Research Article www.acsami.org
Tunable Upper Critical Solution Temperature of Poly(N‑isopropylacrylamide) in Ionic Liquids for Sequential and Reversible Self-Folding Soonyong So†,‡ and Ryan C. Hayward*,† †
Department of Polymer Science and Engineering, University of Massachusetts Amherst, Amherst, Massachusetts 01003, United States ‡ Center for Membranes, Korea Research Institute of Chemical Technology, Daejeon 34114, South Korea S Supporting Information *
ABSTRACT: We demonstrate sequential folding of micropatterned polymer actuators by tuning the upper critical solution temperature (UCST) of poly(N-isopropylacrylamide) (PNIPAM) copolymers in the ionic liquid (IL) 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide. Incorporation of comonomers having different hydrogen-bonding capacities, acrylic acid and methyl acrylate, is shown to shift the UCST of PNIPAM to higher and lower temperatures, respectively. Relying on the ability to tune the transition temperature through copolymerization along with the wide thermal range afforded by the IL as a solvent, we fabricated a photopatterned self-folding device which shows reversible and sequential bending of three sets of hinges. Such sequential and reversible bending of microactuators offers potential for the design of complex self-folding origami and soft robots. KEYWORDS: sequential folding, ion-gel, upper critical solution temperature, poly(N-isopropylacrylamide), microactuator
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INTRODUCTION Self-shaping of 2D sheets into 3D structures through a welldefined sequence of steps has attracted considerable interest in the area of origami-inspired materials and machines because many complex configurations cannot be accessed by systems wherein all folds are activated simultaneously.1−5 To date, two main approaches have been followed to enable sequential activation of two or more active sites incorporating stimuliresponsive materials.3−11 One set of approaches has relied on local increases in temperature provided by Joule heating3,4 or absorption of light6,7 applied in a well-defined sequence to identical actuators. While capable of defining an almost arbitrary sequence of folding events, this approach requires a high degree of spatiotemporal control over the electrical or optical stimulus driving folding. Alternatively, a second set of approaches has taken advantage of several different actuating materials, each exhibiting a distinct response to changes in temperature,5,8−10 absorbing a different wavelength of light,11 or responding to different (electro-)chemical stimuli.12 However, both of these strategies for sequential self-folding have so far been restricted to macroscopic (centimeter-scale) dimensions and have largely made use of heat-shrink or oneway shape memory materials, meaning that subsequent unfolding is either impossible or requires manual flattening.3−11 Hydrogels have been the focus of many studies on selfshaping and actuating materials because they can be rendered © XXXX American Chemical Society
responsive to a wide variety of stimuli (e.g., temperature, solute concentration, pH, magnetic or electric fields) and can often be interfaced with biological systems.13−17 However, their reliance on water as a “working fluid” presents a number of important limitations; for example, the need to work in a relatively narrow temperature range and in aqueous solution or humid atmosphere to avoid water evaporation.17 In this regard, we suggest that the use of responsive gels based on ionic liquids (ILs), which have almost vanishing volatility,18 should represent an attractive alternative for the design of self-shaping materials. The low vapor pressure and high thermal stability (often decomposing only above 400 °C) of many ILs provides a wide window for operation without concerns about solvent evaporation.19−22 Despite this potential, however, there have been relatively few studies focused on tuning the phase transition behavior of polymer/IL solutions23,24 or on thermally responsive swelling of IL-based gels,25,26 and we are not aware of any reports of self-folding systems that take advantage of these materials. Poly(N-isopropylacrylamide) (PNIPAM), having both hydrogen bonding donating and accepting groups, is perhaps the most widely studied temperature-sensitive polymer thanks to its Received: February 28, 2017 Accepted: April 20, 2017
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DOI: 10.1021/acsami.7b02953 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 1. (a) Chemical structure of PNIPAM random copolymers and the IL [EMIM][TFSI]. (b) Temperature dependence of transmittance at 515 nm for PNIPAM copolymer solutions in [EMIM][TFSI] (0.5 wt %) at a heating rate of 1 °C/min. (c) Equilibrium swelling ratio of cross-linked PNIPAM copolymer films in [EMIM][TFSI] upon heating.
