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.     

Influence of molecular structure on the rheology and thermorheology of metallocene polyethylenes

The rheology of linear and branched metallocene polyethylenes (m‐PEs) was investigated. The linear metallocenes were prepared by gas‐phase polymerization, while the branched PEs were commercial resins. Molecular parameters such as M_w, branch type, and molecular weight distribution have influenced the viscoelastic behavior of both linear and branched PEs, whereas branch content (BC) had little influence on viscoelastic properties. Plots of log G′ versus log G″ revealed the effect of comonomer type on the viscoelastic behavior of m‐PEs. Flow activation energy (E) was found to be sensitive to both M_w and BC. Also, E for ethylene‐octene copolymers was observed to be always higher than the butene counterparts, which have been caused by the increase in molar volume of the repeating unit. For the effect of BC on E, different trends were observed for octene and butene m‐LLDPEs. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 1717–1728, 2006     

Gross melt fracture of polyethylene. II: Effects of molecular structure

The effect of molecular structure (MW, MWD and LCB) on the critical tensile stress (σ_c) for the onset of gross melt fracture (OGMF), proposed in Part I (1) as a material‐dependent criterion for fracture, was determined for a group of polyethylenes varying in structure. These included linear low and high‐density polyethylenes and several materials produced using metallocene and constrained geometry catalysts. It was found that the critical stress is independent of M_W, for constant polydispersity but increases with increasing long chain branching and polydispersity. The addition of boron nitride particles had no effect on the σ_c up to a level of 0.5% by weight.     

Long chain branching and polydispersity effects on the rheological properties of polyethylenes

The rheological behavior of linear, and branched polyethylenes is studied as a function of the weight average molecular weight (M_w) and its distribution (MWD) as well as the level of long chain branching in an attempt to identify correlations between long chain branching, polydispersity and rheological properties. It is found that a need for vertical shift of the viscoelastic moduli data to obtain the master curves using the time‐temperature superposition principle is associated with the existence of long chain branching in the structure of the polymer. The degree of vertical shift is found to correlate with the level of long chain branching. This correlation corroborates with the observation that long chain branching correlates with the horizontal flow energy of activation. Plots of atan(G″/G′) vs. G* (known as Van Gurp plots) also reveal some important features that can be used as signs of specific features in the structure of polymers. More specifically, the area included below the Van Gurp curves correlates with the level of long chain branching and polydispersity index. The correlations are presented in graphical form and they can be used to associate rheological properties with the presence of long chain branching and/or polydispersity.     

Untersuchungen zur Langkettenverzweigung in Polyäthylenen

Polyethylene samples of various densities and melt flow indices resulting from different polymerization processes have been investigated with respect to long chain branching (LKV). For that purpose several polymer fractions have been characterized by measurement of weight average molecular weights M_w and intrinsic viscosities [η], the latter ranging from 0,2 to 3,2 with high pressure samples and from 0,2 to 10 with low pressure material. The intrinsic viscosity difference of branched (high pressure) polyethylene compared to linear (low pressure) polyethylene is used as a measure of LKV. With high pressure polyethylene LKV increases with decreasing density. This dependence is strongest within the medium molecular weight range. Samples with varying LKV but constant density can be obtained by appropriate change of polymerization conditions. No LKV has been observed with low pressure polyethylene. This means a marked difference compared to high pressure material of equal density. Branching with low pressure polymers can therefore be ascribed to the short chain type only, which in particular results from copolymerization. Several mathematical approaches have been checked whether or not they can yield suitable information about n, the number of long chain branches per molecule. The best fit with our experimental data is obtained using the expression [η]v/[η]₁ = g^1,3 (n = f(g)) and assuming, that the average concentration of long chain branch points does not depend on molecular weight for fractions of the same sample (n/M = const.). If LKV ist taken into consideration, logarithmic normal molecular weight distributions are obtained for many high pressure polyethylenes (similar to low pressure material). Data are reported in support of the view, that performance characteristics are dependent on LKV. There is some evidence, that melt flow properties of polyethylene are improved with increasing LKV.     

