Opposing Effects of Side-Chain Flexibility and Hydrogen Bonding on the Thermal, Mechanical, and Rheological Properties of Supramolecularly Cross-Linked Polyesters

We report the design of a series of polyesters containing pendant secondary amide groups to probe the cumulative effects of hydrogen bonding and chain flexibility on their thermal, mechanical, and rheological properties. Reported studies on polymers with secondary amide groups have usually focused on the effect of hydrogen bonding interactions on the mechanical, self-assembly, or self-healing properties, whereas the effect of chain flexibility has often been overlooked. In an effort to probe the cumulative effects of hydrogen bonding and chain flexibility, in this work polyesters were designed with either one or two pendant secondary amide-propyl groups and compared to a control polyester with one pendant ester-propyl group. The results show that hydrogen bonding increases glass transition temperature (T-g), Youngs modulus, and polymer brittleness. But at higher temperature (T-g + 50 degrees C), rheometry shows that the polyester containing two amide groups has the shortest chain relaxation time and the lowest zero-shear rate viscosity (eta(0)). These results are counterintuitive, since the polymer with two hydrogen bonding amide groups was expected to relax more slowly and have higher viscosity. Our results demonstrate the opposing effects of side-chain flexibility and hydrogen bonding interactions can be used as a strategy to design materials with desired rheological properties.