Dielectric measurements are reported for alpha and gamma alumina powders exposed to moisture for various periods of time, temperatures and dried in air for various times. The observed behaviour reflects the nature of the surface oxide, topography of the oxide and is influenced by their ability to change as a consequence of thermal treatment. This study highlights the importance of the nature of the surface oxide on the processes of moisture penetration in aluminium bonded structures.

The variation of the viscosity as a function of the degree of conversion for an anhydride cured epoxy resin is modeled using a combination of a Monte Carlo statistical model of the polymerization process and constitutive equations describing the motion of the entities in the mix. The degree of conversion was monitored as a function of time using Fourier transform infrared spectroscopy over the temperature range 80-130 degrees C. The rate data are analyzed using the Kamal equation and activation energy of 57 and 64 kJ mol(-1) obtained from the temperature dependence of the rate constants. The gelation and vitrification times for the cure process measured as a function of temperature were measured using a vibrating probe curometer. The variation of the viscosity measured experimentally and predicted are found to be similar when compared as a function of the degree of conversion. A time-temperature transformation was constructed and indicated that the gelation point occurred at approximately the same degree of conversion for all the temperatures investigated. The influence of the cure process on the final mechanical propeties of the resin is explored through dynamic mechanical thermal analysis. This paper indicates the possibility of predicting the variation of the viscosity prior to gelation based on the cure kinetics for reactive polymer systems.

The property changes occurring when poly(ether ether ketone) (PEEK) is subject to methane and carbon dioxide at high pressures (10(8) Pa) and high temperatures (175-200 degrees C) are reported. Differential scanning calorimetry, density gradient techniques, positron annihilation lifetime spectroscopy, dynamic mechanical thermal analysis and tensile tests measurements were used to monitor the changes which occur during the ageing process. Over the period of the study an overall increase in the tensile strength was noted, with little or no change in 0.2% and 2% proof stresses, whilst there was a decrease in bending modulus and glass transition temperature due to the effects of plasticization. The Young's modulus generally increases for samples exposed to a temperature of 175 degrees C in the presence of a mixture of 90% methane and 10% carbon dioxide, or carbon dioxide alone, but it decreases at 200 degrees C in the presence of carbon dioxide alone. The observed effects are consistent with the polymer undergoing initially a densification of the matrix associated with annealing-induced crystallisation, followed by plasticization as the gases permeate into the disordered regions of the matrix. When de-pressurised, the gas dissolved in the matrix attempts to leave the matrix and morphological changes are observed. The complexity of the changes in crystallinity and plasticization in the disordered phase are reflected in changes in the positron annihilation data. (C) 2013 Published by Elsevier Ltd.

The characterization of the physical and chemical changes that occur in montmorillonite/PDMS nanocomposite elastomers as they are thermally aged is reported. Broadband Dielectric Spectroscopy (BDS) was used to track changes in the physical interaction between the polymer and clay associated with increases in non-oxidative thermal stability (as determined by TGA). The evolution of volatile siloxane species from the elastomers was characterized with Thermal Volatilization Analysis (TVA). Results suggest that the improved thermal stability and the increases in polymer/clay association are a result of significant re-structuring of the polymer network.

This presentation looks at ageing in non isothermal environments of adhesively bonded metal structures

Dielectric measurements are widely used in the laboratory to probe the dynamics of molecules, particularly the dynamics of polymer molecules. The dielectric technique exploits the fact that many molecules, although electrically neutral posses an asymmetric distribution of charges which can be approximated to an electric dipole. The (usually thermal) motion of the molecule can be detected by the interaction of this dipole with a time varying electric field. The great advantage of the technique is that no transducers or sensors are required; the direct application of an electric field produces a directly measurable electric response over a frequency range of MHz to GHz. This paper discusses the practical application of dielectric measurements to composite structures and the information that can be obtained on the state of the polymer in polymer composite matrix materials.

