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Electric-field and magnetic-field alignment of liquid-crystalline clay suspensions and clay/polymer composites

This article has been downloaded from IOPscience. Please scroll down to see the full text article. 2011 IOP Conf. Ser.: Mater. Sci. Eng. 18 062005 (http://iopscience.iop.org/1757-899X/18/6/062005) View the table of contents for this issue, or go to the journal homepage for more

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ICC3: Symposium 3: Nano-Crystals and Advanced Powder Technology IOP Conf. Series: Materials Science and Engineering 18 (2011) 062005

IOP Publishing doi:10.1088/1757-899X/18/6/062005

Electric-field and magnetic-field alignment of liquidcrystalline clay suspensions and clay/polymer composites E Paineau1, I Dozov2, K Antonova3, P Davidson2, M Impéror2, F Meneau4, I Bihannic1, C Baravian5, A M Philippe5, P Levitz6 and L J Michot1 1

Laboratoire Environnement et Minéralurgie, Nancy University, CNRS-INPL UMR 7569, BP40, 54501, Vandoeuvre, France 2 Laboratoire de Physique des Solides, Université Paris Sud, UMR8502 CNRS, 91405, Orsay, France 3 Institute of Solid State Physics, Bulgarian Academy of Sciences, Boulevard Tzarigradzko, 1784, Sofia, Bulgaria 4 Synchrotron SOLEIL, 91192, Gif-sur-Yvette, France 5 Laboratoire d’Energétique et de Mécanique Théorique et Appliquée, Nancy University UMR 7563 CNRS-INPL-UHP, 54501, Vandoeuvre, France 6 Laboratoire Physique de la Matière Condensée, Ecole Polytechnique, UMR7643 CNRS, 91128, Palaiseau, France [email protected] Abstract. Nematic and isotropic aqueous suspensions of beidellite clay sheets have been submitted to magnetic and a. c. electric fields. The nematic suspensions have positive anisotropy of magnetic susceptibility and negative anisotropy of electric susceptibility because the clay sheets orient their normals parallel to the magnetic field but perpendicular to the electric field. Moreover, the isotropic phase shows a large electric-field-induced birefringence. By dissolving acrylamide monomers in the clay suspensions and photopolymerization, clay/polymer composite gels could be elaborated. Aligned and patterned composites could be produced by application of an electric field during polymerization.

1. Introduction Clay minerals are made of stacks of aluminosilicate sheets and can therefore be regarded as lowdimensional solids [1]. When exchanged with monovalent cations, the so-called “swellable clays” spontaneously disperse in water to yield colloidal suspensions of very thin and wide mineral sheets [2]. Some of these suspensions have recently been shown to form a liquid-crystalline phase [3]. Liquid-crystalline phases are intermediate states of matter that are both fluid and anisotropic [4]. They are found between the crystalline state and the usual liquid state upon variation of either the temperature of a pure compound or the concentration of a solute in a solvent. The latter case can be illustrated by considering suspensions of very thin disks (Figure1). The liquid-crystalline nematic phase (N) is anisotropic like a crystal because all disks tend to align in the same direction, called the nematic director n, but it is fluid like a usual liquid isotropic phase (I) because it does not have a crystalline lattice. Because of its anisotropy and fluidity, a nematic phase is a birefringent liquid.

c 2011 Ceramic Society of Japan.

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Published under licence by IOP Publishing Ltd



Nematic liquid crystals are often strongly affected by the application of magnetic and electric fields; this property is at the basis of their use in liquid-crystal displays. In this article, we report on the nematic phase of beidellite clay aqueous suspensions, on its alignment by magnetic and electric fields and on electro-optic effects in the isotropic phase. Moreover, we show how such effects can be exploited to produce aligned or patterned clay/polymer composites.

n

Figure 1. Sketches of the organization in colloidal suspensions of disk-like particles. Left: isotropic phase (I); right: nematic phase (N), n is the nematic director.

