Deformation of Redox-Active Polymer Gel Based on Polysiloxane

Article pubs.acs.org/Langmuir

Deformation of Redox-Active Polymer Gel Based on Polysiloxane Backbone and Bis(benzodithiolyl)bithienyl Scaffold Toshihiro Ohtake,*,† Hideki Tanaka,† Tetsuro Matsumoto,† Akira Ohta,‡ and Mutsumi Kimura*,§ †

Seiko Epson Corporation, 281 Fujimi, Fujimi-machi, Suwa-gun, Nagano 399-0293, Japan Department of Chemistry, Faculty of Science, Shinshu University, Matsumoto, Nagano 390-8621, Japan § Division of Chemistry and Materials, Faculty of Textile Science and Technology, Shinshu University, Ueda, Nagano 386-8567, Japan ‡

S Supporting Information *

ABSTRACT: Redox-active polymer gels consisting of polysiloxane backbone and bis(benzodithiolyl)bithienyl units have been designed and synthesized. The bis(benzodithiolyl)bithienyl units, which undergo interconversion between cyclic form and opened dicationic form, have been incorporated into polysiloxane backbone via hydrosilylation of vinyl-terminated bis(benzodithiolyl)bithienyl derivative and poly(methylhydrosiloxane) (PMHS) or poly(dimethylsiloxane-co-hydrogenmethylsiloxane) (PDMS-co-PMHS), resulting in polymer gels crosslinked with bis(benzodithiolyl)bithienyl units. After the incorporation of M1 into polysiloxane backbone, these polymer gels (P1 and P2) also exhibit redox responses associated with the electrochemical interconversion of the bis(benzodithiolyl)bithienyl moieties. The polymer gels show swelling behavior upon chemical oxidization, and bending behavior has been observed for the polymer gel immobilized poly(vinylidene difluoride) (PVdF) film. These results provide a useful perspective for fabricating redox-triggered polymer gel actuators based on the conformational change of the functional molecular unit.

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INTRODUCTION Gels, which are solidlike materials with viscoelastic properties,1 have been applied in many fields pertaining to everyday life and have made considerable progress in the field of functional materials.2−9 The unique functions of polymer gel materials have attracted a great deal of attention for their potential application to mechanical devices.10 The recently observed mechanical change of polymer gels responding to external stimuli is known to be an important property in terms of application to soft actuators or artificial muscles.11−20 For example, there have been extensive reports of photoresponsive21−23 or temperature24 responsive deformable polymers demonstrating that shape changing of the functional unit and free volume effects are strongly correlated with macroscopic deformation. Only limited examples of redox-triggered deformable materials have been examined, but we do know that electrically controlled polymeric systems25−30 have advantages such as versatility and precise control. In this context, we focus our attention on redox-triggered conformational changing molecules as a functional unit to induce the macroscopic deformation of polymeric gel materials. We have previously reported on the redox-triggered structural interconversion of a bis(benzodithiolyl)bithienyl system.31−33 The structural change involves an intramolecular cyclization and ring-opening reaction34−39 based on significant internal rotation of the bithienyl unit (Scheme 1). In this case, two-electron oxidation of the cyclized form leads to the formation of a dicationic form by ring-opening and twoelectron reduction of the dications yields the initial cyclized © 2014 American Chemical Society

Scheme 1. Redox-Triggered Structural Interconversion of Bis(benzodithiolyl)bithienyl

form. Such large conformational change is an attractive characteristic in terms of stimuli-responsive soft actuator design. Here, our strategy is to incorporate a redox-responsive structural changing unit into a polymer backbone capable of inducing redox-induced conformational change in such a way that the geometry of the two points attached to the polymer repeating unit is changed.40 We previously fabricated a redox-active cross-linked polymer based on PDMS-co-PMHS and redox-active bis(benzodithiolyl)bithienyl scaffold and confirmed the redoxactive properties of the polymer.41 However, the content of the incorporated redox-active moiety was estimated to be 13% with respect to siloxane repeating units, and in order to achieve a more effective deformation of the polymer chain, the content of redox-active moieties needs to be optimized. We thus attempted to use PMHS as polymer backbone instead of Received: October 14, 2014 Revised: November 13, 2014 Published: November 16, 2014 14680

