Upcoming
efficient air-borne wind energy concepts and communication
technologies applying lighter-than-air platforms require high-performance
barrier coatings, which concomitantly and nonselectively block permeation
not only of helium but also of ozone and water vapor. Similarly, with
the emergence of green hydrogen economy, lightweight barrier materials
for storage and transport of this highly diffusive gas are very much
sought-after, particularly in aviation technology. Here the fabrication
of ultraperformance nanocomposite barrier liners by spray coating
lamellar liquid crystalline dispersions of high aspect ratio (∼20 000)
silicate nanosheets mixed with poly(vinyl alcohol) on a PET substrate
foil is presented. Lightweight nanocomposite liners with 50 wt % filler
content are obtained showing helium and hydrogen permeabilities as
low as 0.8 and 0.6 cm3 μm m–2 day–1 atm–1, respectively. This exhibits
an improvement up to a factor of 4 × 103 as compared
to high-barrier polymers such as ethylene vinyl alcohol copolymers.
Furthermore, ozone resistance, illustrated by oxygen permeability
measurements at elevated relative humidity (75% r.h.), and water vapor
resistance are demonstrated. Moreover, the technically benign processing
by spray coating will render this barrier technology easily transferable
to real lighter-than-air technologies or irregular- and concave-shaped
hydrogen tanks.
Intercalation of large organocations into 2:1 clay minerals may be hampered by two problems: on one hand, the solubility of organocations in water is limited and the resulting high selectivity for adsorption in the polar solvent may lead to non-equilibrium structures. On the other hand, the large expansion of the interlayer space will slow down kinetics of ion exchange considerably. The best workaround for these obstacles is to suspend the clay minerals in mixtures of water with more hydrophobic organic solvents that nevertheless trigger a considerable expansion of the interlayer space by swelling. This in turn fosters ion exchange. The current study, therefore, revisited pioneering work by Bradley (1945) and investigated the swelling behavior of synthetic sodium hectorite (Na-hec) as a function of the composition of the swelling solvent, a mixture of acetonitrile and water. Up to a maximum acetonitrile content of 65 vol.%, delamination by osmotic swelling occurred. At even higher acetonitrile concentrations, swelling was limited to the crystalline swelling regime where a step-like adjustment of the d value was observed. Several mixtures were identified yielding a well defined and uniform interlayer height as evidenced by rational 00l-series with the d spacing decreasing with increasing acetonitrile content. Surprisingly, for a specific acetonitrile:water ratio even an ordered interstratification of two strictly alternating interlayer heights with distinctly different solvent compositions was observed.
The
swelling of clay minerals in organic solvents or solvent mixtures
is key for the fabrication of polymer nanocomposites with perfectly
dispersed filler that contain only individual clay layers. Here, we
investigated the swelling behavior of sodium hectorite in different
ternary solvent mixtures containing methanol, acetonitrile, ethylene
glycol, or glycerol carbonate with minimal amounts of water. We found
that in these mixtures, less water is required than in the corresponding
binary mixtures to allow for complete delamination by repulsive osmotic
swelling. A quantitative study of osmotic swelling in a particular
ternary mixture shows that organic solvents resemble swelling behavior
in pure water. At hectorite contents larger than 5 vol %, the separation
of individual layers scales with ϕ–1. At this
concentration, a crossover is observed and swelling continues at a
slower pace (ϕ–0.5) below this value.
Carbon-fiber-reinforced epoxies are frequently used for lightweight applications that require high mechanical properties. Still, there is potential regarding the improvement of the interlaminar-fracture toughness. As matrix toughening with nanoparticles is one possibility, in this study two different layered silicates are used to reinforce carbon fiber composites. The first type is a synthetical K-Hectorite (K-Hect) with outstanding lateral extension (6 µm) that has shown high toughening ability in resins in previous work. The other is a commercial montmorillonite (MMT) with a smaller size (400 nm). The aim of this study is to show the influence of the particles on mode I and mode II fracture toughness, especially the influence of particle size. Therefore, double-cantilever-beam tests and end-notched-flexure tests were carried out. Additionally, the fracture mechanisms were investigated via scanning electron microscopy (SEM). It is concluded, that the larger Hectorite particles are beneficial for mode I fracture behavior because of enhanced toughening mechanisms. One the other hand, the mode II energy dissipation rate is increased by the smaller montmorillonite particles due to sufficient interaction with the formation of hackling structures.
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