2. Experimental 2.1. Preparation of beidellite clay suspensions Natural referenced beidellite SBId-1 was purchased from the Source Clays Minerals Repository of the Clay Mineral Society (Purdue University, USA). Composed of two tetrahedral sheets encompassing an octahedral one, beidellite is a dioctahedral swelling clay mineral with a charge deficit located in the tetrahedra due to isomorphic substitution of Si by Al. According to chemical analyses, the chemical formula can be written as (Si7.27Al0.73)(Al3.77Fe3+0.11Mg0.21)O20(OH)4Na0.67, nH2O. Based on the unitcell parameters, the density of beidellite can be estimated to be around 2.6 g/cm3. Before use, natural clay samples were purified in their lithium or sodium-exchanged form by three exchanges in 1 M solution of the appropriate salt during 24h, followed by several dialyses to remove excess chloride. Miscellaneous impurities (mainly sand-size quartz, feldspar and iron and titanium oxyhydroxydes) were discarded after leaving the suspension to sediment overnight in Imhoff cones. In order to reduce polydispersity, the stock suspension was centrifuged 90 min at 7000g, 17000g and 35000g. The sediment obtained at each step was rediluted in MilliQ water and referred to as size 1 to size 3, respectively. In the following, we mostly focus on size fraction 3 (S3). Batches of suspensions were prepared from the stock suspension at fixed ionic strength (IS = 10-5 M/L) by osmotic stress, using regenerated cellulose dialysis tubes (Visking, MWCO = 14000 Da, Roth) and polyethyleneglycol solutions (PEG 20000, Roth). The experiment lasted one month with renewal of the PEG solutions after two weeks. The beidellite suspensions were then recovered and their mass concentrations were determined by weight loss upon drying. Details of beidellite preparation have been published elsewhere [5]. The average diameter was measured by transmission electron microscopy (TEM) using a CM12 Philips microscope operating at 80 keV. A drop of a dilute beidellite suspension (≈ 20 mg/L) was deposited and air-dried on a carbon-coated copper grid before observation. The polydispersity, defined as the relative standard deviation, σD = ( - 2)1/2/ was determined by analysis of some 150 particles. Beidellite particles occur as large and thin disks of irregular shape with an average diameter of 209 nm and a rather large polydispersity of σD = 38%. The particle thickness, t = 0.7 nm, was previously deduced from Small-Angle X-ray Scattering (SAXS) experiments [5]. This value corresponds to the thickness of a single clay sheet, which shows the perfect exfoliation of the mineral. 2.2. Optical microscopy and small-angle x-ray scattering Microscopy observations in polarized light were performed with a Nikon BX51 microscope equipped with crossed polarizers and photographs were recorded with a Canon Camedia C3030 CCD camera. Samples were introduced into flat capillaries (VitroCom) that were flame-sealed. SAXS experiments were carried out with either an already described in-house setup [6] or at the “Swing”

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beamline of the Synchrotron Soleil [7]. In both cases, samples were held in sealed cylindrical Lindemann glass capillaries of 1mm diameter. 2.3. Application of magnetic and electric fields Samples of nematic beidellite clay suspensions were aligned by submitting them to the 8T magnetic field produced by a superconducting magnet. High frequency (100 kHz < f < 1 MHz) electric fields of amplitudes 0 < E < 200 V/mm were applied along the capillary axis by external electrodes using a signal generators and a wide-band amplifier. The optical response was either measured with a crystalline compensator or recorded with a photomultiplier tube. 2.4. Preparation of polymer/clay composites The monomer suspension was prepared from a commercial 30% solution of acrylamide and N,N'Methylenebisacrylamide (Roth). Photochemical polymerization is initiated by generation of free radicals from riboflavin (Vitamin B2, Sigma) and N,N,N',N'-tetramethylethylenediamine (TMEDA, Sigma) is used as a catalyzer. The final concentration of riboflavin and TMEDA are 5.10-4 and 1.5.10-3 (%w/v), respectively. The monomer suspension was mixed with aliquots (1mL) of concentrated clay suspensions (φ ~ 0.85%) in a ratio of 1:2.5 for the monomer solution. 3. Results and discussion 3.1. Nematic order of beidellite clay suspensions Figure 2 shows a series of test-tubes, filled to the top (white marks) with beidellite clay suspensions of volume fractions φ increasing from left to right, observed between crossed polarizers. All samples are biphasic as they display a top dark, and therefore isotropic, liquid phase that coexists with a bottom bright, and therefore birefringent, nematic phase. The proportion of nematic phase increases with the overall clay volume fraction in the suspension. These observations prove both the existence of a first order I/N phase transition and that the suspensions can reach thermodynamic equilibrium. Actually the suspensions shown in Figure 2 all belong to the I/N biphasic region that is found at volume fractions ranging from ~ 0.4% to ~ 0.6%. At larger volume fractions, the suspensions are completely nematic. At even larger volume fractions, a sol/gel transition occurs and the suspensions are not fluid any more. Therefore, the phase diagram, in volume fraction and ionic strength coordinates, (Figure 3) displays a small region of fluid nematic phase (NL). In this region, the suspensions are strongly affected by external fields; in this respect, these materials are typical examples of “soft” condensed matter.

