Swimming Photochromic Azobenzene Single Crystals in Triacrylate

Swimming Photochromic Azobenzene Single Crystals in Triacrylate Solution. Kenneth Milam ..... To the best of the our knowledge, the sink or swim behav...
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J. Phys. Chem. B 2010, 114, 7791–7796

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Swimming Photochromic Azobenzene Single Crystals in Triacrylate Solution Kenneth Milam,† Garrett O’Malley,§ Namil Kim,† Dmitry Golovaty,‡ and Thein Kyu*,† Departments of Polymer Engineering and Mathematics, UniVersity of Akron, Akron, Ohio 44325, and Department of Physics, Bucknell UniVersity, Lewisburg, PennsylVania 17837 ReceiVed: April 14, 2010; ReVised Manuscript ReceiVed: May 12, 2010

Self-motion of a growing single crystal of azobenzene chromophore in triacrylate solution (TA) is investigated in relation to the solid-liquid phase diagram bound by the solidus and liquidus lines. Upon thermal quenching from the isotropic melt to the crystal + liquid gap, various single crystals develop in a manner dependent on concentration and supercooling depth. During the crystal growth, TA solvent is rejected from the growing faceted fronts, enriching with TA in close proximity to the crystal-solution interface. The concentration gradient that formed as the result of TA expulsion induces convective flows in the solution and generates spatial variability of surface tension usually responsible for Marangoni effect. Either or both of these phenomena may have contributed to the observed self-motion including swimming, sinking, and floating of the azobenzene rhomboidal crystal in TA solution. A stationary rhomboidal crystal is also shown to swim upon irradiation with the UV light because of a mechanical torque generated by the trans-cis isomerization. Judging from the sinking or floating behavior of the azobenzene crystal, it may be inferred that the nucleation occurs at the solution-air interface. Introduction Various topologies of polymer single crystals hitherto reported, ranging from rectangular, square, hexagonal, and diamond shapes to curved single crystals, were grown primarily from polymer solutions.1-3 Occasionally, single crystals of pyramidal shape were found to develop on the bottom substrate, some with and some without a terrace pattern. A natural question is whether the pyramidal-shaped crystal grows downward (e.g., from the solution-air interface) or upward (e.g., from the substrate, like a man-made pyramid). Similar polymer single crystals can be grown from the melt by thermal quenching or slow cooling below the crystallization temperature. Although the final morphologies between the melt-grown and solutiongrown single crystals might be similar, the crystallization pathways traversed by these systems are quite different. To elucidate the structural evolution dynamics of solutiongrown crystals, a nondestructive time-resolved characterization approach is essential. Recently, real-time studies on growth of polymer single crystals from the melt state became possible because of rapid advances of atomic force microscopy (AFM).4,5 In melt-grown single crystals, a considerable difference in the hardness of the solid crystal and that of the surrounding melt surfaces makes the real-time AFM investigations possible. In contrast, the time-resolved AFM investigation of polymer crystallization in a binary solution remains impractical because the polymer crystals in solution are fragile and lack the sufficient mechanical strength. Moreover, the initial polymer crystals (i.e., size of nuclei) thus formed are too small to be seen by most nondestructive in situ techniques. Therefore, it is customary to study the crystallization of small-molecule systems to provide important clues for better understanding of polymer crystallization. This is one of the sources of our motivation for selecting * Corresponding author. E-mail: [email protected]. † Department of Polymer Engineering, University of Akron. ‡ Department of Mathematics, University of Akron. § Bucknell University.

