Detection of low levels of long‐chain branching in polydisperse polyethylene materials

Creep experiments have been applied to probe the zero‐shear viscosity, η₀, of polyethylene chains directly and precisely in a constant‐stress rheometer at 190°C. Such experiments, when combined with precise measurements of the weight‐average molecular weight, M_w, calibrated relative to linear chains of high‐density polyethylene, are shown to provide a very sensitive approach to detect low levels (0.005 branches per 1000 carbons) of long‐chain branching (LCB). This detection limit is shown to be insensitive to whether the molecular weight distribution (MWD) breadth, M_w/M_n, rises from about two to ten. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

The preparation of multiaxially oriented polyethylene morphologies with high mechanical properties in planar direactions

The development of multiaxially oriented films of low molecular weight (M_w ≈ 59,000) high density polyethylene with high mechanical properties in planer directions has been pursued by inducing fibrillar crystallization under curvilinear flow conditions in a contained geometry using an extrudomolding process and by simulating similar crystallization conditions in an optical plate–plate rheometer. The films, like the uniaxially drawn morphologies of the same low molecular weight high density polyethylene by solid-state extrusion, had a high modulus (12–20 GPa) and strength (0.25 GPa) along the residual flow lines but they exhibited also a modulus enhancement (5 GPa) in the transverse direction as a result of the orientation gradient of the molecular chains in the thickness directions.     

Molecular characterization and rheological properties of modified poly(ethylene terephthalate) obtained by reactive extrusion

The use of a tetrafunctional epoxy‐based additive to modify the molecular structure of poly(ethylene terephthalate) (PET) was investigated with the aim of producing PET foams by an extrusion process. The molecular structure analysis and shear and elongation rheological characterization showed that branched PET is obtained for 0.2, 0.3 and 0.4 wt% of a tetrafunctional epoxy additive. Gel permeation chromatography (GPC) analysis led to the conclusion that a randomly branched structure is obtained, the structure being independent of the modifier concentration. The evolution of shear and extensional behavior as a function of molecular weight (M_w), degree of branching, and molecular weight distribution (MWD) were studied, and it is shown that an increase in the degree of branching and M_w and the broadening of the MWD induce an increase in Newtonian viscosity, relaxation time, flow activation energy and transient extensional viscosity, while the shear thinning onset and the Hencky strain at the fiber break decrease markedly.     

Characterization of branched polyethylene fractions from the elution column

Fractions from several elution column runs on samples of up to 6 g. of a well-characterized high-pressure polyethylene were analyzed by absolute molecular weight methods and several other techniques. The M_n and M_w integral distribution curves are free from any reversal, as was the viscosity distribution curve. Fractions with M_w as high as 8 × 10⁶ were recovered, more than 20 times higher than the original sample's M_w. The polydispersity of the fractions increases from M_w/M_n = 1.5 or less in the low molecular weight fractions to a nearly constant value of 4.5–5.0 in fractions above 60% cumulative sample weight. Nonetheless, refractionation on the elution column shows that the fractions are narrowly distributed in terms of solubility, while GPC analysis reveals that the fractions have an extremely narrow size distribution. It is concluded from the combined results that long-chain branching plays an important role in determining the equilibrium solubility and, further, that long-chain branching increases the polymer solubility. Sample calculations are provided, which illustrate the effect of fraction polydispersity on calculated original sample molecular weights and the fit of the fractionation results to several model distribution functions.     

The reactive modification of polyethylene. II: Mathematical modeling

In Part I of this work, experimental data showed that the effect of low concentrations of free radical initiator injected into polyethylene during extrusion depended upon the degree of unsaturation and branching in the feed as well as the molecular weight. This paper shows attempts to quantitatively explain these reactive extrusion results through development of two kinetic models based upon the rate equations for reactions considered dominant. The first model developed incorporated unsaturation via consideration of simultaneous crosslinking-endlinking reactions. It contains two variable parameters: the overall initiator efficiency and a ratio of two rate constants reflecting the reactivity of the unsaturated bonds. The model was able to fit the changes in molecular weight distribution of both the low density and linear low density polyethylene but not the high density polyethylene samples. In addition to fitting the molecular weight distribution, this model also provided reasonable values of initiator efficiencies for crosslinking, endlinking, and chain extension reactions, as well as the number of terminal vinyls of the products. The second model is a special case of the first: it neglects the presence of unsaturation in the feed. This second model is actually the usual “crosslinking” model widely known from a derivation based upon statistical arguments. It was not able to fit most molecular weight distributions obtained. However, the model was shown to be useful for accounting for observed molecular weight distribution for a high density polyethylene sample of low initial unsaturation. Also, it was able to explain the amount of gel formed as a function of initiator concentration.     

