Fully Reversible Shape Transition of Soft Spheres in Elastomeric

Article pubs.acs.org/Langmuir

Fully Reversible Shape Transition of Soft Spheres in Elastomeric Polymer Opal Films Christian G. Schaf̈ er, Daniel A. Smolin, Goetz P. Hellmann, and Markus Gallei* Ernst-Berl-Institute for Chemical Engineering and Macromolecular Science, Technische Universität Darmstadt, Darmstadt, D-64287, Germany ABSTRACT: Core-interlayer-shell (CIS) beads featuring noncrosslinked hard cores were used to prepare large and well-defined elastomeric opal films with remarkably distinct iridescent reflection colors. The matrix of the opal films was cross-linked by UVirradiation after compression molding of the CIS beads mixed with a bifunctional monomer. Stress-induced deformation of the embedded PS cores lead to hexagonally arranged spheroid oblates with an aspect ratio of 2.5. Optical characterization shows that bead deformation provokes a tremendous photonic band gap shift of about 160 nm. Fully reversible shape transition from the spheroid oblates back to the spherical beads and hence full recovery of the original photonic band gap can be achieved.

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INTRODUCTION Photonic band gap materials have attracted a great deal of attention as potential candidates for various optoelectronic applications.1−6 With easily accessible monodisperse colloids, such materials can be prepared by an inexpensive and convenient bottom-up process. A good optical performance with iridescent reflection colors, caused by Bragg diffraction of visible light, is obtained after colloidal crystallization.7−15 This optical feature for 3D photonic band gap materials has been forecast in the pioneering works of John16 and Yablonovitch.17 A recent review dealing with self-assembled photonic materials is given by López et al.18 So-called elastomeric polymer opals, where monodisperse beads with diameters typically in the range of 200−350 nm are embedded in a soft matrix, can be fabricated to yield reversible stretch-tunable films showing remarkable color changes due to a bend or stretch modification of the 111 (200) plane spacing.19−21 The lattice distances in these soft opal filmsand hence the reflected colorshave been varied, e.g., as a function of an applied voltage,11 as well as other external triggers such as organic solvents, pH value, or transition metals are well-known.22−24 Taking advantage of a responsive core/shell structure or polymer matrix led to fascinating tunable materials which are definitely interesting candidates for, e.g., sensing devices or as actuation systems.6 Mechano-responsive materials attracted tremendous attention in recent years for various sensing applications.25−29 The lack of mechanical strength in elastomeric opal films can be overcome by subsequent photocross-linking of the soft matrix after the film preparation, so that good optical performance can be combined with a fully reversible mechanochromic behavior.30 Moreover, the precise arrangement of the particles in such films can be improved by combinations of melting and shearordering methods.31−35 © 2013 American Chemical Society

Theoretical calculations suggested that periodically arranged ellipsoidal structures are promising candidates to create novel optical materials due to their possibility to lift the degeneracy of photonic band gaps (PBG).5,36 In the past, researchers made an effort to obtain such well-defined form-anisotropic particles especially for optical and biological applications.37−44 Van Blaaderen et al. reported the formation of photonic crystals based on inorganic ellipsoidal particles obtained using ion irradiation.45,46 Colloidal crystals embedded in elastomeric films can undergo a spherical shape transition accompanied with color change by applying strain.47 Choi et al. reported the deformation of polystyrene (PS) spheres embedded in a composite colloidal crystal with a remarkable tuning of the stop band while applying compressive stress.48 In the present study, we synthesize monodisperse core interlayer shell (CIS) beads with noncross-linked hard PS cores. In general, the CIS architecture of the latex spheres can be used for the easy-scalable and inexpensive bottom-up manufacturing of large-area opal films. Stretch-tunable opal films of all colors can be prepared by melt compression from these latex spheres varying in diameter.33 For this purpose, only minor changes have to be made in emulsion polymerization protocols, as previously reported. Highly ordered elastomeric polymer opal films are obtained by a shearing process of the corresponding melt of hard core/soft shell CIS beads in the presence of added butanediol diacrylate (BDDA) for matrix cross-linking reaction. This monomer was used for subsequent UV-cross-linking of the soft polymer matrix which led to stretch-tunable opal films with remarkable distinct iridescent color changes while applying strain. Stress-induced deformation Received: June 24, 2013 Revised: August 11, 2013 Published: August 15, 2013 11275