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Scheme 2. (A) Chemical Structures of Vinyl-Terminated Redox-Active Molecule (M1) and Polymer Gels (P1 and P2) Derived from M1 and Polysiloxane Backbone

glass plate and placed in an oven (85 °C) for 3 h. The polymer gel on the ITO glass was then washed with dichloromethane (CH2Cl2) and dried. Sample Preparation for Displacement Measurements. The toluene solution containing M1, PMHS, and 1,3-divinyl-1,1,3,3tetramethylsiloxane)platinum(0) prepared as mentioned above was dropped onto PVdF (Millipore, Durapore membrane filter, HVHP 04700, pore size: 0.45 μm, porosity: 75%), heated at 85 °C for 3 h, washed with CH2Cl2, dried, and cut into pieces for measurements. Displacement Measurements. The evaluation of deformation was carried out after adding an NOBF4 solution in MeCN to a sample tube in which an actuator strip is immersed in acetonitrile (MeCN). The measured displacement δ was transformed into the strain by

PDMS-co-PMHS. The amount of Si−H bond in a polymer chain is strongly correlated to the possible amount of redoxactive moieties being incorporated via hydrosilylation. In the present study, we have designed and prepared functionalized polymer gels consisting of a PMHS or PDMS-coPMHS backbone (P1 and P2, respectively) and redox-triggered functional molecular units (M1), as shown in Scheme 2. We employ bis(benzodithiolyl)bithienyl scaffold31 as an electrochemical functional unit, which undergoes interconversion between closed cyclic form and opened dicationic form. Here, we report the synthesis, electrochemical properties, and deformation behavior upon oxidation of the redox-active polymer gels P1 and P2.

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ε=

MATERIALS AND METHODS

2dδ L2 + δ 2

where L and d refer to the length and thickness of the actuator strip, respectively.42 Here, we used L = 20 mm and d = 0.1 mm to calculate ε.

Materials. All reagents for the synthesis of P1, P2, or M1 were used as received. Polymers PMHS and PDMS-co-PMHS were purchased from Gelest. Details of the experimental procedure to synthesize M1 and P2 have been reported in our previous work.41 Characterization. Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) measurements were performed on Autolab with PGSTAT30 (Metrohm Autolab). The displacement was measured with a laser displacement meter (Keyence LKG152). The morphologies of PVdF film coated with polymer gel were evaluated on a scanning electron microscope (SEM) (Hitachi S-4500). Synthesis of P1. Monomeric molecule M1 (40 mg, 40 μmol) and PMHS (5 mg, 2.2 μmol) dissolved in toluene (70 μL) were placed in a sample tube. A solution of (1,3-divinyl-1,1,3,3-tetramethylsiloxane)platinum(0) in xylene (Pt 2%, 40 μL) was added to the solution, which was heated at 85 °C for 22 h. The resulting gel was washed with toluene (twice) and methanol (three times) and dried to give P1 (27 mg); brown powder; mp 233 °C (decomposition). IR (KBr): 795, 1027, 1244, 1448, 1523, 2152, 2858, 2923 cm−1. Sample Preparation for CV Measurements. M1 (16 mg, 20 μmol) and PMHS (2.6 mg, 1.1 μmol) dissolved in toluene (150 μL) were placed in a sample tube. A solution of (1,3-divinyl-1,1,3,3tetramethylsiloxane)platinum(0) in xylene (Pt 2%) (10 μL) was added to the solution, and the resulting solution was dropped onto an ITO

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RESULTS AND DISCUSSION

The reactive molecular switch bearing terminal vinyl groups M1 was synthesized from 2,2′-diformyl-4,4′-dibromo-3,3′bithienyl43,44 in four steps.41 The X-ray molecular structure and electrochemical properties of M1 were reported in our previous work.41 We attempted to incorporate M1 into the polysiloxane backbone via hydrosilylation45,46 to form redox-responsive polymer gel P1. The vinyl-terminated M1 was reacted with PMHS in the presence of (1,3-divinyl-1,1,3,3tetramethyldisiloxane)platinum(0) complex in toluene at 85 °C to afford functionalized polysiloxane as an orange transparent gel. The gel formations were observed for various amounts of M1 incorporated into the polysiloxane chain, suggesting that the polysiloxane chains were cross-linked by M1 forming stable three-dimensional network structures (Figure S1 and Table S1, Supporting Information). Such gel formation was 14681

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also observed for P2.41 The degree of substitution of SiH bonds has been estimated by IR spectra. The SiH bonds in the polymer backbone have been found to be almost completely substituted with M1 (Figure S2, Supporting Information). The degree of substitution was estimated to be approximately 94% with respect to the number of siloxane repeating units. In order to qualitatively examine the volume change of the gel upon oxidation, we first oxidized P1 by chemical treatment with oxidant. A small piece of P1 was placed between slide glass plates and swollen with MeCN. The solution of oxidant (NOBF4) in MeCN was then injected between glass plates. When the NOBF4 solution was added, rapid color change of the piece of P1 into dark brown and swelling behavior were observed (Figure 1 and movie file, Supporting Information).

