Penetrant Uptake Research Articles

Tuning the selective permeability of polydisperse polymer networks.

We study the permeability and selectivity ('permselectivity') of model membranes made of polydisperse polymer networks for molecular penetrant transport, using coarse-grained, implicit-solvent computer simulations. In our work, permeability P is determined on the linear-response level using the solution-diffusion model, P = KDin, i.e., by calculating the equilibrium penetrant partition ratio K and penetrant diffusivity Din inside the membrane. We vary two key parameters, namely the network-network interaction, which controls the degree of swelling and collapse of the network, and the network-penetrant interaction, which tunes the selective penetrant uptake and microscopic energy landscape for diffusive transport. We find that the partitioning K covers four orders of magnitude and is a non-monotonic function of the parameters, well interpreted by a second-order virial expansion of the free energy of transferring one penetrant from a reservoir into the membrane. Moreover, we find that the penetrant diffusivity Din in the polydisperse networks, in contrast to highly ordered membrane structures, exhibits relatively simple exponential decays. We propose a semi-empirical scaling law for the penetrant diffusion that describes the simulation data for a wide range of densities and interaction parameters. The resulting permeability P turns out to follow the qualitative behavior (including maximization and minimization) of partitioning. However, partitioning and diffusion are typically anti-correlated, yielding large quantitative cancellations, controlled and fine-tuned by the network density and interactions, as rationalized by our scaling laws. We finally demonstrate that even small changes of network-penetrant interactions, e.g., by half a kBT, modify the permselectivity by almost one order of magnitude.

Transport features of nano-hydroxylapatite (n-HA) embedded silicone rubber (SR) systems: influence of SR/n-HA interaction, degree of reinforcement and morphology.

We report the transport characteristics of silicone rubber/nano-hydroxylapatite (SR/n-HA) systems at room temperature with reference to the effects of n-HA loading, morphology and penetrant nature, using toluene, xylene, ethyl acetate and butyl acetate in the liquid phase and methanol, ethanol, 1-propanol, 2-propanol and butanol in the vapour phase as probe molecules. The interaction between the n-HA particles and SR matrix has been confirmed by FTIR analysis. As the n-HA content in the SR matrix increased, the penetrant uptake has been found to decrease. The observations have been correlated with the density and void content of the systems. Scanning electron microscopy images have been found to be complementary to the observed transport features. The reinforcement effect of n-HA particles on the SR matrix has been verified by Kraus equation. Molecular mass between the cross links has been observed to decrease with an increase in n-HA loading. The results have been compared with affine, phantom network, parallel, series and Maxwell models. The transport data have been complemented by observations on biological fluid uptake with urea, d-glucose, KI, saline water, phosphate buffer and artificial urine as the media.

Transport properties of high-density polyethylene/ethylene propylene diene terpolymer blends

Polymer blends based on high-density polyethylene and ethylene propylene diene terpolymer rubber (EPDM) were prepared. The sorption and diffusion of four aliphatic hydrocarbons through the blends were studied in the temperature range of 28–58 °C. Sulfur, dicumyl peroxide (DCP), and a mixture of sulfur and DCP (mixed) were used as cross-linking agents for the blends. Of the three systems, the peroxide vulcanized blends were found to exhibit the lowest penetrant uptake. This has been explained based on the higher rigidity of the –C–C– networks than the polysulfide linkages. The aliphatic liquid penetration through the matrix increased with increase in the EPDM content in the blend. The experimental observations were correlated with the morphology of the blends. Diffusion and permeation coefficients were calculated from the sorption data. A slight deviation from the Fickian trend was observed for the mechanism of transport with an increase in EPDM in the blends. The molar mass between cross-links and thermodynamic parameters of sorption was determined from the swelling data. The experimental observations were compared with various theoretical models.

