Hussein Kalo

Barrier Properties of Synthetic Clay with a Kilo‐Aspect Ratio

Intrinsic anisometry, the appearance in plate-like pseudocrystals (tactoids) with high aspect ratios, and their rich intercalation chemistry (cation-exchange, swelling) are some of the essential features of clays, which render them interesting as functional particles for a variety of applications. [ 1–3 ] Long before Nielsen’s [ 4 ] essay about the tortuous path theory appeared in the late 1960s, composite materials were prepared of inorganic platelets in a continuous macromolecular phase. [ 5 ] Aside from mechanically toughening of polymeric matrices via the incorporation of stiff platelets, [ 6–9 ] diffusion-barrier, [ 10–12 ] and fl ame retardant applications [ 13 ] are the main focus of current research. Since both, experiments and simulations have emphasized the key importance of high aspect ratios ( α ) of fi llers, in particular for gas permeability, [ 14 , 15 ] material scientists continue hunting for higher α exploring a variety of lamellar materials including graphene. [ 16 ]

Nanoplatelets of Sodium Hectorite Showing Aspect Ratios of ≈20 000 and Superior Purity

Applying a combination of melt synthesis followed by long-term annealing a fluorohectorite is obtained which is unique with respect to homogeneity, purity, and particle size. Counterintuitively, the hectorite undergoes a disorder-to-order transition upon swelling to the level of the bilayer hydrate. Alkylammonium-exchanged samples show at any chain length only a single basal spacing corroborating a nicely homogeneous layer charge density. Its intracrystalline reactivity improves greatly upon annealing, making it capable to spontaneously and completely disintegrate into single clay lamellae of 1 nm thickness. Realizing exceptional aspect ratios of around 20,000 upon delamination, this synthetic clay will offer unprecedented potential as functional filler in highly transparent nanocomposites with superior gas barrier and mechanical properties.

Single crystal structure refinement of one- and two-layer hydrates of sodium fluorohectorite

Crystal structures of both one- and two-layer hydrates of sodium fluorohectorite were refined against single crystal data for the first time because melt synthesis yielded a sodium fluorohectorite showing little stacking disorder as compared to natural clays. In both hydrate phases, the relative shift of adjacent 2:1 layers is fixed by hydrogen-bonding between water molecules coordinated to interlayer cations and basal oxygen atoms of tetrahedral sheets encompassing the interlayer space. Despite some apparent diffuse scattering, a decent single crystal refinement of the semi-ordered structure of the one-layer hydrate is achieved, revealing structural details of the interlayer spacing for the first time. For the two-layer hydrate the structural model proposed for vermiculites is confirmed but a different ordering pattern of interlayer [Na(H2O)6]+ is suggested. While in the two-layer hydrate sodium cations reside at the centre of the interlayer space, in the one-layer hydrate sodium is displaced from the centre of the interlayer space either towards the upper or towards the lower tetrahedral sheet. This displacement allows for coordination to the hexagonal cavity on one side while the coordination sphere of sodium is completed by three coordinating water molecules on the other side. These three water molecules in turn are involved in hydrogen bonding to the opposite tetrahedral sheet.

Shear stiff, surface modified, mica-like nanoplatelets: a novel filler for polymer nanocomposites

Synthesis of polymer nanocomposites with novel shear stiff, mica-like nanoplatelets from a synthetic layered silicate is presented. This novel synthetic clay filler shows high aspect ratios while organophilization may be selectively restricted to external surfaces minimizing the organic content of the filler. The obtained nanocomposite shows superior mechanical, thermal and fire properties as compared to commonly used natural clays. Furthermore, the influence of the blending method on the nanocomposite properties was investigated.

How to maximize the aspect ratio of clay nanoplatelets

Melt-synthesis yielded lithium-fluorohectorites (Li-hect(x)) with variable layer charge (x = 0.4, 0.6, 0.8, 1.0). Counterintuitively, both tactoid diameter and intracrystalline reactivity increased concomitantly with increasing layer charge. This way hectorites with very large diameters were obtained (d(50%) = 48 μm) that nevertheless still spontaneously delaminate when immersed into water and nano-platelets with huge aspect ratios (>10 000) are formed. Melt-synthesis of Li-hect(x) has been performed in an open glassy carbon crucible allowing for easy scaling to batches of 500 g. These unprecedented huge aspect ratio fillers promise great potential for flame retardants and barrier applications.

