Toughening of thermosets by the creation of residual compressive stresses

The toughening of thermosets through the creation of residual compressive stresses around microspheres was studied. Expandable, hollow microspheres containing gas with heating were used for the creation of residual compressive stresses. Microscopic compressive residual stresses in the vicinity of the crack tip were graphically analyzed and related to the macroscopic mechanical behavior for mode I fracture. The graphical analysis and experimental results confirmed that toughening was achieved mainly because of residual compressive stresses rather than a postcure effect. As expected, however, no toughening effect was found in mode III fracture. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 4045–4051, 2006     

Characterization of photodegradation of a polyurethane coating by positron annihilation spectroscopy: correlation with cross-link density

Photodegradation of a polyurethane coating by Xe arc-lamp irradiation as a function of time is studied by positron annihilation spectroscopy. Doppler broadening energy spectra (DBES) of annihilation irradiation is measured as a function of incident positron energy (0–30 keV). A significant decrease in the S-defect parameter from DBES is observed as a function of irradiation time. This is interpreted as a loss of free-volume and holes during the photodegradation process. The cross-link density in the same system is measured by a solvent (methylene chloride) swelling method and is found to increase as a function of irradiation time. A direct correlation between the increase in cross-link density and the decrease in the S-defect parameter from the DBES data is observed. These results are used to discuss the photodegradation process in terms of chemical and physical changes due to UV-irradiation in protective polymeric systems.     

A positron annihilation spectroscopy study of carbon-epoxy composites

Many positron annihilation spectroscopy (PAS) studies have been carried out during the last several years for characterizing various polymeric materials. In this work, the PAS technique is used to investigate some positron annihilation characteristics of the combination of carbon fibers with epoxy polymers to form carbon-epoxy composites. These composites are receiving wide and increasing application in industrial markets where light-weight materials with high strength and excellent fatigue characteristics are required. We have found that PAS is capable of detecting different concentrations of carbon fiber in these composites and a relationship is shown to exist between fiber fractions found by PAS and apparent volume and weight fractions found by density measurements. Positron annihilation centers have also been found that we believe were created at the carbon-epoxy interface when the composite was prepared. Characterization of this interface is important for the development of composites with improved mechanical properties.     

Isothermal volume relaxation in aged polycarbonate measured by positron annihilation lifetime spectroscopy

The positron annihilation lifetime spectroscopy (PALS) technique is used to measure volume relaxation in physically aged glassy polycarbonate. The relaxation times and activation energy calculated for the isothermal relaxations in the aged polycarbonate are greater than those parameters calculated for unaged polycarbonate. The activation energy of 8.2 kcal/mol in the aged polycarbonate is used to identify the phenyl group motion or the cooperative carbonate-phenyl interaction as the molecular features responsible for the thermally induced open volume relaxations. It is postulated that the open volume relaxation kinetics as measured by PALS can be used as a nondestructive indication of property differences between aged and unaged polycarbonate.     

Nanoscopic properties of silica filled polydimethylsiloxane by means of positron annihilation lifetime spectroscopy

Positron annihilation lifetime spectroscopy (PALS) was performed on a series of polydimethylsiloxane (PDMS)/fumed silicon dioxide (SiO₂) composites at temperatures between −185 and 100 °C to study the effect of filler content and filler particle size on the free volume properties and the positron annihilation characteristics. The glass transition behavior of the PDMS/SiO₂ composites was determined with differential scanning calorimetry. A clear influence on the o-Ps lifetime (τ₃) in the polymer upon addition of nano-sized fumed SiO₂ was observed at all temperatures. The observed o-Ps lifetime behavior was related to filler content and filler particle size. A transition in the temperature dependence of the o-Ps lifetime was observed close to −35 °C above which temperature PDMS exhibits long o-Ps lifetimes. A relationship between τ₃ and the surface tension, equivalent to the behavior of ordinary molecular liquids was observed in this temperature region. The o-Ps yield was strongly reduced in the crystallization region and by addition of SiO₂. The reduction due to filler addition did, however, in the case of nano-sized SiO₂ not follow a linear relationship with filler weight, which was observed for micron-sized fillers. The nonlinear relationship between filler weight and o-Ps yield could be due to out-diffusion of positrons and/or o-Ps from the filler particles to the matrix.     

Environmental degradation of isotactic polypropylene plates as studied by positron annihilation lifetime spectroscopy

Outdoor degradation of isotactic polypropylene (PPI) plates was studied by means of positron annihilation lifetime spectroscopy (PALS), absorption infrared spectroscopy, differential scanning calorimetry (DSC) and density measurements. Infrared spectra reveal the presence of oxygenated species into the exposed polymer induced by external agents. Results from thermal and density analysis suggest an increase of crystallinity of the sample with exposure time. Positron data strengthen such a conclusion, showing a reduction of the amorphous zones as monitored by the corresponding decrease of positronium (Ps) formation. Furthermore, an estimation of the average sizes of the free volume holes and of the defects in the crystalline regions was obtained.     

