Abstract
Thermoresponsive poly(N-isopropylacrylamide) (PNIPAAm) hydrogels have been attracting attention in a variety of functional materials, such as biomaterials, because they exhibit a volume phase transition phenomenon near physiological temperatures. However, the slow kinetics and small volume shrinkage of bulk PNIPAAm hydrogels upon heating greatly limit their practical application. Here, we report PNIPAAm hydrogels with phase-separated structures that exhibited ultrafast shrinking upon heating. The phase separation into a PNIPAAm-rich phase and a water-rich phase was formed through aqueous polymerization in the presence of NaClO4 salt. Through structural analysis of the hydrogels, a topologically heterogeneous and porous structure was observed, which was highly dependent on the NaClO4 concentration in the polymerization step. Compared to conventional PNIPAAm hydrogels, the phase-separated hydrogels exhibited much faster and larger shrinkage upon heating. Simultaneously, the hydrogels quickly released a large amount of water owing to the effective water channels inside them. The present method can be widely applied to general hydrogels, and it can address the numerous limitations of hydrogels in terms of operating programmability and deformation efficiency.
Highlights
When hydrophilic and stimuli-responsive polymers are three-dimensionally crosslinked and form hydrogels, this can change their volume in response to external stimuli
Redox polymerization was performed in NaClO4 aqueous solutions by using ammonium persulfate (APS) and N,N,N’,N’-tetramethylethylenediamine (TEMED) as the initiator and the accelerator, respectively
lower critical solution temperature (LCST) of the PNIPAAm effectively dropped at the optimized concentration of NaClO4 so that it afforded well-separated PNIPAAm-rich and water-rich domains at the micron level
Summary
When hydrophilic and stimuli-responsive polymers are three-dimensionally crosslinked and form hydrogels, this can change their volume in response to external stimuli. Poly(Nisopropylacrylamide) (PNIPAAm) hydrogels, the most investigated thermoresponsive hydrogels, exhibit sharp, reversible phase transitions in water at approximately 32 ◦ C [1,2,3,4,5] At this temperature, the so-called lower critical solution temperature (LCST), the volume of the hydrogels changes from a swollen state (below 32 ◦ C) to a shrunken state External stimuli are applied from the outside, and the hydrogel forms a hydrophobic skin layer on the surface, preventing water molecules from being released to the outside [13,14,15] This skin layer is the main reason the volume shrinkage of the PNIPAAm hydrogels slows down upon heating. In the case of studies using salts, only NaCl was reported, and the possibility of other salts was not explored
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