Electrospinning of Aqueous Solutions of Atactic Poly(N-isopropylacrylamide) with Physical Gelation

The phase diagram of a given polymer solution is used to determine the solution’s electrospinnability. We constructed a phase diagram of an aqueous solution of atactic poly(N-isopropylacrylamide) (a-PNIPAM) based on turbidity measurements and the rheological properties derived from linear viscoelasticity. Several important transition temperatures were obtained and discussed, including the onset temperature for concentration fluctuations T(1), gel temperature T(gel), and binodal temperature T(b). On heating from 15 °C, the one-phase a-PNIPAM solution underwent pronounced concentration fluctuations at temperatures above T(1). At higher temperatures, the thermal concentration fluctuations subsequently triggered the physical gelation process to develop a macroscopic-scale gel network at T(gel) before the phase separation at T(b). Thus, the temperature sequence for the transition is: T(1) < T(gel) < T(b)~31 °C for a given a-PNIPAM aqueous solution. Based on the phase diagram, a low-temperature electrospinning process was designed to successfully obtain uniform a-PNIPAM nanofibers by controlling the solution temperature below T(1). In addition, the electrospinning of an a-PNIPAM hydrogel at T(gel) < T < T(b) was found to be feasible considering that the elastic modulus of the gel was shown to be very low (ca. 10–20 Pa); however, at the jet end, jet whipping was not seen, though the spitting out of the internal structures was observed with high-speed video. In this case, not only dried nanofibers but also some by-products were produced. At T > T(b), electrospinning became problematic for the phase-separated gel because the enhanced gel elasticity dramatically resisted the stretching forces induced by the electric field.