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In order to improve water and oxygen barrier properties, the surface of two commercial medical grade polyurethane (PU) membranes (Chronoflex® AR-LT and Bionate® II) was modified by a spray deposited film of poly(ethylene-co-vinyl alcohol) (EVOH). The influence of the temperature, the deposited layer thickness and the EVOH ethylene group percentage (27%, 32%, and 44% for EVOH27, EVOH32, and EVOH44, respectively) on the barrier properties of the PU/EVOH multilayered membranes was investigated. The increase of the EVOH layer thickness leads to higher oxygen barrier properties (the highest barrier improvement factor of 412 was obtained). However, in case of the deposited layer thickness higher than 18 μm, microcracks appeared on the treated surface promote a significant loss of the barrier effect. Due to its higher crystallinity degree, EVOH27 provides a higher oxygen barrier effect compared to EVOH32 and EVOH44. On the contrary, an increase of the water barrier properties was observed with the increase of the percentage of ethylene groups. Moreover, the delamination of the EVOH layer was noted after water permeation, especially in case of EVOH44, which is the most hydrophobic layer. Nevertheless, significant decrease of the water and oxygen permeability of the modified PU membranes was achieved, thus showing the benefit of using the EVOH spray deposition for the biomedical application, which requires high performance material with flexible and barrier properties.

Because of their good mechanical properties and low density, natural fibers are more and more considered as reinforcement in composite materials. To improve the adhesion between the polymer matrix and natural fibers and to reduce the water sorption, helium cold plasma and autoclave treatments have been performed on flax fibers. The effect of these treatments on water sorption has been investigated by means of a gravimetric static equilibrium method (IGA). Water sorption isotherms have been deduced from kinetic data. Specific mathematical models have been tested to predict the water sorption isotherms of the treated flax fibers. The Park model based on the three sorption modes: Langmuir, Henry's law, and clustering, was successfully used to simulate the experimental sorption data. The results show no significant effect after plasma treatment while an increase of moisture resistance has been observed after autoclave treatment. This effect of autoclave treatment is interpreted in terms of modifications of cell-wall structure. In a kinetic point of view, for the untreated and treated flax fibers, the variation of the diffusion coefficient with water concentration is in agreement with the three sorption modes of the Park model. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 4281–4289, 2006

This work deals with the role of liquid water on the microstructure of a semiaromatic polyamide (MXD6). One side of injected plates is kept in contact with water during different durations. After immersion, microtomic cuts are performed at different depth and are analyzed by X-ray diffraction, differential scanning calorimetry, and permeation tests. Measurements show that the water plasticizes the initially amorphous plates and induces a spherulitic crystallization, which grows from the wet side to the dry one. As a consequence, the permeation curve is unusual, and the water flux goes through a maximum before reaching a lower plateau when the front of crystallization reaches the opposite dry face. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2604–2616, 2005

Asymmetric membranes, based on Poly[ethylene-co-(vinyl acetate)] (EVA) containing 70 wt.% of vinyl acetate, were prepared by a treatment of unilateral hydrolysis using solutions of sodium hydroxide dissolved in a mixture of water and methanol. The depth of hydrolyzed layers and the concentration of hydroxyl groups in the membranes were controlled by the reaction time. The oxygen permeability, P O2 , of these membranes decrease with the reaction time while the water permeability, P H2O , reaches maximum at 30 min. The ideal separation factors of P H2O to P O2 of the EVA membranes treated for 1h to 4h are in the range of 3840 to 13500, and are greater than that of the EVA membranes. The plasticization effect of the membrane depends on the depth and concentration of hydroxyl groups and the concentration gradient of water in the membranes.

