Micelle-Mediated Self-Assembly of Microfibers Bridging Millimeter

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Micelle-mediated Self-assembly of Microfibers Bridging Millimeterscale Gap to Form 3D-ordered Polysaccharide Membranes Kosuke Okeyoshi, Takeshi Shinhama, Kulisara Budpud, Gargi Joshi, Maiko Kaneko Okajima, and Tatsuo Kaneko Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b03116 • Publication Date (Web): 19 Oct 2018 Downloaded from http://pubs.acs.org on October 24, 2018

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Micelle-mediated Self-assembly of Microfibers Bridging Millimeter-scale Gap to Form 3Dordered Polysaccharide Membranes Kosuke Okeyoshi*, Takeshi Shinhama, Kulisara Budpud, Gargi Joshi, Maiko K. Okajima, Tatsuo Kaneko* Energy and Environment Area, Graduate School of Advanced Science and Technology, Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Nomi, Ishikawa 923-1292, Japan

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ABSTRACT

Micelle-mediated 3D-ordered polysaccharide membranes are constructed by introducing cationic/anionic surfactant into a liquid crystalline polysaccharide solution. Upon drying mixtures of the polysaccharide solution with surfactant such as cetyltrimethylammonium bromide (CTAB) or sodium dodecyl sulfate (SDS), the polymeric microfibers deposit as a nucleus to form a membrane, bridging millimeter-scale gap with high probability. In particular, in a solution with SDS micellar structures, the micro-scale fibers with diameter ~1 µm disassemble into nano-scale fibers with diameter ~50 nm. This transformation allows the polymeric network to become finer in nano-scale, and the vertical membrane is formed much more easily than from a pure polysaccharide solution. Furthermore, it is clarified that the vertical membrane has been successfully formed with three-dimensionally ordered microstructures with a linearly oriented and layered structure. This method will shed light on the preparation of hybrid materials with biocompatibility and responsivity to stimuli such as magnetics, electrics, and optics via hybridization with nanomaterials dispersed by surfactants.

KEYWORDS. fiber, interface, orientation, polysaccharides, self-assembly


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INTRODUCTION The use of hierarchical self-assembly such as polymeric microfibers, micelles, and their combinations is a strong strategy for biomaterials because of advantages in the structural transformations.1-3 Various soft materials have been artificially synthesized at molecular scale, e.g., protein-mimetic microfibers, carbon nanotubes, and functional micelles/nanogels.4-9 However, owing to the challenges in intrinsically achieving biocompatibility and environment adaptability with synthesized materials, natural biopolymers are attracting attention as sophisticated models.1012

Various naturally derived polysaccharides have been extracted and researched, e.g., cellulose

nanofibers,13 schizophyllan,14 and sacran.15 They have hierarchically organized structures from molecular scale to micro-scale in three dimensions. We have researched an anionic suprapolysaccharide, sacran (Figure S1), which forms micro-scale fiber with self-assembled structures of diameter ~1 µm and length > 20 µm,16 showing lyotropic liquid crystallinity (LC) in water at a relatively low concentration of ~0.5 wt%. Sacran is the main constituent of the extracellular matrix of a cyanobacterial colony, Aphanothece sacrum, with an extremely high molecular weight (weight-average molecular weight, Mw > 2 × 107 g·mol-1), and is ten times thicker than typical thickeners such as hyaluronic acid (Mw ≈ 105 g·mol-1). The aqueous solution is a viscous liquid, showing dynamic moduli—storage moduli, G’ ≈ 4 Pa; loss moduli, G” ≈ 2 Pa—for the 0.5 wt% solution at frequency 1 Hz.15, 17 Furthermore, this polysaccharide has unique properties when used as biomaterials, such as super-moisturization, self-orientation, and anisotropic swelling of the hydrogels.18-21 Recently, we successfully found a phenomenon of meniscus splitting via the deposition of a polysaccharide membrane in a limited space by drying its aqueous solution.22-23 Owing to the extremely high viscosity, the polymer on the evaporative interface deposits and bridges milli-scale


