Thermochromatism and structural evolution of metastable polydiacetylenic crystals.
Topochemically polymerized sodium 10,12-pentacosadiynoate (PCDA-Na) microcrystals show an irreversible red-to-blue chromatic transition accompanied by a distinct structural evolution upon initial thermal treatment, and show a subsequent completely reversible blue-to-red chromatic transition upon further thermal stimuli. Visible absorption spectroscopy, X-ray diffraction (XRD), and differential scanning calorimetry (DSC) are used to investigate the thermochromatic transition behavior of the polydiacetylenic microcrystals. Brief quantum mechanical geometry optimization is employed to explain the lattice dimensional change during the irreversible red-to-blue chromatic transition of the metastable polydiacetylenic crystals.
Length scales necessary for proper averaging to characterize polymerization in nanosystems: topochemical polymerization of diacetylene nanocrystals dispersed in a polystyrene matrix as probed by confocal Raman microscopy.
The scales necessary to make appropriate spatial averaging on solid-state polymerization were investigated by confocal Raman microscopy with a mapping resolution of 2 μm. Nanocrystals of an aliphatic diacetylene with an average size of 0.14 μm, each separated by 0.3 μm on average, were dispersed in a polystyrene matrix and were polymerized by UV irradiation. The distribution of nanocrystals was inhomogeneous over approximately 20 μm scale. A large crystal of the same monomer shows that photoinitiation is already averaged at the microscope resolution, while the color transition from the blue to the red form requires a scale greater than 5 μm. For the nanocrystals at low conversion, UV-vis absorption spectroscopy measured over a centimeter scale indicates linear polymerization kinetics and a higher polymer yield at a higher temperature. By contrast, the Raman microscopy reveals that, whereas the 20 μm region of high monomer concentrations yields more polymers at -24 °C, the region of low monomer concentrations gives more polymers at 20 °C. We propose thermal initiation, which is not efficient in the large crystals, as an additional initiation process for the apparent discrepancy, implying that the initiation process is not averaged below 20 μm scale for the dispersed nanocrystals.
Active profiling and polishing for efficient control of material removal from large precision surfaces with moderate asphericity
We present the development of an active fabrication technology for controlling material removal on large precision surfaces of moderate departure from a sphere. The underlying philosophy was established as an efficient solution to the challenging problem of fabricating secondary mirrors of up to nominally 2.5 m in diameter for modern 8 m class telescopes and beyond. The facility described comprises a CNC profiler, two contact profilometers, and a full size active polisher. The trial work-piece was a convex surface of 830 mm in diameter on a zero-expansion ceramic (`Cervit') blank – a 1/3 scale hyperbolic mirror for a proposed 2.5 m diameter f/7 secondary mirror for the 8 m Gemini telescope. Using software error-correction from profilometric metrology data, a factor of 2 improvement in generating the convex aspheric profile was achieved. An active loose-abrasive polishing process is also described, in which a full-size tool is configured to deliver variable, edgeless, sub-diameter removal-footprints. Real-time monitoring of process variables is described, and approximately 10% convergence in each polishing pass is reported.