Molecular transport characteristics of poly (ethylene‐co‐vinyl acetate) membranes in the presence of normal alkanes

AbstractSolvent transport into poly (ethylene‐co‐vinyl acetate) membranes exposed to n‐alkanes has been studied in the temperature interval of 30–60°C. Pure and cross‐linked membranes were prepared. Membranes with different loading of cross‐linking agent were also prepared. It was found that for all liquids, the equilibrium penetrant uptake was influenced by the introduction of cross links. The mechanism of transport has been found to deviate from the regular Fickian behaviour. Transport parameters such as diffusion coefficient, sorption constant and permeability coefficient have been calculated. The influence of temperature on transport was analysed. Transport parameters and activation parameters for the process of diffusion have been calculated. The transport coefficients and the activation parameters showed a dependence on cross‐link density. The Van't Hoff's relationship was used to compute the entropy change. The values of polymer–solvent interaction parameters have been used to calculate the molar mass between cross links of the network polymer. The phantom and affine models were used to analyse the deformation of the networks during swelling. A correlation between theoretical and experimental results was also done. Copyright © 2007 John Wiley & Sons, Ltd.

Molecular transport of aliphatic hydrocarbons through styrene butadiene rubber/ethylene vinyl acetate blends

AbstractPolymer blends based on styrene butadiene rubber and ethylene vinyl acetate (EVA) were prepared. The sorption and diffusion of four aliphatic hydrocarbons through the blends were investigated with temperatures of 26–56°C. Sulfur, dicumyl peroxide (DCP), and a mixed system consisting of sulfur and DCP (mixed) were used as crosslinking agents for the blends. Of the three systems, the peroxide vulcanized blends were found to exhibit the lowest penetrant uptake. The aliphatic liquid penetration through the matrix decreased with an increase in the EVA content in the blends, which was attributed to the semicrystalline nature of the EVA matrix. The experimental observations were correlated with the morphology of the blends. Diffusion and permeation coefficients were calculated from the sorption data. A slight deviation from the Fickian trend was observed for the mechanism of transport with an increase in the EVA content in the blends. The molecular mass between crosslinks and thermodynamic parameters of sorption were determined from swelling data. The experimental observations were compared with different theoretical models. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 2884–2897, 2006

Aqueous marker penetration into ion irradiated polyimide

The penetration of aqueous 6Li + markers into low energy ion irradiated polyimide (PI) foils was examined by the neutron depth profiling technique in combination with a modified tomographic approach. The ion irradiation always leads to an enhancement in marker uptake. After irradiation at low fluence the marker profiles follow the nuclear damage distribution even in three dimensions. At elevated fluences saturation in the marker uptake is clearly seen. The polymer's penetrant uptake can be described well by regular diffusion, with nuclear damage centres acting as saturable traps. These observations are strikingly different from the marker penetration into high-energy heavy-ion irradiated PI.

Gas and vapor sorption and diffusion in poly(ethylene terephthalate)

Equilibrium sorption of n-butane and acetaldehyde in melt-processed poly(ethylene terephthalate) (PET) microtomed flakes is reported. The n-butane sorption isotherm at 35°C is well described by the dual-mode model with the following parameters: kD=0.017cm3 (STP)/(cm3 amorphous polymer cmHg), C′H=1.3cm3 (STP) (cm3 amorphous polymer) and b=0.029cmHg−1. Acetaldehyde isotherms at 35 and 45°C may be described by the Flory–Huggins sorption model, suggesting that penetrant uptake in the non-equilibrium excess volume associated with the glassy polymer made a negligible contribution to the overall sorption level at the conditions of this study. The heat of sorption was essentially equal to the enthalpy of condensation of pure acetaldehyde. At 45°C and acetaldehyde pressures at or above 40.0cmHg, acetaldehyde triggers significant crystallization of PET (up to 37wt%) with increasing concentration. Subsequent sorption experiments at very low penetrant activity reveal solubility coefficients that are markedly higher in the penetrant-crystallized sample than in the initially highly amorphous sample. This result suggests the acetaldehyde-induced formation of microvoids (which act as highly efficient penetrant sorption sites) in the polymer sample. Based on these and literature data, the logarithm of infinite dilution penetrant solubility in amorphous regions of PET was well-correlated with penetrant condensability as characterized by Tc, penetrant critical temperature, or by (Tc/T)2, where T is the experiment temperature. Infinite dilution, amorphous phase penetrant diffusion coefficients in PET decreased according to a power law relation with increasing penetrant critical volume.

