R.S. Lehrle
University of Birmingham
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Featured researches published by R.S. Lehrle.
European Polymer Journal | 1982
R.S. Lehrle; Roy E. Peakman; J.C. Robb
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 CC 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.
European Polymer Journal | 1982
R.S. Lehrle; J.C. Robb; John R. Suggate
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.
Polymer | 1961
A. Barlow; R.S. Lehrle; J.C. Robb
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.
Polymer | 1967
A. Barlow; R.S. Lehrle; J.C. Robb; D. Sunderland
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.
Polymer | 1969
G. Bagby; R.S. Lehrle; J.C. Robb
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.
Polymer Degradation and Stability | 2000
R.S. Lehrle; Ian W. Parsons; M. Rollinson
The thermal degradation of Nylon 6 has been studied over the temperature range 350–500°C by pyrolysis–gas chromatography. Samples in the microgram range were deposited as thin films on a pyrolysis filament with the capacity to reach the degradation temperature in ca. 20 ms, and under these conditions it was found that caprolactam monomer was produced almost exclusively. Kinetic measurements were made by a sequential pyrolysis method. The limiting yields, which are required for the kinetic plots, were not found by taking the degradation to total conversion, but were found by a computation procedure. Using these computed values, first-order log plots were drawn, and these revealed that the monomer is evolved by two independent processes, the faster of which can occur in only a minor portion of the sample. The rate constant for the faster process is two orders of magnitude greater than that of the slow process at 350°C, and one order of magnitude greater at 500°C. This large difference permitted the rate parameters for both processes to be evaluated by a simple kinetic analysis. The slower process was found to have the higher activation energy (170±20 compared with 100±20 kJ mol−1), and also has the higher A factor (2.2±0.3×1010 compared with 2.8±0.7×106 s−1), so the rate of this process approaches that of the faster process as the degradation temperature increases. The associated difference in entropy of activation (75 J mol−1 K−1) is qualitatively interpreted in terms of the change in configurational entropy between the reactant molecule and the transition state for each of the processes. It is proposed that the slow process involves loss of monomer randomly from within any polymer chain, but the faster process is associated with backbiting from the ends of only a restricted number of molecules which have labile ends, leading to sequential loss of monomer. It is suggested that NH2 groups are the active ends which are involved in promoting the fast process. This mechanism suggests the possibility that nylon 6 could be made more thermally stable by modifying or blocking these active ends.
Journal of Analytical and Applied Pyrolysis | 1991
Sally Groves; R.S. Lehrle; Marianne Blazsó; T. Székely
Abstract The technique of pyrolysis—gas chromatography (Py—GC) has been used to study the thermal degradation of natural rubber. The monomer/dimer (M/D) ratio has been measured over the temperature range 300 to 500°C, using sample sizes of the order of 0.3 mg. The M/D ratio has also been measured as a function of sample thickness, using samples in the range 30 nm to 3 μm (0.1 μg to 10 μg). The dependence of M/D on temperature may be interpreted in terms of (a) different activation energies for depropagation versus intramolecular cyclisation, (b) dissociation of dimer to monomer at higher temperatures, or (c) reduced residence time of monomer in the melt at higher temperatures allowing less opportunity for its recombination. The results for the dependence of M/D on thickness (at constant temperature) indicate that interpretation (c) is the most probable. The present work therefore suggests that monomer recombination, possibly by a Diels-Alder mechanism is an important contributor to dimer formation in rubber pyrolysis.
Polymer | 1967
A. Barlow; R.S. Lehrle; J.C. Robb; D. Sunderland
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.
Polymer Degradation and Stability | 1995
M.R. Grimbley; R.S. Lehrle
Abstract Samples of polyisobutylene (PIB) have been degraded thermally as thin films on a thermocouple-controlled filament. In one series of these thermal experiments a temperature of 400°C for 10 s was used in order to ensure only partial pyrolysis, whilst in another series of experiments a temperature of 610°C for 30 s was used to achieve complete pyrolysis. In both cases the yields and molecular weights of the products were assessed by gas chromatography. The weight- and number-distributions of the oligomeric products observed have been compared with those predicted statistically on the basis of random scissions. The results show that the total pyrolysis can be interpreted exclusively in terms of parallel depropagation and random scission mechanisms. However, the partial pyrolysis results are not consistent with random scission statistics but instead imply that either some kinetically favoured scissions occur near the ends of the molecules or secondary reactions take place which favour the production of lower oligomers.
European Polymer Journal | 1992
Sally Groves; R.S. Lehrle
Abstract The formation of oligomers by secondary reactions in polymer pyrolysis can be distinguished from their formation as primary products by studying the dependence of product yields and ratios on sample size. This study has been carried out for natural rubber (cis-1,4-polyisoprene), degraded on a thermocouple-controlled filament at 500°. The sample sizes were in the range 0.2–1.8 μg, deposited from solution on a portion of the filament of area 3.0 mm2. The results show that the monomer and the three principal dimers (D1, D2 and D3) are formed mainly as primary products, though each of the dimers is also formed by secondary processes involving monomer recombination. For sample sizes above 1 μg, there is evidence that D3 isomerizes to D2. In contrast to dimers, trimers are formed very largely by secondary reactions involving monomer and dimer, and all three dimers are reactive in this respect. Trimers may also be formed by a process involving three monomer molecules, but it is considered that this process is more likely to be reaction of monomer with a “transient” dimeric species. Mass spectrometric analysis suggests that probable identities of the dimers observed in the present study are D1 = 1,7,7-trimethyl-bicyclo 2,2,1-hept-2-ene, D2 = 2,5,6-trimethyl 1,3,6-heptatriene and D3 = 1-methyl-4-(1-methylethenyl)cyclohexene; (dipentene/limonene). A reasonance-stabilized biradical structure is proposed for the transient dimeric species.