Minor difference in the hydrogen-bonding group structure has a major impact on the mechanical properties of polymers

Abstract

Toughness of polymer materials can be enhanced by incorporation of reversible interchain interactions such as hydrogen bonds (H-bonds), which are weaker than typical covalent bonds. Diverse interacting motifs have been designed and implemented to largely alter the properties of polymers. Herein, we report that even a subtle difference in the placement of H-bonding groups within a repeat unit leads to distinct mechanical properties of a polymer. We synthesized two types of polynorbornene-based random copolymers which differed only in the relative position of two hydroxymethyl side groups: one in the vicinal arrangement ((2,3)Diol) and the other in the geminal arrangement ((2,2)Diol). The polymer with (2,3)Diol structure showed higher stiffness and superior recoverability, while the one with (2,2)Diol structure exhibited higher stretchability, compared to each other. The combination of viscoelastic characterizations of the polymers and quantum chemical calculations of model compounds revealed that the difference in the structural flexibility of the H-bonded (2,3)Diol and (2,2)Diol structures was the key to the distinct mechanical properties of the two copolymers. Our findings open up a new pathway to flexibly and largely tune the mechanical properties of polymeric materials without a need for considerable changes to the molecular design.