Reconfigurable Photonic Crystals Enabled by Multistimuli-Responsive

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Reconfigurable Photonic Crystals Enabled by Multi-Stimuli-Responsive Shape Memory Polymers Possessing Room Temperature Shape Processability Yin Fang, Sin-Yen Leo, Yongliang Ni, Junyu Wang, Bingchen Wang, Long Yu, Zhe Dong, Yuqiong Dai, Vito Basile, Curtis Taylor, and Peng Jiang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13634 • Publication Date (Web): 23 Jan 2017 Downloaded from http://pubs.acs.org on January 26, 2017

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ACS Applied Materials & Interfaces

Reconfigurable Photonic Crystals Enabled by MultiStimuli-Responsive Shape Memory Polymers Possessing Room Temperature Shape Processability Yin Fang,†,║ Sin-Yen Leo,†,║ Yongliang Ni,‡ Junyu Wang,† Bingchen Wang,† Long Yu,§ Zhe Dong,† Yuqiong Dai,†† Vito Basile,‡‡ Curtis Taylor,‡ and Peng Jiang†,* †

Department of Chemical Engineering, University of Florida, Gainesville, Florida 32611, USA, ‡

Department of Mechanical and Aerospace Engineering, University of Florida, Gainesville, Florida 32611, USA,

§

Department of Materials Science and Engineering, University of Florida, Gainesville, Florida 32611, USA, ††

‡‡

Department of Chemistry, University of Florida, Gainesville, Florida 32611, USA

ITIA-CNR, Industrial Technologies and Automation Institute, National Council of Research, Via Bassini, 15, 20133 Milano, Italy

ACS Paragon Plus Environment

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ABSTRACT Traditional shape memory polymers (SMPs) are mostly thermoresponsive and their applications in nanooptics are hindered by heat-demanding programming and recovery processes. By integrating a polyurethane-based shape memory copolymer with templating nanofabrication, reconfigurable/rewritable macroporous photonic crystals have been demonstrated. This SMP coupled with the unique macroporous structure enables unusual all-room-temperature shape memory cycles. “Cold” programming involving microscopic order-disorder transitions of the templated macropores is achieved by mechanically deforming the macroporous SMP membranes. The rapid recovery of the permanent, highly ordered photonic crystal structure from the temporary, disordered configuration can be triggered by multiple stimuli, including a large variety of vapors and solvents, heat, and microwave radiation. Importantly, the striking chromogenic effects associated with these athermal and thermal processes render a sensitive and noninvasive optical methodology for quantitatively characterizing the intriguing nanoscopic shape memory effects. Some critical parameters/mechanisms that could significantly affect the final performance of SMPbased reconfigurable photonic crystals, including strain recovery ratio, dynamics and reversibility of shape recovery, as well as capillary condensation of vapors in macropores, which plays a crucial role in vapor-triggered recovery, can be evaluated using this new optical technology.

KEYWORDS: shape memory polymers, multi-stimuli-responsive, cold programming, photonic crystals, chromogenic


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1. Introduction The microelectronics revolution sparked by the invention of semiconductor transistors and their very-large-scale integration has significantly affected almost every aspect of our daily lives. In an effort toward further high-density integration and improved system performance, light (or photons) has attracted great recent interest as an alternative to electrons as information carrier, mainly because of its higher travel speed in a dielectric material than that of an electron in a metal wire, the much larger bandwidth of dielectrics, and reduced energy loss.1 However, it is much more difficult to control photons in miniaturized volumes compared to manipulate electrons in integrated circuits.2 Photonic crystals, which are periodic dielectric structures with a forbidden gap (or photonic band gap) for photons, provide enormous opportunities in controlling the flow of light for all-optical integrated circuits and high-speed optical computing.3-4 Unfortunately, traditional photonic crystals with fixed periodic microstructures and photonic band gaps (PBGs) are limited to fabricate passive nanooptical devices (e.g., filters and waveguides).5-6 By contrast, reconfigurable (or tunable) photonic crystals, which can reconfigure various photonic functionalities on a chip to accommodate different application needs, are ideal for developing transformative active and integrated nanooptics.4,7-11 Although various technologies for realizing tunable photonic crystals with adjustable microstructures and PBGs have been demonstrated by using elastic materials, such as elastomers and gels, the intermediate photonic states cannot be fixed and they momentarily return to the original configuration once the external stimuli (e.g., mechanical stress and magnetic field) are removed.12-19 Smart shape memory polymers, which can memorize and transform between multiple permanent and temporary shapes in response to various stimuli, such as heat, light, and solvent,2038

may hold the key to achieve truly reconfigurable photonic crystals. However, traditional SMPs


