[en] Average energy-transfer values are reported from trajectory studies of Xe and He colliding with highly excited azulene-d₀ and azulene-d₈. The calculated isotope effect is small, in agreement with experiments on related systems. This suggests that the low-frequency modes are important in determining the amount of energy transfer. Furthermore, the fact that both calculated and observed changes with deuteration are small for helium suggests that the classical description of the energy transfer is adequate. The biased random walk model for collisional energy transferred is found to reproduce the observation of a small isotope effect. The calculated fraction and magnitude of supercollisions are in accord with extant data and are predicted to be relatively unaffected by deuteration. 33 refs., 2 figs., 2 tabs

[en] Full text: One of the most important means of polymer formation involves initiation by free radicals. A subset of this, especially important in industry, is emulsion polymerization, whereby the polymerization process is dispersed in water: an environmentally-friendly 'solvent'. γ radiolysis as an initiation method for free-radical polymerization can give unique mechanistic information: it can penetrate opaque media (i.e., produce a uniform flux of radicals) yet can be switched off instantly, thereby enabling radical loss mechanisms to be investigated. It also gives a steady radical flux at any temperature, which is particularly convenient for looking at reactions at low temperatures, thereby giving the means of 'tuning out' complications that can occur at elevated temperatures. Data will be presented to show that this has enabled rate coefficients for a variety of free-radical processes to be obtained, often for the first time: for example, radical loss in emulsion polymerization by exit and by termination. A new method enables termination rate coefficients to be obtained by two completely independent means (one of which requires γ), thereby verifying the results from both. However, care must be taken for certain systems, such as those involving vinyl esters, where evidence has been obtained that unusual species are formed which undergo slow subsequent polymerization but rapid termination, thereby limiting the applications of the technique. The knowledge gained from these studies has been used to develop theories for these free-radical processes which are being further refined by testing with further experiments. Moreover, this knowledge has also been used to design new materials

[en] Diffusion-controlled reactions are common in free radical polymerisation reactions, especially in glassy polymer matrices. Such reactions commonly have an important influence on the polymerisation process and final polymer properties. For example, the dominant growth-stopping event (bimolecular termination) is generally diffusion-controlled. In glassy polymer systems, where molecular mobility is very low, the chain growth mechanism (propagation) may become diffusion-controlled. At present, the mechanism for propagation in glassy polymers is poorly understood, but it is expected by the Smoluchowski expression applied to propagation to depend strongly on the diffusion coefficient of monomer. The objective of this study is to measure reliable diffusion coefficients of small tracer molecules in glassy polymers, and compare these with propagation rate coefficients in similar systems, by the prediction above. Samples were initially prepared in a sealed sampled cell containing monomer, inert diluent, and tracer dye. After irradiation for several days, complete conversion of monomer to polymer can be obtained. The diffusion coefficients for two tracer dyes have been measured as a function of weight fraction polymer glassy poly(methyl methacrylate) samples

[en] In most emulsion polymerizations the exhaustion of the separate monomer phase is followed by a decrease in conversion rate due to the decrease in monomer concentration in the latex particles. The unusual feature of vinyl acetate is that the rate remains constant throughout most of the period of decreasing monomer concentration. There are two postulated mechanisms consistent with the observed kinetics. Both require that k_ex = k_tr C_p and this can be tested with a gamma radiolysis experiment. (k_ex = exit rate coefficient, k_tr = rate coefficient for transfer to monomer and C_p monomer concentration in particles). An automatic tracking device (developed by David Sangster) measures the meniscus height in a dilatometer every three seconds. When the dilatometer is removed from the gamma source initiation by radiolysis stops allowing k_ex to be determined directly from the relaxation data. One aspect of relaxation experiments which is often overlooked is the effect of the heat of reaction on the temperature of the reaction mixture. In order to compensate for this effect the temperature and contraction of the reaction mixture were monitored (out of source) while the temperature of the waterjacket was lowered through the range appropriate to the particular experiment. These data were used to construct a calibration curve which, together with a sensitive temperature probe in the dilatometer, was used to correct the conversion/time data obtained during the relaxation experiment

[en] The method devised by Collins and Gilbert for obtaining generalized opacity and deflection functions from reactive and non-reactive cross sections is extended and applied to experimental data on the Ar⁺+D₂→ArD⁺+D reaction and to data on a series of K + halogens reactions. The resulting opacity functions show systematic variations and suggest mechanistic interpretations. (Auth.)

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[en] Abstract only. 1 ref

No abstract available

[en] An emulsion polymerisation typically consists of two phases: the continuous aqueous phase, and the discreet organic phase, which comprises of monomer droplets and latex particles. This system is stabilised by some form of surfactant. While polymerisation mainly occurs in the latex particles, the events which initiate polymerisation usually occur in the aqueous phase. Therefore, the transfer of radical activity from the aqueous phase to the organic phase (otherwise known as entry) is important in the kinetics of polymerisation. Likewise, transfer of radicals from the latex particles to the aqueous phase (exit), is important as a means whereby polymerisation in the latex particle ceases. This study utilizes seeded emulsion polymerisations. Two electrostatically stabilised styrene lattices were grown, of 24 nm and 44 nm particle radius. Poly(acrylic acid), an electrosteric stabiliser, was absorbed onto the surface of the latex particles as a second stage procedure, forming electrosterically stabilised lattices. Dilatometry was used to measure the conversion rate: the steady rate with chemical initiator and relaxation from steady state in a system initiated by γ radiolysis has been used to obtain the exit rate coefficient

[en] Abstract only