small-molecule azobenzene crystal that has interesting photoreversible phase transition. A reversible conformational transition from trans- to cisisomeric states makes azobenzene and its derivatives very attractive, especially for applications such as photoactuator, shape memory, and photo-optical switching.6-9 These photoswitchable polymer films are generally prepared via photopolymerization of azobenzene chromophores/reactive monomer mixtures. Whereas investigating the effect of photoisomerization on the photolithographic patterning of azobenzene chromophore/ triacrylate solution (TA) undergoing photopolymerizationinduced phase separation, the development of azobenzene single crystals was noticed that exhibited a variety of crystal topologies such as pyramidal, rhomboidal, or needle shapes. Such emerged crystal topology is seemingly governed by the concentration and supercooling depth relative to the phase diagram of the constituent pair. On the basis of nondestructive time-resolved video photography, the present azobenzene/TA mixture uniquely affords an in situ investigation of crystal growth phenomena because of the gigantic size (8-10 mm) of azobenzene chromophore crystal and nonvolatile nature of the triacrylate solvent. In this article, the swimming and sinking behavior of the growing single crystal of azobenzene chromophore in TA is examined in relation to the solid-liquid phase diagram bound by the solidus and liquidus lines. Plausible mechanisms of the self-propelling crystals are discussed. In addition, the crystal swimming driven by trans-cis photoisomerization of azobenzene in TA is explored. Experimental Section Azobenzene with a reported molecular weight of Mw ) 182.2 g/mol and trimethylolpropane triacrylate (TA) having Mw ) 296.3 g/mol were bought from Aldrich Chemical and used without further purification. The azobenzene crystal exhibits a melting transition (Tm) at ∼68 °C, whereas TA is noncrystalline. Various concentrations of azobenzene/TA solutions were pre-

10.1021/jp1033454  2010 American Chemical Society Published on Web 05/21/2010

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pared at 70 °C by stirring mechanically until the solution became completely homogeneous. The mixture was sealed in aluminum hermetic pans and used in differential scanning calorimetry (DSC 2920, TA Instruments) measurements. The heating rate was 5 °C/min unless otherwise indicated. We prepared the samples for optical studies by depositing the solution onto a glass slide having a concave circular well with a dimension of 16 mm in diameter and 0.5 mm in depth at the deepest point. For the purpose of comparison, a flat rectangular cell with a dimension of 37 × 8 mm2 in cross-section and 1 mm in depth was used. Spatiotemporal growth of azobenzene crystals at various concentrations was recorded following thermal quenches to room temperature using polarized optical microscope (POM) (PZMTIV, World Precision Instruments). The effect of trans-cis isomerization on crystal growth was examined by illuminating the UV light from one end of the cell, and the crystal motion was monitored. The UV curing unit (ELC403, Electrolite Corporation) was utilized with the intensity of 40 mW/cm2 at the wavelength of 350-380 nm.

Figure 1. DSC endotherms of azobenzene/triacrylate solutions at various concentrations together with the inset for a magnified view, showing the lowering trend of the crystal melting transition. The heating rate was 5 °C/min.

Model Description To verify various coexistence phases, we constructed a theoretical phase diagram by self-consistently solving the free energy density of the crystalline solution that contains contributions from (i) the phase-field free energy of crystal solidification (PF) based on crystalline order parameter (ψ) and (ii) the Flory-Huggins free energy (FH) for liquid-liquid demixing pertaining to concentration (or volume fraction, φ).10-13 The combined free energy densities of PF and FH theories may be expressed as14

f(ψ, φ) ) ζ(T) + ζ0(Tm) 3 ζ(T)ζ0(Tm) 2 1 φW ψ ψ + ψ4 + 2 3 4 [(φ/r1) ln φ + ((1 - φ)/r2) ×

[

]

ln(1 - φ) + χaaφ(1 - φ)] + χcaψ φ(1 - φ) 2

(1) where the first term is a Landau-type asymmetric double-well potential and the second term is the FH free-energy density of liquid-liquid demixing. The third term represents the coupling interaction energy between the crystalline constituent and the amorphous solvent. The crystalline order parameter (ψ) may be interpreted as linear (i.e., 1D) crystallinity of the crystalline constituent, in which ψ ) 1 represents a perfect crystal; otherwise, the crystal is imperfect. The coefficients, ζ(T)and ζ0(Tm) denote the locations of the unstable hump and of the solidification minimum of the Landau-type energy potential, respectively. W is a dimensionless coefficient related to the energy threshold for crystal solidification. The amorphousamorphous interaction parameter is given as χaa ) χFH ) A + B/T, where A and B are constants and T is absolute temperature. The crystal-amorphous interaction parameter is related to the heat of fusion of azobenzene, viz. χca ) ω∆Hu/RT, where the proportionality constant, ω, can be determined from the melting point depression versus solvent concentration plot.15 We determined the solid-liquid phase-transition temperatures by selfconsistently solving the combined PF/FH free energies via minimization with respect to the crystalline order parameter, ψ. Subsequently, we constructed the temperature versus composition phase diagram including liquidus and solidus lines by

