Letter pubs.acs.org/Langmuir
Electric-Field Alignment of Chitin Nanorod−Siloxane Oligomer Reactive Suspensions Maria Yu. Boltoeva,† Ivan Dozov,‡ Patrick Davidson,‡ Krassa Antonova,§ Laura Cardoso,† Bruno Alonso,*,† and Emmanuel Belamie*,†,∥ †
Institut Charles Gerhardt Montpellier, UMR 5253 CNRS/UM2/ENSCM/UM1, ENSCM, 8 rue de l’Ecole Normale, 34296 Montpellier Cedex 5, France ‡ Laboratoire de Physique des Solides, Université Paris-Sud, UMR 8502 CNRS, Bât 510, 91405 Orsay Cedex, France § Institute of Solid State Physics, Bulgarian Academy of Sciences, Tzarigradsko Chaussee 72, 1784 Sofia, Bulgaria ∥ Ecole Pratique des Hautes Etudes, 46 rue de Lille, 75005 Paris, France S Supporting Information *
ABSTRACT: Uniaxially anisotropic chitin−silica nanocomposite solids have been obtained thanks to the electric field-induced macroscopic alignment of liquid-crystalline reactive cosuspensions. We demonstrate how chitin nanorods (260 nm long, 23 nm thick) can be aligned upon the application of an alternating current (ac) electric field, and within water− ethanol suspensions containing reactive siloxane oligomers (Dh ∼ 3 nm). The alignment at the millimeter length scale is monitored by in situ small-angle X-ray scattering (SAXS) and polarized light optical microscopy. The composition and state (isotropic, chiral nematic) of the cosuspensions are proven to be determining factors. For nematic phases, the alignment is preserved when the electric field is switched off. Further solvent evaporation induces sol−gel transition, and uniaxially anisotropic chitin−silica nanocomposites are formed after complete drying of the aligned nematic suspensions. Here, the collective response of colloidal mesophases to external electric fields and the subsequent formation of ordered nanocomposite solids would represent a new opportunity for materials design.
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INTRODUCTION Chitin rodlike nanoparticles extracted from crustacean shells have been shown to form stable suspensions in aqueous media.1,2 Repulsive interactions arising from an excess of positive charges on the particle surface, due to deacetylation of external acetamido groups, prevent flocculation in slightly acidic solutions (pH < 5.5). When brought above a critical concentration, the suspensions undergo a phase transition toward a chiral nematic phase. The chitin volume fraction at which this isotropic/chiral nematic transition occurs is affected by several physicochemical parameters, notably the pH, ionic strength,1,3 and by addition of noninteracting polymers.4 The chirality of these colloidal suspensions is reminiscent of the helical three-dimensional (3D) organization of arthropods carapaces,5 in which chitin is the main fibrous component. In crustaceans, the organic chitin matrix is often reinforced by nanoscopic calcite and amorphous calcium carbonate. Similarly, the bone tissue of vertebrates is comprised of hydroxyapatite nanoplatelets associated to collagen fibrils6,7 organized into cylindrical units of twisted plywood. Mineralization and anisotropic 3D architecture both enhance the mechanical properties of these natural nanocomposites.7−9 Owing not only to their remarkable structural features, but also to subtle interface details and to the resulting mechanical and optical properties, such biological nanocomposites are a source of inspiration for materials chemists.10,11 © XXXX American Chemical Society
Recently, we proposed a novel synthesis route for the elaboration of structured silica-based hybrid and mesoporous materials.12,13 It is based on the formation of hybrid suspensions of chitin nanoparticles and siloxane oligomers, which are chemically stable in ethanol and yet still reactive. Such reactive colloidal suspensions are amenable to a variety of sol−gel processes, including spin-coating or spray-drying. New materials could be obtained, whose organizations were tuned by applying external forces, like for instance shearing or magnetic field, before the sol−gel transition is reached. Interestingly, the organization is preserved after calcination of the organic moiety in the resulting mesoporous silica materials.12,13 When placed in a magnetic field, anisotropic chitin suspensions align with the chiral axis along the direction of the field,1 producing a degenerate alignment where the mesophase retains its helicity. Although periodic chiral organizations mimicking those of some remarkable natural composites are of great interest,8,14,15 there is another challenge in producing solids and porous materials with long-range uniaxially aligned structures and the corresponding anisotropic mechanical, optical and mass transfer properties (membranes, catalysts, sensors, etc.). Here, we report on the uniaxial alignment of chitin and chitin-siloxane Received: April 18, 2013 Revised: June 18, 2013
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mixtures along the direction of an alternating electric field, using in situ small-angle X-ray scattering (SAXS) and polarized light microscopy. Then we assess, using SAXS, optical, and electron microscopies, the transcription of the colloidal suspensions’ alignment into ordered nanocomposites after extensive siloxane condensation and drying.
