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Linear Viscoelasticity of Associating Ionomers

Ralph H. Colby Materials Science and Engineering, Penn State University

Polymers that associate by either hydrophobic interactions, hydrogen bonding or ionic interactions have been of interest since the pioneering 1946 work of Green and Tobolsky. Ionomers can have remarkable mechanical properties that stem from ion associations, with interesting melt rheology, studied more recently by ionomer experts such as Eisenberg, Register and Weiss. Such ionomers typically have very strong ionic interactions, since the surrounding media is hydrocarbon polymer of low dielectric constant, often with high glass transition temperature. In our quest to develop polymer membranes that transport specific ions for a variety of energy materials applications, we have developed a new class of ionomers based on higher dielectric constant polymers that solvate ions. These ionomers are liquids at room temperature, with rather short chains (M 10000) yet they are strongly viscoelastic. Some even exhibit rubbery plateaus that span many decades of frequency despite their short chain lengths.

In this talk we show how simple ideas using a sticky Rouse model with three parameters can be applied to short chain ionomers. The first two parameters, glassy modulus and monomer relaxation time, are common to all molecular polymer models ; they reflect the fact that we do not yet understand well the glass transition. The third parameter is the association lifetime and for ionomers this can be directly observed in dielectric spectroscopy, as a large relaxation where ions start to move. We show examples of how this model works for polyester sulfonate ionomers based on poly(ethylene oxide), polyester sulfonate ionomers based on poly(tetramethylene oxide), and polysiloxane ionomers with a random mixture of phosphonium and poly(ethylene oxide) side chains. In each case, knowledge of the molecular weight distribution and the association lifetime from dielectric measurements allows the full linear viscoelastic response to be understood.