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.     

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.     

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.     

Head to head polymers IX: The rheological behavior of H-H and H-T atactic polystyrenes

The rheological behavior of a sample of H-H polystyrene of M_n of 41,000 and a M_w/M_n of 2 was compared at 160 and 190°C with a sample of H-T atactic polystyrene of similar molecular weight. The melt viscosity of H-H polymer (unlike the H-T polymer) was non-Newtonian at low stresses and decreased more rapidly with stress. This observation seems to indicate a stiffer polymer chain for the H-H polystyrene. The flow activation energy (E*) of H-H polystyrene was found to be dependent on the dynamic shear stress and decreased with increasing dynamic shear stress. The dynamic shear storage modulus of the H-H polymer has a smaller increase of G′ with ω than that of the H-T polystyrene.     

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     

A unifying approach for melt rheology of linear polystyrene

Several studies of melt rheological properties of polystyrene have been conducted over the past 50 years. Several approaches, including empirical models, have been developed to understand the behavior of materials using simple equations. The existing melt rheology models are best suited for high‐molecular‐weight polymers whose T_g does not vary. In this work, a semiempirical viscosity equation has been derived, including the effect of T_g dependence on molecular weight, to describe the melt rheology of low‐molecular‐weight polymers. The equation is derived based on a combination of well‐known concepts, such as the effects of free volume and molecular dynamics on polymer rheology. This provides a better understanding of the rheological behavior in the low‐molecular‐weight regime with respect to temperature and molecular weight. Because of the industrial trend towards lower molecular weight materials for applications such as high solids coatings, this unifying approach, based on the free volume theory with a simple expression, is of extreme practical significance. This equation can predict the zero shear viscosity behavior for different molecular weights, including low‐molecular‐weight regions, and temperatures. Viscosity calculations using the empirical equation agree with published experimental data. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 2597–2607, 2007     

Experimental investigation of flow induced molecular weight fractionation during extrusion of HDPE polymer melts

In this work, two different HDPEs with virtually identical number, M_n, and weight, M_w, average molecular weights were investigated from rheological as well as die drool phenomenon point of view. It has been revealed that long-chain branching, low polymer melt elasticity and shear viscosity significantly reduce die drool phenomenon at the die exit region. It has been concluded that die drool phenomenon of HDPE polymer melts can be explained by the flow induced molecular weight fractionation.     

Molecular weight distribution of polyphenylene sulfide by high temperature gel permeation chromatography

Poly(phenylene sulfide) (PPS) has been characterized using a novel high temperature gel permeation chromatograph (GPC). Samples were injected in slurry form at ambient temperature, and redissolved by an in-line precolumn heater at 250°C. A viscometer consisting of a capillary tube with inlet and outlet taps connected to a sensitive differential pressure transducer was used as sole detector, with deflections converted to concentration using the column calibration. Columns and viscometer were operated at 210°C. Universal calibration was carried out using intrinsic viscosity/molecular weight relations for polystyrene and PPS, determined by light scattering. Satisfactory operation was confirmed by agreement between intrinsic viscosity calculated from GPC with independently measured values, and comparisons with melt flow data. Samples of PPS tested were found to be of relatively narrow distribution, with M_w/M_n typically less than two.     

Application of the rheology of monodisperse and polydisperse polystyrenes to the analysis of injection molding behavior

Monodisperse and polydisperse polystyrenes of equal weight average molecular weight (M _w) are evaluated for melt flow behavior in an Instron capillary rheometer and for injection molding behavior in a 12 ounce in-line reciprocationg screw injection molding machine. The influence of molecular weight distribution on the shape of the flow curves is deconstrated over a wide range of shear rate and temperature. The influence is also reflected in injection molding behavior as defined by pressure to fill or flash the mold at a given melt temperature. Studies of capillary rheometer data for correlation with injection moling beghavior indicate poor agreement when low shear rate viscosity data are used. Good agrement is foun using high shear rate viscosity data in the range 10³ to 10⁴ sec^?1 Striking crossover points on melt rheology and injection colding area diagram curvs are found with the monodisperse and polydisperse polystyrenes of the same M These crossovers shift with melt temperature and make possible the determination of a “controlling shear rate” for the injection molding process. This is found to be 3500 sec^?1 for short shot and 6200 sec^?1 for flash with the ASTM test specimen mold used in this study.     