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significant observations were already made. Beyond a critical chitin concentration (ca. 1.5 wt %), the hybrid suspensions are partly anisotropic. From this concentration up to 2.5 wt %, the samples are biphasic with the coexistence of isotropic and anisotropic phases. This behavior is very similar to that previously observed for suspensions of chitin nanorods in aqueous HCl (10−4 M) showing either isotropic (I) or chiral nematic (N*) phase, or I/N* coexistence.1,2 Therefore, the ability of the chitin nanorods to form a chiral nematic phase seems to be preserved in the hybrid systems. When observed in polarized light between crossed polarizers, these anisotropic suspensions showed strong optical birefringence with typical nematic grainy textures (Figure 1a,b) and
EXPERIMENTAL METHODS
Sample Preparation. Chitin−siloxane hybrid suspensions were prepared by mixing chitin nanorods and siloxane oligomers suspensions in ethanol/aqueous 10−4 M HCl 50/50 wt.%. Chitin nanorods were prepared as described in ref 1 from raw chitin flakes (France Chitin) by hydrolysis in boiling 4 N HCl for 90 min. Stable dispersions of elongated nanoparticles (260 nm long and 23 nm thick) were obtained after ultrasound treatment and exchanged in mixtures of ethanol and aqueous HCl (10−4 M). An alcoholic solution containing siloxane oligomers is obtained by mixing and refluxing (during 4 h) 0.1 mols of TEOS (ABCR, pur. > 99.9%), an acidic aqueous solution (HCl 0.1 M), and absolute ethanol (Sigma-Aldrich, pur. > 99.8%) with molar proportions TEOS 1:H2O 2:EtOH 2. The resulting siloxane oligomers are colloids with an average hydrodynamic diameter determined by dynamic light scattering of Dh = 2.9 ± 0.2 nm, with an average degree of condensation of c.a. 0.75. From the hybrid suspensions, solid chitin-silica samples can be obtained by solvent evaporation and siloxane condensation. In this case, the internal αchitin structure is preserved as verified for bulk samples (X-ray diffraction (XRD) and 13C solid-state NMR). Electric Field Setup. The study of the alignment of chitin suspensions under an alternating current (ac) electric field was performed by using an experimental setup already described.16 The samples were filled and sealed in cylindrical glass capillaries. The electric field, parallel to the capillary long axis, was applied using a pair of external electrodes, flat rings made up of aluminum foil and placed directly on the outer side of the capillary wall, separated at a distance L = 2 mm along the capillary axis. High frequency ( f = 10 kHz to 1 MHz) sinusoidal ac voltage with amplitude U up to 400 V was applied to the electrodes using a function generator and fast amplifier (KrohnHite 7602M). Polarized-Light Microscopy. Optical observations of the texture changes under electric field were carried out with a general purpose Leica stereomicroscope equipped with polarizing filters and images were recorded with a digital camera. In the field of liquid crystals, “texture” refers to the spatial distribution of the orientation of nematic domains.17 In-Situ SAXS under Electric Field. SAXS experiments were performed at the SWING beamline of the SOLEIL synchrotron (Saint-Aubin, France) that was already described in detail. Measurements were carried out using a fixed energy of 9 keV and a sample to detector distance of 6 m. The typical accessible range of scattering vector modulus q was 10−3 − 10−1 Å−1 (q = 4π sin θ/λ, where 2θ is the scattering angle, and λ = 1.38 Å the wavelength). The incident Xray beam transverse dimensions were approximately 600 × 200 μm2 in the horizontal and vertical directions. Two-dimensional (2D) scattering patterns were recorded on an AVIEX 170170 chargecoupled device (CCD) camera formed by four detectors and were corrected for water and glass scattering. Exposure times were typically around 0.5 s. The curves of scattered intensity versus azimuthal angle were obtained by radial integration of the data along a narrow circular strip, at constant q, in the SAXS patterns.
