Low-Temperature Aging of Zirconia Ferrules Limits Optical-Connector Reliability
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smoothed and anisotropically etched samples was evident throughout a wide range of waveguide widths. According to the researchers, this smoothing technology could lead to significant improvements in practical waveguide design for optical devices. TIM PALUCKA
Dynamics Study of Polymer Chains Shows Reduction in Atomic Diffusion Coefficients at a Free Surface In the November 12, 2001, issue of Physical Review Letters, researchers from the State University of New York—Stony Brook reported the relationship between structure and dynamics of polystyrene (hPS) by dynamic secondary-ion mass spectrometry (DSIMS). Their experimental results confirmed the chain-conformation calculation at the free surface by meanfield theory. In order to understand how the center of mass diffusion of polystyrene related to its different molecular weight as a function of distance from the vacuum interface, a special multiple-layer sample used in the DSIMS experiment was designed. It had two parts, each consisting of a 200-nm layer of matrix hPS followed by a 25-nm layer of 20% monodisperse deuterated analogue (dPS) and 80% matrix hPS. From the relationship between the concentration of the tracer polymer (hPS or dPS) volume and the distance from the free surface for the four-layer sample, the diffusion coefficients for diffusion into a semi-infinite and an infinite medium for the top layer and the middle layer were both reduced relative to the bulk value within the distance from the surface. The researchers found that “a purely enthalpic argument” cannot explain the experimental result. They had previously reported a large slowing down of dynamics when polymer chains were oriented parallel to the surface by rubbing the surface. They then hypothesized that the stringent confinement imposed by the large surface tension at the vacuum interface was inducing the chain orientation. This type of orientation was also plausible from the theoretical work of Kurt Binder, who demonstrated that the fluctuation times at the surface of a polymer film were anisotropic—namely, those parallel to the sample surface were much faster than those in the perpendicular direction. To test this hypothesis, self-consistent field theory was used and a one-dimensional equilibrium lattice method was designed, in consideration of the small system size. This method identified the segmental distribution at the surface, that is, the number MRS BULLETIN/JANUARY 2002
of segments a chain has near the surface as a function of its position from the surface. After comparing the theoretical calculation with the experiment results of the chain dynamics, the researchers confirmed that asymmetric segmental fluctuations at the surface resulted in decreased mobility at the surface. This result overturned the long-held assumption that a large enhancement in mobility would be found at the surface, in analogy to monomolecular systems where this effect was observed as a result of the reduced potential. YUE HU
One-Dimensional Metallic Edge States Observed in MoS2
Researchers at the Technical
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