J.C. Robb

Pyrolysis-gas-liquid-chromatography utilised for a kinetic study of the mechanisms of initiation and termination in the thermal degradation of polystyrene

Abstract Pyrolysis-gas-liquid-chromatography (“thermocouple feedback” technique) has been used to study the thermal degradation kinetics of ionically-initiated and free-radical-initiated samples of polystyrene. Although mass-spectrometric measurements confirm that the pyrolysis products from large samples (1 mg) contain oligomers up to at least hexamer in addition to monomer, only monomer is detected when small thin samples (0.1 μg, 102–105 A) are used. This effect is not due to a sensitivity problem in detecting oligomers, nor to the incapacity of such compounds of limited volatility to elute from the GLC apparatus. In studying the kinetics of monomer evolution from thin films, initial work was concerned with the effect of film thickness and the limits of first-order behaviour. Then the specific rate of monomer evolution (kobs) was measured as a function of molecular weight for both types of sample at 723 K and 753 K; the results indicate that the pyrolysis mechanism involves both initiation at the chain-ends and initiation by random scission. Kinetic schemes involving mixed initiation have been proposed, and on this basis the results have been analysed to yield activation energies for scission and end-initiation for both types of sample. Comparison of the activation energies obtained with the quoted value for scission of a CC bond has shown that the depolymerization chain termination process cannot be second order and must be first order in the concentration of long chain radicals. The experimental results also indicate that the ionically-initiated polystyrenes are more stable than free-radical-initiated samples of comparable molecular weight. Possible initiation sites have been discussed with reference to the samples examined and to previous published studies. Several mechanisms leading to first order termination have been proposed; it is suggested that the most probable process involves intramolecular transfer with subsequent scission to give an oligomer radical which is small enough to diffuse readily from the system without further reaction.

An improved (“thermocouple-feedback”) pyrolysis-GLC technique and its application to study polyacrylonitrile degradation kinetics

Abstract An improved pyolysis-GLC unit has been designed in which a micro-thermocouple is spotwelded to the pyrolysis filament. The thermocouple output is used as a feedback signal to control the power supply to the filament. Fast temperature rise-times (0.02–0.1 s) and stable filament temperatures (better than ± 1°) have been achieved in this way. The system has been used to study the pyrolysis of polyacrylonitrile throughout the temperature range 300–800°. It was found that for samples of the order of 1 μm thickness (2.5 μg total mass) the degradation behaviour was independent of sample thickness. Total available yields of the six principal products and two uncharacterized products were measured as a function of temperature. Conversion curves and logarithmic plots permitted first-order rate constants to be evaluated at several temperatures, and Arrhenius parameters have been calculated from the results. Various mechanisms consistent with the results have been proposed.

Direct examination of polymer degradation by gas chromatography: I. Applications to polymer analysis and characterization

Abstract Two techniques of polymer degradation have independently been incorporated into a conventional gas chromatography apparatus, so arranged that the carrier gas sweeps the degradation products directly into the analysis column. The method involving pyrolysis on a filament has proved most versatile, since any temperature up to 1000°C may be selected for the degradation. The chromatogram series for a given polymer at a number of temperatures gives a rapid and unambiguous characterization of the polymer. Quantitative measurements on the chromatograms of copolymers and polymer mixtures enable their composition to be determined with an accuracy better than 2 per cent.

Polymethylmethacrylate degradation—Kinetics and mechanisms in the temperature range 340° to 460°C☆

Abstract Fractionated samples of polymethylmethacrylate have been degraded at temperatures within the range 340° to 460°C, using the micropyrolysis-GLC technique. The conversion at which deviation from first-order kinetics occurs has been found to vary with the temperature of the degradation. Rate constants for the fractions, calculated from the linear regions of the first-order plots, have been plotted as a function of the initial molecular weight of the fractions. The trends of these plots at different temperatures, and the deviations from first-order kinetics, are consistent with the following changes in mechanism throughout the temperature range. At the lowest temperatures, the depropagation reaction is principally initiated at the ends of the molecules, and termination occurs by bimolecular interaction. At intermediate temperatures, chain scission becomes sufficiently important to account for most of the initiation steps, though bimolecular interaction is still the important termination mechanism. At the highest temperatures, initiation by scission is the predominant initiation process, but the majority of the chains are effectively terminated by the diffusion out of the system of the ultimate radical remaining when a chain has completely depropagated. Temperature coefficients measured over this range cannot therefore be regarded as overall activation energies, but must be interpreted in terms of the change of mechanism with temperature.

