A Review on Layered Mineral Nanosheets Intercalated With

DOI: 10.1002/macp.201800142

Article type: Trend

A Review on Layered Mineral Nanosheets Intercalated with

Hydrophobic/Hydrophilic Polymers and their Applications

Danial Sangian*, Sina Naficy*, Fariba Dehghani and Yusuke Yamauchi

Dr. D. Sangian

International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan

Email: [email protected]

Dr. S. Naficy, F. Dehghani

School of Chemical and Biomolecular Engineering, The University of Sydney, Sydney, NSW 2006, Australia

Email: [email protected]

Y. Yamauchi

School of Chemical Engineering, The University of Queensland, Brisbane, QLD 4072, Australia

This is the author manuscript accepted for publication and has undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as doi: 10.1002/macp.201800142.

Keywords: 2D nanosheets, inorganic layered materials, intercalation, exfoliation, nano-actuators.

Abstract

Materials with layered structures at the nanoscale have lately drawn significant attention from engineers and scientists in the fields of physics, chemistry, and mathematics, due to the unique characteristics that originate from their hierarchical structure. The nano-sized free space between each two adjacent layers, so-called the interlayer space, is an attractive place that can be manipulated by incorporating different molecular species to generate novel physical and mechanical behaviors. This review highlights the latest studies and new important developments on possible methods of intercalating popular species such as hydrophobic and hydrophilic polymers into the interlayer spaces of layered materials. It also provides a description of intercalation processes as well as final applications for better understanding as it is believed to be an effective factor in utilizing these materials in research and industry. Finally, this review gives perspectives on the future applicants of inorganic layered materials filled with polymers with different hydrophilicity properties.

1. Introduction Nanomaterials are a growing class of materials with at least one discrete dimension in nanometer scale 1-6. The nanoscale dimensionality of nanomaterials results in new properties that are remarkably different to those at bulk scales 7, such as metallic electrical conductivity in carbon nanotubes, excellent thermal conductivity in boron nitride nanosheets, significantly lower melting temperature and red color in gold nanoparticles and higher solar absorption in photovoltaic cells 8-9. Based on their unique characteristics, nanomaterials have been proposed for numerous task-specific

applications such as cosmetics, intelligent textiles, smart food packaging, controlled drug and gene delivery, tissue engineering, and highly efficient catalysts 10-16.

In nanoscale size, the proportion of atoms located on the surface area of a nanoparticle to the total volume of atoms constructing the nanoparticle is considerably high 17. This high exposure of functional groups and atoms to the environment, which does not exist in macro scales, appears to have a strong impact on the properties of nanomaterials. Various species, from small molecules to large macromolecules, can interact with the functional groups at the surface of nanoparticles. The nature of these interfacial interactions and the type of guest species play important roles in determining the bulk properties of the hybrid systems. However, there are few examples available in which the full characteristics of nanomaterials arisen from their nanoscale structure have been adequately realized. The dilemma here is that while nanomaterials on their own offer unique properties, the nanocomposites made by them do not inherit such characteristics. As such, the key point in creation of new systems based on nanomaterials is to develop methods of fabrication that are capable of translating the nanoscale structural, physical, chemical or biological specifications of nanomaterials from nanometer scale to macroscale 18-20.

Nanomaterials are divided into three major groups based on their molecular structure (Figure 1). When only one dimension of the nanomaterials is within the nanometer range, the particles are layered and referred to as two-dimensional (2D) nanomaterials. The 2D nanomaterials are particularly of interest since they provide the highest exposure of atoms and functional groups per surface area. Graphene, hexagonal boron nitride (hBN), diﬀerent transition metal chacolgenides (TMCs), MXenes and MAX phases belong to this category of nanomaterials 21. Because of their high aspect ratio in two dimensions, the 2D nanomaterials offer unique planar properties including excellent thermal conductivity 22, electrical conductivity 23, and charge carrier mobilities in their planes 24, as well as high mechanical flexibility, and high optical and UV adsorptions 25. 1D nanomaterials (e.g. carbon nanotubes and silver nanowires) have two dimensions in nanometer scale 26. Similarly, 0D nanomaterials, such as quantum dots, are discrete nanoparticles when all their three dimensions are limited to nanometer scale. These 0D nanomaterials are also referred to as “isodimensional” materials 27. Dimensionality of materials plays a pivotal role in determining the properties of nanomaterials: for instance, 0D fullerenes (clusters) 28, 1D nanotubes 29, 2D graphene 30-31 and 3D graphite 32 offer significantly different properties due to their different dimensionality.

In this review, we focus on 2D nanolayered minerals as the most abundant and cost-effective class of nanoparticles. In addition to their availability, 2D mineral nanoparticles can offer unique engineering properties such as high surface reactivity, high adsorption, versatile chemical structure, high chemical resistivity, and processabillity. They often bring significant reinforcements into various properties of virgin organic networks such as polymers after physical intercalation of long polymer chains. These reinforcements in polymer science can include high moduli, increased strength and heat resistance, decrease gas permeability and flammability and biodegradability enhancement. We acknowledge that several excellent reviews33-35 have already been published on layered mineral nanosheets intercalated with organic species, which have broadened the understanding of this area of science. Therefore, in this review we mostly focus on the hydrophilicity behavior and final applications of these materials.

