Analytic Rheology of Long-Chain-Branched Polymers

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The development of new catalysts, including metallocenes, has made possible a wide range of new commercial polymers, including polymers with controlled levels of long-chain branching. Existing analytic methods are not sufficiently sensitive the presence of long-chain branching to detect the very low branching levels that are often present in commercial melts. Yet such low branching levels have large effects on polymer processing, even at levels of less than 1 long-chain branch per 100,000 backbone carbon atoms. Rheology has already been developed as a highly sensitive analytic tool for determination of molecular weight distributions in commercial linear polymers. The PI will use rheology to infer the long-chain branching (LCB) characteristics of commercial polyethylene polymers. Recent theory combines the effects of reptation, primitive-path fluctuations, and constraint release into a unified theory of relaxation of polymers of arbitrary branch composition. The theory has successfully predicted the linear viscoelastic properties of model polybutadiene successfully predicted the linear viscoelastic properties of model polybutadiene linear, star, and H polymers, as well as bidisperse and polydisperse mixtures of linears and of stars with linears. However, the theory is less successful for more complex melts, in large part because of uncertainties about the conditions about the conditions under which constraint release should be thought of as a "tube reorganization" vs. "tube dilation" process. The next stage, proposed here, is to extend these theoretical ideas and test them by obtaining experimental data on more complex mixtures of linear, star, and comb polybutadienes designed to mimic commercial melts. Then, this approach will be carried over to hydrogenated polybutadienes, which are chemically identical to polyethylene, but can be made in ideal, nearly monodisperse form.%%%This work will begin the task of deducing the branching structure of commercial polyethylenes from their rheology. In the process, we expect to acquire a much deeper and more thorough fundamental understanding of relaxation processes in polymers.