Photoinduced Deformation of Rigid Azobenzene-Containing Polymer

Jan 18, 2013 - extended rigid polymer network structure incorporating ... Therefore, polymer gels composed of rigid polymer networks could be the...
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Photoinduced Deformation of Rigid Azobenzene-Containing Polymer Networks Nobuhiko Hosono,†,⊥,* Mayumi Yoshikawa,† Hidemitsu Furukawa,‡ Kenro Totani,† Kyoko Yamada,† Toshiyuki Watanabe,†,* and Kazuyuki Horie§ †

Department of Applied Chemistry, Graduate School of Engineering, Tokyo University of Agriculture and Technology, 2-24-16 Naka-cho, Koganei-shi, Tokyo 184-8588, Japan ‡ Soft & Wet Matter Engineering Laboratory, Department of Mechanical Systems Engineering, Graduate School of Science and Engineering, Yamagata University, 4-3-16 Jonan, Yonezawa-shi, Yamagata 992-8510, Japan § Japan Synchrotron Radiation Research Institute (JASRI/SPring-8), 1-1-1 Kouto, Sayo-cho, Sayo, Hyogo 679-5198, Japan S Supporting Information *

ABSTRACT: Photoresponsive poly(amide acid) (PAA) gels containing multiple azobenzene units in a rigid aromatic backbone were synthesized. A centimeter-long cantilever made up of the photoresponsive PAA gel exhibited reversible bending motions upon blue (442 nm) and visible light (>490 nm) irradiation. The network structure in the PAA gels during alternating photoirradiation of blue and visible light was characterized using in situ scanning microscopic dynamic light scattering (SMILS), which revealed reversible mesh-size changes synchronized with the photoisomerization of azobenzene moieties. The photomechanical responses of the PAA gel were likely due to photoinduced contracting and stretching motions of the polymer backbone. A numerical calculation of photon absorptions revealed that photoisomerization in a very thin layer of the surface (∼40 μm) generated large macroscopic motion and large strain in the gel cantilever. The photoresponsive capability is, however, reduced or eliminated when the PAA gels are transformed to the corresponding polyimide (PI) gels, due to the large shrinkage caused by poor solubility of the backbone in the polyimide state.

1. INTRODUCTION The development of molecularly active materials capable of responding spontaneously to external stimuli provides promising substances for materials chemistry and biomimetic engineering.1−7 Because of practical energy problems currently faced by the human race, there is an urgent need to develop stimuli-responsive materials that produce macroscopic work based on local molecular responses. Photoisomerization reactions of azobenzene derivatives have become the most common power source for molecular machines8−11 and photoresponsive polymers.12−27 Among photomechanical polymeric materials, a large response has been achieved exclusively using azobenzene-containing liquidcrystalline elastomers (azo-LCEs) and networks (azo-LCNs). Finkelmann et al. have developed photoresponsive liquid crystalline elastomers (LCEs) using azobenzene derivatives.12,13 The photomechanical effect is based on volume changes accompanied by isothermal, photoinduced order−disorder (e.g., LC−isotropic phase) transitions. Therefore, the materials were heated and kept at the operation temperature in between the LC phase. By following these pioneering works, Ikeda et al.,14−17 Broer et al.,18,19 and White et al.20−22 have reported that these azo-LCE materials are operative even below the glass © 2013 American Chemical Society

transition temperature (Tg) and expanded as glassy azobenzene liquid crystalline polymer network (azo-LCN). Recently, Tan and White and co-workers reported intriguing photomechanical responses of glassy, amorphous azobenzene polyimide showing relatively large deformation in response to polarized light.23,24 The azo-polyimide cantilevers exhibit bidirectional-bending motions upon polarized laser irradiation. The photomechanical properties are attributed to the photoinduced reorientation phenomenon typically observed in azo-materials upon polarized excitation light. Thus, although a number of efforts have been done to develop polymeric materials capable of transducing light to mechanical work, successful examples of bottom-up direct photomechanical transduction are still limited. In addition, even in above successful cases, either unidirectional alignment of azobenzene domains or polarization control of the excitation light is necessary to induce the macroscopic photomechanical motion. Regarding molecular design of photoresponsive polymers, Irie et al. reported that rigid aromatic polymers Received: October 15, 2012 Revised: December 27, 2012 Published: January 18, 2013 1017

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Scheme 1. Synthesis of Photoresponsive Poly(amide acid) (PAA) and Polyimide (PI) Gels

this system, the internal network structure with spatial inhomogeneities can be characterized statistically.40−43 In the present study, the synthesis and photomechanical behavior of photoresponsive poly(amide acid) (PAA) gels is described, and nanoscopic changes in the network structure upon photoirradiation are characterized. Poly(amide acid) and polyimide gels consisting of rigid aromatic backbones have been developed. 44−48 Here, azobenzene moieties were introduced into the rigid backbone to prepare photoresponsive PAA gels. The centimeter-long PAA gel cantilever rapidly bends and forms a 90° angle within 60 s upon 442-nm He−Cd laser irradiation. The bent cantilever smoothly returns to a straight shape upon successive visible light (>490 nm) irradiation. The reversible photoisomerization reactions of azobenzene moieties incorporated into the poly(amide acid) backbones were monitored by UV−vis spectroscopy. Employing the SMILS technique, the optimal network concentration of the gels was determined to be 7 wt %, since spatial homogeneity of the polymer network might be crucial for nano-to-macro mechanical propagation. Upon alternating blue laser and visible light irradiation, the reversible mesh-size change in the rigid polymer network was observed by the in situ SIMLS analysis. In addition, to have a clear image of the bending mechanism, the trans-to-cis photoisomerization rate in the gel cantilever was estimated by numerical model calculations based on the extinction characteristics of azobenzene chromophores, which determined that photoinduced volume shrinkage occurs only within 40 μm of the surface in 60-s irradiation but leads to a large and rapid deflection of the centimeter-long gel cantilever. Importantly, in contrast to the reported photomechanical actuators,12−27 this polymer gel is completely isotropic and works independently of the polarized direction of the incident light, proving that the operation mechanism is different than those previously reported. These our findings offer a novel material design strategy to realize a nanoscopic-to-macroscopic mechanical transduction.

