Analytic Rheology of Polydisperse and Long-chain-branched Polymers

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9807262 Larson This is a GOALI award between the University of Michigan and Rheometric Scientific Co.,supported jointly by the Division of Materials Research and the Office of Multidisciplinary Activities of the Directorate for Mathematical and Physical Sciences. Recent commercial development of metallocene-catalyzed polymers greatly increases the prospects for molecular tailoring of molecular weight distribution (MWD) and long-chain branching (LCB) distributions for optimal polymer processing characteristics. However, progress requires a much more detailed understanding of the impact of MWD and LCB on the polymer rheological properties. It has already been shown that linear rheological data can be used to infer the MWD of unbranched polymers. Inferring LCB characteristics, however, is much more difficult, and cannot be done using linear viscoelastic data alone, since without additional information, there is no unique linear viscoelastic signature of LCB which could not also be interpreted as a peculiar molecular weight distribution. Fortunately, recent theoretical advances open up the possibility of using nonlinear rheological data to quantify both the MWD and LCB characteristics of entangled melts and concentrated solutions of flexible polymers. To exploit and enhance these new capabilities, an experimental program that combines recent advances in rheological instrumentation with the availability of polymer synthetic methods will be carried out. The group will use a new commercial extensional rheometer and compliance-free and compliance-corrected shear rheometers to study the time dependent nonlinear rheological properties of well characterized blends of linear polymers, and ideal, monodisperse, star and "pom- pom" branched polymers. The nonlinear data will be compared in detail with newly developed theories. Efforts to develop robust methods for distinguishing mixtures of linear polymers from branched polymers, using rheological methods alone will be targeted. %%% Such research has powerful potential applications, since it could turn rheometers into analytic instruments for determining polymer MWD and LCB. In addition, this GOALI research program is an important educational tool, since it will bring graduate students into close contact with a major commercial instrument developer, and will require of them a thorough knowledge both of the most powerful experimental tools and the most recent theoreticai ideas in the field of polymer rheology. ***