Shear and elongational rheology of polyethylenes with different molecular characteristics. I. Shear rheology

Four metallocene polyethylenes (PE), one conventional low density polyethylene (LDPE), and one conventional linear low density polyethylene (LLDPE) were characterized in terms of their complex viscosity, storage and loss moduli, and phase angle at different temperatures. The effects of molecular weight, breadth of molecular weight distribution, and long‐chain branching (LCB) on the shear rheological properties of PEs are studied. For the sparsely long‐chain branched metallocene PEs, LCB increases the zero‐shear viscosity. The onsets of shear thinning are shifted to lower shear rates. There is also a plateau in the phase angle, δ, for these materials. Master curves for the complex viscosity and dynamic moduli were generated for all PE samples. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Formation of branched polyethylenes by ethylene homopolymerization using LNiBr2 homo- and heterogeneous precatalysts: Interpretation of the polymer structures in comparison with commercial LLDPE

Comparative data on the micro-structures and properties of branched polyethylenes (BPE) produced via ethylene homopolymerization over homogeneous N,N-α-diimine LNiBr₂ complexes with different ligand composition (AlEt₂Cl as a cocatalyst) and corresponding supported catalysts LNiBr₂/SiO₂(Al) (Al[iso-Bu]₃ as a cocatalyst) are presented. Noticeable differences were observed between micro-structures of BPEs obtained using homo- and heterogeneous LNiBr₂ complexes as catalysts. Supported catalysts produce BPEs with the majority of methyl branches (17–18 CH₃(1000 C)⁻¹ characterized by different molecular masses (1800–210 kg mol⁻¹) and molecular weight distributions (M_w[M_n]⁻¹ = 5.9 and 2.6). Thermal and mechanical properties of these BPE samples obtained over supported Ni catalysts are similar to those of commercial LLDPE samples prepared with metallocene and Ziegler-Natta catalysts.     

The role of the interface in melt linear viscoelastic properties of LLDPE/LDPE blends: Effect of the molecular architecture of the matrix

The rheological properties of blends consisting of a long chain branched low‐density polyethylene (LDPE) and two linear low‐density polyethylenes (LLDPE) are studied in detail. The weight fractions of the LDPE used in the blends are 5 and 15%. The linear viscoelastic characterization is performed at different temperatures for all the blends to check thermorheological behavior and miscibility in the melt state. Blends containing metallocene LLDPE as the matrix display thermorheologically complex behavior and show evidences of immiscibility in the melt state. The linear viscoelastic response exhibits the typical additional relaxation ascribed to the form deformation mechanism of dispersed phase droplets (LDPE). The Palierne model satisfactorily describes the behavior of these blends in the whole frequency range explored. However, those blends with Ziegler‐Natta LLDPE as the matrix fulfill the time‐temperature superposition, but exhibit a broad linear viscoelastic response, further than the expected for an immiscible system with a sharp interface. The rheological analysis reveals that, in addition to the droplets form relaxation, another mechanism at lower frequencies exists. The broad linear response of the blends with the Ziegler‐Natta LLDPE can be explained by hypothesizing a strong interaction between the high molecular weight linear fraction of the LLDPE and the low molecular weight (almost linear) chains of the LDPE phase, forming a thick interface with its own viscoelastic properties. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Comments on the measurement of long chain branching by size exclusion chromatography

Summary A technique for measuring long chain branching as a function of polymer molecular weight uses SEC with a low angle laser light scattering (LALLS) detector to compare M_W of an eluting species with the molecular weight of the linear counterpart that has the same retention time. This technique is correct only if all species in the SEC detector cells have the same constitution. Evidence is presented that indicates that this condition prevails for low density, high pressure polyethylene and polyvinyl alcohol. Alternative forms of data representations are suggested. A major uncertainty in the data treatment is the value to be assigned to the ratio of radii of gyration of linear and branched polymers with the same molecular weight. A method is suggested to measure this ratio directly, as a function of molecular weight, if the eluting species at any instant are uniform in branching character.     

Measurement of number-average molecular weights and second viral coefficients of polyolefins using an improved vapor pressure osmometer

Operation of an improved design of a vapor pressure osmometer for polyolefins at 140°C is described. Reproducibility of ±10% of the measured number-average molecular weight (M _n) was obtained with a maximum M _n of about 45,000–50,000. Results are reported for some standard and commercial, linear and branched polyethylenes and for commercial polypropylenes.     

Calculation of the effect of macromolecular architecture on structure and thermodynamic properties of linear-tri-arm polyethylene blends from Monte Carlo simulation

A Monte Carlo simulation formalism proposed recently [Peristeras et al. Macromolecules 2007;40:2904-14] is applied here to linear-tri-arm polyethylene blends using atomistic models. Elementary Monte Carlo moves for long chain and branched molecules are used and shown to result in efficient relaxation of long chains. The effect of chain and arm molecular weight and of temperature on the structure and thermodynamic properties of blends is examined. Chemical potential versus composition diagrams are drawn in order to assess the non-ideality of mixing that may lead to phase separation. All of the blends examined are shown to be fully miscible. The microscopic blend structure is examined by calculating the radial distribution function. Finally, the radii of gyration of linear and branched chains are calculated and scaling exponents are evaluated.     