Figure 2. Self-consistently calculated coexistence lines in comparison with the experimental phase diagram and polarized optical micrographs of various single crystal structures after thermal quenching of 25, 30, 35, and 40 wt % azobenzene solutions to indicated points, that is, a-d. The diameter of the concave circular cell is 16 mm and the depth is 0.5 mm at the center.

balancing the chemical potentials of each phase along with a double tangent method, that is, (∂f/∂φ)|φR )(∂f/∂φ)|φβ.14 Results and Discussion Prior to investigating crystal growth dynamics following thermal quenches, it is essential to construct the phase diagram of azobenzene/TA mixtures to provide guidance to kinetic pathway. That is to say, the emerged crystalline structure and growth dynamics strongly depend on thermal quenched depth and concentration in reference to the phase diagram. Figure 1 exhibits the DSC scans at various blend compositions. The melting temperature, Tm, of the pure azobenzene is located at ∼68 °C that shifted systematically to lower temperatures with increasing TA content. The trend of melting point depression can be seen more clearly in the plot of Tm versus composition (Figure 2). This experimental solid-liquid phase diagram is compared with the self-consistently calculated phase diagram using the combined FH/PF free energy density of eq 1. The calculated liquidus line is in good accord with the depressed melting points, whereas the solidus line virtually overlaps with the axis of the pure crystalline constituent with an extremely

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Figure 3. Polarized optical micrographs of (a) the edges of the terrace structure and (b) the core area of the 27 wt % azobenzene sample. The magnification was 50×.

small crystalline solid gap. Therefore, the calculated phase diagram consists of the isotropic liquid (I) and the crystal + liquid (Cr1 + I2) coexistence regions bound by the liquidus and solidus lines. To confirm the existence of the crystal + liquid coexistence region, we undertook several thermal quenches at various concentrations from the melt state at 70 °C to ambient temperature. The crystal that emerged is a rhomboidal single crystal with a diamond shape surrounded by the TA-rich solution (Figure 2a-d). Because azobenzene is a small organic molecule, its single crystal grows to a gigantic size (>8 mm) that is visible to naked eye; the final crystal size is only limited by the size of the container. At 25 wt % azobenzene concentration, a pyramidal shape single crystal with a terrace structure develops. With increasing concentration of azobenzene, the single crystals may stack because of multiple nucleation events (Figure 2b). When the single crystal approaches the container cell wall, the shape gets distorted (e.g., see 35 wt %). At 40 wt %, the whole container is filled with multiple single crystals jamming and infusing together, thereby losing the original needle shape. These observed single crystals are nonequilibrium structures that depend on supercooling depth, that is, the temperature difference between the concentration-dependent phase transition points and the quenched temperature, as indicated by a-d in the phase diagram for each concentration. As typical for a crystal + liquid coexistence region, all single crystals are surrounded by the isotropic solvent. Under the optical microscope at a magnification of 50×, the terrace structure can be discerned very clearly (Figure 3a), exhibiting the multiple terraces with an acute angle of 57°. However, the terrace structure at the core, showing the remnants of the distorted sectorized borders, appears to deplete with the progression of the crystallization (Figure 3b), which is drastically different from that of the polymer single crystals, where the crystal core remained immobile.1-3 The stationary core of polymer single crystals and spiral spherulites has already been demonstrated by our group both experimentally and theoretically using the time-dependent Ginzburg-Landau equations (model C) in conjunction with the combined PF free energy and FH free energy of liquid-liquid demixing.16-19 In polymer crystallization, the single crystal grown from the polymeric melt is considered to nucleate on the substrate, where the crystalline chains are oriented perpendicular to the substrate while folding back-and-forth.1-3 The same nucleation mechanism is perceived to be operative for solution crystallization despite the differences in the crystallization pathway from the melt versus solution. In the case of a small molecule system like azobenzene, however, the nucleation event was not clearly observable, that is, whether the nucleation occurred at the

