Stimuli-Responsive Colloidal Assembly Consisting of Imogolite

【SPECIAL EDITIONS on “NANO FIBER”-Review】 Stimuli-Responsive Colloidal Assembly Consisting of Imogolite, Inorganic Nanotube Kazuhiro Shikinaka Research Institute for Chemical Process Technology, National Institute of Advanced Industrial Science and Technology (AIST), 4-2-1, Nigatake, Miyagino-ku Sendai, Miyagi 983-8551, Japan

Abstract: In this review, I describe stimuli-responsive colloidal assemblies prepared from a rigid rod-like clay mineral called imogolite, and relate them to their structural characteristics. When combined with dicarboxylic acids, imogolite formed gels that exhibited keen thixotropy, and physical anisotropy via orientation of the imogolite particles after flowing and subsequent standing. Robust hydrogels were also obtained by in-situ polymerization of vinyl monomers in imogolite aqueous dispersion. Under strain, these hydrogels showed a reversible isotropic‒anisotropic structural transition. (Received 26 September, 2019; Accepted 24 November, 2019)

1. INTRODUCTION cylindrical polymers. To create the functional colloidal assemblies in a Some living organisms consist of various biomimetic manner, I investigated a single-walled architectures of one-dimensional structures such as aluminosilicate inorganic polymer called imogolite nanofibers and nanotubes. For example, in cell (henceforth denoted as IG), which has a rigid

cytoskeletons, semi-flexible rod-like proteins such as cylindrical structure and the composition (HO)3Al2O3 filamentous actin form network-like architectures via SiOH [9‒14]. With external and internal diameters of non-covalent bonding (e.g., hydrogen bonding and approximately 2 nm and 1 nm, respectively, and a electrostatic interactions) [1]. Tube-like assemblies of length ranging from several tens of nanometers to the same proteins, such as actin and microtubles, form several micrometers, IG has been noted as a perfectly bundle-like architectures by the same mechanism [1]. rigid polyelectrolyte with a high aspect ratio [15]. These architectures realize the motility and other Accordingly, it has been incorporated into inorganic‒ various functionalities of cells through their keen organic nanocomposites [16]. The outer and inner

stimuli-responsive structural transitions. Similar surfaces of IG are covered with Al(OH)2 (proton- materials that respond to stimuli such as light [2] and capturing) and Si(OH) (proton-releasing) groups, temperature [3] have been designed in vitro through respectively. Thus, the charge density of IG surfaces various opportune molecular (assembling) structures, varies with the pH and ionic strength of aqueous empowering both emerging fields of scientific interest media. Consequently, the dispersibility of IG in water and unexplored applications [4]. Stimuli-responsive is highly pH-dependent; in acidic aqueous media (pH ≈ materials have also been built from rigid rod-like 4) with relatively low ionic strength, IG disperses as materials such as thermotropic/lyotropic liquid thin bundles or even as monofilaments, resulting in crystals [5‒8]. Recently, I designed novel stimuli- opaque to transparent solutions. responsive colloidal assemblies consisting of rigid In this review, I discuss some developments of cylindrical inorganic polymers that imitate the stimuli-responsive IG materials and relate them to the evolving architectures in cell cytoskeletons. This structural characteristics of IG. Section 2 describes review describes the novel functional materials the thixotropic gelation of IG nanotubes and the developed by myself and colleagues, which are based characteristics of the obtained IG gels. Section 3 on the supramolecular architectures of rigid discusses how ionic conductivity emerges in IG gel

# corresponding author: Kazuhiro Shikinaka (E-mail: [email protected])

Journal of Fiber Science and Technology (JFST), Vol.76, No. 1 (2020) 1 combined with ionic liquid. Section 4 and 5 describe well known, many kinds of inorganic and organic the structural orderings of IG gels initiated by flow- acids such as carboxylic acids [22] interact strongly shearing and chiral formation, respectively. Finally, I with the outer surface of IG nanotubes. When introduce a robust, stimuli-responsive IG gel carboxylic acids are added to IG, they establish composed of IG and organic network polymers. protonation equilibrium of the aluminol groups of the IG outer surface that form the cationic sites, creating 2. FORMATION AND CHARACTERISTICS strong hydrogen bonds or electrostatic interactions

OF IG-BASED THIXOTROPIC GEL with the molar equivalent of the ‒Al(OH)2 units. When aqueous solutions of IG and maleic acid (MA, a typical 2.1 Thixotropic gelation of IGs by hydrogen bonding short-chain dicarboxylic acid) are combined and aged, I have designed stimuli-responsive IG-based opaque gels (Figure 1 a; denoted as IG‒MA gel) or materials with non-Newtonian fluid behaviors such as hard-gel particle dispersions (phase-separated liquid/ shear thinning. Shear thinning or stimuli-responsive gel mixtures) are formed, depending on the mixing liquid-to-solid phase transitions (also known as ratio. When the mixing ratio is imbalanced (> 2:1 or < thixotropy) are often found in muscle and protoplasm 1:4), the resulting mixtures undergo an apparent [17], and are also important in many industrial phase separation, yielding hard-gel particles in processes (e.g., paints and ceramic sols) [18]. Shear aqueous medium. thinning is thought to emerge from assemblies of At an approximately 1:1 molar ratio of ‒Al(OH)2 colloidal particles, generally called hydroclusters and MA (not the molar quantity of a single ‒COOH [19,20]. In our study [21], IGs purified by appropriate group), the resulting sol‒state mixture gradually procedures were sonicated in pure water to obtain turns into an opaque gel with a thixotropic nature. slightly opaque solutions of nanotubes at a This transition occurs after approximately 1 h at 25 ̊C concentration of 6.4 wt% (i.e. 0.16 mol/L of aluminol (Figure 1 a). In the rotating rheometric test, the aged groups). The average length of the nanotubes was IG‒MA gel (sol) transitioned to the sol (gel) state 68.5 nm. These aqueous solutions were used as within 6 s of agitation (rest), accompanied by perfect starting materials throughout the experiments. As is recovery of its elastic modulus [22]. A 1:1 combination

Fig. 1 (a) Preparative procedure of IG‒MA gel and photographs of the sample tube

inversion test for solid and liquid states. [‒Al(OH)2 of IG] = [MA] = 0.08 mol/L.

(b) Cryo HAADF‒STEM images of quick-frozen IG‒MA gel. [‒Al(OH)2 of IG] = [DA] = 0.08 mol/L. (c) HAADF‒STEM image and EDS elemental mapping of a dried mixture of IG, bis‒2-carboxyethyl germanium(IV)] sesquioxide, andMA. The Ge atoms (i.e., carboxylic acids) localized on the IG nanotubes clearly indicate an interaction between the dicarboxylic acids and the IG nanotubes. Reproduced with permission from [69]. Copyright 2016, Nature Publishing Group