In the fabrication of many composite and adhesively bonded structures it is desirable to determine the extent to which complete cure is achieved. This review briefly discusses the fundamental issues which are involved in the monitoring of the cure process in thermoset resins. The application of the dielectric method is described as a non-destructive probe for the monitor of the cure process in thermoset resin systems. The principles of the technique are illustrated with reference to a simple epoxy-amine structural adhesive. Possible correlations between the changes in the dielectric properties with other physical property changes of a resin, e.g. viscosity, during cure are discussed.

The physical and chemical aging of polysiloxane elastomers incorporating nano-scale particles of differing dimensions and aspect ratios is reported. A series of model polysiloxane nanocomposites have been prepared incorporating montmorillonite nanoclay and polyhedralsilsesquioxane (POSS). Broadband Dielectric Spectroscopy (BDS) has been employed to study the effects of aging on polymer-filler interactions within the nanocomposites by tracking changes in system ionic mobility and filler-induced Maxwell-Wagner-Sillars effects. TGA and DSC have been utilized to study the effects of aging on the non-oxidative stability of the nanocomposites. The complex evolution of volatiles that occurs during aging has been studied using Sub-Ambient Thermal Volatilization Analysis (SATVA). Results indicate that significant physical and chemical changes take place within the nanocomposites upon aging; acid catalyzed hydrolysis, chain backbiting and recombination reactions are re-structuring the polymer-filler network into a more thermodynamically stable form. The nature and magnitude of these processes is dependant on the nano-filler present.

Adhesive-bonded aluminium/dicyandiamide-cured epoxy joints prepared using two different surface treatments, a silica/siloxane and an etched only process, were exposed at 70°C to moisture and their ageing behaviour was studied using broadband dielectric spectroscopy and destructive mechanical tests. The dielectric measurements allow the uptake of moisture and changes in the nature of the surface oxide and pre-treatment to be monitored. Both sets of joints exhibited almost similar changes in their dielectric spectra as a function of exposure time to moisture; small differences being attributed to the influence of the pre-treatment on the moisture absorption behaviour. Changes in the mechanical properties as a result of exposure to moisture were monitored via lap-shear tests. Detailed electron microscopic examination of the surfaces indicated that after prolonged exposure to moisture changes in the physical state of the pre-treatment and oxide layer were observed. Small differences were observed in the dielectric data between the two pre-treatments and are a consequence of the differences in the extent and nature of the oxide to hydroxide conversion and hydration of the surface treatment which occurs on exposure to moisture. Further evidence for the nature of the change that occurred was obtained using X-ray photoelectron spectroscopy of the fracture surfaces. One side of the fractured joint was predominantly resin irrespective of the exposure conditions. The other fracture surface had a more metallic appearance but was covered by a nitrogen-containing organic material with high oxygen content. Predominantly, the initial changes observed in the mechanical strength of the joints are consistent with the plasticization of the adhesive in the joint. The effects observed after prolonged period of exposure to moisture are consistent with an increased contribution from inter-facial failure to the loss of the mechanical properties. The losses in mechanical properties are less dramatic than would be anticipated considering the mild nature of the surface treatments used and the severity of the exposure conditions.

Ageing of adhesive-bonded aluminium-dicyandiamide cured epoxy alumina-filled joints prepared using a variety of different surface treatments and exposed to elevated temperatures and high humidity are reported. The uptake of moisture was followed using broadband dielectric spectroscopy, and attempts are made to correlate these changes with observations of variations in the mechanical properties and surface structure monitored by electron microscopy. It was found that the absorption of moisture, as indicated by the dielectric measurements, is similar for all the joints. Small differences observed may be ascribed to the influence of the pretreatment on the absorption behaviour. There was no evidence of changes in the oxide layer of the substrate. Detailed electron microscopic examination of the surfaces did indicate, after prolonged exposure, that change in the pretreatment is consistent with small differences in the dielectric data. Predominantly, the changes that were observed in the mechanical strength of the joints are consistent with the plasticization of adhesive in the joint rather than failure at the substrate interface. Ageing at 70 degrees C did, however, indicate that there were changes in the interfacial layer and these can be correlated with the change in the failure mechanism. It was also observed that the titanium/zirconium (Ti/Zr) pretreatment showed signs of being less durable than the others used in this study.

