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Stimuli-Responsive Soft Materials Consisting of Cylindrical Inorganic Polymer

Kazuhiro Shikinaka Graduate School of Engineering, Tokyo University of Agriculture and Technology

Introduction
Non-Newtonian fluid behavior such as shear thinning are important in many industrial and natural processes.[1] Shear thinning or stimuli-responsive liquid/solid phase transitions, known as ‘thixotropy’, are often found in clay suspensions, paints, ceramic sols, muscle, and protoplasm. It has been predicted that the formation of assemblies of colloidal particles, generally called hydroclusters, is responsible for the emergence of shear thinning. However, there is still no direct evidence for the relationship between the structural transitions (e.g., formation of hydroclusters and/or continuous networks) and the shear thinning. Here I found the gelation behavior of mixture prepared from a rigid cylindrical inorganic polymer ‘imogolite (IG)’ and a dicarboxylic acid. The obtained hydrogel exhibited thixotropy in response to mechanical shock within the order of seconds or sub-seconds. IG is a single-walled alumino-silicate type rigid nanotube polyelectrolyte with the formula of (HO)3Al2O3SiOH and a well-defined size.[2] The external and internal diameters are approximately 2 and 1 nm, respectively. In contrast, their length ranges from several ten-nanometers to several micrometers. The outer and inner surfaces of IG are covered with aluminol and silanol groups, respectively, where protonation–deprotonation equilibria such as [outer surface] Al(OH)2 + H+ ↔ Al(OH)O+H2 and [inner surface] Si–OH↔ Si–O− + H+ occur. Because of these protonation–deprotonation behavior, IG possesses a point of zero net charge of about 4.0, and the dispersibility of IG in water strongly depends on the pH and ionic strength. That is, when dissolved in neutral to acidic and relatively low-ionic strength aqueous media, IG gives transparent or opaque solutions containing mono-disperse nanotubes or thin bundles. In this study, the structural origin of thixotropic phenomena in the hydrogel composed of IG was directly revealed by estimation of dynamic structural transition of the gel through the latest structural/rheological characterization techniques. Our results are the first to show the relationship between the microscopic structural change and the macroscopic Non-Newtonian fluid behavior. An application of the thixotropic hydrogel for anisotropic materials was also demonstrated.

 

Results and discussion
Here, a stimuli-responsive (i.e., thixotropic) hydrogels prepared from IG nanotube and dicarboxylic acids (DAs) are described (Figure).[3] Due to network structure of IGs connected by DAs, the hydrogel exhibited thixotropy in response to mechanical shock that could be liquefied and re-solidified reversibly by shaking and standing, respectively, within subseconds. The recovery of elastic modulus (i.e., structure) was nearly perfect. The 1:1 combination of IG and Das with 4–6 main chain carbons similarly formed thixotropic gels. Note that the 1:1 mixtures of IG and monocarboxylic acids or dicarboxylic diamides produced fluid solutions, and the combination with dicarboxylic monoamides resulted in phase separation. When IG was combined with oxalic or malonic acid at a ratio of 1:1, the mixture instantaneously formed turbid hard-gel particles dispersed in aqueous solution.


Figure Thixotropic gel consisted of IG and DA

Using the latest structural/rheological characterization techniques, the relationship between the structural transition processes and the fast thixotropy was estimated. The evidence obtained by the experiments revealed the direct relationship between the microscopic structural change and the macroscopic thixotropic behavior. The thixotropic hydrogel has the hierarchical architecture in the combination of IG and DA, i.e., sheathed nanotubes/hydroclusters of cross-bridged nanotubes/frameworks. The formation and disintegration of the network structure upon resting and agitating, respectively, were the origin of gel/sol transition (thixotropy), although the hydroclusters of crossbridged nanotubes were maintained throughout the transition (Figure).Furthermore, after flow-orienting and subsequent standing the sol-state mixture, the uniaxial alignments of IG nanotubes in the centimeter scale were realized in the recovered gel.[4] The degree of orientation of IG was noticed to be dependent on the flow velocity of the sol-state mixture. The self-standing interpenetrated network (IPN) gels were also prepared by the in situ polymerization of the uniaxially oriented gels that were pre-impregnated by vinyl monomer and a cross-linker. The confinement of the IG nanotubes with the uniaxial orientation induced some anisotropic physical properties to the gels such as anisotropic birefringence, mechanical strength and electrochemical characteristics. The specific abilities of the IG thixotropic gel, such as macroscopic supramolecular chiral ordering of the IG nanotubes [5] via combination with chiral DAs, also indicate significant potential for the use of these thixotropic gel as chiral sensing materials, multi-stimuli-responsive actuators, etc.
AcknowledgmentI thank Prof. Dr. Yoshihito Osada (RIKEN), Prof. Dr. Kiyotaka Shigehara, and Dr. Shinya Kajita (Tokyo University of Agriculture and Technology) for their kindly assistance. The synchrotron radiation experiments were performed atBL45XU in SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI). This work was financially supported by grants from JSPS KAKENHI and the JGC-S Scholarship Foundation.

 

References
[1] Larson, R. G., The Structure and Rheology of Complex Fluids, Oxford University Press, Oxford, UK 1999
[2] Donkai, N. ; Inagaki, H. ; Kajiwara, K. ; Urakawa, H. ; Schmidt, M., Makromol. Chem., 186, 2623 (1985)
[3] Shikinaka, K. ; Kaneda, K. ; Mori, S. ; Maki, T. ; Masunaga, H. ; Osada, Y. ; Shigehara, K., Small, 10, 1813 (2014) ;Shikinaka, K., Polym. J., 48(6), 689 (2016) ; Shikinaka, K. ; Mori, S. ; Shigehara, K. ; Masunaga, H. ; Sakai. H., RSC Adv.,6, 52950 (2016)
[4] Kaneda, K. ; Uematsu, K. ; Masunaga, H. ; Tominaga, Y. ; Shigehara, K. ; Shikinaka, K., Sen’I Gakkaishi, 70(7), 137(2014)
[5] Shikinaka, K. ; Kikuchi, H. ; Maki, T. ; Shigehara, K. ; Masunaga, H. ; Sato, H., Langmuir, 32, 3665 (2016).