Figure 1. Microscopic observations in polarized light (P: Polarizer, A: Analyzer) of an anisotropic chitin nanorod−siloxane oligomer hybrid suspension in ethanol/aqueous 10−4 M HCl 50/50 wt %, with CCHI = 5.0 wt %, CSi = 10.7 wt %: (a,b) before application of the field, a few minutes after sucking the suspension into a capillary tube 1 mm in diameter; (c,d) after 5 min application of an electric field E = 140 V·mm−1 (root-mean-square (rms) value) along the main axis of the tube (direction of E indicated by dotted arrow in c and d). The dark regions delimiting the image of the capillary tube are the electrodes. All micrographs were taken with the same exposure time, and are presented here with the same luminosity scale.
little, or no macroscopic alignment (at the millimeter scale) after being sucked into cylindrical 1 mm capillary tubes. Because the hybrid suspensions are more viscoelastic than the pure chitin suspensions, they do not reorganize into typical cholesteric textures on the time scale of the experiments. The corresponding SAXS diagrams sometimes indicated weak flowalignment (along the main axis of the capillary tube) but most often showed complex patterns (Figure 2a1) due to small nematic domains (of a few micrometer size) randomly oriented within the volume probed by the synchrotron beam. In addition, an interference peak is observed at low q values on the radial intensity profiles for suspensions of chitin only (ethanol− aqueous 10−4 M HCl 50/50 wt.%). At 4.3 wt % in chitin, the peak location indicates an average distance between particles dave = 2π/qave ∼ 120 nm, in agreement with our previous data in aqueous media.1 These observations (optical microscopy and SAXS) demonstrate that chitin nanorods form nematic mesophases in aquo-alcoholic solvents containing variable amounts of siloxane oligomers.
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RESULTS AND DISCUSSION We have studied the behavior of hybrid suspensions comprised of chitin nanorods (260 nm long and 23 nm thick) and siloxane oligomers (Dh ∼ 3 nm) in a 50/50 wt % mixture of ethanol and aqueous HCl (10−4 M). Stable hybrid suspensions can be obtained by directly mixing concentrated suspensions of both precursors. The complete study of the phase diagram (outside the scope of this Letter) is currently being achieved, but B
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aligns along the direction of the field, as revealed by a fast increase of Seff to more than 0.70 in less than 15 s. The sample then rearranges more slowly to reach a final value of Seff = 0.75 in a few minutes, very close to the theoretical value of 0.80, which strongly suggests that a single nematic domain is formed under the influence of the electric field, in agreement with the observations by polarized light microscopy (Figure 1c,d). Very importantly, the aligned chitin nanorod−siloxane oligomer suspensions do not relax after the field is switched off and stay aligned for several days (see, for instance, Figure 2c). It is well-known that anisometric colloidal particles in suspension can be aligned in strong electric fields. Three main mechanisms contribute to the electric torque on the particle. The first two of them are quite universal and are related to the particle/solvent contrast of dielectric constant and conductivity: under field, the particle is polarized due to the accumulation of charges (respectively bound or free) on its surface.23,24 The resulting large induced dipole rotates under field, orienting the long axis of the (prolate) particle along the field. The third mechanism, specific for charged particles, is related to the polarization of the counterion atmosphere of the particle.25 In fact, the concentration of mobile charges in the ionic cloud, and hence its conductivity, is very high compared to that of the bulk solvent. Under field, the ionic cloud behaves as a conductive shell, with very strong and anisotropic polarizability. In most cases, up to some relaxation frequency of the order of a few megahertz,16 this mechanism is the dominating one for charged particles in aqueous suspensions. Qualitatively, our observations strongly suggest that for chitin nanorods, the main field-alignment mechanism is the polarization of the counterion cloud. In fact, good macroscopic alignment was observed with chitin suspensions in water or in water−ethanol mixtures under the experimentally accessible fields, E ≤ 140 V·mm−1 rms, f = 0.3−1 MHz. On the contrary, suspensions in pure ethanol did not display any observable field-alignment under the same conditions. This can be explained by the significantly lower charge dissociating power and dielectric constant of ethanol, which decreases the surface charge of the chitin nanorods and thus hinders the main fieldalignment mechanism. The field strength needed to align chitin suspensions in aqueous−ethanol mixtures was therefore comparable to, or slightly lower than, values determined for cellulose nanocrystals suspended in water26 and cyclohexane.27 