Article

Water Uptake of a Sandwich-Structured Composite Film Coated on a Mg-Gd-Y Alloy: Experiments and Model Validation

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Abstract

Water diffusion in a sandwich-structured composite film on a Mg-Gd-Y alloy was experimentally investigated using electrochemical impedance spectroscopy (EIS). The composite film contained an inner integration layer and an outer fluorocarbon (FC) layer. The integration layer was prepared by depositing an environment-friendly self-assembled nanoparticle film on the surface of a plasma electrolytic oxidation film. The resulting structure is therefore referred to as PSF. The measured value of the diffusion coefficient of the PSF film by the EIS method was compared to the value predicted using a mathematical model. The noncontinuity of the water concentration at the interface of the integration and FC layers resulted in a significant difference between the diffusion coefficients obtained by the experimental EIS method and mathematical model. A coefficient "k(e)" was introduced to the Fick's first law of diffusion, and a PSF model validation was used to confirm the concentration jump on the FC-integration boundary of the PSF film. When k(e) << 1, the calculated diffusion coefficient (D-eff(M)) of the PSF film by the mathematical model was found similar to that obtained by the EIS experimental method.

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... The mathematical model for the effective water diffusion coefficient of the composite coating (D com eff ) is described by equation (4) as follows [6][7][8]21 : ...
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Different compositions of poly(ε-caprolactone) (PCL) and (organo-modified) montmorillonite were prepared by melt blending or catalyzed ring opening polymerization of ε-caprolactone. Microphase composites were obtained by direct melt blending of PCL and sodium montmorillonite (MMT-Na+). Exfoliated nanocomposites were obtained by in situ ring opening polymerization of ε-caprolactone with an organo-modified montmorillonite (MMT-(OH)2) by using dibutyltin dimethoxide as an initiator/catalyst. Intercalated nanocomposites were formed either by melt blending with organo-modified montmorillonite or in situ polymerization within sodium montmorillonite. The barrier properties were studied for water vapor and dichloromethane as an organic solvent. The sorption (S) and the zero concentration diffusion coefficient (D0) were evaluated for both vapors. The water sorption increases with increasing the MMT content, particularly for the microcomposites containing the unmodified MMT-Na+. The thermodynamic diffusion parameters, D0, were compared to the value of the parent PCL: both microcomposites and intercalated nanocomposites show diffusion parameters very near to PCL. At variance exfoliated nanocomposites show much lower values, even for small montmorillonite content. In the case of the organic vapor, the value of sorption at low relative pressure is mainly dominated by the amorphous fraction present in the samples, not showing any preferential adsorption on the inorganic component. At high relative pressure the isotherms showed an exponential increase of sorption, due to plasticization of the polyester matrix. The D0 parameters were also compared to those of the unfilled PCL; in this case, both the exfoliated and the intercalated samples showed lower values, due to a more tortuous path for the penetrant molecules.
Article
One of the major factors affecting the acceptability of novel coating in engineering applications is the degradation of the coating by the moistures that will affect the physical and mechanical performances and causing it to fail. Nanoscale reinforcement provides opportunities for enhancing the polymer system with unique properties and increases the performance. These advantages not only lead to an improvement in the mechanical properties but also act as a barrier to slow the water permeation. In this study, an organically modified montmorillonite (MMT) nanoclay and a further modified “X-treated supernanoclay” were used to reinforce an isophthalic unsaturated polyester gelcoat. Five gelcoat systems were prepared and characterised by using a boiling water absorption test (with a saturation time of 12 h) to characterise and quantify the degree of water absorption. All nanocomposite samples show a very convincing result in terms of the coefficient of moisture absorption and reduction in water ingress as a function of weight fraction of nanoclays. The “X-treated supernanoclay” reinforced systems show the greatest improvement compared to conventional nanoclay system.
