Article pubs.acs.org/Macromolecules
Metallo‑, Thermo‑, and Photoresponsive Shape Memory and Actuating Liquid Crystalline Elastomers Brian T. Michal, Blayne M. McKenzie, Simcha E. Felder, and Stuart J. Rowan* Department of Macromolecular Science and Engineering, Case Western Reserve University, 2100 Adelbert Road, Cleveland, Ohio 44106-7202, United States S Supporting Information *
ABSTRACT: A liquid crystalline elastomer incorporating a mesogenic derivative of the 2,6-bisbenzimidazolylpyridine (Bip) ligand has been prepared, and its shape memory and actuating properties have been studied. The reversible liquid crystal to isotropic transition is utilized as the switching mechanism for these stimuli-responsive materials. As such, this material exhibits soft shape memory; that is, flexibility is retained in both the permanent and temporary shapes. In addition to the thermal shape memory/actuating properties exhibited by most liquid crystalline elastomers, the incorporation of the metal ion-binding Bip mesogen into the backbone of the network imparts both (i) photoresponsive properties, via a photothermal conversion process, and (ii) metal-ion-triggered shape recovery/actuation to the material. For the latter process, it is proposed that the metal-binding event induces liquid crystalline to isotropic transition in this material at room temperature, resulting in actuation/recovery of the permanent shape.
■
INTRODUCTION Shape memory polymers (SMPs) are a class of materials characterized by their interesting ability to “fix” a temporary shape and then recover to a “remembered” permanent shape upon exposure to an external stimulus.1−4 Typically, fixing is accomplished by deforming a covalently cross-linked network at a temperature where the incorporated polymer chains are mobile (e.g., above Tg and/or Tm) and then cooling the sample below this transition to limit mobility and lock in the temporary shape. Recovery of the remembered shape is then triggered by reheating the material through the transition temperature, reestablishing mobility, and resulting in recovery of the shape on account of the elastic entropy of the polymer chains. While such temperature-driven systems predominate the shape memory literature, examples can also be found where light,5 exposure to solvents,6 or changes in pH7,8 act as the triggering stimulus. As such, a wide variety of applications for SMPs can be envisaged which utilize either their shape-changing properties directly (biomedical implants, adhesives, sealants, etc.)9−11 or the mechanical actuation that accompanies such a change (molecular muscles, microfluidic pumps, etc.).12 Recently, Mather and co-workers13 showed that liquid crystalline elastomers (LCE) are an interesting class of thermal SMPs. In this approach, a nematic to isotropic liquid crystalline transition (rather than a Tg or Tm) provides the mobility transition within the polymer network required to deform the sample. Importantly, as cooling below this transition to fix the temporary shape does not result in a global vitrification of the sample, the material remains flexible in both its permanent and © XXXX American Chemical Society
temporary shapes (unlike most thermal SMPs, which are generally stiff in their permanent shape). These liquid crystalline “soft” SMPs (along with other examples of soft SMPs14) offer further potential applications in areas of soft lithography and microfluidics.15 Additionally, LCEs have garnered much interest as a consequence of their ability to act as stimuli-responsive actuators.16,17 When a stimulus induces a liquid crystalline to isotropic transition in a liquid crystalline material, the mesogenic units go from an oriented to nonoriented state. In an LCE these mesogens are tethered together in a polymeric network and the molecular motion of transition from an oriented to a nonoriented state is translated to macroscopic actuation. Therefore, by applying a stimulus such as heat,18−24 light,25−31 electrical,32−34 humidity,35,36 or a change in pH,37 a reversible shape change or contraction in the material can be induced resulting in mechanical work or controlled folding.38 We have been working in the field of metallo-supramolecular polymers (MSPs) in which metal−ligand interactions are used to assemble ligand-containing organic components into polymeric species. Specifically, we have focused on the 2,6bis(benzimidazolyl)-4-pyridine (Bip) ligand and its derivatives to access and study a wide variety of new MSPs, with the goal of understanding how both the organic and metal ion component impact the mechanical and stimuli-responsive Received: March 27, 2015 Revised: April 19, 2015
A
DOI: 10.1021/acs.macromol.5b00646 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
would result in a multiresponsive, soft shape-memory and actuating films where heat, light, or metal ions could be used to elicit the response (Figure 2). In such systems, an alignment step would be needed prior to the actuation response in order to achieve the long-range orientation order necessary for large contractile forces. As such we report herein a method of incorporating 1 into a network architecture via thiol−ene photochemistry, the shape-memory properties of this network, using heat, light, or metal ions to trigger the recovery from a temporary shape. Furthermore, we also demonstrate that these materials act as thermo-, photo-, and metalloresponsive actuators.
