Structural insights into self-assembly of a slow-evolving and mechanically robust supramolecular gel via time-resolved small-angle neutron scattering

Abstract

The supramolecular assembly process is a widespread phenomenon found in both synthetically engineered and naturally occurring systems, such as colloids, liquid crystals and micelles. However, a basic understanding of the evolution of self-assembly processes over time remains elusive, primarily owing to the fast kinetics involved in these processes and the complex nature of the various non-covalent interactions operating simultaneously. With the help of a slow-evolving supramolecular gel derived from a urea-based gelator, we aim to capture the different stages of the self-assembly process commencing from nucleation. In particular, we are able to study the self-assembly in real time using time-resolved small-angle neutron scattering (SANS) at length scales ranging from approximately 30 Å to 250 Å. Systems with and without sonication are compared simultaneously, to follow the different kinetic paths involved in these two cases. Time-dependent NMR, morphological and rheological studies act complementarily to the SANS data at sub-micron and bulk length scales. A hollow columnar formation comprising of gelator monomers arranged radially along the long axis of the fiber and solvent in the core is detected at the very early stage of the self-assembly process. While sonication promotes uniform growth of fibers and fiber entanglement, the absence of such a stimulus helps extensive bundle formation at a later stage and at the microscopic domain, making the gel system mechanically robust. The results of the present work provide a thorough understanding of the self-assembly process and reveal a path for fine-tuning such growth processes for applications such as the cosmetics industry, 3D printing ink development and paint industry.