2. 2D Nanolayered Minerals

In the 2D nanolayered materials the constituting atoms are positioned in flat layers. These layers are piled on top of each other like sheets of paper and held together with Van der Waals, polar, or ionic bonds. Disconnecting the staked sheets from each other in the bulk materials to expose the planar surface of each individual particle was an infeasible task until new methods such as oxidation, ion intercalation/exchange, surface and passivation by solvents were developed 36-43. In all these techniques, the space between two individual layers, so called the “interlayer space”, is first expanded by subjecting the bulk material to a combination of certain ions and solvents. Through this process, ions and solvent molecules diffuse into the space between the layers causing the expansion of the interlayer space as a consequence of generated repulsion forces 44-47. For example, water and polar solvents are capable of expanding layered materials with polar interlayer interaction 48. Table 1 categorizes the common types of inorganic layered materials according to their interlayer expansion type and charge carrier. However, the expansion range achieved by these techniques is limited to few nanometers and is not suitable for applications where better access to the surface area of the nanomaterials is required 43, 49. In order to sufficiently increase the interlayer space, weakly connected individual layers can be further separated by rapidly cancelling the atomic attractive forces between the layers using exfoliation (delamination) methods, either chemically or mechanically (shear or ultrasonication) 46-48. The combination of these processes leads to full separation of planar nanosheets leaving them fully suspended in media (Figure 2).

The discovery of exfoliation methods in 1990′s opened up a new chapter in materials science and technology 39, 47, 50, considering that the properties that had been seen in stacked layered materials considerably differ from those of their exfoliated counterparts 43, 51. The immediate observation was that the exfoliation processes could amplify some of the pre-exiting properties of the bulk layered materials, such as electrical conductivity in graphene, magneto-optical effects in Co-Al layered

52-54 double hydroxide (LDH), and photocatalystic activity in MoS2 . As a result, single layers of manganese 55, cobalt 56, tantalum 57, ruthenium 58, titanium 59 oxides and many other perovskite type structures have been produced from the bulk materials. In these examples, protonation via chemical intercalators was used to cause electrostatic repulsion between layers and separate the metal oxides. Frequently used intercalators are tetrabutylammonium (TBA), tetrametylammonium (TMA) and ethylammonium. Loading bulky intercalating agents such as dodecyl sulfate (DS) ions into the interlayer space is another alternative method to exfoliate layered materials such as layered metal hydroxides 60-61. This relatively new method offers excellent control of the shape of isolated layers.

The most commonly used types of nanolayered minerals include clay65, mica66, chlorite67-68, RUB-15

69 70-71 70 72 73 74 kenyaite , magadiite , MCM-22 (MWW) , HLS , AMH-3 (3D layered silicate) and M2Si2O5 (M = H, Li, Na, K, Cs, Rb, and their mixture)75-76. The most widely used category of mineral clays is montmorillonite (MMT) 77. The stable structures and periodical positioning of Si-OH/Si-O- groups existing on the surface of the layered silicate nanosheets such as MMT turn these materials into a very interesting subgroup of mineral nanolayered materials 17, 78. Layered double hydroxides (LDHs)

79-80 are another commonly used class of nanolayered minerals that are widely used in preparation of layered nanocomposites 81-83. LDHs normally consist of inorganic layered sheets with positive charges intercalated with hydrated anions 84-85. Hydrated anions along with water molecules that

exist in the interlayer space of these materials provide charge balance in the system. Hydrogen bonding as well as electrostatic attraction between the positively charged layers and negatively charged anions hold the layers together. Based on previous reports, inorganic anions with higher charge/radius ratio have more tendencies to diffuse into the interlayer space of LDHs due to stronger interaction with inorganic layers. Anionic polymers, depending on their size, geometry, mobility and size/charge ratio are also considered as suitable candidates to be accommodated into the interlayer space of LDHs 86.

3. Layered 2D nanosheets intercalated with long chain polymers

Various molecules so-called “guests” can be accommodated into the interlayer spacing between the adjacent plates of exfoliated 2D nanosheets. The surficial chemical structure of the plates and their interlayer distance determine the size and the type of the guest molecules that can be inserted between nanosheets. The incorporation of guest molecules into the layered nanosheets will lead to new hybrid materials with completely different physical and mechanical properties 87-91.

For layered silicates, the silanol groups (Si-O-H) that decorate the interlayer surface of sheets are very active contact points for covalent or non-covalent grafting of numerous guest molecules with reactive side chains 92-93. The first successful grafting was demonstrated by Rojo and Ruiz-Hitzky in 1980 94, where the interlayer silanol group of a layered silicate was reacted with a silane coupling reagent 94. Later, successful grafting of alcohols 95-97, phosphonic acids 98-99, and other hydroxyl- bearing organic molecules on the interlayer planar surface of exfoliated silicates were demonstrated by various scientific groups 100.

