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Segmented poly[2-methoxy-5-(2-ethylhexloxy)-1,4-phenylene vinylene] (MEH-PPV)-x's, which contain conjugated segments of varying lengths that are interspersed by nonconjugated units along the polymer backbone, were synthesized by selective thermal elimination of precursors containing controlled amounts (x) of a thermally labile group, namely, xanthate or dithiocarbamate (DTC). These precursors were in turn synthesized by competitive nucleophilic substitution of the Wessling polyelectrolyte with varying molar fractions of the respective nucleophiles-potassium ethyl xanthate or sodium diethyl dithiocarbamate. Methanol, used as the reaction medium, also served to introduce the second thermally less labile nucleophilic substituent. This approach for the preparation of segmented MEH-PPV-x is superior to the previous approach that used acetate as the thermally labile group, because it offers greater control over the composition despite a simpler synthetic procedure. Detailed studies of the thermal-elimination kinetics of the three precursors, namely, acetate, xanthate, and DTC, both in solution and in thin films, were carried by in situ monitoring of their ultraviolet-visible spectra. These studies revealed that the rates of elimination followed the order, DTC Ͼ xanthate Ͼ acetate. The activation energies for the elimination were, however, not widely different (ca. 30 kcal/mol), suggesting that the rates primarily reflected differences in the pre-exponential factor. After elimination, the segmented MEH-PPV-x samples exhibited the expected redshift in their absorption and fluorescence spectra with an increasing molar fraction (x) of eliminated segments, which was accompanied by a drastic reduction in the fluorescence quantum yields.

The shot to shot variation in perveance of a planar diode with explosive emission graphite cathode in a range of accelerating gaps 3–12 mm is investigated experimentally. The typical electron beam parameters were 200 kV, 12 kA, 100 ns, with a few hundreds of A/cm2 current density. The diode perveance remains less than the Child–Langmuir value, indicating that only a fraction of the cathode take part in the emission process. A simple statistical analysis of the diode perveance shows that the shot to shot variation is more pronounced for the later part of the accelerating pulse. The cathode plasma expansion velocity and the effective initial emission area have been calculated from the perveance data. It was found that the plasma expansion velocity varies from 3 to 6.5 cm/μs. The mean expansion velocity and the standard deviation increase with the increase in the accelerating gap. The initial emission areas also varies randomly on a shot to shot basis and at the beginning of the accelerating pulse only 4%–35% of the cathode area take part in the emission process. The mean initial emission area and the standard deviation also increase with the increase in the accelerating gap. Experimental result indicates that the larger gaps and lower electric fields suggest a path to more uniform emission.

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