bearing azobenzene moieties exhibited a photoviscosity change caused by photoinduced contraction and expansion of the polymer.28,29 More recently, Hecht et al. demonstrated contraction and stretching behaviors of rigid-rod azobenzene polymers upon photoirradiation.30 These successful examples led to the idea of propagating molecular motion through an extended rigid polymer network structure incorporating azobenzene moieties into the rigid backbone. Here we demonstrate that azobenzene-containing rigid polymer networks, which consist of aromatic poly(amide acid) backbones, show remarkable macroscopic deformation in response to blue (442 nm) and visible light (>490 nm) irradiation. The polymer network structure in the photoresponsive gels during alternating photoirradiation of blue and visible light was investigated using an in situ scanning microscopic light scattering (SMILS)31−33 technique, which revealed reversible mesh-size changes synchronized with photoisomerization of the azobenzene moieties. Therefore, polymer gels composed of rigid polymer networks could be the best architectural motif in which a small molecular deformation can lead directly to a large macroscopic deformation. A dynamic light scattering (DLS) study of polymer networks was first introduced by Tanaka et al.,34 and now has became the conventional method for estimating the mesh-size of a polymer network nondestructively. The relaxation time obtained by DLS analysis is related to the cooperative diffusion constant corresponding to mesh size.35,36 For conventional DLS analysis of gelling systems, a static scattering component due to the inherent inhomogeneity is inevitably included in a DLS signal and needs to be considered.35−39 Such a static component sometimes depends strongly on the measurement position in the polymer network. To overcome this problem, an apparatus was developeda scanning microscopic light-scattering (SMILS) system31−33in which DLS data are successively acquired at different positions in a sample and collected into the ensemble-averaged correlation function (vide inf ra). Using 1018

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nm for UV light irradiation, and a halogen lamp (Luminar Ace LA-150 SAE, Hayashi) with a long-pass filter (VY-50, Toshiba) of λ > 490 nm for visible light irradiation which caused the reverse photoreaction. Thermal reverse isomerization reactions also were measured at 20 °C. 2.6. Scanning Microscopic Light-Scattering (SMILS) Analysis: Theoretical Background. Normally, polymer gels have spatial inhomogeneity, which is inevitably formed through a cross-linking reaction.36 To overcome obstacles in processing such a nonergodic medium, a scanning microscopic light scattering (SMILS) technique was developed and the theoretical basis of the data analysis technique was proposed.31−33 During the data acquisition of SMILS, a number of single DLS measurements were conducted at different positions while scanning the laser spot. The position-dependent static fluctuation (socalled “speckle-pattern”) resulted in the nonrelaxation component in the time-averaged autocorrelation function, g(1) t (q,τ), obtained at one position of the scattering volume. The collected scattering data arrays of g(1) t (q,τ) then were spatially averaged and summarized into the ensemble-averaged autocorrelation function g(1) en (q,τ), which also has the nonrelaxation component that is a unique quantity characterizing the static inhomogeneity of the sample as a whole. For the direct measurements, the time-averaged autocorrelation function was defined as