2 Journal of Fiber Science and Technology (JFST), Vol.76, No. 1 (2020) of IG and dicarboxylic acids with 4‒6 main-chain carbons forms similar thixotropic gels, but requires a longer aging period to obtain the gel state. When IG is combined with oxalic or malonic acid at a molar ratio of 1:1, the mixture instantaneously forms turbid hard- gel particles dispersed in the aqueous solution. Thus, the gelation speed of the mixtures and their stimuli- responsiveness sensitively depends on the number of main-chain carbons, configurations, and steric hindrance of the dicarboxylic acids. 2.2 Gelation/structural transition process of IG thixotropic gel Here, I relate the microscopic structural changes to the thixotropic properties of the well-aged 1:1 IG‒ MA gel (sol). To observe the microscopic structures of Fig. 2 Scattering curves obtained by time-lapse the wet-state IG‒MA gels (including the depth profile), SAXS measurements of a well-aged IG‒MA I directly observed the wet-state gel. The IG portions mixture ([‒Al(OH)2 of IG] = [MA] = 0.08 mol/L) during solid - to - liquid transition. These of the gel comprise the Al and Si atoms, which measurements were performed under 0.1 s X- provided sufficient Z contrast (defined as the contrast ray irradiation at 0.1-s intervals. Reproduced in proportion to atom number Z) for their observation. with permission from [69]. Copyright 2016, The gels were observed by high-angle annular dark- Nature Publishing Group field scanning transmission electron microscopy equipped with a cryo-sample transfer system (cryo high contrast and resolution. Periodic rapid HAADF‒STEM), which obtained the Z contrast measurements were obtained at 0.2-s interval by a images of the silicon, aluminum, and oxygen atoms. tandem vertical undulator synchrotron radiation Figure 1 b shows cryo HAADF‒STEM images of a apparatus with a high photon flux and an X-ray torn sample of a quick-frozen IG‒MA gel, presenting counting two-dimensional pixel detector. Figure 2 the bulk and whole structures. The IG‒MA gel illustrates the scattering profiles (i.e., the SAXS consisted of interconnected spongy frameworks. scattering curves of scattering intensity I(q) versus A contrast-intensified cryo HAADF‒STEM the scattering vector q) of a well-aged IG‒MA gel image (Figure 1 b) partially verifies the individual IG recorded at 0.2-s intervals during the cycles between nanotubes in the frameworks of the IG‒MA gel. As the rest period (2.0 s) and the following vortex-mixer shown in the highly magnified STEM image (Figure agitation period (2.0 s). Here, a rod-like substance can 1b), the IG‒MA gel locally appeared as crossed be inferred from the steep slope of the I(q) ~ q－E nanotubes. Furthermore, as shown in the HAADF‒ relation [23,24]. As the region q = 0.08‒0.3 nm－1 STEM images of a dried mixture of germanium corresponds to the real-space size of IG nanotubes bonded dicarboxylic acids and IG (Figure 1 c), the (average length 68.5 nm; average external diameter ~ dicarboxylic acids attached to the whole IG surface in 2.0nm),E=1and2inthisregionindicate that the IG the mixture. Thus, in the present system, the MA- nanotubes were dispersed as separate entities with no sheathed IG nanotubes behaved as rod-like aggregation and were fully packed in a unit lattice, polyelectrolytes, and the outer ‒COOH groups of the respectively [24]. MA sheaths probably provided the linker groups of During the entire resting and agitating period, the cross-bridged nanotubes. The links were probably the scattering function was characterized by the fixed formed by inter-sheath hydrogen bonding. The asymptotic behavior I(q)~q-1.3 in the region of q = frameworks constructed from the cross-bridged local 0.08‒0.3 nm-1. Here E = 1.3 indicates the formation of structures in the IG‒MA gels conferred the some coarse structures [23,24] corresponding to thixotropic properties of the gels. assembly units such as hydroclusters [12,13] of Because heavy atoms were present, the transient sparsely packed MA-sheathed IG nanotubes. This scattering profiles could be recorded by time-lapse indicates that the hydroclusters always existed and small angle X-ray scattering (SAXS), which requires that the spatial packing density of the sheathed IGs in

Journal of Fiber Science and Technology (JFST), Vol.76, No. 1 (2020) 3 Fig. 3 (a) Time dependence of I(q)atq = 0.1 nm-1 and E in time-lapse SAXS

measurements of a well-aged IG‒MA mixture ([‒Al(OH)2 of IG] = [MA] = 0.08 mol/L) during a gel (solid state)-to-sol (liquid state) transition. These measurements were performed under 0.1 s X-ray irradiation at 0.1 s intervals. (b) Schematic of the gelation and gel-to-sol transition of the IG‒MA mixture (showing fewer rods than in the actual system for simplicity). The IG length distribution [21] is probably responsible for the density inhomogeneity of the connecting points (green circles in (3)) between the hydroclusters in the IG‒ MA gel. Under agitation, these frameworks dissociate into partly assembled hydroclusters. Further details are described in the text. Reproduced with permission from [69]. Copyright 2016, Nature Publishing Group the hydroclusters was unchanged during all resting/ groups located in the middle positions of the sheathed agitating processes. Meanwhile, I(q)atq = 0.1 nm-1 nanotubes, which are densely packed with MA increased (decreased) during gelation (solation) in molecules (Figure 3 b (1)). The inter-sheath hydrogen response to resting (agitating), suggesting that the bonds formed by the non-dissociative outer ‒COOH size of the networks composed of the interconnected groups create cross-bridged nanotubes along with hydroclusters fluctuated with the gelation and intra-sheath (lateral) hydrogen-bonding networks. solation processes. The same phenomena were Meanwhile, the outer ‒COOH groups of the MA reported by other researchers [25]. As summarized in sheath dissociate at the edges of the nanotubes, where Figure 3 a, I(q) (indicating the spatial extension degree the electrostatic repulsion is weakened. When the of a network of interconnected hydroclusters) was intra- or inter-sheath interactions balance the large in the gel state and small in the sol state, dissociation of outer ‒COOH groups at the sheathed although the E value remained at 1.3 during gelation nanotube edges, cross-bridged nanotubes can and solation. The I(q) change was completed within potentially form. These cross-bridged nanotubes 0.4 s of application or removal of agitation, indicating correspond to the hydroclusters (Figure 3b (2)) that the solation and gelation processes were very observed by cryo HAADF‒STEM, EDS, and SAXS rapid; moreover, the gel-to-sol and sol-to-gel [22,26]. These basic hydrocluster units are further transitions were fully reversible. interconnected to form the gel frameworks (Figure 3b Based on the above experimental findings, I (3)). Because the hydroclusters are electrostatically proposed a structural change mechanism of the repulsed as well as hydrogen-bonded, they are thixotropic IG‒MA gel (see Figure 3 b). In the IG‒MA tenuously connected as inhomogeneous gel system, the inevitable electrostatic repulsion frameworks that are easily collapsed at sparsely- suppresses the dissociation of the outer ‒COOH connected points (i.e., the hydrogen bonds between

4 Journal of Fiber Science and Technology (JFST), Vol.76, No. 1 (2020) the hydroclusters) when agitated (Figure 3 b (4)), and mixing ratio of [‒Al(OH)2 in IG]:[MA] = 1:1, with [IG] = are readily re-structured during resting (Figure 3 b 0.08 mol/L (6). Furthermore, reducing the [IG] and

(3)). These thixotropic properties, conferred by the [MA] from 0.08 to 0.04 mol/L (3) decreased the G’0.1 collapse of hydrogen bonds between the and increased the Ttrans. Thus, the viscoelasticity and hydroclusters, presumably cause the shear thinning thixotropic nature of the IG‒MA strongly depend on of the IG‒MA gels shown in Figure 1a. the mixing ratio of the components. 2.3 Kinetics-dominated structure and stimuli- The E values calculated from scattering curves responsiveness of thixotropic IG gel of the IG‒MA are also listed in Table 1. At [IG] = 0.08 This subsection investigates the relationship mol/L, the E value increased with [MA]. As described between the morphology and non-Newtonian previously, the E value was 1.3 at [MA] = 0.08 mol/L, behavior of the IG-dicarboxylic acid (DA) gel, as consistent with loosely packed (i.e., cross-bridged) IG estimated from X-ray scattering and rheological nanotubes [22‒24]. The E value was 1.0 at [MA] = 0.04 measurements of the gel. Here, I reveal the kinetic mol/L, indicating that the IG nanotubes were almost factors that impart a thixotropic character to the monodispersed (with extremely few connections molecular architectures of IG‒DA gels. For the first between them); consequently, the Ttrans was relatively time, I relate the kinetics of the assembly process of high and the G’0.1 was low. In contrast, the E value was rod-like colloids to the macroscopic non-Newtonian 2.0 at [MA] = 0.16 mol/L, indicating that the IG properties based on thermodynamic parameters. To understand the relationship between the viscoelastic nature and composition of the IG‒DA mixture, the bulk mechanical responses of the IG and MA mixtures (IG‒MA) were measured at various concentrations of IG and MA. The IG‒MA became thixotropic at a mixing ratio of [‒Al(OH)2 in IG]:[MA] = 1:1 [22]. The phase state, thixotropic nature, and viscoelasticity of the IG‒MA were evaluated from the period necessary for the completion of gelation Tgel, the solid-to-liquid transition time Ttrans, and the storage modulus G`0.1 under a strain of 0.1%.