behavior of the resulting materials. By mixing and matching different metal ions and oligotopic organic ligands, we have shown that such materials can be used to access easy-to-process thermally stable polymers,39,40 light-emitting, semiconducting polymers with excellent mechanical properties,41,42 chemical warfare agent sensor films,43−45 photohealable coatings,46 multistimuli-, multiresponsive metallogels,47 redox-active viscosity modifying solutions,48 solvo- and mechanochromic films,49 templates for Pt nanoparticle formation,50 and directed assembly at polymer/polymer interfaces.51 If the Bip endcapped macromonomer is a ditopic low-Tg “soft” polymer (e.g., poly(ethylene-co-butylene) or poly(tetrahydrofuran)), then the resulting metallo-supramolecular polymers are phase separated where the Bip:metal complex forms the hard phase.44−46,52 We have shown that by cross-linking the soft phase of these metallo-supramolecular polymers, then it is possible to access photo-, thermo-, and solvent-responsive shape-memory polymers in which the recovery of the original shape is related to the decomplexation of the metal−ligand interaction.53 However, these materials were brittle and exhibited limited elasticity even upon application of the stimulus and as such did not display any significant actuation characteristics. Inspired by work of Piguet and co-workers, who have shown that selected low molecular weight Bip derivatives exhibit liquid crystalline properties,54 we developed Bip derivatives with an alkene containing alkyloxy substituent on the 5′ position (1) and have shown that they exhibit monotropic liquid crystalline behavior upon cooling from an isotropic melt.55 Furthermore, this behavior is disrupted when exposed to various transition metal and lanthanide salts (e.g., Zn(ClO4)2, Eu(NO3)3, etc.), with these complexes exhibiting only isotropic behavior (Figure 1). Acyclic diene metathesis (ADMET) polymerization of 1 allowed access to linear polymers that exhibit enantiotropic liquid crystalline behavior that could be “switched off” by the addition of a metal ion.56 Building on this initial work, it was hypothesized that replacing conventional mesogens in a multidomain LCE with Bip-based mesogens (such as 1)
■
EXPERIMENTAL SECTION
Materials. 1 was prepared according to literature procedures.55 All solvents were purchased from Fisher Scientific. All other chemicals and reagents were purchased from Sigma-Aldrich Co. Reagents were used without further purification.
Figure 2. Incorporation of Bip units into a liquid crystalline elastomer (LCE) network that is subsequently aligned by mechanical stretching to yield a material which undergoes shape change in response to stimuli, such as heat, light, or exposure to metal ions, that stimulate the liquid crystalline to isotropic transition.
Figure 1. Bip mesogens (1) display a conformational change upon binding a metal ion resulting in a change from mesogenic to nonmesogenic behavior. B
DOI: 10.1021/acs.macromol.5b00646 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
Scheme 1. LCEs Containing Bip Are Synthesized by a Two-Step Process Wherein Oligomers (2) Are First Produced via a Photoinitiated Thiol−Ene Reaction of 1 with 1,6-Hexanedithiol; These Oligomers Are Then Cast in a Teflon Mold and CrossLinked Using a Second Thiol−Ene Reaction of the Oligomers with a Tetrathiol Cross-Linking Unit (3), Thereby Creating a Bip-LCE Network (4)
Example Procedure of the Synthesis of the Liquid Crystalline Elastomer (4a). 1 (408 mg, 0.762 mmol), 1,6hexanedithiol (93 μL, 0.610 mmol), and phenylbis(2,4,6trimethylbenzoyl)phosphine oxide (30 mg, 0.070 mmol) were dissolved in dichloromethane (DCM, 3 mL) in a 20 mL vial with a stir bar. The sample was then stirred while being exposed to a light source (Bluepoint 4 Ecocure from Honle UV America Inc.) equipped with a 320−390 nm filter and irradiated for 2 h at an intensity of 50 mW/cm2. An aliquot of the solution mixture that contained oligomer 2 was removed for NMR analysis (Figure S1) and then subsequently replaced. The reaction mixture was dried under reduced pressure, and pentaerythritol tetrakis(3-mercaptopropionate) (3, 37 mg, 0.076) and an additional 30 mg of phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide were added. The contents were dissolved in 3 mL of DCM, and the mixture was cast in a 4 cm × 5 cm Teflon mold and covered with a piece of glass. The sample was again irradiated (320−390 nm filter) for 2 h at an intensity of 50 mW/cm2. Before further characterization, gel fraction treatments were performed on all samples by extracting with DCM (×3) and drying at 25 °C under vacuum to yield a film of 4a. Gel fraction values (mass after extraction/original mass × 100%) were