Thermal and mechanical properties of radiation crosslinked poly(tetrafluoroethylene-co-perfluoromethyl vinyl ether)

The gamma-radiolysis of poly(tetrafluoroethylene-co-perfluoromethyl vinyl ether) (TFE/PMVE) was investigated using chemical and mechanical analyses. The polymer was found to form an insoluble network with a dose of gelation of 15.8 kGy. Tensile and glass transition temperature measurements indicated the predominance of crosslinking, with optimal elastomeric properties reached in the dose range of 120 to 200 kGy. Photoacoustic FTIR spectroscopy CPAS) showed the formation of new carboxylic acid end groups on irradiation. These new end groups were shown to decrease the thermal oxidative stability of the crosslinked network as determined by thermal gravimetric analysis. Electron spin resonance (ESR) studies of the polymer at 77 K indicated the presence of radical precursors. A G-value of 1.1 was determined for radical production at 77 K. Comparison of radical concentrations for a copolymer with a different mole ratio of PMVE, indicated that the PMVE units contribute to scission reactions. (C) 1998 Elsevier Science Ltd. All rights reserved.

Thermal and radiation curing of phenylethynyl terminated macromers

The thermal and gamma-irradiation induced curing of two phenylethynyl terminated composite resin systems, DFB/BPF and PETI5A, was investigated. Thermal curing of these matrix resin samples was performed at a temperature of 360 degrees C, gamma irradiation of the samples was conducted at 300 degrees C at a dose rate of 2.2 kGy h(-1). The reaction and subsequent loss of ethynyl groups in the resins for both cure methods was demonstrated by observing the decrease of the 2215 cm(-1) peak in the Raman spectra of the resins. Fully cured resin samples were found to have glass transition temperatures of 244-246 degrees C and 278-280 degrees C for DFB/BPF and PETI5A respectively. Similar relationships between T-g and fractional conversion were observed in both resins. The apparent polymerization rate, R-p, for thermal cure at 360 degrees C, was found to be 4.79 x 10(-2)% s(-1) in PETI5A and 3.22 x 10(-2)% s(-1) in DFB/BPF. Catastrophic degradation under nitrogen was observed to commence near 450 degrees C and 530 degrees C, with 5% weight losses occurring at 455 degrees C and 540 degrees C for DFB/BPF and PETI5A respectively. Gamma radiation induced cure at 300 degrees C was shown to be feasible, with full cure being reached with doses of 40 kGy for DFB/BPF and 100 kGy for PETI5A.

Thermal characterization of copolymers obtained by radiation grafting of styrene onto poly(tetrafluoroethyleneperfluoropropylvinylether) substrates: thermal decomposition, melting behavior and low-temperature transitions

Differential scanning calorimetric (DSC) and thermogravimetric analysis (TGA) have been used to study the thermal decomposition, the melting behavior and low-temperature transitions of copolymers obtained by radiation-induced grafting of styrene onto poly (tetrafluoroethylene- perfluoropropylvinylether) (PFA) substrates. PFA with different contents of perfluoropropylvinylether (PPVE) as a comonomer have been investigated. A two step degradation pattern was observed from TGA thermograms of all the grafted copolymers, which was attributed to degradation of PSTY followed by the degradation of the PFA backbone at higher temperature. One broad melting peak can be identified for all copolymers, which has two components in the samples with higher PPVE content. The melting peak, crystal-crystal transition and the degree of crystallinity of the grafted copolymers increases with radiation grafting up to 50 kGy, followed by a decrease at higher doses. No such decrease was observed in the ungrafted PFA samples after irradiation. This indicated that the changes in the heats of transitions and crystallinity at low doses are due to the radiation effects on the microstructure of PFA (chain scission), whereas at higher doses the grafted PSTY is the driving force behind these changes. (C) 2001 Elsevier Science Ltd. All rights reserved.

A comparative study of the effects of UV- and gamma-radiation on copolymers of acrylonitrile/butadiene

The radiolysis of nitrile rubbers with different acrylonitrile/butadiene composition and the homopolymers, poly(butadiene) (PBD) and poly(acrylonitrile) (PAN) has been investigated and compared with the photolysis of the same polymers. A significantly different mechanism of degradation was found for the two types of radiation. The results obtained by ESR, FTIR and measurements of soluble fractions of irradiated samples, indicated that the acrylonitrile units of the nitrile rubbers are more sensitive units to gamma-radiation, with the effects of irradiation increasing with the acrylonitrile content. The reactions observed were consumption of double bonds, crosslinking, and cyclization with the formation of conjugated double bonds. No chain-scission reactions were detected. In contrast to gamma-irradiation, the effects of photolysis were centred at the butadiene units, and increases in the acrylonitrile content resulted in a proportional decrease in the sensitivity of the copolymers. Crosslinking and chain scission were identified as the main effects of photolysis of NBR rubbers. (C) 1999 Society of Chemical Industry.

