One Step Synthesis of Shape and Optically Anisotropic Particles




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НазваниеOne Step Synthesis of Shape and Optically Anisotropic Particles
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DOI: 10.1002/adma.((please add manuscript number))


One Step Synthesis of Shape and Optically Anisotropic Particles


By Laura Ramon Gimenez, Jonathan Henry Wilson, Andriy Nych, Igor Musevic and Owain Parri


Dr. O. Parri, Dr. L. Ramon-Gimenez, Dr. J. H. Wilson
Chilworth Technical Centre, University Parkway, Southampton,
SO16 7QD, (UK)


Dr. A. B. Nych, Dr. I. Musevic
J. Stefan Institute, Jamova cesta 39, Ljubljana,
SI-1000 (Slovenia)


Keywords: anisotropic particles, liquid crystals, oil-in-oil emulsion, free radical polymerization.


Optically and shape anisotropic particles have recently attracted significant interest. Particles of this type can be used in a wide range of applications such as scattering polarizers,[1] in microlasing and microresonators[2, 3] or as actuators.[4] Additionally, the ability to manipulate these particles in an electric field makes them potentially useful in applications such as optical switches[5] and their controllable shape anisotropy also makes them attractive for use in electrophoretic displays.[6]

Optically anisotropic spherical particles have been synthesized previously using UV-initiated emulsion polymerization,[5] thermal dispersion polymerization[7] and microfluidics.[8] Effort has been invested on developing techniques to produce shape anisotropic particles from non-liquid crystalline materials. Among these techniques are, for example, hard-templates, microfluidics, particle stretching, and lithographic methods.[9] Using microfluidics, it has been possible to synthesize elastomeric liquid crystalline particles on the scale of hundreds of micrometers, which change shape under temperature cycling.[10]

Here we present for the first time particles with both optical anisotropy and controllable shape anisotropy synthesized by a one-step method. A key advantage of this procedure over those previously described is the possibility of producing these materials on an industrial scale. Using this method, the shape and director distribution is locked in by the process of thermal polymerization of emulsified reactive mesogen (polymerizable liquid crystal) microdroplets to create the particles.


In this work, we present the synthesis and characterization of reactive mesogen (RM) particles exhibiting different anisotropic shapes. The synthesis described here allows control of the particle shape. Through selection of the right reaction parameters it is possible to go from spherical particles to tactoidal particles or to particles which resemble red blood cells. In all cases a defined director configuration is present. Polarized optical microscopy (POM) and scanning electron microscopy (SEM) were used to examine their optical properties and morphology. We were able to manipulate these particles under electric fields, rotating them in a controllable and reversible way; hence, these materials have potential applications especially in the display field.


To achieve the synthesis of these unusual particles an oil-in-oil emulsion polymerization[11] was developed. As the continuous phase, a solvent with the following properties is required: low dielectric constant to allow the manipulation of particles under electric fields; low viscosity for faster response and high boiling point for performance retention. Dodecane was identified as the ideal solvent considering these requirements. Dodecane is also immiscible with most reactive mesogens, facilitating emulsion polymerization. As the internal phase the reactive mesogen RM257 (Merck KGaA) was selected due to its phase behavior, which allows the polymerization temperature in a range appropriate for common thermal initiators. The surfactant used for this polymerization was Kraton 1701, which is an A-B diblock co-polymer, where A is polystyrene and B is a polyolefin. The polystyrene block is compatible with the RMs, while the polyolefin block is soluble in the continuous phase providing steric stabilization. When liquid crystals (including RMs) are used as the internal phase, surfactants also determine the anchoring of the director to the particle or droplet surface. Typically, PVA provides planar anchoring, resulting in a bipolar configuration, while SDS imposes homeotropic anchoring, yielding a radial director configuration. In the present case, we observed that Kraton diblock co-polymer provides planar surface anchoring of LC molecules resulting in bipolar droplets as seen in Figure 1a,b. A sample of the emulsion was taken and observed under the optical microscope prior to thermal polymerization, where typical bipolar texture can be observed in the droplets with two splay director defects at opposite poles.


The emulsion was formed by applying mechanical shear to the two phases until a stable emulsion was obtained. To initiate polymerization, two different thermal initiators were used: V-59 (Wako chemicals) and Luperox P (Sigma-Aldrich).