greater than 95%. This procedure produced a film with an average molecular weight of 4400 g/mol between cross-links (4b). To produce a film with a lower molecular weight between cross-links, the ratio of 1 to hexanedithiol was decreased to 2:1 and the ratio of 2 to 3 altered accordingly to ensure [−SH]:[alkene] of 1:1. Likewise, to produce a film with a higher molecular weight between cross-links, the ratio of 1 to hexanedithiol was increased to 9:8 and the ratio of 2 to 3 altered accordingly. Characterization. Modulated differential scanning calorimetry (mDSC) experiments were performed on a TA Instruments Q2000 DSC equipped with a refrigerated cooling system. The DSC cell resistance and capacitance were calibrated using the Tzero calibration procedure (TA Instruments-Waters LLC); the cell constant and temperature were calibrated using an indium standard. All samples were run under a flowing nitrogen atmosphere. Liquid crystalline phase transition temperatures were determined from appropriate peak maxima on the second cooling traces. Experiments were heated and cooled at a rate of 10 °C/min, during which a linear heating rate and a sinusoidal temperature waveform were applied to the sample with a modulation amplitude of 0.5 °C and a modulation period of 40 s. POM studies were performed using an Olympus BX51 microscope
equipped with 90° crossed polarizers, an HCS402 hot stage from Instec Inc., and a digital camera (14.2 Color Mosaic Model from Diagnostic Instruments, Inc.). Images were acquired from the camera at selected temperatures using Spot software (Diagnostic Instruments, Inc.). Spatial dimensions were calibrated using a stage micrometer with 10 μm line spacing. Either a 10×/0.5 NA or a 20×/0.4 NA achromat long working-distance objective lens (Olympus LMPlanFI) was employed. The Instec hotstage was equipped with a liquid nitrogen LN2-P cooling accessory for accurate temperature control during heating and cooling. XRD data were acquired using a Bruker D8 Discover diffractometer with Cu Kα radiation from a sealed tube X-ray source and a graphite monochromator. Data were collected with a Våntec 500 area detector positioned at a nominal sample-to-detector distance of 20 cm. Sample positioning on an Eulerian 1/4 cradle with x/y/z linear degrees of freedom was accomplished with a laser−video microscope positioning system. System calibration parameters (image x/y center and detector distance) were determined using silver behenate (C22H44AgO2), which exhibits multiple strong peaks at low angles. Data were acquired in transmission with the sample face perpendicular to the incident beam. 15 min exposures were used for both data and background frames. For the smectic ring region a single frame was acquired for each sample, and the detector was positioned with the detector face normal to the beam with a beamstop in place. The sample long axis was horizontal with respect to the presentation of images in this report (vertical with respect to the instrument). For the nematic ring region two frames were acquired for each sample in this region. In first frame (ψ = 0°), the sample orientation was the same as for the smectic ring region. For the second frame (ψ = 90°), the sample was rotated 90° about the beam axis. The ψ = 90° images have been rotated to be consistent with a horizontal sample long axis. For both nematic frames, the beamstop was removed and the detector was rotated about the goniometer axis by 20° to capture the desired 2θ range. Background images were taken in each configuration, and a background subtraction was performed to remove detector artifacts from the X-ray pattern. One-way thermal shape memory experiments were performed on a TA Instruments Q800 DMA. A sample procedure is as follows: the sample is placed in the DMA clamps in its permanent shape, the temperature is raised, and the sample deformed. The deformation is then held while the sample is cooled back to room temperature. Once cooled, the stress on the sample is released and a length measurement (εu, strain after unloading) is taken C
DOI: 10.1021/acs.macromol.5b00646 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules to determine the degree of fixing by dividing by the original length (εm, maximum strain). The sample is then heated again and allowed to recover, with a length measurement again taken (εp, recovered strain). The ability to recover to its original shape is then calculated by taking the difference between εm and εp and dividing by εm. Tensile stress− strain experiments along with actuation force measurements were performed on a Zwick-Roell Z0.5 tensile tester equipped with a 100 N load cell. All tensile stress−strain tests were performed at a strain rate of 10%/min.
