1,721,129 research outputs found
Gas permeability in glassy polymers: A thermodynamic approach
No full textThe permeability of various low molecular weight species (both gases and vapors) in a series of glassy polymers has been extensively analyzed by means of a thermodynamic based approach for solubility and diffusivity, recently proposed and already applied to a few penetrant/polymer systems. The model relies on the thermodynamic description of the solubility behaviors of the solutes provided by the nonequilibrium thermodynamic model for glassy polymers (NET-GP), while the diffusivity is the product of the mobility coefficient, a purely kinetic quantity, and the thermodynamic factor, accounting for the dependence of the penetrant chemical potential on its concentration in the glassy polymer matrix.
The model is applied to permeability data of many penetrant species from very light gases, such as hydrogen or helium, to hydrocarbons and fluorocarbons, in several different glasses, including very high free volume materials, polyimides and fluoropolymers. The model proved to be effective in the representation of all types of permeability behaviors with respect to penetrant upstream pressure, which may be either decreasing, increasing, or with a nonmonotonous trend showing a minimum value at the so-called plasticization pressure
Test methods for the characterization of gas and vapor permeability in polymers for food packaging application: A review
No full textThe assessment of the transport properties of different substances in polymeric materials is of crucial importance
for the evaluation of packaging performances, aiming to protect packed food products and ensure a prolonged
shelf-life. The most relevant substances for such application can be identified in O2, CO2 and C2H4 (together with
N2 as control) as permanent gases, and water vapor as condensable species. All these penetrants are able to alter
significantly food properties, in term of sensory characteristics, textures, and quality in general. All relevant
techniques for the experimental characterization of mass transport properties in polymeric films or packages are
critically reviewed, focusing in detail on standard methods. Each technique is briefly presented in terms of
apparatus requirements and general procedures, followed by a discussion on its main features
Modeling mass transport in dense polymer membranes: cooperative synergy among multiple scale approaches
Full text linkThe modeling description of basic transport phenomena of either liquid, gas or vapor molecules in dense polymeric membranes is of tremendous impact for the separation industry, which relies on solid models for the design of optimal process conditions, for the selection of the most suitable membrane materials as well as for the development of novel ones. Such models need to deal with several physical aspects and phenomena, spanning over broad time and length scales, thus requiring multiple approaches. The solid frameworks now available mainly rely on the solution–diffusion theory, in which equation of state models and free volume theories are applied for the description of thermodynamic and kinetic properties, to be coupled in appropriate transport schemes
Elementary prediction of gas permeability in glassy polymers
No full textThe transport model proposed by Minelli and Sarti for the representation of gas and vapor permeability in glassy polymers has been extensively applied to various systems, and the model results are thoroughly analyzed. The approach is based on fundamental theory for the diffusion of low penetrant species in polymers, in which the diffusivity is considered as the product of the molecular mobility, and a thermodynamic coefficient, accounting for the concentration dependence of the chemical potential. The model relies on the thermodynamic description of the penetrant/polymer systems provided by the NonEquilibrium Thermodynamics for Glassy Polymers (NET-GP) approach. The penetrant mobility is assumed to depend exponentially on penetrant concentration, and the model contains two parameters only: mobility coefficient at infinite dilution and plasticization factor. The model parameters obtained from the analysis of the permeability behaviors of various systems have been examined and general correlations are derived. The mobility coefficient is indeed correlated to the properties of the pure penetrants (penetrant molecular size) and pure polymer (fractional free volume and characteristic energy). This allows the derivation of a simple and general expression for the prediction of the permeability of any penetrant species in glassy polymers in the range of low penetrant pressures, as well as the selectivity of any gas pair. Remarkably, the model predictions are able to represent quite accurately the experimental data available in the literature. Furthermore, the plasticization factor is correlated to the swelling produced by the penetrant into the glassy polymer matrix, obtaining thus a reliable tool for the estimation of the pressure dependence of gas permeability on upstream pressure
In-plane stress measurement in polymer coatings as induced by vapor sorption/desorption processes
No full textThe aim of this work is the design of an apparatus which allows to measure the in plane stress that arises in polymer coating of commercial polystyrene (N2380) below the glass transition temperature (Tg) as result of vapor sorption/desorption processes and its evolution in time. The in plane stress is measured using bending beam cantilever technique. The experiments were performed at different temperatures, using different fugacity of n-pentane inside the system. From the stress state, knowing the material proprieties, the change in the volume recorded during the sorption/desorption process was derived. Through the use ofthe non-equilibrium lattice fluid model (NELF) it is then possible to estimate vapor mass uptake/release in the coating
Predictive model for gas and vapor sorption and swelling in glassy polymers: II. Effect of sample previous history
No full textThe solubility of various gases in several glassy polymers has been analyzed by the nonequilibrium thermodynamics for glassy polymers (NET-GP) framework, coupled with a lattice fluid equation of state model. Moreover, a simple rheological tool recently introduced for the a priori evaluation of the penetrant induced swelling is employed to predict the volumetric behavior of the solute/polymer mixture below Tg. Such model accounts for an additional nonequilibrium parameter for the description of polymer phase that represents its pseudoequilibrium compressibility, and it can be retrieved from pure component pressureâvolumeâtemperature data below Tg. More in detail, the effect of polymer pretreatment and its prior history (e.g. annealing or conditioning by preswelling) on the resulting solubility behavior has been thoroughly analyzed and discussed, and the experimental data obtained for several penetrant/polymer couples have been well described by the present model
