A new procedure to fractionate ethylene/α-olefin copolymers using DSC is presented. This procedure allows melt/melt and melt/solid segregation to occur during thermal cycles that promote self-nucleation, crystallization and annealing processes (Successive Self-Nucleation/ Annealing, SSA). The SSA has been compared with the Step-Crystallization (SC) method proposed earlier in the literature to qualitatively characterize chain branching distribution in a faster and easier way than Temperature Rising Elution Fractionation (TREF). In general, SSA produces better fractionation than SC and the DSC derived chain branching distribution by SSA can be qualitatively comparable to that obtained by TREF. The SSA technique could have important applications for the characterization of polymers that crystallize over a broad temperature range.

Successive self-nucleation/annealing (SSA): A novel technique to study molecular segregation during crystallization

A study was made on the effects of polymerization conditions on the long-chain branching, molecular weight, and end-group types of polyethene produced with the metallocene-catalyst systems Et[Ind]2ZrCl2/MAO, Et[IndH4]2ZrCl2/MAO, and (n-BuCp)2ZrCl2/MAO. Long-chain branching in the polyethenes, as measured by dynamic rheometry, depended heavily on the catalyst and polymerization conditions. In a semibatch flow reactor, the level of branching in the polyethenes produced with Et[Ind]2ZrCl2/MAO increased as the ethene concentration decreased or the polymerization time increased. The introduction of hydrogen or comonomer suppressed branching. Under similar polymerization conditions, the two other catalyst systems, (n-BuCp)2ZrCl2/MAO and Et[IndH4]2ZrCl2/MAO, produced linear or only slightly branched polyethene. On the basis of an end-group analysis by FTIR and molecular weight analysis by GPC, we concluded that a chain transfer to ethene was the prevailing termination mechanism with Et[Ind]2ZrCl2/MAO at 80 °C in toluene. For the other catalyst systems, β-H elimination dominated at low ethene concentrations. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 376–388, 2000

Influence of the catalyst and polymerization conditions on the long-chain branching of metallocene-catalyzed polyethenes

The damping function has been a concept introduced in rheology since more than 30 years ago, and although a similar concept was already earlier implemented in studying rubber materials, its implementation in the modeling of polymer melts was an essential step forward in the classification and understanding of nonlinear viscoelasticity phenomena. It is the objective of this contribution to give an overview on the theoretical background and physical interpretation of the concept of the damping function for different types of deformation, as well as a review on the experimental results including the experimental artefacts to be considered. Besides homopolymers, a summary is given on different investigations of other types of systems, where the concept of the damping function has also been applied, for example, rubbers, rubber-like materials, block copolymers, polymer composites, liquid crystals, polymer blends, suspensions, emulsions, micellar systems, and in food rheology.

The damping function in rheology

In the last decade, there have been several investigations into the route by which a polymer melt that has been subjected to flow transforms into a crystalline state. Crystallization of polymers from flowing or stressed melts is a technologically important problem since most thermoplastics are subjected to intense flow fields during normal processing operations. The renewed interest in this problem is due to the availability of new experimental tools that, coupled with advances in modeling and simulations, are beginning to yield insights into the molecular characteristics that control the crystallization pathways adopted by a stressed polymer melt. Recent evidence conclusively implicates the high molecular weight tail in promoting crystallization on shearing. It appears that the formation of oriented nuclei in polydisperse melts is controlled by chain orientation of a small fraction of high molecular weight chains in a melt. Thus, it is the rheological criteria for “critical” chain orientation that govern...

Crystallization of Polymers from Stressed Melts

Rheological characterization of polyethylene fractions

Material functions of two engineering plastics [a poly(phenylene ether) and a poly(ether imide)] were characterized, including the shear viscosity, first normal stress coefficient, storage and loss moduli, growth and relaxation of shear stress, and first normal stress coefficient and relaxation moduli. The oscillatory shear and relaxation moduli data were employed to determine the temperature‐dependent parameters of Wagner model. Various material functions, which were determined on the basis of this model in conjunction with the fitted parameters agreed reasonably well with the experimental results. The reported data and parameters should facilitate a better understanding of the processability characteristics of these two engineering plastics.

Melt Rheology of Two Engineering Thermoplastics: Poly(ether Imide) and Poly(2,6-Dimethyl-1,4-phenylene Ether)

The effects of the higher alpha-olefin comonomers on the rheology and processing behavior of linear low density polyethylenes were investigated. Four linear low density polyethylene resins polymerized with butene, hexene and octene comonomers and a low density polyethylene were characterized in terms of their shear viscosity, first normal stress difference, storage and loss moduli, shear stress growth, stress relaxation upon step strain, stress relaxation upon the cessation of steady shear, and uniaxial extensional stress growth material functions. Differences in first normal stress difference, relaxation moduli, damping function parameters, and uniaxial extensional stress growth behavior were noted. The observations were elucidated in terms of the processability considerations in blown film extrusion process.

Dong-Woo Yu

Papers

Melt Rheology of Two Engineering Thermoplastics: Poly(ether Imide) and Poly(2,6-Dimethyl-1,4-phenylene Ether)

Rheology and processing of linear low density polyethylene resins as affected by alpha‐olefin comonomers