Crystallization-Stabilized Thermoresponsive Gelation of a PiPOx-containing Hydrogel

LCST-type thermoresponsive physical hydrogels form networks through heat-induced hydrophobic association, which is generally reversible, thereby limiting the operational temperature window and structural stability of the gels. In this study, PiPOx-PEO-PiPOx was designed as a hydrogel system in which a thermally induced physical network can be further stabilized by the crystallization of the PiPOx end blocks. Upon heating above the LCST, the PiPOx domains associate to form physical crosslinks. Gelation was observed at concentrations above 15 wt% during heating, whereas lower-concentration samples mainly showed turbidity without stable gel formation. The gelation process was accompanied by the continuous increase in the storage modulus with time. X-ray diffraction of the gels showed time-dependent growth of crystalline peaks assigned to PiPOx. Moreover, the formed gel retained its macroscopic shape upon cooling below the LCST of PiPOx, suggesting that the thermally formed network was maintained even after removal of the thermal stimulus. These results indicate that the hydrogel develops through a two-step process: thermally induced association first forms physical domains above the LCST, and the subsequent PiPOx crystallization preserves the physical crosslinks. Such behavior makes this new type of hydrogel a promising platform for biomedical applications such as tissue engineering scaffolds.