■
RESULTS AND DISCUSSION Synthesis. The targeted Bip-containing LCEs were prepared using a two-step approach (Scheme 1). A slight excess of the alkene Bip derivative 155 was reacted with 1,6hexanedithiol using a photoinitiated thiol−ene reaction to access the Bip oligomers 2a−c that had terminal alkene moieties (Figure 1). By controlling the ratio of 1:hexanedithiol, three different molecular weights of the oligomers 2 were obtained (as determined by NMR end-group analysis (Figure S1)); molar ratios of 1:hexanedithiol of 2:1, 6:5, and 9:8 yielded Bip oligomers of 1500 g/mol (2a), 4400 g/mol (2b), and 5800 g/mol (2c), respectively. The oligomers 2a−c were then mixed with the tetrathiol cross-linker (3) (at a ratio of 1 thiol to 1 double bond) in dichloromethane, cast into a Teflon mold, and cross-linked using a second photoinitiated thiol−ene reaction. The resulting polymers (4a−c) all had a gel fraction of over 95% and yielded films with different degrees of cross-linking, with 4a being the most cross-linked and 4c having the lowest degree of cross-linking. Thermal and Optical Properties of Liquid Crystal Elastomer 4. With the polymer films 4a−c in hand, initial studies focused on assessing their properties to determine if they did exhibit liquid crystalline properties. All three films of 4 are opaque at room temperature and become transparent upon heating to above 80 °C (Figure 3a). Modulated differential scanning calorimetry (mDSC) of 4b (Figure 3b) shows an exotherm upon cooling at approximately 74 °C, which is slightly lower than the observed liquid crystalline transition for the monomer 1 (82 °C). An endotherm peak is also observed in the mDSC of 4a and 4c at ca. 71 and 76 °C, respectively (Figure S2). These data are consistent with all three films showing a direct isotropic to liquid crystal transition upon cooling.57 To further determine if these films do exhibit liquid crystalline order, XRD and POM experiments were performed on a film of 4b. Birefringence is observed in this film at room temperature (Figure 3c, right), which largely disappears once the sample is heated to 100 °C (Figure 3c, left), again consistent with a transition from a liquid crystalline to an isotropic state. XRD experiments carried out at room temperature show a reflection corresponding to a repeat distance of 30.8 Å, suggesting the presence of smectic order in the cross-linked network, along with a broad reflection corresponding to a distance of 4.13 Å (Figure 4a). It is important to note that the XRD data show a scattering intensity that does not vary with azimuthal angle, confirming that these films are multidomain liquid crystalline elastomers as would be expected given the procedure used to prepare the films. Heating the sample to 100 °C followed by stretching to 100% strain and subsequent cooling in the stretched state leads to significant alignment of the mesogenic units, as observed in the XRD pattern of the stretched sample (Figure 4b). An azimuthal integration of the 30.8 Å reflection in the stretched sample (Figure 4c) gives an orientation parameter of 0.65, which
Figure 3. (a) Pictures of a film of 4b at 25 °C (left) and 80 °C (right), (b) mDSC temperature sweep showing the associated thermal transition in monomer 1 and network 4b, and (c) POM images of 4b at room temperature (left) showing a birefringent pattern indicative of LC ordering and after heating to 100 °C (right) showing a significant decrease in birefringence consistent with a disordered state.
indicates that significant alignment of the mesogens throughout the sample has occurred. Mechanical Properties, Shape-Memory Behavior, and the Effect of Metal Ions. Stress−strain experiments were carried out to characterize the change in material properties upon heating above the liquid crystalline transition or exposure to metal ions. At room temperature 4b (Figure 5) shows good elastomeric properties with ultimate elongation (εu) of 170% and ultimate tensile stress (σu) of 7.9 MPa. Films 4a and 4c show similar behavior (Figure S3) with slight differences consistent with the change in cross-link density; 4a (most cross-linked) εu ca. 130% and σu ca. 9.0 MPa. 4c (least crosslinked) εu ca. 200% and σu is 6.4 MPa. As expected for a LCE film heating them above their clearing point results in a significant increase in its elasticity and a drop in its strength. For example, at 100 °C 4b has a εu of ca. 475% and σu of ca. 0.29 MPa. One of the main goals of this study was to see if metal ions could be used to induce shape recovery and/or actuation of the film. Fe(II) ions were chosen as the metal ion since it is known that Bip complexes with Fe(II) are a deep purple color which allows easy visualization of the incorporation of the metal ion D
DOI: 10.1021/acs.macromol.5b00646 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
Figure 5. Stress strain curves of 4b in the neat state (black), in the neat state at 100 °C (green) after soaking for 20 min in 1 mM Fe(OTf)2 in 1:1 water:THF and subsequent drying (blue), and after soaking for 20 min in 1 mM Fe(OTf)2 in 1:1 water:THF followed by soaking for 1 h in 10 mM diethylenetriamine in 1:1 water:THF and subsequent drying (red).
but it also results in significant swelling of the film to ca. 150% of its original dimensions, which hinders determination of the recovery of the film (Figure S4a) during the shape memory/ actuation experiments. Furthermore, swelling the films with THF alone (i.e., without the metal ion) followed by drying resulted in complete shape recovery, and as such the effect of the metal ion on this process could not be determined. A 1:1 mixture of water and THF was determined to be a good “middle ground” solvent system, which did not induce any significant swelling in the film (Figure S4b) but provided an appropriate environment to infuse the samples with metal ions. Thus, submersion of a film of 4b in 1 mM Fe(OTf)2 dissolved in 1:1 water:THF results in a dramatic change in color of the film (yellow-purple) consistent with the incorporation of Fe2+ into 4b. Upon drying the Fe(II)-imbibed film 4b·Fe(OTf)2 is much more elastic and softer than Fe(II)-free films (εu of 330% and σu of 0.64 MPa), as can been by the stress−strain curves in Figure 5. In fact, these properties more closely resemble the properties of 4b when isotropic (i.e., at higher temperatures). The slight difference in the mechanical properties between the temperature and metal ion induced isotropic films is possibly a consequence of small amount of metal−ligand cross-linking due to the formation of Bip2:Fe2+ complexes that would cause both an increase in material strength and a decrease in strain at break. Furthermore, the 4b·Fe(OTf)2 films exhibit no birefringence under POM (Figure S5) or exotherm upon cooling (via mDSC (Figure S1)) and show only a broad diffuse scattering at 4.16 Å in XRD (Figure S6). As such, these data are consistent with metal ion binding “turning off” the liquid crystallinity of the LCE and converting the film at room temperature to an isotropic state. If this is the case, then removing the Fe(II) from the film should result in recovery of the properties of the material. Gratifyingly, when a film of 4b· Fe(OTf)2 is exposed to a solution of 10 mM diethylenetriamine, the deep purple color of the film is removed and both the birefringent pattern (Figure S5) the mechanical properties (Figure 5) of the demetalated film are restored to close to those of the original film.