A systematic study of the microwave and thermal cure kinetics of the TGDDM/DDS and TGDDM/DDM epoxy-amine resin systems

The microwave and thermal cure processes for the epoxy-amine systems N,N,N',N'-tetraglycidyl-4,4'-diaminodiphenyl methane (TGDDM) with diaminodiphenyl sulfone (DDS) and diaminodiphenyl methane (DDM) have been investigated. The DDS system was studied at a single cure temperature of 433 K and a single stoichiometry of 27 wt% and the DDM system was studied at two stoichiometries, 19 and 32 wt%, and a range temperatures between 373 and 413 K. The best values the kinetic rate parameters for the consumption of amines have been determined by a least squares curve Ft to a model for epoxy-amine cure. The activation energies for the rate parameters for the MY721/DDM system were determined as was the overall activation energy for the cure reaction which was found to be 62 kJ mol(-1). No evidence was found for any specific effect of the microwave radiation on the rate parameters, and the systems were both found to be characterized by a negative substitution effect. Copyright (C) 2001 John Wiley & Sons, Ltd.

The radiation chemistry of ultem at 77 K as revealed by ESR spectroscopy

Radical formation in ultem following gamma-radiolysis has been reassessed, and the G(R*) values at different temperatures have been determined by ESR spectroscopy. The radical assignment and radical reactivity have been re-examined by photobleaching and thermal annealing studies. Photobleachable radical anions were found to comprise approximate to40% of the total number of radicals formed on radiolysis at 77 K. Spectral subtraction methods, ESR spectral simulations, measurement of g-values and the hyperfine splitting constants were used to identify the other radical intermediates. The principal chain scission radicals are formed due to scission of the main-chain at (i) the ether linkage, (ii) the isopropylidene group and (iii) the imide ring in the main chain. The side chain methyl groups of the isopropylidine units also lose hydrogen to form methylene radicals. The five-line spectrum observed to decay in the temperature range 370-430 K, which has not been assigned previously, has been identified as being characteristic of a di-substituted benzyl radical. (C) 2002 Elsevier Science Ltd. All rights reserved.

A systematic study of the microwave and thermal cure kinetics of the DGEBA/DDS and DGEBA/DDM epoxy-amine resin systems

The microwave and thermal cure processes for the epoxy-amine systems (epoxy resin diglycidyl ether of bisphenol A, DGEBA) with 4,4'-diaminodiphenyl sulphone (DDS) and 4,4'-diaminodiphenyl methane (DDM) have been investigated for 1:1 stoichiometries by using fiber-optic FT-NIR spectroscopy. The DGEBA used was in the form of Ciba-Geigy GY260 resin. The DDM system was studied at a single cure temperature of 373 K and a single stoichiometry of 20.94 wt% and the DDS system was studied at a stoichiometry of 24.9 wt% and a range of temperatures between 393 and 443 K. The best values of the kinetic rate parameters for the consumption of amines have been determined by a least squares curve fit to a model for epoxy/amine cure. The activation energies for the polymerization of the DGEBA/DDS system were determined for both cure processes and found to be 66 and 69 kJ mol(-1) for the microwave and thermal cure processes, respectively. No evidence was found for any specific effect of the microwave radiation on the rate parameters, and the systems were both found to be characterized by a negative substitution effect. Copyright (C) 2002 John Wiley Sons, Ltd.

An investigation of the thermal and tensile properties of PFA following γ-radiolysis

For some applications for fluoropolymers they must be subjected to high-energy radiation, e.g., when they are grafted with styrene using an irradiation method to produce fuel cell membranes or matrix supports for combinatorial chemistry. In some of these applications they may be subjected to mechanical stress or elevated temperature, so it is important to elucidate the effects of the radiolysis on these properties. In the present work the effect of gamma-radiolysis on the glass transition, melting behavior, and thermal stability of PFA has been studied as well as the effect of the radiolysis on the tensile properties of the polymer.