Exfoliated layered nanosheets that are intercalated with polymer chains can be called “layered nanocomposites” 101-105. There are two general categories of polymer intercalated-layered materials depending on the nature of the components and the preparation methods 101-102. The first type is called intercalated nanocomposite, where polymer chains are fully integrated into the interlayer spacing of the layered materials. It has been highlighted that, almost in all such cases the process of polymer chains intercalation increases the interlayer spacing of the layered materials up to ~1-3 nm 106. The second type is called exfoliated (delaminated) nanocomposites in which the polymer chains are loaded into the interlayer space of well separated nanosheets with the inter-sheet distance of up

to ~8 nm. Figure 3 illustrates these two types of polymer-layered materials nanocomposites 101, 107- 108.

As illustrated in Figure 4, polymer chains can be incorporated into the interlayer space of layered materials via four preparation routes 109. The layers can be exfoliated using a suitable organic or aqueous solvent in which polymer chains are also soluble. In this case, the exfoliation phenomenon takes place by overcoming the weak interaction forces between the layers. Subsequently, the dissolved polymer chains will be absorbed into the interlayer space of exfoliated layers, which then will be stacked between the layers after solvent is evaporated. The ultimate product normally is a polymer intercalated – layered inorganic material offering a multilayer structure 110-112. The second method is called “in situ intercalative polymerization”. In this method the interlayer expansion of layered materials occurs in a solution carrying monomers and initiators followed by a polymerization process inside the interlayer space utilizing either heat or radiation to initiate the polymerization 113. The third method is a solvent free method in which the layered material and polymer chains are mixed at temperatures above the melting point of the polymer (or glass transition temperature in the case of non-crystalline polymers). If the surface of layered materials is compatible with the polymer, the polymer chains will then diffuse into the interlayer space and produce an intercalated or exfoliated nanocomposite 114-116. The forth method so called “template synthesis” is only suitable for water soluble polymers 117. Basically, the layered material can be generated in situ in an aqueous solution containing polymer chains on the basis of self-assembly forces 118. In this case, nucleation and growth of inorganic layers occurs with the help of polymer, therefore, the polymer chains are stacked inside the interlayer space 119-121. In most cases the polymer chains and the layered materials are not connected chemically and the polymer chains are only physically trapped inside the interlayer space122.

Essential requirements to achieve fully intercalated polymers in layered silicates are large surface area, high aspect ratio, and good interfacial interactions between the quest polymers and the nanosheets 123. Amongst layered silicates, montmorillonite, hectorite, and mica are known to be the most popular host materials because of their unique intercalation abilities such as lower surface

energy which improves the wetting process with the polymer matrix, various functional groups of inorganic cations that react with the polymer chains to improve adhesion between the inorganic phase and the matrix as well as high intensity of silicate particles to disperse into individual layers 112, 124-125. Other types of layered silicates such as magadiite and kenyaite are not as useful due to their low interfacial reactivity126.

4. Intercalation with hydrophobic polymers

The layered nanocomposites normally offer significantly enhanced properties in comparison to their neat polymeric counterparts, such as higher thermal stability, enhanced rigidity, higher chemical and mechanical resistance, higher degree of optical transparency, and higher electrical and thermal conductivity, as well as impermeability to gases 101, 127-130. Because of their unique mechanical and physical properties, layered nanocomposites offer great potentials in task specific applications such as membranes for fuel cell application 131, intelligent membranes for separation devices 122, 132, food packaging, photovoltaic devices, photo catalysts133-134, chemical and biochemical sensors 135-136, smart microelectronic devices 137, micro optic devices 138, new cosmetics 139-140, sustained release of active molecules 141, and advanced ceramics 142. Biodegradable polymers such as aliphatic polyesters, polyhydroxybutyrates, polyhydroxyvalerates and polylactides have also been intercalated into the interlayer space of layered materials such as layered silicates to create biodegradable composites with mechanical and physical properties resembling those of polyolefins (e.g. polypropylene (PP), polyethylene (PE)), polystyrene (PS), poly(vinyl chloride) (PVC) 130. Such biodegradable layered nanocomposites are promising candidates to replace petroleum derivatives in packaging production 143-144.

The first successful layered nanocomposite made of homogeneously dispersed montmorillonite in polymer matrix (nylon-6) was introduced by Toyota company in 1992 145. The intercalation process was carried out at 250-270 oC for 48 hours in order to polymerize the e-caprolactam monomer as shown in Figure 4 for polymer melting intercalation process. The interlayer space of mineral layered clay has also been used for polymerization of vinyl monomers such as cis and trans-butene-2 (Friedlander et al. 146), methyl methacrylate (Blumstein et al. 147), acrylonitrile (Sugahara et al. 148 and Kooli et al. 149-150) and styrene (Kato et al. 151). Akelah et al. 152 synthesized PS within the interlayer space of montmorillonite via free radical polymerization. The interlayer space of montmorillonite

(MMT) was firstly swollen by styrene monomers which were subsequently polymerized. The adsorption of monomer into the interlayer space was facilitated by the dipole moment of the monomer. The XRD analysis revealed that the interlayer distance of MMT was increased from 0.96 to 2.5 nm after polystyrene polymerization 152. The resulting composite exhibited significantly lower hydrophilicity compared to the neat MMT. Intercalation of hydrophobic PS chains into the nanospace of MMT altered the nature of system from hydrophilic to ganophilic.

