316: Carbon dioxide as a trigger for hydrogel phase transitions

Abstract: Nature inspires the design of innovative and sustainable materials. For example, CO2-switchable pores in plant leaves enable gases to enter and exit the leaf as a function of CO2 concentration, enabling the plant to increase CO2 uptake when needed and without losing too much moisture. These CO2-switchable pores inspire the design of synthetic CO2-switchable polymeric materials. One approach towards the design of CO2-switchable polymers involves exploiting the rich phase behavior displayed by certain polymers. Notably, certain crosslinked hydrogels exhibit a volume phase transition, which is characterized by an abrupt change from a collapsed, dehydrated state to a swollen, hydrated state. This transition controls the absorption and release of water from the hydrogel and is typically modulated by a temperature change across the volume phase transition temperature. This presentation will describe and characterize a hydrogel displaying a CO2-switchable volume phase transition in which the phase transition is controlled by the isothermal addition and removal of CO2. Specifically, we study the effects of hydrogel composition on the transition temperature shifts (i.e., the change in transition temperature upon CO2 addition and removal), swelling kinetics, and swelling extent. Over the range of compositions tested, we observe up to a five-fold increase in mass due to swelling caused by an isothermal CO2-induced phase transition. By activating repeated swell-shrink cycles via CO2 addition and removal, the reversibility of the transition temperature shift and swelling response was studied, and the morphological changes occurring through this cycle were observed using scanning electron microscopy. This easily fabricated and tunable system may help guide the design of CO2-switchable surfaces for applications including responsive membranes, smart filtration, and controlled release coatings.