solution-air interface or solution-substrate interface needed to be established in situ. To elucidate the mechanism of nucleation, the spatiotemporal emergence of the azobenzene single crystal was videotaped at the 35 wt % azobenzene concentration. Figure 4a shows the time-lapsed video images of azobenzene single crystal growing at two locations. (See the picture at 52 s.) The two rhomboidal single crystals approach each other, whereas their crystalline dimensions keep increasing with time. (See Figure 4a at 69 s.) When the two crystals are in close proximity, the smaller one submerges. (See the picture at 76 s.) This behavior is presumably influenced by the neighboring larger crystal. At 87 s, the submerged crystal floats back into view. The observed sink and float behavior implies that the crystal nucleation must occur at the solution-air interface. When the submerged crystal on the right approaches the cell wall, multiple small rhomboidal single crystals suddenly form and rapidly swim away from the larger crystal. The same but more pronounced behavior is observed for the crystal on the left at 93 s. Two possible mechanisms of this phenomenon may be hypothesized. First, the tip of a large crystal can break off in the collision with the wall, and the broken fragments can serve as new nucleation sites for the baby single crystals. Second is the surface-tension-driven growth instability. As the crystal approaches the container wall, the solutal convective flow may be enhanced, thereby creating spatial variation of concentration near the interface. When the spatially varying local concentration exceeds a critical value, crystal nucleation can be triggered. When the fluid flow bounced back (or directed away) from the container wall, the new-borne crystals move toward the free zone, that is, the middle, as shown in Figure 4. To the best of the our knowledge, the sink or swim behavior of azobenzene single crystals in a TA solution is the first observation of such phenomenon reported in open literature. The azobenzene chromophore is known to undergo elastic deformation due to the mechanical torque generated by the trans-cis isomerization upon irradiation with UV light.20,21 Although such isomerization can contribute to the crystal swimming, it is inconsequential in the present experiment because only a weak diffuse white light is used in probing the growth dynamics of azobenzene single crystal. It should be emphasized that liquid-liquid phase separation occurred in the azobenzene/TA solution in competition with crystallization of azobenzene during thermal quenching from the single phase temperature into the crystal + liquid coexistence region. It is this competing liquid-liquid and liquid-solid phase separation that has led to the rejection of TA solvent from the growing faceted azobenzene single crystal fronts in the lateral directions (i.e., in the plane of the surface) and (to the lesser extent) in

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Figure 4. (a) Self-motions of azobenzene crystals in 35 wt % solution, showing swimming, sinking, floating, and birth of baby single crystals and (b) the sketch on the left is a conjecture of self-motion due to the unbalanced forces of the rejected solvent creating a concentration/surface tension gradient from the lateral crystal growth fronts, propelling the rhomboidal crystal to swim on the surface; the drawing in the middle shows the stationary crystal growth as the forces and the surface tension gradients on each facet are balanced, and the sketch on the right represents the solvent rejection in the vertical direction causing the pyramid crystal to float.

the downward direction. It may be hypothesized that the rejected solvent creates spatial concentration variation in the solution in the vicinity of the growth front. It is this gradient of solute concentration that in turn leads to the spatial gradient of surface tension, responsible for the Marangoni flow.22,23 When the growth front of the single crystal deviates from the symmetric diamond shape and one sharp end of the crystal is replaced by a new facet, the forces due to the surface tension gradients and the forces due to vertical convective flows would be unbalanced. Furthermore, the rates of growth and the rates of solvent rejection for differently oriented facets may be different. The unbalanced solvent rejection might have propelled the faceted single crystal to swim in a resultant direction; otherwise, the crystal remains stationary (Figure 4b). The solvent rejection in the vertical direction can exert downward thrust, making the nearly pyramidal crystal buoyant; a similar role may be played by the surface tension at the advancing crystal fronts. When the solution crystallization is near completion, the influence of the solvent diminishes and the single crystal may no longer be able to stay afloat. Consequently, the rhomboidal single crystal sinks to the bottom because of gravity. To remove the uncertain effect of container cell geometry, we further performed the crystallization in a flat rectangular cell having 1 mm depth. The crystal growth by a “seeding” method can provide another avenue for investigating of crystal swimming. Because the seed crystal is solid, no solvent is expected to be involved. To our astonishment, we observed the rhomboidal crystals swimming, sinking, and then floating back, similar to what was observed in the case of the concave container cell (Figure 5). It may be hypothesized that when the seed crystal is in contact with the azobenzene/TA solution, the solute molecules will be pulled into the crystal front during growth; therefore the spatial variation of solvent concentration would be created near the solid-liquid interface. In principle,

