Modulating mechanical properties of self-assembled polymer networks by multi-functional complementary hydrogen bonding

Abstract

The mechanical properties of reversible polymer networks have been modulated successfully at room temperature with a high degree of control over a large magnitude exclusively by altering the complementary hydrogen bonding interactions used for the inter-chain crosslinking process. For these studies, norbornene-based copolymers have been synthesized with multiple functional side-chains that offer different hydrogen bonding motifs. By adding small molecule crosslinking agents with complementary motifs to solutions of these copolymers, self-assembled polymer networks with tunable mechanical properties were obtained. The hydrogen bonding motifs utilized in this study are based on thymine/2,4-diaminotriazine and cyanuric acid/Hamilton wedge pairs. It was found that the mechanical properties of the self-assembled polymer networks strongly depend upon the type of hydrogen bonding motif used for the inter-chain crosslinking as well as the concentration of crosslinking agent. We were able to modulate the rheological properties of the networks from highly viscous to highly elastic and vary the dynamic moduli over five orders of magnitude at room temperature. This degree of control over the network's mechanical properties was achieved without changing the copolymer backbone architecture. Finally, competitive hydrogen bonding of various motifs was used to de-crosslink and re-crosslink the network at room temperature through the selective addition of various crosslinking agents. In addition to the more common thermal responsiveness of hydrogen bonded networks, competitive binding offers an additional parameter to control the mechanical properties of the self-assembled polymer networks at ambient temperature.