Ageing studies on adhesive-bonded aluminium-dicyandiamide cured epoxy joints prepared using a variety of different surface treatments were carried out using 100% humidity and cycling the joints between 42-48-42 degrees C every hour. The pretreatments include a chromic acid, silica/siloxane pretreatment, nonrinse chrome pretreatment, titanium/zirconium (Ti/Zr) pretreatment, and anodised substrate with no pretreatment. Dielectric spectroscopy measurements were used to characterise the rate of the water uptake and monitor the conversion of the surface oxide to hydroxide. These data were correlated with the changes that occurred in the mechanical strength of the bonds. The changes in the surface structure were observed using electron microscopy and elemental analysis conducted using X-ray photoelectron spectroscopy. The dielectric permittivity changes observed were similar for all the different pretreatments, indicating that the predominant process was water absorption. However, small differences were observed that reflect the different surface treatments used. Ageing at an elevated temperature of 70 degrees C provided definite evidence of hydration of the surface oxide layer. Electron microscopy of the fracture surface indicated oxide-to-hydroxide conversion and was reinforced by X-ray photoelectron spectroscopy. Predominantly, the changes in the mechanical strength observed at low temperature are consistent with the plasticization of the adhesive. However, at the elevated temperature of 70 degrees C, evidence for weakening of the interfacial layer by hydration becomes evident as a reduction in the mechanical strength. The dielectric measurements allowed the changes in the bond to be followed nondestructively. The mechanical strength of the etched-only pretreatment aged surprisingly well and at the lower temperature of ageing was comparable with the no-rinse chrome and titanium/zirconium pretreatments. The least durable was the PT2Cr-free treatment; however, all treatments showed a significant level of durability at low temperature.

Dielectric spectroscopy has been developed as a non-destructive technique for assessment of moisture content and structural integrity of adhesively bonded joints. Knowledge of these parameters is particularly crucial for the aerospace industry, since environmental degradation of adhesive joints presents a major limit on their utilization. High and low frequency measurements have been carried out on joints assembled from CFRP adherend, and a commercially available adhesive (AF 163-2K). The samples have been aged in deionised water at 75 degrees C to chart the effect water ingress has on bond durability. In addition, some joints have been exposed to cryogenic temperatures to mimic the conditions joints experience whilst an aircraft is in flight. In this way it has been possible to determine the extent of degradation caused by freezing of water within the joint structure. Dielectric behaviour of the joints was studied in both the frequency and in the time domain. Frequency domain analysis allows the amount and effects of moisture ingress in the bondline to be assessed, whereas the time domain highlights the onset of joint defects with increasing exposure time. Mechanical testing of the joints has been carried out to enable correlation between changes in strength and failure mechanism due to moisture ingress, with changes in the dielectric data. In addition, dielectric studies of the neat adhesive have been undertaken, as have gravimetric and dynamic mechanical thermal analysis. These have helped reveal the effects of ageing upon the adhesive layer itself.

Semi-crystalline Biopol™ copolymers are biodegradable materials which are being intensively probed as suitable systems for embossed substrates in tissue regeneration applications. However, it has been shown that an embrittlement process occurs on storage at room temperature which places restrictions as to their application possibilities. In order to study this process we have made use of Biopol™ systems which have aged for 3 years since extrusion as thin films, and subsequently rejuvenated at elevated temperatures. Utilising varying techniques we have attempted to shed light on the mechanisms responsible for the rejuvenation, with particular emphasis on the observed dielectric properties. We highlight the possibility of on-going secondary crystallisation on ageing and attempt to rationalise the rejuvenation in terms of release of amorphous material from the rigid amorphous phase of semi-crystalline systems.