NMR spectroscopy and free volume analysis of the effects of copolymer composition on the swelling kinetics and chain dynamics of highly crosslinked copolymers of acrylic acid with PEG-containing multiacrylates

Novel ionizable polymer networks were prepared from oligo(ethylene glycol) (OEG) multiacrylates and acrylic acid (AA), employing bulk radical photopolymerization techniques. The properties of these materials exhibited a complex dependence on the network structure and composition. Penetrant sorption experiments demonstrated that the crosslinked structure of the copolymers depended very strongly on the AA content as well as the number of ethylene glycol groups. The impact of varying the AA content and the oligo(ethylene glycol) chain length on the polymer chain dynamics was examined using diffusion and 13 C NMR relaxation studies. The penetrant uptake studies indicated a coupling of Fickian and relaxation-driven contributions to the swelling behavior. The effect of increasing the AA content on the characteristic chain relaxation time was reversed as the oligo(ethylene glycol) chain length was varied, indicating that chain relaxation is controlled by structural considerations, for shorter oligo(ethylene glycol) chains, and by compositional considerations, for longer oligo(ethylene glycol) chains. Measured compositional effects on solid state 13 C NMR relaxation times supported these conclusions.

On the penetration of aqueous solutions into pristine and radiation damaged polyimide

Abstract The penetration of aqueous solutions at different pH values into pristine and ion-irradiated commercial polyimide (PI, “Kapton”) foils was examined, by using Li ions as tracers to probe the corresponding penetrant depth distributions. These distributions were recorded for different polymer exposure times and temperatures. For comparison, examinations have also been made on the time dependence of the polyimide conductivity during water uptake. The results derived here for PI are compared with those of other polymers. For both pristine and low-fluence irradiated PI, an initially rapid solution penetration was observed up to some maximum penetrant uptake value. The penetrant depth distributions point at nanocapillaric penetration mechanisms in this initial stage. At longer exposure times of the polymer, the penetrant concentration diminishes transiently, suggesting that polymer swelling in the confined track zone leads to a reduction of the available free volume. In the subsequent stage of penetrant uptake this process is reversed again which we explain by crazing and overall swelling. Whereas thereafter saturation is reached for non-alkaline penetrants, alkaline solutions in irradiated PI show a pronounced continuous uptake, which is correlated with the ion track etching process. Whereas low-fluence ion irradiation of PI leads to a reduction in penetrant uptake compared with the pristine material, high-fluence irradiation results in a pronounced increase of the uptake capability.

THE EFFECT OF DRUG DISSOLUTION ON DRUG RELEASE FROM SWELLING POLYMERIC MATRICES: MATHEMATICAL MODELING

A mathematical model of simultaneous dissolution and diffusion controlled drug release from swellable, non-erodible viscoelastic polymer matrices is developed. The model assumes that the drug is loaded into the dry matrix as a solid phase. Two unsteady-state mass balances are required for both the drug and the incoming external penetrant. The Camera-Roda and Sarti equation is considered for the determination of the penetrant uptake, while Ficlc's equation accounting for the density gradient developing in the matrix is considered for the drug diffusion. Assuming that upon dissolution a drug phase transition may occur which affects the drug solubility without altering the diffusion coefficient. This analysis revealed that the features of the drug release kinetics are sensibly affected by the viscoelastic properties of the swollen matrices and by the drug dissolution and phase transition rates.

MODELING OF DRUG RELEASE FROM A SWELLABLE MATRIX

In many polymeric controlled release systems (CRSs) the drug release kinetics is mainly ruled by the penetrant absorption characteristics which, in turn, depend on the viscoelastic properties of the polymer/penetrant system. The kinetics of the drug release process can follow either a typical Fickian behavior or a non-Fickian one (anomalous release), depending on the polymer/ penetrant viscoelastic properties. The aim of this work is to model such physical evidences by means of the appropriate flux equations describing the penetrant uptake and the corresponding drug release. As a main result of our work we prove that, by resorting to the Sarti and Camera-Roda flux equation for the penetrant, and to the Fick flux equation for the drug, it is possible to simulate both Fickian and non-Fickian drug release from swelling CRSs characterized by different viscoelastic properties of the polymer/penetrant system. Furthermore, we demonstrate that the effect of the density gradient developing inside the matrix during the drug release does not sensibly affect the drug release kinetics.

