Tortuous Diffusion Path Research Articles

Physical and Gas Transport Properties of Asymmetric Hyperbranched Polyimide-Silica Hybrid Membranes

Physical and gas transport properties of the asymmetric hyperbranched polyimide (HBPI) -silica hybrid membranes prepared with a dianhydride, 4,4’-(hexafluoroisopropylidene)diphthalic anhydride (6FDA), and an asymmetric triamine, 2,4,4’-(triaminodiphenyl)ether (TADE), were investigated and compared with those of the symmetric HBPI-silica hybrid membranes prepared with a symmetric triamine, 1,3,5-tris(4-aminophenoxy)benzene (TAPOB). The HBPI-silica hybrid membranes were prepared via sol-gel reaction using hyperbranched polyamic acid of which end groups were modified with silane coupling agents, water and tetramethoxysilane. The thermal mechanical and dynamic mechanical analysis measurements confirmed that the rigidity of asymmetric HBPI was higher than that of symmetric HBPI because of the rigid and asymmetric structure of TADE monomer. In addition, the degree of branching of asymmetric HBPI is lower than that of symmetric HBPI because of the different reactivity of the three amino groups included in TADE. The rigidity and linearity of HBPIs had an effect on the progression of sol-gel reaction, consequently the gas transport properties. The increasing of the gas permeability coefficient of the asymmetric dianhydride(DA)-HBPI-silica hybrid membranes with increasing silica content was smaller than those of symmetric DA- and amine(AM)-HBPI-silica hybrid membranes. In addition, the gas permeability coefficient of the asymmetric AM-HBPI-silica hybrid membranes decreased with increasing silica content. This was due to the fact that the dispersibility of silica in the asymmetric HBPI-silica hybrids, of which polymer chain was more rigid and linear than those of symmetric HBPI-silica hybrid, was not as fine as in the symmetric HBPI-silica hybrids, and that the long and tortuous diffusion path was newly formed by hybridization with silica.

Oxygen scavenging polyolefin nanocomposite films containing an iron modified kaolinite of interest in active food packaging applications

Abstract A synthetic iron containing kaolinite was evaluated as an oxygen scavenger additive for food packaging plastics having an active performance of ca. 43 ml O 2 /g at 100%RH and of ca. 37 ml O 2 /g of additive at 50%RH. The corresponding polyolefin films containing 10 wt.% of the active filler also exhibited significant oxygen scavenger activity (up to 4.3 ml of oxygen/g composite, at 24 °C and 100%RH). The effect of oxygen scavenging capacity of HDPE films with temperature and %RH was also assessed. The oxygen permeability tests carried out after inactivation of the additive suggested that the iron containing clay plays in fact a dual oxygen fighting role: active performance (trapping and reacting with molecular oxygen) as well as a passive barrier performance (imposing a more tortuous diffusion path to the permeant). Migration tests into two food simulants indicated that iron (from active ingredient) and aluminum (from clay migration) are hardly detectable in commonly used simulants. This study suggests that there is significant potential for the use of this novel oxygen scavenger additive to constitute active packaging of value in the shelf-life extension of oxygen sensitive food products. Industrial relevance Oxygen penetration in food and beverages leads to, for instance, oxidative rancidity of unsaturated fats, loss of vitamin C, browning of fresh meat, oxidation of aromatic flavor oils and pigments and fostering the growth of aerobic spoilage microorganisms. These undesirable effects on the product quality indicate that the elimination or exclusion of oxygen is one of the main targets for preserving foods and beverages in the food packaging industry. The industrial relevance of this study is very significant because it proves that the technology generated and characterized in the paper can uniquely fight oxygen by two means, i.e active and passive performance. This should enable the food packaging industry to package foods in relatively inexpensive, low barrier commodity plastic materials such as polyolefins, that will allow the shelf-life extension of the packaged products with a safe use, since migration of the components is negligible.

Synthesis and Characterization of High-Throughput Nanofabricated Poly(4-Hydroxy Styrene) Membranes for In Vitro Models of Barrier Tissue

Commercially available permeable supports with microporous membranes have led to significant improvements in the culture of polarized cells because they permit them to feed basolaterally and thus carry out metabolism in a more in vivo-like setting. The porous nature of these membranes enables permeability measurements of drugs or biomolecules across the cellular barrier. However, current porous membranes have a high flow resistance due to great thickness (20-40 μm), low porosity, and a wide pore size distribution with tortuous diffusion paths, which make them low-throughput for permeability studies. Here we describe an alternate platform that is more flexible, allows for more control over physical parameters of the membranes, and is high-throughput. This study reports on the synthesis, nanofabrication, and surface characterization of a 3-μm-thick transparent membrane based on poly(4-hydroxy styrene) (PHOST). The membranes are nanofabricated using electron beam lithography and deep ion plasma etching to achieve an organized array of straight pores from 50 to 800 nm in diameter, with at least 23 times less flow resistance. It also shows for the first time the potential utility of PHOST as a cell culture substrate without cytotoxicity, and suitability for nanofabrication processes due to temperature stability.

