Effects of DMF cosolvent addition on the extensional rheology of PNIPAM solutions

Poly(N-isopropylacrylamide) (PNIPAM), a thermoresponsive polymer known for its lower critical solution temperature (LCST) of ~32 °C in aqueous media, has been studied extensively for a variety of applications including drug delivery, sensing, and smart coatings. PNIPAM chains undergo a dramatic conformational change from hydrated coils in solution below the LCST to collapsed globules that often aggregate above the LCST. Polymer chain conformation affects the amenability of solutions to extensional flow-dominated processes such as spraying and printing, which are of interest for manufacturing coatings and devices at-scale. Adding a cosolvent can change the PNIPAM solution phase windows and thus be leveraged to tune processing windows. Herein, we use dripping-onto-substrate extensional rheometry (DoS) to measure the extensional flow behavior of PNIPAM in dimethylformamide (DMF)/water mixtures. We demonstrate the effects of DMF fraction on solution elasticity. Most notably, while the LCST exhibits a maximum with increasing DMF fraction as determined by turbidimetry measurements, the extensional relaxation time increases monotonically within the same range. Our findings demonstrate that preferential PNIPAM-DMF interactions evolve with solvent composition, in line with other studies.