Shear and elongational flow properties of peroxide‐modified wood/low‐density polyethylene composite melts

Adding fillers to a polymer melt may result in a strain softening behavior in elongational flow in long‐chain branched materials, showing strain‐hardening behavior when compared with unfilled one. To improve the strain‐hardening properties in wood/LDPE composites, the effect of peroxide concentration on both the molecular architecture and molar mass distribution, and the rheological quantities in shear and elongation is studied. Addition of wood flour increases the viscosity according to a logarithmic mixing rule, as expected from the large particle size and the filler fractions used. The peroxide has multiple effects on the molar architecture of the polymer. First, a gel fraction of cross‐linked material is formed, the concentration of gel being dependent of the amount of peroxide used. Second, a higher molar mass component is detected, leading to higher value of M_w and to a broader molar mass distribution. Finally, the degree of long‐chain branching unexpectedly decreases with increasing peroxide content. The changes in molecular architecture are hardly influenced by addition of the wood flour. The peroxide treatment leads to an improved strain‐hardening behavior, detected by elongational viscosity and melt strength measurements. However, the addition of wood flour decreases the amount of strain hardening.POLYM. COMPOS., 33:2084–2094, 2012. © 2012 Society of Plastics Engineers     

Dynamic rheology of branched poly(ethylene terephthalate)

Branched poly(ethylene terephthalate)s (BPET) of varying molar mass have been synthesized with glycerol and pentaerythritol as branching comonomers, and their rheological behaviour has been measured. In this study, we describe the use of dynamic and steady shear measurements to examine the influence of the proportion and type of branching comonomers on the melt viscosity of BPET. Steady shear rheology has been used to measure the shear rate dependence on the apparent viscosity. Dynamic (oscillatory) measurements have been used to obtain the complex viscosity η* (ω) and the storage modulus G′ (ω) as a function of frequency. G′ (ω) represents the elastic component of the viscoelastic melt; this variable was measured as a function of frequency at various temperatures in the linear viscoelastic domain. Linear poly(ethylene terephthalate) (LPET) exhibited nearly Newtonian behaviour, while BPET became shear thinning at relatively low shear rates. The viscosity and elasticity increased with increase in molar mass and specific branching composition. This was attributed to increasing chain entanglements at higher molar mass and to increasing branching of the BPET. At higher shear rates or frequencies, the BPET show much greater shear thinning character than LPET and this is more pronounced with higher branching proportions. © 2000 Society of Chemical Industry     

A study on the onset surface melt fracture of polypropylene materials with foaming additives

The melt fracture behaviors of linear and branched polypropylene resins with foaming additives were investigated. The effects of branching, processing temperature, additives, and blowing agent on the surface melt fracture of polypropylene materials were thoroughly studied. A CCD camera was installed at the die exit to precisely observe the onset of surface melt fracture of extruded foams. The critical wall shear stress was determined for various linear and branched polypropylene resins using a capillary die. It was found that the branching required to foam polypropylene resins also promotes melt fracture: the critical shear stress was decreased by 0.0175 MPa with an increase of 0.1 n/1000c in long‐chain branching. It was also observed that the dissolved blowing agent (butane) significantly suppressed the melt fracture of both linear and branched polypropylene resins. On the other hand, a noticeable increase in the critical shear stress of branched polypropylene materials was observed with the nucleating agent (talc) and the aging modifier (glycerol mono stearate), whereas almost negligible effect of the additives on the critical shear stress was observed for linear polypropylene materials. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

The characterization of rheological properties of melt grafting polypropylene for foaming