Figure 2. Aqueous suspensions of size 3 beidellite (IS = 10-4 M) observed between crossed polarisers. (a) φ = 0.40%; (b) φ = 0.42%; (c) φ = 0.44%; (d) φ = 0.46%; (e) φ = 0.48%; (f) φ = 0.50%.

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Figure 3. Phase diagram of sizeselected beidellite suspension (size 3); IL = Isotropic liquid, B = Biphasic liquid, NL = Nematic Liquid, G = Gel.



3.2. Influence of a magnetic field on the nematic phase of beidellite clay suspensions When observed in polarized light microscopy, a nematic sample submitted to a magnetic field appears dark when its director n is parallel either to the polarizer or analyzer axis and very bright when n lies at 45° from these axes. This actually proves the complete alignment of the nematic phase in the field.

1mm mm Figure 4. (a,b) Optical textures and (c) SAXS pattern of a fluid nematic phase of beidellite suspension (size 3, φ = 0.62%, 10-4 M, 1mm cylindrical glass capillary) in an 8 T magnetic field. The small perpendicular white arrows in (a,b) show the polarizer and analyzer axes; the large white arrows represent the direction of the magnetic field and therefore the nematic director. The SAXS pattern (Figure 4c) of such an aligned nematic sample of clay suspension displays a series of equidistant diffuse spots. Such diffuse (i.e. non-resolution limited) scattering features are typical of a liquid phase with only short-range positional order but the scattering pattern is not isotropic. Therefore, the phase is not a usual liquid phase but is actually a nematic phase with longrange orientational order. Moreover, the diffuse spots are due to a local lamellar order arising from electrostatic repulsions between the charged beidellite disks. The orientation of the pattern with respect to the field direction shows that the platelets orient their normal parallel to the magnetic field. In other words, the anisotropy of magnetic susceptibility of the nematic phase is positive, ∆χm = χ// -χ⊥ > 0, where χ// (resp. χ⊥) represents the susceptibility along (resp. perpendicular to) the nematic director. The positive sign of ∆χm is due to the very large aspect ratio of the clay sheets. The production of a single domain of the nematic phase allows for the determination of its birefringence by using an optical compensator. For the sample shown in Figure 4, ∆n= ne – no = -(4.1± 0.3)×10-4. The birefringence was always found to be negative. 3.3. Influence of an electric field on the nematic and isotropic phases of beidellite clay suspensions The nematic phase of beidellite suspensions is also readily aligned by an electric field (Figure 5). Although formally very similar to Figure 4, the orientation of the row of diffuse spots with respect to the electric field shows here that the clay platelets align their normal perpendicular to the electric field. This means that the anisotropy of electric susceptibility of the nematic phase is actually negative, ∆ε = ε// -ε⊥ < 0. Upon application of the electric field, many aligned nematic domains form with their directors perpendicular to the field. The alignment is therefore degenerated. However, sometimes, probably due to planar anchoring onto the capillary flat walls, one of the domains eventually grows at the expense of the others and the sample may completely align with the nematic director oriented both

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perpendicular to the field and parallel to the glass plates. Other suspensions of mineral nanosheets, such as Sb3P2O143- [8], behave in exactly the same way when submitted to an a.c. electric field.