2. EXPERIMENTAL SECTION 2.1. Materials. All reagents were purchased from Tokyo Chemical Industries, Wako Pure Chemical Industries and Lancaster Chemicals, and were used without further purification unless otherwise noted. Dehydrated N,N′-dimethylformamide (DMF) was used as received from Wako Pure Chemical Industries. The 4,4′(hexafluoroisopropylidene)diphthalic anhydride (6FDA), oxidiphthalic anhydride (ODPA), and pyromellitic dianhydride (PMDA) were received from Wako Pure Chemical Industries and used after recrystallization from acetic anhydride. The triamine cross-linker, 1,3,5-tris(aminophenyl)benzene (TAPB), was synthesized according to a previously published procedure45 and used after recrystallization from methanol. 2.2. General Instrumentation. Analytical size-exclusion chromatography (SEC) was performed at 20 °C using a JASCO model JBIP-I GPC system using DMF with 30 mM LiCl as the eluent at a flow rate of 1.0 mL/min on a polystyrene gel column (GMHHR-M, Tosoh). The molecular weight calibration curve was obtained using standard polystyrenes. Electronic absorption spectra were recorded on a JASCO model VP-570 UV/vis spectrophotometer. Infrared (IR) spectra were recorded at 25 °C on a JASCO model FT/IR-4100 Fourier transform infrared spectrometer with a diffuse reflectance accessory. The solid samples for IR spectroscopy were prepared by drying under vacuum at room temperature. KBr powder was used as a support matrix. Light intensity at the irradiated area was measured using a Newport model 840-C optical power meter for a He−Cd laser, and Ushio model UVD-S405 for a high-pressure mercury UV lamp. 2.3. Preparation of Photoresponsive Poly(amide acid) and Polyimide Gel. 4,4′-Diaminoazobenzene (DAA) and 4,4′(hexafluoroisopropylidene)diphthalic anhydride (6FDA) were dissolved at a predetermined concentration (5, 6, 7, 8, 10, and 12 wt %) in DMF at room temperature in a 4:3 molar ratio for the preparation of amide acid oligomers (Scheme 1). The resulting 6FDA/DAA amide acid oligomer solution was kept in an iced water bath for 4 h to minimize its molecular weight distribution by decreasing the weightaverage molecular weight (Mw). Characteristics of the resulting 6FDA/ DAA oligomer included the values of Mn = 2100 g/mol and Mw/Mn = 1.15 on SEC. A DMF solution of TAPB prepared at the above concentration was added to the 6FDA/DAA oligomer solution with a stoichiometric ratio of the amine group to the terminal acid anhydride group. The mixed solution became viscous within several minutes, affording a pale yellow PAA(6FDA/DAA) gel after standing overnight. The corresponding polyimide gels, PI(6FDA/DAA), were synthesized by allowing PAA(6FDA/DAA) gels to stand in a mixed solvent of acetic anhydride/pyridine/DMF (1/1/3, v/v) for seven days, then replacing the mixed solvent with pure DMF. Diffuse reflectance FT-IR spectra of PAA(6FDA/DAA) and PI(6FDA/DAA) gel are given in Figure S1.49 2.4. Bending Experiments of Photoresponsive Gel Cantilever. A cylinder-shape gel cantilever (1.5 cm-long, 0.2 mm-diameter) was prepared using a glass tube as a mold with an inner diameter of 0.2 mm. For bending experiments and in situ SMILS measurements, a network concentration of 7 wt % was adopted (vide inf ra). The gel was removed from the mold and placed into a vial with DMF. Photoirradiation for the bending experiments and in situ SMILS measurements was conducted at 20 °C using a He−Cd laser (Kimmon model IK5351R-D, 35 mW output power, 20 mW at irradiated area, 1 mm beam diameter) of λ = 442 nm for blue laser irradiation, and a halogen lamp (Luminar Ace LA-150 SAE, Hayashi) with a long-pass filter (VY-50, Toshiba) of λ > 490 nm for visible light irradiation. The bending motion of the cantilever was recorded by a video camera and analyzed on a computer. 2.5. UV−vis Spectroscopy of Film Samples. Film samples of PAA(6FDA/DAA) gels were fabricated by preparing pregel solutions between two fused silica glass plates. The gap between the two glass plates was not controlled, but was pressed as hard as possible to obtain thin films. The estimated thickness of films was ca. 1 μm as determined from absorbance measurements. Photoirradiation of the film samples was conducted at 20 °C using a high-pressure mercury lamp (USH250, Ushio) with a band-pass filter (C-39A, Toshiba) of λ = 360−470

g t(2)(q, τ ) ≡

⟨I(q, t )I(q, t + τ )⟩t ⟨I(q, t )⟩t ⟨I(q, t + τ )⟩t

(1)

where q is the scattering vector, τ is the correlation time, I(q,t) is the scattering intensity, and ⟨···⟩t indicates the time-averaging operation. For inhomogeneous media such as gels, the extended version of socalled Siegert relationship was used to obtain gt(1)(q,τ) from 31,32,50 g(2) t (q,τ):

g t(1)(q, τ ) = γ −1 1 + g t(2)(q, τ ) − g t(2)(q, 0)

(2)

where γ (0 ≤ γ ≤ 1) is the coherence factor, which takes into account the incoherence effect arising from the finite area of the photodetector. (1) Taking the position dependence of g(1) t (q,τ) into account, gen (q,τ) is calculated as the space average of the time-averaged correlation function:31,40 (1) gen (q, τ ) =

=

⟨⟨I(q, t )⟩t g t(1)(q, τ )⟩sp ⟨I(q, t )⟩en ⟨⟨I(q, t )⟩t γ −1 1 + g t(2)(q, τ ) − g t(2)(q, 0) ⟩sp ⟨I(q, t )⟩en

(3)

where ⟨···⟩sp means space-averaging operation. If one focuses only on the dynamic component of the dynamic structure factor, the normalized dynamic component of g(1) en (q,τ) can be calculated by the following equation: (1) Δgen (q, τ ) =

=

(1) (1) gen (q, τ ) − gen (q, ∞) (1) (1) gen (q, 0) − gen (q, ∞)

⟨⟨I(q, t )⟩t 1 + g t(2)(q, τ ) − g t(2)(q, 0) ⟩sp − ⟨⟨I(q, t )⟩t 2 − g t(2)(q, 0) ⟩sp ⟨I(q, t )⟩en − ⟨⟨I(q, t )⟩t 2 − g t(2)(q, 0) ⟩sp

(4) (1) Using the baseline of gen (q,τ), the static and the dynamic components, ⟨Is(q)⟩en and ⟨Id(q)⟩en, of the ensemble-averaged scattering intensity ⟨I(q,t)⟩en can be obtained separately. Thus, ⟨Is(q)⟩en and ⟨Id(q)⟩enwere determined using: (1) ⟨Is(q)⟩en = ⟨I(q, t )⟩en gen (q, ∞)

