Graeme A. George

High energy radiation grafting of fluoropolymers

Fluoropolymers are known as chemically inert materials with good high temperature resistance, so they are often the materials of choice for harsh chemical environments. These properties arise because the carbon-fluorine bond is the strongest of all bonds between other elements and carbon, and, because of their large size, fluorine atoms can protect the carbon backbone of polymers such as poly(tetrafluoroethylene), PTFE, from chemical attack. However, while the carbon-fluorine bond is much stronger than the carbon hydrogen bond, the G values for radical formation on high energy radiolysis of fluoropolymers are roughly comparable to those of their protonated counterparts. Thus, efficient high energy radiation grafting of fluoropolymers is practical, and this process can be used to modify either the surface or bulk properties of a fluoropolymer. Indeed, radiation grafted fluoropolymers are currently being used as separation membranes for fuel cells, hydrophilic filtration membranes and matrix substrate materials for use in combinatorial chemistry. Herein we present a review of recent studies of the high energy radiation grafting of fluoropolymers and of the analytical methods available to characterize the grafts

Cure kinetics and mechanisms of a tetraglycidyl-4,4′-diaminodiphenylmethane/diaminodiphenylsulphone epoxy resin using near i.r. spectroscopy

The cure of tetraglycidyl-4,4′-diaminodiphenylmethane with 27 wt% diaminodiphenylsulphone at 160°C was studied using near i.r. spectroscopy. The changes in concentrations of epoxy, hydroxyl, ether and primary, secondary and tertiary amine groups during cure were calculated and empirical reaction rate curves derived for the various functional groups involved. An autocatalytic reaction mechanism was observed for both the primary and the secondary amine group reaction with epoxide. The secondary amine-epoxy reaction showed a substitution effect and is affected by the changes in rheological properties of the resin during gelation and vitrification. Two separate epoxy-hydroxyl reactions are observed, as well as a cyclization reaction due to impurities.

Physical spreading of oxidation in solid polypropylene as studied by chemiluminescence

Abstract Highly sensitive chemiluminescence equipment was applied to study the oxidative behaviour at 125–150°C of individual polypropylene powder particles (10–500 μm) and film sections pressed from the powder. The chemiluminescence curves from the oxidation of single isolated powder particles revealed a range of induction periods equivalent to different intrinsic stabilities of the individual particles. Physical contact of the particles, however, resulted in a relatively early oxidation of the combined sample, i.e. a single induction period, as a result of the apparent initiation by the weakest particle and a subsequent spreading of the oxidation through the ensemble of particles. Evidence of this oxidative spreading was also found in polypropylene film samples. The physical spreading of the oxidation from an initial highly reactive centre has therefore been established as being the predominant step in the oxidation of a larger sample, and thus the weakest centre can control the overall oxidation of a sample. This mechanism was found to be consistent with the sigmoidal curve shape of the chemiluminescence curve obtained from the oxidation of both powder and film. The observed spreading is believed to be a highly efficient process, as the oxidation can ‘jump’ from particle to particle over distances of approximately 100 μm. In stabilized film the oxidation at the end of the induction period shows features different from that of an unstabilized film, and it is proposed that the spreading of the oxidation is limited by residual stabilizer in zones of lower reactivity.

A heterogeneous model for the thermal oxidation of solid polypropylene from chemiluminescence analysis

From an analysis of chemiluminescence (CL) curves from the isothermal oxidation of polypropylene powder and film, it is proposed that the usual homogeneous kinetic analysis of the sigmoidal increase in apparent oxidation rate with time is inappropriate and should be considered instead as a statistical accumulation of rapidly oxidizing centres. Over the temperature range 90-150°C it is found that the CL curves up to the maximum emission intensity may be reduced to a single sigmoidal master curve by plotting the data after the induction period in relative coordinates. Identical activation energies are observed for the initial emission and the maximum emission intensities, which suggests that throughout the oxidation there is no change in the CL mechanism, which requires the occurrence of a degenerately branched chain reaction within the oxidizing centres. The induction period shows a different temperature dependence and in the heterogeneous model is interpreted as the time required for the oxidation to spread from the initially localized centres. This model is able to explain the observations of volatile formation in the earliest stages of oxidation and the rapid crack formation at the end of the induction period as well as other anomalies which cannot be rationalized in a homogeneous kinetic model.

Characterisation and degradation studies of peroxide and silane crosslinked polyethylene

Abstract Crosslinked polyethylene (XLPE) used as an insulation material is increasingly employed in aerial bundle cable systems for domestic power supply. This has resulted in the need to investigate the long term performance and properties of the polymer under thermal and UV ageing conditions. In the current study it has been attempted to characterise various silane and peroxide crosslinked low density polyethylene samples and to investigate the fundamental processes occurring during the thermal and UV degradation of these materials. The technique of solvent extraction used to determine the gel content and solvent swelling of the XLPE was shown to be ideally suited to characterise and to monitor the degradation of the XLPE. The two materials differ in relation to the mechanical properties, melting characteristics and network properties of the XLPE, with silane XLPE being generally less homogeneously crosslinked than peroxide XLPE. Various degradation studies revealed that this fundamental difference may also influence the mechanism of the degradation. Silane XLPE was shown to be more sensitive to the degradation and may degrade in a more heterogeneous manner than the peroxide XLPE.

