letters to nature 146 NATURE | VOL 400 | 8 JULY 1999 | www.nature.com Our interpretation uses the approximately linear semiclassical relation G(R) between conductance and wire cross-section, equation (1). Corrections may arise from two mechanisms. First, back scattering on defects (or phonons) may shift the peak positions. The fact that strong scattering would tend to smear the peak structure, and the close agreement between the experimental and theoretical periodicity both suggest that this shift is small. Second, the mechanism of level-bunching leading to¯uctuations in R should also give rise to¯uctuation corrections to G(R), which by itself would lead to peaks in the histograms even for a perfectly smooth distribution of wire diameters. Peaks due to this mechanism would be found at those points where the increase of the conductance with wire diameter is slow, and indeed we believe the peaks at low conductance (near 1, 3 and 6 G 0 ) are due to this mechanism. However, we argue that the main structure in Fig. 3 is due to the shell structure in R.The main argument in favour of this interpretation comes from the temperature dependence shown in Fig. 2. As in the cluster experiments 11 , the electronic shell structure becomes observable at higher temperatures, where the increased mobility of the atoms allows the system to explore a wider range of neighbouring atomic con®gurations in order to ®nd the local energy minima. At low temperatures, the nanowires are frozen into the shape which evolves from mechanical deformations in the breaking and indenting processes. The decrease in amplitude of the sharp quantization peaks at low conductance can probably be explained by thermally induced breaking of the contact when it consists of only a few atoms. If the¯uctuations in G(R) were solely responsible for the observed peak structure, it could be argued that the higher temperature would favour formation of a smoother and longer wire, leading to less backscattering of electrons and less tunnelling corrections, respectively. Backscattering is responsible for the shift to lower values of the conductance peaks near 1, 3, 5 and 6 G 0 (ref. 20), and we ®nd that the position of these peaks does not change with temperature. An indication of the length of the wire may be obtained from the global variation of the conductance with elongation 21 , and the time evolution of conductance for ®xed elongation 22 . In our experiment, both variations show that the effective wire length decreases for increasing temperature. With these arguments, an interpretation of the temperature dependence of Fig. 2 in terms of G(R)¯uctuations alone is ruled out.The correspondence between the shell structure in clusters and in nanowires can be explained by comparing the energy levels for a three-dimensional spherical potential well and a two-dimensional cylindrical geometry. The levels are obtained from the zeros of the spherical and ordinary Bessel functions, respectively. Apart from a small constant shift, the distribution of levels for the two systems has...

We report measurements of rapid ordering and preferential alignment in block copolymer films zone annealed below the order-disorder transition temperature. The orientational correlation lengths measured after approximately 5 h above the glass-transition temperature ( approximately 2 microm) were an order of magnitude greater than that obtained under equivalent static annealing. The ability to rapidly process polymers with inaccessible order-disorder transition temperatures suggests zone annealing as a route toward more robust nanomanufacturing methods based on block copolymer self-assembly.

Bovine synovial fluid and albumin solutions of similar concentration are rheopectic (stress increases with time in steady shear). This unusual flow characteristic is caused by protein aggregation, and the total stress is enhanced by entanglement of this tenuous protein network with the long-chain polysaccharide sodium hyaluronate under physiological conditions. Neutron scattering measurements on albumin solutions demonstrate protein aggregation and all measurements are consistent with a weak dipolar attraction energy (of order 3kT) that is most likely augmented by hydrophobic interactions and/or disulfide bond formation between proteins. Protein aggregation appears to play an important role in the mechanical properties of blood and synovial fluid. We also suggest a connection between the observed rheopexy and the remarkable lubrication properties of synovial fluid.

Photopolymerized hydrogels from poly(ethylene glycol) dimethacrylate (PEGDM) and similar derivatives have been extensively used as scaffolds for tissue regeneration and other biological applications. A systematic investigation into the structure and mechanical properties of PEGDM hydrogels was performed to characterize the relationships between the network structure and gel properties. Gels were prepared from oligomers of different molecular masses (1000−8000 g/mol) at various concentrations. Small-angle neutron scattering was used to characterize the structural features of hydrogels with respect to their semidilute solution precursors. A well-defined structural length scale manifested as a maximum in the scattering intensity was observed for hydrogels derived from high molecular mass PEGDMs and/or high oligomer mass fractions. Mechanical testing showed that PEGDM hydrogels are mechanically robust, consistent with gel structures that contain reinforcing cross-linked clusters. Shear moduli were determined for hydrogels swollen to various degrees in water. The concentration dependence of shear modulus for these nonideal hydrogels exhibits a power law behavior with an exponent close to 1/3.

We investigate the effect of the ordering temperature (T) and film thickness (h(f)) on the surface morphology of flow-coated block copolymer (BCP) films of asymmetric poly(styrene-block-methyl methacrylate). Morphology transitions observed on the ordered film surface by atomic force microscopy (AFM) are associated with a perpendicular to a parallel cylinder BCP microphase orientation transition with respect to the substrate with increasing h(f). "Hybrid" surface patterns for intermediate h(f) between these limiting morphologies are correspondingly interpreted by a coexistence of these two BCP microphase orientations so that two "transitional" h(f) exist for each T. This explanation of our surface patterns is supported by both neutron reflectivity and rotational SANS measurements. The transitional h(f) values as a function of T define upper and lower surface morphology transition lines, h(fu) (T) and h(fl) (T), respectively, and a surface morphology diagram that should be useful in materials fabrication. Surprisingly, the BCP film surface morphology depends on the method of film formation (flow-coated versus spun-cast films) so that nonequilibrium effects are evidently operative. This morphological variability is attributed primarily to the trapping of residual solvent (toluene) within the film (quantified by neutron reflectivity) due to film vitrification while drying. This effect has significant implications for controlling film structure in nanomanufacturing applications based on BCP templates.