Kinetic measurements by micropyrolysis-GLC: Thermal degradation of polymethylmethacrylate possessing lauryl-mercaptyl end groups

Abstract A sample of polymethylmethacrylate possessing lauryl-mercaptyl end groups has been fractionated by gel permeation chromatography, and measurements of the thermal degradation of the fractions have been performed by micro-pyrolysis-gas-liquid-chromatography. The rates of pyrolysis were measured between 330°C and 463°C by a resistive heating technique incorporating initial boost; between 450°C and 510°C heating by induction (‘Curie-point’) was attempted. Changes in mechanism with temperature have been deduced from: (a) Trends in specific reaction rate with initial molecular weight at different temperatures, (b) Inflections in the Arrhenius plots for the different fractions, and (c) Trends in molecular weight with conversion, for both fractionated and unfractionated samples. End-initiation is the predominant mechanism at low temperatures, but random scission becomes more important at higher temperatures. The predominating termination process also depends upon temperature; above 400°C the majority of chains unzip to the end provided the initial molecular weight is not too high, but at the lower temperatures termination occurs by a first order process during the unzip.

Direct examination of polymer degradation by gas chromatography II—Development of the technique for quantitative kinetic studies

The following difficulties may be encountered when direct-pyrolysis gas chromatography is aplied to obtain quantitative kinetic measurements: (a) poor reproducibility of the measurements, arising from the method of mounting the sample, (b) time and temperature errors, arising principally from the pre-effect of the temperature/time profile of the filament, and (c) dependence of the observed degradation rate on sample thickness, even for samples in the microgramme range. The first two problems have been surmounted by depositing the sample within a limited region of a ribbon filament, and supplying an initial current boost to bring the filament to the desired degradation temperature within one second. If in addition the degradation is effected in the carrier-gas stream of a capillary column GLC apparatus incorporating a detector sensitive to better than 10−10g, the pyrolysis may be studied under conditions where the rate becomes independent of sample thickness, i.e. 5 × 10−8g samples, 200 A thick. The requirements of the technique and the choice of operating conditions are described. The principal advantages of the method are that only submicro samples are required, and that quantitative kinetic measurements over a wide temperature range may be performed rapidly.

Polyacrylonitrile degradation kinetics studied by the micropyrolysis—g.l.c. technique

Abstract The thermal degradation of thin films of polyacrylonitrile (thickness ∼ 750 A ) has been studied in the temperature range 200–850°C by the micropyrolysis—g.l.c. technique. Throughout the temperature range 300–850°C the following products were characterized: ammonia, hydrogen cyanide, monomer, acetonitrile and methacrylonitrile. From studies of (a) the limiting yields of products, (b) the conversion curves and kinetics of the evolution of some of the products, (c) the influence of water on the degradation and (d) stoichiometric considerations, it is concluded that three independent processes occur: (1) ammonia evolution, possibly by a stepwise process leaving a laddered polymer containing some five-membered rings, (2) hydrogen cyanide evolution, either from head-to-head linkages or by a chain reaction, leaving double bonds randomly within the polymer molecules, and (3) monomer evolution by a depropagation process, involving intermolecular and intramolecular transfer steps leading to acetonitrile and methacrylonitrile production. A fourth independent process is the only one observed at 200°C; this is probably a laddering process which leads to the stabilization of the polymer towards subsequent degradation at higher temperatures. High-temperature kinetic studies of polymer samples which had been pre-heated at 200°C have indicated the limitations of this stabilization process and have shown that the parts of the polymer which are not laddered degrade in the same way as normal polymer.

Telomerization of styrene with bromotrichloromethane

Abstract The photo-initiated reactions between styrene and bromotrichloromethane have been studied at 30. The analysis of the telomer products by gel permeation chromatography has been refined since the previously published work; the relative proportions of several smaller telomers have been estimated. The results show a considerable increase in the magnitude of the transfer constant over the first few propagation steps of the reaction, with a limiting value after the incorporation of three monomer units into the growing polymer radical.

1,1,1-Trichloro-3-bromo-3-phenylpropane as a photoinitiator in the polymerization of styrene

Abstract The photoinitiated reaction between styrene and 1,1,1-trichloro-3-bromo-3-phenylpropane at 30° has been studied. The latter compound undergoes transfer with polystyryl radicals less readily than styrene dibromide and much less readily than bromotrichloromethane. The kinetic behaviour is consistent with a scheme involving primary radical termination, which would be enhanced by the regeneration of primary radicals in the transfer step. An alternative explanation invokes the participation of monomer in a rate-controlling initiation reaction with primary radicals which have escaped from their solvent cages. Rate constants for the individual steps are quoted.