(5)

(1) ⟨Id(q)⟩en = ⟨I(q, t )⟩en [1 − gen (q, ∞)]

(6)

The dynamic component of the ensemble-averaged correlation function, Δg(1) en (τ), was analyzed with an inverse Laplace transform (1) (τ) can be approximately (ILT) method.31,32 In general, Δgen expressed by a superposition of many exponential functions as: 1019

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n ⎛ τ ⎞ (1) ⎟⎟ Δgen (τ ) = N ∑ Pen(ln τR, i) exp⎜⎜− ⎝ τR, i ⎠ i=1

(7)

where τR,i is a geometric progression of relaxation time expressed as

τR, i = τR,min(τR,max /τR,min)(i − 1)/ n

(i = 1, 2, ..., n)

and N is a normalized factor defined as τR,max 1 N = ln n τR,min

(8)

(9)

The distribution function of relaxation time Pen(ln τR) satisfies the following relation: ln τR,max

∫ln τ

Pen(ln τR ) d[ln τR ]

R,min

n

=

∑ Pen(ln τR, i) i=1

ln τR,max − ln τR,min n

=1 (10)

By using eq 7, the distribution function of relaxation time, Pen(ln τR), can be obtained directly from Δg(1) en (τ). The fitting was performed with τR,min = 10−6 s, τR,max = 101 s, and n = 70 for all cases of ILT analysis in the present work. For the quantitative analysis of dynamics, the distribution function was fitted to a logarithmic Gaussian distribution: ⎡ (log τ − μ)2 ⎤ 10 R ⎥ Pen(q, τR ) = A exp⎢ − ⎥⎦ ⎢⎣ 2σ 2

Figure 1. Schematic illustration of the scanning microscopic lightscattering (SMILS) apparatus customized for the experiments with three types of light sources. (a) Typical setup for SMILS analysis. (b) Typical setup for He−Cd laser irradiation. The light sources can be switched between a He−Cd laser (442 nm) and a halogen lamp equipped with a VY-50 filter (>490 nm) for blue and visible light irradiation, respectively.

(11)

where A is amplitude, μ is the average of the logarithmic relaxation time, and σ 2 is the dispersion. From σ, the distribution width of the relaxation time can be estimated. Here, the average relaxation time was given by ⟨τR⟩ = 10μ with a standard deviation of σ(⟨τR⟩) given by ⟨τR⟩ ± σ(⟨τR⟩) = 10μ±√2σ. 2.7. Customized Setup for in situ SMILS upon Photoirradiation. The scanning microscopic light-scattering (SMILS) system31,32 adjusted for the present study is illustrated in Figure 1. Three types of light sources were used. For light scattering measurements, a 22 mW He−Ne linearly polarized laser (633 nm) was used. For trans-to-cis photoisomerization, a 35 mW He−Cd linearly polarized laser (Kimmon model IK5351R-D, 442 nm) was used. For cis-to-trans photoisomerization, a halogen lamp (Luminar Ace LA-150 SAE, Hayashi) with a VY-50 long-pass filter (λ > 490 nm) was used. Incident beams from the two lasers were focused on the sample with an objective lens with a very long working distance. The beam diameter at scanning volume was about 3 μm. The sample holder was moved vertically by a computer-controlled stepping motor with a minimum step of 1 μm.

in light intensity. These bending and unbending of the cantilever could be altered by changing the wavelength of the excitation light. When a photoabsorptive material shows mechanical responses under the exposure to, in particular, strong actinic light such as laser, one has to consider about the contribution of photothermal effects.22 If the bending motion of a cantilever was based on the photothermal mechanism, the visible light could not give any effect on the recovery motion. As shown in Figure 2b, upon visible light irradiation, the bent cantilever rapidly unbends and recovers to form the initial straight form. On the contrary, under the thermal backward reaction (the region indicated by Δ in Figure 2c), it takes much longer time (∼1.5 h) for the complete recovery. This fact that the visible light irradiation causes rapid recovery of the bent gel clearly indicates the photomechanical motion of our system is predominantly based on the photoisomerization of azobenzene. In addition, since the cantilever was irradiated in DMF, the absorptive heat could be immediately diffused into the medium and not to be accumulated on a surface of the cantilever. The He−Cd laser, we used for the bending experiments, is linearly polarized. In order to examine the bending motions under the exposure to different polarization directions, we irradiated the cantilevers with the He−Cd laser, which is polarized parallel or orthogonal to the long axis of the cantilever. In either polarization direction, as a consequence, the cantilevers bent in the same way in the identical time. The bending PAA gel contains 93 wt % of DMF, and made up through a random cross-linking reaction with TAPB. Therefore, the gel is totally isotropic, and does not have any ordered structure. Taking these facts into account, we concluded that our system operates in fundamentally different mechanism

3. RESULTS AND DISCUSSION 3.1. Photoresponse of PAA(6FDA/DAA) Gel Cantilever. Exposure of a cantilever composed of PAA(6FDA/ DAA) gel in DMF to a He−Cd laser at 442 nm, which causes trans-to-cis photoisomerization of the azobenzene moiety, produced bending in the irradiation direction of the actinic light (Figure 2a). The time course of the extent of bending and unbending of the cantilever (1.5 cm-long, 0.2 mm-diameter) is shown in Figure 2b, where x/l represents the ratio of displacement, x, of the bottom of the cantilever against the length, l, of the bending part. When laser intensity was 20 mW, the deflection was nearly completed in 60 s. When the bent cantilever was exposed to visible light above 490 nm, which causes cis-to-trans backward photoisomerization of the azobenzene chromophore, unbending of the cantilever occurred immediately and the initial straight form was restored in 5 min. The time difference observed between the trans-to-cis and cis-to-trans photoisomerizations may be due to differences 1020