Figure 4. (a) XRD patterns for unstretched 4b. Left image: scattering at lower angles with a reflection corresponding to d-spacing of 30.8 Å. Middle and right images: higher angle scattering with the sample in the same orientation as the first image (middle) and after rotating the sample by 90° (right), showing a diffuse scattering with a d-spacing of 4.13 Å. (b) XRD patterns for 4b in the stretched state (after being heated to 80 °C, stretched to 100% strain, and cooled to room temperature). Left image: scattering at low angles with the reflection at 30.8 Å showing a strong azimuthal dependence with strong scattering in the stretching direction and almost no scattering perpendicular to the stretching direction. Middle and right images: higher angle scattering with the sample in the same orientation as the first image (middle) and after rotating the sample by 90° (right), showing the reflection at 4.13 Å is weak in the stretching direction and stronger perpendicular to the stretching direction. (c) Azimuthal integration of the reflection at 30.8 Å shows no azimuthal dependence for the unstretched film and a strong dependence for the stretched film.
into the film.58 The method chosen to introduce the Fe(II) into the film was to place it in a solution of the metal ion and allow the metal ion to diffuse into the film. With an eye toward metal ion induced switching studies it was important to find a solvent that allows the Fe(II) salt to diffuse into the film but does not result in any solvent-induced recovery of the material. Water does not swell the film; however, in aqueous Fe(OTf)2 salt solutions the Fe(II) ions do not appear to diffuse into the sample, presumably as water is a bad solvent for the hydrophobic polymer. Conversely, tetrahydrofuran (THF) does allow significant diffusion of metal salts into the polymer, E
DOI: 10.1021/acs.macromol.5b00646 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules The shape-memory properties of the LCE 4b were then evaluated using dynamic mechanical thermal analysis (DMA) (Figure 6a). Heating the films to 100 °C, straining them to 75%, and then cooling them before releasing the strain showed the films exhibited a degree of fixing of ca. 99% (as defined by εu/εu where εm is the maximum strain and εu is the strain after unloading). Reheating the stretched fixed film to 100 °C results in the films recovering to ca. 98% of its original length. A similar degree of fixing and recovery is observed over at least five cycles (Figure 6b). Having determined that films of 4b exhibit excellent thermal shape memory properties, the next goal was to ascertain if metal ions can be used to induce recovery of this material. As we have shown above, the 4b·Fe(OTf)2 film is isotropic so the goal was to verify that Fe2+ ion diffusing into a strained film of 4b can bind to the Bip ligand, induce the LC to isotropic transition, and in turn cause the film to recover its original shape. In initial studies the films of 4b were prestrained into a temporary shape by heating to ca. 80 °C followed by stretching the film with tweezers (Figure 7a,b). When this stretched temporary shape is exposed to a 1 mM Fe(OTf)2 solution in 1:1 water:THF, the film turns deep purple, becomes isotropic
Figure 7. Pictures of the metalloresponsive behavior of 4b. A sample film of 4b (a) is first fixed in a temporary shape by heating and stretching (b). (c) The film is exposed to a 1 mM Fe(OTf)2 in 1:1 water:THF, showing recovery of the film and change in color. (d) Removal of a significant amount of the Fe(OTf)2 can be achieved by exposure to a 10 mM diethylenetriamine solution in 1:1 water:THF.
(as discussed earlier), and the material recovered to a size similar to its permanent shape (Figure 7c). It should be noted that the rate of this process is diffusion controlled, and thus the rate of response presumably depends on the dimension of the films and the concentration of the Fe(II). For the films studied here (dimensions 0.3 mm × 4 mm × 9 mm) the response occurs over the course of ca. 20 min. In a control experiment where the temporary stretched shape is submerged in 1:1 water:THF with no Fe(OTf)2, only partial shape recovery is observed after 20 min (Figure S7). As discussed above, the metal-binding process is also reversible, as exposure to a 10 mM diethylenetriamine solution in 1:1 water:THF (which will competitively bind with the Fe2+ ions), the purple color of the film is mostly removed and the mechanical properties are predominantly restored. Finally, the ability of these films to act as an actuator was evaluated. In these experiments a film of 4b, which had previously been heated above the liquid crystalline clearing temperature, stretched to 100% strain and cooled, was attached to a 10 g weight (Figure 8). The sample was then suspended in
Figure 8. A film of 4b that had been previously heated, stretched to 100% strain, and then cooled in the stretched state holds a 10 g mass in a beaker of 1:1 water:THF. After 20 min no actuation is observed. Fe(OTf)2 is added and stirred to produce a 1 mM solution. After 20 min the sample had noticeably lifted the mass. No further actuation was observed after 20 min.