When the reaction is performed at 140 °C, where the RM is in its isotropic state, spherical particles are obtained independently of the thermal initiator used (Figure 1d). When the polymerization is carried out at 85 °C, where the RM is in its nematic phase, particles exhibit anisotropic shape. All particles from the same batch show the same shape, and shape changes depending on the nature of the initiator used. Thus, when reactive mesogens are used as monomers in colloidal synthesis, the initiator and temperature choice is crucial to control the particle shape. Using Luperox P, particles can exhibit tactoidal shape (Figure 1c); however when V-59 is used, particles which resemble red blood cells can be obtained (Figure 1e). Studies under a polarized optical microscope revealed that both systems possess, besides an anisotropic geometry, a well defined director configuration (see Figure 2b,c and Figure 3d).


Tactoidal particles of isotropic materials have already been made by stretching PVA films containing spherical polymeric particles, achieving high aspect ratio.[12] Fernandez-Nieves stretched PVA films, which contained reactive mesogen droplets, obtaining tactoidal, optically anisotropic particles.[13] However, these methods involve several steps including preparation of the film, stretching to the desired aspect ratio and redissolving the film. This is an expensive, time consuming and low-yield method of preparing tactoidal shaped particles.


The one pot method described in the present paper allows the synthesis of tactoidal particles exhibiting a very well-defined shape, as shown by the SEM images in Figure 2a. The average size of the tactoids is about 7 µm, however, particle size ranges from 2 to 15 µm (long axis of the particle). In this particular example, size distribution is moderate resulting in a well particle packing. Typical aspect ratio is 1:1.4 but we were able to obtain up to 1:1.9 under certain reaction conditions.


Analysis under polarized optical microscopy (POM) revealed that all tactoids retained the bipolar configuration (see Figure 2b) observed initially in the emulsified droplets prior to polymerization (see Figure 1). Particle shape can also be studied in solution under parallel polarizers, (Figure 2b). Rotation of the particles under crossed-polarizes shows changes in birefringence which confirms the bipolar configuration (Figure 2c right).[14]


A schematic director distribution inside a tactoid is shown in Figure 2c left. The two splay director defects, called boojums, are situated at the extremes of the long axis of the particle. Tactoid-shaped particles, showing same director distribution, have already been observed by Fernandez et al.[8] and in lyotropic liquid crystal dispersions.[15, 16] This phenomenon has attracted considerable interest and theoretical models describing shape, formation conditions, and behavior of tactoids have been elaborated.[17]

Toroidal particles from isotropic polymers have been already synthesized by dispersion polymerization,[18] microfluidics[19] and electrospray.[20] By tuning the synthetic conditions it was possible to control particle shape from spheres to red blood cell-like particles and to tores. However, none of these particles have been synthesized using RMs as monomers so far.


In Figure 3a, a SEM image of red blood cell-like particles made of RM257 is shown. The materials present well-defined shape. The contrast difference in the SEM signal between the middle and shoulder of the particle may suggest the presence of a hole. However, when the particles are studied under crossed-polarizers, significant birefringence is observed in the central part. Thus, particles are only slightly depressed possess a thin membrane in the middle part (see Figure 3d), mimicking the shape of red blood cells. Observation of the particles from the top under crossed-polarizers revealed a maltese cross texture (four brush defect), which does not change upon rotation (Figure 3c). This texture is typical for radial or concentric director configurations. However, when the particle is analyzed from the side, maximum retardation was found at 45° with respect to the crossed-polarizers, which extinguishes when rotating to 0° or 90° (see Figure 3c). These observations indicate that first, the director has to be parallel or perpendicular to the side axis of the particle. If we consider the particle as a deformed sphere, the global director distribution could be then concentric or bipolar. To distinguish between them, a lambda wave plate of λ = 530 nm with slow optical axis at 45° with respect to polarizer/analyzer was introduced. Figure 3e and 3f show the color shift when the lambda plate is introduced. When the director is parallel to the optical axis of the lambda plate, birefringence is added and the color is shifted to the blue yellow region comparing to the colors on the Michel-Levy chart.[14]

If the director orientation is perpendicular to the optical axis of the lambda plate, birefringence is subtracted and the color is shifted to the yellow blue of the second order of interference. The distribution of colors in the particles in Figure 3e, reveals a radial configuration when the particles are seen from the top (see scheme in Figure 3b right). The particle marked with a circle in Figure 3e is seen from the side. It exhibits a blue color shift, meaning that the director is aligned parallel perpendicular to the optical axis of the lambda plate and, therefore, parallel perpendicular to the long axis of the side particle (see scheme in Figure 3b left). This director configuration can be confirmed when the sample is rotated by 90° (Figure 3f), because particles viewed from the top do not change color, while particles viewed from the side change color from blue to yellow.