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Figure 2. Photographs of PAA(6FDA/DAA) gel cantilever (1.5 cm long, 0.2 mm thick) upon (a) He−Cd laser (442 nm) irradiation and (b) visible light (>490 nm) irradiation. (c) Time course of the extent of bending, x/l, of the PAA(6FDA/DAA) gel cantilever upon alternating exposure to a He−Cd laser (442 nm) and visible (>490 nm) light, and the thermal reverse reaction (Δ) at 20 °C, where x is the displacement of the bottom of the cantilever and l is the length of the bending part of the cantilever.

Figure 3. Spectral changes in UV−vis absorption spectra of PAA(6FDA/DAA) gels. (a) Trans-to-cis photoisomerization during UV light (360−470 nm) irradiation. (b) Time course of absorption at λmax = 380 nm during UV irradiation. (c) Cis-to-trans photoisomerization after visible light (>490 nm) irradiation. (d) Time course of absorption at λmax = 380 nm during visible light irradiation. (e) Reversible absorption change in PAA(6FDA/DAA) gel upon alternating exposure to UV (360−470 nm) and visible (>490 nm) light, and the thermal reverse reaction (Δ) at 20 °C.

from previously reported LCEs/LCNs and glassy polymers, in which either a macroscopically oriented poly/monodomains or polarization control of actinic light is always crucial for the photomechanical response.12−27 In our system, the cross-linked PAA(6FDA/DAA) main chain at the irradiated surface contracted isotropically, upon trans-to-cis photoisomerization of the azobenzene, leading to the bending motion of the cantilever. 3.2. Photoisomerization of Azobenzene Moieties Followed by UV−vis Spectroscopy. Using UV−vis spectroscopy, the photoisomerization behavior of azobenzene units incorporated into the rigid polymer network was investigated (Figure 3). Before the measurements, a thin film of PAA(6FDA/DAA) gel sandwiched between two fused silica glasses was exposed to visible light (>490 nm) until the cis-totrans photoisomerization reached the photostationary state at 20 °C. Typical absorption spectra of trans-azobenzene moieties of PAA(6FDA/DAA) gels in DMF exhibit an absorption maxima at 380 nm due to π−π* transitions and at 500 nm due to n−π* transitions ascribed to the lone pair electrons of nitrogen.51 Results showed that UV light (360−470 nm, 29 mW cm−2) irradiation induced trans-to-cis isomerization of the azobenzene moieties, even in the gel form. The absorbance at 380 nm decreased rapidly followed by a slight increase in the absorbance at 470 nm as shown in Figure 3a,b. Cis-to-trans isomerization also was performed by illuminating visible light (Figure 3c,d). The reverse photoisomerization proceeded at 20

°C with recovery of the absorption band of the π−π* transition at 380 nm. On the basis of the absorption change, trans-to-cis photoisomerization kinetics in 6FDA/DAA oligomer and PAA(6FDA/DAA) gel was investigated.49 When trans-azobenzene groups with initial concentration [tr]0 are irradiated with a continuous wave of excitation light, the rate of the change in trans-azobenzene concentration, [tr], as it approaches its equilibrium value, [tr]∞, is determined under continuous light exposure from eq 12:49,52,53 ⎛ [tr] − [tr]∞ ⎞ [tr]0 ln⎜ 0 t ⎟ = 2.3 × 103I0εtf Φt − c [tr]0 − [tr]∞ ⎝ [tr] − [tr]∞ ⎠ (12) −2

−1

where t is exposure time, I0 (ein cm s ) is incident light intensity, εt (L mol−1 cm−1) is the molar extinction coefficient of trans-azobenzene groups, f is the compensation factor, and Φt−c is the quantum yield of the trans-to-cis photoisomerization of azobenzene groups. The change in concentration of transazobenzene groups, [tr], was calculated from the absorbance at 405 nm using εt = 4.16 × 104 M−1 cm−1. The photoisomerization reaction proceeded as first order in oligomer solution, but in gel films it proceeded with the same rate as in 1021

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the solution up to 86% conversion and then deviated from the first-order kinetics (Figure S2, Supporting Information). This slow down in rate only at the final stage of conversion is similar to the photochemical reaction in an amorphous polymer matrix below Tg.52,53 The value of Φt−c for azobenzene groups in the main chains of a PAA(6FDA/DAA) gel is approximately one-fifth of that for azobenzene in organic solvent, and is the same as that for azobenzene groups in 6FDA/DAA oligomer. This is reasonable because the photoisomerization of azobenzene groups is not constrained by the network structure but is restricted by the existence of 4,4′-substituted chains directly attached to the phenyl rings of the azobenzene moieties. The obtained and reported quantum yields of azobenzene are summarized in Table 1. Table 1. Quantum Yields for Trans-to-Cis Photoisomerization of Azobenzene Moieties in 6FDA/DAA Oligomer and PAA(6FDA/DAA) Gel as Well as Those for Azobenzene in Organic Solvents49 system

temperature (°C)

