James H. O'Donnell

Ageing phenomena in bacterial poly[(R)-3-hydroxybutyrate]: 1. A study on the mobility in poly[(R)-3-hydroxybutyrate] powders by monitoring the radical decay with temperature after γ-radiolysis at 77 K

Poly[(R)-3-hydroxybutyrate] (PHB), a bacterial polyester, is subject to a detrimental ageing process, which hampers its application possibilities. This ageing phenomenon is usually studied using moulded specimens, and was found to be related to a loss in segmental mobility. The presence of additives and orientation in moulded PHB samples might influence this ageing process. As a reference for our ageing studies, we therefore conducted a study on the ageing effects in virgin PHB powder. For this purpose a method is required to measure the mobility in a powder without the employmet of heat or mechanical strain, since these are known to induce deageing. This paper describes a suitable technique based on the principle that the variation of the radical concentration with temperature after γ-irradiation at 77 K provides a measure of the changes in the segmental mobility of the polymer chains.

Nature of the side chain branches in low density polyethylene: volatile products from gamma radiolysis

Measurements of the yields of low molecular weight hydrocarbons after γ-irradiation of low density polyethylene (LDPE) in the solid and liquid phases have confirmed that the main products are ethane and butane. Alkene yields were small, contrary to some previous reports. Irradiation at 150°C doubled the total yield of hydrocarbons, but the relative proportions of different hydrocarbons did not change markedly. The compatibility of these results with 13C n.m.r. determinations of the distributions of short chain branches in the same samples of LDPE is considered.

An electron spin resonance study on γ-irradiated poly(l-lactic acid) and poly(d,l-lactic acid)

The effect of gamma irradiation on poly(l-lactic acid) (l-PLA) and poly(d,l-lactic acid) (d,l-PLA), has been examined using ESR spectroscopy and through analysis of the changes in molecular weight. The G values for radical formation of both polylactic acids have been calculated at 77 and 300 K; G(R) = 2.0 at 77 K and G(R) = 1.5 at 300 K for l-PLA and G(R) = 2.4 at 77 K and G(R) = 1.2 at 300 K for d,l-PLA. The ESR spectrum at 300 K for the polymers was assigned to one radical, resulting from H atom abstraction from the quaternary carbon atom. The G values for crosslinking and scission have also been determined for the polymers at 300 K; G(S) = 2.3 and G(X) = 0.0 for d,l-PLA, G(S) = 2.4 and G(X) = 0.28 for l-PLA.

Kinetic parameters for polymerization of methyl methacrylate at 60°C

Abstract Fourier transform infrared spectroscopy (FTi.r.) and electron spin resonance spectroscopy (e.s.r.) have been used to follow the kinetics of the polymerization of methyl methacrylate to high conversion at 60°C. The FTi.r. absorbance at 6152 cm−1 was used to monitor the time dependence of the concentration of double bonds, and the concentration of the polymer-chain propagation radicals was monitored using the e.s.r. absorption spectrum. These data were analysed to obtain instantaneous estimates of the kinetic rate parameters for propagation and termination across the range of conversion, and of the initiator efficiency at high conversion. The kinetic parameters were found to be consistent with values obtained by other methods and with the predictions of recent theories.

Development of wear-resistant thermoplastic polyurethanes by blending with poly(dimethyl siloxane). II: a packing model

It has been shown in a previous article that melt blending of low levels of commercial poly(dimethyl siloxane) (PDMS) fluid with commercial thermoplastic polyurethanes has a significant positive impact on the coefficient of friction (CoF) and on the mechanical and wear properties of the polyurethanes. The improvements in CoF and wear resistance were expected due to surface modification of the polymer; however, the improvements in the mechanical properties were much more significant than expected. Evidence presented in the earlier publication suggests that the changes in the wear and mechanical properties are not due to surface modification alone, but are largely due to modification of the bulk by PDMS. In this article a model is presented that accounts for the observed relationship between PDMS content and the properties of the blends. It is proposed that the addition of PDMS facilitates an improved packing efficiency (antiplasticization) in the polyurethane soft domain, leading to improved material performance. Beyond an optimum PDMS concentration of 1.5–2.0%, phase separation of PDMS becomes significant, plasticization sets in, and mechanical properties then begin to diminish rapidly. This model has been rigorously investigated and has proven to be highly robust.