Sorption and diffusion of aliphatic hydrocarbons into crosslinked natural rubber

The sorption and transport of four aliphatic hydrocarbons into natural rubber crosslinked by different vulcanization systems [conventional (CV), efficient (EV), peroxide (DCP) and a mixed system consisting of sulfur and peroxide (mixed)] were investigated in the temperature interval of 28–60°C. Of the four systems, natural rubber vulcanized by DCP exhibited lowest penetrant uptake. It was observed that the kinetics of liquid sorption in every case deviates from the regular Fickian trend, characteristic of sorption of liquids by rubbers. The diffusion coefficient, activation energy of sorption, enthalpy, entropy, and the rubber-solvent interaction parameter were evaluated for the four systems from the swelling data. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 725–734, 1997

Concentration and temperature dependence of diffusional deborah number during dodecane transport in crosslinked polystyrene

The diffusional Deborah number, De, defined as the ratio of characteristic relaxation time to characteristic diffusion time was determined as a function of concentration and temperature in the dodecane transport process in polystyrene. The characteristic relaxation time was obtained from the viscoelastic properties of dodecane/polystyrene systems measured by a dynamic mechanical analyzer. The characteristic diffusion time was obtained from the concentration and temperature dependence of the diffusion coefficient measured by NMR PGSE method. Above the room temperature the Deborah number changed significantly during isothermal transport process; however, the transport mechanism did not deviate from the Fickian transport as the order of De was still greater than 1. Around the room temperature the transport mechanisms represented by the resulting diffusional Deborah number changed significantly as the order of magnitude approached 1 with the temperature decreasing from 50°C to the room temperature. The transport mechanism predicted from the diffusional Deborah number was verified by the diffusional exponent, n, of an exponential time-dependence of the penetrant uptake.

Sorption and Diffusion Profiles of Monocyclic Aromatic Liquids into Polymeric Blends of Ethylene-Propylene Random Copolymer and Isotactic Polypropylene∗

Abstract Molecular transport of monocyclic aromatics into polymeric blends of ethylene-propylene random copolymer and isotactic polypropylene is investigated in the temperature interval 25°-70°C. For all liquids, the penetrant transport rate, equilibrium penetrant uptake, and degree of penetrant overshoot are influenced by the type of the penetrant and the experimental temperature. Sorption-desorption-resorption-redesorption (S-D-RS-RD) experiments have been performed to determine the true equilibrium sorption and diffusion coefficients. These S-D-RS-RD experiments are a convenient method of determining the continuous weight loss during long-term solvent exposure, a phenomenon associated with the release of residuals. The molar mass between crosslinks of the polymer, the kinetic rate constants, and the activation parameters for diffusion, permeation, and sorption are obtained. Transport coefficients and the activation parameters are affected by the type and size of the penetrant molecules. The sorption ki...

Molecular transport characteristics of Santoprene thermoplastic rubber in the presence of aliphatic alkanes over the temperature interval of 25 to 70°C

Solvent transport into Santoprene thermoplastic rubber samples exposed to –alkanes (C 5 to C 16), 2,2,4-trimethylpentane, cyclohexane and 1,2,3,4-tetrahydronaphthalene in the temperature interval 25–70°C has been studied. For all liquids, equilibrium penetrant uptake and degree of penetrant overshoot have been influenced by factors such as penetrant size, shape, polymer morphology and temperature. Sorptio–desorptio–resorptio–redesorption experiments have been performed for some typical solvents at 25°C to determine the true sorption equilibrium and transport parameters. The values of the polymer-solvent interaction parameters have been used to calculate the molar mass between crosslinks of the network polymer. Kinetic rate constants and the activation parameters for the process of diffusion, permeation and sorption/desorption have been calculated. The transport coefficients and the activation parameters showed a dependence on the size of the penetrant molecules. The transport phenomenon was found to follow the Fickia–type mechanism.

Controlled delivery of pilocarpine. 1. In vitro characterization of Gelfoam matrices.

The potential of Gelfoam absorbable gelatin sponge as a carrier for ophthalmic delivery of pilocarpine was examined. Prolonged in vitro release of pilocarpine was achieved through pharmaceutical modification of the device by embedding a retardant in the pores. The device embedded with cetyl ester wax released pilocarpine in a zero-order pattern (release exponent = 0.93 +/- 0.04) for up to 5 hr. This result corresponded well with a linear penetrant uptake by this device. The device impregnated with polyethylene glycol 400 monostearate exhibited anomalous drug transport with a release exponent of 0.63 +/- 0.02. The absorption of water by this retardant and the formation of a gel layer on the surface slowed the penetration of the release medium into the deeper sections of the matrix, as well as the rapid outward diffusion of drug, resulting in a prolonged release of pilocarpine.

