Effects of 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 for a wide range of applications including drug delivery, sensing, and smart coatings. PNIPAM chains undergo a conformational change from hydrated coils in solution below the LCST to collapsed globules that often aggregate above the LCST. Polymer chain conformation significantly 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 two PNIPAM polymers of different molecular weight and dispersity. Most notably, while the LCST exhibits a maximum with increasing DMF content as determined by turbidimetry measurements, the extensional relaxation time (λ_E) increases monotonically within the same range. At a given x_c, λ_E exhibits an anomalous trend with polymer concentration regardless of polymer molecular weight and dispersity. These results suggest that preferential PNIPAM-DMF interactions evolve with solvent composition in line with prior studies.