Chemiluminescence imaging of the oxidation of polypropylene

A commercial high-sensitivity CCD camera has been applied to image the weak chemiluminescence from the oxidative degradation of polypropylene. Images of powder and film samples were obtained after various oxidation times and showed the heterogeneous characteristics of oxidation. The results are consistent with a model for oxidative spreading from zones with a high local rate of oxidation.

Heterogeneous and homogeneous kinetic analyses of the thermal oxidation of polypropylene

Abstract The evidence from chemiluminescence analysis for the heterogeneous model for the oxidation of polypropylene powder and film is re-examined. In this model, the polymer contains a small number of localized zones in which oxidation occurs at a high rate and from which it then spreads. Stabilizer, if present, prevents the spreading for a period of time (the induction period) and also lowers the rate of subsequent spreading through the polymer. By considering a model of progressive ‘infection’ of the unoxidized polymer fraction a sigmoidal chemiluminescence growth curve is predicted and depends on several parameters including the initial oxidized fraction and the spreading coefficient. It is shown that a homogeneous kinetic analysis based on an autoaccelerating free radical reaction initiated by trace hydroperoxides fails to provide a good fit to the observed CL data. The activation energies for the chemiluminescence at the onset of oxidation, the maximum in the sigmoidal profile, and the induction period are reinterpreted with consideration given to the activation energy for the chemiluminescence quantum yield, as well as the influence of instrumental sensitivity on measured values. From these results and the observation that the extent of oxidation at the end of the induction period is not a constant, independent of temperature, it is concluded that a homogeneous kinetic model cannot be applied.

Change of mechanical and electrical properties of polypyrrole films with dopant concentration and oxidative aging

The process of loss of mechanical and electrical properties of polypyrrole (Ppy) was studied for up to 12 months of aging in air at room temperature. Tensile strength of films highly doped with p-toluene sulphonate was lower than that of lightly doped ones. This difference is attributed to the presence of stress concentrations on the nodular surfaces of the films with high dopant concentrations. Highly doped films were electrically more stable, with a lower rate of loss of conductivity with time, compared with lightly doped ones. The conductivity of the latter initially decreased at a different rate than that observed after a prolonged time, exhibiting two regions in the aging behavior. In contrast, highly doped PPy films followed first-order kinetics with a very small loss of conductivity over the aging period. The rate of decline of conductivity was dependent on aging time, dopant type, and concentration, suggesting a complex mechanism of conductivity decay. Examination of FT-IR spectra of aged Ppy films revealed an increase in intensity of an α, β-unsaturated conjugated carbonyl peak, which may be correlated with the loss in conductivity.

Real-time analysis of the thermal oxidation of polyolefins by FT-IR emission

Quantitative FTIR-emission spectroscopy was applied to investigate the real-time thermal oxidation of polyolefin samples and shown to be a useful method for studying polymer degradation. The high sensitivity of the technique was demonstrated by studying the oxidation of individual pressed polypropylene (PP) and high-density polyethylene (HDPE) reactor powder particles and microtome cuttings of crosslinked polyethylene (XLPE). A detailed analysis of the growth in carbonyl bands during the degradation showed no changes in the product distribution with time. Secondary oxidation products, such as γ-lactones in the case of polypropylene, were observed from the onset of the degradation, which is consistent with a model of oxidation occurring in localized zones with high reactivities. Differences in the oxidation sensitivity of these materials were identified and attributed to variations in the stability of the PP particles, catalyst residues in the HDPE and the degree and method of crosslinking of the XLPE.

Preparation of graphene oxide/epoxy nanocomposites with significantly improved mechanical properties

The effect of graphene oxide (GO) on the mechanical properties and the curing reaction of Diglycidyl Ether of Bisphenol A/F and Triethylenetetramine epoxy system was investigated. GO was prepared by oxidation of graphite flakes and characterized by spectroscopic and microscopic techniques. Epoxy nanocomposites were fabricated with different GO loading by solution mixing technique. It was found that incorporation of small amount of GO into the epoxy matrix significantly enhanced the mechanical properties of the epoxy. In particular, model I fracture toughness was increased by nearly 50% with the addition of 0.1 wt. % GO to epoxy. The toughening mechanism was understood by fractography analysis of the tested samples. The more irregular, coarse, and multi-plane fracture surfaces of the epoxy/GO nanocomposites were observed. This implies that the two-dimensional GO sheets effectively disturbed and deflected the crack propagation. At 0.5 wt. % GO, elastic modulus was ∼35% greater than neat epoxy. Differential ...