Φt−c

6FDA/DAA oligomer in DMF49 PAA(6FDA/DAA) gel in DMF49 azobenzene in ethyl acetate52 azobenzene in ethanol52

20 20 15 15

0.037a 0.039a 0.09,b 0.25c 0.12,b 0.15c

π−π* excitation upon 360−470 nm irradiation. bπ−π* excitation upon 360 nm irradiation. cn−π* excitation upon 440 nm irradiation. a

Figure 3e shows the reversible absorbance change induced by alternating UV and visible light irradiation to a PAA(6FDA/ DAA) gel film. Absorbance changes were clearly the result of the reversible trans−cis photoisomerization of azobenzene units in the polymer network. Using the mercury lamp (λ = 360−470 nm, 29 mW cm−2), saturation of the absorbance change was attained in ca. 2−3 min upon UV irradiation. Response times of the forward and reverse reactions were nearly the same. The thermal reverse reaction proceeded relatively slowly, requiring more than several hours. 3.3. Determination of Optimal Network Concentration by SMILS. As described above, submicrometer density fluctuations of the polymer chains are frozen during the crosslinking reaction, resulting in inherent spatial inhomogeneities. These inhomogeneities are strongly dependent on the initial concentration of the polymer chains involved in the crosslinking reaction. Thus, first the optimal polymer concentration of PAA(6FDA/DAA) gels was determined to prepare the most homogeneous network structure. Several PAA(6FDA/DAA) gels composed of stoichiometric amounts of 6FDA/DAA oligomer and TAPB with varying total solid concentrations (wt %) were prepared in DMF. Using standard SMILS measurements,31 the static component of ensemble-averaged scattering intensity ⟨Is(q)⟩en was quantified as a function of polymer concentration as shown in Figure 4a. The value of ⟨Is(q)⟩en was related to the static inhomogeneities on a submicrometer scale. The value of ⟨Is(q)⟩en strongly depended on the polymer concentration in the gel. The dynamic component of scattering intensity ⟨Id(q)⟩en was rather small compared to the static component (Figure 4a). A modest minimum of ⟨Is(q)⟩en occurred in the intermediate region (8 wt %). At lower polymer concentrations, the gels tended to form a loose network structure containing many defects (fractal gel) and scattering intensity decreases. In contrast, at greater polymer

Figure 4. SMILS data for the PAA(6FDA/DAA) gel prepared at different concentrations. (a) Static ⟨I s (q)⟩ en and dynamic ⟨Id(q)⟩encomponents of ensemble-averaged scattering intensity ⟨I(q,t)⟩en. (b) Concentration dependence of cooperative diffusion constant Dcoop. (c) Concentration dependence of the standard deviation σ of dynamic relaxation time τR measured at different angles.

concentrations, the gel network became highly entangled with many waste chains existing as pendant chains (entangled gel), and the gel apparently became homogeneous again due to a high concentration of the polymer chains. Assuming that the dynamic relaxation time is based on gel mode, then the cooperative diffusion coefficient, Dcoop, of the network is obtained as 1 Dcoop = 2 q ⟨τR ⟩ (13) The values of Dcoop as a function of polymer concentration in the PAA(6FDA/DAA) gels are shown in Figure 4b. The Dcoop curve shows a moderate peak at 7 wt %, which corresponds to the minimum of ⟨Is(q)⟩en at 8 wt %. This tendency is quite different than that of vinyl polymer gels,31 which fit well to a power law function of the volume fraction of the gel. For the nanoscale inhomogeneity, σ shows a minimum at 6 wt %, similar to the Dcoop curve (Figure 4c). The increase in 1022

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Dcoop and decrease in σ at concentrations greater than 10 wt % were caused by the crossover of the system to the entangled gel region. These experimental results suggest that a homogeneous network structure can be realized when the polymer concentration is about 6−8 wt % for the PAA(6FDA/DAA) gel. Taking these results into account, the present experiments of photosensitive PAA gels were conducted with a polymer concentration of 7 wt %. 3.4. In situ Characterization of Photoinduced MeshSize Change by SMILS. Using the customized SMILS system for in situ measurements, the changes in the mesh size of the PAA(6FDA/DAA) gel cantilever could be measured in DMF upon photoirradiation. During alternating irradiation using He−Cd blue laser (442 nm) and visible (>490 nm) light, reversible changes in relaxation time from dynamic diffusion of the polymer strands in the network were observed successfully (Figure 5). The relaxation time obtained by SMILS for the PAA(6FDA/DAA) gel with the cis-azobenzene moieties after He−Cd laser irradiation shifted to the faster side as compared

to the gel consisting of trans-azobenzene moieties after visible light irradiation (Figure 5a). The correlation length ξ is estimated from Dcoop using the following Einstein−Stokes relation ξ=

kBT 6πηDcoop

(14)

where kBT is the Boltzmann energy and η is the viscosity of the solvent (DMF) at a given temperature. Thus, apparent mesh size of the polymer network can be defined by 2ξ. The mesh size was estimated through the SMILS procedure, and was ca. 2.1 nm in the trans form, and 0.83 nm in the cis form (Figure 5b). The reversible mesh-size change remained constant after several cycles and was accompanied by macroscopic deformation of the PAA(6FDA/DAA) gel cantilever. 3.5. A Numerical Calculation of Photoisomerization Rate in Irradiated Gels. To understand the bending mechanism schematically, an estimate of the depth of excitation light penetration into the gel upon continuous irradiation was determined. The isomerized fraction of azobenzene moieties upon blue light irradiation can be provided by a numerical model calculation. For the simplified model, a flat gel sheet containing azobenzene moieties was considered at the actual concentration of DAA monomer in the PAA(6FDA/DAA) gel (7 wt %). When the gel sheet is exposed to excitation light, the number of photons absorbed in between l and (l + dl) thick region in the gel is expressed as: d I (l ) = I (l ) − I (l + d l )