Large enhancements of the melt strength of polypropylene (PP) were achieved by the introduction of specific unsaturated linear polyester (ULP) branches using melt grafting. The transient torque curves and optical rheology microscope images indicated that branching reactions took place and the ULP had been grafted onto the PP backbone. Shear rheological behaviors of three kinds of PP were investigated using rotational rheometer under dynamic shear mode with periodic shear rate. These PP samples are foamable PP (FPP) with sparse branches obtained by grafting ULP, commercial high melt strength PP (HMS PP) for foaming and conventional linear PP (EPS). It was found that the rheological properties of FPP, the HMS PP, were distinctly different from those of conventional PP. Storage modulus, steady state compliance and zero shear viscosity increased in comparison with EPS, while shear viscosity decreased. This result implied the presence of branching structures that was not revealed in conventional PP. In melt flow measurements, extrusion swell that was a prominent behavior of branching PP was observed also for FPP and PF. Compared to linear PP, FPP and PF showed distinct sag-resistant property and lower melt flow index. On the other hand, to estimate the extent of branching, a detailed method was applied using the obtained zero shear viscosity. The result showed that FPP was grafted by sparse ULP. From these results, it was found that FPP showed obvious enhancements in rheological behaviors similar to PF, although its melt strength was lower than that of PF due to the presence of shorter branching chains grafted on the backbone of FPP.     

The rheological and physical properties of linear and branched polypropylene blends

The rheological, thermal, and mechanical properties of blends consisting of a linear high melt flow rate polypropylene (PP) and two branched PPs are characterized in detail. Blends containing branched PPs display evidence of miscibility in the melt state and exhibit high melt elasticity together with significant strain hardening in extensional deformation while retaining good flow properties. Out of the two blend systems examined the blends containing linear and branched PPs with similar melt flow rates have better mechanical properties, higher crystallization temperatures, and higher crystallinities. POLYM. ENG. SCI., 47:1133–1140, 2007. © 2007 Society of Plastics Engineers     

过氧化物交联HDPE热水管材料性能分析

通过核磁共振、凝胶渗透色谱、差示扫描量热、粒径、熔体流动速率和力学性能等分析手段，研究了进口过氧化物交联高密度聚乙烯（HDPE）热水管材料及国产HDPE的结构与性能。结果表明，进口样品具有支化度小、双键含量高、相对分子质量高、相对分子质量分布窄、熔融温度高、熔体流动速率小、合适的粒径及分布以及较高的力学性能等特点；而国产HDPE则不适合用于过氧化物交联热水管。     

Understanding the foamability and mechanical properties of foamed polypropylene blends by using extensional rheology

In this article, the influence of the rheological behavior of miscible blends of a linear and a high melt strength, branched, polypropylene (HMS PP), on the cellular structure and mechanical properties of cellular materials, with a fixed relative density, has been investigated. The rheological properties of the PP melts were investigated in steady and oscillatory shear flow and in uniaxial elongation in order to calculate the strain hardening coefficient. While the linear PP does not exhibit strain hardening, the blends of the linear and the HMS PP show pronounced strain hardening, increasing with the concentration of HMS PP. Related to the cellular structure, in general, the amount of open cells, the cell size, and the width of the cell size distribution increase with the amount of linear PP in the blends. Also mechanical properties are conditioned by the extensional rheological behavior of PP blends. Cellular materials with the best mechanical properties are those that have been fabricated using large amounts of HMS PP. The results demonstrate the importance of the extensional rheological behavior of the base polymers for a better understanding and steering of the cellular structure and properties of the cellular materials. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42430.     

Study of mechanical and rheological behaviors of linear and branched polycarbonates blends

A study of the mechanical and rheological properties of linear and branched polycarbonates blends is presented. Phase separations of the blends were checked through DSC and SEM, and, subsequently, mechanical and rheological properties were investigated. Phase separations were not observed in the blends. The mechanical properties were examined through tensile, flexural, and impact tests. All the mechanical properties of the blends were relatively independent of the compositions. For study of the rheological properties, melt viscosity, storage and loss moduli, and melt tension of the blends with various compositions were examined at various temperatures. The dependence of the viscosity on the molecular weight was also studied. As the content of branched polycarbonate increases, the dependence of the viscosity on the molecular weight and the shear thinning behavior became more marked. Melt tensions were also increased as the branched polycarbonate content increased in the blends for all tested temperatures. In this study, the blend systems which have same mechanical properties but different rheological properties can be obtained through blending of linear and branched polycarbonates. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 1814–1824, 2001     

Molecular structure and rheological properties of a poly(ethylene terephthalate) modified by two different chain extenders