Figure 2. (A) Cyclic voltammogram (scan rate: 50 mV/s) and (B) differential pulse voltammogram for P1 immobilized on an ITO electrode in MeCN containing Bu4NBF4 (0.1 M) as the supporting electrolyte. The solid curve and dashed curve in DPV chart indicate the scan from −0.66 to 1.64 V (vs Fc/Fc+) and its reverse scan, respectively.

Figure 1. Photographs of P1 swollen with MeCN (A) before and (B) after the treatment with NOBF4.

The volume change by swelling was estimated to be 44% (Supporting Information), and the color change can be attributed to the formation of the dicationic form of bis(benzodithiolyl)bithienyl moieties.31,33 The swelling of P1 may be caused by an osmosis47 through the formation of ionic moiety in P1, Coulomb repulsion48 between positively charged bis(benzodithiolyl)bithienyl moieties, and an increased free volume effect49 owing to the conformational change of polymer chain accompanied by intramolecular ring-opening oxidation of bis(benzodithiolyl)bithienyl moieties. We evaluated the electrochemical properties of P1 by CV and DPV measurements. Polymer gel P1 was directly formed on a transparent ITO electrode, and electrochemical analysis was performed for the modified electrodes. The first scan was taken for the CV curve because the current density of redox peaks dramatically decreases after the second scan (Figure S3, Supporting Information). This may be due to the peeling off of the gel from the electrode. As shown in Figure 2A, a similar CV curve to that of M1 is observed for P1, suggesting that the redox properties of M1 are maintained even after being incorporated into the polymer backbone. The cyclic voltammogram of P1 showed irreversible anodic and broad cathodic peaks at +1.43 and −0.24 V (vs Fc/Fc+), respectively. The large peak separation between oxidation and reduction can be attributed to the redox response of bis(benzodithiolyl)bithienyl moieties, indicating the redox-induced C−C bond formation and cleavage.31,41 Clear oxidation and reduction currents are also observed by DPV (Figure 2B). Similar results have previously been observed for P2.41 Next, we examined the deformation behavior of P1 and P2 induced by the oxidation with NOBF4. Figure 3 shows the experimental procedure used to prepare PVdF film coated with these polymer gels. Unfortunately, self-standing films of these polymer gels50 could not be obtained, so we used PVdF as a base film to evaluate the deformation behavior of P1 and P2. The layered structure of two materials with different swelling ratios is expected to induce significant shape change in the

Figure 3. Schematic illustration of the preparation of PVdF film coated with polymer gel (P1 or P2) for deformation measurement: (i) dropping the precursor solution, (ii) forming polymer gel, and (iii) washing and cutting into pieces.

film.48 Moreover, the porous structure of PVdF film should be favorable for the polymer gel to transmit its swelling to PVdF film because of the large surface area of PVdF. A toluene solution containing M1, PMHS (or PDMS-co-PMHS), and Pt catalyst was dropped onto side A of the PVdF film, and the film was then heated to 85 °C to form a polymer gel deposited on the PVdF base film. The amount of polymer gel deposited on the PVdF was ca. 5.7 and 2.3 mg per 1 cm2 of the film for P1 and P2, respectively. The resulting films were rinsed and cut into strips for deformation measurement (Figure S4, Supporting Information). Preliminarily, the morphology of the porous surface of polymer gel formed PVdF was examined by SEM observation (Figure 4). On side A, the polymer gel incorporated into the porous structure of the PVdF film could be seen, whereas the same morphology as the original PVdF (Figure S5, Supporting Information) was observed on side B. This indicates that the polymer gel was incorporated only onto side A. Such deviation in the amount of incorporated polymer gel may cause the gradient of expansion ratio associated with swelling by oxidation with respect to the direction parallel to the film thickness, which consequently leads to bending behavior of the PVdF film. As shown in Figure 5A,B, the deformation of P1 or P2 deposited PVdF was evaluated by measuring the displacement 14682

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Figure 4. SEM photographs of the surface of PVdF film coated with P1 on (A) side A and (B) side B. Figure 5. Evaluation of the deformation of P1 and P2 by the oxidation with NOBF4: (A) experimental setup for measurements; (B) schematic image in detail of displacement measurements; (C) changes of displacement for the PVdF films coated with polymer gels as a function of time. The curve for P1 is in red, for P2 in blue, and for bare PVdF (reference) in black.