Mechanical degradation of high molecular weight polymers in dilute solution

The mechanical shear degradation of polydisperse polyisobutene and monodisperse polystrene in oils of different viscosities in the concentration range of 0.1% to 1% was studied using a high-shear concentric cylinder viscometer under laminar and uniform well-defined shear field conditions. Molecular weight distributions (MWDs) were measured by gel permeation chromatography (GPC). Degradation of polydisperse polyisobutene solutions narrows the distributions principally through the breaking down of large molecules. Degradation of monodisperse polystyrene broadens the distributions at lower shear stress. At higher shear stresses, the distributions do not broaden as much but are still broader than those of the original polymer. The final M_w/M_n ratios are considerably different from the value of 2 expected for random degradation. Hence, the degradation is likely a nonrandom process. It was found that the extent of degradation has a negative concentration dependence coefficient at relatively high molecular weight and a positive concentration dependence at lower molecular weight. Competing mechanisms of “stretching” and “entanglements” for degradation were postulated to explain the results. The degradation data indicate that the shear stress is the controlling parameter, not the shear rate. The shear degradation is independent of initial molecular weight and viscosity of the solvent.     

Structure‐property relationships of blends of polycarbonate

Three grades of bisphenol‐A polycarbonate—high molecular weight linear, high molecular weight branched and low molecular weight linear—and their blends have been studied by GPC, DMTA, DSC, rheometry and impact measurements. The molecular weight distribution of the blends agred with that predicted from the component's distributions, indicating that no transesterification reactions had occurred during melt blending. The T_g of the blends varied with blend composition according to the Fox equation and was related to the reciprocal molecular weight predicted by the Flory‐Fox equation. The low shear rate viscosity of the blends agreed with a logarithmic rule of mixtures and showed power‐law dependence on the weight average molecular weight. At higher shear rates, shear thinning was observed. The steady shear viscosity correlated well with the dynamic viscosity, as suggested by the Cox‐Merz relation. The stress relaxation behavior of the melt was very sensitive to the blend composition and molecular weight and correlated well with the real modulus. Temperature studies of the dart impact energy showed that only the low molecular weight polymer underwent a brittle‐duetile transition at ea ?30°C and that all the blends were tough at room temperature. The enhanced stress triaxiality inherent in the notched lzod test caused the impact strenght at room temperature to decrease almost linealy with blend composition.     

Flow properties of ABS (acrylonitrile–butadiene–styrene) terpolymer

ABS (acrylonitrile–butadiene–styrene) terpolymer is a two-phase thermoplastic with SAN (styrene–acrylonitrile) copolymer constituting the continuous phase (matrix). The flow properties of ABS with varying molecular parameters were studied using a capillary viscometer at the shear rate range encountered in its processing. The viscosity-average molecular weights (M_v) of matrix SAN with 26% acrylonitrile content are in the range of 90,000 to 150,000, and M_v of poly-butadiene-are in the range of 150,000 to 170,000. The weight-average molecular weight of the matrix SAN is the main controlling factor for the flow properties of ABS at low shear rate, while the molecular weight distribution of the matrix SAN becomes increasingly important with the increase of shear rate. The presence of SAN grafted polybutadiene increases the melt viscosity of ABS by 40–60% over comparable free SAN copolymer and also decreases the activation energy at constant shear stress to 24–25 kcal/mole from the 33–36 kcal/mole for free SAN. The die swell of ABS and SAN can be correlated with the dynamic shear modulus G′, and the melt fracture of ABS and SAN starts at G′ equal to 3.6 × 10⁶ dynes/cm².     