The interlayer surface of most layered nanosheets is decorated with charged species. As such, doped conductive polymers with counter charges can ionically interact with the surface of nanosheets. Polyacetylene 153, polyaniline 154, polypyrrole 155, poly(p-phenylene) 156 and polythiophene 157 are popular conductive polymers that have been utilized as the guest for intercalation of silicate nanosheets (Table 2). These polymers, when doped, offer conductivities in the range of 106, 103, 600, 500 and 200 S cm-1, respectively 158.

Kanatzidis et al. 159 successfully intercalated/polymerized aniline monomers into the interlayer space of vanadium oxide (V2O5.nH2O) xerogels. The reaction was conducted in water at room temperature in order to increase the reaction rate. In this reaction, aniline monomers were oxidatively

4+ polymerized while the V2O5 layers were reduced that generated V centers resulting in an increase of interlayer space of in the xerogels to 13.94 Å (Figure 5). The final product was a flexible and quasi free-standing film. The authors reported an increase in conductivity of V2O5 from 0.5 to 2 S/cm at 300 K after intercalation of polyaniline 160. This increase in conductivity can be attributed to the

161 alignment of polymer chains in the interlayer space of V2O5 .

Taking advantage of the 2D structure of zirconium phosphate (ZrP), Herzog and coworkers 162 demonstrated the intercalation of aniline into the interlayer space of ZrP with both α and γ structures. The authors reported that, the α and γ zirconium phosphate was brought to contact with either pure aniline or aniline diluted in acetone at various concentrations for 7 days to intercalate aniline into ZrP interlayer space (Figure 6). The intercalated aniline was polymerized to achieve ZrP- PAn composites.

Challier et al. 163 reported the intercalation of polyaniline into the interlayer space of thin sheets of layered double hydroxides (LDHs) using chimie douce pathways. The dried layered materials (TA/Cu/Cr LDH and CF/Cu/Al LDH) were refluxed for 24 hours in pure aniline and subsequently the final product was collected by centrifugation and washed with acetone, methanol and DMSO. XRD analysis was used to investigate the change in basal spacing of LHDs after polymer intercalation. It was revealed that, no change in basal space was recorded when LDH/aniline were treated at room temperature, which indicates that no intercalation was occurred. Successful intercalation was achieved under reflux depending on the ageing and the drying temperature of the precursor. XRD patterns showed in Figure 7 proves that the basal spacing was increased from 10.5 to 13.3 Å for TA/Cu/Cr LDH and from 11.8 to 13.5 Å for CF/Cu/Al LDH after aniline aromatic rings intercalation.

The authors used IR spectroscopy analysis to prove the polymerisation of aniline using diagnostic frequencies reported in the literature 164. In general, the IR diagram of polyaniline shows four absorptions at 1600 (weak), 1500 (strong), 1290 (medium) and 820 (weak) cm-1 related to the ring- breathing modes, a phenyl-nitrogen mode and an out of plane CH bending mode, respectively. Reportedly, the peak intensity at 1600 and 1500 cm-1 are corresponding to the qualitative amount of oxidation state of the polymer. Furthermore, two small bands in the ranges of 770-730 and 710-630 cm-1 are corresponded to the five adjacent ring hydrogens and are an indication of the presence of short oligomers. Here, IR spectroscopy proves the absence of monomers in the interlayer space as absorptions peaks of 3520-3450 and 3420-3350 and combination band at 2650 cm-l are missing (Figure 8). For aniline-TA/Cu/Cr LDH sample the polyaniline fingerprint at 1487 and 1299 cm-1 were clearly detected. The main purpose of using Cu2+ in this study was to promote the oxidative polymerisation of aniline in the interlayer space of LDH materials.

5. Intercalation with hydrophilic polymers

The interlayer space of layered materials can be expanded or swelled by diffusion of ions and water 165. Figure 9 depicts two different swelling mechanisms that have been described in literature for layered materials 166-168. The intracrystalline swelling occurs when the interlayer lattice dimension of the layered materials increases due to the hydration of the gallery species with water molecules

(Figure 9 a). The osmotic swelling refers to the process in which a significant amount of water diffuses into the interlayer space of the layered materials as a result of a positive osmotic pressure, resulting a remarkable increase in the interlayer space 46-47, 169. This osmotic pressure is generated because of the difference between concentrations of ionic species between the layers and the outside environment (Figure 9 b). The diffusion of high amount of water into the interlayer space overpowers the existing Van der Waals, ionic or polar bonds between the layers. Therefore, this process is believed as a critical and instable step immediately prior the exfoliation process166. The Instability and irreversibility of this step is currently a challenge for precise manipulation of the nanostructure of the resulting hybrid systems 165.