(c)

500 µm µm µm

E

Figure 5. Optical textures (a,b) and SAXS pattern (c) of fluid nematic samples of beidellite suspension in a 1mm cylindrical glass capillary aligned in a 4×104 V/m, f = 500 kHz field. (a,b) size 2, IS = 10-5 M, φ = 0.61%; (c) size 3, φ = 0.61% ,10-4 M. Unexpectedly, the electric field also has a very strong influence on the isotropic phase of beidellite clay suspensions (Figure 6). The isotropic phase, that is completely dark between crossed polarizers in zero-field, becomes quite birefringent (and therefore anisotropic) when submitted to a rather weak field of 100 V/mm. Moreover, the saturation of the induced birefringence versus field intensity (Figure 6, right hand side) around 200 V/mm implies that all the clay platelets are completely aligned with their normals perpendicular to the field intensity. This demonstrates that clay platelet alignment can be controlled with a weak external electric field.

d.∆n (nm)

30

E= 0 20

10

E

0 0

100

200

ERMS (V/mm)

E= 100 V/mm

Figure 6. Effect of an electric field on the isotropic phase of a beidellite clay suspension (size 3, f = 500 kHz). Left: optical textures of the phase in the absence (top) or presence (bottom) of an electric field. Right: induced birefringence versus field intensity.

3.4. Production of patterned clay/polymer composites The clay platelet alignment produced by application of an electric field on an isotropic suspension can be frozen in a composite clay/polymer material. Indeed, mixtures of monomer, crosslinker, initiator, catalyzer, and beidellite suspensions (see subsection 2.4) can be prepared and show a similar phase diagram as that of Figure 3. Therefore, it is possible to produce clay/polymer composite materials with liquid-crystalline nematic organization. Moreover, when an isotropic clay/monomer suspension is submitted to an electric field, it becomes anisotropic as in Figure 6. It can then be photopolymerized and the field-induced alignment of the clay platelets is frozen and persists for months (Figure 7, left).

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In a slightly more complicated experiment, the electric field was alternatively switched on and off while the photopolymerization took place under the microscope. The microscope light produces free radicals that diffuse within the sample along the capillary axis, away from the center in two directions. This creates two polymerization fronts that propagate through the sample. The regions crossed by the front when the field is switched on are aligned and appear bright between crossed polarizers whereas those crossed by the front in absence of the field remain isotropic and appear dark. It is thus possible to produce a striped texture whose period is simply controlled by the “off” and “on” alternation sequence (Figure 7, right). Patterned clay/polymer composites with periods ranging from about 50 µm to 1 mm could thus easily be obtained.

Figure 7. Polarized light microscopy images of (i) left: an isotropic clay/monomer suspension polymerized in a 20 V/mm electric field (ii) right: the same suspension polymerized during a sequence during which the field was alternatively switched on and off. The polarizer and analyzer axes are parallel to the sides of the figures. The flat glass capillaries are 2 mm wide. 4. Conclusion Beidellite clay aqueous suspensions form isotropic and nematic phases which are readily aligned by a magnetic or an electric field. Such behavior provides a cheap and simple way to control the orientation of the clay platelets. Moreover, adding a water-soluble monomer to these colloidal suspensions makes it possible to freeze the field-induced alignment and other more subtle electro-optic effects in polymer/clay composite chemical gels. This procedure, illustrated here in the case of beidellite, can in principle be generalized to colloidal suspensions of other low-dimensional solids. References [1] Ross C S and Hendricks S B 1945 Minerals of the Montmorillonite Group US Geological Survey Professional Paper 205B pp 23-79 [2] Güven N 1988 Hydrous Phyllosilicates (Exclusive of Mica) vol. 19 ed Bailey S W (Washington DC: mieralogical Society of America) chapter 13 pp 497-559 [3] Michot L J, Bihannic I, Maddi S, Funari S, Baravian C, Levitz P and Davidson P 2006 Proc. Nat. Acad. Sci. USA 44 16101 [4] De Gennes P G 1979 The Physics of Liquid Crystals (Oxford: Clarendon Press) [5] Paineau E, Antonova K, Baravian C, Bihannic I, Davidson P, Dozov I, Impéror-Clerc M, Levitz P, Madsen A, Meneau F and Michot L J 2009 J. Phys. Chem B 113 15858 [6] Impéror-Clerc M and Davidson P 1999 Eur. Phys. J. B 9 93 [7] http://www.synchrotron-soleil.fr/portal/page/portal/Recherche/LignesLumiere/SWING [8] Gabriel J C P, Camerel F, Lemaire B J, Desvaux H, Davidson P and Batail P 2001 Nature 413 504

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