Characterizing Dynamic Gelation of ThermoresponsiveMicrofibrillated Cellulose

Microfibrillated cellulose (MFC) has potential applications as a rheological modifier to change the flow characteristics of a product, including changing the viscosity or enabling phase transitions in response to external stimuli. Our work focuses on MFC from wood, with the goal of repurposing paper waste into new consumer products. Properties of MFC, such as inherent biodegradability, strength, and a nonionic surface make it appealing for inclusion in a wide range of formulations. In this work, we characterize thermoresponsive MFC, which is synthesized by grafting a thermoresponsive polymer, Jeffamine polyetheramine M2005, onto the surface of the fibers. Thermoresponsive MFC is stable and in good solvent conditions in the temperature range of 4–16 °C. Between 16–30 °C, solvent conditions change to a bad solvent, and MFC aggregates and has a higher elasticity. Thermoresponsive MFC is characterized with bulk rheology to measure dynamic temperature-dependent rheology during transitions as a function of fiber concentration and temperature. We define the interaction concentration, <i>c</i>**, as the concentration when the material transitions from the semidilute to concentrated regime. The interaction concentration is calculated as <i>c</i>** ≃ 0.2 wt %. Below <i>c</i>**, 0.17 wt %, this material transitions from a viscous fluid to a structure with a higher elastic than viscous component as temperature increases. Above <i>c</i>**, 1.4 wt %, we measure elastic behavior for all temperatures due to initial associated fibers and an increase in elastic moduli above the lower critical solution temperature (LCST) of the grafted polymer. We also characterize hysteresis in the moduli during thermal cycles due to association–dissociation kinetics of the grafted polymer. Bulk rheology measurements indicate that structural transitions occur near the grafted polymer LCST. The achievable elasticity range is primarily determined by the concentration relative to <i>c</i>** and is varied within this range by changing the temperature. This information can be used in the future design of consumer products that require specified structure and properties during temperature changes.