Processability of branched polymers

Rokudai, M., Mihara, S., and Fujiki, T., Influence of Shearing History on the Rheo-iogical Properties and Processability of Branched Polymers. II. Optical Properties of Low-Density Polyethylene Blown Films, /. Appl. Polym. Set, 23, 3289 (1979)... 

A special feature of a process of branched polymer formation is the possibility of the appearance in the reactor of a macroscopic structure whose size is comparable with that of the reactor. The main peculiarity of such a phenomenon, known as gelation, is that the range of conversions... 

Rokudai, M. and Fujiki, T., Influence of shear histories on the rheological and processibility of branched polymers, Contemp. Topics Polym. ScL, 4, 245 -258 (1984). 

Rokudai, M., Mihara, S., Fujiki, T. J. Influence of shearing history on the rheological properties and processability of branched polymers. /. AppL Polym. Sci (1979) 32, pp. 463 71... 

CLFs are a key mechanism for the relaxation at earlier times and also the basis for hierarchical relaxation processes of branched polymers, where CLFs are considered to be the fundamental process facilitating the release of side branches. 

A simple algorithm [17] makes it possible to find the probability of any fragment of macromolecules of Gordonian polymers. Comparison of these probabilities with the data obtained by NMR spectroscopy provides the possibility to evaluate the adequacy of a chosen kinetic model of a synthesis process of a particular polymer specimen. The above-mentioned probabilities are also involved in the expressions for the glass transition temperature and some structure-additive properties of branched polymers [18,19]. 

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]. 

However, branched COC chains increase the flexibility and processability of the polymers without significantly weakening the optical properties (45). 

Rheological measurements are of central importance in the processing of siloxane polymers. Typical studies would include determination of the dependence of the bulk viscosity of the material on the average molecular weight, molecular weight distribution, and rate of shear. Characterization of the effects of any branched chains or reinforcing fillers present is also of great importance.16... 

Polyvinylacetate (PVAc) has not been used in the pharmaceutical held until recently. During the polymerization, especially at high conversion, free radicals are transferred to dead polymers, resulting in the formation of branched polymers. These branched polymers are susceptible to deterioration. Because the PVAc latex particles are produced by an emulsion polymerization technique, this provides a good process for the water-based dispersion in him coatings. The main purpose of this polymer is the him coating of sustained release dosage forms. The polymer is used as a precursor in the production of polyvinylalcohol (PVA), which cannot be prepared directly by polymerization due to the unstable, isomeric monomer of acetaldehyde. 

Both statistical and kinetic methods can be used, in principle, for the description of destruction, the process inverse to polymerization. However, the main problem limiting their applicability consists in difficulties of taking into account of all opportunities for breaking bonds inside the polymeric molecules. This problem is solved for chain polymers and some of the simplest kinds of branched polymers, for instance, polymers containing a limited number of units and those with a specified structure, but not for the general case of macromolecules. 

In this system conversion affects polymer properties. We typically cannot go to high conversion because of molecular weight or heat removal constraints (if adiabatic). There may also be a large increase in viscosity that affects the heat removal, agitation, and processability of the polymer solution. Here conditions dictate the kind of molecular weight distribution. The polymer is often affected by impurities and chain transfer agents that determine the amount of branching and termination. 

The reason for this difference in behaviour of these two rubbers might be the difference in molecular structure. Mainly linear rubber molecules are polymerised in the solution process, while the emulsion process produces a product with a considerable amount of branched polymer chains. 

Hence, the performed above analysis has shown that different solvents using in low-temperature nonequilibriiun polycondensation process can result not only in symthesized polymer quantitative characteristics change, but also in reaction mechanism and polymer chain structure change. This effect is comparable with the observed one at the same polymer receiving by methods of equilibrium and nonequilibrium polycondensation. Let us note, that the fractal analysis and irreversible aggregation models allow in principle to predict symthesized polymer properties as a function of a solvent, used in synthesis process. The stated above results confirm Al-exandrowicz s conclusion [134] about the fact that kinetics of branched polymers formation effects on their topological structures distribution and macromolecules mean shape. 

Table 9.12 shows how polybutadiene microstructure and macrostructure (i.e., molecular weight, Mw, Mn, polydispersity, and branching) can affect the processability of a polymer (Kumar et al 1996). A study with both cobalt- and neodenium-catalyzed polybutadiene showed the relationship between polydispersity or molecular weight distribution and increases in stress relaxation. Increases in stress relaxation, as measured by the Mooney viscometer, will infer greater difficulty in compound processing, gauge control, nerve, and extrudate or calendered sheet shrinkage (Waddell et al 2004). 

Alkyl chains are the most commonly used side chains in conjugated polymers. They can be easily introduced to the conjugated polymers, which could improve the solubility and processing of the polymer without changing the conjugation of the main chain. Linear and branched alkyl chains are the two forms of alkyl chains. Hexyl, octyl, and dodecyl... 

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