Using ethanol−water (HCl 10−4 M) mixtures as solvent, the trends presented above for an anisotropic hybrid suspension of given composition are also valid for other chitin and siloxane concentrations. However, we observed that increasing the concentration in both chitin nanorods or siloxane oligomers result in slowing down the alignment kinetics. Beyond a chitin concentration of 6 wt % and/or a silica content of 10 wt %, the field-induced alignment of the sample is either completely prevented or rapidly arrested. This is consistent with previous observations of a gelled nematic state for chitin aqueous suspensions at high volume fractions, which is most likely due to jamming.28−30 Moreover, condensation reactions between the reactive siloxane oligomers may also induce gelation, thus preventing the chitin nanorods from aligning along the electric field direction. In contrast, dilute suspensions (CCHI below 1.5 wt %, with or without siloxane oligomers), at rest, form isotropic phases since they appear dark in polarized light and their SAXS patterns are isotropic (Supporting Information Figure S2a1). Applying an electric field induces a clear and fast response, but the value of
Figure 2. SAXS patterns (a1−a4) of an anisotropic chitin nanorod− siloxane oligomers hybrid suspension in ethanol/aqueous 10−4 M HCl 50/50 wt %, with CCHI = 5.0 wt.%, CSi = 10.7 wt % before (a1) and after (a2−a4) application of an electric field as a function of time. The direction of the field with respect to the SAXS patterns is indicated by the black arrow in a2. (b) Corresponding azimuthal intensity profiles I = f(ψ) before (black) and after 5 (blue), 10 (green), and 300 (red) seconds of application of the field. (c) Order parameter S inferred from I = f(ψ) traces as a function of duration of the electric field application. The two last data points were taken more than 80 s after the field was switched off. The dashed line is a guide to the eye.
These anisotropic samples are shown to quickly respond upon application of an alternating electric field, as evidenced by the fast changes in the SAXS patterns recorded in situ (Figure 2a2). The appearance within seconds of such anisotropic SAXS patterns proves the alignment of the nematic phase director (and thus of the chitin nanoparticles) along the direction of the field. Accordingly, before application of the field, the azimuthal intensity profiles, I = f(ψ), are irregular and strongly vary when the X-ray beam is scanned over the sample. In contrast, after application of the field, the profiles are much smoother and exhibit a strong peak located at an angle (here 180°) corresponding to the direction perpendicular to that of the electric field. This peak narrows as the sample is kept longer in the field (Figure 2b), illustrating the gradual alignment of the nematic phase. The degree of alignment of the chitin nanorods can be estimated from the analysis of the peaks in the SAXS azimuthal intensity profiles.17−19 Such analysis provides an effective nematic order parameter, Seff, which represents a statistical average of the nanorods orientation not only within a nematic single domain (i.e., the intrinsic nematic order parameter, S), but also over the distribution of nematic domains probed by the X-ray beam (i.e., the nematic texture), as the sample is not usually a single domain. The intrinsic order parameter of the nematic phase at coexistence is theoretically expected to be S ∼ 0.80,20,21 as it was shown previously for pure chitin suspensions in aqueous media.1 Indeed, some texture distribution is expected for chitin suspensions (and many other systems) that are intrinsically chiral and tend to form cholesteric textures.1,2,22 Therefore, the apparent order parameter inferred from the nearly flat azimuthal trace before application of the electric field is rather low (0.50 in Figure 2c). However, once the field is applied, the nematic domains anneal and the phase C
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Figure 3. Dry chitin−silica nanocomposites after complete evaporation of the solvent. SAXS analysis: (a) 2D pattern of the region initially placed in the field and (b) corresponding azimuthal intensity profiles I = f(ψ) with the actual data as open circles and the fit as the continuous line. Microscopy analysis (sample orientation and field direction are indicated by yellow arrows): (c) polarized light micrographs taken between crossed polarizers. The field direction with respect to the polarizers is rotated by 45° between the upper and lower pictures. In the lower picture, the directions of the polarizer (P) and analyzer (A) are indicated by white arrows and the bar is 200 μm. (d) Scanning electron micrograph and (e) transmission electron micrograph after resin embedding and sectioning. Initial compositions of the suspensions in ethanol/aqueous HCl 10−4 M (50/50 wt %) placed under electric field were CCHI = 5.0 wt %, CSi = 10.7 wt % for (a) and (b), and CCHI = 6.0 wt %, CSi = 2.4 wt % for (c), (d), and (e).