The transport of water vapor and gases (oxygen or carbon dioxide) through poly(ethylene-co-vinyl acetate) (EVA) films of different VA contents and through EVA 70 /PVC and EVA 70 /PVC/gluten blend films, was analysed by permeation measurements. In the case of water, a plasticization effect on the material is observed for EVA films with more than 33% wt. of VA content and also for the EVA 70 /PVC blend,. For EVA of 19 wt.% VA, there is no plasticization, while for LDPE (low density polyethylene) and EVA of 4.5 wt.% VA, the water diffusion coefficient decreases with increasing the water content. A negative plasticization effect was accounted for by an empirical model and attributed to the formation of water clusters in the non polar polymers. The increase in water sorption extent with the VA content leads to a steady increase in the water permeability in the EVA copolymers while for the EVA 70 /PVC blend, the reduced water permeability is explained by the interaction between chlorinated units and polar groups. In the case of gas permeation, both for O 2 and CO 2 and whatever the VA content of the copolymer used, the experimental curves are characterized by a constant diffusion coefficient. This result is not surprising as it is generally admitted that, gases sorb into rubbery polymers according to Henry's law. By mixing PVC with the EVA of 70% wt. VA, the diffusion coefficients of CO 2 and O 2 are greatly reduced (6 times).

The diffusion and permeation properties of liquid water through different polar and nonpolar polymers and copolymers were studied with a highly sensitive permeameter. The transient permeation fluxes through the polar polymer films could be fitted well only with an exponential equation for the diffusivity concentration dependence; this empirical exponential equation represented the diffusion plasticization effect of water on the materials. For the hydrophobic polyolefins, this exponential equation was no longer valid, and another form of the equation was empirically found to account for the reduction of the water diffusivity with the extent of the permeation. Such a negative plasticization effect might be attributed to the formation of water clusters in the polyolefins. The values of the diffusion coefficient of water in the dry polar polymers were smaller than those in dry polyolefins, but the opposite behavior was found for the permeability because it was much more favorable for water sorption in the polar polymers than in the hydrophobic polyolefins. For the ethylene–vinylacetate copolymers, the plasticization effect of water on its own diffusion was negative for the sample with a low vinyl acetate (VA) content; it became nil at 19 wt % VA and positive at higher VA contents. This increase in the extent of the water sorption with the increase in the VA content led to a steady increase in the water permeability in the poly(ethylene-co-vinylacetate) copolymers. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1998–2008, 2000

A water-specific permeameter was developed to study water diffusion in an unsaturated polyester; these polyesters are often used as high-barrier materials. Low water enrichment of a dry sweeping gas is measured via the dew point temperature, with two hygrometers, one of which is used because of its fast response and the other because of its accuracy. This high-performance device is just as suited to pervaporation as to permeation tests and allows the transient and stationary fluxes to be characterized. At first sight, the experimental data seem in good agreement with D = D0eγC. However, a more thorough study has shown a time-dependence of D. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 3380–3395, 1999

Water diffusion through Unsaturated Polyester Resin (UPR) is investigated. From differential scanning calorimetry, we observe that the plasticization effect due to water sorption leads to large variations of T g and is a reversible phenomenon. The transport of water through the polymer is characterized with a new instrument developed in our laboratory which shows that the water transport deviates from Fickian behavior. It is found that the diffusion coefficient is not a constant and experimental data are in good agreement with D = D 0 e γC where γ is a plasticization coefficient.

In this work, the curing kinetic of an unsaturated polyester resin, mixed with styrene as curing agent, was studied by means of diffential scanning calorimetry (DSC) and infrared (IR) spectroscopy. Investigations were made in situ during curing and post-curing periods. The enthalpy (ΔHcop) characterizing the styrene homopolymerization and its copolymerization with the polyester chains was determined from isothermal DSC investigations. The residual enthalpy (ΔHres) was determined from nonisothermal DSC measurements. We find that the quantity (ΔHres + ΔHcop) depends on the curing temperature (Tiso). Styrene homopolymerization and/or copolymerization lead to characteristic bonds modifications. The variations of the most characteristic bonds versus curing duration were studied from Fourier transform IR investigations. The data allow the determination of a relationship between transformation rate and curing duration using a time constant (τ) characteristic of each reaction involved during the liquid to solid-state transformation. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67: 695–703, 1998