Physical aging of ultrathin glassy polymer films tracked by gas permeability

Membrane-based separations play a key role in energy conservation and reducing greenhouse gas emissions by providing low energy routes for a wide variety of industrially-important separations. For reasons not completely understood, membrane permeability changes with time, due to physical aging, and the rate of permeability change can become orders of magnitude faster in films thinner than one micron. The gas transport properties and physical aging behavior of free-standing glassy polysulfone and Matrimid^® films as thin as 18 nm are presented. Physical aging persists in glassy films approaching the length scale of individual polymer coils. The films studied ranged from 18–550 nm thick. They exhibited reductions in gas permeability, some more than 50%, after 1000 h of aging at 35 °C, and increases in selectivity. The properties of these ultrathin films deviate dramatically from bulk behavior, and the nature of these deviations is consistent with enhanced mobility and reduced T_g in ultrathin films. The Struik physical aging model was extended to account for the influence of film thickness on aging rate, and it was shown to adequately describe the aging data.     

Physical aging of thin glassy polymer films monitored by gas permeability

The physical aging at 35 °C of three glassy polymers, polysulfone, a polyimide and poly(2,6-dimethyl-1,4-phenylene oxide), has been tracked by measurement of the permeation of three gases, O₂, N₂, and CH₄, for over 200 days. Several techniques were used to accurately determine the thickness of films (∼400 nm-62 μm) in order to obtain absolute permeability coefficients and to study the effects of film thickness on the rate of physical aging. Each film was heated above the polymer T_g to set the aging clock to time zero; ellipsometry revealed that this procedure leads to isotropic films having initial characteristics independent of film thickness. A substantial pronounced aging response, attributed to a decrease in polymer free volume, was observed at temperatures more than 150 °C below T_g for thin films of each polymer compared to what is observed for the bulk polymers. The films with thicknesses of approximately 400 nm of the three polymers exhibit an oxygen permeability decrease by as much as two-fold or more and about 14-15% increase in O₂/N₂ selectivity at an aging time of 1000 h. The results obtained in this study were compared with prior work on thickness dependent aging. The effects of crystallinity on physical aging were examined briefly.     

Physical aging of layered glassy polymer films via gas permeability tracking

The physical aging of polymers in confined environments has been an area of intensive study in recent times. The rate of physical aging in thin films of many polymers used in gas separation membranes is dependent on film thickness and accelerated relative to bulk. In this study, the physical aging of polymer films with alternating glassy polysulfone and rubbery polyolefin layers was monitored by measuring the gas permeability of O₂ and N₂ as a function of aging time at 35 °C. The alternating layer structures were formed by a melt co-extrusion process. The polysulfone layers have thicknesses ranging from 185 to 400 nm, and the overall thicknesses of the films are on the order of 80-120 μm. The aging of freestanding thin films of polysulfone is rapid and exhibits clear thickness dependence, whereas the aging of multilayered films was observed to be similar to bulk and showed no dependence on layer thickness. At 1000 h of aging time, a 400 nm freestanding PSF film decreased in O₂ permeability by 35%, whereas on average the bulk and multilayered films only experienced a decline of 10-15%. A slight increase in O₂/N₂ selectivity for the multilayered films was observed over the course of aging.     

Physical aging of 6FDA-based polyimide membranes monitored by gas permeability

The aging behaviors, as judged by gas permeability, of two glassy 6FDA-based polyimides, 6FDA-DAM and 6FDA-mPDA, in thick and thin film forms are reported. Their O₂ and N₂ gas permeabilities were monitored over several thousands of hours. In general, the properties of these thin films deviate dramatically from their bulk behavior, by showing much more rapid decrease in gas permeability and increase in selectivity. Owing to the high free volume, the 6FDA-DAM thin films have very high permeability at the very early aging time, followed by an order of magnitude decrease in permeability relative to the initial value over the course of 1000 h of aging. On the other hand, a thick 6FDA-DAM polymer film maintained a high permeability over thousands of hours of aging. 6FDA-mPDA thin films have moderate aging rates comparable to thin films made from other polyimides such as Matrimid^®. Effect of PDMS coating on gas permeability and aging was examined using a few 6FDA-mPDA thin film membranes and was found to be insignificant.     

