Single-walled carbon nanotube (SWNT)/poly(methyl methacrylate) (PMMA) nanocomposites were prepared via our coagulation method providing uniform dispersion of the nanotubes in the polymer matrix. Optical microscopy, Raman imaging, and SEM were employed to determine the dispersion of nanotube at different length scales. The linear viscoelastic behavior and electrical conductivity of these nanocomposites were investigated. At low frequencies, G‘ becomes almost independent of the frequency as nanotube loading increases, suggesting an onset of solidlike behavior in these nanocomposites. By plotting G‘ vs nanotube loading and fitting with a power law function, the rheological threshold of these nanocomposites is ∼0.12 wt %. This rheological threshold is smaller than the percolation threshold of electrical conductivity, ∼0.39 wt %. This difference in the percolation threshold is understood in terms of the smaller nanotube−nanotube distance required for electrical conductivity as compared to that required to impede polymer mobility. Furthermore, decreased SWNT alignment, improved SWNT dispersion, and/or longer polymer chains increase the elastic response of the nanocomposite, as is consistent with our description of the nanotube network.

The intention of this study is to discuss scientific advances toward one very important challenge in the polymer processing industry: How does one increase the crystallization rate of slow-to-crystallize polymeric materials, thereby facilitating processing and enabling peak product performance? In the medical device field, where both government-controlled regulatory entities and medical professionals closely scrutinize the biocompatibility of added crystallization rate enhancers, achieving these twin goals has always been challenging. Herein, we present a review of various chemical and physical approaches used to tune the crystallization rate of semicrystalline polymers, with a strong emphasis on two novel approaches recently discovered and developed in our laboratories.

A material strained beyond its yield point typically suffers substantial irrecoverable deformation. Surprisingly, this is not the case for ethylene/methacrylic acid (E/MAA) copolymers and ionomers, for which significant permanent deformation does not result until the applied strain exceeds 50-150%, far beyond the yield strain of 5-10%. At room temperature, strain recovery is complete on the order of hours or days following the removal of the applied load. Interestingly, the onset of permanent deformation coincides with a broad maximum or shoulder in the plot of stress versus strain. Two-dimensional X-ray scattering studies of both initially isotropic samples and highly aligned blown films reveals that this ''second yield shoulder,'' commonly observed in the stress-strain curves of ethylene/a-olefin copolymers, is fundamentally associated with polyethylene crystal fracture, resulting in fragments of reduced lateral extent. Connections formed between these crystalline fragments lock in the deformed conformations of the amorphous intercrystalline segments, preventing the specimen from retracting to its initial dimensions. Additional recovery is possible through heating; complete melting of the deformed specimens results in full recovery up to applied strains of 200%, beyond which strain-induced chain disentanglement begins. V V C 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys

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In the idealized two-phase model of a semicrystalline polymer, the amorphous intercrystalline layers are considered to have the same properties as the fully-amorphous polymer. In reality, these thin intercrystalline layers can be substantially influenced by the presence of the crystals, as individual polymer molecules traverse both crystalline and amorphous phases. In polymers with rigid backbone units, such as poly(etheretherketone), PEEK, previous work has shown this coupling to be particularly severe; the glass transition temperature (T g ) can be elevated by tens of degrees celsius, with the magnitude of the elevation correlating directly with the thinness of the amorphous layer. However, this connection has not been explored for flexible-chain polymers, such as those formed from vinyl-type monomers. Here, we examine T g in both isotactic polystyrene (iPS) and syndiotactic polystyrene (sPS), crystallized under conditions that produce a range of amorphous layer thicknesses. T g is indeed shown to be elevated relative to fully-amorphous iPS and sPS, by an amount that correlates with the thinness of the amorphous layer; the magnitude of the effect is severalfold less than that in PEEK, consistent with the minimum lengths of polymer chain required to make a fold in the different cases.