CO2-switchable non-Newtonian fluids

Abstract

Fluidic materials that exhibit an engineered non-Newtonian response are highly desirable in industrial and military applications. For example, shear thickening is highly intriguing in the case of flexible body armor or damping devices; however, shear thinning is more suitable in industrial processes with confining flow or painting. Herein we present a new class of non-Newtonian polymeric fluids with switchable rheological properties being mediated by carbon dioxide (CO₂). Several pH-sensitive associative amphiphilic polymers were designed by copolymerizing N,N-dimethylacrylamide (DMA) and N,N-dimethylaminoethyl methacrylate (DMAEMA), followed by hydrophobic modification. In the beginning, polymers carrying an optimal balance of hydrophilic and hydrophobic moieties exhibit shear thinning properties in an alkaline environment. After CO₂ aeration, the protonation of tertiary amine groups on the polymer backbone induces coil expansion and promotes interchain cross-linking under shear, leading to an abnormal shear thickening. The fluidic material can regain its shear thinning properties after sparging the solution with N₂. Such a gas-controlled process is reversible and clean, which can be performed over multiple cycles. This study taps the potential of CO₂ utilization in the dynamic fluidic materials science.