientational movements in polymeric solids is proposed: in a mechanical step-function experiment, we thermally stimulate the response at a constant stress. The great advantage of the thermostimulated creep (TSC) technique is to allow the experimental resolving of the mechanical retardation time spectrum. In low density polyethylene, the existence of a discrete spectrum of mechanical retardation times is demonstrated; it has the same fine structure than the spectrum of dielectric relaxation times deduced from the study of depolarization thermocurrent (DTC) on the same sample.     

DSC studies of linear low density polyethylene. Insights into the disrupting effect of different comonomers and the minimum fold chain length of the polyethylene lamallae

This paper reports a DSC examination of a series of linear low density polyethylenes (LLDPE) prepared variously with propylene, 1-butene or 4-methyl-1-pentene comonomers. The enthalpy of fusion of the crystalline regions of the LLDPE decreases progressively with increasing comonomer content and is effectively independent of the comonomer type. Disruption of polymer crystallinity is thus determined principally by the number and distribution of the chain irregularities. A sequence length of n = 14 was calculated as the critical chain dimension below which polyethylene segments cannot be readily packed into a stable crystal lattice.     

Calorimetric study of blends of low density polyethylene (LDPE) and linear low density polyethylene (LLDPE) temperature rising elution fractionation (TREF) fractions

Preparative temperature rising elution fractionation (TREF) on commercial linear low density polyethylene (LLDPE) and low density polyethylene (LDPE) samples has been performed. The resulting fractions exhibited a bimodal and a unimodal distribution, respectively. Two LLDPE fractions of low (5 CH₃/1000 C) and high (21 CH₃/1000 C) short-chain branching content were solution-mixed with the LDPE central fraction (16 CH₃/1000 C). Indirect evidence based on differential scanning calorimetry results suggest that the fractions with similar branch contents are more miscible than those with dissimilar ones.     

Infrared spectroscopic studies on polyethylene, 6. Linear low density polymers

The infrared spectra of five linear low density polyethylenes, containing methyl, ethyl, butyl, isobutyl and hexyl branches, respectively, were measured. Particular interest attaches to the region 800–1200 cm^?1 where the spectra, although weak, show significant differences. The findings of an earlier study on the methyl rocking mode, in the vicinity of 900 cm^?1, are confirmed and extended. In particular, no band was detected for the ethyl branched polymer and it must be an order of magnitude weaker than for methyl, butyl and hexyl branches. Conversely, it is displaced in frequency and is markedly stronger for isobutyl branches. The methyl wagging mode band is at about 1 150 cm^?1 with the methyl branched polymer. It is relatively broad and probably consists of two overlapping components, specific for methyl branches in crystalline and amorphous regions. This mode is not of detectable intensity for ethyl, butyl and hexyl branches but, as with the rocking mode, it is anomalously strong for isobutyl branches.     

Morphology,crystallization, and thermal behaviour of isotactic polypropylene/low density polyethylene blends

The morphology, the crystallization and thermal behaviour of isotactic polypropylene (IPP) in its blends with two different samples of low density polyethylene (LDPE) was investigated at temperatures high enough to prevent any solidification of LDPE. It is found that pre-existing liquid LDPE domains are incorporated in intra-spherulitic regions during the isothermal crystallization of iPP. The radial growth rate of spherulites is almost unaffected by the LDPE content. The overall rate of crystallization of iPP, on the contrary, is strongly depressed by the addtion of LDPE. A depression of the equilibrium melting temperature of iPP, due to kinetic and morphological effects, is also observed. The depression of the overal kinetic rate constant is accounted for by the negative effect (decrease in the number of nuclei) that the addition of LDPE has on the primary nucleation process of iPP.     

The influence of spherulitic size on the environmental stress cracking of low density polyethylene

The influence of morphology in a low density, high melt index polyethylene was studied with particular reference to environmental stress cracking (ESC). Test samples of the polyethylene were molded under constant conditions, only the subsequent rate of cooling to room temperature being changed. Several techniques were used to study the resultant polymer including ESC tests, X-ray diffraction, optical microscopy and constant strain rate stress/strain tests. It was found that the faster the polymer was cooled after molding, the smaller was the typical diameter of the spherulites and the greater the resistance to stress cracking. A hypothesis is proposed to explain the differences in terms of the ease with which a crack can propagate, assuming that propagation is easier in amorphous material than within spherulites.     

Ethylene-butadiene copolymers, 2. Long branching,double bond distribution and their effect on polymer properties

The principal structural characteristics of ethylene-butadiene copolymers prepared with a modified vanadium catalyst are described. The distribution of the butadiene monomeric units, though non-random, is sufficiently homogeneous at low monomeric units content of butadiene (1–3 mole-%) that a ready and efficient vulcanization with sulfur based systems is easily achieved. Long chain branching, which has a detrimental effect on the copolymer properties, can be measured rheologically and controlled at a very low level by the catalyst system. Vulcanized ethylene-butadiene copolymers show improved mechanical properties with respect to high density polyethylene and good processability before curing.     