As shown in Table 1 (entries 4‒7), the Tgel at [IG] = 0.08 mol/L decreased as the [MA] increased from 0.04 to 0.16 mol/L. At [IG] or [MA] = 0.02 mol/L (1 and 2), the IG‒MA did not form a gel. The Ttrans of the IG‒MA was 3.3 times shorter at [IG] = [MA] = 0.08 mol/L (6) Fig. 4 SAXS curves of the IG‒DA mixture after 1 than at [IG] = 0.08 mol/L and [MA] = 0.04 mol/L (5). At week of aging ([‒Al(OH)2 of IG] = [DA] = [MA] = 0.16 mol/L (7), the IG‒MA yielded hydrogels 0.08 mol/L). Reproduced with permission from that were phase- separated by agitation but did not re [74]. Copyright 2016, the Royal Society of -solidify. The G’0.1 of the IG‒MA was highest at the Chemistry

Table 1 Tgel, Ttrans, G’0.1, and E values of the IG‒MA at various ratios of [IG] and [MA] after 1 week of aging.

[IG] / mol/L [MA] / mol/L Tgel /min Ttrans /sec G’0.1 /Pa E 1 0.02 0.02 No gelation - - N.D. 2 0.02 0.08 No gelation - - N.D. 3 0.04 0.04 600 70 15.8 N.D. 4 0.08 0.02 No gelation - - N.D. 5 0.08 0.04 210 20 26.5 1.0 6 0.08 0.08 53 6 195 1.3 7 0.08 0.16 10 Phase separation 5.09 2.0 8 0.08 0.48 Precipitation* - - N.D. * Hard-gel particles + aqueous fluid.

Journal of Fiber Science and Technology (JFST), Vol.76, No. 1 (2020) 5 nanotubes were packed into bundle-like assemblies in an elasticity change (i.e., assembly of hydroclusters this q region. In this fully lateral packing environment, and their network formation), respectively [22]. the IG nanotubes formed inhomogeneous aggregates Thermodynamic parameters linked to the molecular in the mixture, which precipitated during agitation. assembly of IG‒DA were calculated from the RFDO As shown in the SAXS analyses, the shapes of the curves and are summarized in Tables 3 and 4. hydroclusters in the IG‒MA clearly depended on From the RFDO curves in Figure 5, we can their compositions and affected their viscoelasticities obtain the binding constant (K ) and other and thixotropic natures. thermodynamic parameters of the adhesion of DA to The viscoelastic behaviors and shapes of the IG, hydrocluster formation, hydrocluster assembly, hydroclusters (Figure 4) were investigated in IG‒DA and network formation (see caption of Figure 5 for mixtures containing different DA species: fumaric details). acid (FA, a geometric isomer of MA), citraconic acid As shown in Table 3, the thermodynamic (CTA, 2-methylmaleic acid), and mesaconic acid (MSA; interaction parameters were increasingly correlated

2-methylfumaric acid). This analysis revealed the with the DA-to-IG adhesion (K0d) and hydrocluster detailed relationship between the viscoelastic/ formation (ud) in the following order: IG‒CTA mixture structural properties and the molecular nature of the < IG‒MA mixture < IG‒MSA mixture < IG‒FA IG‒DA mixture. mixture. This trend suggests a strengthening The IG‒FA mixture (entry 2 of Table 2) rapidly interaction among the IG, DA, and IG‒DA nanotubes. gelled and obtained a lower G’0.1 than the other IG‒DA The tendencies of the thermodynamic interaction mixtures. Although FA is a geometric isomer of MA, parameters correlated with the interaction properties the IG‒FA mixture was phase-separated by agitation differed in the between-hydrocluster and within- and did not re-solidify like its IG‒MA counterpart hydrocluster cases (cf. Tables 3 and 4).

(entry 1). The Tgel was longer in the IG‒CTA (3) and The thermodynamic interaction parameters IG‒MSA (4) mixtures than in the IG‒MA and IG‒FA correlated with the interaction between mixtures. The IG‒MSA mixture delivered the hydroclusters (K0p) were much higher in the IG‒MA shortest Ttrans (3 s) and the highest G’0.1 (222 Pa) among and IG‒MSA mixtures than in the IG‒FA and IG‒ the IG‒DA mixtures. The E value of the IG‒DA CTA mixtures. Investigating the relationship mixtures was negatively correlated with G’0.1.In between the E values (Table 2) and K0p values (Table particular, the IG‒FA mixture with E = 2.0 formed 4), we find that the loosely packed hydroclusters (low non-thixotropic hydrogels with low G’0.1. E) interacted quickly (high K0p) owing to their low The shape and hierarchical assembly of critical concentration (i.e., large excluded volume). hydroclusters are assumed to be determined by the The ud and thermodynamic interaction interaction processes between IG and DA. The kinetic parameters correlated with the network formation factors of the interaction between IG and DA have value (up) are related to the formed IG frameworks (i.e., been thermodynamically derived from rigid-body free the hydroclusters formed by IGs and the networks damping oscillation (RFDO) measurements [22,27] of formed by hydroclusters). The frameworks are the an incubating IG‒DA mixture prior to its molecular main components of hydrogels in the IG‒DA mixture. assembly. In the RFDO data, the decrease in The difference between ud and up (Δu) corresponds to a logarithmic damping ratio and the increase in time lag between hydrocluster formation and oscillation period reflected a viscosity change (i.e., network formation in the IG‒DA mixture. The Δu adhesion of DA to IG and hydrocluster formation) and increases in the order of IG‒FA < IG‒CTA < IG‒MA

Table 2 Tgel, Ttrans, G’0.1, and E values of various IG‒DA mixtures with [‒Al(OH)2 in IG] = [DA] = 0.08 mol/L after 1 week of aging.

Species Tgel Ttrans G’0.1 E of DA /min /sec /Pa

1 MA 53 6 195 1.3 2 FA 19 Phase separation 1.19 2.0 3 CTA 300 27 122 1.5 4 MSA 180 3 222 1.2

6 Journal of Fiber Science and Technology (JFST), Vol.76, No. 1 (2020) Fig. 5 Typical RFDO curves of an IG‒DA mixture during the initial gelation stage.

Here, [‒Al(OH)2 of IG] = [DA] = 0.08 mol/L. The RFDO curves give the binding constant (K) and other thermodynamic parameters of the DA-to-IG adhesion, hydrocluster formation, interaction between hydroclusters, and network formation in accordance with a theory in ref 28-31. The binding constant is

given by K = K0u = 1/(Ts)0.5, where K0 is the binding constant of an IG nanotube to DA or of a hydrocluster to another isolated hydrocluster (initiation process),

(Ts)0.5 is the reaction time at β=0.5 (β defines the ratio of average oscillation period or logarithmic damping ratio to its maximum), and u is a cooperative parameter indicating the extra interaction energy between the IG nanotubes sheathed with DA or between the hydroclusters (propagation process). The value of u can be calculated from the slope of the RFDO curves at the half-

< IG‒MSA; that is, Δu is negatively proportional to E evidences, I proposed an assembly process of stimuli- and directly proportional to G’0.1 (see Figure 6). responsive IG‒DA gels. The process is schematized in The linear relationships in Figure 6 indicate that Figure 7. the sheathed IG‒DA nanotubes are stepwise- Here, the stepwise development of the IG assembled into networks, forming loosely connected assemblies, i.e., the fast and slow propagation of frameworks of IG nanotubes, and imbuing the IG‒DA networks relative to the hydrocluster formation, gels with their sharp thixotropic nature (i.e., rapidly results in loosely connected frameworks and stimuli- reversible solid‒liquid transition) and high storage responsive gels. In contrast, the simultaneous modulus. Based on my experimental and analytical development of IG assemblies, i.e., the fast

Table 3 Thermodynamic interaction parameters correlated with the total molecular interactions: viscosity change (Kd), adhesion of DA to IG (K0d), and hydrocluster formation (ud), estimated from the damping-ratio vs. reaction-time curves.

2 2 Species of DA Kd [10 ] K0d ud [10 ] MA 2.4 0.55 4.4 FA 6.7 1.3 5.3 CTA 2.0 0.54 3.7 MSA 4.5 0.95 4.8

Table 4 Thermodynamic interaction parameters correlated with the total molecular interactions: elasticity change (Kp), interaction between hydroclusters (K0p), and network formation (up), estimated from the period vs. reaction-time curves.

