Low Levels Of Long Chain Branching Research Articles

Detecting very low levels of long-chain branching in metallocene-catalyzed polyethylenes

The detection of long-chain branches (LCBs) is an issue of significant importance in both basic research and industrial applications, as LCBs provide excellent means to improve the processing behavior, especially in elongation-dominated processing operations. In this article, different methods for the detection of very low amounts of LCBs in metallocene-catalyzed polyethylene are presented and compared with respect to their sensitivity. Depending on the molar mass, the zero shear rate viscosity increase factor η 0/ $\eta_{0}^{\rm lin}$ , the steady-state elastic recovery compliance $J_{e}^{0}$ , the complex modulus functions G′(ω) and G″(ω), and the thermorheological complexity were found to be sensitive. In general, the higher the molar mass, the more important the transient quantities become and the easier finding the long-chain branches gets. Although rheology is very sensitive, rheological methods in combination with size exclusion chromatography proved to be the most sensitive combination to detect even very low amounts of LCBs. Especially methods involving the elastic properties (G′(ω), $J_{\rm e}^{0}$ , and J r(t)) react very sensitively, but these are also very distinctly influenced by the molar mass distribution.

Long-chain branching in PE from Cr/aluminophosphate catalysts

Phillips Cr/silica catalysts have long been known to produce long-chain branching (LCB) in polyethylene (PE) through macromer incorporation. In fact, the manipulation of LCB levels through catalyst variables is a very useful way of controlling properties and molding behavior in commercial PE resins. Using alumina as the support, instead of silica, is one way of achieving very low levels of LCB. Adding small amounts of phosphate to the alumina base as a modifier causes a large rise in LCB levels. Phosphate also promotes the activity and melt index potential of the catalyst by contributing a large new low-MW population of PE chains. In this study LCB levels were measured and found to correlate nicely with the amount of phosphate added. The polymer from such a catalyst was then solvent-fractionated to determine the location of the LCB. It was found to be concentrated in this same low-MW polymer, to which the phosphate contributes, which suggests that the phosphate-associated Cr sites have a tendency to produce LCB. However, the incorporation of 1-hexene resulted in a flat short-chain branch (SCB) profile, indicating that there was no parallel behavior among the different sites between SCB and LCB incorporation. These results indicate a slightly different mechanistic pathway between the two monomers (macromer and 1-hexene). Furthermore the Cr sites must have incorporated macromers that were produced only at the same site, and not (as is usually portrayed) randomly from a general pool of vinyls contributed by all sites. To explain this finding, alternatives to the widely accepted random-incorporation mechanism are proposed, including a “tethered” vinyl end-group, and intra-, rather than the traditional inter-, molecular insertion of macromer.

Thermorheological properties of LLDPE/LDPE blends: Effects of production technology of LLDPE

AbstractThe thermorheological behavior of a number of LLDPE/LDPE blends was studied with emphasis on the effects of the production technology of the linear low‐density polyethylene (LLDPE) and the effects of long chain branching (LCB). Two Ziegler‐Natta LLDPE's (LL3001.32 and Dowlex2045G) and two metallocene LLDPEs (AffinityPL1840 and Exact 3128) were blended with a single low‐density polyethylene (LDPE), with all LLDPEs having distinctly different molecular weight. The weight fractions of the LDPEs used in the blends were 1, 5, 10, 20, 50, and 75%. DSC analysis has shown that the blends with metallocence LLDPEs are miscible in the crystal state, whereas for the Ziegler‐Natta, apart from the two distinct peaks of the individual components, a third peak appears which indicates the existence of a third phase that is created from the cocrystallization of components from the two blended polymers. The linear viscoelastic characterization was performed and mastercurves at 150 °C were constructed for all blends to check miscibility using the time temperature superposition principle. In addition, Van Gurp Palmen and zero‐shear viscosity versus composition were constructed to check the thermorheological behavior of all blends. In general, good agreement is found among these various methods. It was concluded that metallocene LLDPEs are more compatible with LDPE at all LDPE compositions when compared with their Ziegler‐Natta counterparts. Finally, the extensional properties of all blends were studied to examine the effects of different levels of LCB on their extensional rheological properties. It was concluded that extensional rheology is a sensitive tool capable of detecting subtle changes in the polyethylene macrostructure, that is, low levels of LCB. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1669–1683, 2008

Long-Chain-Branching Index for Essentially Linear Polyethylenes

Low levels of long-chain branching (LCB) in an otherwise linear polyethylene, such as HDPE or LLDPE, are known to affect dramatically the polymer melt rheological properties. However, reliable techniques to detect and quantify LCB at these levels (i.e., ≪1 LCB/1000 C) have not been available. In this work, the long-chain-branching index (LCBI) is developed for essentially linear, polydisperse polyethylenes, which are those containing a small level of LCB, such that the measured intrinsic viscosity is the same, within experimental error, to that calculated from MWD data, assuming the resins are linear. The long-chain-branching index (LCBI) is fundamentally related to the viscosity enhancement due to long-chain branching, and it is independent of molecular weight (MW) and molecular weight distribution (MWD). It is given by the relation: LCBI = ηo0.179/4.8[η] − 1, where ηo (P) is the zero shear viscosity at 190 °C, and [η] (dL/g) is the intrinsic viscosity in trichlorobenzene at 135 °C. A suitable average mo...

Olefin polymerization using supported metallocene catalysts: development of high activity catalysts for use in slurry and gas phase ethylene polymerizations

Reaction of hydroxylated silica and alumina supports with methyl aluminoxane in toluene suspension provides chemically modified supports suitable for use in slurry and gas-phase polymerizations of ethylene or propylene on treatment with a variety of metallocene dichloride complexes. In particular, aluminas derived from calcination of sol–gel precursors feature high degrees of surface hydroxylation in comparison with commercially available silica (or even alumina) of similar surface area and total porosity. This feature provides a mechanism for increasing the amount of aluminoxane on the former supports, such that commercially acceptable productivities (>10 kg PE/g support×h) are observed at relatively low, total levels of aluminoxane or other alkylaluminum compounds in slurry or gas-phase polymerizations, respectively. A variety of evidence indicates that leaching of active catalyst from these alumina supports occurs to a minor extent under slurry conditions, particularly at higher temperatures in the presence of additional aluminoxane. At lower temperatures, this does not occur to an appreciable extent but the morphology and bulk density of the polymer formed is unsuitable for use in a gas-phase process. This can be attributed to the method for synthesis of the sol–gel alumina precursor which results in irregular particles with a broad particle size distribution. Copolymerization of ethylene with 1-octene or 1-hexene results in formation of linear, low density, PE with a narrow composition distribution as revealed by temperature rising elution fractionation. These studies indicate that less comonomer is incorporated using these supported metallocene catalysts than their soluble analogues under otherwise identical conditions. Finally, some of the resins prepared under slurry conditions (and to a lesser extent in a gas-phase process), exhibit properties consistent with the presence of low levels of long-chain branching; this feature appears to be reasonably general for a variety of simple metallocene complexes.