From the previous observations it can be concluded that the director profile inside the red blood cell-like particles remains bipolar after polymerization, as it was in the original droplets. When the spherical droplet is deformed during polymerization, the director distribution deforms in order to adapt to the new particle shape, while trying to retain the original bipolar configuration, dictated by parallel surface alignment of LC molecules.

The mechanism of particle formation is governed by many factors including temperature, surface tension, elastic constants, activation energy, solubility of the initiator, etc.

Elasticity of polymerized liquid crystals, namely splay and bend elastic constants, are known to be higher than those for low molecular weight liquid crystals.[21, 22] In our case the final crosslinked network makes the polymerized material stiffer and harder to deform compared to the non-polymerized state. One may, therefore, expect increased values for its splay and bend elastic constants. One of the critical factors responsible for the change of shape from spherical shaped droplets to anisotropically shaped particles tactoids during polymerization could be a mechanism to relax the elastic deformations. Luperox P has a 10 hour half life at 103 °C and it is fully soluble in both continuous and internal phase. Polymerization occurs slowly and allows the droplet to take its optimum form, the tactoidal shape.


Thus in case of tactoidal particles, the shape of a single particle is most likely defined mostly by change the increase of elastic properties constants of the liquid crystal during polymerization. In contrast to this, formation of blood-cell shaped particles cannot be driven by the elasticity of the liquid crystal because bringing the two boojum defects inside a particle closer to each other results in an elastic energy increase. Some external force or/and torque (with respect to the liquid crystal) is required to explain the red blood-cell shape. This might be explained by a different polymerization mechanism when V-59 is used as initiator. Formation of torus or red blood-cell particles made of non-liquid crystalline polymers has been attributed to a crust formation due to a non-uniform solidification of the polymer, since the process of particle formation involves solvent evaporation.[18, 20]

V-59 is only partially soluble in dodecane and possesses a 10 hours half life activation temperature of 67 °C. Initiation in the monomer droplet is through the V-59 entering, mostly from solid material dissolving on the surface of that droplet. This, coupled with the high initial radical concentration means that initiation occurs on the surface of the monomer droplet creating a crust surrounding the molten RM monomer. Around the poles where boojum defects are allocated, polymerized crust may already have some mechanical stress originating from non-uniform director configuration which makes the sphere more likely to collapse from these weaker points, resulting in particles with red blood-cell shapes.

Due to the partial solubility of both initiators in the internal and continuous phase a more complex polymerization mechanism is occurring and this is hypothesized to be a mixture between an emulsion and a suspension polymerization. Thus the true mechanism of formation of shape anisotropic particles is still unknown to us and will require a deeper investigation to elucidate the full polymerization mechanism.

We believe that this method, by tuning reaction conditions, will allow greater control of the particle shape to obtain tactoids with higher aspect ratios and tune the membrane of the red blood like cell-particles to produce liquid crystalline torus.


Initial experiments carried out on the manipulation of the red blood cell-like particles revealed that they can be rotated by electric fields. The particle dispersion is filled into the cell by capillary action at room temperature, used as obtained from the synthesis without any further purification. Experimental cells were made using flat glass plates coated with transparent indium/tin oxide (ITO) electrodes and 25 µm spacers to control the cell gap The cell was attached to a function generator and a square wave voltage was used for the experiment. The voltage is applied across the cell as shown in Figure 4d, perpendicular to the microscope’s optical axis.


Straight after filling the cell, particles are lying parallel to the ITO plates (Figure 4a,c). When the voltage is applied, the particles quickly rotate from horizontal orientation to vertical perpendicular to the ITO plates (Figure 4b,d) due to electric field torque[23]. When the voltage is removed, gravity or surface interactions pulls the particles back to their initial horizontal position. However, it is not clear what drives the off-state. This process is reversible and can be followed in the video given in the Supporting Information. The minimum voltage needed to rotate the particles can be as low as 0.5 V µm-1 using a frequency of 100 Hz. For lower frequencies, continuous flipping of the particles was observed, which follows the frequency of the applied voltage. We are currently working on understanding the switching and relaxation mechanism of the particles. In general, particles act like a whole entity, showing a similar switching behavior to low molecular weight liquid crystals; however no alignment layer is needed to achieve on and off states.