2 3 Species of DA Kp [10 ] K0p up [10 ] MA 1.7 14 1.3 FA 5.0 9.1 5.5 CTA 1.7 8.8 2.0 MSA 2.9 18 1.6

Journal of Fiber Science and Technology (JFST), Vol.76, No. 1 (2020) 7 nanotubes are frequently dissociated from the edges of the nanotubes, where the electrostatic repulsion becomes strong (the “repulsion” zone in Figure 7). When the intra- or inter-sheath interactions balance the dissociation of free ‒COOH groups at the IG‒DA nanotube edges, one can envisage cross-bridged nanotubes that correspond to hydroclusters. These basic hydroclustered units are further interconnected to form the gel frameworks. Because the hydroclusters experience both electrostatic repulsion and hydrogen bonding, they are tenuously connected into gel frameworks that easily collapse at the loosely connected points (i.e., the hydrogen bonds between

Fig. 6 Relationship between G’0.1,E,andΔu. The G’0.1 the hydroclusters) when agitated, and subsequently and E are plotted as the red closed and green reform when rested. The collapse of hydrogen bonds opened circles, respectively. Reproduced with between the hydroclusters causes the thixotropy of permission from [74]. Copyright 2016, the the IG‒MA/CTA/MSA mixtures [22]. Royal Society of Chemistry On the other hand, an IG‒FA mixture gives propagation of networks relative to the hydrocluster hydroclusters with a bundled shape and a densely formation, leads to densely connected frameworks packed architecture with no shear-thinning property and gels without stimuli-responsiveness. (which exists in the IG‒MA/‒MSA/‒CTA mixtures). When IG and DA are mixed, the surfaces of the The high tendency for hydrogen bonding between IG nanotubes are fully covered by DA molecules via FA molecules, occurring by strong intermolecular + - interactions such as Al(OH)O H2… OOC-(CH2)n-COOH interactions between the trans-type dicarboxylic (n > 1), as shown in my previous work [22]. In groups in FA, directs the bundling of IG nanotubes in thixotropic IG ‒ DA mixtures, the inevitable the IG‒FA mixture. In mixed IG and MA, MSA or electrostatic repulsion among the densely packed DA CTA, the hydrogen bonding tendency is lowered by molecules on the sheath suppresses the dissociation of the free rotation of cis-type DA groups in the MA/ the free ‒COOH groups (i.e., the ‒COOH groups that CTA molecules, and by the restricted intermolecular did not attach to Al‒OH of IG) located at the middle interaction between trans-type DA groups in the MSA positions of the IG‒DA nanotubes. Such non- molecules (imposed by steric hindrance by the methyl dissociative free ‒COOH groups form inter-sheath group of MSA). These factors produce hydroclusters hydrogen bonds (H-bonding in Figure 7), thereby of IG nanotubes with cross-bridged shapes in the IG‒ cross-bridging the nanotubes and forming intra- DA mixture. sheath ( lateral ) hydrogen - bonding networks. In the IG‒MA/‒MSA/‒CTA mixtures with Meanwhile, the free ‒COOH groups of the IG‒DA subdued hydrogen-bonding potential between the

8 Journal of Fiber Science and Technology (JFST), Vol.76, No. 1 (2020) cross-bridged hydroclusters, the hydroclusters slowly agitating. The present results explain the self- agglomerate into loosely connected networks organization mechanism of rod-like colloidal low- (relatively low u) despite their relatively high collision dimensional compounds with a well - defined frequency (relatively high K0p) and large exclusion morphology, which accords with the thermodynamic volume originating from their cross-bridged shape interaction parameters. The present study also (this state is called the reaction control state). By provides insights into the molecular assembly kinetics, contrast, the IG‒FA mixtures with greater hydrogen- enabling the design of supramolecular architectures bonding potential between the bundled hydroclusters with non-Newtonian properties (e.g., thixotropic form densely connected frameworks. In these hydrogels [54‒56]). mixtures, the hydroclusters aggregate quickly

(relatively high up) despite their relatively low collision 3. APPLICATION OF IONIC LIQUID frequency (relatively low K0p) and small exclusion IG-DA GELS AS QUASI-SOLID volume arising from their bundled shapes (this state ELECTROLYTES defines the diffusion control state). The high tendency for hydrogen bonding in the IG‒FA mixture is also 3.1 Preparation of IG thixotropic gel consisting of observed in the IG‒CTA mixture, in which the ionic liquid rotation of cis-type dicarboxylic groups is sterically Electrolytes are fundamental materials in various hindered by the methyl group. This restriction causes industrial applications such as batteries [57]. Liquid the relatively dense packing of IG nanotubes (i.e., E= electrolytes, although widely used, lack safety and

1.5) and their slow thixotropic behavior (the Ttrans is moldability because they include organic solvents and nine times larger in IG‒CTA (27 s) than in the IG‒ are inherently fluid [58]. Solid electrolytes are MSA mixture (3 s)). promising alternatives with higher safety and The resulting architectures in the IG‒DA moldability than liquid electrolytes [59,60], but their mixture are thus determined by the rate-limiting step ionic conductivity is lower than that of liquid of the interactions between the hydroclusters, which electrolytes. Recently, a gel-type electrolyte [61] that control their non-Newtonian behaviors such as combines the high ionic conductivity of liquid thixotropy. Furthermore, these reaction kinetics seem electrolytes with the safety of solid electrolytes [62,63] to depend on the molecular structure of DA (i.e., its has been reported. However, the gel-type electrolyte potential for hydrogen bonding). The present study needs various specific techniques for molding [52]. In newly reveals the relationship between the this section, I propose a quasi-solid electrolyte that microscopic molecular assembly processes and the overcomes the drawbacks of ordinary electrolytes. macroscopic non-Newtonian nature that accords with The proposed quasi-solid electrolyte is a the extensively discussed thermodynamic parameters thixotropic gel swelled with an ionic liquid (IL). ILs are [32‒45]. molten salts consisting of weakly coordinated cations In this section, I revealed the assembly kinetics of and anions. They are characterized by melting points the molecules in stimuli-responsive hydrogels prepared from IG (a rigid rod-like low-dimensional compound) and DAs. The stepwise development of IG assemblies in the hydrogels usually produces loosely connected IG networks with a fast-responding thixotropic property. The stepwise organization of rod-like colloidal low-dimensional compounds, such as filamentous actin [46] and microtubules [47‒49], has been previously attributed to non-equilibrium conditions during the assembly processes, which promote macroscopically expanded, loosely packed assemblies without inhomogeneous aggregation. Rod- Fig. 8 Photographs, TEM images, and preparation like low-dimensional compounds such as filamentous routes of IG-IL and IG-DA-IL. Reproduced fd virus [50], amyloid fibrils [51], carbon nanotubes [52], with permission from [72]. Copyright 2017, the and microtubules [53] respond similarly to heating and Royal Society of Chemistry

Journal of Fiber Science and Technology (JFST), Vol.76, No. 1 (2020) 9 below 100 ̊C, low vapor pressure, high thermal [MeSO3] solid. The ionic conductivity is greatly stability, and high ionic conductivity [64‒68]. Unlike reduced by interaction of the IL anions with the ordinary liquid/solid/gel electrolytes, thixotropic networked IGs in the IG‒IL solid, which dramatically electrolytes can be freely molded to the shape of the decreases the mobility of the anions. The effect is conduction interface. The proposed thixotropic gel is similar to the diffusion inhibition of anions in a solid a quasi-solid IG electrolyte. polymer electrolyte [70]. In the IG-[EmIm][MeSO3] In Section 2, I explained that mixtures of IG and solid, the ionic conductivity would be remarkably － DAs in water form homogeneous gels of networked decreased by strong interaction of the SO3 with the IGs at certain mixing ratios [22,69]. To apply the IG‒ Al‒OH group in IG [71]. In contrast, the ionic DA gel as a quasi-solid electrolyte, I replaced the conductivity of the IG‒IL liquid is almost identical to water solvent with an IL. After mixing equal amounts that of neat IL. The ionic conductivity depends on the of the as-synthesized IG water dispersion and the IL physical nature of the IG‒IL (solid or liquid), which and evaporating off the water, I obtained a solid-state itself depends on the length of the IG. The dispersed material with an IG: IL weight ratio of 6.4: 93.6, in IG aggregates shown in the TEM image in Figure 8 which the IG units formed a network-like architecture are expected to allow ionic diffusion through the IG‒ (Figure 8; later denoted as IG‒IL solid). The average IL, as occurs in the neat IL. IG length in this formulation exceeded 1.6 µm, so this Following the strategy depicted in Figure 8, I IG is later denoted as long IG. Entanglement of long prepared mixtures of short IG, a DA (MA; FA; CTA;

