Understanding the ion-induced transition of charged cellulose nanofibrils/nanocrystals (CNFs/CNCs) from a dispersed state to a volume-spanning arrested state (VAS) is crucial in processes to create new advanced materials from nanocellulose [1]. Ideally, the nanoscale structural changes can be characterized using small-angle X-ray scattering (SAXS), where the influence of radiation can be neglected by studying a continuously flowing mixing system. However, this kind of <i>in-situ</i> mixing experiment to target the time-resolved kinetics is challenging, especially due to the dramatic change in material viscoelasticity as well as shear flow smearing out the probed time scales. Here we demonstrate a flow-focusing mixing device in combination with SAXS to study the VAS transition of nanocellulose. The flow of nanocellulose is detached from the walls during focusing, providing a core flow with constant velocity and minimal shear that in turn allows for a trivial conversion between spatial coordinates and mixing times. Using this technique, we studied the VAS transition of dispersed CNFs and CNCs when mixed with a sodium chloride solution as gelation agent. By scanning the X-ray beam through the stationary mixing flow, we obtain a 2D image of the structural features and keep track of the core flow interface and center position. The results provide important insights into the time scales of the system, which are found to be very different when comparing CNCs and CNFs. For CNCs, it is observed how locally ordered <i>tactoids</i> are disrupted when the added monovalent ions affect the electrostatic interactions, which in turn leads to a loss of CNC alignment through enhanced rotary diffusion. For CNFs, the observed nano-structural transition can be understood through the formation of segmental aggregates leading to a connected network and reduced Brownian motion, whereby an aligned structure can be preserved.

Time-resolved ion-induced phase transition of nanocellulose using shear-free flow-focusing mixing and scanning SAXS

ACS Spring Meeting 2021

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.