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gap through a drastic increase in the polymer concentration from 0.5 to 100 wt%, differing from other polysaccharides such as hydroxypropyl cellulose showing LC with higher concentration ~60 wt%. This bridging and formation of a membrane is accompanied by orientation of the micro-scale fibers along the descending air–liquid–solid contact line (Figure 1A). On the evaporative air– liquid interface, the fibers are condensed with orientation, and they restrict water discharge similar to the skin layers of a deswelling gel.24-25 This evaporative interface in a narrow gap induces accumulation of small depositions at multiple specific points with intervals of ~1 cm and partitions the macro-space. However, the role of microfibers on the bridging of milli-scale gap and the formation of membrane is yet to be verified. Furthermore, whereas the characteristics of mixtures of polysaccharide and surfactant have been clarified,26-27 the relationship among the selfassemblies during the condensation is yet to be clearly verified. If a polysaccharide membrane having micelles is prepared, the clarification of the condensation will help designing of biomaterial models for drug delivery systems. In this study, to clarify the behaviors of micro-scale fibers on the evaporative interface during the depositions, anionic/cationic surfactants are introduced into an aqueous polysaccharide solution initially (Figure 1B). With their coexistence in the microfiber matrix, the surfactants will not only reduce the surface tension on the air–liquid interface but also electrostatically affect the microfibers by the micellar structures. In fact, the negatively charged sacran shows sensitivity to multivalent metal cations and forms crosslinking points18, the interactions among the sacran fiber and the ionic micelles should be effective. With cationic micelles such as cetyltrimethylammonium bromide (CTAB), the electrostatic interaction between the negatively charged sacran microfibers should be attractive. This situation induces the flocculation to be a nucleation deposition. In contrast, with anionic micelles such as sodium dodecyl sulfate (SDS), the electrostatic interaction


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between the negatively charged fibers should be repulsive. Subsequently, the fibers are easily aligned along the contact line of the air-liquid-solid interface. In these ways, the cationic/anionic surfactants coexisting with microfibers substantially promote the integration near the interface and bridging of the gap as the nucleus.

METHODS Materials. Sacran was extracted from Aphanothece sacrum using the procedure reported previously.15 CTAB and SDS were purchased from Nacalai Tesque, Japan and Wako Pure Chemical Industries, Ltd, Japan, respectively. Preparation of polymer solutions and measurement of viscosity. After dissolving sacran in pure water at ~80 °C, the mixture was cooled down to room temperature at ~25 °C to obtain aqueous solution with a concentration of 0.5 wt%. The sacran solution was purified by precipitating small amount of insoluble impurities using centrifuge (Avanti HP-26 XP, Beckman equipped with JA20 rotor) under 2 × 104 rpm at 4 °C for 1 h (three times), and the supernatant solution was used. The viscosity was measured at room temperature using a rheometer (Physica MCR301, AntonPaar). Drying experiments. The aqueous solutions at room temperature were poured into a top-side-open cell composed of two non-modified glass slides sandwiching silicone rubber as a spacer. The silicone spacer was prepared in U-shape by cutting the rubber sheet. Subsequently, they were placed in an oven at 60 °C under atmospheric pressure with an air circulator. Microscopic observations. To characterize the submicron scale structures of the nucleus and membrane, the samples were observed using SEM (JCM-6000plus, JEOL) or FE-SEM (S-5200, Hitachi). The samples used in SEM were coated with Au using a magnetron sputtering system


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(MSP-IS, Vacuum Device), and the samples used in FE-SEM were coated with Os using an osmium coater (Neoc-Pro, Meiwafosis co., ltd.). Polarized microscopic observations were obtained using an optical microscope (IX73, Olympus) equipped with a charge-coupled device camera (DP80, Olympus). A first-order retardation plate with λ = 530 nm was placed into the light path.

RESULTS AND DISCUSSION First, pure sacran solution for the formation of membrane was obtained with the initial concentration of 0.14 wt%, which is close to the critical concentration of LC, ~0.2 wt%.17 The aqueous solution composed of the 0.14 wt% sacran and pure water showed neutral pH (pH7.4 ± 0.1), and the experiments were carried out at the neutral pH. We confirmed that the sacran could form the microfibers with ~1 µm diameter at ~pH7 (Fig. S2). It shows high viscosity in the initial state, and dynamic moduli, G’, G” ≈ 0.4 Pa, at the frequency 1 Hz.15 The viscous solution was poured into a top-side-open cell composed of two glass slides and a silicone spacer with an Xwidth of 25 mm and Y-thickness of 1 mm. The sample was placed at 60 °C under atmospheric pressure in an oven with an air circulator. As shown in Figure 2A, immediately after the drying, the deposited polymer bridged the gap of 1 mm between two glass slides and a thin membrane grew in the YZ-plane. Similar to the case with the initial concentration of 0.5 wt%,22-23 the formation of membrane was confirmed immediately even at three times lower concentration. This indicates that the deposition bridging is strongly affected by the evaporative speed rather than the initial concentration. Once the macrodomains are integrated into size ~1 mm on the evaporative interface, the deposition bridging occurs easily.