IGs formed a solidified network of long IGs and IL. MSA), and IL. The molar ratio of [Al(OH)2 and The long IG was miscible with 1-butyl-3- carboxyl group in the DA was maintained at 1:1 in all methylimidazolium tetrafluoroborate ([BmIm][BF4]), 1- mixtures (hereafter denoted as IG‒DA‒IL). Gelation ethyl-3-methylimidazolium tetrafluoroborate ([EmIm] was observed only in the mixture of short IG, MA,

[BF4]), and 1 - ethyl - 3 - methylimidazolium and [BmIm][BF4] or [EmIm][MeSO3]. Mixing other methanesulfonate ( [ EmIm ] [ MeSO3 ]), but was DAs (FA, CTA, MSA) with the short IG and IL immiscible with 1-butyl-3-methylimidazolium bis yielded precipitates of IG and DA in IL. The (trifluoromethanesulfonyl)imide ([BmIm][TFSI]). transparent gelated IG‒MA‒IL showed thixotropic The dispersibility of IG in ILs was also estimated behavior. In rotating rheometric tests, the IG‒MA‒IL by the strategy shown in Figure 8. The IG water gel (sol) transitioned to a sol (gel) when agitated dispersion (6.4 wt/v%; i.e., 0.16 M of ‒Al(OH)2 groups) (rested). Furthermore, the recovery speed of the was mixed with an equal amount of IL. In this elastic modulus (i.e., structure) depended on the IL formulation, the average IG length was 131 nm [26], so species [72]. The solid-to-liquid transition speed this IG is hereafter denoted as short IG. After (structure recovery speed) was much lower in the IG‒ evaporating off the water, the obtained mixture of MA‒IL mixtures than in aq. IG‒MA [22], because the short IG with [BmIm][BF4], [EmIm][BF4], and [EmIm] hydrogen bonds between the individual IGs in the

[MeSO3] (hereafter denoted as IG‒IL liquid) formed a network architecture were fewer than in water. Also, homogeneous transparent dispersion. The water the structure recovery speed is lower in IG‒MA‒ content of the short IGs in [BmIm][BF4] was 0.1871%, [BmIm][BF4] than in IG‒MA‒[EmIm][MeSO3], because suggesting that water contamination of the IG‒IL the high polarity in the former induces strong liquid can be ignored. Interestingly, the short IGs did electrostatic interactions. The mixture of short IG and not disperse in [BmIm][TFSI]. As short IG does not MA in DMSO or DMF did not gelate or precipitate, disperse in methanol and THF but disperses in but formed a transparent liquid composed of densely dimethylsulfoxide (DMSO) and N, N - packed (sheaths) of DMSO/DMF on the surfaces of dimethylformamide (DMF), the miscibility of IG with the IG nanotubes. When the ionic (molecular) size of the solvent can be attributed mainly to the the IL was larger than that of DMSO/DMF, the dense hydrophobicity of the solvent. Transmission electron packing of solvents on the IG surface was prevented, microscopy (TEM) of the IG‒IL liquid (Figure 8) and the MA molecules could attach to the IG surface showed that the short IGs dispersed in the IL, forming to afford the network structure [22]. small bundle-like aggregates. 3.2 Physical characteristics and ionic conductivities IG‒IL solids have lower ionic conductivities than of IG thixotropic gel consisting of IL neat ILs. This tendency typifies the IG-[EmIm] The IG‒MA gel formed in water transformed

10 Journal of Fiber Science and Technology (JFST), Vol.76, No. 1 (2020) Fig. 9 (Left) TEM image of IG‒MA‒[BmIm][BF4] gel (upper) and photographs of the

sample tube inversion test for solid and liquid states of IG‒MA‒[BmIm][BF4] (lower). (Right) Arrhenius plots of ionic conductivity in the IG‒MA‒IL gels. Reproduced with permission from [72]. Copyright 2017, the Royal Society of Chemistry into the sol state above 60 ̊C as the hydrogen bonds Surprisingly, the thixotropic IG‒MA‒IL gels between the IGs were cleaved by the extreme exhibited high ionic conductivities over the 25‒80 ̊C molecular motion of water. In contrast, IG‒MA‒IL range, similarly to neat ILs (Figure 9). This result remained in the gel-state even at 150 ̊C. As no phase- affirms that adding IG and MA as the gelator does not separation peaks (e.g., only peak from IL [72]) disturb the ionic conductivity of the IL. The ionic appeared in the -80‒200 ̊C range of the DSC curve conduction on the MA-sheathed IG nanotubes [69] in [72], I assumed that IG‒MA‒IL maintains a stable the thixotropic gels can raise the ionic conductivity of network structure over a wide temperature range. IG‒MA‒IL. Thus, a thixotropic gel consisting of IG, The gelated IG‒MA‒IL forms a network of IG MA, and IL is a quasi-solid electrolyte with nanotubes, as shown in the TEM image (Figure 9). transparency, moldabilty, thermal stability, and high The aggregation state of IG in IG‒DA‒IL can be ionic conductivity. The shorter IG in IG‒MA‒IL than estimated by synchrotron X-ray scattering. Figure 9 in the IG‒IL solid might improve the mobility of IG confirms a network-like architecture in IG‒MA‒IL, and the anions attached to the IG surface, thereby and a bundle-like architecture in IG‒FA/CTA/MSA‒ increasing the ionic conductivity of IG‒MA‒ILs. IL. The network and bundle architectures of IG and In this section, I described a novel electrolyte DA induce gelation and precipitation of the IG‒DA‒IL, material, specifically, a quasi-solid electrolyte, created respectively. The different manufacturability of by combining an IG thixotropic gel with an IL. By hydrogen bonds between the DA species might cause virtue of its transparency, moldability, thermal the structural differences between the IG substances stability, and high ionic conductivity, the quasi-solid (phase states of IG‒DA‒IL), as described in the above electrolyte is suitable for various applications such as section [74]. Especially, the higher hydrophobicity of freely moldable conductive and anti-icing coatings, FA/CTA/MSA relative to MA should induce more and battery electrolytes. The transparency of the aggregation in the IG and FA/CTA/MSA mixtures quasi-solid electrolyte would prevent damage to the than in IG and MA, causing different phase behaviors base materials. in the IG‒DA‒ILs. The mixed IG and CTA/MSA yielded a homogeneous gel with thixotropic behavior 4. FLOW-INDUCED ORIENTATION OF in water but not in ILs. Because the proton IG THIXOTROPIC HYDROGELS dissociation ratio of -COOH is lower in IL than in water, the hydrogen bonds between the CTA/MSA 4.1 Preparation of IG thixotropic gel with an oriented molecules are stronger in IL than in water, so dense IG structure aggregations (bundles) of IG‒CTA/MSA nanotubes The phase changes during thixotropic solid-to- are expected in ILs. liquid or liquid-to-solid transitions are extremely fast.