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suffer from two major drawbacks that significantly limit their widespread applications in nextgeneration integrated nanooptics. First, most of the currently available SMPs are thermoresponsive and their nanooptical applications are hindered by heat-demanding programming and recovery steps.39-44 Shape memory programming, which involves deforming a SMP from its permanent shape to a temporary one, is usually done by heating the sample above a specific transition temperature (Ttrans), such as the polymer glass transition temperature (Tg), to take advantage of the compliant nature of the polymer chains at high temperature.23-33,35-36,45 Similarly, shape memory recovery is commonly triggered by reheating the deformed SMP in the “frozen” temporary configuration to above Ttrans, which increases macromolecular chain mobility and allows the polymer to return to its permanent shape via entropy elasticity.28-31,46 By contrast, SMPs that enable all-room-temperature operations for the entire shape memory cycle (from programming to storage to recovery), which could greatly enhance the processability and broaden the applications of SMPs, are rare.47-53 Second, current SMP applications mostly focus on leveraging the macroscopic shape memory effects, where the deformation length scale is large (on the order of centimeters or larger).21-33,35-36 The bulky volume changes associated with these macroscopic shape memory transitions could greatly limit the response speed of the final SMP devices. This limitation is particularly challenging for SMP-based nanooptical devices (e.g., optical switches) that typically require fast response speed. Nanoscopic shape memory effects, which involves much less amount of polymer chains during the shape transitions, could significantly boost the device response rate and vastly expand the applications of SMPs, especially in reconfigurable nanooptics.43-44 However, this intriguing potential for all SMPs has received little examination.4144,54-55


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To resolve the aforementioned drawbacks of existing SMPs for nanooptics, we have recently exploited a new series of SMPs, which are copolymers of commercial ethoxylated trimethylolpropane triacrylate (ETPTA) and polyethylene glycol (600) diacrylate (PEGDA) oligomers with varying volumetric ratios.9,56-57 The active components of these SMPs are thin macroporous photonic crystal layers (only a few µm thick) which are fabricated by using selfassembled, three-dimensional (3D) highly ordered colloidal crystals as structural templates. As demonstrated in our previous work,9 “cold” programming involving deformation of the ordered macropores can be achieved at room temperature by large capillary pressures created by the evaporation of water entrapped in the templated macropores. Equally important, the shape memory recovery of the permanent 3D photonic crystal structure can also occur under ambient conditions, triggered by various external stimuli, including static pressure,9 a variety of vapors and solvents,56 and lateral shear stress created by direct writing.57 Although these hydrophilic SMPs enable allroom-temperature operations and instantaneous shape recovery, they suffer from weak mechanical strengths (with a typical tensile strength of ~ 7-8 MPa), very low Tg (~ –42 C), and high water sensitivity. These limitations significantly impede the durability and environmental stability of the templated macroporous SMP photonic crystals. Additionally, liquid water is required to trigger the autonomous “cold” programming process.9 This “wet” step is potentially detrimental to the integration of SMP-based reconfigurable photonic crystals with other optoelectronic components. Here we report a polyurethane-based shape memory copolymer that enables unusual allroom-temperature shape memory cycles. This hydrophobic copolymer with a glass transition temperature of ~ 32.3 C is significantly stronger than the above ETPTA-co-PEGDA SMPs, and is completely insensitive to water. Its relatively low Tg allows for “cold” programming at room temperature induced by “dry” static compression or dynamic writing processes. Importantly,


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multiple stimuli, including a large variety of vapors and solvents, heat, and microwave radiation, can be applied to trigger the rapid shape memory recovery of the permanent 3D photonic crystal structure. Reconfigurable photonic crystals with well-defined optical stop bands and high optical reversibility/reproducibility can thus be patterned, erased, and regenerated on the templated macroporous SMP membranes.We have also demonstrated that the striking chromogenic effects associated with the shape memory transitions can be utilized as a sensitive and noninvasive optical technology for investigating the intriguing shape memory effects at nanoscale. 2. Experimental Section 2.1. Templating Nanofabrication of Macroporous SMP Photonic Crystal Membranes. Monodispersed silica microspheres with 290 nm diameter were synthesized by the standard Stöber method.58 The as-synthesized particles were purified in 200-proof ethanol by multiple centrifugation and redispersion cycles (at least six times). The purified silica microspheres were redispersed in ethanol to make a colloidal suspension with particle volume fraction of 1.0%. The convective self-assembly technology was then utilized to assemble silica particles into 3D highly ordered colloidal crystals on glass substrates.59 The typical crystal thickness is ~ 3 m or ~ 15 colloidal layers. A sandwich cell was constructed by stacking a bare glass slide onto the glass substrate with the assembled colloidal crystal on its surface, separated by a double-sided adhesive tape spacer (~ 1 mm thick). The commercial oligomer mixture (CN982A75, Sartomer, viscosity 1,115 cps @ 60 C, refractive index 1.4793), which is an aliphatic polyester/polyether-based urethane diacrylate oligomer (75 wt.%) blended with tripropylene glycol diacrylate monomer (25 wt.%, SR 306, Sartomer, MW 300, refractive index 1.4485), was preheated at 75 C for 10 min to change the viscous mixture to a watery fluid. Darocur 1173 (2-hydroxy-2-methyl-1-phenyl-1propanone, BASF, 1 wt.%) was added as photoinitiator. The oligomer mixture was then injected