Table 1. Molecular Characteristics of PNIPAM Copolymers polymer
Mna (kg/mol)
Đb
xNIPAMc
PNIPAM P(NIPAM-AAc) P(NIPAM-MA) P(NIPAM-BP) P(NIPAM-BP-AAc) P(NIPAM-BP-MA)
29.5 23.4 30.3 19.2 23.4 16.5
2.3 3.6 3.0 2.6 3.6 2.7
1.00 0.94 0.90 0.962 0.893 0.899
xAAc
xMA
xBP
0.06 0.10 0.079 0.074
0.038 0.029 0.026
a
Number-average molecular weight (Mn) of polymers determined by SEC equipped with a refractive index detector. bDispersity (Mw/Mn) of polymers determined by SEC, where Mw is weight-average molecular weight. cMole fraction of comonomers determined by 1H NMR spectroscopy.
soluble in [EMIM][TFSI],36,37 and we thus expect copolymers containing AAc to exhibit higher UCSTs. In contrast, PMA is highly soluble in [EMIM][TFSI];36,37 thus, we expect copolymers containing MA to exhibit lower UCSTs. To test this expectation, 3 different linear copolymers with similar number-average molecular weights (Mn) of 20−30 kg/mol were prepared (Figure 1a and Table 1), and their cloud points as 0.5 wt % solutions in [EMIM][TFSI] were measured with a UV−vis spectrometer. As shown in Figure 1b, Tc for the PNIPAM homopolymer was 41 °C (taken as the temperature where the transmittance increases to 50%),38 but this value increased to 75 °C with the incorporation of 6 mol % AAc and decreased to 32 °C with 10 mol % MA. To enable fabrication of photopatterned films, 3−4 mol % of photo-cross-linkable acrylamidobenzophenone (BP) comonomers39 were introduced into the copolymers. Notably, the incorporation of BP as a comonomer led to negligible changes in the UCST (Figure S1), presumably because its hydrogen bonding capacity is similar to that of NIPAM. After crosslinking films (∼6 μm in thickness) under 365 nm UV irradiation followed by developing with solvent to remove un-cross-linked material and releasing from the substrate, the temperature-dependent swelling behavior of each copolymer was monitored using an optical microscope equipped with a heating stage. The temperature was increased in increments of 10 °C and held for at least 20 min at each temperature, which
accessible lower critical solution temperature (LCST) phase transition behavior in water,27−30 which can easily be tuned through incorporation of either hydrophobic or hydrophilic comonomers.31−35 In ILs, however, PNIPAM often shows UCST behavior, as first reported by Ueki and Watanabe for the solvent 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide ([EMIM][TFSI]), arising from the positive enthalpy of mixing between PNIPAM and IL, presumably due to the presence of hydrogen bonds between PNIPAM segments at low temperatures.26 Although the UCST of PNIPAM is known to depend on polymer molecular weight and the type of IL,36 we are not aware of studies aimed at tuning the UCST of PNIPAM in a given IL through copolymerization. Here, we develop a simple approach to tune the UCST of PNIPAM copolymers in an IL and incorporate these polymers into a micropatterned device that represents the first example of reversible and sequential autonomous self-folding.
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RESULTS AND DISCUSSION Our approach to vary the UCST of PNIPAM in an IL, [EMIM][TFSI], relies on the incorporation of comonomers with different hydrogen-bonding capabilities, namely acrylic acid (AAc) and methyl acrylate (MA) (Figure 1a). Strongly hydrogen bonding polymers such as PAAc, poly(methacrylic acid), and poly(2-hydroxyethyl methacrylate) are generally not B
DOI: 10.1021/acsami.7b02953 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 2. (a) A schematic illustration of the design for a sequential self-folding device in plan view (top) and a 3D perspective view showing the area around a hinge (bottom). (b) Schematic illustration showing how the bending angle (θ) is determined from the projected length of the hinge. (c) Optical micrograph of a micropatterned device in [EMIM][TFSI] at room temperature before folding.