on side A. Upon addition of NOBF4, bending behavior toward side B (Figure 5C) and rapid color change to dark brown (Figure S6, Supporting Information) were observed for both P1 and P2, whereas clear bending behavior was not evident for a bare PVdF. The maximum displacements were 1.1 and 0.6 mm for P1 and P2, respectively, corresponding to respective strains of 0.05% and 0.03%. Such low strains may be due to the quite small amount of polymer gel penetrated into the pores of PVdF. In this case, the amount of polymer gel in the PVdF is not enough to transform the swelling of the polymer gel into the macroscopic deformation of the PVdF base film. The P1 deposited PVdF film has a larger bend tendency than P2. The amount of P1 deposited onto the PVdF is larger than P2, and the ratio of the redox-active functional units incorporated into the polymer backbone is higher for P1 than P2, which may cause the difference in the strain of the PVdF base film. The bending behavior suggests that the polymer gels swell with proceeding oxidation in such a way that the degree of expansion on side A is higher than that on side B. In the case of photodriven bending behavior of polymer actuators based on the photoisomerization of azobenzene units, the gradient in isomerization through film thickness is a driving force behind mechanical deformation.51−53 Thus, in our case, the gradient in the amount of polymer gel formed through the film thickness of PVdF film may lead to a similar bending behavior, as schematically illustrated in Figure 6. During the oxidation by NOBF4, the degree of swelling is larger on side A than on side B, which results in the greater bending toward side B.

Figure 6. Schematic illustration of the possible mechanism of the bending behaviors of PVdF film with a gradient in the amount of formed polymer gels through the direction of film thickness.

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CONCLUSIONS Redox-active polymer gels consisting of a polysiloxane backbone and redox-responsive functional units were designed and synthesized. The polymer gels P1 and P2 exhibit redoxactive properties derived from the redox-triggered interconversion of bis(benzodithiolyl)bithienyl moiety and its dication. Clear swelling behavior was observed for P1 by chemical oxidation with NOBF4. The polymer gels were formed on porous PVdF film, and bending behaviors of the polymer-gel deposited PVdF film by chemical oxidation were observed. We attribute such bending behavior to the combination of swelling of the polymer gel and the gradient in the amount of deposited polymer gel in the direction of film thickness. In this study, the observed strains were smaller than those previously reported. For further improvement, design of actuators in which the volume change of polymer gels can be effectively transformed 14683