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in the above sandwich cell. The rapid infiltration of the refractive-index-matching oligomer mixture into the interstitials between the assembled silica microspheres made the final sample transparent. A pulsed UV curing system (RC 742, Xenon) was used to cure the oligomer mixture for 4 s. The crosslinked polymer membrane was finally immersed in a 2 vol.% hydrofluoric acid aqueous solution to selectively remove the templating silica microspheres. After rinsed with deionized water and blow-dried using compressed air, the resultant free-standing macroporous SMP photonic crystal membrane shows striking iridescent colors. 2.2. Mechanically Deforming Macroporous SMP Photonic Crystal Membranes at Room Temperature. The templated macroporous SMP copolymer membrane with a typical size of 2 × 4 cm2 was covered by a glass or silicon piece (1 × 1 cm2). The sample was transferred to the stainless steel chamber of a manual hydraulic press (Carver Model C) and a clamp force of 50 lbf was applied for 5 s at room temperature. The pressure was then released and the glass or silicon cover piece was removed to release the underneath deformed region, which transformed from iridescent to transparent appearance. To create a specific letter pattern (e.g., letter “F”), the letter was first cut in a PET film (6.35 mm thick, McMaster-Carr) and then put on a shining macroporous SMP photonic crystal membrane. The above mechanical deformation process was then applied to deform the regions underneath the PET mold. 2.3. Shape Memory Recovery Triggered by Acetone Vapor, Liquid Ethanol, Heat, and Microwave. The deformed macroporous SMP membrane was placed at ~ 1 cm above the surface of liquid acetone (15 mL) contained in a centrifuge tube for ~ 5 s and then removed from the tube (see Video S2 in Supporting Information). The deformed region instantaneously changed color from transparence to shining green triggered by the acetone vapor exposure. To quantitatively evaluate the dynamics of the vapor-induced shape memory recovery process, an Ocean Optics


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reflectance probe (R600-7) connected to an Ocean Optics HR4000 spectrometer was positioned at ~ 1 cm above the deformed SMP membrane to automatically collect time-resolved optical reflection spectra at a speed of 1 spectrum per second. A homemade shutter device consisting of a glass substrate with a drilled 9 mm diameter hole on it and a plastic cover piece was designed to control the exposure of the SMP sample to acetone vapor. To trigger solvent-induced shape memory recovery, the mechanically deformed SMP membrane was immersed in various solvents, such as ethanol, acetone, and hexane, for 2 s. After drying the sample using a Kimwipes paper, the shining greenish color of the original macroporous SMP membrane was recovered. The same chromogenic recovery process can also be activated by putting the deformed SMP membrane (with the macropores faced downwards) on a Fisher Scientific Isotemp RT digital hotplate preset at 45.0 C. The same reflectance probe setup as described above was utilized in assessing the recovery dynamics of the heat-triggered shape memory effects. A microwave oven (Sharp R-305C) with an operating power of 1000 W was used in evaluating the shape memory recovery of a deformed macroporous SMP membrane under microwave radiation. An ennoLogic dual laser infrared thermometer was utilized in measuring the surface temperature of the microwave-activated sample. 2.4. Sample Characterization. SEM imaging was carried out on a FEI Nova NanoSEM 430 unit. A thin layer of gold (~ 5 nm) was sputtered onto the samples prior to imaging. Amplitudemodulation atomic force microscopy (AM-AFM) was performed using a MFP-3D AFM (Asylum Research, Inc.) with a Nanosensor PPP-NCHR probe (tip radius < 10 nm). In-situ nanoindentation tests were conducted on a MFP-3D NanoIndenter (Asylum Research, Inc.) using a spherical sapphire indenter (tip radius ~ 500 µm). Such configuration of the instrument enables a force and displacement resolution less than 3 µN and 1 nm, respectively. Detailed calculations of Young’s


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modulus, plastic deformation, and surface roughness are discussed in Supporting Information. Differential scanning calorimetry (DSC) thermograms were obtained from –75 to 250 °C at a heating rate of 10 °C min-1 using a Seiko DSC 6200 instrument and an empty pan as reference. The DSC data shown in this paper was extracted from the second full heat-cool-heat cycle. Thermogravimetric analysis was carried out under nitrogen with a TGA Q5000 analyzer and a platinum crucible between 25 and 500 °C at a heating rate of 20 °C min-1. Dynamic mechanical analysis (DMA) was carried out by using a Thermogravimetric Analyzer Q800 DMA instrument, under a 1 Hz constant frequency with a heating rate of 3 °C min-1. This DMA characterization was repeated 3 times with 3 heating and cooling cycles for each test. The sample dimensions for DMA tests are 23.0  11.5  2.4 mm. Normal-incidence optical reflection spectra were obtained using the Ocean Optics HR4000 high-resolution vis-NIR spectrometer with the reflection probe (R6007) and a tungsten halogen light source (LS-1). Absolute reflectivity was obtained as the ratio of the sample spectrum and a reference spectrum, which was the optical density obtained from an aluminum-sputtered (1000 nm thick) silicon wafer. The swelling ratios of the templated macroporous SMP membranes in various solvents (water, ethanol, and acetone) were evaluated by measuring the masses of the membranes immersed in these solvents for different durations. 2.5. Scalar Wave Approximation Optical Modeling. The scalar wave theory developed for modeling the optical properties of periodic dielectric structures60 was implemented to calculate the normal-incidence optical reflection spectra from macroporous SMP photonic crystals. In this theory, Maxwell’s equations are solved for a periodic dielectric assuming that one may neglect diffraction from all but one set of crystalline planes (e.g., the (111) planes in our case). The SWA calculation contains no adjustable parameters, since the size of the templated macropores and the


