Aqueous solutions of atactic poly­(N-isopropylacrylamide) (PNIPAM) exhibit complex phase transitions at 20–33 °C, that is, the phase behaviors of lower critical solution temperature (LCST) and physical gelation. The LCST phase behavior has been successfully described by the “pearl-necklace” chain model (Macromolecules 2005, 38, 4465); however, the formation of the physical gel is still elusive. In this study, atactic PNIPAM (a-PNIPAM) was used and the gel point (GP) of semidilute solutions was validated by observing the frequency-independent loss tangent (Winter–Chambon criterion) from the oscillatory shear measurements to derive the gel temperature T gel. It was found that the relaxation exponent n at GP is independent of solution concentration to be 0.76 with the fact that entanglement couplings play no effect on n. T gel decreases with a-PNIPAM concentration from 29.5 °C for the 5 wt % unentangled solution to 25 °C for the 12 wt % entangled solution. The binodal point (T b) was obtained from the extrapolated cloud-point temperature at zero heating rate, at which an initial drop of the light transmittance was observed. Based on these derived data, a phase diagram was constructed to show three typical phase domains composed of an one-phase solution at T < T gel, a clear gel at T gel < T < T b, and an opaque gel at T > T b. At 30 °C, the clear gels of 5 and 12 wt % samples possess extremely low equilibrium moduli of 0.2 and 41 Pa, respectively, suggesting that many a-PNIPAM single chains are associated and connected in between two gel junctions. Synchrotron small-angle/wide-angle X-ray scattering was performed to disclose the radius of gyration of gel junctions (with functionality f ≥ 3) to be 30–55 Å above T gel. Within the gel junctions, the collapsed subchains (pearls), which belong to different chains, become more compact with the interchain distance decreasing from 15 Å at 20 °C to 11 Å at 30 °C for the 12 wt % solution. We proposed that the origin of physical gelation is relevant to the inter-amide hydrogen bonding between collapsed subchains in the gel junctions to develop a strong physical bonding for interchain connectivity. At an elevated temperature approaching the GP but still below the spinodal temperature, the physical crosslinking of the developing pregel clusters is further facilitated by the enhanced concentration fluctuations with a small-q Fourier-mode driven by the interchain associations, eventually giving rise to the critical gel at T gel prior to solution phase separation.

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 T1, gel temperature Tgel, and binodal temperature Tb. On heating from 15 °C, the one-phase a-PNIPAM solution underwent pronounced concentration fluctuations at temperatures above T1. At higher temperatures, the thermal concentration fluctuations subsequently triggered the physical gelation process to develop a macroscopic-scale gel network at Tgel before the phase separation at Tb. Thus, the temperature sequence for the transition is: T1 < Tgel < Tb~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 T1. In addition, the electrospinning of an a-PNIPAM hydrogel at Tgel < T < Tb 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 > Tb, electrospinning became problematic for the phase-separated gel because the enhanced gel elasticity dramatically resisted the stretching forces induced by the electric field.

Herein, the direct morphological evidence of the extension‐induced phase‐separated structures in the electrospinning jet observed by high‐speed video imaging and by light scattering technique is reported. Model solutions of poly(vinyl alcohol) (PVA)/water are electrospun. Two types of internal structures, that is, long strings and short ellipsoids, are found. A light scattering model is derived for the Vv scattering configuration to account for the scattered intensities contributed from the liquid jet itself and those from the internal structures. For the severely stretching jet of PVA/water, the Vv intensity profile is dominant by the internal structures to mask the scattering contribution from the jet itself. Moreover, the Hv intensity profile reflects the anisotropy of the oriented chains parallel to the jet axis. For the 7 wt% solution, the derived extension rate in the vicinity of the Taylor cone apex is about 3420 s−1, which is higher than the Rouse relaxation rate measured by rheometer. It is concluded that extension‐induced phase separation of the single‐phase PVA solution is likely to occur in Taylor‐cone apex to trigger the self‐assembly process for producing strings (and/or bulges) in the flowing jet, which eventually transform to become the nanofibers, after solvent removal, to be collected on the grounded collector.

Aqueous solutions of atactic poly(N-isopropylacrylamide) (a-PNIPAM) undergo complex phase transitions at 20–33 °C. In this temperature range, the a-PNIPAM solution exhibits a phase behavior of lower critical solution temperature at the binodal temperature (Tb) and physical gel formation at the gel temperature (Tgel). On slow heating of the one-phase solution containing linear a-PNIPAM chains, branched chains are gradually developed to proceed with the physical gelation before phase separation considering that Tgel < Tb. Thus, the phase separation temperature determined from the conventional approaches, either by turbidity to derive the Tb or by scattering to derive the spindal temperature (Ts) from the Ornstein–Zernike analysis, is strictly the transition temperature associated with the a-PNIPAM hydrogel (or highly branched chains newly developed at elevated temperatures), rather than the initial a-PNIPAM solution prepared. Herein, the spinodal temperatures of a-PNIPAM hydrogels (Ts,gel) of various concentrations were determined from rheological measurements at a heating rate of 0.2 °C/min. Analyses of the temperature dependence of loss modulus G″ and storage modulus G′ give rise to the Ts,gel, based on the Fredrickson–Larson–Ajji–Choplin mean field theory. In addition, the specific temperature (T1) above which the one-phase solution starts to dramatically form the aggregated structure (e.g., branched chains) was also derived from the onset temperature of G′ increase; this is because as solution temperature approaches the spinodal point, the concentration fluctuations become significant, which is manifested with the elastic response to enhance G′ at T > T1. Depending on the solution concentration, the measured Ts,gel is approximately 5–10 °C higher than the derived T1. On the other hand, Ts,gel is independent of solution concentration to be constant at 32.8 °C. A phase diagram of the a-PNIPAM/H2O mixture is thoroughly constructed together with the previous data of Tgel and Tb.