Rheology of metallocene-catalyzed monomodal and bimodal polyethylenes

The effect of molecular architecture on the dynamic viscoelastic properties of new metallocene high density polyethylenes has been analyzed. Bimodal molecular weight distribution metallocene polyethylenes show features different from conventional polydisperse and bimodal polyethylenes. Higher values of Newtonian viscosity (η_o) at the same values of weight average molecular weight (M_w) and stronger frequency dependence of dynamic viscosity (η′) than in conventional HDPE-s have been observed; this leads to lower values of the characteristic frequency for the onset of non-Newtonian behavior (ω_o) and higher values of the power law index (α). These features are probably due to the presence of very small amounts of long chain branching (LCB). The implications of these results in polymer processing are analyzed comparing extrusion rheometer data, which leads to the conclusion that extrusion difficulties in metallocene catalyzed polyethylenes can be overcome with bimodal molecular weight distributions.     

Long-chain branching in low-density polyethylene

A sample of a commercial low-density polyethylene was fractionated and values of number ? M_n and weight-average M_w, molecular weights obtained together with intrinsic viscosities [η], measured in decalin at 135° and in a theta-solvent, diphenyl at 118°. Results are compared with those obtained using samples of high-density polyethylene, of narrow molecular weight distribution, in decalin at 135° and in diphenyl at 125°. Values of the z-average mean square radius of gyration (S^?2)_z, are converted to the weight-average unperturbed state. The branching parameters g and g¹ thus obtained, indicate that long-chain branching increases with increasing molecular weight. Intrinsic ivscosities under theta-conditions for the low-density polyethylene fractions lead to a relationship [η]_θ = K _w^0·20, agreeing with the treatment of Zimm and Kilb. Some of the approximations involved in the estimation of long-chain branching are discussed.     

Homo- and copolymerizations of ethylene and α-olefins in <Emphasis Type="Italic">n</Emphasis>-hexane catalyzed by Ni(ii)-α-diimine catalyst

Ni(II)-α-diimine catalyst [(2,6-i-Pr)₂C₆H₃-DAB(An)]NiBr₂ plus methylaluminoxane was successfully used in the homopolymerization of ethylene, 1-hexene, and 1-octene and the copolymerization of ethylene with 1-hexene and 1-octene in n-hexane. The polymerization of 1-octene was conducted in a controlled manner with a narrow molecular weight distribution (M _w/M _n = 1.2–1.5) and with the weight-average molecular weight increasing linearly with the monomer conversion. The molecular weights, T _g, T _m, branching degree, and density of the obtained (co)polymers were greatly controlled by ethylene pressure and polymerization temperature. Compared with that of ethylene homopolymer, the branching degree of the copolymers prepared by the copolymerization of ethylene with 1-hexene or 1-octene increased, whereas the molecular weight, density, T _m, and catalyst activity decreased. However, compared with those of the homopolymer of 1-hexene or 1-octene, the copolymer density, T _m, and catalyst activity increased, whereas the branching degree declined.     

