Quantifying the dynamic phase transition of thermoresponsive microfibrillated cellulose with multiple particle tracking microrheology

Rheological modifiers adjust flow behavior, tune rheology, and induce phase transitions in products. Microfibrillated cellulose (MFC), specifically sourced from wood, is a potential rheological modifier due to a high aspect ratio, which enables changes in rheology with minimal added material. Results from this work will inform design of rheological modifiers that use cellulose waste from the paper industry. In this work, we characterize thermoresponsive MFC, which is made by grafting Jeffamine polyetheramine M2005 onto the colloid surface. Thermoresponsive MFC is controllably degraded or gelled by equilibrating the material at different temperatures, enabling the structure and rheology to be measured. Multiple particle tracking microrheology (MPT) characterizes these properties during phase transitions by measuring the thermal motion of embedded particles. Using MPT, we identify the state of the material and structure at the phase transition in response to temperature changes. At low temperatures (4–16 °C), the system is a sol. Increasing the temperature (T≥16°C) induces a sol-gel phase transition. During network formation, the critical transition temperature, Tc, and critical relaxation exponent, n, are quantified using time-cure superposition. Tc=15.7±1.5°C, which is within error of the lower critical solution temperature of the grafted polymer, 16°C. This indicates that the polymer conformation and phase transition on the MFC surface is similar to that in solution. This information can be used in future design of consumer products that requires specific structure and properties over a temperature range.