Article
An in-situ FTIR–ATR method for simultaneously obtaining both kinetic and structural information during liquid sorption into polymers was presented. The kinetics and diffusion profile of the sorption of liquid water and liquid methanol into poly(ethylene terephthalate) (PET) were compared and contrasted. The diffusion of water into PET was shown to follow Fickian kinetics, without significant swelling and the calculated diffusion coefficients (D) varied between for a crystallinity range of 4–25%. The D values decreased non-linearly with the increase in crystallinity. The average spherulitic crystal size was thought to play a significant role in determining the rate of water sorption. Conversely, the sorption of liquid methanol was accompanied by significant swelling and crystallisation and hence showed non-Fickian or anomalous kinetics. The sorption data were fitted to a dual sorption model which indicated that the rate of diffusion of liquid methanol was faster than that of liquid water, probably due to the accompanying swelling. Increasing the level of crystallinity was shown to decrease the capacity for the polymer to swell and reduced the calculated diffusion coefficients.
Article
Poly(4,4′-oxydiphenylene pyromellitimide) (PMDA—ODA) films of 10–109 μm thickness were prepared from its poly(amic acid) precursor by thermal imide-ring-closure formation at various temperatures. Water sorption in the films was measured at 25°C over 22–100% relative humidity using an electromicrobalance. Water diffusion in all the films was a nearly Fickian process despite the morphological heterogeneity due to the ordered and less ordered phases. Depending upon humidity, film thickness and imidization history, the diffusion coefficient and water uptake varied in the ranges of 1 × 10−9 to 3 × 10−9 cm2 s−1 and 0.4 to 4.5 wt%, respectively. Overall, both the diffusion coefficient and the water uptake increased with increasing humidity and film thickness, but decreased as the imidization temperature and time increased. The water sorption results were interpreted by consideration of morphological variations (molecular order, chain orientation and microvoids) due to film thickness and imidization history.
Article
The diffusion of small molecules in polymers was measured using Fourier transform infra-red-attenuated total reflection (FTi.r.-a.t.r) spectroscopy, a new approach which allows one to study liquid diffusion in thin polymer films in situ. The system used to test this technique was the diffusion of liquid water in polyacrylonitrile (PAN). The diffusion coefficients that were measured were in good agreement with those reported in the literature for high-activity water vapour in PAN. The test also demonstrated both the reproducibility and accuracy of FTi.r.-a.t.r. experiments for measuring the diffusion coefficient. The diffusion coefficient and the amount of water sorbed at equilibrium appear to be dependent on the temperature at which the films were treated. The method described here shows great potential for elucidating penetrant-penetrant and penetrant-polymer interactions during the diffusion process owing to the spectroscopic nature of the technique.
Article
Sorption and diffusion data were obtained for water vapor in four different polymers: poly (methylmethacrylate) (PMMA), poly (2-hydroxyethylmethacrylate) (PHEMA), poly (N-vinyl-2-pyrrolidone) (PVP) and poly (acrylonitrile) (PAN) at 35 °C using a gravimetric sorption method. Highest sorption was for PVP, followed by PHEMA. PMMA and PAN sorbed very little water. All the polymers exhibit a BET type III sorption isotherm; the large upturn at high activity for PVP and PHEMA is probably due to plasticization of the polymers by water vapor. Sorption data were interpreted using Flory–Huggins theory and the Zimm and Lundberg cluster integral.Fickian diffusion is observed for PHEMA. For PVP, the fractional uptake Mt/M∞ is linear with the square root of the time up to Mt/M∞=0.6−0.8 for all water activities aw, but it shows a clear water sorption overshoot at aw=55.3% and aw=72.1%, probably due to macromolecular relaxation. PMMA sorption kinetics is also characterized by a maximum in the water uptake. The diffusion coefficient increases significantly with water concentration for PVP and PHEMA, weakly for PMMA, but it is independent of concentration for PAN.
Article
In the present paper, we have intensively investigated the effects of porosity on the electrical characteristics of current-limiting BaTiO3-based positive-temperature-coefficient (PTC) ceramic thermistors coated with electroless nickel–phosphorous electrode by employing scanning electron microscopy (SEM) and ac current inrush test. It was found that the PTCR effect, dielectric property, ac current inrush capability and internal stress of specimens were influenced by porosity. In fact, porosity in BaTiO3 ceramics was related to not only organic and inorganic inclusions in powders but also formation pressure of green compacts. PTCR effect of BaTiO3-based PTCR ceramic thermistors with various porosities could be evaluated by the barrier height of Heywang’s double-Schottky barrier model, and dielectric constants could be estimated by the modified Niesel’s equation and Maxwell relationship. The results suggested that their PTCR effect and current-withstanding capability of thermistors could be improved by increasing the porosity in ceramic disks. In order to obtain the current-limiting PTCR ceramic disks with high performance, the porosity of disks should be between 6 and 14%, while formation pressure should be chosen 500–1550 kg cm−2.
