We report the production and characterization of polymer nanocomposites with single-walled carbon nanotubes having improved mechanical properties and exceptional nanotube alignment. High-pressure carbon monoxide nanotubes (HiPco) were efficiently distributed in polystyrene (PS) and polyethylene (PE) with a twin-screw compounder. Nanotube concentrations were 1, 5, 10, and 20 wt% in PE composites and 0.7 wt% in PS composites. PE composites were melt-spun into fibers to achieve highly aligned nanotubes. Polarized Raman spectroscopy shows that the degree of alignment increases with decreasing fiber diameter and decreases with increasing nanotube loading. The orientation distribution function of a 1 wt% HiPco/PE composite had a full width at half-maximum of approximately 5 degrees. The elastic modulus increases up to 450% relative to PE fibers for 20 wt% nanotube loading at an intermediate fiber diameter of 100 microns.
The neutron pair distribution function (PDF) of RbsCso at 10 K reveals local deviations from the Stephens model of merohedral disorder. The experimental PDF in the 3 -7 A. range is best represented by a greater fraction of near-neighbor pairs with opposite standard orientations than would be expected from purely random decoration, indicating the presence of short-range "antiferromagnetic" orientational correlations.The average crystal structure of the prototype fullerene superconductor Rb3C6o has been firmly established by Rietveld refinement of powder x-ray difFraction data. The molecules form a face-centered cubic lattice, the tetrahedral and octahedral voids are occupied by Rb ions, and the C60's are all oriented with twofold axes parallel to cube edges but are randomly distributed over two equally populated "standard orientations" A and B which dier by a vr/2 rotation about any twofold axis. This is referred to as merohedral disorder, space group Em3m. The disorder persists at all accessible temperatures ' but may not be the equilibrium ground state.Given the sensitivity of the observed superconducting properties ' and the calculated densities of states to difFerences in orientational correlations, it is important to determine if short-range deviations &om the average structure exist.Here we describe the results of pulsed neutron atomic pair distribution function (PDF) analysis in determining the nature of the local structural disorder in Rb3C60 at 10 K. In this approach [which has been successfully applied to pure Cso (Ref. 10)], both the Bragg and diffuse scattered intensities are included without assuming long-range order (and preserving the short-range information), and the comparison between data and structure models is made in real space rather than reciprocal space, with emphasis on difFerentiating between random and short-range correlated atomic displacements. ' Indeed we find a distinct preference for A-B contact compared to what would obtain with random decoration, but only on the scale of first neighbors. There is no inconsistency with the globalLy equal &actions of A and B, nor is there any evidence for intermediate-or long-range orientational order. The 1 g sample of nominal Rb3C60 was taken from a larger batch prepared by reacting weighed amounts of Rb and C60. The latter was purchased &om TERM USA; purity was checked by HPLC to be better than 99.9%, and the orientational ordering transition was observed at the ideal value 261 K by diA'erential scanning calorimetry. The C60 was first ground to a fine powder in an argonfilled glove box. After vacuum sealing in pyrex with the Rb, the solid state reaction was carried out at 433 K for 3 days followed by 4 days at 733 K, then slowly cooled to 300 K over ll h. High-resolution x-ray difFraction at 300 K revealed no detectable impurity phases at the 2% level. dc magnetization showed a superconducting onset at 28 K with 100% shielding and 14% Meissner fractions (measured at 10 G). The Hebel-Slichter coherence peak in spin-lattice relaxation rate was o...
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