Application of the composite model for the mechanical properties of high density polyethylene

The existing models of high density polyethylene (HDPE) fail to tackle effectively the development of mechanical anisotropy on drawing at different temperatures. This paper is concerned with the theoretical interpretation of the anisotropic elastic properties of HDPE by applying the composite model proposed by the authors. The model takes into account the change in orientation and crystallinity on drawing. As far as the orientational changes on drawing are concerned, it is seen that the pseudo affine deformation law tan θ = ?(n) · tan θ′ with ?(n) = n^?3/2 is applicable at ?60°C only. It is further found that the two parameter analytical form of ?(n) reproduces the correct orientational changes on drawing for the entire temperature range from ?60 to 100°C. The agreement of the calculated values of E₀ and E₉₀ over the entire temperature range at all drawing ratios is quite satisfactory.     

Lipoproteins in human atherosclerotic vessels. I. Biochemical properties of arterial low density lipoproteins, very low density lipoproteins, and high density lipoproteins.

Lipoproteins extracted from the human aortic intima into 1.65 M NaCl were quantitated and characterized biochemically and by electron microscopy following separation in the preparative ultracentrifuge. The arterial lipoproteins, although separated and designated according to the density classes used for the serum lipoproteins, were distinctly different from their serum counterparts. The amount of lipoproteins in the low density range of d 1.063 to 1.006 (arterial LDL) and in the very low density range of d < 1.006 (arterial VLDL) extracted from arterial intima increased with increasing intimal lipid content. In contrast, the concentration of lipoproteins in the high density range of d 1.210 to 1.063 (arterial HDL) was small and did not change with the severity of atherosclerosis.Arterial VLDL, LDL and its subfractions, LDL₁ (d 1.006 to 1.019) and LDL₂ (d 1.019 to 1.063), were markedly heterogenous and contained unusually large particles, which were isolated by Bio-Gel A-150. These particles showed a pitted and cratered appearance by scanning electron microscopy and were immunochemically unreactive and had no electrophoretic mobility. The lipid and amino acid composition of the arterial VLDL and LDL fractions as well as their electrophoretic, chromatographic and analytical flotation behavior was distinctly different from that of their serum lipoprotein counterparts. Arterial VLDL, in sharp contrast to serum VLDL, was rich in cholesteryl ester and poor in triglycerides. Arterial VLDL also showed no electrophoretic mobility and only half of the preparations reacted to LDL antisera. Acid mucopolysaccharides were detected in the arterial VLDL and LDL fractions in association with the large size particles which lacked electrophoretic mobility and immunochemical reactivity and showed only a “saw tooth” pattern in the analytical ultracentrifuge. Arterial LDL and LDL₂ contained a smaller sized population of particles as separated by Bio-Gel A-150. These particles exhibited a reaction of complete identity with serum LDL when reacted against LDL antiserum. However these particles had a greater electrophoretic mobility and different amino acid composition than did serum LDL and LDL₂. An asymmetrical peak with a mean SF of 7.3 was demonstrated by these particles in the analytical ultracentrifuge.The over-all studies suggest that lipid deposition in atherosclerotic plaques is associated with the accumulation of lipoproteins with biochemical and ultrastructural properties unlike those of serum lipoproteins. The presence of these lipoproteins in the arteries may be a result of the interaction of serum and arterial lipoproteins with acid mucopolysaccharides and of lipoprotein synthesis and degradation in the arteries.     

Fretting wear properties of hydroxyapatite, alumina containing high density polyethylene biocomposites against zirconia

Considering the importance of wear on the materials performance in biomedical applications, the major objective of the present work is to investigate the friction and fretting wear behavior of various HDPE-based composites against zirconia counterbody, both in air and simulated body fluid (SBF) environment. Both Al(2)O(3) and/or HAp fillers (upto 40 vol %) have been incorporated in HDPE to improve the hardness and elastic modulus of HDPE. The fretting wear study indicates that extremely low COF (approximately 0.055-0.075) as well as higher wear resistance (wear rate in the order of approximately 10(-6) mm(3)/N m) can be achieved with the newly developed composites in SBF. A low wear depth of 3-7 microm is recorded, irrespective of fretting environment. Besides reporting the phenomenological tribological data, major focus has been on to understand the underlying mechanism of material removal at fretting contacts. Such understanding has been established in discussing the wear mechanisms in terms of deformation of polymer matrix, tribolayer formation, and wear debris generation.     

Lipoproteins in human atherosclerotic vessels. II. Biochemical properties of the major apolipoproteins of arterial low density and very low density lipoproteins.

