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

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 Tgel. 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. Tgel 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 (Tb) 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 < Tgel, a clear gel at Tgel < T < Tb, and an opaque gel at T > Tb. 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 Tgel. 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 Tgel prior to solution phase separation.