Adhesive nanocomposites of organically modified montmorillonite (OM) and polyurethane have been synthesized and their permeability to oxygen and water vapor has been measured. The gas permeation through the composites was correlated to the volume fraction of the impermeable inorganic part of the OM. The incorporation of small volume fractions of the platelike nanoparticles in the polymer matrix decreased the gas transmission rate, when the interface between the two heterogeneous phases was properly designed. The oxygen transmission rate decayed asymptotically with increasing aluminosilicate volume fraction and a 30% reduction was achieved at 3 vol %, when the clay was coated with bis(2-hydroxyethyl) hydrogenated tallow ammonium or alkylbenzyldimethylammonium ions. In contrast, coating the clay surface with dimethyl dihydrogenated tallow ammonium ions leads to an increase in the gas transmission rate with augmenting inorganic fraction. This was attributed to a probable change in morphology resulting from phase separation at the interface between the apolar pure hydrocarbon clay coating and the relatively polar PU. The water vapor permeation through the PU nanocomposites was more strongly reduced than oxygen and a 50% reduction was observed at 3 vol % silicate fraction. This was attributed to stronger interactions and hydrogen bonding of the water molecules with the PU matrix as well as to their clustering. Differences in the hydrophobicity of the clay coating influenced the water transmission rate. No spectroscopic evidence could be obtained for a reaction between the hydroxyl groups of the clay organic coating and the isocyanate groups of the prepolymer. A mixed morphology, that is, exfoliated layers and intercalated particles was observed in all composites. WAXRD and TEM gave a qualitative picture of the microstructure of the nanocomposites but no conclusive information. Some of the problems to be solved before a correlation between the nanocomposite properties and their microstructure can be established have been outlined.

Monolayers of mono-, di-, tri-, and tetraalkylammonium cations of varying chain length (C 4 , C 8 , and C 18 ) were self-assembled on montmorillonite platelets. The structure and chain dynamics of these SAMs were probed by infrared spectroscopy (IR), nuclear magnetic resonance spectroscopy (NMR), X-ray diffraction (XRD), and differential scanning calorimetry (DSC). Depending on the cross-sectional area, the available area/cation, and the alkyl chain length, the molecules adopt a two-dimensional order or a disordered state at ambient temperatures. Short alkyl chains lie flat, disordered on the substrate surface as long as there is enough space. With increasing volume of the organic layer between two silicate layers facing each other, the chains force the layers to enlarge their basal-plane spacing but remain disordered. At a certain length and number of chains, the molecules adopt an ordered state due to increasing chain interactions and packing density. To minimize their conformational entropy and maximize their packing density, the chains attached to platelets facing each other interdigitate. The average molecular axis in the organic thin film is inclined to the montmorillonite surface normal by an angle, which depends on the packing environment and the geometry of the molecules. In the ordered state, the alkyl chains preferentially assume an all-trans conformation. With increasing temperature, conformational transformation of the chains takes place, leading to a dynamically disordered phase (liquidlike). Although the translational freedom of the chains is restricted by the electrostatic binding of the headgroups to the substrate, the conformational transformation leads to chains with random conformation and destroys the two-dimensional order. The phase transition manifests itself in an increase in basal-plane spacing as well as in IR absorption frequency and carbon resonance shifts accompanied by an entropy change. The density of the organic ultrathin film confined between two silicate layers seems to decrease on heating across the phase transition, leading to an increase in volume and consequently in the organic layer thickness and in d-spacing, respectively. The basal-plane spacing of 3C18 and 4C18 is appreciably larger than that of C18 and 2C18, which is advantageous for exfoliation in the synthesis of polymer nanocomposites.

Monolayers of dialkyldimethylammonium (C 10 -C 18 ) cations have been electrostatically bound to the surface of mica platelets. The structure and chain dynamics of these well defined self-assembled alkane monolayers have been probed by X-ray diffraction (XRD), Fourier transformation infrared spectroscopy, nuclear magnetic resonance, and differential scanning calorimetry (DSC). All results indicate that at low temperatures the alkyl chains preferentially assume an all-trans conformation, leading to a highly ordered two-dimensional lattice. With increasing temperature, the all-trans conformation is gradually transformed to a mixture of trans and gauche conformers. Although the translational freedom of the chains is limited by the electrostatic binding of the headgroups to the substrate, the conformational transformation destroys the two-dimensional lattice leading to disordered molecules with a liquidlike conformation. This phase transition manifests itself in IR-absorption frequency and carbon resonance frequency shifts as well as in an enthalpy change. The methylene carbon resonance frequencies of the trans and gauche conformers are better resolved than their IR-absorption frequencies and show clearly that the change in conformation starts at a considerably lower temperature than that indicated by IR spectroscopy and extends over a wide temperature range. The relatively sharp transitions seen in IR-absorption maxima plots are due to poor resolution of the heterogeneously broadened lines and consequent sluggish detection of the onset of the trans/gauche transformation. The phase transitions observed by DSC also seem to be relatively sharp but are really broad if the onset of the enthalpy change is considered. The phase transition temperatures depend on the length of the alkyl chains and their packing. The transition temperatures of SAMs electrostatically bound to a planar mica surface are higher than those assembled on planar or curved (nanoparticles) gold surfaces. XRD revealed that the average molecular axis of a dioctadecyl monolayer is inclined to the mica surface by ca. 50°. The thickness of this ultrathin film increased by ca. 5.5% across the transition temperature indicating a decrease in density.