Figure 3. (a) Optical images of a self-folding micropatterned device in [EMIM][TFSI] at different temperatures throughout heating. The sample was exposed for at least 20 min to reach equilibrium at each temperature. Scale bar: 200 μm. (b) Bend angle determined from the micrographs.
of d/d0 ≈ 1.15 and 1.2, while P(NIPAM-BP-AAc) has still not swelled appreciably. Taking advantage of the distinct temperature-dependent swelling of these three photo-cross-linkable copolymers, we proceeded to study sequential actuation of a simple self-folding microstructure, as summarized in Figure 2. A cross-like microstructure with four active hinges on a photocurable passive layer of Norland Optical Adhesive 68 (NOA 68) was prepared by consecutive photo patterning and developing. P(NIPAM-BP-MA) was first drop cast (≈ 6 μm thick film) on a silicon wafer precovered with a Ca2+ cross-linked poly(acrylic acid) (PAAc) sacrificial layer, and a pair of 68 × 96 μm rectangles (marked blue in Figure 2a) was UV cross-linked using a digital micromirror array device (DMD),41,42 followed by developing in an ethanol:water mixture (2:1, volume ratio). Equivalent steps were then taken to pattern rectangles of P(NIPAM-BP-AAc) (red in Figure 2a) and P(NIPAM-BP) (green in Figure 2a), and finally, a cross shape (gray in Figure 2a) of NOA 68 was patterned to cover the four hinges, using acetone as a developer. The NOA 68 layer was ≈10 μm thick, except in the regions where it covered the PNIPAM copolymer films, where its thickness was reduced to ≈4 μm due to the planarizing effect of spin-coating. To differentiate the four arms, those having P(NIPAM-BP-MA) hinges were patterned with rectangular ends, while half-circular and triangular shapes were
was sufficient for the degree of swelling to equilibrate (see Figure S3), as anticipated based on the report by Ueki and coworkers that PNIPAM gel particles of ∼10 μm size reached swelling equilibrium in [EMIM][TFSI] within less than 10 min.26,40 The swelling ratio was determined as the ratio of the diameter d of a gel disk at a given temperature to its size d0 in the dry, as-cross-linked state. As shown in Figure 1c, films of each copolymer swell from d/d0 ≈ 1 at room temperature to a value of ≈1.3 by 170 °C, consistent with the UCST behavior of the linear polymers in the IL. Importantly, the onset of swelling is comparable to Tc for each polymer: P(NIPAM-BP) starts to swell at around 45 °C, whereas P(NIPAM-BP-MA) and P(NIPAM-BP-AAc) start to swell at around 30 and 60 °C, respectively. The swelling transition is much broader than the liquid−liquid phase-separation transition of the linear polymer solutions, however, such that the gels continue to increase in swelling to temperatures well above Tc. Although the crosslinked films swell gradually upon heating, the onset of swelling follows the same order as the UCSTs of the linear polymers, and appreciable differences in swelling ratio are attained at temperatures between 30 and 170 °C. For example, at 45 °C, P(NIPAM-BP-MA) has already swelled by a factor of 1.13 before the other two copolymers have begun to swell. At 60 °C, P(NIPAM-BP) and P(NIPAM-BP-MA) show respective values C
DOI: 10.1021/acsami.7b02953 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
Figure 4. Optical images of a micropatterned device in [EMIM][TFSI] at different temperatures during unfolding by cooling. The sample was held for at least 20 min at each temperature. Scale bar: 200 μm.
strength within, and between, the NIPAM and AAc comonomers. The separation in fold angles adopted by successive generations of folds in Figure 3b is fairly consistent at about 15−20° between a temperature of 20 and 70 °C. Although this is somewhat smaller than the separation of ∼45−60° that has been achieved (as a function of time, rather than temperature) for 3 generations of folds in shape memory polymers,5 the approach here relies on an actuation sequence programmed through thermodynamic differences in UCST rather than differences in folding rate at a given temperature. This means that the structure can be held in a partially folded state for arbitrary periods of time and also can be reversibly unfolded, as summarized in Figure 4. As temperature is decreased from 170 °C, unfolding follows the reverse sequence, with P(NIPAM-BPAAc) hinges unfolding first, followed by P(NIPAM-BP) and P(NIPAM-BP-MA). However, while the angles adopted during unfolding initially match those during folding (Figure S4), substantial hysteresis is developed for each hinge at temperatures below ∼60−80 °C, suggesting that 20 min at each temperature is insufficient to reach the equilibrium fold angles on cooling. Indeed, the samples continue to unfold slowly at room temperature, e.g., as revealed in Figure 4, where the structure becomes flatter following 4 d at 25 °C. The reason for these slow kinetics in the last stages of unfolding are at present unclear and require further study. However, as the free-standing gel layers themselves are able to reversibly swell and deswell on time scales of ∼20 min or less, we expect the slow unfolding kinetics are the result of either the NOA 68 capping film or the grafting of a thin layer of (IL-insoluble) PAAc from the sacrificial layer onto the PNIPAM copolymer films.
