Complementary Hydrogen-Bonded Thermoreversible Polymer Networks with Tunable Properties

Abstract

Complementary hydrogen bonded cross-linked polymer networks based on two distinct hydrogen bonding recognition motifs have been synthesized by using a combination of ring-opening metathesis polymerization and hydrogen bonding interactions and were subsequently characterized in solution using rheometry. The hydrogen bonding recognition units were based on either three-point cyanuric acid−2,4-diaminotriazine or six-point cyanuric acid−Hamilton wedge interactions. Through the addition of “ditopic cross-linking agents”, the polymer scaffold, which was functionalized with cyanuric acid functional groups, was noncovalently cross-linked in solution through complementary interchain hydrogen bonding interactions. The extent of cross-linking could be controlled by varying the amount of the cross-linking agent added. These networks are thermally reversible and have highly tunable mechanical properties that are controlled by the molecular structure of the cross-linking agent. While the addition of the Hamilton wedge cross-linking agent to the polymer solution led to high-viscosity fluids, the 2,4-diaminotriazine cross-linking agent produced highly viscoelastic gels. It is hypothesized that this is due to a higher degree of connectivity between the cross-linking agent and the polymer in spite of the inherently weaker hydrogen bonding (three- vs six-point). The study shows that the microstructure plays an important role in the macroscopic mechanical properties of these hydrogen bonded networks in solution. By varying the hydrogen bonding motif, materials with tunable rheological properties were obtained from the same parent polymer backbone. Such a strategy will allow for materials design by tailoring the network microstructure via the molecular architecture of the cross-linking agents.