Richard A. Vaia

Polymer/layered silicate nanocomposites as high performance ablative materials

The ablative performance of poly(caprolactam) (nylon 6) nanocomposites is examined. A relatively tough, inorganic char forms during the ablation of these nanocomposites resulting in at least an order-of-magnitude decrease in the mass loss (erosion) rate relative to the neat polymer. This occurs for as little as 2 wt.% (∼0.8 vol.%) exfoliated mica-type layered silicate. The presence of the layers does not alter the first-order decomposition kinetics of the polymer matrix. Instead, the nanoscopic distribution of silicate layers leads to a uniform char layer that enhances the ablative performance. The formation of this char is only minutely influenced by the type of organic modification on the silicate surface or specific interactions between the polymer and the aluminosilicate surface, such as end-tethering of a fraction of the polymer chains through ionic interaction to the layer surface. Thus, the enhancement in ablative performance should be general for the class of exfoliated layered silicate/polymer nanocomposites.

Framework for nanocomposites

Materials and material development are fundamental to our very culture. We even ascribe major historical periods of our society to materials such as the stone age, bronze age, iron age, steel age (the industrial revolution), polymer age, silicon age, and silica age (the telecoms revolution). This reflects how important materials are to us. We have, and always will, strive to understand and modify the world around us and the stuff of which it is made. As the 21st century unfolds, it is becoming more apparent that the next technological frontiers will be opened not through a better understanding and application of a particular material, but rather by understanding and optimizing material combinations and their synergistic function, hence blurring the distinction between a material and a functional device comprised of distinct materials.

Relaxations of confined chains in polymer nanocomposites: Glass transition properties of poly(ethylene oxide) intercalated in montmorillonite

The relaxation behavior of poly(ethylene oxide) (PEO), intercalated in montmorillonite, a naturally occurring mica-type silicate, was studied by differential scanning calorimetry ( DSC and thermally stimulated dielectric depolarization ( or thermally stimulated current, TSC). The materials were synthesized by melt or solution-mediated intercalation. In both intercalates, the PEO chains were confined to ca. 0.8-nm galleries between the silicate layers. The solution intercalate contained a fraction of uninterca-lated PEO chains which exhibited a weak and depressed PEO melting endotherm in DSC. In contrast, the melt intercalate was starved such that almost all the PEO chains were effectively intercalated. For these melt intercalates, no thermal events were detected by DSC. TSC thermal sampling technique was used to examine the glass transition regions and to estimate the extent of cooperativity of chain motions. The motions of the intercalated PEO chains are inherently noncooperative relative to the cooperative T g motions in the amorphous portion of the bulk polymer. This is presumably due to the strong confining effect of the silicate layers on the relaxations of the intercalated polymer.

Thermal characterization of organically modified montmorillonite

Abstract Polymer/organically modified layered silicate (PLS) nanocomposites are a new class of filled polymers with ultrafine phase dimensions. They offer an outstanding combination of stiffness, strength and weight that is difficult to attain separately from the individual components. Additionally, the nanoscopic phase distribution as well as synergism between polymer and the layered silicate result in additional properties, such as flame retardency, enhanced barrier properties and ablation resistance, which are not observed in either component. These nanocomposites are synthesized by blending the organically modified layered silicate (OLS) into the polymer melt. Thus, understanding the relationship between the molecular structure and the thermal stability (decomposition temperature, rate, and the degradation products) of the organic modification of the layered silicate is critical. During this study, modern thermal analysis techniques combined with infrared spectroscopy and mass spectrometry (TGA–FTIR–MS) were used to obtain information on the thermal stability and degradation products. The effect of chemical variation (alkyl chain length, number of alkyls, and unsaturation) of organic modifiers on the thermal stability of the organically exchanged montmorillonite are discussed. A range of interesting results is observed, however, not all are currently understandable.