any concentration fluctuation at the interface can lead to the spatial variation of the surface tension. Therefore, solvent rejection may not be the only criterion contributing to the crystal swimming, but it certainly affects the concentration gradient, which in turn can exert profound influence on the surface tension driven phenomena. As long as there is a spatial variation of concentration gradient at the solid-liquid interface, the surface tension can propel the floating crystals to move. In a regular thermal-quench experiment at 38 wt % azobenzene in TA solution without the addition of a crystal seed, we are able to observe the same crystal nucleation and the same swimming, floating, or sinking behavior in a flat-bottomed container (data not shown). Therefore, it is reasonable to conclude that the container geometry plays little or no role in the observed swim or sink phenomena. Figure 6 demonstrates the effect of UV intensity gradient on crystal growth in the 26 wt % azobenzene solution. Because the T-quench is shallow for this concentration, the emerged rhomboidal crystal grows with little or no movement, and after some elapsed time, the crystal virtually remains stationary. Then, UV light is irradiated on this stationary crystal from one end of the cell (top right of Figure 6). It can be seen clearly that the rhomboidal crystal moves away from the light source upon irradiation. According to Nakayama et al.,24 azobenzene crystal itself is capable of changing not only their molecular shape in solution but also the crystal surface topology under the UV light, that is, narrowing the layered spacing of terrace surface. In the present case, the emerged single crystal is in a solvated form, and thus the azobenzene molecules would have greater mobility and more free space to meander. Because the trans-cis conversion of the crystal front close to the light is higher than that of the opposite side, the mechanical torque generated by the trans-cis isomerization can propel the crystal to move. Another possibility is that the trans-cis isomerization increases

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Figure 5. Self-motions of rhomboidal azobenzene crystals in a flat bottom rectangular cell, following the seeding with a foreign azobenzene crystal, showing swimming, sinking, and floating behavior of two individual crystals, as indicated by the solid and open arrows.

zene single crystal can sink or float because of the downward thrust pumped by the solvent rejection in competition with the gravitational force. Furthermore, the present work unambiguously confirmed that the azobenzene crystal nucleation occurs at the solution-air interface, unlike the nucleation of the solution-grown polymer single crystals, which is perceived to take place on the substrate. Trans-cis photoisomerizationinduced mechanical deformation is an additional source of swimming of the azobenzene crystal, which otherwise may be stationary. Figure 6. Snapshot of a rhomboidal single crystal at 26 wt % azobenzene solution under the UV illumination exhibiting the crystal swimming away from the light source.

the polarity of azobenzene and thus the solubility of azobenzene crystal in triacrylate solvent might have been reduced. This change in solubility makes the system become unstable, which in turn drives phase segregation, and thus the solvent rejection rates from the growing crystal front close to the UV source may be different from that of the opposite end. This differential flow rate can further propel the crystal to swim away from the light. It should be emphasized that the present swimming behavior of the azobenzene single crystal driven by the UV irradiation is similar to the liquid crystalline elastomers grafted with azobenzene that swim away from the light.21 Conclusions We have demonstrated the spatiotemporal emergence of a gigantic single crystal of azobenzene in TA at various concentrations following temperature quenches into the solid-liquid coexistence regions of the phase diagram. We are intrigued by the fact that the emerged single crystal swims, which may be propelled by the surface tension (or concentration) gradient caused by the solvent rejection. Moreover, the gigantic azoben-