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water is added continuously within 1 h (PEA interlayer). After 30 min of additional reaction time, a monomer emulsion of 125.8 g EA, 0.40 g SDS, 0.2 g KOH, and 160 g water is added continuously over a period of 5 h (PEA shell). Opal Film Preparation. For preparation of elastomeric polymer opal films, the latices (containing 30 wt % polymer) are coagulated in methanol, containing a small amount of saturated sodium chloride solution, filtered, and dried. The elastomeric mass, together with 0.05 wt % of carbon-black powder (Special Black 4, Degussa), 1 wt % benzophenone, 1 wt % Darocure 1173 and 10 wt % BDDA, are extruded at 80 °C into millimeter-sized ribbons using a lab microextruder (micro1, DSM Research). In order to produce thin films, the opaline samples used for the experiments are preformed with a chill roll (CR136-350, Dr. Collin) at 80 °C. Extruded strands of the opal polymer mixture with 10 wt % of BDDA are continuously fed to the roller, which has a compressive downward pressure of 200 bar, and are directly squeezed between polyethylene terephthalate (PET) foil (Mylar A 75, DuPont) into multimeter long and approximately 4 cm wide opal film sandwiches. The gap between the two rollers is typically set to produce thicknesses on the order of 100 μm. Then, the preformed opal film sandwiches are processed over a coated stainless steel hot plate, with an apex angle of 45° and a radius of curvature Tg (PS)

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arrangement before and after deformation of the polymer opal building blocks. Furthermore, we could demonstrate that the compression molding process does not destroy the periodic structure of polymer opal embedded in the highly cross-linked elastomeric matrix. Reversible Shape Transition of the Soft Spheres. The compression molding process at elevated temperature resulted both in the deformation of the macroscopic PS/PEA film and the individual PS building blocks embedded by cross-linked PEA in the elastomeric opal. This process is completely reversible since the thermodynamically stable form for the polymer beads in the molten state is spherical. The PS particles are glassy at room temperature and transform into a viscous liquid above their Tg. As evidence that these oblate spheroids reveal nonequilibrium shapes, we heated the compressed films to 120 °C, i.e., above the Tg of PS (Tg = 108.5 °C). After 3 min, the film was allowed to cool to room temperature. As can be drawn from Figure 9 (right), a strong red reflection color

Figure 9. Optical images of an elastomeric polymer opal film after compression molding (right) and temperature-induced relaxation (left).

became visible after compression molding. Subsequent thermal treatment led to the initial state of the film (Figure 9, left). For this reason, complete reversibility of compression- and temperature-induced shape transition of the polymer opal building blocks embedded inside the opal could be assumed. One might wonder why the elastomeric opal film in Figure 9 (left) appears blue, although the stop band of the film is located in the NIR. As can be drawn from the transmission spectra in Figures 5 and 8, the whole spectra are dominated by a baseline which increases considerably to smaller wavelengths. The origin of this baseline had been discussed earlier.31 This background, which is the only phenomenon in the transmission curve of completely disordered samples, represents the typical Mie scattering for colloids at the particles and lattice defects. Since the stop band of as-prepared elastomeric opals is located in the NIR, this is the only observable optical effect under white light irradiation. As a consequence, the sample in Figure 9 (left) features a violet color. The TEM images in Figure 10 clearly show that the oblate PS particles regained their spherical shape when heated, i.e.,

Figure 8. UV/vis transmission spectra of elastomeric polymer opal during the compression molding process as a function of angle θ: original elastomeric polymer opal (top) and film after compression molding (bottom).