We have presented in this work the first optical and shape anisotropic particles synthesized in a one-step reaction by an oil-in-oil emulsion polymerization. Particles show a well-defined director configuration and their shape can be controlled by varying the reaction conditions. The use of dodecane as a continuous phase has the advantage of compatibility with most electro-optic applications. Red blood cell-like particles are able to switch when a voltage is applied, and relax back when the voltage is removed. We believe that with this synthetic method it will be possible to generate a variety of controllable shaped particles with the potential to exhibit macroscopic liquid crystalline behavior. The combination of shape and optical anisotropy, additionally, to the results obtained so far, make these materials very promising candidates for a wide range of applications, especially in the display technology field.


Experimental


Materials: RM257 was received from Merck KGaA, V-59 was purchased from Wako chemicals, Kraton G1701 EU was received from Kraton and Luperox P and dodecane were purchased from Sigma-Aldrich. Materials were used without any further purification.


Particle Synthesis: RM257 (5.0 g, 8.49 mmol) is placed in a beaker and heated until liquid. In another beaker dodecane (50.0 g) and Kraton G 1701 EU (500 mg) are combined and stirred at 150 °C until homogeneous. The solutions are combined and homogenized at 150 °C with a Silverson homogenizer for 8 minutes at 2000 rpm. The stable emulsion is transferred into a 3-necked 100 mL round bottomed flask which is pre-heated to 85 °C and flushed with nitrogen. The flask is equipped with a reflux condenser and an overhead stirrer with an impeller blade set at 400 rpm. Once the temperature of the emulsion is stabilized, V-59 (thermal initiator) (200 mg, 1.04 mmol) or Luperox P (200 mg, 1.03 mmol) is added to the reaction. Stirring and heating of the reaction is maintained for 2 hours to complete polymerization. The reaction mixture is cooled to room temperature and the emulsion is filtered through a 50 micrometer pore size nylon cloth.


Characterization: SEM characterization was carried out on a Leo 1455VP SEM equipped with a 30 kV LaB6 filament with an Everhart-Thornley secondary electron detector and a Cambridge four quadrant backscatter detector.


Acknowledgements

Support from Marie Curie, under the Hierarchy Program (FP7) is gratefully acknowledged. L. Ramon wants to thank specially Andriy Nych for fruitful discussions and Igor Musevic (J. Stefan Institute in Ljubljana) for letting the authors use the microscope. I. Musevic would like to thank Primoz Ziherl for valuable discussion on the shape changes in lipid bilayer vesicles. Supporting Information is available online from Wiley InterScience or from the author.


Received: ((will be filled in by the editorial staff))
Revised: ((will be filled in by the editorial staff))
Published online: ((will be filled in by the editorial staff))


[1] I. Amimori, J. N. Eakin, G. P. Crawford, N. V. Priezjev, R. A. Pelcovits, in Digest of Technical Papers - Society for Information Display International Symposium Vol. 33, 2002, pp. 834.

[2] M. Humar, I. Musevic, Optics Express 2010, 18, 26995.

[3] G. Cipparrone, A. Mazzulla, A. Pane, R. J. Hernandez, R. Bartolino, Advanced Materials 2011, 23, 5773.

[4] C. Ohm, N. Kapernaum, D. Nonnenmacher, F. Giesselmann, C. Serra, R. Zentel, Journal of the American Chemical Society 2011, 133, 5305.

[5] D. R. Cairns, M. Sibulkin, G. P. Crawford, Applied Physics Letters 2001, 78, 2643.

[6] M. Goulding, A. Smith, L. Farrand, N. Greinert, M. James, H. Wilson, C. Topping, R. Kemp, E. Markham, J. Canisius, Nippon Gazo Gakkaishi 2011, 50, 135.

[7] M. Vennes, S. Martin, T. Gisler, R. Zentel, Macromolecules 2006, 39, 8326.

[8] A. Fernández-Nieves, G. Cristobal, V. Garcés-Chávez, G. C. Spalding, K. Dholakia, D. A. Weitz, Advanced Materials 2005, 17, 680.

[9] T. J. Merkel, K. P. Herlihy, J. Nunes, R. M. Orgel, J. P. Rolland, J. M. DeSimone, Langmuir 2010, 26, 13086.

[10] C. Ohm, E.-K. Fleischmann, I. Kraus, C. Serra, R. Zentel, Advanced Functional Materials 2010, 20, 4314.