Figure 6. (a) Shape memory DMA plot quantifying thermal shape memory properties for 4b (εm = maximum strain, εu = strain after unloading, εp = recovered strain, fixing = εu/εm, recovery = (εm − εp)/ εm). (b) Five cycles of the thermal shape memory experiment. F
DOI: 10.1021/acs.macromol.5b00646 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
readings were taken every minute for the first 10 min and then every 10 min until the force equilibrated. The maximum force achieved using this method was 0.148 N (Figure 9). As a control, this experiment was repeated using a 1:1 water:THF solution with no metal ions. Under these conditions no significant actuation force was observed (Figure 9).
a beaker of 1:1 water:THF (Figure 8, middle). After 20 min no actuation was observed (Figure 8, left). Fe(OTf)2 was then added to the solution along with brief stirring to produce a 1 mM solution. After 20 min the film had noticeably lifted the mass (Figure 8, right). This demonstrated that simple plasticization of the sample with the solvent was insufficient to achieve appreciable actuation of the sample, but the binding of the metal ion to the Bip can indeed generate enough force to lift a 10 g weight. To further quantify the actuation force generated by this material, a film with dimensions of 0.3 mm × 4 mm × 10 mm was heated above the LC transition, stretched to 100% strain, and then cooled to room temperature. The sample was then clamped in a tensile tester under zero load and zero strain rate. The sample was irradiated with UV light with an intensity of approximately 500 mW/cm2 in the wavelength range of 320− 390 nm and a λmax of 365 nm. The Bip ligand is known to absorb UV light in this range with significant conversion to heat, resulting in a dramatic increase in sample temperature.46,53 As the sample absorbs light it is heated above the LC transition, which in turn triggers the contraction. The resulting contraction force is measured by the load cell on the tensile tester under the isostrain conditions and showed that the maximum force achieved by the sample under these conditions was 0.254 N (212 kPa) (Figure 9), which is typical for an LCE actuator.16 The actuation force upon exposure to the metal ion salt solution (1 mM in 1:1 water:THF) was then examined. A sample with dimensions 0.3 mm × 4 mm × 10 mm was heated above the LC transition, stretched to 100% strain, and cooled to room temperature. The sample was then clamped in a tensile tester under zero load and zero strain rate. A submersion chamber was fitted to the apparatus and was filled with a 1 mM Fe(OTf)2 solution in 1:1 water:THF. The resulting actuation force was measured by the load cell in the isostrain state, and
■
CONCLUSIONS With the goal of preparing multiresponsive polymer actuators, we have incorporated liquid crystalline metal-binding Bip monomers into polymeric networks via thiol−ene chemistry. These cross-linked films exhibit liquid crystalline behavior which can be harnessed to access soft shape memory with excellent thermal fixity and recovery. The permanent shape of these multidomain LCEs can also be recovered via exposure to metal ions at room temperature, presumably as the metal ion binding results in a liquid crystal to isotropic transition. This process is reversible, as the removal of metal ions from the film with diethylenetriamine yields a return toward the original properties of the material. Furthermore, we have shown that long-range alignment of the mesogens can be achieved through physical stretching above the LC transition temperature. The conversion from the stretched aligned to contracted isotropic state can be achieved via a thermo-, photo-, or metallostimulus resulting in a contractile force in the material which can be as high as 0.254 N.
■
ASSOCIATED CONTENT
S Supporting Information *
1
H NMR spectrum of 2b; mDSC cooling traces of 4a−4c; tensile stress−strain curves of 4a−4c; swelling test of 4b in THF and 1:1 water:THF; POM images of 4b after exposure to Fe2+ and subsequent removal of Fe2+; solvent induced shape recovery of 4b in 1:1 water:THF; and a video showing the photoinduced actuation of a film of 4b. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b00646.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (S.J.R.). Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the National Aeronautics and Space Administration (Grant NNX11AN50H to BTM) and the Kent H. Smith Charitable Trust and the Army Research Office (Grant W911NF-12-10339). We also thank Dr. Richard Rogers from NASA Glenn Research Center for help with the XRD experiments.
■
Figure 9. Actuation force generated by a 0.3 mm × 4 mm × 10 mm vs immersion time in a 1 mM Fe(OTf)2 solution in 1:1 water:THF (■) and 1:1 water:THF with no metal ion (▲). The dashed line represents the maximum force achieved for the same sample when irradiated with UV light with an intensity of approximately 500 mW/cm2 in the wavelength range of 320−390 nm with a peak at 365 nm. In all instances the sample had previously been heated above the LC transition, strained to 100% strain, and then cooled to room temperature.