Effect of UV crosslinking and physical aging on the gas permeability of thin glassy polyarylate films

The effect of crosslinking by UV irradiation on the gas permeation properties of thin films (thickness ≤1 μm) made from two benzophenone-based polyarylates were examined. In addition to the permeation response to UV crosslinking in these two polymers, the effects of crosslinking on the rate of physical aging was also explored. The sequence of physical aging and crosslinking, as well as reversal of the aging process was studied in order to separate the similar effects of aging and crosslinking. The results show that crosslinking very thin films can greatly improve the long-term performance of membranes when compared to noncrosslinked films of similar thickness.     

Noble gas permeability of polymer films and coatings

Permeabilities of noble gases, particularly argon, krypton, and xenon, were measured through a number of polymer films and coatings. Extrapolation of the log of the permeation coefficient versus the square of the gas molecular diameter was used to estimate radon permeability. An equation has been developed that can predict permeability to these noble gases as a function of the base polymer structure of the coating.     

Physical aging kinetics of syndiotactic polystyrene as determined from creep behavior

Creep experiments in uniaxial extension have been performed to explore the kinetics of the physical aging process in semicrystalline syndiotactic polystyrene (sPS) having two processing histories. Classical time-aging time superposition behavior was found for both materials at temperatures from 70 to 95°C, with the shift rate μ decreasing as temperature was increased. Virtually no aging was seen at 95°C, the DSC determined glass transition, T_g. This behavior was atypical for a semicrystalline polymer and reminiscent of the behavior of glassy amorphous thermoplastics. Some evidence for a separate crystalline aging mechanism > T_g, which manifests itself as only vertical shifts without timescale shifts, is seen in experiments at T > 100°C. Finally, the two different materials age differently, suggesting that some control of aging can be obtained by altering processing conditions or morphology.     

Physical aging in polystyrene: Comparison of the changes in creep behavior with the enthalpy relaxation

Physical aging in polystyrene was studied by annealing samples isothermally (at 22°, 57°, and 93°C) for various lengths of time and measuring the changes in the enthalpy and the flexural creep property. The enthalpy decrease reached a constant value after about 10 hours of aging at 93°C, indicating an apparent attainment of equilibrium, whereas the creep behavior continued to change with prolonged aging even after 10 hours. At room temperature it took about a year of aging to induce a measurable change in enthalpy, while the creep behavior showed changes clearly only after hours of aging. These and other items of evidence indicate that the effect of aging on the creep behavior cannot be interpreted solely in terms of the free volume concept. Thus the hope of being able to predict the mechanical properties of aged polymers from the measurement of specific volume or enthalpy alone is not likely to be realized.     

Mechanical spectroscopy of thin polystyrene films

Mechanical spectroscopy is applied to thin polystyrene films of 7.5-730 nm thickness spin coated on a thin silicon reed. Below a thickness of 100 nm, the α-relaxation peak (glass transition) broadens considerably and shifts to lower temperatures by a few degrees. These effects are attributed to a different polymer dynamics at the polymer/vacuum and the polymer/silicon interfaces.     

Photo-oxidation of polystyrene film: 1. Photo-oxidation of polystyrene films with light absorbed by polymer

The photo-oxidation of polystyrene films irradiated with light absorbed by the polymer has been studied. The overall quantum yield of hydroperoxide as well as the relationship between the concentration of acetophenone-type carbonyl products and α,β-enone and diketone products were found. The spatial distribution of the different types of carbonyl products was investigated.     

Controllably crazed polystyrene: Morphology and permeability

The effects of n-heptane and heat treatment on the structural and transport properties of polystyrene films (biaxially oriented and unoriented) were studied to determine whether these treatments improve the film as selective barriers for separation of molecules differing only slightly in size and shape. n-Heptane treatment of biaxially oriented polystyrene produces a sandwich structure composed of expanded, crazed, surface layers surrounding an apparently unaffected central core. The crazed layers contain a continuous network of interconnected channels. The core provides the total resistance to gas permeation, hence, the overall effect of n-heptane treatment is fabrication of a thinner more permeable membrane. By restricting the stress-cracking treatment to one face of the film, it should be possible to make high flux, anisotropic membranes—a type of membrane which is required for successful development of membrane separation processes. n-Heptane treatment of cast, annealed polystyrene results also in a crazed polymer, but the crazing is in the form of spherical voids, and the films, even with a residual uncrazed core, are too weak to be useful as separation membranes. The crazing process in both types of polymer specimens is characteristic of case II non-Fickian diffusion in which the kinetics are apparently controlled by polymer relaxation processes. Sorption of isopentane into cast, annealed polystyrene does not cause visible crazing but the kinetics are again non-Fickian. Desorption of isopentane into n-heptane-treated polystyrene releases the appreciable residual n-heptane in the film which could not be removed by long-term exposure to vacuum. Analysis of D(0) values for isopentane in n-heptane treated films indicates that the polymer surrounding the visible voids in the film is essentially unaltered polystyrene with only a small fraction of the voids being interconnected by open channels.     