(15)

which can be simply defined by the following Lambert−Beer relation: I(l + dl) = I(l)10−OD dl ∴ dI(l) ≅ OD dl I(l)

(16)

where I(l) is intensity of the excitation light at depth l from the irradiated surface of the gel sheet and OD is optical density attributed to azobenzene chromophores in the PAA(6FDA/ DAA) gel. Here, the gel sheet can be considered a laminated object with a layer thickness of dl. Upon blue light irradiation, azobenzene moieties undergo trans-to-cis photoisomerization. The photoisomerization brings about enormous change in the absorption in the π−π* transition region. The values for ODt and ODc, the optical density, were defined for the ideal PAA(6FDA/DAA) gel with 100% trans- and 100% cis-azobenzene, respectively. Thus, the number of photons absorbed by trans-azobenzenes, dIt(l), is described as: dIt(l) =

ODt dnt(l) d I (l ) ODt dnt(l) + ODc dnc(l)

(17)

where dnt and dnc are the abundance ratio of trans- and cisazobenzene, respectively, at a depth l. Therefore, the time increment for the amount of trans-azobenzene, dnt+, that isomerizes to the cis form in unit time dt is determined as: dnt +(l) = Φt − cdIt(l) dt

(18)

The time evolution and depth profile of the trans-azobenzene fraction in the gel sheet was calculated by successive iteration of eqs 17 and 18 in terms of dl and dt. In this numerical calculation, cis-to-trans backward isomerization was not considered and the gel sheet was assumed to consist initially

Figure 5. Reversible changes in (a) relaxation-time distribution function P(τR) of the gel mode and (b) relaxation time τR and estimated mesh-size 2ξ of PAA(6FDA/DAA) gel obtained by SMILS analysis during alternating 30-min irradiations using He−Cd laser (442 nm) and visible (vis) light (halogen lamp with VY-50 filter, >490 nm). 1023

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of 100% trans-azobenzene. The increment value for dl and dt were fixed at 0.01 μm and 1 s, respectively. An azobenzene concentration of 0.083 mol L−1 and Φt−c of 0.039 were used. The molar extinction coefficient and optical density at the selected wavelength used in this calculation are summarized in Table 2. Table 2. Molar Extinction Coefficient ε and Optical Density OD of PAA(6FDA/DAA) Gel ε × 104 cm−1 mol L−1

OD (103 cm−1)

wavelength (nm)

trans, εt

cis, εc

trans, ODt

cis, ODc

405 442

4.3 1.7

0.11 0.79

3.6 1.4

0.091 0.66

Two calculations using different light intensities were performed with respect to a PAA(6FDA/DAA) gel irradiated with a 405-nm high-pressure mercury lamp (29 mW cm−2; photon density 9.82 × 10−8 ein cm−2 s−1) and a gel irradiated with 442 nm He−Cd laser (2.5 × 103 mW cm−2; photon density 9.42 × 10−6 ein cm−2 s−1). Resulting depth profiles are shown in Figures 6a and 6b. The light intensity decays exponentially in the gel medium. For gels exposed to 405 nm light, the excitation light gradually penetrates into the surface and the trans component becomes nearly extinct in 30 s (Figure 6a). In the gel exposed to 442 nm He−Cd laser, the calculated depth profile of the trans form became steep and the isomerization proceeded up to penetration 40-μm deep into the surface during 60 s irradiation (Figure 6b). The trans form fraction in the 1-μm thick region (gray area in Figure 6a) is plotted in Figure 6c as a function of irradiation time. As shown in Figure 6c, the calculated decay curve can be superimposed with the observed absorption decay of 1-μm thick gel film exposed to 405 nm light irradiation. Considering these calculations, the bending motion of the centimeter-long cantilever could be driven by photoisomerization within a very thin layer of the surface (∼40 μm). Since incident photons are absorbed by azobenzenes in the gel, the photogenerated contractions will diminish with the penetration depth. Bending behavior of the cantilever is originated from the contractile/expansive strain gradients generated in between the irradiated surface and the inner part.54,55 Generally, magnitudes of the photogenerated strains on glassy azo-LCNs are considerably small (∼2%)17−21 when compared with the photogenerated strain on the elastomeric azo-LCEs (∼20%).12 In our systempolymer network swelling a large amount of solventit could be possible that the network squeezes the solvent out when exposed to the actinic light since the network architecture still has much free volume to attain a larger contraction. Therefore, the photoinduced deformation of the network could provide a sufficient strain to the irradiated surface and lead to such a large magnitude of deflection. 3.6. Photoresponsive Capability of Rigid Polymer Networks with Different Backbones. To confirm that this phenomenon was caused by photoisomerization, a similar gel cantilever of the same shape without azobenzene moieties was made using 4,4′-oxidianiline (ODA) instead of DAA. This PAA(6FDA/ODA) gel, as expected, did not show any photoresponse upon He−Cd laser irradiation. Dependence of photoresponsive ability on the chemical structures of poly(amide acid) (PAA) and polyimide (PI) gels also was investigated (Table 3). A series of photoresponsive