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To investigate the microstructure formed in the bridging deposition, membranes were peeled off from the substrate and cut from another substrate for multilateral observation using scanning electron microscopy (SEM) (Figure 2B). The images i, ii, and iii show the nucleus near the portion peeled off from the substrate, the membrane cut in the Y-direction, and the perspective of the membrane around the nucleus, respectively. In image i, we can observe lines on the XZ-plane near the substrate radially directed to the nucleus center. Moreover, the top portion of the nucleus (images iv and v) has straight lines along the Y-direction. These drying records suggest that the microfibers on the evaporative interface are oriented along the air–liquid–solid contact line (Figure 2C). The layered structures in the YZ-plane can be observed on the cut edges (images ii, vi, vii, and viii). Considering the membrane thickness of ~10 µm and the diameter of the microfiber of ~1 µm, the contact line is effective even inside the membrane where capillary force is the dominant factor for orientation rather than the gravity force. To clarify the effects of evaporative interface on the deposition nucleation and the formation of a vertical membrane, two types of surfactants, CTAB and SDS, were introduced into the sacran aqueous solution (Figure 3A). Although an anionic surfactant, sodium hexadecyl sulphate (SHDS, C16H33SO4Na) having the same alkyl chain length with that in CTAB can be a candidate for the experiments, the saturating concentration is too low that we use SDS. For the drying experiment, the initial concentration of the surfactant was arranged to have dispersion stability macroscopically (Figure S3). First, the viscosity of the solution with different concentrations of surfactants was measured using a rheometer without evaporation (Figure 3B). The mixture with 0.14 wt% sacran and 0.012 wt% CTAB was more viscous than the pure 0.14 wt% sacran solution; however, the mixture with 0.078 wt% CTAB was less viscous than the pure sacran solution. These results indicate that cationic CTAB can be a crosslinker among the anionic sacran microfibers via


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electrostatic interaction. With an increase in the concentration of CTAB, sacran fibers became flocculated as shown in Fig. 1B, and phase separation occurred at a concentration higher than the critical micelle concentration (C.M.C., 0.036 wt%) (see Figure S3). This flocculation causes the drastic decrease of the viscosity. In contrast, the viscosity of the solutions with SDS (0.29 wt% and 0.87 wt%) showed no significant difference as compared with that of the pure sacran solution. In addition, the solutions with higher SDS concentrations than the C.M.C. (0.23 wt%) were clear without macroscopic flocculation or phase separation. The transparency of the mixtures obviously shows the effect of the electrostatic interaction by the surfactants (Figure S3B). These results should be due to the electrostatic repulsion among the negatively charged sacran microfibers and the SDS micelles (see Figure S4). Thus, the dispersion stability of sacran is low in CTAB but high in SDS. Upon drying the mixtures containing CTAB or SDS at 60 °C from a top-side-open cell, both solutions showed nucleation and formation of a vertical membrane, similar to the case where pure sacran solution is used (Figure 3C). Furthermore, multiple nuclei could be observed in both cases, especially, from the mixture of sacran and SDS. These results indicate that the surfactants promote integration of deposited macrodomains on the air–liquid interface during the drying (see Figure 1B). In fact, water evaporation causes drastic condensation of sacran fibers and SDS, which allows phase separation near the air–liquid interface (see Figure S5). This situation induces integration of macrodomain growing into a nucleus point. Not only the number of nuclei but also the probabilities of nucleation and membrane growth were drastically increased by the introduction of surfactants, CTAB or SDS, into sacran solution (Figure 3D). This indicates that the surfactants support the deposition nucleus for bridging the millimeter scale gap.