Thermal and rheological studies on the molecular composition and structure of metallocene‐ and Ziegler–Natta‐catalyzed ethylene–α‐olefin copolymers

The relationship between the molecular structure and the thermal and rheological behaviors of metallocene‐ and Ziegler–Natta (ZN)‐catalyzed ethylene copolymers and high‐density polyethylenes was studied. Of special interest in this work were the differences and similarities of the metallocene‐catalyzed (homogeneous) polymers with conventional coordination‐catalyzed (heterogeneous) polyethylenes and low‐density polyethylenes. The short‐chain branching distribution was analyzed with stepwise crystallization by differential scanning calorimetry and by dynamic mechanical analysis. The metallocene copolymers exhibited much more effective comonomer incorporation in the chain than the ZN copolymers; they also exhibited narrower lamellar thickness distributions. Homogeneous, vanadium‐catalyzed ZN copolymers displayed a very similar comonomer incorporation to metallocene copolymers at the same density level. The small amplitude rheological measurements revealed the expected trend of increasing viscosity with weight‐average molecular weight and shear‐thinning tendency with polydispersity for the heterogeneous linear low‐density polyethylene and very‐low‐density polyethylene resins. The high activation energy values (34–53 kJ/mol) and elevated elasticity found for some of our experimental metallocene polymers suggest the presence of long‐chain branching in these polymers. This was also supported by the comparison of the relationship between low shear rate viscosity and molecular weight. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 1140–1156, 2002     

Effect of molecular structure and topology on network formation in peroxide crosslinked polyethylene

The present study compared the crosslinking performance of single site linear low density polyethylenes (LLDPE) with high pressure, free radical polymerised, low density polyethylenes (LDPE). The difference in crosslinking performance is not fully explained by different structural parameters such as molar mass distribution (MMD), M_n, MFR₂ value and vinyl groups but is related more to the phenomenon of a long chain branched LDPE macromolecule being smaller in size in the molten state than a macromolecule of a linear LLDPE sample of the same molar mass. The result of the difference in size is that the LDPE will contain a larger number of intramolecular crosslinking points than the LLDPE, which, on the other hand, will contain a larger fraction of intermolecular crosslinking points. The crosslinking points mentioned are of either a physical or chemical nature. From the perspective of the network build-up, the intermolecular crosslinking points are the most efficient. To compensate for the larger fraction of intramolecular crosslinking points in LDPE, more peroxide can be added or vinyl groups can be introduced.     

Crystalline and viscoelastic properties of branched polyethylenes synthesized using bidentate nickel (II) catalyst

In this work, five branched polyethylenes with different branching units were synthesized using bidentate nickel (II) catalyst containing -diimine ligands. For comparison, one linear polyethylene was also prepared using tridentate iron (II) catalyst containing -diimine ligand. The crystalline structure of the polyethylenes was investigated using X-ray diffraction (XRD) and polarized optical microscope. The crystalline properties were also measured by differential scanning calorimeter (DSC). Viscoelastic properties of the polyethylenes were investigated using rheometric dynamic analyzer. The DSC and XRD results showed that highly branched polyethylenes exhibit no melting points and no predominating crystalline forms, while the linear polyethylene exhibits clear orthorhombic (110) and (200) reflections on XRD pattern and a clear melting point at 118 °C. The viscoelastic properties of the branched polyethylenes were very complicated due to the combined effect of the molecular weight difference and the degree of chain branching as well as the branching structure.     

Influence of long-chain branching on linear viscoelastic flow properties and dielectric relaxation of polycarbonates

The dynamic viscoelastic property, creep and creep recovery behavior, and dielectric relaxation of long-chain branched Bisphenol A polycarbonates were measured in parallel plate rheometer and dielectric analyzer. The linear polycarbonate (PC-L) as reference and three branched polycarbonates (PC-Bs) have similar molecular weights and molecular weight distributions, while the PC-Bs have different branching degrees, below 0.7 branch points/chain and above twice of M_c. The long-chain branched polycarbonates exhibit higher zero-shear viscosities, more significant shear shinning, higher flow activation energies, and much longer relaxation times. It was also found that long-chain branches increase the elasticity of melt characterized by the steady-state recoverable compliance and the storage modulus. The ‘dissident’ rheological behavior of long-chain branching exhibiting mainly in addition polymers such as polyolefin, is confirmed in condensation polymers. These behaviors resulted from additional molecular entanglements of long-chain branches can be understood qualitatively in terms of the tube model for topological constraints. The dielectric α-relaxation of linear polycarbonate and branched polycarbonates has been fitted with Vogel-Fulcher-Tammann-Hesse (VFTH) equation and the shape of relaxation time curves is also analyzed. The long-chain branched polycarbonates present longer relaxation times, but divergent α-relaxation temperatures, because the latter is dominated by the free volume.     