Oriented crystallization of isotactic polystyrene in films prepared by friction transfer

The ability to orient polymer chains by applying external forces opens up the possibility to obtain polymeric surfaces with ordered structures. Here, we employed a friction-transfer approach by moving a pin of isotactic polystyrene (i-PS) across a smooth silicon counterface at controlled velocity, pressure and temperature which led to the deposition of a molecularly thin layer of highly oriented i-PS chains. The observed morphology of the resulting film (ribbons oriented in the sliding direction) indicated that the transferred molecules were highly oriented. This was confirmed after isothermal crystallization which led to the formation of so-called “shish-kebab” crystals aligned in the sliding direction. Thus, after crystallization all polymers were preferentially oriented with their chain axis in the shearing direction. Our results strongly suggest that by extending the polymer chain conformations in the shearing direction we can introduce a significant reduction of the nucleation barrier. Accordingly, friction transfer allows to align not only the transferred polymer chains but also the subsequently forming crystalline domains within the transferred films.     

Electron conduction in fe-doped polystyrene films

The DC electrical conductivity of pure and Fe³⁺-doped polystyrene films has been studied at various bias field strengths in the temperature range 304–392K. Doped films showed increased conductivity compared with undoped films up to 0.5% dopant concentration; beyond this concentration there was a decrease in the conductivity. The increase in conductivity for lower doping concentrations (0.25% and 0.5%) is attributed to the formation of charge transfer complexes. The decrease in the electrical conductivity and the increase in the activation energy at more than 0.5% dopant concentration is due to the formation of molecular aggregates. For undoped films the activation energies in the low temperature region were found to be higher than those at higher temperatures. Also, with increasing bias field strength, the activation energies showed a decrease at all temperatures studied. It is suggested, on the basis of the magnitudes of the activation energies, that electronic conduction is the dominant charge transport mechanism both at low and high temperatures.     

Inverse gas chromatographic characterization of polystyrene and organo-montmorillonite/polystyrene nanocomposites

Polymer nanocomposites of a polystyrene (PS) matrix containing 2% and 5% organo-montmorillonite (OMt) by mass were prepared using the solution blending method with sonication. Hexadecyl trimethyl ammonium bromide was used to modify the montmorillonite after its surface was saturated with Na⁺ ions. X-ray diffraction and transmission electron microscopy revealed the mixed nanomorphology of nanocomposites. The majority of OMt is dispersed in the polymer matrix in the form of an ordered tactoid (multilayer particles) structure consisting of few silicate layers and a small amount of exfoliation was achieved. The synthesized nanocomposites showed a higher decomposition temperature in comparison with pure PS according to the thermogravimetric analysis. Inverse gas chromatography under infinite dilution conditions was applied to evaluate the surface properties of the PS and OMt/PS nanocomposites. The IGC results were in agreement with conceptual and theoretical expectations. The dispersive component of the free energy of adsorption and specific interactions (acid–base properties) were determined using polar and non-polar adsorbates (probes) of known properties in the temperature range 40–70 °C. The IGC data showed that the introduction of a very small amount of OMt into the polymer matrix significantly changed the surface characteristics of the final material.     

Electrical conduction in sulfonated polystyrene films

The d.c. electrical conductivity of 14% (by weight) sulfonated polystyrene films was measured as a function of temperature from 20° to 60°C in a 25-micron vacuum. Sulfonated films in the silver, cadmium, and methylene blue sorbed forms were prepared for these electrical measurements. Plots of log specific conductance versus reciprocal temperature gave straight lines with activation energies of 0.49 eV for methylene blue, 0.60 eV for Ag, and 0.98 eV for Cd. Large conductivity ansiotropies were also measured for the sulfonated polystyrene films in the Ag and methylene blue forms. Attempts with Hall-effect measurements to determine the sign and concentration of the charge carriers were unsuccessful. Photoconduction studies were also carried out with Ag-, Cd-, and methylene blue-doped sulfonated polystyrene films in the film lateral direction of highest photocurrent. Rise of photocurrent under d.c. illumination was found to be exponential with time constants of 29, 3.5, and 1.0 sec for Ag-, Cd-, and methylene blue-doped samples. The rise and decay of photocurrent in these films exhibited similar responses. Photocurrent was ohmic up to 400 V and varied linearly with light intensity up to 200 milliwatts.     

