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 degrees 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 similar to 31 degrees 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 stDructures 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.

Automated summary

The phase diagram of a polymer solution can determine its electrospinnability and transition temperatures. Low-temperature electrospinning can successfully produce uniform nanofibers, while electrospinning within a specific temperature range is feasible, but it becomes problematic at higher temperatures due to the enhanced gel elasticity.