An electron spin resonance study of the radiation chemistry of poly(hydroxybutyrate)

Abstract The spectra of the radical species resulting from gamma irradiation of poly(hydroxybutyrate) and its copolymers with hydroxyvalerate have been obtained after radiolysis at 77 and 303 K, and the corresponding radiation chemical yields have been calculated. Photobleaching and annealing experiments have allowed the major radical species to be identified and their reactivities to be assessed. Radical anions, as well as neutral radicals, were observed to be present at 77 K, but the radical anions were found to decay to neutral radicals at temperatures around 140K. Above about 250 K a multi-line component of the spectrum was lost, but the radical associated with this spectral change has not been unequivocally assigned, though it is believed to be a radical resulting from chain scission. Annealing over the temperature range 300–350 K resulted in the loss of a triplet, which accounts for most of the radicals present. This triplet has been assigned to a radical located on the carbon atom adjacent to the carbonyl group. This radical is believed to exist in two possible conformations. All of the radicals decayed at temperatures above 400 K.

A study of a simulated low Earth environment on the degradation of FEP polymer

Spacecraft flying in a low Earth orbit environment require thermal blankets to provide protection from direct solar heat from the sun. Fluorinated ethylene propylene copolymer is one of the major components of these thermal blankets. In this study, the effect of a simulated low Earth orbit environment on FEP was investigated. UV and VUV degradation of fluorinated ethylene propylene (FEP) copolymer was studied using ESR and XPS. The ESR study revealed the formation of a terminal polymer chain radical. The stability of this radical has been investigated under different environments. An XPS study of FEP film exposed to VUV and atomic oxygen showed that oxidation takes place on the polymer surface. The study revealed also that the percentage of CF in the polymer surface decreased with exposure time and the percentage of CF, CF and carbon attached to oxygen increased. SEM micrographs of FEP film exposed to VUV and atomic oxygen produced a rough surface with regular undulations similar to sand dunes.

The crosslinking mechanism in gamma irradiation of polyarylsulfone: evidence for Y-links

Measurements of molecular weights, soluble fractions and examination of NMR spectra of bisphenol-A polysulfone, after gamma irradiation in vacuum at 150 degrees C were used to elucidate the mechanism of crosslinking. Excellent agreement was obtained between G(S) and G(X) determined from measurements above and below the gel dose when a Y-linking mechanism was assumed, whereas poor agreement was obtained when an H-link mechanism was assumed, which is the mechanism normally believed to be responsible for crosslinking. New resonances were observed in the C-13 NMX spectra of the irradiated polymer which were consistent with the formation of Y-links involving phenylene units in the backbone chain

Thermal degradation of polymethacrylonitrile

The thermal degradation properties of polymethacrylonitrile (PMAN) have been studied by isothermal heating and thermogravimetric analysis. There are two initiation processes for weight loss from PMAN degraded in nitrogen, namely chain-end and random scission initiation. There is also an internal cyclization reaction which forms a thermally stable residue during the thermal degradation process. The activation energies of the weight loss and formation of stable residues have been calculated. X-ray photoelectron spectroscopy has been used to investigate the structure of the stable residue and thus to confirm the degradation mechanism.

The effects of γ-radiation on polyacrylonitrile

The effect of γ-radiation on polyacrylonitrile has been examined using ESR spectroscopy and through analysis of the changes in molecular weight. The G-values for radical formation of polyacrylonitrile have been calculated at 77 and 300K; G(R·) ≅ 2·8 at 77K and G(R·) ≅ 4·2 at 300K. The ESR spectrum at 300K was assigned primarily to two radicals; one resulting from H abstraction from the methylene group, to form a chain radical, and the other from radical addition to the nitrile group, to form a polyimine radical. These radicals are consistent with a dominant cross-linking reaction. The G-values for cross-linking and scission have been determined at 300K; G(S) ≅ 0·0 and G(X) ≅ 0·59. The cross-linking possibly results from addition of backbone radicals to nitrile groups on adjacent chains.