Cosolvent addition causes anomalous extensibility in thermoresponsive polymer solution

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 electroseparations, drug delivery, 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 electrodes 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. Interestingly, while the LCST exhibits a maximum as determined by turbidimetry measurements, the extensional relaxation time anomalously increases monotonically within the same range. Complementary shear rheology and light scattering measurements reveal that while the polymer radius of gyration decreases over this DMF composition range, the apparent hydrodynamic radius increases. This evidence, paired with subsequent studies at higher DMF content and on polydimethylacrylamide (PDMA) solutions, strongly suggests that DMF molecules can bridge PNIPAM chains, leading to a higher 'effective' PNIPAM molecular weight and the observed enhanced elasticity. This interpretation of 'bridging' is also in agreement with prior computational studies. Subsequent studies reveal that while this effect is observed across a wide range of PNIPAM solutions, the strength of this bridging effect depends on polymer concentration and molecular weight. Our results reveal that cosolvent incorporation can be used to tune solution extensibility in ways that are unexpected based on solvent quality or shear rheology alone.