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photonic crystal thickness were independently determined from SEM characterization, and the refractive index of the SMP film is known. 3. Results and Discussion 3.1. Preparation and Characterization of New Macroporous SMP Photonic Crystals. The schematic illustration in Figure 1 shows the unusual shape memory cycles of the reconfigurable macroporous photonic crystals enabled by a multi-stimuli-responsive SMP, which is a photocured copolymer of a commercial oligomer mixture (CN982A75, Sartomer) consisting of an aliphatic polyester/polyether-based urethane diacrylate oligomer (75 wt.%) blended with tripropylene glycol diacrylate monomer (25 wt.%). To fabricate self-standing macroporous SMP photonic crystal membranes, this oligomer mixture was allowed to penetrate into the interstitials between silica microspheres of a convectively self-assembled multilayer colloidal crystal through strong capillary actions.59 After photocuring the oligomer mixture and selectively removing the templating silica microspheres, shining macroporous SMP photonic crystal membrane was resulted. Figure 2a shows an iridescent macroporous sample templated from 290 nm silica particles. The striking greenish color of the film is caused by Bragg diffraction of visible light from the 3D periodic arrays of SMP macropores, which are evident in the cross-sectional scanning electron microscope (SEM) image as shown in Figure 2b.


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Figure 1. Schematic illustration showing the microstructural transitions between the permanent (3D ordered) and the temporary (disordered) geometries of a macroporous SMP membrane triggered by an unusual “cold” programming process and 4 different types of external stimuli.


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Figure 2. Photographs and typical cross-sectional SEM images showing the microstructural transitions during a room temperature shape memory cycle. (a,b) Permanent structure with 3D ordered macropores. (c,d) Temporary structure (imprinted letter “F”) with disordered macropores. (e,f) Recovered structure triggered by exposing the deformed sample to acetone vapor.


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The templated macroporous SMP photonic crystal membranes are hydrophobic with an apparent water contact angle of ~ 90 measured using a goniometer (Ramé-Hart Inc.). Once dried out of water, the iridescent colors of the membranes will retain even after re-immersing them in water, indicating the dewetting of the dried samples by water. The intrinsic water content of the macroporous SMP membrane was determined by thermogravimetric analysis (TGA) as shown in Figure S1 (Supporting Information). From the TGA curve and the corresponding weight loss rate, it is apparent that the water content of the SMP membrane is quite small, and the thermal degradation of the copolymer starts at ~ 250 C. The tensile strength, percent elongation, and Young’s modulus of the shape memory copolymer, measured by Sartomer, is 40.7 MPa, 42%, and 151.7 MPa, respectively. The glass transition temperature of the copolymer was evaluated by differential scanning calorimetry. The typical DSC plot of a macroporous SMP membrane in Figure 3 shows a Tg of ~ 32.3 C. Additionally, no apparent crystallization dips show up in the DSC plot for temperature from –75 °C to 250 °C, revealing the copolymer is amorphous in this wide temperature range. The storage modulus and tangent delta as a function of temperature of the polyurethane-based SMP are shown in the temperature-sweep DMA plot in Figure 4. This DMA test reveals that the SMP exhibits a temperature-dependent modulus. When the SMP is below the glass transition temperature, it shows a relatively high storage modulus and the modulus decreases with increasing temperature. The storage modulus shows an obvious drop in the glass transition region upon heating. The examination of the tangent delta plot shows that the glass transition temperature of the SMP is around 35.6 C, which is close to that of the above DSC test. Compared with the hydrophilic and weak ETPTA-co-PEGDA shape memory copolymers used in our previous studies,9,56-57 which suffer from small tensile strength and very low Tg, the polyurethanebased hydrophobic copolymer is much less sensitive to water, and is significantly stronger. The


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high mechanical strength of the copolymer renders good structural stability of the templated macroporous photonic crystal when dried out of water (see Figure 2a). By contrast, large capillary pressures created by the evaporation of water can easily deform macroporous ETPTA-co-PEGDA copolymers with significantly smaller Young’s modulus.9,56-57 These desirable properties make the polyurethane-based shape memory copolymer a better candidate for fabricating truly reconfigurable photonic crystals with superior durability and environmental stability. 2.4

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Figure 4. Storage modulus and tan delta as a function of temperature of the polyurethane-based SMP sample.

