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.     

Aging and carbon dioxide plasticization of thin polyetherimide films

Industrial gas separation membranes have selective dense layers with thicknesses around 100 nm. It has long been assumed that these thin layers have the same properties as thick (bulk) films. However, recent research has shown that thin films with such thickness experience accelerated physical aging relative to bulk films and, thus, their permeation properties can differ significantly from the bulk. Thin films made from Extem^® XH 1015, a new commercial polyetherimide, have been investigated by monitoring their gas permeability. The permeability of the thin films is originally greater than the thick films but eventually decreases well below the permeability of the thick film. The CO₂ plasticization of Extem thin films is explored using a series of exposure protocols that indicate CO₂ plasticization is a function of film thickness, aging time, exposure time, pressure and prior history.     

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 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.     

Induced slim ferroelectric hysteresis loops and enhanced energy-storage properties of Mn-doped (Pb0·93La0.07)(Zr0·82Ti0.18)O3 anti-ferroelectric thick films by aerosol deposition

In this study, the high ferroelectric hysteresis loss of (Pb_0·93La_0.07)(Zr_0·82Ti_0.18)O₃ (PLZT 7/82/18) antiferroelectric (AFE) ceramics was reduced by employing a combinatorial approach of Mn acceptor doping followed by thick film fabrication via an aerosol deposition (AD) process. The grains of the as-deposited PLZT 7/82/18 AFE AD thick films were grown by thermal annealing at 550 °C to enhance their electrical properties. Investigation of the electrical properties revealed that Mn-doping results in improved dielectric and ferroelectric properties, increased dielectric breakdown strength (DBS), and energy-storage properties. The Mn-doped PLZT AFE AD films possess a frequency-independent high dielectric constant (ε_r ≈ 660) with low dielectric loss (tan δ ≈ 0.0146), making them suitable candidates for storage capacitor applications. The bipolar and unipolar polarization vs. electric field (P-E) hysteresis loops of PLZT 7/82/18 AFE AD thick films were found to be slimmer than those of their bulk form (double hysteresis) with significantly reduced ferroelectric hysteresis loss, which is attributed to the AD-induced mixed grain structure. The Mn-doped PLZT 7/82/18 AFE AD thick films exhibited a low remnant polarization (P_r ≈ 9.22 μC/cm²) at a high applied electric field (~2750 kV/cm). The energy-storage density and energy efficiency of the Mn-doped PLZT AFE AD thick films were calculated from unipolar P-E hysteresis loops and found to be ~38.33 J/cm³ and ~74%, respectively.     

Structural,optical and photocatalytic properties of ZnO nanorods: Effect of aging time and number of layers

ZnO thin films were prepared by sol–gel dip coating method onto glass substrates. The effects of aging time of the starting solution (2, 10 and 30 days) and the number of coats (2, 5 and 10 coatings) on structural, morphological and optical properties were investigated. Photocatalytic efficiency was also assessed. X-ray diffraction analysis indicates that all the films exhibit a Zincite-type structure with a preferred grains orientation along the [002] direction. The preferred orientation factor (POF) increases with aging time while the crystallite size decreases. The field emission scanning electron microscopy observations reveals nanorods morphology. The length of ZnO nanorods increase with increasing number of layers whereas their length decreases as a function of aging time while adopting a random orientation. A high optical transparency is observed for all ZnO thin films, ranging from 90 up to 96%. Methylene Blue (MB) dye photocatalytic degradation was found increases with aging time, reaching almost 94% after 10 h under UV irradiation. The apparent reaction rate (K_app) obtained by Langmuir-Hinshelwood model increases with increasing aging time from, from 0.218 h⁻¹ for 2 days to reach a steady state around 0.270 h⁻¹. Nevertheless, a small variation of K_app was recorded when varying the number of coats; 0.223–0.226 h⁻¹.     

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.     

Carbon dioxide plasticization and conditioning effects in thick vs. thin glassy polymer films

Recent studies have shown that thin glassy polymer films undergo physical aging more rapidly than thick films. This suggests that thickness may also play a role in the plasticization and conditioning responses of thin glassy films in the presence of highly-sorbing penetrants such as CO₂. In this paper, a carefully designed systematic study explores the effect of thickness on the CO₂ plasticization and conditioning phenomena in Matrimid^®, a polyimide commonly used in commercial gas separation membranes. Thin films are found to be more sensitive than thick films to CO₂ exposure, undergoing more extensive and rapid plasticization at any pressure. The response of glassy polymers films to CO₂ is not only dependent on thickness, but also on aging time, CO₂ pressure, exposure time, and prior history. Finally, thin films experiencing constant CO₂ exposure for longer periods of time exhibit an initial large increase in CO₂ permeability, which eventually reaches a maximum, followed by a significant decrease in permeability for the duration of the experiment. Thick films, in contrast, do not seem to exhibit this trend for the range of conditions explored.     

