Spatial Heterogeneity Accompanying Gel Formation of Poly(N-isopropylacrylamide) Aqueous Solution at a Temperature below Cloud Point

Poly(N-isopropylacrylamide) (PNIPAM) is a representative thermoresponsive polymer, and its aqueous solution becomes phase-separated at a temperature higher than the cloud point (Tcp) at ∼304 K, which plays an important role in various biomedical applications. Thus, to further promote the use of PNIPAM as a functional material, it is necessary to gain a better understanding of its phase behavior especially at a temperature just below Tcp. To this end, we examined the local rheological properties of atactic PNIPAM solutions with a concentration of 10 wt % by a particle tracking technique, in which the thermal motion of probe particles in the solution was tracked, in conjunction with Fourier-transform infrared spectroscopy, small-angle X-ray scattering measurements, fluorescence spectroscopy, and confocal laser scanning microscopy. At temperatures far below Tcp, the solution exhibited one-phase homogeneous characteristics. After the PNIPAM aqueous solution was left undisturbed at a temperature just below Tcp (302 K), it formed a physical gel. Changing the size of probe particles for the tracking measurements, we found that the resultant gel was spatially heterogeneous in terms of its rheological properties at a length scale of approximately 50-100 nm. The gelation of the solution promoted by the formation of hydrophobic pearls in PNIPAM chains led to the formation of network junctions with heterogeneity. The knowledge obtained here should be useful for understanding and controlling the phase behavior of the PNIPAM solution, thereby leading to the further development of thermoresponsive functional materials.