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.     

Crystallization and morphology of metallocene polyethylenes with well‐controlled molecular weight and branching content

The crystallization and morphology of some metallocene polyethylenes with well‐controlled molecular weight and branching content were investigated by DSC, WAXD, PLM and SALS. The banded spherulites observed in linear PE are not seen in crystallization of branched PEs. The small spherulites with small lamellae or fringed micelle crystals are formed when branching content is higher, as suggested by PLM and SALS. The expansion of the unit cell was observed by WAXD as the molecular weight and branching content increased. At even higher branching content (more than 7 mol%), a shrinkage of the unit cell was seen, probably due to a change of crystal morphology from lamellar‐like crystals to fringed micelle‐like crystals. Crystallization temperature, melting point and crystallinity are greatly decreased for branched PEs compared with linear PEs. The equilibrium melting temperature cannot be determined via the Hoffman–Weeks approach for branched PEs since T_m is always 5–6 °C higher than T_c and there is no intercept with the T_m = T_c line. Our results show a predominant role of branches in the crystallization of polyethylene. © 2003 Society of Chemical Industry     

Synthesis, thermal and mechanical properties and biodegradation of branched polyamide 4

A series of linear and branched polyamide 4 were prepared and characterized in order to study the effect of the structure on thermal and mechanical properties. Polybasic acid chlorides were effective initiators for the synthesis of the branched polyamide 4. The melting points of the polyamide 4 for the high molecular weight region were near 265 °C and showed no significant difference depending on their chain structure. On the other hand, it was found that the branched polyamide 4 showed remarkable increase of tensile strength, compared to similar molecular weight of the linear polyamide 4 (e.g. four-branched type M_w=9.28×10⁴, tensile strength=72 MPa).When the initiator having branched structure were used, gel was also formed at the initiator concentration over a certain value (e.g. 3.0 mol% for 4-branched type).The biodegradation of the branched polyamide 4 was evaluated using a standard activated sludge (e.g. four-branched type M_w=8.25×10⁴, biodegradation 41%).     

Effect of molecular weight on high pressure crystallization of linear polyethylene. I. Kinetics and gross morphology

Ten linear polyethylenes ranging from M_w = 4.9 × 10⁴ to 4.6 × 10⁶ were crystallized in a dilatometer at 0.51 GPa and 242°C and then cooled slowly. Volume vs time data were used to follow the kinetics of the crystallization. The dilatometer data for the isothermal part of the crystallization were fitted to the Avrami equation. The time exponent was independent of molecular weight and the average was n = 2.2. Electron microscopy of fracture surfaces showed that all of the polyethylenes crystallized in extended chain morphology. The crystalline order and maximum extended chain crystallite thickness decreased with increasing molecular weight. The dominant morphological feature of the crystallized high molecular weight samples was a strand-like network superstructure. Attempts to stabilize the hexagonal structure formed in the isothermal part of the crystallization failed, and all specimens had only the usual orthorhombic crystal structure.     

Linear viscoelastic relaxation modulus of polydisperse poly(dimethylsiloxane) melts containing unentangled chains

This work analyzes the relationship between the shear relaxation modulus of entangled, linear and flexible homopolymer blends and its molecular weight distribution (MWD) when a fraction of the sample contains chains with molecular weight M lower than the effective critical molecular weight between entanglements Mc_eff. This effective critical parameter is defined in terms of the critical molecular weight between entanglements M_c of the bulk polymer that forms the physical network and the effective mass fraction Wc_eff of the unentangled chains. In the terminal zone of the linear viscoelastic response, the double reptation mixing rule for blended entangled chains and a modified law for the relaxation time of chains in a polydisperse matrix are considered, where the effect of chains with M<Mc_eff is included. Although chain reptation with contour length fluctuations and tube constraint release are still the relevant mechanisms of chain relaxation in the terminal zone when the polydispersity is high, it is found that the presence of a fraction of molecules with M<Mc_eff modifies substantially the tube constrain release mode of chain relaxation. In this sense, a modified relaxation law for polymer chains in a polydisperse entangled melt that includes the effect of the MWD of unentangled chains is proposed. This law is validated with rheometric data of linear viscoelasticity for well-characterized polydimethylsiloxane (PDMS) blends and their MWD obtained from size exclusion chromatography. The short time response of PDMS, which involves the glassy modes of relaxation, is modeled by considering Rouse diffusion between entanglement points of chains with M>Mc_eff. This mechanism is independent from the MWD. The unentangled chains with M<Mc_eff occluded in the polymer network also follow Rouse modes of relaxation although they exhibit dependence on the MWD.