Thermal degradation of polyethylene in a nitrogen atmosphere of low oxygen content. II. Structural changes occuring in low-density polyethylene at an oxygen content less than 0.0005%

Samples of low-density polyethylene, free from additives, were heated at temperatures between 284° and 355°C under high-purity nitrogen. Changes in molecular weight distribution (MWD), molecular weight averages, and degree of long-chain branching (LCB) were followed by gel chromatography (GPC) and viscosity measurements. Other structural changes were investigated by infrared spectroscopy and differential scanning calorimetry (DSC). At 284° and 315°C, the MWD's were shifted toward higher molecular weights and the M?_w values increased. At 333° and 355°C, the MWD's shift toward lower molecular weight, but the high molecular weight, tail is largely retained. M?_w decreases slowly at 333°C. At 355°C, M?_w undergoes a rapid initial drop which levels off. M?_w/M?_n and the degree of LCB increase with heating time and temperature. Olefinic unsaturation increases. The vinyl groups show a larger relative increase than do the trans-vinylene and vinylidene groups. At 355°C, the peak of the unimodal DSC thermogram is shifted to ～3°C higher temperature. A lower melting peak then develops, and after 72 and 90 min the two peaks are about equal in size. The density increases from 0.922 g/cm³ to 0.930 g/cm³ for samples heated at 355°C, and the weight loss was 1.5% after 90 min. A reaction scheme for the thermal degradation of polyethylene is discussed. Initiation is suggested to be accomplished by scission of allylic C? C bonds. Propagation proceeds by both intra- and intermolecular hydrogen abstraction, followed by β-scission. Termination can occur by both combination and disproportionation. Combination reactions are suggested to account for the observed formation of LCB and high molecular weight material. Due to changes in the degree of LCB during the degradation, viscometry alone will not give a proper measure of the changes in molecular weight.     

Postponing sharkskin of metallocene polyethylenes at low temperatures: the effect of molecular parameters

The effect of temperature on extrusion rheometry of single site metallocene-catalyzed polyethylenes and polyethylene copolymers is investigated. Samples of molecular weight, M_w, ranging from 90,000 to 330,000 and short-chain branching degree (SCB) from 0 to 21.2 CH₃/1000C, as well as samples with a small amount of long-chain branching, are analyzed. It is observed that all the samples display a low temperature region, limited by induced crystallization and gross melt fracture, in which smooth extrudates are produced at shear rates similar to those of industrial extrusion. A characteristic temperature of this region, T_s, is defined as the highest temperature at which sharkskin disappears. Clear symptoms of non-slip conditions at the capillary wall, are detected in this low temperature region. We assume that the necessary slip-stick conditions to produce sharkskin, would only be produced at shear rates above those involved in gross melt fracture. The analysis of the effect of the molecular parameters, leads to the conclusion that only SCB has a direct effect on T_s. A linear correlation between T_s and SCB level is established, showing the decrease of the former as the latter is increased. Considering the wide spectrum of the molecular characteristics of our samples, we claim that decreasing temperature is a sound route to postpone sharkskin of any polyethylene.     

The effect of processing on rheological and molecular characteristics of a low density polyethylene

The viscoelastic responses of some molten polymers, and particularly of low density polyethylene (LDPE), are known to vary with processing history. Reasons for the variations include the effects of shear history on morphological states of the polymer, or on its molecular weight parameters. A typical low density polyethylene has been used to test the shear-history dependence concept following a variety of processing steps. The polymer was sheared in single-screw and twin-screw extruders, and in a high speed melter / mixer (Gelimat). Samples also were precipitated from very dilute solutions in trichlorobenzene and in p-xylene. GPC analyses showed that, in general, these procedures did not affect the various moments of molecular weight. An exception was the Gelimat-mixed sample, for which mild reductions in M_n and M_w were noted. In contrast, melt viscosity and elasticity readings, the former from low shear evaluations and the latter from extrudate swelling, were affected by the various procedures. A drop in melt viscosity and in elasticity was observed, being most pronounced for precipitated and twin-screw extruded versions of the LDPE. Reductions also were observed in the specimen sheared in the Gelimat instrument. Following conditioning at the test extrusion temperature (170°C), viscous and elastic responses tended to revert to those of the unsheared control sample, the exception again being the sample sheared in the Gelimat melter / mixer. Of the various mechanisms proposed in the literature to account for transient property changes such as those reported, temporary changes in the degree of chain entanglement appear the most satisfactory explanation. Irreversible alterations in viscoelastic properties may be associated with changes in molecular weights due to processing at high shear.     