Water sorption in microfibrillated cellulose (MFC): The effect of temperature and pretreatment
Full text linkWater sorption behavior of two different microfibrillated cellulose (MFC) films, produced by delamination of cellulose pulp after different pretreatment methods, is examined at various temperatures (16â65 °C) and up to 70% RH. The effect of drying temperature of MFC films on the water uptake is also investigated. The obtained solubility isotherms showed the typical downward curvature at moderate RH, while no upturn is observed at higher RH; the uptakes are in line with characteristic values for cellulose fibers. Enzymatically pretreated MFC dispersion showed lower solubility than carboxymethylated MFC, likely due to the different material structure, which results from the different preparation methods The experimental results are analyzed by Park and GAB models, which proved suitable to describe the observed behaviors. Interestingly, while no significant thermal effect is detected on water solubility above 35 °C, the uptake at 16 and 25 °C, at a given RH, is substantially lower than that at higher temperature, indicating that, in such range, sorption process is endothermic. Such unusual behavior for a cellulose-based system seems to be related mainly to the structural characteristics of MFC films, and to relaxation phenomena taking place upon water sorption. The diffusion kinetics, indeed, showed a clear Fickian behavior at low temperature and RH, whereas a secondary process seems to occur at high temperature and higher RH, leading to anomalous diffusion behaviors
Non-equilibrium thermodynamics of glassy polymers: Use of equations of state to predict gas solubility and heat capacity
No full textThe use of Non-Equilibrium Thermodynamics for Glassy Polymers (NET-GP) model to predict infinite dilution gas solubility coefficient is revised in this work and its extension to the analysis of apparent constant pressure heat capacity in polymeric materials below the glass transition temperature Tg is developed. Use is made of different Equations of State (EoS), in the class of tangent-hard-spheres-chain theories, to predict the thermal expansion coefficient below Tg, resulting in consistent representations of the Henry's coefficient for gaseous species in the same temperature range, for the case of different versions of Statistical Associating Fluid Theory (SAFT) EoS. With reference to calorimetric properties, the analysis here performed indicates that NET-GP endowed with EoS tuned on melt phase equilibrium properties does not allow for the prediction of the bulk modulus of the glassy polymeric phase, and only qualitative behavior for the apparent heat capacity are reproduced. On the other hand, after the use of experimental data from structural relaxation experiments to evaluate the bulk modulus in glassy state, a satisfactory prediction of the excess heat capacity is obtained within the same framework, for the case of different non-equilibrium conditions. Conclusion are finally drawn for the need to account for additional order parameters in NET-GP approach in order to address the representation of the complex calorimetric behavior exhibited by glassy polymeric materials
Effect of relative humidity and temperature on gas transport in Matrimid®: Experimental study and modeling
No full textThe influence of water vapor on the gas permeability of a commercial polyimide, Matrimid® 5218, has
been extensively investigated at three different temperatures (25, 35 and 45 1C), and with four different
penetrant gases (CH 4 , N 2 , CO 2 and He), varying the relative humidity in the range 0–75%.
In all tests performed, the permeability coefficient decreases as the concentration of water vapor in
the membrane increases. In particular, the influence of the presence of water on gas permeability is very
similar for all penetrants, as the same permeability decrease is found, at a given relative humidity,
despite the different thermodynamic and kinetic characteristics of the probe gases considered.
As temperature is raised, the gas permeability is enhanced, as expected. On the other hand, its decrease
with respect to the dry polymer values, as relative humidity increases, is not affected by temperature, and it
remains substantially unaltered from 25 to 45 1C, suggesting that such phenomenon can be directly related
to the amount of water dissolved in the membrane, which is also unaffected by temperature.
Based on the experimental evidence, a simple model is proposed to describe the permeation process
under humid conditions, in the framework of the free volume theory. In particular, it has been considered
that absorbed water molecules influence gas permeability by occupying polymer free volume, reducing its
availability to other penetrants with lower condensability. The model describes accurately the experi-
mental data using only two adjustable parameters for the polymer-water-penetrant system, once the water
solubility is estimated from sorption measurements
Thermodynamic Modeling of Gas Transport in Glassy Polymeric Membranes
No full textSolubility and permeability of gases in glassy polymers have been considered with the aim of illustrating the applicability of thermodynamically-based models for their description and prediction. The solubility isotherms are described by using the nonequilibrium lattice fluid (NELF) (model, already known to be appropriate for nonequilibrium glassy polymers, while the permeability isotherms are described through a general transport model in which diffusivity is the product of a purely kinetic factor, the mobility coefficient, and a thermodynamic factor. The latter is calculated from the NELF model and mobility is considered concentration-dependent through an exponential relationship containing two parameters only. The models are tested explicitly considering solubility and permeability data of various penetrants in three glassy polymers, PSf, PPh and 6FDA-6FpDA, selected as the reference for different behaviors. It is shown that the models are able to calculate the different behaviors observed, and in particular the permeability dependence on upstream pressure, both when it is decreasing as well as when it is increasing, with no need to invoke the onset of additional plasticization phenomena. The correlations found between polymer and penetrant properties with the two parameters of the mobility coefficient also lead to the predictive ability of the transport model
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