Rheological properties branched polymers

Yan, D., W. J. Wang, and S. Zhu. 1999. Effect of long chain branching on rheological properties of metallocene polyethylene. Polymer 40 1737-1744. 

An important class of commercial polymers is that of copolymers of ethylene and alpha-olefins, which are commonly referred to as linear low density polyethylenes (LLDPE). The use of a copolymer introduces short-chain side branches onto the polyethylene backbone, and the effect of these short-chain branches on rheological properties depends very much on the method of polymerization. If a heterogeneous, Ziegler catalyst is used, the side-chains tend to be distributed in blocks rather than randomly along the backbone, and Wardhaugh and Williams [74] point out that this can lead to microphase separation in the melt, which could have an important effect on rheological behavior. 

Favorable rheological properties are an essential requirement for the commercialization of polyolefins like polyethylene. The ease of processability of the polymer melt, obtained through modifications in the microstructural features, is as important as the end use mechanical properties of these polymers. Presence of long-chain as well as short-chain branching, LCB and SCB, respectively, more or less dictates the rheological behavior of most commercial... 

If combs represent one extreme of the topological family of branched polymers, then another extreme is given by the case of dendritic polymers, which retain a branched structure at all timescales. The study of tree-like branched architectures is also motivated by the important commercial low density polyethylene (LDPE), which has remarkable rheological properties making it suitable for many processing operations [3]. 

The final chapter develops the most modern insights in the relation between the rheological properties and the large scale architecture of polymers. Indeed, the largest effects of branching are encountered in their melt relaxation properties. In the absence of reptation, which dominates relaxation processes in Hnear polymers, a rich variety of other relaxation processes becomes apparent. The control ot the melt properties of polymers by means of their long-chain branch architecture will continue to lead to new industrial applications. 

Rokudai, M., Mihara, S., and Pujiki, T., Influence of Shearing History on the Rheological Properties and Processability of Branched Polymers. II. Optical Properties of Low-Density Polyethylene Blown Pilms, /. Appl. Polym. Set, 23, 3289 (1979) MaiUefer, C., Screw for Extrusion Apparatus, U. S. Patent 3,358,327 (1967)... 

Graessley and co-workers have studied the rheological properties of solutions of branched PVAc in diethyl phthalate (178, 188), using polymer concentrations of 0.17, 0.225, and 0.35 g ml-1. At the lowest concentration, the low shear-rate viscosity was simply related to [17], so that it was lower for branched polymers the equation ... 

Nam GJ, Yoo JH, Lee JW (2005) Effect of long-chain branches of polypropylene on rheological properties and foam-extrusion performances. J Appl Polym Sci 96 1793-1800... 

The chains of industrially produced polymers contain hundreds to thousands of monomeric units. The properties of polymers strongly depend on their structure, on the manner of attachment of the monomeric units in the chains, and on the distribution of chain lengths and shapes. Branched macromolecules in solution or melt exhibit completely different rheological properties compared with their linear counterparts. When the length and number of branches exceed some critical limit, difficulties in their processing usually occur. 

---