Self-assembled double networks of chitin nanocrystals and wormlike surfactant micelles exhibiting heat-induced viscosity and elasticity enhancement

Abstract

A novel fully self-assembled double network is proposed. It consists of a supramolecular network of entangled wormlike micelles (WLMs) of cationic surfactant cetyltrimethylammonium bromide (CTAB) and hydrotropic salt sodium salicylate and a rigid network of chitin nanocrystals (ChNCs). For the first time, the structure of the network of ChNCs was visualized using cryo-tomography. It revealed the formation of very long fibrillar-like aggregates of nanocrystals, composed of bundles of parallel stacked nanocrystals that bind to each other at their ends. A new type of junction between ChNC and WLM was visualized, formed by the lateral adhesion of a large fragment of WLM to the side of ChNC. Increasing ChNC concentration induces a strong increase of viscosity and elasticity due to the growth of ChNC aggregates and a microphase separation with the formation of bicontinuous ChNC-rich and WLM-rich phases, which effectively concentrate ChNCs and WLMs locally, thereby strengthening both nanocrystal and micellar networks. The first time, it was observed that heating induces the gelation of ChNC/WLM suspensions, although the WLMs become shorter. The heat-induced gelation is found to be due to the growth of ChNC aggregates both in length and width. These properties make the ChNC/WLM suspensions promising for use in fracturing fluids in the oilfield industry especially at elevated temperatures, where pure WLMs would lose their viscoelasticity.