A simple approach for producing high aspect ratio fluorohectorite nanoplatelets utilizing a stirred media mill (ball mill)

Abstract The potential of platy nanofillers like clays in polymer nanocomposites is mostly determined by their aspect ratio. The degree of improvement that may be achieved in respect to reinforcement, gas-barrier properties and flame retardancy critically depends on the aspect ratio. Thus, increasing the aspect ratio is highly desirable in order to explore the full potential of the clay filler. Mechanical shear stress as generated in the grinding chamber of a stirred media mill (ball mill) induced an efficient exfoliation of highly hydrated and therefore ‘shear-labile’ synthetic Mg-fluorohectorite in aqueous dispersion. The attainable degree of exfoliation can be tuned and controlled through the shear forces applied by changing process parameters such as solid content and grinding media diameter. Characterization and evaluation of the exfoliation efficiency during milling was achieved by combining and cross-validating data obtained by powder X-ray diffraction (XRD), static light scattering (SLS), specific surface area measurements applying the Brunauer-Emmett-Teller (BET) equation, and scanning electron microscopy (SEM). This led to the identification of optimal processing parameters, allowing for control of the degree of exfoliation and, consequently, the aspect ratio of the nanoplatelets. Not surprisingly, besides exfoliation, increasing the magnitude of the shear stress also resulted in some reduction in platelet size. The clay platelets obtained showed a high average aspect ratio (>600), several times greater than that of original synthetic fluorohectorite. The increase of aspect ratio was reflected in a significant enhancement of both specific surface area and cation exchange capacity (CEC) of the external basal surfaces. This method has substantial advantages compared to microfluidizer processing with respect to feasibility, batch size and particle diameter size preservation. The exfoliated nanoplatelets obtained by milling have great potential to improve mechanical properties of polymer layered silicate nanocomposites (PLSN).

Encapsulation of Functional Organic Compounds in Nanoglass for Optically Anisotropic Coatings

A novel approach is presented for the encapsulation of organic functional molecules between two sheets of 1 nm thin silicate layers, which like glass are transparent and chemically stable. An ordered heterostructure with organic interlayers strictly alternating with osmotically swelling sodium interlayers can be spontaneously delaminated into double stacks with the organic interlayers sandwiched between two silicate layers. The double stacks show high aspect ratios of >1000 (typical lateral extension 5000 nm, thickness 4.5 nm). This newly developed technique can be used to mask hydrophobic functional molecules and render them completely dispersible in water. The combination of the structural anisotropy of the silicate layers and a preferred orientation of molecules confined in the interlayer space allows polymer nanocomposite films to be cast with a well-defined orientation of the encapsulated molecules, thus rendering the optical properties of the nanocoatings anisotropic.

Pressure-Induced Water Insertion in Synthetic Clays†

High pressure behaviour of Na-hectorite in water media shows surprisingly strong similarities to graphite oxide compression in liquid media considering rather different chemical composition and structure of these two types of materials. Crystalline swelling with pressure induced insertion of additional solvent layer (not correlated with pressure point of liquid media solidification) was observed previously for GO immersed in methanol, ethanol, DMF and acetone [2].

In vitro biostability and biocompatibility of ethyl vinyl acetate (EVA) nanocomposites for biomedical applications

The in vitro biostability and biocompatibility of ethyl vinyl acetate (EVA) nanocomposites incorporating organically modified montmorillonite (organo-MMT) were investigated as new candidate material for biomedical applications. The in vitro treatment of neat EVA and EVA nanocomposites was performed by immersing the materials in oxidizing and hydrolytic agents, at a temperature of 37 °C, for 4 weeks. The in vitro mechanical properties of the materials under these environmentally challenging conditions were assessed. Based on morphology studies, the degree of MMT dispersion and exfoliation decreased as the nanofiller loading increased. The EVA containing 1 wt% organo-MMT exhibited the best nanofiller dispersion and exfoliation characteristics. The surface degradation features of this nanocomposite were seen to be smoother than those of neat EVA and other EVA nanocomposites. Furthermore, the EVA nanocomposites have improved mechanical properties in comparison with the neat EVA, and these properties were less affected by the in vitro conditions. The best in vitro mechanical properties were achieved when 1 wt% of organo-MMT was added into the EVA. It was postulated that the presence of a better dispersed and exfoliated organo-MMT layered structure introduced a more tortuous path for the diffusion of oxidants and water molecules, thereby decreasing their permeation towards the EVA molecular chains. Therefore, the degradation kinetics within the EVA molecular chains were at a lower rate, which resulted in enhanced biostability. Furthermore, the toughness of the hydrated EVA (exposed to PBS at 37 °C) was greatly enhanced with the addition of the 1 wt% organo-MMT. The biocompatibility assessment suggests that the EVA nanocomposites are not cytotoxic, and thus have fulfilled the prerequisite to be further developed as a biomedical material.

Controlled Exfoliation of Layered Silicate Heterostructures into Bilayers and Their Conversion into Giant Janus Platelets

Ordered heterostructures of layered materials where interlayers with different reactivities strictly alternate in stacks offer predetermined slippage planes that provide a precise route for the preparation of bilayer materials. We use this route for the synthesis of a novel type of reinforced layered silicate bilayer that is 15 % stiffer than the corresponding monolayer. Furthermore, we will demonstrate that triggering cleavage of bilayers by osmotic swelling gives access to a generic toolbox for an asymmetrical modification of the two vis-à-vis standing basal planes of monolayers. Only two simple steps applying arbitrary commercial polycations are needed to obtain such Janus-type monolayers. The generic synthesis route will be applicable to many other layered compounds capable of osmotic swelling, rendering this approach interesting for a variety of materials and applications.