[11] M. Klapper, S. Nenov, R. Haschick, K. Mueller, K. Muellen, Accounts of Chemical Research 2008, 41, 1190.

[12] C. C. Ho, A. Keller, J. A. Odell, R. H. Ottewill, Colloid and Polymer Science 1993, 271, 469.

[13] A. Fernandez-Nieves, Soft Matter 2006, 2, 105.

[14] P. Drzaic, Liquid Crystal Dispersions Vol. 1, World Scientific, 1995.

[15] A. Kaznacheev, M. Bogdanov, S. Taraskin, Journal of Experimental and Theoretical Physics 2002, 95, 57.

[16] Y. A. Nastishin, H. Liu, T. Schneider, V. Nazarenko, R. Vasyuta, S. V. Shiyanovskii, O. D. Lavrentovich, Physical Review E 2005, 72, 041711.

[17] P. Prinsen, P. van der Schoot, Physical Review E 2003, 68, 021701.

[18] L. Alexander, K. Dhaliwal, J. Simpson, M. Bradley, Chemical Communications 2008, 3507.

[19] B. Wang, H. C. Shum, D. A. Weitz, ChemPhysChem 2009, 10, 641.

[20] C. H. Park, N.-o. Chung, J. Lee, Journal of Colloid and Interface Science 2011, 361, 423.

[21] X.-J. Wang, Q.-F. Zhou, Liquid Crystalline Polymers World Scientific, 2004.

[22] D. Demus, J. Goodby, G. W. Gray, H. W. Spiess, V. Vill, in Handbook of Liquid Crystals Set, Wiley-VCH Verlag GmbH, 2008.

[23] L. D. Landau, E. M. Lifshitz, Electrodynamics of continuous media, Pergamon Press, 1984.




Figure1. Bipolar configuration observed in RM257 droplets in dodecane before thermal polymerization under parallel polarizers (a) and same image under crossed-polarizers (b). SEM images of polymerized particles at 85 °C using Luperox P as initiator (c), particles polymerized particles at 140 °C (d) and particles polymerized particles at 85 °C using V-59 as initiator (e). The two diagrams represent particle shape evolution during polymerization.


20 µm



45°

P

A

b)

a)

c)


Figure2. SEM images of tactoidal particles obtained in a one-step reaction when Luperox P is used as a thermal initiator (a). Tactoidal particles observed under parallel polarizers (b). Scheme of director distribution in a tactoidal particle, dot lines represent mesogens (c, left). Particle rotation under parallel (c right above) and crossed-polarizers (c right below). Red arrows show the long axis of the tactoidsdirector orientation. Scale bars are 20 µm.




Figure3. SEM image of red blood cell-like particles synthesized using V-59 as thermal initiator (a). Scheme of director configuration from the side view (b left) and front view (b right). Optical microscope images under parallel (c above) and crossed-polarizers(c below) of blood cell-like particles at 0 (3 left) and 45 degrees respect to polarizer/analyzer. Crossed-polarizers image of a particle seeing from the top (d). Particles under crossed-polarizers with a lambda wave plate inserted at 45 degrees angle (e) and same image after 90 degree rotation (f). Optical axis of the λ-plate is drawn in the corner of the image.





E


25 µm


E=0


b)

a)

c)

d)


Figure4. Crossed-polarizers images of red blood cell-like particles in ITO cells before applying an electric field (a), after applying 60 V (b). Scheme of the ITO cell and particles position before (c) and after (d) applying a voltage. Scale bars are 20 µm.


The table of contents entry should be fifty to sixty words long, written in the present tense, and refer to the chosen figure.


Optically and shape anisotropic particles can be synthesized in one step reaction using an oil-in-oil emulsion polymerization. Shape anisotropy is obtained after polymerization of bipolar spheres and can be controlled by the nature of the thermal initiator. Internal liquid crystalline alignment is present in all particles and they respond under electric fields.


Keyword: anisotropic particles, liquid crystals, oil-in-oil emulsion, free radical polymerization.


L. Ramon-Gimenez, J. H. Wilson, A. Nych, I. Musevic, O Parri *


One Step Shape and Optically Anisotropic Particles





ToC figure ((55 mm broad, 50 mm high, or 110 mm broad, 20 mm high))


Supporting Information should be included here (for submission only; for publication, please provide Supporting Information as a separate PDF file).





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