Physical aging behavior of miscible blends containing atactic polystyrene and poly(2,6-dimethyl-1,4-phenylene oxide)

The influence of blend composition on physical aging behavior was assessed for miscible blends of atactic polystyrene (a-PS) and poly(2,6-dimethyl-1,4-phenylene oxide) (PPO). At aging temperatures of 15 and 30°C below the midpoint glass transition temperature (T_g), the a-PS/PPO blends exhibited volume relaxation rates that were retarded compared to additivity based upon the aging rates for pure a-PS and PPO. This negative deviation diminished with increased undercooling, and eventually the volume relaxation rates displayed a nearly linear trend with respect to composition at the greatest undercooling of 60°C that was employed. The compositional nature of unaged glassy density and secondary relaxation intensity, both influenced by the presence of specific attractive interactions in the blend system, were likely causes for the variation of volume relaxation rate with composition and undercooling. For aging at 30°C below T_g, the dependence of enthalpy relaxation rate on composition was similar to that observed for volume relaxation. Mechanical aging rates determined from time–aging time superposition of creep compliance data showed significantly less than additive behavior for the blends aged at T_g−30°C, but unlike the volume relaxation results, this trend persisted at the 60°C undercooling.     

Dependence of thin polystyrene films stability on the thickness of grafted polystyrene brushes

We investigate the stability of thin polystyrene (PS) films on chemically identical grafted brushes of various thickness and grafting density. We observe an essential influence of the brush thickness on the stability of the PS films. For brushes with a thickness of 20-35 nm no de-wetting of the PS film occurs, while considerably thicker or thinner PS brushes lead to de-wetting of the PS top layer. We suggest that in the thin brush-like layers, the unfavorable interactions with underlying silica favor de-wetting. The tendency to de-wet is reduced once the brush is sufficiently thick to insulate the free PS layer from the surface. Beyond that point, the de-wetting process speeds up as the brush becomes thicker and has a higher grafting density with a substantial increase of the interfacial tension between the brush and the free polymer.     

Units of gas permeability constants

Gas permeability constants of polymers are generally expressed by composite units which contain Numerous combinations of various units to express each component units are used. However, permeability constants of nongaseous substances in polymer membranes are generally expressed by the units of cm²/sec, which are identical to the units of the diffusion constant. The relationship between constants in units of cm²/sec and conventional composite units for gas permeability constants is presented, and the advantage of using the units in cm²/sec to express gas permeability constants is discussed.     

Physical aging of nylon 66

The physical aging behavior of nylon 66 as a function of aging temperature, magnitude of applied stress, and aging time has been investigated. Creep tests were performed for samples quenched from a stabilization temperature of 160°C to aging temperatures between 41°C and 65°C, for aging times ranging from 0.5 to 64 h, and at stresses levels from 4 MPa to 24 MPa. Volume recovery was investigated for samples quenched from 160°C to aging temperatures of 41°C and 65°C. The creep compliance curves at different aging times were able to be superimposed. The double logarithmic shift rate was calculated and its dependence on aging temperature and applied stress was determined. Also, it was found that the volume recovery rate depends on the aging temperature. The time-temperature and stress-temperature superpositions were not possible for our data.     

Gas permeability of deformed polyethylene films

Low-density polyethylene films strained up to 35% exhibit an initial increase of diffusivity and permeability which soon reach a maximum and subsequently drop to steadily decreasing values below those of the unstrained starting material. The sorption steadily increases and seems to approach a plateau. The maximum and the subsequent decrease are probably caused by significant, recoverable plastic deformation which seems to depress the tortuosity factor but not the free volume, as one concludes from the opposite trend of diffusion and sorption. Permanently deformed drawn or rolled films on the strain range from 0.5 through 3.0 exhibit a continuous decrease of diffusivity and permeability with an almost constant reduction of sorption. This postulates a decrease in free volume and a steadily decreasing tortuosity factor as a consequence of the gradually increasing fraction of the new, practically impermeable fibrous structure.