Mark C. Piton

An ‘‘on’’‐gated photomultiplier circuit for the determination of phosphorescence lifetimes

An ‘‘on’’‐gated photomultiplier circuit is described which possesses a cutoff ratio >103 by switching dynodes 1 and 3 of a nine‐stage side‐on phototube (Hamamatsu models 1P28, R955, etc.). ‘‘On’’‐gate delay and gate‐width times are continuously variable between 150 ns and 10 ms. The ‘‘ringing’’ usually observed after the gate opening in similar circuit designs, due to the propagation delay in the dynode chain and the capacitive coupling between the anode and the switched dynodes, has been greatly reduced using a ‘‘spike compensator’’ circuit. When −1000 V was applied to the anode of the phototube, the resulting output of the circuit was linear 80 ns after the unit was triggered. The circuit will also operate as an ungated photomultiplier.

The Measurement and Meaning of Rate Coefficients in Free-Radical Polymerizations

New experimental data and theory are reported for obtaining three types of rate parameters controlling kinetics and molecular weight distributions in free-radical polymerizations. (1) (a) Propagation rate coefficients (kp) for butyl aery late are found using pulsed-laser polymerization, with the novel variant of carrying out the procedure at very low temperatures (−10 to −40 °C); this obviates some problems hitherto found with the technique, (b) A means of calculating kp from first principles from transition state theory is given, using accurate quantum mechanical computation of the transition state properties. These calculations give quantitative accord with experiment for ethylene, and reveal that the magnitude of the frequency factor for propagation is dominated by three hindered rotors in the transition state. Since these rotations are influenced by the penultimate unit, this suggests that significant penultimate unit effects are likely in copolymerizations. The effect of these rotations also explains the observed effect of deuteration on kp. (2) A new technique for measuring the transfer rate coefficient (k tr) is presented: initiation by pulsed laser, whence k tr can be obtained from the high molecular weight slope of a plot of ln(number molecular weight distribution) against molecular weight. The method, which can be employed even when extensive termination is taking place, is tested with transfer from MMA to triethylamine. (3) The rate coefficient for termination can be successfully modelled from a knowledge of the diffusion coefficients for oligomeric species as functions of the degree of polymerization and the polymer fraction. New PFG NMR diffusion data for a series of model compounds in polystyrene/benzene give results for this dependence which are in accord with a semiempirical scaling law suggested previously [Piton MC, Gilbert RG, Chapman BE, Kuchel PW (1993) Macromolecules 26: 4472], suggesting that this scaling should be a useful basis for predicting termination rate coefficients.