3.2. Room Temperature Shape Programming of Macroporous SMP Photonic Crystals. In addition to the unique combination of hydrophobicity and good mechanical strength, another major merit of the polyurethane-based shape memory copolymer is that it enables unique room temperature shape programming, which is in stark contrast to heat-demanding programming generally required by almost every class of traditional SMPs.47 As shown by the scheme in Figure 1, the 3D highly ordered macropores (permanent geometry) can be mechanically deformed into disordered arrays (temporary geometry) at room temperature (22.5 ± 0.5 C), which is lower than the Tg of the copolymer. Our extensive experiments showed that both static mechanical compression and dynamic lateral shear stress could induce this microscopic order-disorder transition under ambient conditions. As shown in Figure 2c, the transparent letter “F” imprinted on the shining SMP photonic crystal membrane was prepared by compressing a polyethylene terephthalate (PET) film with the same letter pattern onto the shining macroporous sample using


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a manual hydraulic press. The discoloration of the deformed region was caused by the loss of the original, 3D ordered diffractive structure, which is clearly shown by the cross-sectional SEM image in Figure 2d. Besides the apparent collapse of the periodic lattice structure, the photonic crystal layer thickness was measured to decrease from 2.64 ± 0.03 µm to 1.39 ± 0.23 µm during the static compression process. A large variety of materials ranging from hard glass and silicon to soft elastomers (e.g., polydimethoxysiloxane) can be used as molds to mechanically deform the macroporous SMP membranes. Video S1 in Supporting Information illustrates a dynamic direct writing process using the round tip of a plastic pen to write arbitrary patterns on a shining SMP sample. The lateral shear stresses created by the moving pen deformed the written regions, leading to the transparent “UF” marks left on the greenish membrane. Technically important, the deformed states generated by both static compression and dynamic writing are structurally stable for a long period of time. The samples prepared about 7 months ago did not show any apparent change when stored under ambient conditions. To gain fundamental insights into the “cold” programming process (as the Tg of the shape memory copolymer is higher than room temperature), as well as determine the critical mechanical stress that can trigger the microscopic order-disorder transition of macropores, we conducted insitu nanoindentation tests on the templated macroporous SMP photonic crystal membranes. Figure 5a shows a typical indentation force-depth curve when a 30 µN force (corresponding to ~ 100 kPa static pressure after normalized to the contact area) was applied on a macroporous SMP sample with 290 nm macropores using a spherical sapphire indenter (tip radius ~ 500 µm). The hysteresis between the tip approaching and retracting (or loading and unloading) stages indicates the extents of the elastic and plastic deformations induced by the mechanical compression process. 61 Our extensive nanoindentation experiments revealed that the hysteresis is nearly negligible when the


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normal load was smaller than 30 µN, and the indents could be fully recovered after unloading the tip. Plastic deformation was observed under a critical normal load of ~ 30 µN, and it became more dominant with increasing indentation forces. For example, the plastic deformation generated under a 200 µN indentation load, which corresponds to a contact pressure of ~ 300 kPa, was measured to be ~ 280 nm (Figure 5b).

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By considering the above mechanical characterization and Tg measurements, the unusual room temperature shape programming of the shape memory copolymer can be explained as follows. First, the copolymer has a Tg only slightly higher than room temperature. Therefore, to some extent it shows good elasticity under ambient conditions, as evidenced by the relatively large percent elongation (42%). Second, during the mechanical deformation process, the large external stress (either tensile or shear) could cause the so-called reversible-plastic strain,47 which is the portion of strain that can be recovered due to shape memory and differs greatly from the classical, unrecoverable plastic strain.62 Additionally, the van der Waals attractions between the macromolecules of the collapsed macropores (see Figure 2d) could also facilitate to make the macroporous membrane retain the deformed structure after the removal of the external stress. Some of the mechanical energy will then be stored in the deformed, temporarily configured polymer chains. The strained polymer networks have a strong tendency to recover back to their permanent, stress-free configurations, when an appropriate external stimulus is applied to the deformed SMP membrane.63 3.3. Room Temperature Vapor-Triggered Shape Memory Effects. One unusual recovery mechanism enabled by the shape memory copolymer is triggered by a large variety of vapors (e.g., acetone, ethanol, hexane, benzene, and dichloromethane) under ambient conditions. As shown in Figure 2e and Video S2 in Supporting Information, the mechanically deformed samples with the imprinted patterns, such as the letter “F” in Figure 2c, were fully recovered after exposing the membranes in acetone vapor, which was in equilibrium with liquid acetone (at room temperature) contained in a centrifuge tube, for a few seconds. The iridescent colors and the 3D highly ordered structure of the original macroporous photonic crystals were restored in the activated sample (see Figure 2e,2f). The recovered macroporous layer thickness is measured to be 2.57 ± 0.07 µm, which