REFERENCES
(1) Xie, T. Polymer 2011, 52, 4985−5000. (2) Zhao, Q.; Behl, M.; Lendlein, A. Soft Matter 2013, 9, 1744−1755. (3) Hu, J.; Zhu, Y.; Huang, H.; Lu, J. Prog. Polym. Sci. 2012, 37, 1720−1763. (4) Huang, W. M.; Zhao, Y.; Wang, C. C.; Ding, Z.; Purnawali, H.; Tang, C.; Zhang, J. L. J. Polym. Res. 2012, 19, 9952−9985. (5) Lendlein, A.; Jiang, H.; Junger, O.; Langer, R. Nature 2005, 434, 879−882. (6) Du, H.; Zhang, J. Soft Matter 2010, 6, 3370−3376.
G
DOI: 10.1021/acs.macromol.5b00646 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules (7) Chen, H.; Li, Y.; Liu, Y.; Gong, T.; Wang, L.; Zhou, S. Polym. Chem. 2014, 5, 5168−5174. (8) Han, X. J.; Dong, Z. Q.; Fan, M. M.; Liu, Y.; Li, J. H.; Wang, Y. F.; Yuan, Q. J.; Li, B. J.; Zhang, S. Macromol. Rapid Commun. 2012, 33, 1055−1060. (9) El Feninat, F.; Laroche, G.; Fiset, M.; Mantovani, D. Adv. Eng. Mater. 2002, 4, 91−104. (10) Li, G.; King, A.; Xu, T.; Huang, X. J. Mater. Civ. Eng. 2013, 25, 393−402. (11) Eisenhaure, J. D.; Xie, T.; Varghese, S.; Kim, S. ACS Appl. Mater. Interfaces 2013, 5, 7714−7717. (12) Liu, C.; Qin, H.; Mather, P. T. J. Mater. Chem. 2007, 17, 1543− 1558. (13) Burke, K. A.; Mather, P. T. J. Mater. Chem. 2010, 20, 3449− 3457. (14) Ishida, K.; Hortensius, R.; Luo, X.; Mather, P. T. J. Polym. Sci., Polym. Phys. 2012, 50, 387−393. (15) Buguin, A.; Li, M. H.; Silberzan, P.; Ladoux, B.; Keller, P. J. Am. Chem. Soc. 2006, 128, 1088−1089. (16) Jiang, H.; Li, C.; Huang, X. Nanoscale 2013, 5, 5225−5240. (17) Ohm, C.; Brehmer, M.; Zentel, R. Adv. Mater. 2010, 22, 3366− 3387. (18) Krause, S.; Dersch, R.; Wendorff, J. H.; Finkelmann, H. Macromol. Rapid Commun. 2007, 28, 2062−2068. (19) Thomsen, D. L.; Keller, P.; Naciri, J.; Pink, R.; Jeon, H.; Shenoy, D.; Ratna, B. R. Macromolecules 2001, 34, 5868−5875. (20) Yang, H.; Buguim, A.; Taulemesse, J. M.; Kaneko, K.; Mery, S.; Bergeret, A.; Keller, P. J. Am. Chem. Soc. 2009, 131, 15000−15004. (21) Ohm, C.; Kapernaum, N.; Nonnenmacher, D.; Giesselmann, F.; Serra, C.; Zentel, R. J. Am. Chem. Soc. 2011, 133, 5305−5311. (22) Ohm, C.; Serra, C.; Zentel, R. Adv. Mater. 2009, 21, 4859−4862. (23) Wei, R.; He, Y.; Wang, X.; Keller, P. Macromol. Rapid Commun. 2014, 35, 1571−1577. (24) Pei, Z.; Yang, Y.; Chen, Q.; Terentjev, E. M.; Wei, Y.; Ji, Y. Nat. Mater. 2014, 13, 36−41. (25) Beyer, P.; Zentel, R. Macromol. Rapid Commun. 2005, 26, 874− 879. (26) Camacho-Lopez, M.; Finkelmann, H.; Palffy-Muhoray, P.; Shelley, M. Nat. Mater. 2004, 3, 307−310. (27) Ikeda, T.; Nakano, M.; Yu, Y. L.; Tsutsumi, O.; Kanazawa, A. Adv. Mater. 2003, 15, 201−205. (28) Kondo, M.; Yu, Y. L.; Ikeda, T. Angew. Chem., Int. Ed. 2006, 45, 1378−1382. (29) Yu, Y. L.; Nakano, M.; Ikeda, T. Nature 2003, 425, 145. (30) Deng, W.; Li, M. H.; Wang, X. G.; Keller, P. Liq. Cryst. 2009, 36, 1023−1029. (31) White, T. J.; Serak, S. V.; Tabiryan, N. V.; Vaia, R. A.; Bunning, T. J. J. Mater. Chem. 2009, 19, 1080−1085. (32) Lehman, W.; Skupin, H.; Tolksdorf, C.; Gebhard, E.; Zentel, R.; Kruger, P.; Losche, M.; Kremer, F. Nature 2000, 410, 447−450. (33) Huang, C.; Zhang, Q. M.; Jakli, A. Adv. Funct. Mater. 2003, 13, 525−529. (34) Fuchigami, Y.; Takigawa, T.; Urayama, K. ACS Macro Lett. 2014, 3, 813−818. (35) Harris, K. D.; Bastiaansen, C. W. M.; Lub, J.; Broer, D. J. Nano Lett. 2005, 5, 1857−1860. (36) de Haan, L. T.; Verjans, J. M. N.; Broer, D. J.; Bastiaansen, C. W. M.; Schenning, A. P. H. J. J. Am. Chem. Soc. 2014, 136, 10585− 10588. (37) Shishmanova, I. K.; Bastiaansen, C. W. M.; Schenning, A. P. H. J.; Broer, D. J. Chem. Commun. 2012, 48, 4555−4557. (38) Ware, T. H.; McConney, M. E.; Wie, J. J.; Tondiglia, V. P.; White, T. J. Science 2015, 347, 982−984. (39) Knapton, D.; Rowan, S. J.; Weder, C. Macromolecules 2006, 39, 4069−4075. (40) Burnworth, M.; Rowan, S. J.; Weder, C. Macromolecules 2012, 45, 126−132. (41) Knapton, D.; Rowan, S. J.; Weder, C. Macromolecules 2006, 39, 651−657.