Journal of Fiber Science and Technology (JFST), Vol.76, No. 1 (2020) 11 Therefore, after flowing a liquid‒state IG gel through desired X-shape, but the ordinary HEA gel (Figure 10c a cylindrical capillary cell (Figure 10), the recovered right) was deformed by the inevitable inhomogeneity solid-state IG gel exhibited uniform birefringence induced by the network formation. The IGs in the IPN under a polarized optical microscope (POM). Complete gel reduced the inhomogeneity of the HEA network. extinction was observed at 0̊ and 90̊ with respect to Because the IPN method prepares gels with the the flow direction, while maximum brightness was desired shapes, it is expected to provide various types obtained at 45̊ (Figure 10 b). Positive birefringence of soft materials requiring fixed and complicated was also confirmed, indicating that the IG nanofibers shapes, such as artificial cartilages and blood-vessel uniformly aligned parallel to the flow direction, i.e., biomaterials. the flow orientation of IG in the sol state was frozen The flow-orientation properties of IG‒MA gel on the instantaneous gelation state. The uniform mixed with HEA are similar to those of IG‒MA gel alignment of IGs is collapsed by mechanical impacts (Figure 10 b). Because the birefringence was caused by the thixotropic nature of the oriented IG‒ unchanged before and after polymerization of HEA MA gels; the alignment may plausibly be stabilized and TEGDA, the resulting IPN gels maintained the and fixed by introducing interpenetrated networks uniform alignment enforced by the flow orientation [75]. (IPNs) to the IG gel. 4.2 Physical properties of an IPN gel composed of Figure 10 c compares the shaping properties of oriented IGs IPN gels prepared by polymerization of 2- This subsection analyzes the anisotropic hydroxyethyl acrylate (HEA) and tetra(ethylene mechanical properties of a large, self-standing glycol) diacrylate (TEGDA) in the IG‒MA gel. After oriented IPN gel (henceforth denoted as an oriented polymerization in an X-shaped cell and equilibration IPN gel). The stress‒strain curves of the gel (with in water, the IPN gel (Figure 10c left) maintained the dimensions as large as 42 mm × 71 mm × 2.0 mm, see

Fig. 10 (a) Schematic of the experimental setup of the flow-orientation capillary cell system. (b) POM images of the oriented IG‒MA gel in the capillary cell. The A and P axes represent the setting directions of the analyzer and polarizer, respectively. The angle between the gel and analyzer is shown in the lower left part of (b). Blue lines in (b) delineate the capillary glass-wall positions. (c) Photographs of IPN gel (water content = 92.4%) and HEA gel as a reference (water content = 96.8%) prepared by in situ polymerization in an X-shaped cell and subsequent equilibration in water for 7 d. Reproduced with permission from [69]. Copyright 2016, Nature Publishing Group

12 Journal of Fiber Science and Technology (JFST), Vol.76, No. 1 (2020) Fig. 11 (a) Photographs (upper) and POM images (lower) of the oriented IPN gels prepared in a rectangular cell ([42 × 71 × 2] mm3). Background illustrations of the POM images correspond to the oriented IPN gels shown in the photographs. The birefringence Δn of the oriented IPN gels grew to 1.64 × 10-4 at flow rate v = 34.9 mL/min. (b) Typical tensile stress‒strain curves of wet-state oriented IPN gel under parallel and perpendicular stretching with respect to the IG orientation. Reproduced with permission from [69]. Copyright 2016, Nature Publishing Group

Figure 11 a) are plotted in Figure 11 b. The maximum values of the equivalent circuits are summarized in fracture stress and initial tangent corresponding to Table 5. Z1 is the DCE derived from proton conduction the Young’s modulus were ~1.5 and 3 times larger between the IG and HEA networks [78], and Z2 is the under parallel elongation (with respect to the IG DCE derived from proton conduction through the orientation) than under perpendicular elongation. The hydrogen bridges among the protonated aluminol maximum fracture stress of the IPN gel elongated groups on the IG surface, and from the carboxylic parallel to the IG orientation was 139 kPa. groups in the MA molecules attached to IG [79]. The + The current‒voltage characteristics of the hydrogen bridges form along the Al(OH)O H2… oriented IPN gel, measured by the AC impedance bonding motif of IG, and the contributing carboxyl method, were anisotropic. The complex impedance groups are the outer ‒COOH groups of the MA plane plots (Cole‒Cole plots) [76] of the oriented IPN sheath. gel depended on the direction of the applied electric The Rs and Z1 values were almost independent of field relative to the IG orientation (Figure 12). These the direction of the electric field. In contrast, the

Cole‒Cole plots were separable into three components capacitance (C2) and resistance (R2)ofZ2 were 1.5 and with different equivalent circuits (see Figure 12 b). 0.5 times higher, respectively, when the electric field

Here, Rs is the high-frequency limiting resistance was applied parallel to the IG orientation than when corresponding to the resistance of the liquid phase, the field was perpendicular to the IG orientation; i.e., which consists of water and a small amount of free (C2//)/(C2┴) 㾮 1.5 and (R2//)/(R2┴) 㾮 0.5, where // and ┴ MA molecules [76]. The other two circuits are parallel indicate the parallel and perpendicular directions,

RC circuits with impedances of Z1 and Z2, respectively respectively. Owing to the hydrogen bonds aligned

(DCE) [77]. The estimated resistance and capacitance along the IG long axis, the C2 was enlarged by the Table 5 Conductance C (F) and resistance R (Ω) of oriented IPN gels, estimated from the equivalent circuits shown in Figure 12b.

Rs Z1 Z2

3 /10 C1 R1 C2 R2 /1011 /103 /109 /103 // 2.59 5.23 3.00 7.95 0.363

┴ 2.34 5.74 3.57 5.27 0.684

// /┴ 1.1 0.9 0.9 1.5 0.5

a) Symbols // and ┴ indicate that the electric field was applied parallel and perpendicular to the orientation of the IG‒MA nanofibers, respectively.

Journal of Fiber Science and Technology (JFST), Vol.76, No. 1 (2020) 13 large synchronized dipole under the parallel electric macromolecules [83‒88]. More specifically, these field; accordingly, (C2//)>(C2┴). Furthermore, the molecules assemble covalently or non-covalently into hydrogen bonds aligned along the IG long axis helical supramolecules, which may be gels, liquid enhanced the proton conduction and reduced the R2 crystals, or bulk crystals [89]. However, obtaining value under the parallel electric field, so R2// < R2┴. defect-free, macroscopically continuous hierarchical Consequently, the unidirectional alignment of the IG ordering is a challenging task. In this section, I explain nanofibers provided anisotropic proton conduction how chiral linkage induces millimeter - scale properties through the lateral hydrogen bridges on continuous hierarchical helical ordering of IG achiral the sheath. nanotubes in their thixotropic gel. An achiral IG nanotube is formed by symmetric 5. HELICAL ASSEMBLIES OF IGs helical alignment of its constituent molecules (i.e., gibbsite sheets). The aluminol groups on the IG Chiral molecules realize one-handed molecular surface are helically arranged with a 55̊ tilt from the ordering that reflects their configuration. Chiral long axis of the IG (see Figure 13) [15]. Therefore, linkages of materials are especially desired in when a DA with asymmetric carbons interacts with hierarchical architectures [80‒82]. On the molecular IG, it can induce one-handed helical alignment of DA level, the helical handedness corresponds to the on the aluminol groups of IG, encouraging the linkage between chiral monomers and hierarchical ordering (e.g., spiral architectures) of IG nanotubes in the oriented IG‒DA gel. Assuming the above hypothesis, I designed oriented IG‒DA gels composed of chiral DAs such as malic acid. The alignment of aluminol groups on the IG surface is shown in the red square of Figure 13. Chiral linkage (i. e., chiral DA) should induce one-handed helical alignment of the IG nanotubes, as shown in the bottom right of Figure 13. IG and chiral DAs such as malic acid and tartaric acid (hereafter denoted as MaA and TA, respectively),

mixed at an ‒Al(OH)2: DA molar ratio of 1:1, formed a thixotropic gel after ~20 min of aging. These mixtures also exhibited birefringence. IG mixed with succinic acid (SA), an achiral DA with a similar molecular structure to MaA, exhibited the same properties as Fig. 12 (a) Cole‒Cole plots of oriented IPN gel the IG and MaA mixture. measured under an electric field applied The vibrational circular dichroism (VCD) spectra parallel (circles) or perpendicular (squares) to the IG orientation. (b) Equivalent circuits of (Figure 14) reveal the chirality of the IG‒MaA gel. the oriented IPN gel. Reproduced with The peaks in the spectra originated from the permission from [69]. Copyright 2016, Nature hydroxyl groups in the MaA of the gel. These peaks Publishing Group were absent in the VCD spectra of the individual gel

14 Journal of Fiber Science and Technology (JFST), Vol.76, No. 1 (2020) flow-orientation capillary cell system described in Figure 10 a. POM images of the chiral gels, obtained at an analyzer angle of 0̊, revealed contiunuous strip- like patterns such as pitch bands, which originate from helical structures in various liquid crystals [91]. These millimeter-scale bands appeared at both the left and right halves of the cylindrical capillary cell of

the oriented IG‒DMaA gel. This result confirms a

Fig. 14 Vibrational circular dichroism spectra of the helical arrangement of the IG‒DMaA nanotubes in the