For as-extruded amorphous and biaxially orientated polyester films based on poly(ethylene terephthalate), poly(ethylene naphthalate), and copolymers containing poly(ethylene terephthalate) and poly(ethylene naphthalate) moieties, permeability, diffusion, and solubility coefficients are interpreted in terms of chain mobility. The influence of polymer morphology is determined by comparison of the data for as-extruded amorphous sheets and materials produced with different biaxial draw ratios. The crystallinities of the samples were assessed using differential scanning calorimetry and density measurements. Changes in mobility at a molecular level were investigated using dielectric spectroscopy and dynamic mechanical thermal analysis. The study, in conjunction with our earlier work, leads to the conclusion that the key to understanding differences in gas transport is the difference in local chain motions rather than in free volume. This was illustrated by the permeability results for He, Ar, N-2, and O-2 in the range of polyesters. However, the permeability of CO2 was found to require alternative explanations because of polymer-penetrant interactions. For biaxially oriented samples, the differences in diffusivity are not only due to differences in local chain motions, but also additional constraints resulting from the increased crystallinity and chain rigidity-which also act to hinder segmental mobility. The effectiveness of the reduction in permeability in the biaxially oriented films is consequently determined by the ability of the polymer chains to effectively align and form crystalline structures. (C) 2004 Wiley Periodicals, Inc.

The modern dielectric spectrometer is capable of real time observation of changes in the dipole activity of molecular systems. This article discusses the issues involved in such measurements and their application to a range of problems. Data are presented on the use of dielectric spectroscopy to monitor the cure of thermosetting resin systems, water absorption in polymers, physical ageing in thermoplastics and film formation and coalescence in polymer latexes. The dielectric technique is coming of age and its application to a range of interesting kinetic problems is now possible.

Broadband dielectric measurements (10 −2 to 3 GHz) are reported on the effects of exposure of thick film adhesive-bonded structures to moisture. Measurement of the dielectric properties over a broad frequency range allows identification of water both in voids and as a molecular dispersion in the matrix. Changes in the low frequency region of the dielectric spectrum can be attributed to a combination of processes associated with plasticisation of the adhesive, interfacial polarisation effects, and hydration of the surface oxide layer. The data obtained are complemented by mechanical testing and failure analysis of the bond structure measured as a function of the time of exposure. This study indicates that for thick film adhesives the ageing characteristics are apparently independent of the surface treatment. In one of the joints studied an additional feature is identified which appears to correlate with the premature aging of the joint structure.

Semi-crystalline poly(3-hydroxybutyrate-co-3-hydroxyvalerate) copolymers are biodegradable systems with potential as substrates for use in tissue regeneration. Previous studies have shown that severe embrittlement occurs on storage at room temperature restricting their application possibilities. Concepts such as secondary, advancing crystallisation causing changes in the amorphous/crystalline ratio have been mooted as the cause of the embrittlement. Using films prepared by extrusion and compression moulding procedures we have attempted to probe not only the pure amorphous and crystalline phases but also the interfacial region. Interpretation of dynamic mechanical and dielectric data highlights the changes in the nature of the interfacial region on processing. Moreover, the use of the Thermally Stimulated Discharge technique is a powerful probe for highlighting the morphological changes induced in multiphase systems by the processing step.

Permeability, diffusion and solubility coefficients are reported for biaxially orientated polyester films based on poly(ethylene terephthalate) [PET], poly(ethylene naphthalate) [PEN] and copolymers containing PET and PEN moieties. Data for cast amorphous sheets and materials produced with different biaxial draw ratios are compared. The crystallinity of the samples was assessed using differential scanning calorimetry and density measurements. The changes in the void structure at a molecular level were investigated using positron annihilation lifetime spectroscopy (PALS). The variation of the gas diffusion behaviour with the gas used (carbon dioxide, nitrogen, argon, helium and oxygen) reflects the effects of change in morphology on the solubility and diffusivity components of the permeability. The diffusivity of the gas is influenced not only by both the changes in the void size and content at a molecular level, but also by the effects of crystallinity on the percolation behaviour of the gas through the matrix. Changes in the extent of chain alignment also have a profound affect on the solubility of the gas in the matrix. The observed behaviour for the gas permeation can be interpreted as being the result of the complex interplay of changes in the crystalline content, the polymer chain alignment and the void structure of the amorphous phase.