This paper compares the molecular structure and rheological properties of a commercial poly(ethylene terephthalate) (PET) after reactive processing with different concentrations of either pyromellitic dianhydride (PMDA) or a multifunctional epoxide (Joncryl®ADR-4368) as a chain extender. By size exclusion chromatography with triple detection, an increase of molar mass, a broadening of molar mass distribution, and the generation of long-chain branched molecules were found for both chain extenders. While gel-free materials were obtained with PMDA, the processing with Joncryl leads to the formation of gels. The effect of branching, indicated by the Mark–Houwink exponent, is more pronounced for materials with Joncryl compared to PMDA and points to a more compact branching structure of the PET/Joncryl molecules. Rheological measurements in shear and elongation support the analysis from SEC and reveal a complex tree-like branching structure for both chain extenders. In addition, the role of the two modifiers with respect to processing was assessed.     

Melt rheological and thermodynamic properties of polyethylene homopolymers and poly(ethylene/α-olefin) copolymers with respect to molecular composition and structure

Various types of polyethylene homopolymers and copolymers, including linear high-density polyethylene (HDPE), branched low-density polyethylene (BLDPE), poly(ethylene vinyl acetate) copolymer (EVA), heterogeneous linear poly(ethylene/α-olefin) copolymer (het-LEAO) or commonly known as linear low-density polyethylene, homogeneous linear poly(ethylene/α-olefin) copolymer (hom-LEAO), and homogeneous branched poly(ethylene/α-olefin) copolymer (hom-BEAO), were evaluated for their melt rheological and thermodynamic properties with emphasis on their molecular structure. Short-chain branching (SCB) mainly controls the density, but it has little effect on the melt rheological properties. Long-chain branching (LCB) has little effect on the density and thermodynamic properties, but it has drastic effects on the melt rheological properties. LCB increases the pseudo-plasticity and the flow activation energy for both the polyethylene homopolymer and copolymer. Compared at a same melt index and a similar density, hom-LEAO has the highest viscosity in processing among all polymers due to its linear molecular structure and very narrow molecular weight distribution. Small amounts of LCB in hom-BEAO very effectively reduce the average viscosity and also improve the flow stability. Both hom-LEAO and hom-BEAO, unlike het-LEAO, have thermodynamic properties similar to BLDPE. © 1996 John Wiley & Sons, Inc.     

Melt rheology of poly(lactic acid): Consequences of blending chain architectures

The rheology of blends of linear and branched poly(lactic acid) (PLA) architectures is comprehensively investigated. Measurement of the melt rheological properties of PLA is complicated by degradation effects but the addition of 0.35 wt% tris(nonylphenyl) phosphite (TNPP) provides excellent stabilization over a range of temperatures. Master curves of dynamic viscosity constructed using time‐temperature superposition show significant dispersion for unstabilized samples; this behavior is accompanied by a loss of molecular weight. TNPP stabilized samples show excellent superposition throughout the entire frequency range and minimal loss in molecular weight. For the linear architecture, the Cox‐Merz rule is valid for a large range of shear rates and frequencies. The branched architecture deviates from the Cox‐Merz equality and blends show intermediate behavior. Both the zero shear viscosity and the elasticity (as measured by the recoverable shear compliance) Increase with increasing branched content. The viscosities of both the unstabilized samples and the TNPP stabilized samples roughly obey a log additivity mixing rule. The recoverable shear compliance is monotonic in blend composition and a mixing rule for this property is also presented. For the linear chain, the compliance is independent of temperature but this behavior is apparently lost for the branched and blended materials. Tensile and thermal properties of the blends are also measured and found to be roughly equal within the statistical error of the experiments. The results suggest that excellent control over rheological behavior of PLA is possible through blending chain architectures without compromising mechanical properties.     