used for arms with P(NIPAM-BP-AAc) and P(NIPAM-BP) hinges, respectively. After the microstructure was released by dissolving the sacrificial film in an aqueous medium (1 × 10−3 M NaCl), the sample was transferred to the IL medium and dried under vacuum overnight at room temperature to remove water. As shown in Figure 2c, the sample remained nearly planar in the IL at room temperature, consistent with the minimal swelling of each PNIPAM copolymer in this state. The layer thicknesses and hinge dimensions were chosen to yield a bending angle of roughly θ ≈ 90° for each arm when the PNIPAM copolymers reached their limiting swelling ratio of d/ d0 ≈ 1.3 at high temperature. Using a heating stage to increase the temperature from 25 to 170 °C, the actuation behavior was monitored by optical microscopy, as shown in Figure 3. The structures are imaged in plan view with the central panel resting flat against the underlying silicon wafer. Thus, as a panel bends, it becomes defocused, and the apparent position of the panel tip moves closer to the center of the cross due to foreshortening. The bending angle (θ) was calculated using the formula θ = cos−1(l/l0) by measuring the projected length (l) and the original length (l0) of each arm from the center of the hinge. The P(NIPAM-BP-MA) hinges begin to bend almost immediately above room temperature and are the first to saturate at θ ≈ 100° at 60 °C, followed by P(NIPAM-BP), which begins to bend at 40 °C and saturates by about 90 °C, and finally P(NIPAM-BP-AAc), which begins bending at roughly 50 °C and reaches a fold angle close to 90° only by 170 °C. This folding sequence is as expected based on the ordering of the UCSTs and the swelling ratios of the copolymers. Although the data in Figure 3 were gathered with a slow heating rate (held for at least 20 min at each temperature), we note that even at the maximum heating rate allowed by our experimental system (3.2 °C/min), folding in 3 distinct stages could still be discerned. Interestingly, the P(NIPAM-BP-AAc) arm showed a change in bending angle with temperature more gradual than those of the others, consistent with the more gradual increase in degree of swelling of freestanding gels seen in Figure 1c. We speculate that this more gradual transition may reflect the presence of three different types of hydrogen bonding interactions of varying
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SUMMARY In summary, we showed how the incorporation of comonomers with different hydrogen-bonding capacities into PNIPAM allows for the UCST of the copolymers to be effectively tuned. By also incorporating a benzophenone comonomer, we used photo-cross-linkable PNIPAM copolymers to generate a self-folding structure with three different types of hinges with different UCST values. Coupled with the thermal stability and D
DOI: 10.1021/acsami.7b02953 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Vario) with a temperature stage (INSTEC HCS621 V) and a CCD camera (PixeLINK). The same setup was used to record sequential actuation of self-folding structures. For both measurements, the temperature was first increased in increments of 10 °C, held for at least 20 min at each temperature, and subsequently decreased using liquid nitrogen cooling.
nonvolatility of the IL, this allows for sequential and reversible bending of hinges, offering a simple approach for sequential actuation that may find application in microscale soft robotics or complex autonomous origami structures.
