Photo-induced actuator using temperature and light dual responsive azobenzene containing ion gel in ionic liquid
Graphical abstract
Introduction
Soft actuators, which can change shape in response to different external stimuli such as electricity [1], [2], [3], [4], heat [5], [6], [7], [8], solvent [9], [10], [11], humidity [12], [13], [14], [15], [16], [17], [18], and light [19], [20], [21], [22], [23], [24], have received considerable attention in scientific and engineering fields, because they can convert those external energies into two- or three- dimensional movements. Actuators have been widely applied into many fields, including robotics [18], artificial muscles [25], [26], switches [27], motors [28], [29], and sensors [1]. Among those actuators driven by diverse stimuli, light induced actuators possess distinctive advantages, such as wireless actuation and remote control [30].
In general, many light-driven actuators have been developed using polymer gels as base materials. Polymer gels as light-driven actuators have attracted much attention because polymer gels contain fluid in their three-dimensional network structures, which offer softness and exhibit a discontinuous change in volume between the swollen state and the collapsed state. This characteristic of polymer gels could be useful in the development of sensors and biomimetic materials. There are generally two designs for photo-driven polymer gel actuators: by introducing photochromic molecules in polymer chain and by adding nano-materials with photothermal properties into thermo-sensitive polymer gels (nano-composite polymer gel actuator). The motions of light-driven polymer gel actuators containing photochromic molecules is induced by size or polarity change of photochromic molecules: azobenzene [19], [21], [28], [30], spiropyran [31], [32], [33], diarylethene [20], [29], and so forth [34], [35]. The actuation of nano-composite polymer gel actuator is caused by volume change of thermo-sensitive polymer gels, as heat is provided by photothermal conversion of nano-materials, (e.g., gold nanorods, carbon nanotubes, graphene oxide) [24], [27], [36], [37], [38], [39], [40], [41], [42], [43]. However, these actuators have a serious problem of durability under atmospheric conditions caused by evaporation of solvents, which are indispensable as diffusion media. For example, hydrogel actuators become inactive when they work in an open atmosphere.
Ionic liquids (ILs) exhibit unique properties such as thermal and (electro)chemical stability, nonvolatility, nonflammability, and high ionic conductivity. Consequently, they have been an area of intense interest [44]. Ion gels are polymer networks swollen by an IL or a mixture of ILs [45], which combine the advantages of polymer gels and the merits of ILs [46], [47]. Ion gels have promising applications in many fields such as actuators [48], [49], gas separation membranes [50], [51], [52] and batteries [53]. Electrochemical actuators based on ion gels have been developed over the past decades and have been applied in robotic devices [48], [49], [54]. However, there is still no report on photo-induced ion gel actuators by utilizing the difference between LCST of thermo-/photo- sensitive polymers under UV and LCST of thermo-/photo- sensitive polymers under visible light irradiation, to the best of our knowledge.
We develop a photo-induced actuator using a thermo-/photo-induced ion gel comprising a chemical cross-linked random copolymer of BA and AzoMA and a hydrophobic IL: 1-butyl-3-methylimidazoliumbis(trifluoromethanesulfone)amide ([C4min][NTf2]). Previously, Lodge and coworkers reported a thermo-sensitive polymer poly(butyl methacrylate) that showed LCST-type phase behavior in [C4min][NTf2] [55]. In this study, we extend from thermo-sensitive polymer to thermo-/photo-sensitive polymer through copolymerization of AzoMA with butyl acrylate (BA) to form a random copolymer P(AzoMA-r-BA) (Scheme 1a and b). P(AzoMA-r-BA) exhibits different LCST under light irradiation of different wavelengths. The photo-induced phase separation of P(AzoMA-r-BA) is reversible at suitable temperatures chosen. Based on these properties, a thermo-/photo-sensitive ion gel (BA-AzoMA gel) is prepared by free radical copolymerization of BA and AzoMA with ethylene glycol dimethacrylate (EGDMA) as crosslinker in [C4min][NTf2] (Scheme 1c and d). BA-AzoMA gel exhibits high temperature contraction and low temperature expansion behavior. The contraction starts at different temperatures, depending on whether the gel is irradiated by visible or UV light. At a bistable temperature, photo-induced contraction-expansion of BA-AzoMA gel is reversible, making it a photo-induced actuator.
Section snippets
Materials
S-1-dodecyl-S′-(α, α′-dimethyl- α″-acetic acid) trithiocarbonate (CTA) and AzoMA were synthesized and characterized according to the previously reported procedure [56], [57], [58]. [C2mim][NTf2] and [C4mim][NTf2] was purchased from Lanzhou Greenchem™. BA, EGDMA and 2, 2′-azobisisobutyronitrile (AIBN) were purchased from Aladdin™. 1, 4-dioxane, acetone and methanol are purchased from Sinopharm™ Chemical Reagent Co., Ltd. AIBN was recrystallized from methanol prior to use. The inhibitors in BA
Characterization of P(AzoMA-r-BA)
The P(AzoMA-r-BA) random copolymers were characterized by gel permeation chromatography (GPC) and 1H NMR. The composition of AzoMA was calculated from the integrated intensity ratio between peaks (h) from AzoMA and peaks (c) from BA (Fig. 1). The number average molecular weight (Mn) and the polydispersity were determined by the GPC using tetrahydrofuran (THF) as the carrier solvent. The characterization results are showed in Table 1. The GPC curves show a single peak, without high molecular
Conclusions
The P(AzoMA-r-BA), synthesized by RAFT polymerization, exhibits LCST-type phase separation behavior in [C4mim][NTf2]. The LCST depends on the photoisomerization states of the azobenzene moiety in the polymers. At a bistable temperature, the photo-induced phase transition was reversible. Thermo- and photo-responsive ion gel (BA-AzoMA gel) has been prepared by free radical copolymerization of BA and AzoMA with EGDMA as the crosslinker in [C4min][NTf2]. BA-AzoMA gel has shown low temperature
CRediT authorship contribution statement
Xiaofeng Ma: Conceptualization, Methodology, Writing - original draft. Xiaoyu Lan: Data curation, Investigation. Linlin Wu: Visualization, Funding acquisition. Lei Wang: Validation. Qun Gu: Methodology, Writing - review & editing. Yijun Shi: Writing - review & editing. Xiaoli Gu: Methodology. Zhenyang Luo: Supervision.
Declaration of Competing Interest
We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled.
Acknowledgements
This work was supported by the Natural Science Foundation of Jiangsu Province (BK20160992), the Natural Science Foundation of the Jiangsu Higher Education Institutions (15KJB430018), and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).
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These authors contributed equally.