described by Bragg’s law, while the maximum absorbance decreased slightly. Notably, a second feature is observed in Figure 8: an additional peak emerged caused by the photonic stop band or Bragg diffraction at lattice planes in the (200) direction. The (200) planes are expected to diffract light of smaller wavelengths compared to the (111) planes, which redshifted with a decreasing incident angle. In Figure 8 (top), this feature can be observed starting from θ = 60° with an increasing intensity up to θ = 40°, until the resonances of the (111) and (200) are coincident at θ ≤ 30°. Consequently, only one peak for both resonances is observed there. In Figure 8 (bottom), the resonance of the (200) lattice plane after compression is clearly visible in transmission spectra in the range of θ = 40° to 20° with increasing intensity. The peak position of the (200) stop band should reveal a red-shift after deformation of the building blocks, because the lattice spacing in the inclined (200) direction decreased due to the decreasing major axis of the oblate spheroids. Surprisingly, the stop band red-shifted only 40 nm compared to the original film, indicating that the deformation of the building blocks only slightly affected the oblique reclining (200) plane of the fcc lattice. To sum up the investigated spectral features, well-defined distinct opaline resonances have been evidenced for all angles of incidence, which additionally indicated the remarkable 3D

Figure 10. TEM images of ultrathin sections of the elastomeric polymer opal film in different film directions after thermal treatment. Scale bars correspond to 300 nm. 11281

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the elastomeric opal films. Exactly the same position of the photonic band gap could be achieved after thermal treatment as for the original opal films. The fascinating optical feature of the opal films caused by the fully reversible switching of the embedded noncross-linked cores to either oblates or spheres can be utilized, e.g., in deformation sensor applications.

heating allowed the original shape recovery. Furthermore, the periodic structure of polymer opal fully maintained due to elastomeric feature of the PEA matrix. This demonstrates that the shape-transition of the polymer opal building blocks inside the elastomeric polymer opal was completely reversible. In addition to the TEM investigations, angle-dependent transmission spectra after heating were recorded and directly compared with the spectra of the original film in Figure 8 (left). After heating, complete reversible and reproducible tuning of the stop band was achieved (Figure 11). The stop band of the

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors want to thank the Landesoffensive zur Entwicklung Wissenschaftlich-ö k onomischer Exzellenz (LOEWE Soft Control) for financial support of this work.

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ABBREVIATIONS ALMA, allyl methacrylate; BDDA, butanediol diacrylate; CIS, core-interlayer-shell; DLS, dynamic light scattering; DSC, differential scanning calorimetry; EA, ethyl acrylate; MMA, methyl methacrylate; PBG, photonic band gap; PEA, polyethyl acrylate; PET, polyethylene terephthalate; PS, polystyrene; S, styrene; SDS, sodium dodecylsulfate; SPS, sodium peroxodisulfate; TEM, transmission electron microscopy; Tg, glass transition temperature

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Figure 11. UV/vis transmission spectrum of elastomeric polymer opal after thermal relaxation as a function of the angle θ.

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(111) lattice plane returned to the same wavelength (λ111 = 780 nm) compared to the original film, implying that the periodicity of the opal structure was not influenced by the heating process. Melting of the PS building blocks led to a reproducible 160 nm red-shift of the Bragg peak position due to the increasing small axis and the decreasing major axis of the oblate spheroids back to the average bead diameter of the original beads of 220 nm. Upon heating, the angle-dependent optical properties of the polymer opal returned to their original states, causing a reversible red-shift of the (111) lattice plane resonance, while the resonance of the (200) planes was fully reversible blueshifted.

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CONCLUSIONS The easy-scalable preparation of elastomeric opal films featuring noncross-linked hard cores and a cross-linked matrix was reported. TEM investigations of ultrathin slices in three directions of the films furnished proof that the cores had a stringent spherical shape even after the applied melt shear process. The stretch-tunable opal films showed remarkable iridescent colors due to the Bragg reflection of the (111) lattice plane. Stress-induced deformation of the embedded PS cores led to hexagonally arranged and well-defined spheroid oblates with an aspect ratio of 2.5. UV/vis characterization revealed that the bead deformation was accompanied with a tremendous photonic band gap shift of about 160 nm compared to the photonic band gap for films composed of spherical beads. The fully reversible shape transition from spheroid oblates back to spherical beads was again evidenced by TEM characterization of ultrathin slices as well as studying the optical properties of 11282

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