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EXPERIMENTAL SECTION
Materials. PNIPAM homopolymers and copolymers were prepared via free radical polymerization with azobis(isobutyronitrile) (AIBN, Aldrich) as the initiator. For P(NIPAM-BP), N-isopropylacrylamide (NIPAM, Tokyo Chemical Industry Co., Ltd.), BP,43 and AIBN (recrystallized from methanol) were dissolved in 1,4-dioxane (Aldrich), followed by N2 purging for 30 min at room temperature. Then, the reaction was conducted at 80 °C for 15 h under N2 atmosphere. The polymers were precipitated in diethyl ether, filtered, and then dried under vacuum overnight. Other copolymers were synthesized in the same manner, but with AAc (Aldrich) or MA (Aldrich) added as comonomer. The copolymer composition was measured by 1H NMR spectroscopy (Bruker AvanceIII 500), while the Mn and dispersity (Đ = Mw/Mn) were measured by size exclusion chromatography (SEC) using tetrahydrofuran (or dimethylformamide/0.01 M LiCl for polymers having AAc contents) as the eluents. [EMIM][TFSI] was synthesized by an ion exchange reaction in water with 1-ethyl-3-methylimidazolium bromide ([EMIM][Br], Io-LiTec, 99%) and lithium bis(trifluoromethylsulfonyl)imide ([Li][TFSI], HQ115, 3M) at 70 °C overnight.44 The phase separated [EMIM][TFSI] was washed with deionized water 3 times and dried under vacuum at 70 °C for 3 days. NOA 68 was purchased from Norland Products, Inc. and used as received. Cloud Point Measurements. Polymers were dissolved in dichloromethane, and then [EMIM][TFSI] was added for 0.5 wt % solutions in [EMIM][TFSI] after evaporating dichloromethane under N2 for 30 min and vacuum at 100 °C for 12 h. A UV−vis spectrometer (Hitachi U-3010) equipped with a temperature controlled cuvette holder (Quantum Northwest) was used to determine cloud points. Transmittance was measured at 515 nm with a heating rate of 1 °C/ min. Film Preparation and Patterning. Copolymers were dissolved in 1-propanol at 100 mg/mL. A PNIPAM copolymer solution was drop cast (15 μL of 100 mg/mL) on a silicon wafer (≈1 × 1 cm) precovered with a Ca2+ cross-linked PAAc sacrificial layer. The solvent was evaporated at 70 °C for 20 min in a convection oven. Then, the desired pattern was prepared using an inverted optical microscope (Nikon ECLIPSE Ti) equipped with a DMD (DLP Discovery 4100, 0.7 XGA, Texas Instruments) which can project a programmed pattern of UV light (365 nm, pE-100, CoolLED) on the sample. After 7 min of exposure, a cross-linked pattern was developed by an ethanol:water mixture (2:1, v/v), a solvent to dissolve un-cross-linked regions. The rectangular hinges were patterned and developed in the order of P(NIPAM-BP-MA), P(NIPAM-BP), and P(NIPAM-BP-AAc). Then, highly viscous NOA 68 (∼5000 cP) was spin-coated at a speed of 7000 rpm for 60 s, patterned by UV exposure for 1 min, and finally developed using acetone. The thickness of the films was measured using a Veeco Dektak stylus profilometer. Patterned copolymer films were released by dissolving the sacrificial film in an aqueous medium (1 × 10−3 M NaCl) and washing by fresh water several times to remove salt in the releasing aqueous solution. The washed sample was transferred to the IL medium and dried under vacuum overnight at room temperature to remove residual water. For swelling ratio measurements, a free-standing gel of each copolymer was prepared in a manner similar to that described above. A polymer solution was drop cast (15 μL of 100 mg/mL) on a bare silicon wafer and developed using an ethanol:water mixture (2:1, v/v) after patterning. Finally, the patterned gel film was released by swelling in the IL at 150 °C. Swelling Ratio and Actuation Measurements. For swelling ratio measurements, a cross-linked square of each copolymer was fully dried before adding the IL to determine its side length in the dry state (d0). Next, the swelled length in the IL (d) was measured at different temperatures using an upright optical microscope (Zeiss, Axiotech
ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b02953. Temperature dependence of transmittance (Figure S1), 1 H NMR spectra (Figure S2), equilibrium swelling ratio (Figure S3), and bend angles (Figure S4) (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Ryan C. Hayward: 0000-0001-6483-2234 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Science Foundation through Grant NSF DMR-1609972. We acknowledge Nakul P. Bende and Dr. Seog-Jin Jeon for helpful discussions.
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REFERENCES
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