Figure 6. Calculated time-dependent depth profiles of the trans form of azobenzene mol fraction in the photoresponsive gel irradiated using (a) UV lamp (405 nm, 29 mW cm−2) and (b) He−Cd laser (442 nm, 2.5 × 103 mW cm−2). The horizontal axis indicates penetration depth of the incident light in the gel sample. (c) Superimposed plots of the calculated time-evolution of the trans form of the azobenzene mol fraction at a 1 μm-thick region in the irradiated gel [gray colored area in part a (red circles)] and the absorption decay at 380 nm of 1 μmthick gel film irradiated with a high-pressure mercury lamp with C-39A filter (405 nm, 29 mW cm−2) [blue squares, the same data shown in Figure 3b].

PAA gels with different acid dianhydrides, oxidiphthalic anhydride (ODPA), and pyromellitic dianhydride (PMDA), were prepared by a procedure identical to that for the PAA(6FDA/DAA) gels. These two PAA gels, designated PAA(ODPA/DAA) and PAA(PMDA/DAA) gels, respectively, also showed reversible photomechanical responses. PAA(PMDA/ODA) gels swelling 7 wt % of DAA monomer in DMF did not show photochemical bending behavior although it shows photoisomerization. This fact indicates that azobenzene moiety should be embedded into molecular network to propagate the microscopic shape change to macroscopic deformation. The bending behavior of PAA gels was observed only in good solvents for aromatic poly(amide acid) chains such as DMF, N-methyl-2-pyrrolidone (NMP), and N,N′-dimethylacetamide (DMAc). After chemical imidiza1024

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Table 3. Photoresponsive Capabilities of Poly(amide acid) (PAA) and Polyimide (PI) Gelsa

diamine

crosslinker

6FDA 6FDA 6FDA ODPA ODPA PMDA PMDA PMDA

ODA DAA DAA DAA DAA DAA DAA ODA

TAPB TAPB TAPB TAPB TAPB TAPB TAPB TAPB

gel type

photoresponseb

PAA PAA PI PAA PI PAA PI PAA (swelled with DAA monomer)c

N Y N Y N Y Δ N

ASSOCIATED CONTENT

S Supporting Information *

IR spectra of PAA(6FDA/DAA) and PI(6FDA/DAA) gels and photoisomerization of azobenzene in PAA(6FDA/DAA) gel. This material is available free of charge via the Internet at http://pubs.acs.org/.

monomer anhydride

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (N.H.);[email protected] (T.W.). Telephone/Fax: +81-42-388-7289 (T.W.). Present Address ⊥

Laboratory of Macromolecular and Organic Chemistry, Institute for Complex Molecular Systems, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands.

a All samples were prepared at 7 wt % solid concentration. bY: reversible response. N: inactive. Δ: irreversible response. cThis sample was prepared by immersing PAA(PMDA/ODA) gel in a 7 wt % DMF solution of DAA monomer.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS N.H. is thankful to the Japan Society for the Promotion of Science (JSPS) Young Scientist Fellowship. This work was supported by a Grant-in-Aid for Scientific Research (B) from the Ministry of Education, Culture, Sports, Science and Technology, Japan (No. 20360112 and No. 23360109).

tion with pyridine and acetic anhydride (Scheme 1), the resulting polyimide PI(6FDA/DAA) and PI(ODPA/DAA) gels did not show any macroscopic photoresponse. Interestingly, only the PI(PMDA/DAA) gel possessed a photoresponsive ability. The PI(PMDA/DAA) gel cantilever exhibited a bending motion upon He−Cd laser irradiation; however, it did not return to its initial state upon irradiation with visible light. The extinct or depressed photoresponsive capability of the PI gels could be due to backbone aggregation caused by poor solubility of the polyimide in DMF, since high volume shrinkage of the gels occurred after imidization. The rigid and highly extended polymer chain may promote propagation of the nanoscopic deformation to the macroscopic level, while sufficient free spaces between the networking chains are required to produce such large and rapid volume changes.

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REFERENCES

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4. CONCLUSIONS Photoresponsive poly(amide acid) (PAA) gels that show reversible rapid and sharp bending motions upon alternating blue (442 nm) laser and visible light (>490 nm) irradiation were prepared successfully. For the PAA(6FDA/DAA) gel, nanoscopic in situ observation of the network structure was accomplished using scanning microscopic light scattering (SMILS) analysis. Upon alternating irradiation of blue and visible light, the cantilever composed of PAA(6FDA/DAA) gel exhibited dynamic photoinduced deformation. These reversible changes in the nanoscopic mesh size were characterized by SMILS as ca. 2.1 nm for the trans state and 0.83 nm for the cis state. This is the first report describing the measurement of dynamic mesh-size changes during photoinduced structural deformations of polymer networks. The calculations based on photon extinctions successfully described the time-evolution of the photoisomerized fraction in the gel, and indicated that photoisomerization within a very thin layer of the surface (∼40 μm in 60 s) generated large macroscopic motion of the centimeter-long cantilever. This bottom-up concept for propagating molecular motion through an extended rigid polymer network structure could produce nano-to-macro mechanical transduction. These findings provide a novel molecular design approach for creating gel actuators, in which phase transitions, osmotic changes, or ion transfers played a crucial role in their operation.1−7 1025

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