Thermogravimetric analysis was used to investigate the surface cleanliness of calcite fillers and to determine the optimal amount of fatty acid needed to coat the particles with an organic monolayer. The use of excess surfactant led to the formation of a bilayer as well as to the presence of free acid molecules. Both species could be detected by TGA. Optimal coating of calcite with stearic acid gave a monolayer of calcium stearate bicarbonate in which one acid molecule is attached to every Ca2+ present on the surface. The alkyl chains in the monolayer are vertically oriented to the surface and are restricted in their motion. 13C NMR and IR spectroscopy revealed that the chain conformation shows a high trans population, which leads to an ordered solidlike phase. Monolayers of saturated fatty acids with long alkyl chains (≥C10) showed similar behavior, while the lower homologues (≥C10) gave monolayers with dynamically disordered phases. The immobilization of oleic acid molecules by tethering them to the calcite surface rendered them liable to thermal polymerization at relatively low temperatures. In other words, calcite can be coated with a monolayer of oleic acid molecules, which is polymerized later to give a polymeric monolayer.

The structure and phase transitions of an alkyl monolayer tethered to a mica surface have been studied by X-ray, infrared (IR) spectroscopy, and differential scanning calorimetry. All results indicate that the alkyl chains attend an all-trans conformation at low temperatures, leading to a two-dimensional crystalline film that undergoes a first-order transition to an isotropic liquid upon heating. Although the molecules are fixed to the surface at one end, which restricts their translational freedom, they undergo a melting process. It seems that the trans-gauche transformation is enough to destroy the two-dimensional lattice. A model in which the molecules assume a tilted upright position to the mica surface can explain the results obtained. To minimize the conformational entropy and maximize the packing density of the molecules, the chains attached to different mica platelets interdigitate to build an organic interlayer. A crystal−crystal transition was also observed, which led to an increase in the thickness of the organic interlayer. This increase was attributed to a change in the tilt angle of the chains to the mica surface. The phase behavior of the alkyl monolayer is quite similar to that of bulk alkanes but the transition temperatures are higher probably because of the chain end fixation. The solid−liquid transition influenced the IR spectrum but not the film thickness, whereas the solid−solid transition did the contrary. However, both phase transitions were observed in the differential scanning calorimetry. It was possible to differentiate between bonded and intercalated molecules by thermogravimetric analysis, because their decomposition temperatures were different.

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Epoxy-OM (organo-montmorillonite) nanocomposites have been synthesized, and their permeability to oxygen and water vapor has been measured. The chemical structure of the organic monolayer ionically bonded to the montmorillonite surface has been varied, and its influence on the swelling, intercalation, and exfoliation behavior of the OM has been studied. Exfoliated aluminosilicate layers build a barrier for the permeating gas molecules, while the polymer intercalated tactoids do not contribute much to the permeation barrier performance. The gas permeation through the composites was correlated to the volume fraction of the impermeable inorganic part of the OM. The incorporation of small volume fractions of the platelike nanoparticles in the polymer matrix decreased its permeability coefficient when the interface between the two heterogeneous phases was properly designed. Long alkyl chains enhanced the polymer intercalation but increased the permeability coefficient probably due to phase separation at the interface between the polymer and the inclusions. Matching the surface energy of the OM with that of the matrix as well as tethering polymer molecules to the silicate layers surface enhanced the exfoliation and decreased the permeation coefficient. The exfoliation process is governed by interplay of entropic and energetic factors. A macroscopic volume average of the aspect ratio of montmorillonite platelets was deduced from the relative permeability of the nanocomposites by comparing the measured values to numerical predictions of gas permeation through composites of misaligned disk-shaped inclusions. The permeability coefficient of the epoxy matrix was reduced to one-fourth at 5 vol % Bz1OH loading, and the reduction was attributed to the tortuous pathway the gas molecules have to cover during their random walk to penetrate the composite. The transmission rate of water vapor through the composites is more influenced by the permeant−composite interactions and hence the hydrophobicity of the monolayer covering the inclusions surface. At 5 vol % BzC16 loading, the relative vapor transmission rate was reduced to half.

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