Fabrication and characterization of food‐grade fibers from mixtures of maltodextrin and whey protein isolate using needleless electrospinning

Food‐grade fibers were fabricated from dispersions of maltodextrin and whey protein isolate (WPI) using needleless electrospinning. Two maltodextrins (DE 2) from different starch sources were used and the maltodextrin/WPI ratio was varied. Molecular weight, intrinsic viscosity, entanglement concentration, shear stability, and electrical conductivity were studied as function of maltodextrin type and mixing ratio and correlated to fiber production rate and morphology. The results show that a high molecular weight of the maltodextrin was beneficial to its spinnability. Waxy potato starch maltodextrin (P‐MD) (M_w = 129.6 kDa) and WPI produced fibers with diameters between 1.40 and 1.67 µm at production rates up to 1.65 g/h; while the dispersions with waxy maize starch maltodextrin (M‐MD) (M_w = 85.9 kDa) showed poor spinnability and ruptured fibers. P‐MD/WPI dispersions had a higher viscosity and stronger shear thinning behavior attributed to a stronger entangled polymer network which is beneficial to electrospinning. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46328.     

Effect of molecular weight and molecular weight distribution on the rheological properties of aqueous poly(ethylene oxide) solution

The effect of the molecular weight and the molecular weight distribution on the rheological properties of aqueous poly(ethylene oxide) (PEO) solutions has been investigated with four PEO samples differing in their M_w, M_w/M_n and purity. The main result of this study is that the steady shear viscosity as well as the complex dynamic viscosity of the samples with broad molecular weight distribution greatly differed from the viscosities of the samples having a narrow molecular weight distribution. Furthermore, the samples with broad molecular weight distribution showed a distinct molecular weight dependent non-Newtonian behavior at increasing shear rates and frequencies. This behavior was not observed for the sample with a narrow molecular weight distribution. Both effects are mainly attributed to the influence of the high molecular weight fraction in the PEO samples of broad molecular weight distribution. The often reported degradation of PEO solutions was not observed within the time scale of our experiment.     

The effect of shear rate on the molecular weight determination of acrylamide polymers from intrinsic viscosity measurements

The rheological response of dilute solutions of high molecular weight polyacrylamides at low shear rates has been measured using a capillary viscometer that provided for a fivefold variation in shear rate at each concentration. The non-Newtonian effects were found to be significant for polyacrylamides with number-average molecular weights exceeding 10⁶. The molecular weight average–intrinsic viscosity relationship most widely used in the literature, [η] = 6.80 × 10^?4M , was found to be valid when [η] was measured at high shear rates where the polymer solutions approached Newtonian behavior. A new relationship was developed relating M _n to the intrinsic viscosity extrapolated to zero shear rate.     

Dependence of intrinsic viscosity and molecular size on molecular weight of partially hydrolyzed polyacrylamide

As a typical water-soluble polymer, ultra-high molecular weight (UHMW) partially hydrolyzed polyacrylamide (HPAM) has been widely used in various industries as thickeners or rheology modifiers. However, precise determination of its critical physical parameters such as molecular weight, radius of gyration (R_g) and hydrodynamic radius (R_h) were less documented due to their high viscosity in aqueous solution. In this work, the molecular structure of five UHMW-HPAM samples with different MW was elucidated by ¹H and ¹³C NMR spectroscopy, and their solution properties were characterized by both static and dynamic light scattering. It is found that all the second virial coefficient (A₂) values are positive and approaching zero, indicating of a good solvent of 0.5 M NaCl for UHMW-HPAM. The weight-average molecular weight (M_w) dependence of molecular size and intrinsic viscosity [η] for these series of HPAM polymers with MW ranging from 4.81 to 15.4 × 10⁶ g·mol⁻¹ can be correlated as R_g = 3.52 × 10⁻²M_w^0.51, R_h = 1.97 × 10⁻²M_w^0.51, and [η] = 6.98 × 10⁻⁴ M_w^0.91, respectively. These results are helpful in understanding the relationship between molecular weight and coil size of HPAM polymers in solution, and offer references for quick estimation of molecular weight and screening of commercial UHMW-HPAM polymers for specific end-users.     