Extrusion of broad‐molecular‐weight‐distribution polyethylenes

The extrusion (single‐screw) characteristics of four high‐molecular‐weight, broad‐molecular‐weight‐distribution (MWD) polyethylene resins are discussed with an emphasis on the output rate. Despite the high molecular weights of the subject polyethylenes, their broad MWD (M_w/M_n range: 10 to 50) does not limit the pressure and torque developed during extrusion. However, the specific output of the four polymers was quite varied. First, the dynamics of the solids conveying section were examined with the highest‐molecular‐weight polyethylene exhibiting lower solids‐conveying rate than the other three. Further, a simple and quick method to evaluate the relative solids‐conveying efficiencies for various polyethylenes is presented. Finally, the dependence of the specific output on the melt rheology of the polymers is also addressed; specifically, the shear‐thinning extent of the melt in the metering section was found to influence output rate. The unique and counterintuitive temperature‐dependence of the shear‐thinning character for one of the four polymers will also be addressed in relation to its extrusion characteristics. Polym. Eng. Sci. 44:2266–2273, 2004. © 2004 Society of Plastics Engineers.     

Molecular structure,crystallization behavior,and morphology of fractions obtained from an extrusion grade high-density polyethylene

An extrusion-grade of high density polyethylene (HOPE) (3 ethyl groups per 1000 carbons) has been divided into 16 fractions by preparative GPC and selective p-xylene extraction. The fractions, with molecular weights ranging from 900 to 1,000,000, have been studied by IR spectros-copy, DSC, WAXS, polarized microscopy, and small-angle light scattering (SALS), The average degree of chain branching (percent C₂H₅) is 0.5 percent for the part of the sample having a molecular weight lower than 10,000 and it decreases monotonically with increasing molecular weight, finally approaching 0.1 percent C2H5. A crystallinity depression with respect to linear PE equivalent to 20 percent/(percent C₂H₅) is recorded for all samples except for the very low molecular weight samples for which the crystallinity depression is much larger (30 to 35 percent/ (percent C₂H₅)). The unit cell volume increases with increasing percent C₂H₅, presumably due to the inclusion of ethyl groups in the crystals as interstitlals at 2gl kinks. The concentration of ethyl groups in the crystals (?_c) unanimously follows the relationship: ?_c(percent) = 0.32 + 0.25 log(percent C₂H₅) except for the low molecular weight fractions which have significantly lower values for ?_c. Our admittedly speculative explanation for this major discrepancy between high and low molecular weight samples is based on the idea that segments with ethyl groups close to chain ends have a greater difficulty in crystallizing than segments containing ethyl groups located at positions far from the chain ends. The fractions obtained from the extrusion-grade HDPE show a solidification temperature depression with respect to linear PE which can only be explained by the presence of chain branches in these samples. The depression is particularly pronounced for the low molecular weight samples as is expected from the data on molecular structure. Well-developed non-banded spherulites are observed in rapidly cooled (crystallized at about 35 K supercooling), low molecular weight samples (6,000 < M_w < 8,000)from the extrusion-grade HDPE in contrast to the axialites observed in linear PE of the same molecular weight and thermal treatment. This discrepancy in morphology has been related to the presence of ethyl groups in the extrusion grade HDPE fractions. Higher molecular weight samples (20,000 < M_w < 1,000,000)from the extrusion-grade HDPE and linear PE both display well-developed banded spherulites of similar nature as is expected due to the similarity in molecular structure of the two sets of sample.     

On the accuracy of SEC analyses of molecular weight distributions of polyethylenes

Molecular weights of National Bureau of Standards SRM 1476 polyethylene have been reported by six laboratories. The measured values are in remarkably good agreements and all show that M _w from SEC/LALLS analyses is significantly lower than the same average determined by LALLS on the whole polymer itself. This is shown to be due to the presence of high molecular weight species which become too diluted on passage through the SEC columns to be observed in the LALLS detector. The resulting error in M _w and higher averages may vary from slight to very serioius, depending on the molecular weight distribution of the particular polyethylene. A procedure is described to detect the presence of such high molecular weight species.     