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In contrast to the mixture of sacran and CTAB in which CTAB induces sacran flocculation and phase separation, the mixture of sacran and SDS shows a clear solution even if it is concentrated during the drying. To clarify the structures in submicron scale, the dried vertical membrane was observed using field-emission SEM (FE-SEM) (Figure 4A). Over the entire area, the surface is smooth with crystal-like thin plates or creases, and it has a thin layer with thickness less than 20 nm (Figure 4A, i and ii). This appears to be an SDS lamellar layer and it covers the polysaccharide membrane. The thin layer is constructed on the evaporative contact line of air/liquid/polymeric membrane. In the thickness direction, the sample has layer structures even with the coexistence of SDS. Thus, the sacran fiber has structural stability to form a vertical membrane with a threedimensionally ordered structure. We observed nano-scale fibers with outer diameter ~50 nm under the surface thin layer (Figure 4A, yellow region of i and iii). This fiber is the polysaccharide coexisting with SDS micellar structures in the liquid. Furthermore, most of the nano-scale fibers are oriented in the Y-axis. This fact is supported by the similar direction of straight cracks in the membrane (see Figure S6). Considering the micro-scale fibers around the nucleus, this nano-scale fiber was a result of disassembly of the micro-scale fiber in SDS (Figure 4B). By condensation which induces a drastic increase of the SDS concentration in the mixture (>> C.M.C.), the sodium ions as monovalent cations on the micelles should cause loosening of hydrogen bonds among carboxylic groups in the microfiber and disassembling the microfiber into nano-scale fibers. This disassembled state as a bundle of nano-scale fibers is also observed by atomic force microscopy.15 Consequently, the selfassembled microfibers exhibit a morphological change into nano-scale fibers with finer polymeric networks during the drying. Furthermore, considering that the SDS forms micellar structure at less than ~35 wt% and lamellar structure at more than ~70 wt%28-30, the SDS in coexistence of sacran


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fibers might behave as shown in Figure 4B. Due to technical limitations for the observation of nano-scale fibers in aqueous solution, the dynamic changes of assembly/disassembly of microfibers and micelles/lamellae of the surfactant is now under consideration: preparation and use of a TEM holder for wet samples; use of other types of liquids.

CONCLUSIONS Drying-induced formation of a three-dimensionally-ordered membrane was investigated by introducing a cationic/anionic surfactant into an LC polysaccharide solution. With the coexistence of surfactant such as CTAB and SDS in the polysaccharide solution, sacran, the polymeric macrodomain easily deposits a nucleus bridging the milli-scale gap through integration of macrodomains on the evaporative interface. Especially, with the coexistence of SDS micellar structures, by disassembling micro-scale fibers with diameter ~1 µm into nano-scale fibers of diameter ~50 nm, the polymeric network became finer in nano-scale than the network composed of micro-scale fibers. Therefore, the vertical membrane was formed much more easily from the mixture of sacran and SDS than from the pure sacran solution. Furthermore, from SEM observations, it was clarified that the vertical membrane has three-dimensionally ordered microstructures with a linearly oriented and layered structure. With the coexistence of the SDS, a thin lamellar layer was formed on the surface of the polysaccharide membrane in the drying process. We envision that this method with the use of surfactant will shed light on the preparation of hybrid materials with biocompatibility and responsivity to stimuli such as magnetics, electrics, and optics via hybridization with nanomaterials dispersed by surfactants.


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Experimental details including evaluation for microfibers, evaluations for mixture of sacran and surfactant (PDF) AUTHOR INFORMATION Corresponding Author E-mail: [email protected] (K.O.) E-mail: [email protected] (T. K.) Author Contributions K. O. wrote the paper. T. S. carried out the experiments. K. B. and G. J. adjusted the conditions of sacran microfibers. M. K. O. prepared sacran. K. O. and T. K. supervised the projects. All the authors approved the manuscript. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was supported by Grant-in-Aid for Young Scientists (16K17956), Grant-in-Aid for Challenging Exploratory Research (16K14077) from the Ministry of Education, Culture, Sports, Science, and Technology of Japan, The Asahi Glass Foundation, and Innovation inspired by Nature Research Support Program, Sekisui Chemical Co. Ltd. G. J. is grateful for the research fellowships of the Japan Society for the Promotion of Science for Young Scientists (18J11881).


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The authors also acknowledge Mr. Nobuaki Ito (JAIST, Center for Nano Materials and Technology) for his useful discussion on the SEM observations.