An empirical model relating the molecular weight distribution of high-density polyethylene to the shear dependence of the steady shear melt viscosity

An empirical model has been developed to relate molecular weight distribution to the shear dependence of the steady shear viscosity in high-density polyethylene melts. It uses a molecular weight, M_c, which partitions molecular weights into two classes; those below M_c contribute to the viscosity as they do at zero shear, and those above M_c contribute to the viscosity as though they were of molecular weight M_c at zero shear. Each individual molecular weight species contributes on the basis of its weight fraction. M_c is proposed to be a unique function of the shear rate. Using this method of treating the molecular weight distribution, and the zero shear relation for relating η0 to molecular weight, the calculated steady shear viscosities at various shear rates for polyethylene samples of widely varying polydispersities agree well with experimental results. The model makes no judgment on the existence or importance of entanglements in non-Newtonian behavior since it has no specific parameters involving an entanglement concept. Use of the model suggests that for the samples studied, only the upper portion of the molecular weight distribution contributes toward the experimentally observed decrease of steady shear viscosity with shear rate for shear rates of up to 10,000 sec^?1. The lower molecular weight species are assumed to behave in a Newtonian manner.     

Thermal response of low molecular weight poly-(<Emphasis Type="Italic">N</Emphasis>-isopropylacrylamide) polymers in aqueous solution

Temperature-induced phase transition of three low-molecular-weight samples (M _w  < 1.2 × 10⁴) of poly(N-isopropylacrylamide) was studied with the aid of turbidimetry, dynamic light scattering, and rheology. We have demonstrated that the lower critical solution temperature depends on the length of the chain and the concentration of the polymer in the low molecular weight range. The turbidity results show a transition peak in the turbidity curve at intermediate temperatures. This peak, as well as the cloud point, is shifted toward lower temperatures when the molecular weight and the concentration of the polymer increase. The DLS measurements disclose a fast and a slow relaxation mode, which in both cases are found to be diffusive. The fast mode is linked to the diffusion of small species in the solution, and the slow mode is associated with the formation of large aggregates. The formation of these aggregates is less pronounced in solutions of polymers with low molecular weight and the incipient aggregation is shifted to higher temperatures. The shear viscosity measurements show the formation of weak aggregates, which are easily broken in solutions of short polymers. This effect is less pronounced when the molecular weight of the sample is increased. At certain shear rates, temperature-induced transition peaks of the viscosity are observed.     

The relation between melt flow properties and molecular weight of polyethylene

The non-Newtonian behavior of commercial linear polyethylene samples and their fractions were studied at 190°C. The viscosity η versus shear rate \documentclass{article}\pagestyle{empty}\begin{document}$ \dot \gamma $\end{document} curves of whole polymers could be superimposed onto a single master curve despite the variations of their molecular weights and molecular weight distributions. For fractions, however, the same master curve was inapplicable, and the sensitivity of the viscosity to shear rate was found to be greater than those of the whole polymers. The zero-shear viscosities η₀ of fractions were related to the 3.42 power of the weight-average molecular weight M_u as follows: For whole polymers, the zero-shear viscosities were found to be considerably higher at the same M_w and markedly lower at the same z-average molecular weight M_z than those of the fractions. Thus, it was concluded that η₀ corresponds to an average of molecular weight between M_w and M_z. It was found that the molecular relaxation time τ is proportional to M_z^5.3 for whole polymers and to η₀M_w for fractions. Using these relations it was possible to relate the flow ratio, the ratio of flow rates at two different shear stresses, with the molecular weight distribution.