Temperature dependence of polymer crystalline morphology in nylon 6/montmorillonite nanocomposites

Abstract The influence of nanodispersed montmorillonite layers and process history on the crystal structure of nylon 6 between room temperature and melting is examined with simultaneous small- and wide-angle X-ray scattering and modulated differential scanning calorimetry. For the examined process history, nylon 6 exhibits predominantly α-phase behavior from room temperature to melting, with a gradual shift in chain–chain and sheet–sheet spacings from ∼100°C to melting. In contrast, the presence of aluminosilicate layers stabilizes a dominant γ-crystal phase, which persists, essentially unmodified, until melting. The temperature dependence of the total crystallinity and the relative fractions of α- and γ-phases is strongly dependent on the layered silicate content and the interaction between the nylon 6 and the aluminosilicate layers. Additionally, the temperature dependence of the α- and γ-phases imply that the γ-phase is preferentially in the proximity of the silicate layers, whereas the α-phase exists away from the polymer–silicate interphase region: In general, process history and use-temperature will determine the relative fraction of the crystalline polymer phases in semi-crystalline polymer nanocomposites, and thus have significant influence on the stability of the crystalline region at elevated temperatures.

A high frequency photodriven polymer oscillator

High frequency and large amplitude oscillations are driven by laser exposure in cantilevers made from a photosensitive liquid crystal polymer.

Polymer nanocomposites: a new strategy for synthesizing solid electrolytes for rechargeable lithium batteries

Abstract The ionic conductivity and lithium electrode-electrolyte interfacial stability have been measured for composite polymer electrolytes using micrometer- and nanometer-size alumina (Al2O3) with polyethylene oxide (PEO) and lithium tetrafluoroborate (LiBF4). The influence of the nanometer-size alumina particles increases ionic conductivity by an order of magnitude compared with micrometer-size particles. The interfacial stability is increased a factor of two. Characterization of layered nanocomposite polymer electrolytes based upon melt intercalation of PEO in layered silicates (montmorillonite) show that the intercalated PEO is amorphous.

Nanocomposites: issues at the interface

Carbon nanotubes (CNTs), whether single- or multi-walled (SWNT or MWNT, respectively), have, in an unparalleled fashion, grabbed the attention of both researchers and business leaders within the polymer community. The vast potential afforded by the unprecedented combination of mechanical, electrical, and thermal properties within one nanoscale additive opens new vistas for commodity plastics, elastomers, adhesives, and coatings, as well as new specialty systems with never-before-realized combinations of material properties within a processible plastic or fiber 1 , 2 , 3

Depletion-induced shape and size selection of gold nanoparticles.

For nanoparticle-based technologies, efficient and rapid approaches that yield particles of high purity with a specific shape and size are critical to optimize the nanostructure-dependent optical, electrical, and magnetic properties, and not bias conclusions due to the existence of impurities. Notwithstanding the continual improvement of chemical methods for shaped nanoparticle synthesis, byproducts are inevitable. Separation of these impurities may be achieved, albeit inefficiently, through repeated centrifugation steps only when the sedimentation coefficient of the species shows sufficient contrast. We demonstrate a robust and efficient procedure of shape and size selection of Au nanoparticles (NPs) through the formation of reversible flocculates by surfactant micelle induced depletion interaction. Au NP flocculates form at a critical surfactant micelle molar concentration, C(m)* where the number of surfactant micelles is sufficient to induce an attractive potential energy between the Au NPs. Since the magnitude of this potential depends on the interparticle contact area of Au NPs, separation is achieved even for the NPs of the same mass with different shape by tuning the surfactant concentration and extracting flocculates from the sediment by centrifugation or gravitational sedimentation. The refined NPs are redispersed by subsequently decreasing the surfactant concentration to reduce the effective attractive potential. These concepts provide a robust method to improve the quality of large scale synthetic approaches of a diverse array of NPs, as well as fine-tune interparticle interactions for directed assembly, both crucial challenges to the continual realization of the broad technological potential of monodispersed NPs.

Polarization-controlled, photodriven bending in monodomain liquid crystal elastomer cantilevers

We report on the fast and optically controlled angular bending of cantilevers made from liquid crystal polymer networks functionalized with azobenzene moieties (azo-LCN). For potential applications such as adaptive optics, photoresponsive cantilevers should rapidly deform to controlled angles while maintaining a modulus that can accomplish appreciable mechanical work. This work demonstrates cantilevers made of monodomain azo-LCN containing pendant (side chain) azobenzene mesogens with a storage modulus of 1.4 GPa bend 85° in less than 300 ms upon exposure to 442 nm irradiation. Moreover, the bending angle of these monodomain azo-LCN cantilevers can be controlled by the polarization angle of the source relative to the long-axis of the cantilever. The bending performance (deformation angle and speed) of this monodomain system is compared to a polydomain analogue. The impact of azobenzene concentration, laser intensity, and thickness on these parameters is also presented.