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(12) Yoshida, R. Self-Oscillating Gels Driven by the BelousovZhabotinsky Reaction as Novel Smart Materials. Adv. Mater. 2010, 22, 3463−3483. (13) Takada, K.; Tanaka, N.; Tatsuma, T. A Redox Actuator Based on Reversible Formation of Bond between Poly(acrylic acid) Gel and Cu2+ Ion. J. Electroanal. Chem. 2005, 585, 120−127. (14) Thérien-Aubin, H.; Wu, Z. L.; Nie, Z.; Kumacheva, E. Multiple Shape Transformations of Composite Hydrogel Sheets. J. Am. Chem. Soc. 2013, 135, 4834−4839. (15) Urayama, K.; Okuno, Y.; Kawamura, T.; Kohjiya, S. Volume Phase Transition of Liquid Crystalline Gels in a Nematic Solvent. Macromolecules 2002, 35, 4567−4569. (16) Hayata, Y.; Nagano, S.; Takeoka, Y.; Seki, T. Photoinduced Volume Transition in Liquid Crystalline Polymer Gels Swollen by a Nematic Solvent. ACS Macro Lett. 2012, 1, 1357−1361. (17) Liu, D.; Bastiaansen, C. W. M.; den Toonder, J. M. J.; Broer, D. J. (Photo-)Thermally Induced Formation of Dynamic Surface Topographies in Polymer Hydrogel Networks. Langmuir 2013, 29, 5622−5629. (18) Zhang, Y.; Ionov, L. Actuating Porous Polyimide Films. ACS Appl. Mater. Interfaces 2014, 6, 10072−10077. (19) Ma, Y.; Zhang, Y.; Wu, B.; Sun, W.; Li, Z.; Sun, J. Polyelectrolyte Multilayer Films for Building Energetic Walking Devices. Angew. Chem., Int. Ed. 2011, 50, 6254−6257. (20) Dai, M.; Picot, O. T.; Verjans, J. M. N.; de Haan, L. T.; Schenning, A. P. H. J.; Peijs, T.; Bastiaansen, C. W. M. HumidityResponsive Bilayer Actuators Based on a Liquid-Crystalline Polymer Network. ACS Appl. Mater. Interfaces 2013, 5, 4945−4950. (21) Yu, Y.; Nakano, M.; Ikeda, T. Photomechanics: Directed Bending of a Polymer Film by Light. Nature 2003, 425, 145. (22) Yamada, M.; Kondo, M.; Mamiya, J.; Yu, Y.; Kinoshita, M.; Barrett, C. J.; Ikeda, T. Photomobile Polymer Materials: Towards Light-Driven Plastic Motors. Angew. Chem., Int. Ed. 2008, 47, 4986− 4988. (23) Liu, D.; Bastiaansen, C. M. W.; den Toonder, J. M. J.; Broer, D. J. Light-Induced Formation of Dynamic and Permanent Surface Topologies in Chiral−Nematic Polymer Networks. Macromolecules 2012, 45, 8005−8012. (24) Naciri, J.; Srinivasan, A.; Jeon, H.; Nikolov, N.; Keller, P.; Ratna, B. R. Nematic Elastomer Fiber Actuator. Macromolecules 2003, 36, 8499−8505. (25) Yu, H.-H.; Xu, B.; Swager, T. M. A Proton-Doped Calix[4]arene-Based Conducting Polymer. J. Am. Chem. Soc. 2003, 125, 1142− 1143. (26) Norman, L. L.; Badia, A. Microcantilevers Modified with Ferrocene-Terminated Self-Assembled Monolayers: Effect of Molecular Structure and Electrolyte Anion on the Redox-Induced Surface Stress. J. Phys. Chem. C 2011, 115, 1985−1995. (27) Hempenius, M. A.; Cirmi, C.; Song, J.; Vancso, G. J. Synthesis of Poly(ferrocenylsilane) Polyelectrolyte Hydrogels with Redox Controlled Swelling. Macromolecules 2009, 42, 2324−2326. (28) Takada, K.; Haseba, T.; Tatsuma, T.; Oyama, N.; Li, Q.; White, H. S. In Situ Interferometric Microscopy of Temperature- and Potential-Dependent Volume Changes of a Redox Gel. Anal. Chem. 1995, 67, 4446−4451. (29) Pei, Q.; Inganaes, O. Electrochemical Applications of the Bending Beam Method. 1. Mass Transport and Volume Changes in Polypyrrole during Redox. J. Phys. Chem. 1992, 96, 10507−10514. (30) Kim, O.; Kim, S. Y.; Park, B.; Hwang, W.; Park, M. J. Factors Affecting Electromechanical Properties of Ionic Polymer Actuators Based on Ionic Liquid-Containing Sulfonated Block Copolymers. Macromolecules 2014, 47, 4357−4368. (31) Ohta, A.; Ueki, C.; Uchiyama, Y.; Fujimori, K. Synthesis and Properties of Novel Bis(1,3-benzodithiolium)-Type Dications Containing a Biaryl Unit: New Redox Systems Undergoing Reversible Structural Changes by Electron Transfer. Heterocycles 2006, 69, 365− 375. (32) Uchiyama, Y.; Ohta, A.; Fujimori, K. Synthesis and Properties of Bis(1,3-benzodithiole)-Type Redox Systems Containing a Xylyl

to the shape change of the actuator is necessary. Although the extent of the contribution of the conformational change of bis(benzodithiolyl)bithienyl unit on bending is currently under investigation, the results in this study should open the way to build redox-triggered polymer gel actuators based on the conformational change of the molecular unit. To achieve redoxdriven actuators exhibiting reversible action, electrochemically controlled operation system should be constructed. The preparation of electrochemical actuators based on our polymer gels with flexible and high surface area carbon electrodes is now in progress.

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ASSOCIATED CONTENT

* Supporting Information S

Photos for the gelation of P1, IR spectra of P1 and PMHS, all CV scans of P1, procedure to estimate the volume change of P1 by oxidation, photograph of sample pieces for displacement measurements, SEM image of bare PVdF film, photographs of the PVdF film with P1 during oxidation; movie file recording the changes of volume and color of P1 during oxidation. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: (T.O.) [email protected]. *E-mail: (M.K.) [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. Masahiro Furusawa and Takuro Yasuda of Seiko Epson for fruitful discussions and SEM measurements, respectively.

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REFERENCES

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dx.doi.org/10.1021/la504055m | Langmuir 2014, 30, 14680−14685