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is very close to the original photonic crystal thickness (2.64 ± 0.03 µm), indicating a nearly perfect shape memory recovery triggered by acetone exposure at room temperature. To complement structural characterization using cross-sectional SEM imaging, atomic force microscopy (AFM) was used in evaluating the surface morphology and roughness of the SMP samples during acetone vapor-triggered shape memory recovery process. Figure 6 compares the AFM images and the corresponding depth profiles of the regions of the mechanically deformed letter “F” in Figure 2c prior to (Figure 6a,6b) and after (Figure 6c,6d) acetone vapor exposure. Apparently, the deformed region possesses a much rougher surface than that of the recovered area, with an average root mean square roughness (Rq) of 32.7 ± 3.7 nm and 11.4 ± 0.3 nm, respectively. In addition, the permanent long-range hexagonal ordering of the templated macropores, which was lost in the temporary geometry (Figure 6a), was recovered in the activated configuration (Figure 6c).


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Figure 6. Typical AFM images and the corresponding height profiles showing the difference in the surface microstructure of a deformed macroporous sample (a,b) and the same sample after exposing to acetone vapor (c,d). The easily perceived color changes associated with the entire shape memory order-disorderorder cycle can be quantitatively characterized by in-situly measuring the normal-incidence reflection spectra of the macroporous SMP membranes using an Ocean Optics optical spectroscopy. Figure 7a compares the spectra obtained from the samples shown in Figure 2a, 2c, 2e with the ordered permanent state (red curve), deformed state (black curve), and acetone vaporrecovered state (blue curve). The original macroporous photonic crystal membrane with the shining greenish color shows a distinct optical stop band centered around 520 nm. The welldefined Fabry-Perot fringes demonstrate the high crystallization quality of the templated


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macroporous photonic crystal. Associated with the microscopic order-disorder transition induced by the “cold” programming process (see Figure 2c,2d), the Bragg diffraction peaks disappear, complying with the discoloration of the deformed sample. Once activated by acetone vapor, the restoration of the permanent 3D photonic crystal structure leads to the nearly perfect overlap of the recovered reflection spectrum with the original one. We have also conducted scalar-wave approximation (SWA) optical modeling,60 which assumes a perfect face-centered cubic (F.C.C.) crystalline lattice of 290 nm air cavities in a polymer with a refractive index of 1.48, to complement the optical measurements. The simulated reflection spectrum (purple curve) matches well with the experimental spectra obtained from the original and the acetone vapor-recovered SMP membranes. This good agreement further confirms the near-unity strain recovery ratio (Rr), which quantifies the ability of the SMP to memorize its permanent shape and is a measure of how far a programmed strain is recovered in a shape memory transition.31


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60 40 20 0 O1

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Figure 7. (a) Normal-incidence specular reflection spectra showing the permanent, deformed, and two consecutive acetone-vapor-recovered states of a macroporous SMP membrane with 290 nm macropores. The SWA-simulated spectrum is also shown to compare with the experimental results. (b) Reflection amplitudes at  = 520 nm obtained from the above SMP membrane which was cyclically deformed (D1, D2, D3, D4, D5) and then triggered by acetone vapor (A1, A2, A3, A4, A5). O1 indicates the original membrane with 3D ordered macropores. Above we have shown that the polyurethane-based shape memory copolymer enables allroom-temperature shape memory cycle including mechanical stress-induced “cold” programming and vapor-triggered recovery. This unique characteristic could significantly enhance the shape


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processability to accommodate broader application requirements. Another attractive feature is that these all-room-temperature shape memory transitions are repeatable. The cyan curve in Figure 7a, which was obtained after the second “cold” programming/acetone vapor activation cycle, overlaps with the spectrum of the same sample after the first shape memory cycle, showcasing good reversibility and reproducibility. As the amplitude of the optical stop band of a macroporous photonic crystal is a good indicator of its strain recovery ratio (or the recovered macroporous layer thickness),64 we compared the absolute reflection amplitudes at 520 nm wavelength for the same sample shown in Figure 7a after cyclically deformed and then triggered back by acetone vapor for 5 times (Figure 7b). Clearly, good optical repeatability was achieved by this sample. Our extensive experiments showed that the all-room-temperature shape memory cycle can be repeated for at least 50 times without apparent degradation of the optical performance of the templated SMP photonic crystal membranes. This outstanding reversibility is critically important for developing truly reconfigurable/rewritable photonic crystal devices. The striking chromogenic effects induced by the microscopic disorder-order transition also render a sensitive and noninvasive optical approach for in-situ investigating the rapid dynamics of the vapor-triggered shape recovery process. Figure 8a shows a 3D plot of time-resolved, colorcoded normal-incidence specular reflection spectra continuously obtained from a macroporous SMP membrane with 290 nm macropores exposed to acetone vapor. A homemade shutter device was constructed to precisely control the starting moment when the deformed SMP sample was exposed to acetone vapor. The rapid reach of the plateau in reflection amplitude within ~ 6.5 s is clearly shown in the 2D plot of the time-resolved spectra in Figure 8b.