The properties of the interlaced ions and charge density are known as important factors to have a direct impact on magnitude of the swelling 167-168. For example, in layered metal oxides with high charge density, swelling process is infeasible unless by replacing the interlayer species with protons followed by reaction in basic solutions 170. The acid-base intercalation of protonated titanate

171-172 (H0.7Ti1.825O4.H2O and H1.07Ti1.73O4.H2O) was studied by Sasaki et al. . It was found that, the hydrated alkali metal ions and organoammonium ions can diffuse into the interlayer space resulting in creation of crystalline swollen phases depending on the type of cations incorporated into the system. Additionally, penetration of a remarkable amount of water was observed when quaternary ammonium hydroxides, such as tetrabutylammonium hydroxide (TBAOH) or tetramethylammonium hydroxide (TMAOH) were used as an electrolyte 39, 173.

The process of osmotic swelling of protonic titanate of HxTi2-x/4 O4.H2O at high electrolyte concentration was reported by Watanabe et al. 47 ,where decreasing TBA caused extensive expansion of interspace between the sheets and subsequently resulted in exfoliation. Sasaki and his co-workers 165 reported a dramatic and reversible monolithic crystalline swelling of layered materials by water/polar amine diffusion. As shown in Figure 10, the interlayer space was reportedly expanded to 100-fold with a maximum periodicity of 90 nm. This stable and reversible 100-fold expansion in volume was reported to be very rapid (a few seconds). The optical microscopy was employed to record the real time of swelling process in which it was reported to be 1.4 seconds immediately after adding DMAE, showing 200 µm increase in the crystal size. Adding HCL resulted in removal of water molecules from the interlayer space and therefore a reversible crystal expansion was achieved. Such a short response time can be employed in fabricating of nano-actuators where a

response time as fast as 1 second is required 174-175. Surprisingly, the layers remained strongly held without exfoliation, maintaining a nearly perfect three-dimensional lattice structure of 43000 layers. The main impetus behind the diffusion of polar amines and/or water into the galleries was known to be the acid–base reaction using a controllable and precise process. The stability of the swollen phase was associated with the chemical nature of the ions that were present in the gallery between nanosheets and their interactions with the surrounding H2O molecules (Figure 10).

Geng et al. 166 reported a significant degree of reversible swelling for protonated titanate 176 in aqueous solutions of various amines such as tertiary amines, quaternary ammonium hydroxides and primary amines. Using these solutions, the height of the crystal gallery of protonated titanate was expanded up to ∼100-fold (90 nm interlayer distance). The concept of the osmotic pressure balance between the ions within the gallery and the surrounding aqueous solution were employed to successfully control the degree of swelling or inﬂow of H2O in this study. Both factors were dependent on molarity and relatively independent of the type of electrolyte but were. In solutions of tertiary amines and quaternary ammonium hydroxides, by increasing the external concentration of these solutions the diffusion of ammonium ions was observed to increase linearly reaching a saturation plateau of ∼40%. Different swelling behavior was observed due to the attractive forces between amine solute molecules on the solution osmotic pressure. It was reported that, ions with higher polarity and smaller sizes offer more stable swollen structure while ions with lower polarity and larger sizes leads to exfoliation.

The swelling behavior of layered materials was found to be related on the positioning and dihedral angle of each individual layer in the dielectric medium 177. Furthermore, the quantity of repulsion force between a pair of electrically charged layers immersed in a dielectric medium (i.e. water) highly depends on the dihedral angle of the layers with respect to each other, where the maximum repulsive force mostly occurs at dihedral angle of 0o so called cofacial orientation (Figure 11). This is the fundamental principle of structural stability of layered materials in solutions and should be carefully monitored 178-179.

One approach to promote the water molecules to diffuse into the interlayer spaces of layered materials is to fill the interlayer space with hydrophilic polymers. Hydrogels180-185, passive or active,

are good candidates to be loaded into the interlayer space of mineral layered nanosheets. Hydrogels are hydrophilic polymer chains which are cross-linked either chemically or physically 186. These materials are capable of absorbing a remarkable amount of water (up to 90% of its volume) when immersed into the aqueous solutions and therefore display a reversible swelling behavior 187-188. The swelling ratio of hydrogels can be controlled by environmental signals such as temperature 189-192, light 193, certain chemicals 194-195, pH 196-199, solution ionic strength 200-201 and external electric fields 202. This dynamic response of hydrogels has been widely utilized in fabricating of many devices such as artificial muscles (actuators) 203-204, drug delivery systems 205 and sensors 206. However, traditional hydrogels are mechanically weak at their highly swollen state and their response time is restricted by slow diffusion of water molecules through bulky networks. Incorporating hydrogels as guest materials into the interspace of nanosheets can enhance the overall mechanical properties and improve the response time. The partial substitution of hydrating water molecules available inside the interlayer space of clay with uncharged linear polymers such as poly(vinylalcohol), poly(vinylpyrrolidone), poly(ethylene glycol), and polyacrylamide through diffusion has been explored by various research groups 207-210. The main driving force for this phenomenon is a gain in entropy. However, the substitution efficiency is low due to the small entropy gain, making this method not practical.

