Elsevier

European Polymer Journal

Volume 123, 15 January 2020, 109446
European Polymer Journal

Photo-induced actuator using temperature and light dual responsive azobenzene containing ion gel in ionic liquid

https://doi.org/10.1016/j.eurpolymj.2019.109446Get rights and content

Highlights

  • We prepared azobenzene containing random copolymers (P(AzoMA-r-BA)) and ion gel (BA-AzoMA ion gel).

  • LCST of P(AzoMA-r-BA) depends on photo-isomerization states of azobenzene.

  • Photo-induced LCST transition of P(AzoMA-r-BA) was reversible at certain temperatures.

  • BA-AzoMA ion gel shows reversible thermo- and photo-induced volume changing.

  • Photo-induced actuator with reversible bending behavior can be realized.

Abstract

A series of well-defined random copolymers comprising butyl acrylate (BA) and 4-phenylazophenylmethacrylate (AzoMA) (P(AzoMA-r-BA)) are prepared successfully by reversible addition fragmentation chain transfer (RAFT) polymerization. P(AzoMA-r-BA)s show LCST-type phase transition in 1-butyl-3-methylimidazolium bis(trifluoromethanesulfone)amide ([C4mim][NTf2]). LCST depends on the photoisomerization state of azobenzene, as well as on the AzoMA composition in the random copolymers. LCST of (P(cis-AzoMA-r-BA) is significantly higher than that of P(trans-AzoMA-r-BA), because cis-AzoMA and trans-AzoMA behave as solvato-philic and solvato-phobic comonomer, respectively. At a bistable temperature, photo-induced phase separation is completely reversible. Secondly, based on this phenomenon, a thermo-/photo- responsive ion gel (BA-AzoMA ion gel) can be prepared by free radical polymerization of BA and AzoMA using ethylene glycol dimethacrylate (EGDMA) as crosslinker in [C4mim][NTf2]. BA-AzoMA ion gel shows high temperature contraction and low temperature expansion behavior, due to LCST-type phase behavior of polymer system consisting of AzoMA and BA. Contraction temperature of BA-AzoMA ion gel depends on photoisomerization state of the azobenzene group in polymers. At a bistable temperature, photo-induced expansion-contraction is reversible. Finally, a Photo-induced actuator can be realized using BA-AzoMA ion gel at suitable temperatures, featuring reversible bending by alternate irradiating with visible and UV light. The bending behavior is also demonstrated.

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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