Article
Water sorption, desorption, and permeation in and through Nafion 112, 115, 1110 and 1123 membranes were measured as functions of temperature between 30 and 90 °C. Water permeation increased with temperature. Water permeation from liquid water increased with the water activity difference across the membrane. Water permeation from humidified gas into dry nitrogen went through a maximum with the water activity difference across the membrane. These results suggested that the membrane was less swollen in the presence of water vapor and that a thin skin formed on the dry side of the membrane that reduced permeability to water. Permeation was only weakly dependent on membrane thickness; results indicated that interfacial mass transport at the membrane/gas interface was the limiting resistance. The diffusivity of water in Nafion deduced from water sorption into a dry Nafion film was almost two orders of magnitude slower than the diffusivity determined from permeation experiments. The rate of water sorption did not scale with the membrane thickness as predicted by a Fickian diffusion analysis. The results indicated that water sorption was limited by the rate of swelling of the Nafion. Water desorption from a water saturated film was an order of magnitude faster than water sorption. Water desorption appeared to be limited by the rate of interfacial transport across the membrane/gas interface. The analysis of water permeation and sorption data identifies different regimes of water transport and sorption in Nafion membranes corresponding to diffusion through the membrane, interfacial transport across the membrane–gas interface and swelling of the polymer to accommodate water.
Article
The sorption kinetic and equilibrium data obtained at different water activities allowed us to determine the water sorption isotherms and mean diffusion coefficient in the Nafion® membrane. The water sorption isotherm is reminiscent of systems in which there is water clustering at high water activities, with a marked Langmuir sorption at low water activity for the acid form of the membrane. The variation of the diffusion coefficient with the water activity showed a maximum value whose coordinates depend on the counter-cation nature. The smaller the counter-cation, i.e. the larger the counter-cation hydration energy, the higher the diffusion coefficient and the lower the water activity at the maximum point.
Article
Both sorption and desorption transport coefficients of water and the inhibitor (HPO42−) in a series of epoxy resins (model paints) of different cross-linking density have been reported. Water diffusion is unaffected by low electrolyte concentrations (0.1 μm) but is retarded at 0.6 M chloride ion levels, presumably due to water–polymer–polymer interaction shielding. Swelling is small, but may be significant for polymer chain relaxation. The FTIR data have been used to add a molecular level interpretation of the four different pseudo-Fickian processes observed at room temperature. These data are consistent with rapid sorption onto polymer active (OH, NH), sites followed by slower incorporation of water into polymer microvoids where cluster sizes are smaller. The reverse process (on desorption) is also evident from the water, ν(OH), spectral distribution. No evidence of significant accumulation of ‘bulk’ water at the polymer–substrate interface was found.
Article
The water uptake of a commercial alkyd paint system is studied using the impedance spectroscopy technique. The first part of the paper is devoted to establish the experimental setup to accurately measure capacitance values in the pF range, characteristic of the studied system. It was concluded that electrochemical instrumentation is not adequate to perform this task. The second part of the paper discusses the water uptake of the paint system under two different immersion conditions: deionized water and 5% NaCl solution. This analysis leads to the conclusion that the diffusion process seems to follow the ideal-Fickian behaviour in both systems, the corresponding diffusion coefficients were calculated.
Article
In this investigation, water sorption behaviour of six epoxy systems with different chemical structure of amines and epoxies were monitored at five temperatures ranging from 35 to 75 °C. The sorption process was analyzed by gravimetric measurements, positron annihilation lifetime spectroscopy (PALS), and attenuated total reflection Fourier transform infrared spectroscopy (ATR-IR). PALS measurements showed that the free volume fraction is not a decisive factor in governing the equilibrium water content, while the polarity of the system is the essential factor. ATR-IR curve fitting were consistent with the theoretical calculated results, and correlated to the diffusion coefficients from gravimetric measurements.