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Tunable Polymer Networks Based on Specific Hydrogen Bonding Motifs

Kamlesh P. Nair, School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, GA 30332, Marcus Weck, Department of Chemistry, New York University, New York, NY 10003, and Victor Breedveld, Chemical and Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Drive, Atlanta, GA 30332-0100.

We have investigated the rheology of polymer networks in which the interpolymer crosslinks are based on controlled hydrogen bonding between polymer side groups and small-molecule crosslinking agents. Hydrogen bonds are noncovalent interactions, like metal coordination, Coulombic and dipole-dipole interactions, and therefore reversible under the appropriate environmental conditions. Hydrogen bonds are readily broken at elevated temperatures and can thus be used to form reversible, thermosensitive polymer networks.

Another unique property of hydrogen bonds as intermolecular interactions is that multiple bonds are often combined in specific molecular recognition motifs. Thymine and cyanuric acid are both well-known examples of compounds that form strong, multiple hydrogen bonds with receptor molecules that possess complementary acceptor-donor motifs. Thymine, one of the bases in DNA, forms 2-point hydrogen bonds with adenine in DNA, but can also form 3-point hydrogen bonds with diaminotriazines. Cyanuric acid forms 3-point hydrogen bonds with melamine or diaminotriazine, and even stronger 6-point hydrogen bonds with Hamilton wedge receptors.

For this study, a variety of co- and terpolymers was synthesized via ring-opening metathesis polymerization (ROMP) of three norbornene-based monomers with different functional side chains: an inert alkane (C8), cyanuric acid, and thymine. The alkane-based spacer monomers were used to dilute the functional groups and enhance solubility of the polymers in 1-chloronaphthalene. Selective ditopic crosslinking molecules were also synthesized: one with two Hamilton wedge motifs and another with two diaminotriazine groups. The objective was to create a tunable system in which the strength of the network can be manipulated by selective crosslinking different polymer side chains. As controls, we also used monotopic Hamilton wedge and diaminotriazine, which should only be able to bind to one side chain at a time and therefore not act as crosslinkers.

Shear and oscillatory rheology will be shown for solutions of spacer/cyanuric acid and spacer/thymine copolymers in the absence and presence of the small molecule hydrogen-bonding receptors. It was found that, depending on the combination of polymer and crosslinking agent a wide range of rheological properties could be obtained, from highly viscous fluid to strongly elastic gel. The variability of rheological properties reflects significant variations in the underlying molecular structure, which are currently not fully understood. The tunability of polymer networks based hydrogen bonding will be highlighted by presenting the rheology of solutions of spacer/thymine/cyanuric acid terpolymers in the presence of all cross-linking agents in varying combinations and concentrations. We were able to manipulate the rheology of the terpolymer solution by taking advantage of the competitive binding of the cyanuric acid and thymine residues with the various cross-linking agents.