Haraguchi et al. 211 introduced a nanocomposite hydrogel (NC gel) system comprised of hydrophilic polymers and water swellable inorganic clay. Inorganic clay nanosheets were exfoliated and uniformly dispersed in an aqueous media. In situ free-radical polymerization method was used to polymerize N-isopropylacrylamide (NIPA) or acrylamide monomers in the presence of water swollen inorganic clay. After polymerization, the individual clay nanosheets were connected by long polymer chains, acting as multifunctional physical crosslinking agents for highly stretchable polymer networks. No chemical crosslinker was required in formation of the NC gels. Unlike robust NC gels with no covalent crosslinking, using an organic crosslinker such as N,N’-methylenebisacrylamide (BIS) resulted in brittle gels 211

The ordered topology of the exfoliated nanosheets can be used as structure-directing templates to create hydrogels with highly ordered structures and very rapid response times. This new category of nanocomposite hydrogels can be referred to as nano-actuating layered materials. Miyamoto et al. 212 reported a temperature sensitive hybrid hydrogel based on poly(N-isopropylacrylamide) (PNIPA)

infused in nanosheets of fluorohectorite (FHT). PNIPA is hydrophilic at ambient temperatures and becomes hydrophobic upon heating to temperatures above 32 oC 213-214. Anisotropic PNIPA gels were facilely synthesized by radical polymerization of N-isopropylacrylamide as a monomer in the presence of the FHT and BIS as chemical crosslinkers. FHT nanosheets used in this study with the chemical composition of Na0.46[Mg2.60Li0.46Si4O10F2.00] had an average lateral size and thickness of 2.2 µm of 0.9 nm, respectively. The highly dispersed state of the nanosheets and mesoscopic scale lamellar structures in the gel system was confirmed using small angle x-ray diffraction technique. Prior to starting the polymerization process, the basal spacing of the sheets was calculated using the detected peaks of q: 0.17, 0.36 and 0.56 nm-1 and was found to be 35 nm. The structure is mostly similar to lyotropic liquid crystal phase with a swollen lamellar structure. Following the polymerization process the peaks were shifted to higher q (smaller d-value) indicating the reduction of basal spacing to 8.8 nm. It was likely due to the compression of ordered domains by the emerging excluded volume of the polymer chains and/or the bridging of the nanosheets by the polymer chains. The basal spacing increased again to 30.4, 48.2 and 60 nm after 15, 30 and 60 min, respectively, after immersing the composite system into the water. The increase in basal spacing led to anisotropic but reversible change of the total system which is a very essential property for fabricating actuator systems. As shown in Figure 12, the cylindrical nanocomposite gel (1 mm in diameter) exhibits considerable anisotropic shrinkage upon heating.

Kim and co-workers 215 have lately introduced a layered structure hydrogel system consists of cofacially oriented electrolyte nanosheets. 215. Monomers of temperature sensitive PNIPA (i.e. N- isopropylacrylamide) were inserted into the interlayer space of TiNSs nanosheets followed by applying magnetic fields to align the nanosheets in a cofacial position followed by synthesizing the polymer network via in situ radical polymerization method (Figure 13A). The maximum repulsion force between the sheets can normally be achieved via cofacial positioning 216. As a consequence of the electrostatic repulsion force between the nanosheets, the hydrogel system presented a compression resistance orthogonal to the nanosheets and an easy deformation along the parallel

sheets. The electric field has also been used to align block copolymers, however, it was reported that due to the weak effect of this mechanism a strong electric field is required, which normally leads to damaging the polymer chains217. Based on this fundamental characteristic, a temperature responding nano-actuation system was developed. The traditional PNIPA hydrogels deswell at temperatures above 32 oC because of thermodynamic phase transitioning from fully solvated polymer chains to collapsed chains that expel water molecules from their structure. The deswelling mechanism of nanocomposite hydrogel actuators was amplified by parallel nanosheets of TiNS. Similar to neat PNIPA hydrogels, the temperature sensitive hydrogel between TiNS nanosheets offers a rapid reversible volume change on external heating/cooling (32 oC) stimulation due to difference in amount of water uptake/release. On heating above the lower critical solution temperature (LCST ∼32 oC) water molecules are released from the PNIPA gel network and triggered the intrinsic electrostatic permittivity. The electrostatic repulsion between the cofacially oriented TiNSs was therefore amplified due to the increase in electrostatic permittivity of the guest material. Consequently, the plane-to-plane distance between TiNSs increases, and the hydrogel expands in a direction orthogonal to the TiNS plane. As shown in Figure 13 B, the change in the properties of the intercalated hydrogel in nanoscale resulted in reversible elongation and contraction of this hybrid system in macroscale. It is important to note that in the absence of TiNS nanosheets the PNIPA hydrogel shrinks at temperatures above 32 oC. In order to investigate the actuation behavior of this new composite a long rod-shaped sample with height and cross-sectional diameter of 15 and 0.6 mm, respectively was prepared using a glass capillary, the TiNS plane was orthogonal to the longer axis of the capillary. The rod-shaped sample was immersed in the waters baths having different temperatures of 15 and 50 oC. Following the immersion in to the higher temperature bath the height of the sample rapidly increased by170% in just a second and returned to its original shape after dipping into the cooler bath (uniform cooling). The deformation rate was reported to be 70 %s−1, which is very practical for artificial muscle applications where high deformation rates are needed218- 219.