Numerical modelling of thermal effects in rats due to high-field magnetic resonance imaging (0.5-1 GHz)

A finite-difference time-domain (FDTD) thermal model has been developed to compute the temperature elevation in the Sprague Dawley rat due to electromagnetic energy deposition in high-field magnetic resonance imaging (MRI). The field strengths examined ranged from 11.75-23.5 T (corresponding to H-1 resonances of 0.5-1 GHz) and an N-stub birdcage resonator was used to both transmit radio-frequency energy and receive the MRI signals. With an in-plane resolution of 1.95 mm, the inhomogeneous rat phantom forms a segmented model of 12 different tissue types, each having its electrical and thermal parameters assigned. The steady-state temperature distribution was calculated using a Pennes 'bioheat' approach. The numerical algorithm used to calculate the induced temperature distribution has been successfully validated against analytical solutions in the form of simplified spherical models with electrical and thermal properties of rat muscle. As well as assisting with the design of MRI experiments and apparatus, the numerical procedures developed in this study could help in future research and design of tumour-treating hyperthermia applicators to be used on rats in vivo.

Thermal Cycling Stability of Silica Membranes for Gas Separation

Hydrogen is being seen as an alternative energy carrier to conventional hydrocarbons to reduce greenhouse gas emissions. High efficiency separation technologies to remove hydrogen from the greenhouse gas, carbon dioxide, are therefore in growing demand. Traditional thermodynamic separation systems utilise distillation, absorption and adsorption, but are limited in efficiency at compact scales. Molecular sieve silica (MSS) membranes can perform this separation as they have high permselectivity of hydrogen to carbon dioxide, but their stability under thermal cycling is not well reported. In this work we exposed a standard MSS membrane and a carbonised template MSS (CTMSS) membrane to thermal cycling from 100 to 450°C. The standard MSS and carbonised template CTMSS membranes both showed permselectivity of helium to nitrogen dropping from around 10 to 6 in the first set of cycles, remaining stable until the last test. The permselectivity drop was due to small micropore collapse, which occurred via structure movement during cycling. Simulating single stage membrane separation with a 50:50 molar feed of H2:CO2, H2 exiting the permeate stream would start at 79% and stabilise at 67%. Higher selectivity membranes showed less of a purity drop, indicating the margin at which to design a stable membrane separation unit for CO2 capture.

Thermal effects in the evolution of initially layered mantle material

A simplified model for anisotropic mantle convection based on a novel class of rheologies, originally developed for folding instabilities in multilayered rock (MUHLHAUS et al., 2002), is extended ¨ through the introduction of a thermal anisotropy dependent on the local layering. To examine the effect of the thermal anisotropy on the evolution of mantle material, a parallel implementation of this model was undertaken using the Escript modelling toolkit and the Finley finite-element computational kernel (DAVIES et al., 2004). For the cases studied, there appears too little if any effect. For comparative purposes, the effects of anisotropic shear viscosity and the introduced thermal anisotropy are also presented. These results contribute to the characterization of viscous anisotropic mantle convection subject to variation in thermal conductivities and shear viscosities.

Quantum noise in optical fibers. I. Stochastic equations

We analyze the quantum dynamics of radiation propagating in a single-mode optical fiber with dispersion, nonlinearity, and Raman coupling to thermal phonons. We start from a fundamental Hamiltonian that includes the principal known nonlinear effects and quantum-noise sources, including linear gain and loss. Both Markovian and frequency-dependent, non-Markovian reservoirs are treated. This treatment allows quantum Langevin equations, which have a classical form except for additional quantum-noise terms, to be calculated. In practical calculations, it is more useful to transform to Wigner or 1P quasi-probability operator representations. These transformations result in stochastic equations that can be analyzed by use of perturbation theory or exact numerical techniques. The results have applications to fiber-optics communications, networking, and sensor technology.

Growth of Bose-Einstein condensates from thermal vapor

We report on a quantitative study of the growth process of 87Rb Bose-Einstein condensates. By continuous evaporative cooling we directly control the thermal cloud from which the condensate grows. We compare the experimental data with the results of a theoretical model based on quantum kinetic theory. We find quantitative agreement with theory for the situation of strong cooling, whereas in the weak cooling regime a distinctly different behavior is found in the experiment.

NMR study of the radiation-induced cross-linking of poly(tetrafluoroethylene-co-perfluoromethyl vinyl ether)

The gamma-radiolysis of poly(tetrafluoroethylene-co-perfuoromethyl vinyl ether) (TFE/PMVE) was investigated using solid state F-19 and C-13 NMR spectroscopy. Chain scission products identified in the polymer were saturated chain ends -CF2CF3 (G = 1.0), methyl ether end groups -CF2OCF3 (G = 0.9), acid end groups -CF2COOH (G = 0.5), and a small amount of terminal unsaturation -CF=CF2 (G = 0.2). A mechanism for the formation of these scission products was proposed and the G value for main chain scission, G(S), was determined to be 1.4. Cross-linking of TFE/PMVE was found to proceed via a Y-linking mechanism. The G value for cross-linking, G(X), was determined to be 0.9. A maximum of 0.2 mol % cross-links were formed under the experimental conditions.