Our continuing study of the mechanism of flammability reduction of polymer−layered-silicate nanocomposites has yielded results for polypropylene-graft-maleic anhydride and polystyrene−layered-silicate nanocomposites using montmorillonite and fluorohectorite. Cone calorimetry was used to measure the heat release rate and other flammability properties of the nanocomposites, under well-controlled combustion conditions. Both the polymer−layered-silicate nanocomposites and the combustion residues were studied by transmission electron microscopy and X-ray diffraction. We have found evidence for a common mechanism of flammability reduction. We also found that the type of layered silicate, nanodispersion, and processing degradation have an influence on the flammability reduction.

The static and dynamic properties of polymer-layered silicate nanocomposites are discussed, in the context of polymers in confined spaces and polymer brushes. A wide range of experimental techniques as applied to these systems are reviewed, and the salient results from these are compared with a mean field thermodynamic model and non-equilibrium molecular dynamics simulations. Despite the topological constraints imposed by the host lattice, mass transport of the polymer, when entering the galleries defined by adjacent silicate layers, is quite rapid and the polymer chains exhibit mobilities similar to or faster than polymer self-diffusion. However, both the local and global dynamics of the polymer in these nanoscopically confined galleries are dramatically different from those in the bulk. On a local scale, intercalated polymers exhibit simultaneously a fast and a slow mode of relaxation for a wide range of temperatures, with a marked suppression (or even absence) of cooperative dynamics typically associated with the glass transition. On a global scale, relaxation of polymer chains either tethered to or in close proximity (<1nm as in intercalated hybrids) to the host surface are also dramatically altered. In the case of the tethered polymer nanocomposites, similarities are drawn to the dynamics of other intrinsically anisotropic fluids such as ordered block copolymers and smectic liquid crystals. Further, new non-linear viscoelastic phenomena associated with melt-brushes are reported and provide complementary information to those obtained for solution-brushes studied using the Surface Forces Apparatus.

. Here we show that model elastomeric artifi cial skins wrinkle in a hierarchical pattern consisting of self-similar buckles extending over fi ve orders of magnitude in length scale, ranging from a few nanometres to a few millimetres. We provide a mechanism for the formation of this hierarchical wrinkling pattern, and quantify our experimental fi ndings with both computations and a simple scaling theory. Th is allows us to harness the substrates for applications. In particular, we show how to use the multigeneration-wrinkled substrate for separating particles based on their size, while simultaneously forming linear chains of monodisperse particles.Wrinkling, buckling and other mechanical instabilities have been typically treated as a nuisance to be avoided rather than an exquisite pattern to be exploited. Although this view is changing with the growing understanding of how ubiquitous these phenomena are 9 , the utilization of wrinkling in applications has been hampered by the absence of a detailed understanding of the phenomena, as well as the ability to control it experimentally. Here we focus on the tunable hierarchical wrinkling of model stiff elastomeric artifi cial skins supported on a soft base. These wrinkles are fabricated by uniaxially stretching poly(dimethyl siloxane) (PDMS) network sheets (thickness ∼0.5 mm, Young modulus ∼1 MPa) 10 in a custom-designed stretching apparatus 11 and exposing them to ultraviolet/ozone (UVO) radiation for extended periods of time (30-60 minutes). Previous studies established that the UVO treatment of PDMS converts the fi rst ∼5 nm of the PDMS surface into a stiff 'skin' 12 , whose density is approximately a half of that of silica 13 . Optical microscopy and scanning force microscopy (SFM) experiments confi rm that the surfaces are originally fl at in the presence of strain.After the UVO treatment, the strain is removed from the specimen and, the skin buckles perpendicularly to the direction of the strain. The buckle morphology depends on the strain removal rate. Specifically, stretched and UVO-modified specimens released at a fast rate (strain removed abruptly)

The synthetic routes and materials properties of polypropylene/montmorillonite nanocomposites are reviewed. The nanocomposite formation is achieved in two ways: either by using functionalized polypropylenes and common organo-montmorillonites, or by using neat/ unmodified polypropylene and a semi-fluorinated organic modification for the silicates. All the hybrids can be formed by solventless melt-intercalation or extrusion, and the resulting polymer/inorganic structures are characterized by a coexistence of intercalated and exfoliated montmorillonite layers. Small additionsstypically less than 6 wt %sof these nanoscale inorganic fillers promote concurrently several of the polypropylene materials properties, including improved tensile characteristics, higher heat deflection temperature, retained optical clarity, high barrier properties, better scratch resistance, and increased flame retardancy.

Poly(vinyl alcohol)/sodium montmorillonite nanocomposites of various compositions were created by casting from a polymer/silicate water suspension. The composite structure study revealed a coexistence of exfoliated and intercalated MMT layers, especially for low and moderate silicate loadings. The inorganic layers promote a new crystalline phase different than the one of the respective neat PVA, characterized by higher melting temperature and a different crystal structure. This new crystal phase reflects on the composite materials properties. Namely, the hybrid polymer/silicate systems have mechanical, thermal, and water vapor transmission properties, which are superior to that of the neat polymer and its conventionally filled composites. For example, for a 5 wt % MMT exfoliated composite, the softening temperature increases by 25 °C and the Young's modulus triples with a decrease of only 20% in toughness, whereas there is also a 60% reduction in the water permeability. Furthermore, due to the nanoscale dispersion of filler, the nanocomposites retain their optical clarity.

The segmental dynamics of 1.5-2.0 nm polymer films confined between parallel solid surfaces is investigated with dielectric spectroscopy in polymer/silicate intercalated nanocomposites. The confinement effect is evident by the observation of a mode, much faster than the bulk-polymer alpha relaxation and exhibiting much weaker temperature dependence. This is discussed in relation to either the interlayer spacing restricting the cooperative volume of the alpha relaxation or to the dominance of the more mobile interphase regions as predicted by simulations; the data qualitatively support the former.

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Monte Carlo and molecular dynamics computer simulations are used to explore the atomic scale structure and dynamics of intercalated PEO/montmorillonite nanocomposites. Particular attention is paid to the configuration of the polymer within these confined spaces. A layered, but disordered and liquid-like, structure is observed, in contrast to the all-trans or helical extended interlayer structures traditionally suggested. The cations primarily reside near the silicate surface. Molecular dynamics simulations also provide information on the interlayer mobility of Li+ ions, which is related to the ionic conductivity in polymer electrolyte nanocomposites.