6. Applications

2D layered mineral nanosheets exhibit highly tuneable properties and are suitable candidates for applications in a large number of fields such as drug delivery 220, flame retardant221, nanohybrids222- 223, antibacterial224, rheology modifiers225, superior thermal transporter22, 37 and food packaging226. Recently, there have been an enormous number of new developments using 2D layered minerals intercalated with polymer chains to load and in vivo deliver biologically active materials such as porphyrins 227-228, nucleoside phosphates 229-230, drugs 231 vitamins 232, amino acids 233-234, and fatty acids 235. It has been reported that 2D layered minerals acting as the host lattice can carry the intercalated biomolecules through stomach and small intestine to the specific targets inside the body. The main function of 2D layered minerals in this technique is to safely transport the intercalated biomolecules into the bloodstreams without any deintercalation. The unfavourable repulsive interaction between the negatively charged cell membrane during the transformation of drug into mammalian cells has been eliminated in this technique. Taking advantage of instability properties of layered double hydroxides (LDHs) in acidic conditions, utilizing these 2D layered minerals are very popular to encapsulate biologically active materials. Once inside the cell, LDHs can normally be dissolved by lysosomes (acidic) and therefore maximum drug release can be achieved 220. Flame retardant chemicals are widely used to delay or stop the fire propagation in textiles, plastics and coatings 236-237. 2D layered minerals such as montmorillonite (MMT) based nanocomposites have already been used as flame retardant additives. For instance, the flammability of MMT-nylon6 and MMT-PS nanocomposites reduced by 50 to75 % compared to the neat polymers 238. 2D layered minerals intercalated with polymer chains are used either solely or with other flame- retardant additives to improve the fire resistance property of any material while enhancing other properties of the final product. 2D layered minerals intercalated with polymer chains have also widely been employed in food packaging industry 226, 239. These materials can fulfill the essential requirements of this industry by offering captivating properties such as barrier properties against oxygen, carbon dioxide, ultraviolet, moisture and volatiles are perhaps the most important properties that a nanocomposite food packaging can offer 240-241. For example, the barrier properties of the net polymers can significantly improve by layered shape minerals by increasing the impermeable property which is very useful in food packaging applications. Song et al22 have fabricated high thermal conductive poly (vinyl alcohol)/boron nitride films by mechanically stretched and therefore align the embedded BN nanosheets. These results were competitive with or beyond those based on graphene sheets. Lin et al 225 have developed a new class of rheology modifiers and

dispersing agent for epoxy packing of white LED. The new polyetheramine-modified organoclays enabled the homogenous dispersion of inorganic phosphorescent phosphor powder, yttrium aluminum garnet Y3Al5O12 (YAG), in epoxy which can be employed for packing of light-emitting diode (LED) devices. The authors reported a significant enhancement in the viscosity of the anhydride/epoxy resins (a non-Newtonian shear-thinning behavior) by adding the POP-diamine- based organoclays. Recently, Lin et al 224 have also fabricated a thermal-sensitive nanohybrids for antibacterial applications using silver nanoparticles (AgNPs) in combination with silicate nanoplatelets (NSP) and thermally sensitive poly(N-isopropylacrylamide) (PNiPAAm). PNiPPAAm were chemically grafted on the NSP using atom-transfer radical polymerization method. The nanoparticles of AgNPs then were adsorbed on NSP-PNiPAAm nanosheets through in situ reduction reaction of AgNO3 in aqueous dispersion. The key point behind this study was the shrinkage of PNiPAAm at 37 oC, which led to the aggregation of Ag nanoparticles and therefore enhancement of antibacterial ability.

7. Conclusion

2D layered minerals as an important class of materials science have attracted a great deal of attention because of their interesting and promising properties. They offer potential usage in various ﬁelds including catalysis, biosensors, data storage, food packaging, flame retardancy and drug delivery. Recently, hydrophilic polymers such as hydrogels were inserted into the interlayer space of layered materials showing a very interesting behavior. Basically, by using this technique it is possible to reversibly and rapidly change the distance between each individual layer. This phenomenon normally occurs in response to external stimuli such as temperature and light showing very similar behavior to actuators in nanometer scale. Previously paraffin, a temperature sensitive material, has been used in manufacturing micro actuators and machines. In the future, it is worthy to investigate the possibility of employing this unique behavior in 2D layered minerals in manufacturing nanoscale smart devices and machines that can offer reversible and fast actuating like behavior in nanoscale.

Acknowledgments

DS would like to thank Japan Society for Promotion of Science (JSPS) for providing the JSPS standard postdoctoral fellowship. FD and SN acknowledge the support of the University of Sydney’s Centre of Excellence for Advanced Food Enginomics.

