Abstract
To overcome the slow kinetics of the volume phase transition of stimuli-responsive hydrogels as platforms for soft actuators, thermally responsive comb-type hydrogels were prepared using synthesized poly(N-isopropylacrylamide) macromonomers bearing graft chains. Fast responding light-responsive hydrogels were fabricated by combining a comb-type hydrogel matrix with photothermal magnetite nanoparticles (MNP). The MNPs dispersed in the matrix provide heat to stimulate the volume change of the hydrogel matrix by converting absorbed visible light to thermal energy. In this process, the comb-type hydrogel matrix exhibited a rapid response due to the free, mobile grafted chains. The comb-type hydrogel exhibited significantly enhanced light-induced volume shrinkage and rapid recovery. The comb-type hydrogels containing MNP were successfully used to fabricate a bilayer-type photo-actuator with fast bending motion.
Highlights
Can absorb visible light and convert photo energy into thermal energy, thereby heating the matrix and inducing volume shrinkage of the hydrogel[6,15,16]
To synthesize PNIPAm macromonomers (PNM), NIPAm was first polymerized in N,N-dimethylformamide (DMF) at 75 °C for 24 h using 2,2-azobisisobutyronitrile (AIBN) as an initiator and 2-aminoethanethiol (AESH) as a chain transfer agent. (Figure 1) AIBN is well-known thermal initiator for radical polymerization, which is commonly used under moderate heating at 50 ~ 75 °C for effective generation of radical species
PNM with a vinyl end group that can participate in the radical crosslinking process was successfully synthesized. (Figure 1) The degree of polymerization (DP) of PNM was determined via NMR analysis; the number-average molecular weights were measured by gel permeation chromatography
Summary
Can absorb visible light and convert photo energy into thermal energy, thereby heating the matrix and inducing volume shrinkage of the hydrogel[6,15,16]. The phase-separation method during polymerization under mixed solvent system, which resulted in generation of heterogeneous structure, has been reported to enhance the swelling kinetics[26]. The former strategies were limited by poor mechanical property due to the porous structure, while the latter strategy is not feasible to prepare composite due to the use of organic solvent that may precipitate the MNP dispersed in water. In light of the enhanced response of the grafted hydrogel system containing MNP, we fabricated photothermally driven bilayer-type photo-actuators that show reliable bending motion with maximized volume change and minimized response time
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