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(19) Okajima, M. K.; Mishima, R.; Amornwachirabodee, K.; Mitsumata, T.; Okeyoshi, K.; Kaneko, T. Anisotropic swelling in hydrogels formed by cooperatively aligned megamolecules. RSC Advances 2015, 5, 86723–86729. (20) Joshi, G.; Okeyoshi, K.; Okajima, M. K.; Kaneko, T. Directional control of diffusion and swelling in megamolecular polysaccharide hydrogels. Soft Matter 2016, 12, 5515–5518. (21) Okeyoshi, K.; Joshi, G.; Rawat, S.; Sornkamnerd, S.; Amornwachirabodee, K.; Okajima, M. K.; Ito, M.; Kobayashi, S.; Higashimine, K.; Oshima, Y.; Kaneko, T. Drying-Induced Self-Similar Assembly of Megamolecular Polysaccharides through Nano and Submicron Layering. Langmuir 2017, 33, 4954–4959. (22) Okeyoshi, K.; Okajima, M. K.; Kaneko, T. Emergence of polysaccharide membrane walls through macro-space partitioning via interfacial instability. Sci. Rep. 2017, 7, 5615. (23) Okeyoshi, K.; Joshi, G.; Okajima, M. K.; Kaneko, T. Formation of Polysaccharide Membranes by Splitting of Evaporative Air- LC Interface. Adv. Mater. Inter. 2018, 7, 1701219. (24) Yoshida, R.; Uchida, K.; Kaneko, Y.; Sakai, K.; Kikuchi, A.; Sakurai, Y.; Okano, T. Combtype grafted hydrogels with rapid deswelling response to temperature changes. Nature 1995, 374, 240–242. (25) Suzuki, A.; Yoshikawa, S.; Bai, G., J. Chem. Phys. 1999, 111, 360–367. (26) Biswas, S. C.; Chattoraj, D. K., Polysaccharide-surfactant interaction. 1. Adsorption of cationic surfactants at the cellulose-water interface. Langmuir 1997, 13, 4505–4511. (27) Ropers, M. H.; Novales, B.; Boué, F.; Axelos, A.V. Polysaccharide/surfactant complexes at the air-water interface – effect of the charge density on interfacial and foaming behaviors. Langmuir 2008, 24, 12849-12857. (28) Kékicheff, P.; Grabielle-Madelmont, C.; Ollivon, M. Phase diagram of sodium dodecyl sulfate-water system. J. Colloid Interface Sci. 1989, 131, 112–132. (29) Marques, E.; Khan, A.; Miguel, M. G.; Lindman, B. Self-assembly in mixture of a cationic and an anionic surfactant: the sodium dodecyl surfate-didodecyldimethylammonium bromidewater system. J. Phys. Chem. 1993, 97, 4729–4736. (30) Baalbaki, N. H.; Kasting, G. B. A pseudo-quantitative ternary surfactant ion mixing plane phase diagram for a cationic hydroxyethyl cellulose with dodecyl sulfate conterion complex salt. Colloids and Surfaces A: Physicochem. Eng. Aspects 2017, 522, 361–367.


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Figure 1. Drying-induced bridging deposition of a gap and formation of a vertical membrane from a mixture of liquid crystalline polysaccharide and surfactant. A. Schematic illustration of deposition nucleation and formation of vertical membrane on the evaporative interface. B. Nucleation in coexistence positively charged CTAB micelles, where the attractive interaction with negatively charged sacran microfibers results in flocculation. Nucleation in coexistence with negatively charged SDS micelles, where repulsive interaction causes no flocculation. Polysaccharide: sacran forming microfibers with diameter ~1 µm.


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Figure 2. Membrane formation with linear orientation of microfibers and layer structure. A. Dryinginduced vertical membrane formation in a top-side-open cell (25 × 1 × ~20 mm) and schematic illustration of formation of a vertical membrane in a top-side-open cell with a gap. Initial concentration of polysaccharide, sacran: 0.14 wt%. Drying atmosphere: 60 °C under air pressure. B. SEM images of vertical membrane. C. Proposed schematic for the vertical membrane with layer structure in the YZ-plane.


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Figure 3. Effects of surfactant coexistence on deposition nucleation and formation of a vertical membrane. A. Chemical structures of sacran, CTAB, and SDS. B. Viscosity as a function of shear rate for given aqueous solutions: mixture of sacran and CTAB, and mixture of sacran and SDS at 25 °C. C. Polarized images of formation of vertical membrane from a mixture of 0.14 wt% sacran and 0.011 wt% CTAB, and a mixture of 0.14 wt% sacran and 0.29 wt% SDS. Dimensions of top-side-open-cell (25 × 1 × ~20 mm). Drying atmosphere: 60 °C under air pressure. D. Probabilities of deposition nucleation and formation of vertical membrane from the mixtures. Number of trials is 17.


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Figure 4. Microscopic structures of vertical membrane deposited from a mixture of sacran and SDS. A. SEM images of vertical membranes in the YZ-plane. B. Schematic illustration of the micro-scale fiber and nanoscale fiber depending on the concentration of SDS. With an increase in the concentration of SDS in the mixture (CSDS >> C.M.C), the sodium ions on the micelles cause loosening of hydrogen bonds among carboxylic groups in the microfiber and disassemble the microfiber into nano-scale fibers.


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Micelle-Mediated Self-Assembly of Microfibers Bridging Millimeter

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