IG‒DMaA gel (blue circles), IG‒LMaA gels (red gels. In Figure 16 a, this arrangement appears as a circles), and IG water dispersion (black plots). chiral smectic C or a cholesteric liquid crystalline The arrows indicate that the peak originated phase. The cell contained a distinct (white) centerline from the hydroxyl groups in the MaA region, which developed uniformly in all regions of the molecules. Reproduced with permission from [90]. Copyright 2016, American Chemical cell when the analyzer angle was increased to +45̊. In Society contrast, the oriented IG‒SA gel exhibited a uniform sand-like texture at both analyzer angles (Figure 16 b). components (namely, the IG water dispersion and According to these POM results, the chirality of the MaA solution). The chirality change in MaA IG‒DA gels induced millimeter-scale regulated manifested as a peak inversion (i.e., the peaks became ordering and uniform alignment of the IG nanotubes. valleys), indicating reversed handedness of the IG‒D

MaA and IG‒LMaA gels. To determine the inner structure of the IG‒MaA gel, I collected TEM images of gels prepared by the freeze-edging replica method. The TEM images revealed the one-handed helical architecture (Figure

15 a) of the IG nanotubes in the IG‒DMaA gel. As this architecture was not observed in the IG‒SA gel (Figure 15 b), its formation was attributed to interaction between the IG nanotubes and DMaA. In addition, the chirality of DMaA should impose a right- handed architecture of the DMaA-sheathed IG Fig. 16 POM images of the oriented (a) IG‒DMaA and nanotubes (hereafter denoted as IG‒DMaA nanotubes). (b) IG‒SA gels. The A and P axes denote the The one-handed helical architecture shown in the directions of the analyzer and polarizer TEM image is expected to induce a hierarchical settings, respectively. The shear flow was chirality-based ordering of the IG nanotubes through applied from right to left on the 0̊ images. their flow orientation in the IG‒DA gel. To test this Reproduced with permission from [90]. idea, oriented IG-chiral DA gels were prepared by the Copyright 2016, American Chemical Society

Fig. 15 TEM images of the (a) IG‒DMaA and (b) IG‒SA gels, obtained by the freeze- edging replica method. The scale bars are 50 nm. Below the TEM images are corresponding schematics of the IG nanotubes (yellow lines) and helical pitch- like structures (green loops). Reproduced with permission from [90]. Copyright 2016, American Chemical Society

Journal of Fiber Science and Technology (JFST), Vol.76, No. 1 (2020) 15 The oriented IG‒DA gels composed of LMaA or D/LTA the equatorial lines, a Λ-shaped alignment of the IG exhibited similar optical textures to those of the nanotubes appeared in the front view of the oriented oriented IG‒DMaA gel. The striation separations IG‒DMaA gel. From these SAXS images, the IG was (pitch bands) were smaller in the oriented IG‒MaA inferred to be aligned as shown in Figure 18 b. gel than in the oriented IG-TA gel (4.6 µm in IG‒DMaA The TEM (Figure 15), POM (Figure 16), and versus 3.3 µm in IG-DTA). The different chemical SAXS (Figure 17) results can be summarized as structures of MaA and TA (e.g., different number of follows: carbon atoms in their functional groups) should 1. TEM images of the IG‒DMaA gels revealed control the helical pitches in their oriented IG‒DA helically structured chiral nanotubes (i.e., IG‒DMaA gels. nanotubes).

To reveal the fine details of the IG‒MaA gel 2. SAXS images of the oriented IG‒DMaA gels in structure, the gel sample of diameter 1.0 mm was the cylindrical capillary cell revealed that the IG‒D subjected to SAXS measurements using a microbeam MaA nanotubes were uniformly aligned parallel to the of diameter 0.0034 mm (see Figure 16). Figure 17 a long axis of the cell, with a symmetric inclination to shows the anisotropic scattering patterns along the the cell central line (i.e., a Λ-shaped alignment). equatorial-line direction, measured by irradiating the 3. POM images of the oriented IG‒DMaA gels in X-ray beam at different positons of the gel. The the cylindrical capillary cell revealed optical textures, results clarify that the IG nanotubes were aligned in such as contiunuous strip-like patterns indicating a the long-axis direction of the gels. Furthermore, when millimeter-order helical arrangement of IG‒DMaA the anisotropic scattering patterns were tilted from nanotubes in the gels.

Based on these findings, I surmise that IG‒DMaA

nanotubes in the oriented IG‒DMaA gel formed a giant spiral architecture governed by the handedness

of the IG‒DMaA gel, which was revealed in the VCD spectrum (Figure 14). Furthermore, the chiral inversion of MaA (i.e.,

Fig. 17 (a,b) Typical two-dimensional scattering images at various positions of the ordered IG

alignments in oriented (a) IG‒DMaA and (b) IG

‒LMaA gels. The top-left image is overlaid with a schematic of the IG, in which the nanotube alignment corrresponds to anisotropic scattering. The numbers in colored font give the intervals between the Fig. 18 Schematic of IG ordering in the oriented IG‒ SAXS measurement points. (c,d) Schematics MaA gel. (a) Some of the achiral helically showing the ordered alignments of IGs in the aligned hydroxyl groups on the IG nanotube

(c) IG‒DMaA and (d) IG‒LMaA gels, surface. (b) One-handed alignment of MaA determined from the SAXS images. The hydroxyl groups on the IG surface; this shear flow was applied in the upper direction alignment depends on the chirality of the of figures. The red and blue lines correspond MaA (upper part, in boxes) and the giant to the equatorial and meridional lines, spiral alignment of the helically structured respectively. Reproduced with permission IG‒MaA nanotubes (lower part). Reproduced from [90]. Copyright 2016, American with permission from [90]. Copyright 2016, Chemical Society American Chemical Society

16 Journal of Fiber Science and Technology (JFST), Vol.76, No. 1 (2020) conversion of DMaA to LMaA) caused an unexpected chirality of the linkage molecule. A self-standing IPN alignment change of the IG‒MaA nanotubes from the gel with an ordered structure can be prepared by Λ-shape to a V-shape (see Figure 17 c and d). The polymerizing pre - impregnated monomers, as nanotube ordering in the oriented IG-chiral DA gel is described in previous sections. Therefore, the schematized in Figure 18. presented chirality-regulated architectures are The aluminol groups were helically arranged on potentially applicable to chiral sensing, chiral the IG surface (Figure 18 a) [15]. When the MaA resolution, lightwave electronics, and similar molecules interacted with the aluminol groups of the technologies. IG, their carboxyl groups exhibited one-handed helical alignment, depending on the chirality of MaA. 6. ROBUST HYDROGELS COMPOSED OF

Consequently, the carboxyl groups in DMaA and IG AND ORGANIC POLYMERS

LMaA pointed in the right- and left-handed directions, respectively, on the IG surface (boxed upper parts in 6.1 Synthesis of IG gels composed of various Figure 18 b). This structural character of the IG‒MaA organic polymers nanotubes induced the direction-driven giant spiral As shown in Section 4, IG combined with an alignment of the IG nanotubes through their flow organic polymer network (i.e., HEA gel) yielded a self- orientation in the IG‒MaA gel. This alignment standing hydrogel. Motivated by this result, I demonstrates a chirality - reflected hierarchical synthesized robust hydrogels from IG and various ordering, governed by the handedness of the IG‒MaA organic polymers: poly(acrylic acid) (PAA), poly(HEA), gel (lower part of Figure 18 b). and poly(acrylamide) (PAAm). The hydrogel In this section, I described the macroscopic formation depended on the interactions between IG hierarchical ordering (i.e., millimeter-scale spiral and the organic polymers. architectures) of achiral IG nanotubes. In this The phase states of the mixed IG and organic investigation, I introduced chiral linkage and polymers are shown in Figure 19 a. Different mixing subsequent flow shearing into thixotropic IG gels. The methods of the IG and polymers obtained different millimeter-scale regularity of the nanotube alignment phase states of the mixtures. For example, the (i.e., the supramolecular chirality) is apparently combination of preformed PAAm (4.0 mol/L) and IG controlled by the handedness of the nanotube-linkage (5.0 wt/v%) did not form a gel, although PAAm units complex, which depends on the angstrom-scale and IG nanotubes are expected to interact through