Structural relaxation in poly(ethylene terephthalate) studied by positron annihilation lifetime spectroscopy

Positron lifetime technique was used to study the physical ageing process in poly(ethylene terephthalate) (PET). Positron lifetime results show that the structural relaxation processes in PET encompass two different time regimes, one short and the other long. The relaxation function constructed from the measured o‐Ps intensity I₃ exhibits non‐exponential character, which can be best fitted with two additive exponentials. The Narayanaswamy model (Kohlraush‐William Watt (KWW) function) is invoked to extract the stretching parameter β indicating the extent of deviation from exponential relaxation. Based on the relaxation times, the activation energies calculated seem to label the different kinetic units of PET structure participating in the relaxation process. © 2002 Society of Chemical Industry     

Characterization of methyl‐substituted polyamides used for reverse osmosis membranes by positron annihilation lifetime spectroscopy and MD simulation

Three kinds of polyamides were synthesized from three diamines and 1,3,5‐benzenetricarbonyl trichloride (TMC). The diamines used were m‐phenylene diamine, N‐methyl‐m‐phenylenediamine, and N,N′‐dimethyl‐m‐phenylenediamine. The average free volume sizes of the polyamides were measured by positron annihilation lifetime spectroscopy (PALS), and the free volume fractions were evaluated by molecular dynamics (MD) simulations. The methyl substitution on amino groups of diamines brought about an increase in interstitial space of molecular chains of the polyamides. In addition, reverse osmosis (RO) membranes were prepared by interfacial polymerization from the three diamines and TMC. The increase in the degree of methyl‐substitution of diamines led to increased chlorine resistance and decreased salt rejections of the polyamide RO membranes. Thus, the methyl‐substitution of diamines significantly influenced membrane performance. The vacancy sizes and fractional volumes in polyamides evaluated by PALS measurement and MD simulation were well correlated with salt rejection of polyamide RO membranes. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Infrared spectroscopy measurements of residual stresses in polypropylene fibers

Average residual stresses in polypropylene fibers, produced in a full-scale short-spin line, are estimated from the shift of the IR absorption peak at 975 cm^-1. Linearly polarized incident IR radiation is used to separate between stress components parallel and perpendicular to the fiber axis. For all the fibers in this study, the axial stress component is larger than the average stress perpendicular to the fiber axis. Axial and perpendicular stresses are correlated with draw ratio and polydispersity index. For narrow molecular weight distributions (M_w/M_n < 4–5), the axial stress increases with increasing draw ratio. Average stress perpendicular to the fiber axis decreases with increasing draw ratio for all the molecular weight distributions in this study (M_w/M_n = 3–6). Measurements of residual stresses are consistent with other results characterizing molecular structure and mobility. An expression for estimating the stored deformation energy is discussed. © 1993 John Wiley & Sons, Inc.     

Influence of starch and glycerol on the properties of chitosan by positron annihilation spectroscopy

Two types of chitosan (CS), α and β, were blended with different concentrations of starch and cast to obtain films. The addition of 1% glycerol was used as a plasticizer to increase film flexibility. The properties of the obtained films were studied by positron annihilation lifetime spectroscopy, X‐ray diffraction, and scanning electron microscopy. The results indicate that pure β‐CS had smaller size free‐volume holes with high fractions than pure α‐CS; this was attributed to the difference in bonding of main chains in β‐CS. The addition of starch (>20% up to 50%) reduced the size of the free‐volume holes and increased their fraction because of the close packing of chain segments. The effect of 1% glycerol to the CS starch blends indicated that some modification took place. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Surface acidity by positron annihilation lineshapes

An alternative new method to measure the specific acidity of catalytic materials with large internal surface areas by using positron annihilation spectroscopy was introduced. Results of lineshape parameter evaluations from two-dimensional angular correlation and Doppler broadened annihilation radiation measurements for a zeolite ZSM-5 were presented.     

Antiplasticization of a polyamide: a positron annihilation lifetime spectroscopy study

In order to modify the transport properties of the amorphous polyamide Trogamid-T™, several additives with structural characteristics that produce antiplasticization have been tested. Positron annihilation lifetime spectroscopy (PALS) has been used to determine both the size of free volume holes and the number of holes (free fractional free volume). Polyvinyl phenol (PVPh), 1,5 dihydroxy naphthalene (Ndiol) and a hexafluorinated Bisphenol A (HFBA) were chosen according to the results of a preliminary study based on density and PALS measurements with 15% of additive blends. Their blends with Trogamid show different behaviours: HFBA behaves clearly as a plasticizer while the polyvinyl phenol acts as an antiplasticizer. Ndiol has a more special effect: blend densities are greater than additive whereas the hole size decreases.     

Compatibilized linear low-density polyethylene/isotactic polypropylene blends studied by positron annihilation lifetime spectroscopy

Positron annihilation lifetime spectroscopy (PALS), capable of probing free volume, is used to study the effect of compatibilizer concentration, compatibilizer type, and the effect of blend processing on the morphology and properties of an immiscible linear low-density polyethylene/polypropylene system. It is proposed that improvement of fracture toughness due to compatibilization can be attributed to the packing (and bonding) at interfaces. Improved interfacial packing and bonding result in lower free volume concentration than expected from component additivity, with a concomitant increase in plastic deformation on impact.     

