Direct evidence of stress-induced chain proximity in a macromolecular complex

Abstract

The mechanical properties of many supramolecular materials are often determined by non-covalent interactions that arise from an interplay between chemical composition and molecular microstructural organization. The reversible nature of non-covalent interactions gives supramolecular materials responsive properties that are otherwise difficult to obtain, such as becoming rigid as a response to mechanical stress. How exactly non-covalent interactions emerge from microstructure, how they might change in response to applied force or deformation is not understood. Here we combine Nuclear Magnetic Resonance (NMR) and rheology to directly probe the role of chain proximity in polymer complexes. We observe an increase in chain proximity in response to imposed flow, which we hypothesize to originate from enhanced hydrogen bonding. The chain proximity is directly correlated to rod climbing and shear banding. Flow persists only when applied stresses are low, suggesting a stress-induced thickening mechanism. We verify that hydrogen bond disruptors can turn off both the non-trivial flow behavior and the spectroscopic evidence of chain proximity. The combined rheo-NMR approach shows that it is possible to directly observe the molecular origins behind supramolecular mechanics, paving the way for further study into mechano-chemical properties of supramolecular materials.