Mechanical and rheological properties,and morphology of polyamide-6/organoclay/elastomer nanocomposites

The effects of ethylene-methyl acrylate-glycidyl methacrylate (E-MA-GMA) terpolymer and three types of organoclays (Cloisite^® 15A, 25A, and 30B) on mechanical and rheological properties, and morphology of impact modified polyamide-6/montmorillonite ternary nanocomposites were investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), parallel disk rheometry, melt flow index measurements, and tensile and impact tests. The materials were prepared by melt blending using a co-rotating twin-screw extruder. XRD and TEM analyses showed that exfoliated-intercalated nanocomposites were formed in both polyamide-6/Cloisite^® 25A and Cloisite^® 30B binary nanocomposites and in ternary systems. SEM micrographs showed that rubber domain sizes were larger in the nanocomposites than in their corresponding polyamide-6/elastomer blends. Generally, tensile strength, Young's modulus, and elongation at break decreased with the addition of elastomer to polyamide-6/organoclay binary nanocomposites. In the melt state, liquid-like behavior of polyamide-6 slightly turned to pseudo solid-like in the binary and ternary nanocomposites. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

PE-LD电缆料支链的表征及其对加工性能的影响

研究了不同低密度聚乙烯电缆料的力学性能差异,并对其支链结构进行了研究。通过动态流变性能分析和表征各电缆料的长支链含量指数（LCBI）,进一步采用拉伸流变对长支链含量进行了表征和验证,同时利用红外分析法测定了不同电缆料的总支链含量。结果表明,在相对分子质量、总支链含量相近的前提下,短支链含量的增加有利于降低反应活化能,提高加工性能,有利于交联;而长支链可增加缠结点,增大反应活化能,不利于交联。     

A rheological evaluation of linear and branched controlled-rheology polypropylenes

Commodity linear and branched polypropylene resins have been modified by means of peroxide initiated chemical degradation in a reactive extrusion process. Data collected from molar mass and linear viscoelastic property measurements have been used to evaluate the L“crossover modulus” and “modulus separation” rheological polydispersity measures and a theoretical justification is provided for the modulus separation index. In the past, these empirical methods have been used successfully to relate molar mass characteristics to rheological properties. Results obtained in this study confirm the validity of the modulus separation index for linear polymers and suggest that it should be used carefully in the analysis of data from branched polymers. Linear viscoelastic data are used to estimate the terminal relaxation time spectra of both the linear and branched materials and a new correlation between modulus separation and relaxation time polydispersity is given.     

Prediction of rheological properties of well-characterized branched polyethylenes from the distribution of molecular weight and long chain branches

An empirical model suggested earlier by B. H. Bersted predicting the rheological properties of linear polyethylene from the molecular weight distribution is modified to account for the rheological properties in steady shear flow of branched polyethylene as well. The shear dependent viscosity, the steady shear compliance, and extrudate swell could be calculated from the distributions of molecular weight and long-chain branching in good agreement with experimental data for eight commercial low density polyethylenes. Tanner's equation was used to calculated die-swell from the modified Bersted model. The steady shear compliance was found to be related to \documentclass{article}\pagestyle{empty}\begin{document}$(\overline {gM} )_z .(\overline {gM} )_{z + 1} /(\overline {gM} )_w^2 .$\end{document}.     

Rheological Properties of Different Film Blowing Polyethylene Samples Under Shear and Elongational Flow

Summary: The rheological behavior of polyethylenes is mainly dominated by the molecular weight, the molecular weight distribution and by the type, the amount and the distribution of the chain branches. In this work a linear metallocene catalyzed polyethylene (m‐PE), a branched metallocene catalyzed polyethylene (m‐bPE), a conventional linear low density polyethylene (LLDPE) and a low density polyethylene (LDPE) have been investigated in order to compare their rheological behavior in shear and in elongational flow. The four samples have similar melt flow index and in particular a value typical of film blowing grade. The melt viscosity has been studied both in shear and in isothermal and non‐isothermal elongational flow. The most important features of the results are that in shear flow the m‐PE sample shows less pronounced non Newtonian behavior while in the elongational flow the behavior of m‐PE is very similar to that of the linear low density polyethylene: the narrower molecular weight distribution and the better homogeneity of the branching distribution are reasonably responsible for this behavior. Of course the most pronounced non‐linear behavior is shown, as expected, by the LDPE sample and by the branched metallocene sample. This similar behavior has to be attributed to the presence of branching. Similar comments hold in non‐isothermal elongational flow; the LDPE sample shows the highest values of the melt strength and the other two samples show very similar values. As for the breaking stretching ratio the opposite is true for LDPE while m‐PE and LLDPE show higher values. The transient isothermal elongational viscosity curves show that the branched samples show a strain hardening effect, while LLDPE and m‐PE samples present a linear behavior.