Microstructure,ferroelectric and piezoelectric properties of Bi4Ti3O12 platelet incorporated 0.36BiScO3-0.64PbTiO3 thick films for high temperature piezoelectric device applications

Bi₄Ti₃O₁₂ (BiT) platelet incorporated 0.36BiScO₃-0.64PbTiO₃ (BS-PT) thick films were successfully developed for high temperature piezoelectric device applications. Their microstructure and ferroelectric and piezoelectric properties were systematically investigated to demonstrate the effect of the BiT template. It was found that the incorporation of BiT template was not responsible for textured grain growth of the BS-PT thick films; however, it could result in grain growth and densification of the BS-PT thick films by optimizing the sintering temperature and amount of BiT template. In particular, a 4 wt% BiT-incorporated BS-PT thick film sintered at 1000°C exhibited a pure perovskite structure and excellent piezoelectric properties of d₃₃ (440 pC/N) and k_p (53%), despite the presence of micro-pores caused by molten BiT. The BS-PT thick film, exhibiting good ferroelectric characteristics of P_r of 39.4 μC/cm² and E_c of 3.0 kV/mm at 1 kHz, also had an extremely high T_c of approximately 402°C. This implies that the BiT platelet incorporated BS-PT thick film is applicable for high temperature piezoelectric devices.     

Effects of annealing temperature on the phase formation,optical, photoluminescence and magnetic properties of sol-gel YFeO3 films

YFeO₃ (YFO) thin films were deposited onto quartz substrates via sol-gel spin-coating technique and annealed at different temperature ranged between 650 and 900 °C. The impact of annealing temperature on the phase formation, microstructural, optical, photoluminescence (PL) and magnetic properties of the films were systematically investigated. X-ray diffraction analysis revealed an amorphous structure in film annealed at 650 °C and formation of hexagonal-YFO (h-YFO) phase in films annealed at 750–800 °C. The films annealed at 850–900 °C exhibited an orthorhombic-YFO (o-YFO) structure. Atomic force microscopy images of h-YFO films showed homogeneous surface with uniform particles size and shape. The particle size increased and had irregular shape in o-YFO films. The average particle size was 44 and 117 nm, while the root square roughness was 1.38 and 2.55 nm for h- and o-YFO films annealed at 750 and 850 °C, respectively. The optical band gap (E_g) was 2.53 and 2.86 eV for h- and o-YFO films annealed at 750 and 850 °C, respectively. The PL spectra of h-YFO films were red-shifted compared with that of o-YFO films. The PL emission related to near band edge was observed at 459.0 and 441.9 nm for h- and o-YFO films annealed at 750 and 850 °C, respectively. The magnetization was enhanced with the increasing of annealing temperature and has the value of 4.8 and 12.5 emu/cm³ at 5000 Oe for h- and o-YFO films annealed at 750 and 850 °C, respectively.     

Synthesis and microstructural characterization of Bi12SiO20 (BSO) thin films produced by the sol–gel process

We have produced Bi₁₂SiO₂₀ (BSO) thin films using the sol–gel process. The stable sol was synthesized using Bi(NO₃)₃·5H₂O and Si(OC₂H₅)₄ (TEOS) as the precursors, acetic acid and 2-ethoxyethanol as the solvents, and ethanolamine as the stabilizer. The stability of the solution, which depends on the concentration and the R_h value (R_h = [H₂O]/[M]), directly affects the microstructure of the BSO thin film. We determined that the optimal concentration for the preparation of BSO thin films is 0.76 M. The influences of the substrates, the annealing temperature, the concentration and the R_h = value of the solution on the microstructure of the Bi₁₂SiO₂₀ thin films were investigated. X-ray diffraction (XRD) showed that the Bi₁₂SiO₂₀ starts to form at 500 °C and that single-phase Bi₁₂SiO₂₀ polycrystalline thin films are formed at 700 °C. The coated films were characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM).     