Molecular weight distribution of commercial PVC

The molecular weight distribution (MWD) of commercial suspension grade poly(vinyl chloride) (PVC) resins with K values from 50 to 93 and mass grade PVC resins with K values from 58 to 68 has been determined by size exclusion chromatography (SEC), using literature Mark‐Houwink coefficients. The MWD is characterized by the number average molecular weight (M_n), the weight average molecular weight (M_w) and the polydispersity (M_w/M_n). Our results for M_w are consistent with recently published data, but we find different results for M_n and consequently for M_w/M_n. The polydispersity of PVC increases with increasing K value. This effect can be explained by two mechanisms. The first mechanism is a reduced terminating reaction rate between two growing polymer chains (disproportionation) at higher molecular weight owing to the reduced mobility of the polymer chains. The second mechanism is long‐chain branching of molecules with high molecular weight which lets the molecules grow at two ends. For two examples graphs of the measured MWD are compared with the theoretically expected MWD.     

Melt rheology of high-density polyethylene

The melt rheology of high density polyethylene was investigated. Linear viscoelasticity, capillary flow properties, and molecular weight parameter were measured with a plate relaxometer, capillary rheometer, and gel permeation chromatography, respectively. Intimate correlations among the slope of relaxation modulus curve, non-Newtonian flow behavior, Barus effect, and molecular weight parameter, M_z(M_z+1)/M_w, respectively, were found.     

Influence of molecular structure on the rheological and processing behavior of polyethylene resins

The rheological and processing behavior (melt fracture performance) of linear lowdensity polyethylenes (LLDPEs) is studied as a function of both the weight average molecular weight (M_w) and its distribution (MWD). A number of LLDPE resins having different molecular characteristics were tested, with essentially one characteristic (M_w or MWD) changing at a time. The first series of resins consisted of nine samples having a wide range of polydispersities (3.3–12.7) and nearly constant M_w and short chain branching. The second series had six resins with varying M_w (51,000–110,000) but fixed MWD (about 4). The influence of M_w and MWD on the viscosity profiles, linear viscoelastic moduli as expressed by means of a discrete spectrum of relaxation times, extrudate swell, and melt fracture behavior for these resins is reported. Correlations between the molecular characteristics of the resins and their rheological and processing behavior are also reported. It is found that for a given molecular weight, the optimum melt fracture performance is obtained at a specific polydispersity value, and it is characterized by a minimum relaxation time for the resin defined in terms of recoverable shear.     

Rheological properties of polypropylenes with different molecular weight distribution characteristics

Relationships between the rheological properties and the molecular weight distribution of two polypropylene series with different molecular weight distribution characteristics were studied. The end correction coefficient in capillary flow is determined by the molecular weight M_w and the molecular weight distribution M_w/M_n, and is higher as both characteristic values are larger. The die swell ratio at a constant shear rate depends on M_w, M_w/M_n, and M_z/M_w, and is higher as the three characteristic values are larger. The critical shear rate at which a melt fracture begins to occurs depends on the molecular weight M_w and the molecular weight distribution M_z/M_w, and is proportional to M_z/M_w² in a log–log plot. The critical shear stress does not depend on the molecular weight, and is higher as M_z/M_w is higher. The zero‐shear viscosity is determined by a molecular weight of slightly higher order than M_w, and the characteristic relaxation time is determined by M_z. The storage modulus at a constant loss modulus scarcely depends on the molecular weight, and is higher as the molecular weight distribution M_w/M_n is higher. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 2128–2141, 2002     

The degradation of polystyrene during extrusion

An investigation was made of the magnitude and mechanism of shear degradation of a narrow distribution, high molecular weight (M_w = 670,000) polystyrene. An Instron rheometer was used to perform the extrusion at temperatures from 164° to 250°C. The change in molecular weight distribution was studied by gel permeation chromatography. The maximum shear stress employed was 5.83 kg/cm². It was found that degradation could be induced at high stress at temperatures of 50°C lower than degradation of polystyrene would occur exclusively due to thermal forces. An activation energy for the degradation, calculated at constant shear rate, was +20.2 kcal/mole. The direction and magnitude of this value are consistent with degradation induced through a mechanical reduced activation for thermal degradation.