Comparative study of the release kinetics of osmotically active solutes from hydrophobic elastomeric matrices combined with the characterization of the depleted matrices

The release process of three osmotically active solutes with various solubilities in water (NaCl, CsNO₃, and CsCl) from silicone rubber matrices is presented. The kinetics of release for different initial loads of the salts were supplemented by measurements of the kinetics of concurrent water uptake. To gain insight on the relevant non‐Fickian transport mechanisms, the morphology, the diffusion and sorption properties and the physicochemical state of water in the salt‐depleted matrices were studied. In addition, both salt‐loaded and salt‐depleted matrices were characterized with respect to their mechanical properties. The combined information, derived from these techniques, supported the operation of a release mechanism carried out through the formation of microscopic cracks, interconnecting the permanently formed cavities inside the matrices. The results indicate that these microscopic cracks may have healed upon drying. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Vapor‐induced swelling of supported methacrylic and siloxane polymer films: Determination of interaction parameters

White light reflectance spectroscopy is applied to monitor vapor‐induced thickness changes of polymer films, supported on suitable silicon substrates. Assuming unidirectional swelling due to the constraining support, the equilibrium volume swelling of four methacrylic polymers and two siloxane‐based copolymers upon exposure to various activities of water, methanol, ethanol, and ethyl acetate vapor, at 30°C is evaluated. The deduced sorption isotherms were fitted to the Flory‐Huggins equation and interaction parameters, as well as solubility coefficients at infinite solute dilution, were deduced for each binary system. The relative sorption capacity of the different classes of polymers toward the four vapors are in line with the expected solubility interactions between solvent and solute. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Phenomena affecting the interferometric determination of concentration profiles of micromolecules diffusing along a stiff-chain polymer film

A detailed examination of the interferometric determination of penetrant concentration profiles during longitudinal penetration of micromolecular liquids in glassy polymer films, nonrigidly clamped between glass plates, is presented. Penetration of methylene chloride and methyl alcohol (representing strong and relatively weak swelling agents, respectively) into cellulose acetate films, characterized, respectively, by a degree of acetylation of 2.45 (CA-2.45) and 2.0 (CA-2.0) acetyl groups per monomer unit, was studied, and the importance of certain factors, which should be taken into account in order to obtain reliable concentration profiles, is demonstrated. In particular, conversion of the interferometrically determined optical path difference profiles to concentration profiles requires taking account of (i) the deviation from linearity of the refractive index–concentration relation, due to the filling of the excess free volume of the polymer by the penetrant and (ii) swelling of the clamped polymer film in the thickness direction, with particular attention to deviations from uniform specimen thickness. In the example of strong swelling agent penetration (CA-2.45/MC), no detectable increase in film thickness over that of the original dry film was detected, as a result of the fact that the polymer becomes sufficiently plasticized to deform plastically along the penetration axis under the clamping pressure. During weak swelling agent penetration (CA-2.0/MA), on the other hand, significant swelling in the thickness direction was observed, because the polymer is less plasticised and can undergo plastic deformation as above only to a limited extent. In this case, it is important to guard against deviations from uniform specimen thickness. The relevant strain birefringence profiles provided significant information which helped to establish the above picture. The calculated concentration profiles are discussed in the light of the measured penetration kinetics and in relation to the magnitude of the clamping pressure. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 65:317–327, 1997     

Moisture‐induced effects on the tensile mechanical properties and glass‐transition temperature of poly(vinyl alcohol) films

In this study, the effect of sorbed water on the tensile mechanical properties of noncrosslinked, thermally treated poly(vinyl alcohol) (PVA) films was studied. The Young's modulus, elongation at break, and tensile strength of the PVA films equilibrated at different relative humidities (0–86%) are reported, together with the depression of the glass transition of the polymer at each equilibrating humidity, as determined by temperature‐modulated differential scanning calorimetry. The results indicate that drastic changes in the tensile properties were correlated with the transition of the hydrated polymer from the glassy to the rubbery state. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Vapor sorption in thin supported polymer films studied by white light interferometry

In the present study, we apply a white light interferometric methodology to study sorption of moisture and methanol vapor in thin films of poly(2-hydroxyethyl methacrylate) [PHEMA] and poly(methyl methacrylate) [PMMA], supported on oxidized silicon wafers. The measured equilibrium thickness expansion of each film, exposed to different activities of the vapor penetrant, is used to determine the sorption isotherm of the system. Results for relatively thick films (100 nm < L_o < 600 nm) are compared with corresponding literature data for bulk, free-standing films, obtained by direct gravimetric methods. Furthermore, PMMA films of thicknesses lower than 100 nm were employed in order to study the effect of the dry film's thickness, and of substrate, on fractional swelling.     