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Figures and captions

Figure 1. Schematic illustration of four different types of nanomaterial dimensionality. 3D to 0D structures from A) to D).

Figure 2. Schematic description of exfoliation mechanism of layered materials. A) Mechanical (shear, ultrasonication) or thermal exfoliation, B) chemical exfoliation using solvents. Reproduced from 43.

Figure 3. Schematic of two main types of composites produced via inserting the polymer chains into the interlayer space of layered nanomaterials.

Figure 4. Schematic illustration of different methods/steps of intercalating polymer chains into the interlayer space of layered materials.

Figure 5. Schematic of polyaniline intercalation into the interlayer space of V2O5.H2O layered material and increasing the inlayer space to 1.39 nm.

Figure 6. Powder X-ray diffraction pattern of A) α-ZrP-An and B) γ-ZrP-An obtained from pure aniline C) γ-ZrP-An obtained from aniline diluted in acetone 1.

Figure 7. XRD pattern of a) TA/Cu/Cr LDH, b) aniline/TA/Cu/Cr LDH, c) CF/Cu/Al LDH and d) aniline/CF/Cu/Al LDH. (basal spacings are indicated in Å) 2.

Figure 8. FTIR spectra of aniline-intercalated LDHs: a) aniline/TA/Cu/Cr LDH b) aniline/CF/Cu/Al LDH. Key bands of polyaniline are marked with an asterisk 2.

Figure 9. Schematic illustration of swelling and exfoliation process of layered materials with water intercalation.

Figure 10. Illustration of reversible basal space expansion of titanate sheets due to water intercalation into the interlayer space 3.

Figure 11. Schematic illustration of positioning of two individual layers indicating the highest and lowest repulsion forces at cofacial and orthogonal positions, respectively (red arrows represent the repulsion force directions) 177.

Figure 12. A) The anisotropic properties of the gels in thermally-induced volume phase transition, the relative sizes of the cylindrical gel along the directions (a) parallel and (b) perpendicular to the nanosheets orientation and (c) the relative gel volume. In B), the microscopic images of the anisotropically colored gel immersed in the dye solution is shown4.

Figure 13. A) Schematic of preparation steps of PNIPA/TiNSs composite system containing cofacially oriented TiNSs in a magnetic ﬂux of 10 T. B) Actuation behavior of the PNIPA/TiNSs composite system caused by the dehydration and hydration of its PNIPA gel network (red arrows represent the electrostatic force generated as a result of the nanosheets similar charges) 215.

Tables and captions

Table 1. Classification of typical inorganic layered materials and their corresponding interlayer expansion methods 62.

Group subgroup Example

Ion-exchangeable Cation- Clay minerals (montmorillonite, hectorite) exchageable Metal phosphates (α-Zr(HPO4)2)

Niobates and titanates (K4Nb6O17,H xTi2−x/4.x/4O4)

Manganates (KxMnO2.nH2O)

Layered silicates 63

Magadiite 64

Anion- Lyered doube hydroxide (LDH) exchangeable Hydroxide salt (La(OH)2NO3)

Non-ion- Nonpolar Graphite exchangeable Metal dichalcogenides (MoS2)

Black phosphorus

Polar Metal carbides/nitrides (MXene; Ti3C, Ti3C2)

Metal phosphate (VOPO4.2H2O)

Table 2. Chemical structure of frequently used conductive polymers in producing layered nanocomposite field.

Polymer Chemical structure

Polyacetylene

Polyaniline

Polypyrrole

Poly(P-phenylene)

Polythiophene

Biographies

Danial Sangian received his Ph.D. from the University of Wollongong, Australia, in December 2016. During his Ph.D., he focused on fabricating new types of actuators using temperature sensitive materials. Afterwards, he joined NIMS as a Japan Society for Promotion of Science postdoctoral fellow where he worked on actuating behavior of layered materials and synthesis of related functional materials. Since March 2018, he is a Humboldt postdoctoral fellow at Technical University of Berlin/University of Potsdam. His current research interests include inorganic layered materials, elastomers, smart materials and actuators.

Sina Naficy is a polymer engineer with years of experience in development and processing of functional and tough polymer gels and hydrogels. Sina was awarded his Ph.D. form the University of Wollongong, Australia, in December 2011. He is currently a Research Fellow at the University of Sydney, working on development of soft sensors and actuators for smart packaging and soft robotics. His research is mainly focused on creating soft and functional gel systems that are processable via printing and additive manufacturing.

Yusuke Yamauchi received his B.S. degree (2003), master’s degree (2004), and Ph.D. degree (2007) from Waseda University, Japan. After receiving his Ph.D. degree, he joined NIMS to start his own research group. In 2016, he moved to the University of Wollongong, Australia, as a professor. Recently, he joined the School of Chem. Eng. & AIBN at the University of Queensland as a full-time professor. He concurrently serves as an honorary group leader of NIMS, and a visiting professor at several universities.

TOC

A Review on Layered Mineral Nanosheets Intercalated with Hydrophobic/Hydrophilic Polymers and their Applications

D. Sangian*, S. Naficy*, F. Dehghani, Y. Yamauchi