Fabrication and residual stresses characterization of novel non-oxide multilayer ceramics

A multilayer composite was produced by the tape-casting technique in the AlN–SiC–MoSi₂ system, coupling insulating and electrically conductive layers. The composite was highly dense and delamination-free. The values of residual stresses due to the coupling of layers of different composition were assessed using the indentation technique and Raman piezo-spectroscopy. The experimental techniques agreed in respect to the sign and magnitude of the residual stresses, revealing compression in the insulating layers and tension in the conductive layers. The experimental results were lower than the expected values calculated by lamination theory. The choice of the parameter values needed for the theoretical calculations appears to be critical from this point of view.     

Free volume behavior in spincast thin film of polystyrene by energy variable positron annihilation lifetime spectroscopy

Free volume behavior in polystyrene thin films with thickness ranging from 22 to 1200 nm on silicon substrates was studied by energy variable positron annihilation lifetime spectroscopy (EVPALS). The films were prepared by spincasting from toluene solutions of 0.5-5.0 wt% polystyrene with M_w = 1?090?000 g/mol. Distinct deviations from bulk polystyrene in thermal expansion of the free volume holes and the glass transition temperature associated with free volume behavior were observed for the thinnest film with 22 nm thickness, indicating its exclusively high chain mobility. Comparison of the polystyrene concentration in the precursor solution around the overlap concentration suggests that the high chain mobility is due to less entangled chains caused by rapid removal of the solvent from the diluted solution in order to prepare very thin film.     

Thermal residual stresses in bilayered,trilayered and graded dental ceramics

Layered ceramic systems are usually hit by residual thermal stresses created during cooling from high processing temperature. The purpose of this study was to determine the thermal residual stresses at different ceramic multi-layered systems and evaluate their influence on the bending stress distribution. Finite elements method was used to evaluate the residual stresses in zirconia-porcelain and alumina-porcelain multi-layered discs and to simulate the ‘piston-on-ring’ test. Temperature-dependent material properties were used. Three different multi-layered designs were simulated: a conventional bilayered design; a trilayered design, with an intermediate composite layer with constant composition; and a graded design, with an intermediate layer with gradation of properties. Parameters such as the interlayer thickness and composition profiles were varied in the study. Alumina-porcelain discs present smaller residual stress than the zirconia-porcelain discs, regardless of the type of design. The homogeneous interlayer can yield a reduction of ~40% in thermal stress relative to bilayered systems. Thinner interlayers favoured the formation of lower thermal stresses. The graded discs showed the lowest thermal stresses for a gradation profile given by power law function with p=2. The bending stresses were significantly affected by the thermal stresses in the discs. The risk of failure for all-ceramic dental restorative systems can be significantly reduced by using trilayered systems (homogenous or graded interlayer) with the proper design.     

Dynamic water absorption characteristics of chitosan-based hydrogels. An investigation by positron annihilation lifetime spectroscopy

The water content dependence of the free volume in chitosan-based hydrogels was studied using the positron annihilation lifetime spectroscopy (PALS) technique. The dynamic water absorption behavior was correlated with the variation in the mean lifetime (τ₃) and intensity (I₃) of the o-Ps component. PALS data suggest that water clusters coexist with individual water molecules at the beginning of swelling. The comparison between the calculated and measured values of τ₃ and I₃ implies that the free volume of the hydrogel network has a great effect on the state of water.     

Electric charge trapping, residual stresses and properties of ceramics after metal/ceramics bonding

The use of ceramic components in electrical engineering and mechanical applications is rapidly increasing. Most of these applications require the use of ceramics bonded with metal. In this paper, we have studied the role of residual stresses occurring after joining between an industrial alumina ceramic (Al₂O₃) and Ni-based super-alloy, on the dielectric behaviour of ceramics. The electric charging phenomenon i.e. trapping-detrapping or diffusion of electric charges is studied by Scanning Electron Microscope Mirror Effect (SEMME) coupled with the Induced Current Method (ICM). Knowing that localized trapped charges in ceramics is a source of damage, the correlation between residual stress intensity, apparent-toughness of ceramics and ability to trap charges near the interface was demonstrated: the SEMME and ICM measurements of the quantities of trapped charges near the interface, highlighted the changes in the ceramic properties related to residual stresses due to both thermo-mechanical effect and diffusion of metallic species in the ceramics, during the bonding process.     

γ-Irradiation effects on polyethylene terephthalate studied by positron annihilation lifetime spectroscopy

Structural changes in commercial polyethylene terephthalate subjected to γ-irradiation were investigated by means of differential scanning calorimetry, viscosity measurements and positron annihilation lifetime spectroscopy. Irradiated samples show a lower molecular weight and increased crystallinity, which are attributed to chain scissions of the macromolecules. Positron results support such an interpretation and supply an estimation of the average sizes of the free volume holes and of the defects in the crystalline regions of the polymer.