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Figure 8. Time-resolved, color-coded normal-incidence specular reflection spectra continuously obtained from a macroporous SMP membrane with 290 nm macropores exposed to acetone vapor. (a) 3D plot. (b) Corresponding 2D plot. The in-situ dynamic optical measurements could also provide insights into the fundamental understanding of the vapor-triggered shape memory recovery process. As shown in Figure 8b, when equilibrium was reached with acetone vapor above liquid acetone at room temperature (the partial pressure of acetone vapor is ~ 229 mmHg at 25 C), the optical stop band shifts from ~ 520 nm in air (see Figure 7a) to ~ 593 nm in acetone vapor. This large red-shift of the Bragg diffraction peaks indicates the capillary condensation of acetone vapor in the templated macropores.56,65 The


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condensed liquid acetone increased the effective refractive index (neff) of the diffractive medium, leading to the red-shift of the diffraction peaks. By using the Bragg diffraction equation,

max  2neff d sin  , where d is the inter-plane distance and  is /2 for normal incidence, neff is calculated to be ~ 1.252 for the acetone vapor-activated sample. If we assume the templated macropores are close-packed and the volume fraction of air (VFair) in a dry macroporous SMP membrane is 0.74, we can then calculate the volume fraction of the condensed acetone (VFacetone) using neff = nSMP × 0.26 + nair × (0.74  VFacetone) + nacetone × VFacetone, where nSMP, nair, and nacetone is 1.48, 1.0, and 1.359, respectively. By this way, the equilibrium VFacetone is determined to be 0.354, indicating about half volume of the air cavities (original VFair = 0.74) was filled with condensed acetone. We believe the condensed liquid acetone in the interconnected SMP macropores plays a critical role during the rapid, vapor-triggered shape recovery process. Considering the microscopic thickness of the active photonic crystal layer (only a fewµm thick) and its high porosity, the condensed acetone should easily diffuse into the macroporous polymer layer. We therefore evaluated the uptake of acetone by the macroporous shape memory copolymer by measuring the weight change after immersing the sample in acetone for different durations (see Figure S2a in Supporting Information). The swelling ratio of the SMP by acetone, which is defined as Wwet  Wdry Wdry

, where Wwet and Wdry are the weights of the copolymer membrane after and before

immersing in acetone, was observed to keep increasing with longer immersion durations. Previous studies have revealed that entrapped solvents could significantly reduce the glass transition temperatures of various SMPs through the plasticizing effect.66-70 As the Tg of the shape memory copolymer is only slightly higher than room temperature, the effective reduction of its Tg to room


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temperature or even below room temperature by the absorbed acetone is anticipated. This could significantly increase the polymer chain mobility and allows the strained polymer networks to recover back to their permanent, stress-free configurations. Additionally, solvent-induced polymer swelling can also facilitate the macropore recovery by expanding the intermolecular distance and thus reducing the macromolecular interactions.67 3.4. Room Temperature Solvent-Triggered Shape Memory Effects. As demonstrated in the above section, the unique macroporous thin film structure and the thermomechanical properties rendered by the templated SMP photonic crystals enable vapor-triggered shape memory effects at room temperature. In addition to acetone, our further polymer swelling experiments revealed that a large variety of other solvents, such as ethanol, benzene, hexane, and dichloromethane, can also noticeably swell the macroporous shape memory copolymer membranes. It is thus not surprising to observe solvent-activated shape memory effects under ambient conditions. Figure S2b in Supporting Information shows that an equilibrium swelling ratio of ~ 2.5% can be rapidly reached when a macroporous SMP sample was immersed in liquid ethanol. The normal-incidence optical reflection spectra shown in Figure S3a (Supporting Information) demonstrate a nearly perfect overlap of the spectra of an ethanol-recovered SMP photonic crystal (after immersing the “cold” programmed sample in liquid ethanol for 2 s, followed by drying) and the same sample in its original, intact state (i.e., 3D ordered configuration). Moreover, the excellent shape memory reversibility and reproducibility was confirmed by the consistency in the reflection spectra (Figure S3a) and the reflection amplitudes at 520 nm wavelength (Figure S3b) measured using the same sample after different shape memory cycles. Along with optical characterization, the crosssectional SEM (Figure S4a in Supporting Information) and AFM (Figure S5a) images also demonstrate the 3D highly ordered structure of the liquid ethanol-recovered macroporous SMP