The biochemical behavior of the major apoproteins of low density lipoproteins (LDL) and very low density lipoproteins (VLDL) isolated from human atheroslerotic arteries was examined. On Sephadex G-150, delipidated arterial LDL behaved like delipidated serum LDL and eluted as a single peak (apoLDL) at the void volume. The apoLDL fractions migrated as a single band when examined by electrophoresis on cellulose acetate or acrylamide gel and had a mean sedimentation coefficient of 3.2. Upon double immunodiffusion arterial apoLDL formed immunopreciptin lines of complete identity with serum apoLDL, when reacted against antisera to either human serum apoLDL or LDL. However the amino acid composition of arterial and serum apoLDL was significantly different.When delipidated arterial VLDL was examined on Sephadex G-150, it separated into two major apoprotein fractions, SF-1 and SF-3, and into a minor SF-2 fraction. These fractions appeared to have similar elution volumes and immunochemical reactivity as the corresponding Sephadex fractions of delipidated serum VLDL. Arterial and serum SF-1, SF-2, and SF-3, when reacted respectively against apoLDL antisera, HDL antisera and apo C-III antisera, formed immunoprecipitin lines of complete identity. The sedimentation coefficients of arterial and serum SF-1 appeared similar, as were the coefficients of arterial and serum of SF-3. However the amino acid composition of arterial SF-1 and SF-3 was clearly distinguishable from that of their corresponding serum apoprotein fractions. When examined by acrylamide electrophoresis arterial and serum SF-3 showed a similar band pattern but with different electrophoretic mobility. SF-1 of arterial apoVLDL behaved much like apoLDL from arterial LDL as indicated by electrophoretic and immunochemical behavior and amino acid composition. Although the mean sedimentation coefficients of these apoprotein fractions were similar, the analytical pattern suggested SF-1 of arterial apoVLDL to be more polydisperse in its composition. The present studies indicate that major differences exist between the arterial and serum apoproteins especially in regard to their amino acid composition. They also suggest that arterial LDL and VLDL contain apoproteins which have some characteristics that are similar to the apoB and apoC proteins contained in serum LDL and VLDL.     

Synthesis of a polyethylene-graft-polystyrene copolymer and its compatibilization for linear low density polyethylene/poly(phenylene oxide) blends

Based on unsteady diffusionkinetics, polyethylene(PE)-graft-polystyrene (PS) copolymers were designed and synthesized with a heterogeneous high yield titanium-based catalyst by copolymerization of ethylene with a PS-macromonomer using 1-hexene as a short main agent to promote the incorporation of the PS-macromonomer. The presence of 1-hexene facilitated the diffusion of the PS-macromonomer, giving rise to the significantly increased incorporation of the PS-macromonomer. Compatibilization of blends of linear low density polyethylene (LLDPE)/poly(phenylene oxide) (PPO) with the PE-g-PS copolymer were investigated using scanning electron microscopy (SEM) and dynamic mechanical analysis (DMA).     

Determination of long-chain branching distribution in polyethylene by combination of gel permeation chromatography and viscometry

Direct measurement of intrinsic viscosity on gel permeation chromatography effluent was combined with use of the universal calibration concept in order to determine long-chain branching in polyethylene. The experimental procedure, involving an axial dispersion correction for the limited resolution of our chromatographic system, is presented. Results obtained for three branched polyethylene samples are compared with those provided by the well-established method of preparative fractionation by column elution and examination of the fractions by viscometry and light scattering.     

Synthesis and properties of branched polyethylene/high‐density polyethylene blends using a homogeneous binary catalyst system composed of early and late transition metal complexes

Branched polyethylene/high‐density polyethylene blends (BPE/HDPE) with a wide range of molecular weights, melt flow indexes (MFI), and intrinsic viscosity were prepared using the homogeneous binary catalyst system composed by Ni(α‐diimine)Cl₂ ( 1 ) (α‐diimine = 1,4‐bis(2,6‐diisopropylphenyl)‐acenaphthenediimine) and {Tp^Ms*}TiCl₃ ( 2 ) (Tp^Ms* = hydridobis(3‐mesitylpyrazol‐1‐yl)(5‐mesitylpyrazol‐1‐yl)) activated with MAO and/or TIBA in hexane at two different polymerization temperatures (30 and 55 °C) and by varying the nickel loading molar fraction (x_Ni). At all temperatures, a non‐linear correlation between the x_Ni and the productivity was observed, suggesting the occurrence of a synergistic effect between the nickel and the titanium catalyst precursors, which is more pronounced at 55 °C. The molecular weight of the BPE/HDPE blends considerably decreases with increasing Al/M molar ratio. The melt flow indexes (MFI) and intrinsic viscosities (η) are strongly affected by x_Ni, but the melting temperatures are nearly constant, 132 ± 3 °C. Dynamic mechanical thermal analysis (DMTA) shows the formation of different polymeric materials where the stiffness varies according to the x_Ni and temperature used in the polymerization reaction. The surface morphology of the BPE/HDPE blends studied by scanning electron microscopy (SEM) revealed a low miscibility between the PE phases resulting in the formation of a “sandwich structure” after etching with o‐xylene.