Acknowledgment. We thank the donors of the American Chemical Society Petroleum Research Fund (PRF#48735-ND7) for partial support of the present study. This research is in part supported by Collaborative Center for Polymer Photonics sponsored by Air Force Office of the Scientific Research, Wright-Patterson Air Force, and The University of Akron and also by the National Science Foundation through grant numbers DMR 0514942 and DMR-REU 0648318 to The University of Akron. We would like to express our sincere appreciation to Professor Anna C. Balazs, Department of Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh, PA for her valuable comments and suggestions. References and Notes (1) Geil, P. H. Polymer Single Crystals; Interscience: New York, 1963. (2) Schultz, J. M. Polymer Materials Science; Prentice Hall: Englewood Cliffs, NJ, 1974. (3) Bassett, C. Principles of Polymer Morphology; Cambridge University Press: Cambridge, U.K., 1981. (4) Li, L.; Chan, C.-M.; Li, J.-X.; Ng, K.-M.; Yeung, K.-L.; Weng, L.-T. Macromolecules 1999, 32, 8240–8242. (5) Magonov, S. N.; Yerina, N. A.; Ungar, G.; Reneker, D. H.; Ivanov, D. A. Macromolecules 2003, 36, 5637–5649. (6) Yu, Y.; Ikeda, T. Angew. Chem., Int. Ed. 2006, 45, 5416–5418. (7) Harris, K. D.; Cuypers, R.; Scheibe, P.; van Oosten, C. L.; Bastiaansen, C. W. M.; Lub, J.; Broer, D. J. J. Mater. Chem. 2005, 15, 5043–5048.

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(8) Jiang, H.; Kelch, S.; Lendlein, A. AdV. Mater. 2006, 18, 1471– 1475. (9) Yamane, H.; Kikuchi, H.; Kajiyama, T. Polymer 1999, 40, 4777– 4785. (10) Harrowell, P. R.; Oxtoby, D. W. J. Chem. Phys. 1987, 86, 2932– 2942. (11) Wheeler, A. A.; Boettinger, W. J.; Mcfadden, G. B. Phys. ReV. A 1992, 45, 7424–7439. (12) Kobayashi, R. Physica D 1993, 63, 410–423. (13) Flory, P. J. Principles of Polymer Chemistry; Cornell University Press: Ithaca, NY, 1953. (14) Matkar, R. A.; Kyu, T. J. Phys. Chem. B 2006, 110, 12728–12732. (15) Rathi, P.; Huang, T.-M.; Dayal, P.; Kyu, T. J. Phys. Chem. B 2008, 112, 6460–6466. (16) Kyu, T.; Chiu, H.-W.; Guenthner, A.; Okabe, Y.; Saito, H.; Inoue, T. Phys. ReV. Lett. 1999, 83, 2749–2752.

Milam et al. (17) Mehta, R.; Keawwattana, W.; Guenthner, A. L.; Kyu, T. Phys. ReV. E 2004, 69, 061802/1-061802/9. (18) Xu, H.; Matkar, R.; Kyu, T. Phys. ReV. E 2005, 72, 011804/1011804/9. (19) Xu, H.; Chiu, H.-W.; Okabe, Y.; Kyu, T. Phys. ReV. E 2006, 74, 011801/1-011801/7. (20) Yu, Y.; Nakano, M.; Ikeda, T. Nature 2003, 425, 145. (21) Camacho-Lopez, M.; Finkelmann, H.; Palffy-Muhoray, P.; Shelley, M. Nat. Mater. 2004, 3, 307–310. (22) Scriven, L. E.; Sternling, C. V. Nature 1960, 187, 186–188. (23) Levich, V. G. Physicochemical Hydrodynamics; Prentice-Hall: Englewood Cliffs, NJ, 1962. (24) Nakayama, K.; Jiang, L.; Iyoda, T.; Hashimoto, K.; Fujishima, A. Jpn. J. Appl. Phys. 1997, 36, 3898–3902.

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