Fig. 19 (a) Phase states of mixtures of IG (5.0 w/v%) and polymers. (b) Typical tensile stress‒strain curves of in situ polymerized (1) IG gels and (2) their reference gels. AAX , AAmX , and HEAX respectively indicate the gels composed of X mol/L of AA, AAm and HEA and 0.1 mol% of TEGDA per vinyl monomer. Percentages on the plots indicate the degrees of swelling. Reproduced with permission from [69]. Copyright 2016, Nature Publishing Group

Journal of Fiber Science and Technology (JFST), Vol.76, No. 1 (2020) 17 their -NH2 or >C=O and Si(OH) groups, respectively and 83%, respectively. [92]. It appears that PAAm did not interact with the The breakdown stress of the IG‒AA gel was outer walls of IG, meaning that macromolecular much higher than that of the IG‒AAm gel. Strong strands can rarely penetrate the hollow structure of anchoring is possible only when most aluminol group the IG nanotubes. Based on these observations, I draw on the outer wall of IG interacts with an AA molecule; the following two conclusions: the theoretical limit of such anchoring is 0.83 mol/L 1) Gels exhibit high mechanical strength because out of the existing 4.0 mol/L. In-situ polymerization monomeric AAm molecules can penetrate the hollows produced polymerized sheaths, from which the PAA and then polymerize. strands grew outward by consuming the remaining 2) A few PAAm strands can be anchored to the 3.17 mol/L portion. The sheaths then became mouth of an IG nanotube, creating pseudo- crosslinked by radical recombination of the crosslinking. Such pseudo-crosslinking might confer propagating PAA strands. an elastic property of the IG‒PAAm gel under large Although polymer sheaths also formed in the IG‒ deformation. HEA gel, the breakdown stress and strain of this gel IG combined with preformed PHEA solution were much lower than those of the IG‒AA gel. In the yielded a homogeneous mixture of [IG] (5.0 wt/v%) absence of IG nanotubes, ordinary AA gel is and [PHEA] (1 mol/L). When added at higher mechanically weaker than ordinary HEA gel, concentrations, the PHEA solution formed local hard indicating that the physical properties of IG gels gel-like precipitates indicating sufficient interactions strongly depend on the molecular structures of their between IG and PHEA. Further investigation by in- constituent organic polymers. Both the IG‒AA and IG situ polymerization revealed that macroscopically ‒HEA systems exhibited strong mechanical-strength homogeneous gels formed at HEA concentrations up variations, owing to more interaction points between to 4.0 mol/L. IG and the organic polymer than in the IG‒AAm gel. Poly(carboxylic acid)s are known to interact with Whereas both the C=O and COOH groups of PAA the outer Al(OH)2 groups of IG by hydrogen bonding interact with Al‒OH [93] on IG in the IG‒AA gel, only and related polar interactions [22,93]. Such strong the OH group of PHEA interacts with Al‒OH on IG in interactions cause instantaneous phase separation the IG‒HEA gel. It seems that the number of and the formation of hard solid particles when IG is interacting groups between IG and the constituent mixed with preformed poly(acrylic acid) (PAA) in the organic polymer determines the mechanical concentrated state. However, when IG (5.0 wt/v%) properties of IG‒organic polymer gels. was mixed with monomeric AA (4.0 mol/L), it yielded The physical properties of IG gels are thus a clear, homogeneous solution, and clear, largely influenced by the interaction manner (tying or macroscopically homogeneous gels were obtained sheathing) between IG and the organic polymer. after subsequent in-situ polymerization. When the polymer is tied to the IG nanotubes (IG‒ 6.2 Mechanical strength of IG gels composed of AAm system), stretchable IG gels are formed; in various organic polymers contrast, sheathing the polymer on the IG nanotubes Figure 19 b plots the stress‒strain curves of the (IG‒AA and IG‒HEA systems) yields stiff IG gels. The IG‒AAm, IG‒HEA, and IG‒AA gels estimated from stiffness depends on the strength of the IG‒organic the tensile test results. The IG gels were tough and polymer interaction. their breakdown stresses and strains and Young’s 6.3 Strain-induced reversible isotropic‒anisotropic moduli greatly depended on their vinyl monomer structural transition in IG gels composed of types. The IG‒AAm gel exhibited a larger breakdown various organic polymers strain than the other two gels, implying that this gel The hydrogels composed of IG and organic was more elastic than the other gels. This elasticity polymers were birefringent only upon stretching. was attributed to fewer pseudo-crosslinking IG-to-IG Such ordering of IG (Figure 20 a) was reversible tying points than were formed on the outer sidewall during stretching and releasing cycles of tensile anchors in IG‒HEA and IG‒AA. This tying difference strains below the gel breakdown point. The was also reflected in the spatial densities and degrees birefringence Δn of the IG‒AAm gel ([AAm] = 2.0 mol of swelling; the water contents of the swollen IG‒ /L) is plotted as a function of strain in Figure 20 b. AAm, IG‒HEA, and IG‒AA gels were 93‒94%, 86%, The Δn of the IG‒AAm gel increased with tensile

18 Journal of Fiber Science and Technology (JFST), Vol.76, No. 1 (2020) strain up to 3.0 mm/mm. At this point, the Δn of the Δn‒strain curve of the IG‒AAm gel in Figure exceeded 5.3 × 10－5. The IG‒AAm gel was optically 20 b was the sum of the positive and negative isotropic when unstretched (strain = 0), but became birefringence effects [98] derived from the ordering of positively birefringent at strains between 0.5 and ~8.5 the stretched IG filaments and PAAm strands. Other mm/mm as the elongation oriented the IG molecules. robust IG hydrogels (IG‒AA/HEA gels) also show After further elongation, the birefringence became reversible Δn changes under strain cycles [26]. negative (see the optical retardation photographs in Figure 20 b). As elucidated from the POM and optical 7. CONCLUSIONS retardation images [21], IGs in pure water showed an optically positive birefringence, whereas PAAm is In this review, I described stimuli-responsive known to become negatively birefringent after materials based on a rigid rod-like clay mineral, elongation-induced molecular orientation [94‒97]. imogolite (IG). The obtained materials exhibited Even in ordinary crosslinked PAAm gel without IG, thixotropy under mechanical shock [22], physical the birefringence monotonously decreased from zero anisotropy due to their oriented structure [75,90], and into the negative region with increasing strain (black reversible isotropic‒anisotropic structural transition plots in Figure 20 b). Therefore, the bell-shaped profile under strain [21,26]. Furthermore, when combined with DA IL, IG yielded thixotropic gels with high ionic conductivity [72]. Adequately designed interactions between IG and various organic molecules achieved materials with various stimuli-responsiveness. The supramolecular assembling methods described in this study might be applicable to the design of other hybrid materials containing rod-like polymers [53]. The presented experimental system yields structure- derived functional materials based on nanofiber assemblies. This system will provide a basal knowledge for industrial/biomaterial applications of supramolecular architectures such as mechno- chemical sensors, artificial biological tissue valves, and similar complex structures.

ACKNOWLEDGMENT

I am deeply grateful to Professor Emeritus Kiyotaka Shigehara of Tokyo University of Agriculture and Technology, Professor Emeritus Yoshihito Osada of Hokkaido University (RIKEN) for their appropriate guidance and to all the collaborators for developing the studies. This work was financially Fig. 20 (a) POM images and two-dimensional supported by a grant for JSPS KAKENHI Grant SAXS patterns of IG‒AAm gel under a Numbers 26870179, 2555055, 18 K 05252, the AIST H strain of 3.0, under no strain, and after release from the 3.0 strain. (b) Strain 29 Exploratory Research (SMACTIVE), and the JGC-S responses of Δn in IG‒AAm gel without Scholarship Foundation (No. 1335). The synchrotron crosslinker (red plot), and ordinary AAm radiation experiments were performed at BL 40 B 2 gel (black plot). Measurements were and BL 45 XU in SPring-8 with the approval of the carried out until the gel fractured. Inserted Japan Synchrotron Radiation Research Institute images are optical retardations (L = 530 (JASRI) (Proposal No. 2011 A 1116, 2011 B 1407, and nm) of IG‒AAm gel under strain = 0 (A), 4.0 (B), 9.0 (C), and 10.0 (D). Reproduced 2012 B 1140). with permission from [69]. Copyright 2016, Nature Publishing Group

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