Sorption and dilation of silicone elastomer composites at high gas pressures: The role of interfacial quality

AbstractIn elastomers the crosslink density, the presence of filler particles, and the volumetric confinement toward sorptive dilation can influence the extent of gas mass uptake. In this study the effects of filler particles on the high‐pressure gas mass sorption and the volumetric dilation of a silicone elastomer matrix has been investigated. Glass beads, ca. 30 μm radii, with different surface treatments were incorporated as inclusions in various specimens at relatively low concentrations of ca. 10% by volume. The high‐pressure gases used were N2 and CO2 up to a maximum ambient pressure of ca. 25 MPa at ca. 20°C and 42°C, respectively. The gas mass sorption was determined by a vibrating reed technique. The sorptive dilation was measured by an ultrasonic transducer operating as a displacement probe. In certain systems the absorbed CO2 gas was able to disrupt the internal interfaces. This led to an increased gas mass uptake in the corrupted specimen. The N2 gas did not affect the interfacial bonds. The amount of penetrant uptake was found generally to be reduced when the internal interfaces were not disrupted. The presence of various internal interfaces restrained the sorptive dilation of the elastomeric matrix. These hindrances to the natural sorptive dilation of the elastomer network suppressed the extent of the gas sorption process. This effect has also been investigated separately in detail using model ‘poker chip’ type of specimens of various aspect ratios. The sorptive dilational characteristics have been correlated with the mechanical properties of similar specimens. The influence of an almost complete volumetric confinement on the gas sorption capacity of the silicone elastomer specimen has also been studied. © 1992 John Wiley & Sons, Inc.

Sequential vapor sorption of additional penetrants in preswollen polymers

AbstractUptake and removal of a second penetrant in polymer samples preexposed to another vapor dictate many industrially important processes. Both the kinetics and equilibrium sorption can be strongly affected by the presence of the first permeant in the polymeric matrices. A novel experimental setup was constructed to study penetrant uptake in sequence. Sorption of the second vapor took place while the partial vapor pressure of the first vapor was maintained, so that diffusion into the preswollen polymer approximated transport in a pseudobinary system. Polybutylene was chosen in this work to illustrate the capability of this versatile experimental system. Both the rate of diffusion and equilibrium sorption of the second vapor were found to depend on the prevailing composition of the preswollen polymer for all penetrant pairs studied.

Solute and penetrant diffusion in swellable polymers: VIII. Influence of the swelling interface number on solute concentration profiles and release

Penetrant-activated solute release was modeled in polymers undergoing Case-II penetrant uptake. The rate of solute release can be nearly constant for systems with Case-II penetrant uptake behavior, depending on the values of solute diffusion coefficients and the functional form of concentration dependence. It is shown that swelling of a polymer slab can have a significant effect on solute release behavior.

Anomalous penetrant transport in glassy polymers VI. Effect of temperature on transport

AbstractTransport of liquid cyclohexane through well characterised, initially glassy, crosslinked polystyrene slabs was Investigated at 20, 30, 40, and 50°C. The samples used were produced by bulk polymerization of styrene and divinyl benzene (DVB) at 90°C for 43 hr using benzoyl peroxide as an initiator. The samples tested had initial cross‐linking ratios, X, between 5 and 25 × 10−3 mol DVB/mol styrene. The initial thickness of the samples tested varied from 0.25 mm to 1.80 mm, and the aspect ratio (length to thickness ratio) was maintained above 10;1 in order to analyze the results by one‐dimensional transport equations. Cyclohexane uptake was followed as a function of time along with dimensional changes in the thickness and area of the samples. The results of penetrant uptake as a function of time were analyzed using a simplified exponential expression and employing all the data points from the beginning of the experiment until the time of observation of the maximum in uptake. Good correlations were established between the penetrant uptake and the transport temperature. These results were interpreted in terms of competitive relaxational and diffusional mechanisms.