(42) Burnworth, M.; Mendez, J. D.; Schroeter, M.; Rowan, S. J.; Weder, C. Macromolecules 2008, 41, 2157−2163. (43) Knapton, D.; Burnworth, M.; Rowan, S. J.; Weder, C. Angew. Chem., Int. Ed. 2006, 45, 5825−5829. (44) Kumpfer, J. R.; Wie, J. J.; Swanson, J. P.; Beyer, F. L.; Mackay, M. E.; Rowan, S. J. Macromolecules 2012, 45, 473−480. (45) Kumpfer, J. R.; Jin, J.; Rowan, S. J. J. Mater. Chem. 2010, 20, 145−151. (46) Burnworth, M.; Tang, L.; Kumpfer, J. R.; Duncan, A. J.; Beyer, F. L.; Fiore, G. L.; Rowan, S. J.; Weder, C. Nature 2011, 472, 334− 337. (47) (a) Beck, J. B.; Rowan, S. J. J. Am. Chem. Soc. 2003, 125, 13922− 13923. (b) Weng, W.; Beck, J. B.; Jamieson, A. M.; Rowan, S. J. J. Am. Chem. Soc. 2006, 128, 11663−11672. (c) Weng, W.; Li, Z.; Jamieson, A. M.; Rowan, S. J. Soft Matter 2009, 5, 4647−4657. (d) Weng, W.; Li, Z.; Jamieson, A. M.; Rowan, S. J. Macromolecules 2009, 42, 236−246. (48) Miller, A. K.; Li, Z.; Streletzky, K. A.; Jamieson, A. M.; Rowan, S. J. Polym. Chem. 2012, 3, 3132−3138. (49) Kumpfer, J. R.; Taylor, S. D.; Connick, W. B.; Rowan, S. J. J. Mater. Chem. 2012, 22, 14196 −14204. (50) Lee, S. W.; Kumpfer, J. R.; Lin, P. A.; Li, G.; Gao, X. P. A.; Rowan, S. J.; Sankaran, R. M. Macromolecules 2012, 45, 8201−8210. (51) Kumpfer, J. R.; Rowan, S. J. ACS Macro Lett. 2012, 1, 882−887. (52) Beck, J. B.; Ineman, J. M.; Rowan, S. J. Macromolecules 2005, 38, 5060−5068. (53) Kumpfer, J. R.; Rowan, S. J. J. Am. Chem. Soc. 2011, 133, 12866−12874. (54) Terazzi, E.; Suarez, S.; Torelli, S.; Nozary, H.; Imbert, D.; Mamula, O.; Rivera, J.-P.; Guillet, E.; Benech, J.-M.; Bernadinelli, G.; Scopelliti, R.; Donnino, B.; Guillon, D.; Bunzli, J.-C. G.; Piguet, C. Adv. Funct. Mater. 2006, 16, 157−168. (55) McKenzie, B. M.; Miller, A. K.; Wojtecki, R. J.; Johnson, J. C.; Burke, K. A.; Tzeng, K. A.; Mather, P. T.; Rowan, S. J. Tetrahedron 2008, 64, 8488−8495. (56) McKenzie, B. M.; Wojtecki, R. J.; Burke, K. A.; Zhang, C.; Jakli, A.; Mather, P. T.; Rowan, S. J. Chem. Mater. 2011, 23, 3525−3533. (57) Mukherjee, P. K. J. Mol. Liq. 2014, 190, 99−111. (58) Krumholz, P.; Orquima, S. A. Inorg. Chem. 1965, 4, 612−616.
H
DOI: 10.1021/acs.macromol.5b00646 Macromolecules XXXX, XXX, XXX−XXX