Polymer-BaTiO3 composites: Dielectric constant and vapor sensing properties in chemocapacitor applications

In this work, composite materials, prepared by inclusion of various amounts of BaTiO₃ nanoparticles in PBMA and PHEMA polymer matrices, were characterized in respect to their dielectric properties and then used as the sensing layer of capacitive-type sensors. BaTiO₃ was found to be less effective in enhancing the permittivity of PHEMA, as compared to PBMA, in the range of 1 kHz–1 MHz, possibly due to the observed lower quality of dispersion in the polymer matrix and the higher polarity of the polymer in the former case. The response of the composite-based chemocapacitors to four vapor analytes, covering a wide range of dielectric constants, was studied in relation to the BaTiO₃ load. In all cases, with increasing amount of BaTiO₃ load, the absolute value of capacitance response was increased, due to the corresponding increase of the initial capacitance of the sensing composite layer. However, the corresponding normalized (to the initial capacitance of the sensing composite layer) capacitance values were reduced, due to the decreased volume fraction of the sorbing polymer material in the composite matrix. An exception to this trend, observed upon exposure of the PBMA/BaTiO₃ chemocapacitors to low humidity levels, is also presented and discussed. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Anomalous sorption kinetics in the methanol vapor–poly(methyl methacrylate) system

The non‐Fickian sorption kinetics of methanol vapor in poly(methyl methacrylate) films 8 and 51 μm thick at 25°C are presented. The behavior of the system was studied in series of interval and integral absorption runs. The relevant diffusion coefficient and viscous relaxation processes were studied separately by kinetic analysis of the first and second stages of sorption kinetic curves. The sorption isotherm concaved upward at high activities, this being typical of Flory–Huggins behavior, whereas it exhibited a convex‐upward curvature at low methanol vapor activities, this indicating sorption in the excess free volume of the polymer matrix. After excess free‐volume fill‐up, the concentration dependence of the diffusion coefficient was found to be well represented by the free‐volume theory of Vrentas and Duda. Relaxation frequencies calculated from the second stage of two‐stage curves exhibited a weak dependence on the concentration. Integral sorption experiments indicated that the system exhibited nearly case II kinetics at high methanol vapor activities. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 1184–1195, 2005     

Melting and recrystallization processes in a rubbery polymer detected by vapor sorption and temperature-modulated DSC methods

Series of successive absorption runs of n-hexane vapor in films of semi-crystalline poly(dimethylsilatrimethylene) at 25 °C are presented. Sorption overshoots were observed in the uptake kinetic curves of first series of absorption runs in as-prepared films. The said overshoots were absent in the second series of runs, conducted after desorption, while the sorption isotherms of the two series were identical. The overshoots are attributable to penetrant-induced melting and recrystallization phenomena. Strong support to this interpretation is given by temperature-modulated DSC data obtained from the pure polymer, which reveal similar thermally induced phenomena at slightly higher temperatures.     

Hydrophilic modification of silicone elastomer films: Thermal,mechanical and theophylline permeability properties

The improvement of the hydrophilicity of silicone rubber (SR) is sought in many biomedical applications. In the present work, we have prepared neat films coming from condensation-type SR, modified either through blending or end-linking reaction with low molecular weight poly(ethylene glycol) (PEG). The films were studied with respect to their water uptake capacity, stability of embedment of ethylenoxy moieties, and mechanical and thermal properties. Subsequently, we prepared drug-loaded films with theophylline, a relatively hydrophilic model drug, and studied the release performance of these systems. Results on neat films showed that blending increases significantly the overall water uptake of the films and, at the same time, has a limited detrimental effect on their mechanical properties. On the other hand, end-linked films loaded with theophylline exhibited better rate-controlling properties in vitro, due to better dispersion of the sorbed water.     

Release mechanisms of semipolar solutes from poly(dimethylsiloxane) elastomers: Effect of a hydrophilic additive

The release profiles of three xanthine derivatives from matrices of pure poly(dimethylsiloxane) (PDMS), and the effect of incorporating 10% w/w polyethylene glycol (PEG)?3000 in the matrix are presented. Theobromine (TBR), theophylline (TPL), and caffeine (CAF), although structurally very similar, are characterized by different physicochemical properties. In addition, differential scanning calorimetry and scanning electron microscopy measurements indicate different physical interactions with the polymeric matrices. The observed rates of release from pure PDMS matrices loaded at 0.065 g/g with each one of the drugs increase in the order TBR < TPL < CAF. The same order holds for the corresponding permeabilities derived from the release kinetic data. The slopes of the release curves versus the square root of time were linearly correlated to the square root of calculated solubilities in the polymer, as expected by the Higuchi equation for diffusive transport of solutes of the same diffusivity. The incorporation of 10% w/w PEG in the PDMS matrix accelerates the release rate of each drug, because of the concurrent water uptake induced by the hydrophilic additive. The extent of the corresponding permeability enhancement for theophylline was close to that predicted by the Maxwell equation for a composite two‐phase system, consisting of a PDMS continuous phase characterized by a much lower permeability than that of the fully swollen PEG dispersed phase. The corresponding enhancement of permeability was higher for TBR and lower for CAF. Possible reasons for these differences are discussed. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40782.