Permeability and partition coefficient of aqueous sodium chloride in soft contact lenses

AbstractTransport of physiologic saline through soft contact lenses is important to on‐eye behavior. Using a specially designed Stokes‐diaphragm cell, we measure aqueous NaCl permeabilities through commercial soft contact lenses at 35°C. The permeabilities increase exponentially with the water content of the lenses spanning a range from 10−7 to 10−5 cm2/s. Equilibrium partition coefficients are obtained by the back‐extraction of lenses initially immersed in 1M aqueous NaCl. Partition coefficients also increase with lens water content but over a smaller range, from 0.1 to 0.7. Because the partition coefficient values are smaller than the water content of the lenses, ideal theory is not followed. Donnan exclusion, bound water, and excluded volume are proposed explanations. The diffusion coefficients of aqueous NaCl through soft contact lenses increase with increasing lens water content following free‐volume theory. Aqueous NaCl diffusivities in the lower water‐content lenses are smaller than the diffusion coefficient of NaCl in water by factors up to 100 indicating very tortuous diffusion paths. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

Membrane formation temperature-dependent gas transport through thermo-sensitive polyurethane containing in situ-generated TiO2 nanoparticles

Thermo-sensitive polyurethane (TSPU) solution containing varying concentration of in situ-generated TiO2 nanoparticles was successfully prepared via an organic–inorganic hybrid technique, and the final nanocomposite membranes were formed via solution casting. Depending on the membrane formation temperature (Tmf) during casting, completely-opposite gas transport behaviors of the TSPU nanocomposite membranes were observed. When Tmf was lower than the melting point (Tm) of soft segment (PCL10000) in the TSPU, gas permeability coefficients of the nanocomposite membranes were found to increase significantly with increasing nano-TiO2 concentration. Conventional composite theory failed to explain such observation because filler particles are generally considered to create more tortuous diffusion path in the polymer for penetrants, and should thereby lead to a systematic reduction in gas permeability. This counter-intuitive phenomenon was then rationalized by postulating that rigid TSPU chain could not pack efficiently around the TiO2 nanoparticles during solvent evaporation, which resulted in polymer layer with higher free volume at the interface than the bulk polymer regions. Evidences supporting such presumption were provided by (1) density measurement that found negative deviation of actual densities of TSPU nanocomposite membranes from theoretical prediction; (2) positron annihilation lifetime spectroscopy that demonstrated increased free-volume size and concentration within TSPU upon the in situ generation of TiO2 nanoparticles. However, when Tmf was elevated above the Tm of the soft segment, flexible TSPU chain seemed able to pack around the TiO2 nanoparticles as efficiently as in the bulk polymer. In this case, gas transport behavior of the TSPU nanocomposite membranes followed the prediction of conventional composite theory.

Direct Measurement of VOC Diffusivities in Tree Tissues: Impacts on Tree-Based Phytoremediation and Plant Contamination

Recent discoveries in the phytoremediation of volatile organic compounds (VOCs) show that vapor-phase transport into roots leads to VOC removal from the vadose zone and diffusion and volatilization out of plants is an important fate following uptake. Volatilization to the atmosphere constitutes one fundamental terminal fate processes for VOCs that have been translocated from contaminated soil or groundwater, and diffusion constitutes the mass transfer mechanism to the plant-atmosphere interface. Therefore, VOC diffusion through woody plant tissues, that is, xylem, has a direct impact on contaminant fate in numerous vegetation-VOC interactions, including the phytoremediation of soil vapors and dissolved aqueous-phase contaminants. The diffusion of VOCs through freshly excised tree tissue was directly measured for common groundwater contaminants, chlorinated compounds such as trichloroethylene, perchloroethene, and tetrachloroethane and aromatic hydrocarbons such as benzene, toluene, and methyl tert-butyl ether. All compounds tested are currently being treated at full scale with tree-based phytoremediation. Diffusivities were determined by modeling the diffusive transport data with a one-dimensional diffusive flux model, developed to mimic the experimental arrangement. Wood-water partition coefficients were also determined as needed for the model application. Diffusivities in xylem tissues were found to be inversely related to molecular weight, and values determined herein were compared to previous modeling on the basis of a tortuous diffusion path in woody tissues. The comparison validates the predictive model for the first time and allows prediction for other compounds on the basis of chemical molecular weight and specific plant properties such as water, lignin, and gas contents. This research provides new insight into phytoremediation efforts and into potential fruit contamination for fruit-bearing trees, specifically establishing diffusion rates from the transpiration stream and modeling volatilization along the transpiration path, including the trunk and branches. This work also has importance in other plant-VOC interactions, such as potential uptake from the atmosphere for hydrophobic compounds and also uptake from vapor-phase soil contaminants.

Concentration dependence of diffusion–adsorption rate in activated carbon

A new method based on transfer rate measurements of a radioactive solute inside activated carbon grains is presented which allows to isolate the relative contributions of the chemical (reversible adsorption) and geometrical (tortuous diffusion paths) effects on the overall effective diffusion coefficient. Transfer rate measurements were performed with two different initial conditions: carbon grains either initially saturated with a solution of the same concentration as the bath ( self-diffusion) or free of solute. In the self-diffusion case the adsorption equilibrium for the solute is undisturbed during the process and the adsorption dynamics for tracer particles becomes linear. These processes can be analyzed in terms of an effective self-diffusion coefficient ( D eff). The concentration dependence of the transfer rate is shown to be controlled by nonlinearities of the adsorption isotherm. We also show that the self-diffusion coefficient ( D s ) is one order of magnitude lower than in free bulk diffusion. Finally, in the second case (grains initially free of solute) we predict satisfactorily sorption rate curves, as well as the dependence of the transfer rate on the initial concentration, using results obtained in the self-diffusion case.