Two-Step Freezing Polymerization Method for Efficient Synthesis of High-Performance Stimuli-Responsive Hydrogels.

The widespread use of stimuli-responsive hydrogels is closely related to their synthesis efficiency. However, the widely used thermal-responsive poly(-isopropylacrylamide) (PNIPAM) hydrogels usually require a time-consuming synthesis process to produce (more than 12 h) and exhibit a relatively slow response speed in the field of cryo-polymerization. In this study, a sequence of thawing polymerization after freezing polymerization by a two-step method of free radical polymerization for the efficient synthesis of PNIPAM hydrogels (merely 2 h) with an excellent comprehensive performance is demonstrated. Results show that the overall performance of the as-synthesized PNIPAM hydrogels is at the top level among reported works despite the significantly reduced preparation time. Moreover, after incorporating multi-walled carbon nanotubes (MWNTs), the PNIPAM hydrogels exhibit a rapid near-infrared (NIR) light-response and programmable shape-morphing capability. It is believed that such a viable and time-saving synthetic method for producing PNIPAM hydrogels of high performance will lay a solid foundation for drug delivery and smart actuators.