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photonic crystal. The root mean square roughness of the sample is 11.4 ± 0.4 nm, which is almost the same as that of the acetone vapor-activated membrane. This indicates that the solvent-triggered shape recovery is as complete as the vapor-triggered one, as the absorbed solvent-induced polymer swelling and Tg reduction dominate both recovery processes. In addition to ethanol and acetone, our further experiments have demonstrated that a large variety of solvents, including methanol, toluene, benzene, and dichloromethane can all recover the deformed macroporous SMP samples. The recovery speed is nearly the same for these solvents. 3.5. Heat-Triggered Shape Memory Effects. Besides the athermal shape memory effects triggered by various vapors and solvents, traditional thermoresponsive shape recovery can also be stimulated in the “cold” programmed macroporous SMP photonic crystals. Similar to the striking chromogenic effects induced by the athermal processes, the normal-incidence reflection spectra in Figure 9a and the reflection amplitudes at 520 nm wavelength in Figure 9b measured during repeated mechanical deformation (at room temperature) and thermal treatment (heated on a digital hotplate preset at 45.0 C) cycles also reveal the near-unity strain recovery ratio and good reversibility triggered by heat. The long-range ordering of the heat-recovered macropores is further confirmed by the cross-sectional SEM and AFM images in Figure S4b and Figure S5b, respectively. The dynamics of the heat-triggered shape recovery was in-situly characterized by continuously collecting time-resolved normal-incidence reflection spectra (Figure 10). To enhance the heat transfer rate from the heat source to the active photonic crystal layer, the deformed macroporous layer was placed directly on the surface of the digital hotplate. Both 3D (Figure 10a) and 2D (Figure 10b) plots of the stacked, color-coded spectra show that the recovery of the ordered macropores starts at ~ 10 s, and it takes ~ 25 s to reach the maximal reflectivity. The heat-induced shape recovery speed, which is mainly affected by the limited heat transfer rate from the hotplate


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to the rough surface of the deformed SMP membrane (see Figure 6a), is a bit slower than that of the vapor-triggered recovery. This heat-triggered shape memory effect is caused by entropy elasticity28-31,46 as the recovery temperature is higher than the glass transition temperature of the polyurethane-based copolymer (~ 32.3 C). Although this near room temperature Tg facilitates both programming and recovery under ambient conditions, it limits the thermal stability of the temporary configuration in the shape memory cycle. The deformed state will rapidly recover back to the permanent state when the temperature is slightly higher than room temperature.


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3.6. Microwave Radiation-Triggered Shape Memory Effects. In addition to direct heating, the recovery of the mechanically deformed macropores can also be activated by indirect heating created using a microwave oven operating at 2.45 GHz and 1,000 W for 5 min. However, due to the low water content and the limited microwave absorption of the shape memory copolymer membranes, the recovery speed induced by microwave radiation is significantly slower


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than that of direct heating. The surface temperature of a macroporous SMP sample with 290 nm macropores after 5 min microwave treatment was measured to be 53.3 ± 0.26 C using an infrared thermometer. The cross-sectional SEM image in Figure S4c and the AFM surface scan in Figure S5c, both in Supporting Information, demontrate the full recovery of the permanent, 3D ordered arrangement of the templated macropores after microwave heating. Figure S6 in Supporting Information illustrates the good match of the normal-incidence optical reflection spectra obtained after two consecutive microwave-triggered shape memory cycles with the original spectrum. 4. Conclusions In conclusion, we have exploited a polyurethane-based shape memory copolymer that combines unique characteristics including “cold” programming, multi-stimuli responsiveness, fast recovery dynamics, and all-room-temperature shape memory cycles. The integration of this SMP with the templated macroporous structure enables the fabrication of truly reconfigurable photonic crystals that can transit between a temporary disordered configuration and a permanent 3D ordered geometry. The microscopic thin-film configuration also renders fast recovery speed when triggered by both athermal and thermal stimuli, including a large variety of vapors and solvent, heat, and microwave radiation. The instantaneous transition between the temporary and the permanent states of the macroporous SMP photonic crystals leads to an easily perceived color change. This striking chromogenic effect provides a sensitive and noninvasive optical technology for investigating the intriguing shape memory effects (e.g., strain recovery ratio, shape recovery dynamics, and shape memory reversibility) at nanoscale. These smart multi-stimuli-responsive materials

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ASSOCIATED CONTENT Supporting Information. Thermogravimetric analysis and swelling tests of the templated macroporous SMP membranes. Optical reflection spectra, cross-sectional SEM, and AFM images characterize the recovered SMPs triggered by liquid ethanol, heat, and microwave radiation. Videos show the deformation and vapor-activated recovery processes of a macroporous SMP membrane. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors *Peng Jiang. E-mail: [email protected]. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ║These authors contributed equally. ACKNOWLEDGMENT This work was partially supported by the US Defense Threat Reduction Agency, Basic Research Award # HDTRA1-15-1-0022, to University of Florida and an Early Stage Innovations grant (Award No. NNX14AB07G) from NASA’s Space Technology Research Grants Program. Acknowledgments were also made to the US National Science Foundation (NSF) under Award No. CMMI-1562861 and Marie Curie IRSES Project 247614-NET4m.


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Reconfigurable Photonic Crystals Enabled by Multistimuli-Responsive

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