Seff for such isotropic suspensions remains lower than for nematic samples (Figure S2b). Most importantly, isotropic suspensions rapidly relax back to an orientationally disordered state when the field is switched off (in less than 1 s for a suspension of chitin nanorods at 2 wt % devoid of siloxane oligomer). We have demonstrated previously that chitin−silica nanocomposites with oriented textures can be obtained when the sol−gel transition occurs in a strong and uniform magnetic field.12 In the present case of anisotropic suspensions uniaxially aligned by an electric field, we have tried and obtained oriented nanocomposites without continuously applying the field. A suspension initially aligned in the field (that of Figure 2) has been allowed to dry in absence of field for several weeks. After complete evaporation of the solvent, the resulting solid gave an anisotropic SAXS pattern, due to an oriented texture (Figure 3a), from which an order parameter of Seff ∼ 0.32 was determined. Similarly, polarized light microscopy (Figure 3c) reveals the long-range uniform alignment along the direction of the field for such samples. This shows that the initial electricfield-induced alignment was partly preserved during the sol−gel transition and further solidification due to solvent evaporation and siloxane condensation, even though the sample was not kept in the field during drying, yielding an aligned chitin−silica nanocomposite materials. Scanning (SEM) and transmission electron microscopy (TEM) of the resulting materials (Figure 3d,e and SI) allows observing elongated nanoparticles with
dimensions close to typical values measured for the purified chitin nanorods (260 nm long and 23 nm thick).1 Again, these elemental units appear globally aligned along the direction of the electric field applied to the initial colloidal suspensions. Fourier transforms of SEM and TEM images and related azimuthal profiles reveal similar orientational distributions about a main direction as inferred from SAXS (see Supporting Information). This set of results clearly demonstrates that the axial alignment of suspended chitin nanorods obtained after a short application of an electric field is transcribed into the final chitin−silica solid composites.
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CONCLUSIONS In summary, we showed that reactive cosuspensions of rodlike chitin particles and siloxane oligomers can be strongly aligned in an alternating electric field. In the case of chiral nematic phases formed by these systems, the collective reorientation occurred within seconds and yielded an effective order parameter of Seff ∼ 0.75, i.e., an almost defectless nematic single domain. These phases can stay aligned for several weeks after application of the electric field for only few minutes. In the case of isotropic cosuspensions, alignment upon application of an electric field also occurs, but it vanishes once the field is switched off. Mixtures of acidic water and ethanol (50/50 wt %) proved to be a good compromise to guarantee both colloidal and chemical stability, and to enable polarization of the counterion cloud and hence a large enough aligning torque. D
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Finally, by solvent removal from a field-aligned nematic phase, uniaxially aligned chitin-silica nanocomposites (Seff ∼ 0.3) have been obtained.
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ASSOCIATED CONTENT
S Supporting Information *
Additional observations of textures in polarized light, SAXS patterns for an isotropic suspension, optical and electron (scanning and transmission) microscopy of solid samples. This material is available free of charge via the Internet at http:// pubs.acs.org
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AUTHOR INFORMATION
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge financial support from the ANR (HYSIKIT project, programme blanc) and CNRS (MaProSu program), as well as Université Montpellier 2 (B.A. and E.B.). The authors are grateful to the SOLEIL synchrotron facility for the allocated beamtime and to Florian Meneau at the Swing beamline for help during the experiments. Thomas Cacciaguerra and Franck Fayon are acknowledged for complementary analysis.
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REFERENCES
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