Rheology is more sensitive to polymer architecture, including molecular weight distribution and long-chain branching distribution, than is any other measurable property. Thus there is great motivation to carry out rheological measurements on well-chosen polymer melts and solutions that will lead to improved ability to link rheology to polymer molecular weight and branching distributions. Three new projects are proposed. First, the effects of polydispersity in arm and backbone length on the linear viscoelasticity of well defined entangled H polymers, which have two branch points and four arms, will be studied. Recent models show that H-branched polymers should have rheology that is very sensitive to polydispersity, so that these studies, along with corresponding theoretical work, will allow strong tests of these theories to be carried out, and help in the development of a more accurate theory of branch point motion. Second, using model materials from Dow Chemical Co., linear viscoelastic data will be obtained that will be sensitive to higher order effects of branch point motion and used to test the ability of an existing algorithm, the hierarchical model, to predict accurately these data. The third problem will be to test the principle of universal scaling of linear and nonlinear rheological properties in polymer solutions, which holds that all such properties, when measured at a fixed value of the ratio of c/ce, the concentration to the entanglement concentration superpose, when plotted in dimensionless form. This concept will be tested thoroughly by measuring sets of linear and nonlinear data for different molecular weights, at several fixed values of c/ce and by extending these tests to binary blends of linear polymers.
NON-TECHNICAL SUMMARY
Over 200 billion pounds of polymers are produced commercially worldwide, and shaped into a wide variety of products. The shaping processes depend on their flow properties, or rheology, which, in turn, depends on molecular characteristics, such as molecular weight distribution and branching distribution. Thus, there is a huge industrial interest in better defining the relationship between polymer molecular structure and rheology. The proposed work will increase knowledge of this relationship, leading to polymers that are designed better at the molecular level. Insights from this grant, and past grants from the same NSF program, will assist in collaborations the proposer has with Dow Chemical Co. and Procter and Gamble. A new book, co-authored by the proposer, Molecular Structure and Rheology of Molten Polymers, published this year, contains work from the previous NSF grants, and the work to be carried out under the new grant will similarly be well publicized, both to U.S. industry and overseas, through international collaborations. The work data will also lead to improvements in a commercial software package IRIS, for predicting rheological properties of polymer melts, that is used widely in industry.