Dimensionless flow curves of different polyethylene samples.     

Triple-detector GPC characterization and processing behavior of long-chain-branched polyethylene prepared by solution polymerization with constrained geometry catalyst

Fourteen long-chain branched (LCB) polyethylene (PE) samples were prepared by a constrained geometry catalyst. The PE samples had average branching frequencies of 0.06-0.98 branches per polymer chain, as determined by the nuclear magnetic resonance spectroscopy (¹³C NMR). These samples, as well as five linear PEs were characterized using a gel permeation chromatography (GPC) coupled with online three-angle laser light scattering (LS), differential refractive index (DRI), and viscosity (CV) detectors. The root mean-square radius of gyration intrinsic viscosity ([η]), and molecular mass (M) of the PEs were measured for each elution fraction. Based on the comparison of the long-chain branching (LCB) PEs with their linear counterparts and the Zimm-Stockmayer equation, the distributions of long-chain branch frequency (LCBF) and density (LCBD) as function of molecular mass were estimated. It was found that although the LCBF increased with the increase of molecular mass, the LCBD showed a maximum value in the medium molecular mass range for most of the PE samples. The average LCBD data from the GPC analysis were in good agreement with the ¹³C NMR measurements. The rheological properties and processing behavior of these samples were also assessed. While the long chain branching showed significant effects on the modulus and viscosity, it did not improve the processing. Compared to linear PE, polymer melt flow instabilities such as sharkskin, stick-slip and gross melt fracture developed in extrusion of LCB PEs occurred at lower wall shear stresses and apparent shear rates.     

Dynamic mechanical and rheological properties of metallocene-catalyzed long-chain-branched ethylene/propylene copolymers

The dynamic mechanical and rheological properties of five long-chain branched (LCB) and three linear ethylene/propylene (EP) copolymers were investigated and compared using a dynamic mechanical analyzer (DMA) and an oscillatory rheometer. The novel series of LCB EP copolymers were synthesized with a constrained geometry catalyst (CGC), [C₅Me₄(SiMe₂N^tBu)]TiMe₂, and had various propylene molar fractions of 0.01-0.11 and long-chain branch frequencies (LCBF) of 0.05-0.22. The linear EP copolymers were synthesized with an ansa-zirconocene catalyst, rac-Et(Ind)₂ZrCl₂ (EBI), and contained similar levels of propylene incorporation as the CGC copolymers, but no LCB. In dynamic mechanical analysis, the dynamic storage moduli (G′) and loss moduli (G″) of the copolymers decreased with an increase of propylene molar fraction. The α- and β-transitions of the CGC copolymers were overlaid with each other. High damping (tan δ) values were found with the CGC copolymers at temperatures below 0 °C. In oscillatory rheological analysis, compared to the linear EBI counterparts, the LCB CGC copolymer melts showed higher zero shear activation energies, broader plateaus of δ and larger elastic contributions, which are essential characteristics of LCB polymers. It was found that the long chain branching was the determining factor in controlling rheological properties of the polymer melts while the short chain branching from propylene incorporation played a decisive role in affecting dynamic mechanical properties. This work represents the first rheological evidence of LCB in EP copolymers synthesized with CGC.     

Control on the topological structure of polyolefin elastomer by reactive processing

The feasibility of preliminary tailoring of the long chain branched (LCB) polymer through complex flow field was evaluated in the torque rheometer, for the reaction of melt polyolefin elastomer (POE) with peroxides at elevated temperatures. With the compensation of temperature, the strength of complex shear flow could be the only factor affecting the reaction kinetics and mechanism. The results of sample characterization by the rheological and dilute polymer solution methods indicated that the degradation mainly made the length of LCB arm shorter and shorter as the rotational speed increases. Extremely, a certain amount of LCB degraded to be linear chains again due to the scission approaching the branching point at intense mixing condition. One new LCB index (D_LCB) was defined from nonlinear oscillatory shear, and a nearly linear relationship between it and long chain branching index (LCBI) was found, which can be a map to quantify LCB level by Fourier Transform Rheology (FTR).