Cellulose esters for forward osmosis: Characterization of water and salt transport properties and free volume

The fundamental salt and water transport properties of cellulose ester polymers have been studied and correlated with their hydrated free volume, as probed by positron annihilation lifetime spectroscopy (PALS), to provide scientific insights for material development in seawater desalination. It has been found that the hydrated free volume is strongly dependent on water uptake. The water sorption in polymeric films reduces the fractional free volume (FFV) at low degrees of hydration due to the overwhelming hole-filling effect, but gradually increases FFV when more water sorption takes place as a result of the swelling effect. The diffusivity and permeability of both water and salt vary with changes in wet-state FFV. Diffusion selectivity of H₂O/NaCl increases dramatically as the hydrated FFV decreases, while solubility selectivity of H₂O/NaCl is dependent on both hydrated free volume and chemical structure. Moreover, thin freestanding films with no sub-layer support have been prepared by spin casting these polymers on silicon wafers for the purpose of eliminating internal concentration polarization (ICP) in the forward osmosis (FO) process. Based on the theoretical prediction from the solution-diffusion model, the performance ratio (i.e., the ratio of the experimental to theoretical water flux) of the sub-layer free films can be calculated. It can reach as high as around 50%, which is 2–3 times of the values reported elsewhere when testing traditional asymmetric membranes. Using a model 3.5 wt.% NaCl feed solution and a 2M NaCl draw solution, a water flux of 21.8 LMH has been observed, which is the highest value ever reported. The high water flux in the FO desalination process indicates that the concept of sub-layer-free thin films is promising for the next generation FO membranes.     

Preparation of c-axis-oriented Bi4Ti3O12 thick films by templated grain growth

Highly c-axis-oriented Bi₄Ti₃O₁₂ thick films were successfully fabricated by templated grain growth. The effects of template particles and sintering conditions on grain orientation in thick films were investigated. SEM micrographs and X-ray diffraction (XRD) patterns exhibited that thick films were c-axis-oriented. The degree of grain orientation (Lotgering factor, f) increases with increasing sintering temperature and soaking time. Highly c-axis-oriented thick film (orientation degree of ∼ 0.98) is obtained with the use of only 5 wt.% template particles by sintering at 1000 °C for 2 h. This film exhibits a better temperature-independent dielectric constant and a lower dielectric loss.     

Fast preparation of (111)‐oriented β‐SiC films without carbon formation by laser chemical vapor deposition from hexamethyldisilane without H2

(111)‐oriented β‐SiC films were prepared by laser chemical vapor deposition using a diode laser (wavelength: 808 nm) from a single liquid precursor of hexamethyldisilane (Si(CH₃)₃–Si(CH₃)₃, HMDS) without H₂. The effects of laser power (P_L), total pressure (P_tot) and deposition temperature (T_dep) on the microstructure, carbon formation and deposition rate (R_dep) were investigated. β‐SiC films with carbon formation and graphite films were prepared at P_L ≥ 170 W and P_to ≥ 1000 Pa, respectively. Carbon formation strongly inhibited the film growth. β‐SiC films without carbon formation were obtained at P_tot = 400‐800 Pa and P_L = 130‐170 W. The maximum R_dep was about 50 μm·h^?1 at P_L = 170 W, P_tot = 600 Pa and T_dep = 1510 K. The investigation of growth mechanism shows that the photolytic of laser played an important role during the depositions.     

High-performance (Na0.5Bi0.5)(Ti0.97Fe0.03)O3-based heterostructure thin films for energy storage capacitors

This work presents a simple process to fabricate Na_0.5Bi_0.5(Ti_0.97Fe_0.03)O₃/Ba_(1–x)Sr_xTiO₃–based heterostructure thin films with a compositional graded sequence, and their energy storage properties are investigated systematically. A simulation technique is used to predict the experimental results. Interestingly, improved energy storage properties are obtained in the “up–graded” film, which is associated with the stress/strain. Furthermore, the “up–graded” film after aging with treating exhibits a higher breakdown strength (E_BD = 3176.4 kV/cm), recoverable energy storage density (W_rec = 67.18 J/cm³) and efficiency (η = 75.65%). This can be attributed to the ordered defect dipoles induced by aging with treating driving domain switching, which is favorable for high (P_m–P_r) and large W_rec. These results provide an approach and guidance to design thin film-based devices with optimal energy storage performance.     

Enthalpy recovery and structural relaxation in layered glassy polymer films

Recent studies of physical aging in confined polymer glasses have revealed that aging behavior in confinement often differs from bulk behavior. This study used DSC to characterize physical aging and structural relaxation in bulk polysulfone (PSF) and co-extruded multilayered films of PSF and an olefin block copolymer (OBC) that have average PSF layer thicknesses of 640 nm, 260 nm, and 185 nm. The films were aged isothermally at 170 °C, and the recovered enthalpy upon reheating was measured over time. The films with 640 nm and 260 nm PSF layers had aging rates very similar to that of bulk PSF, while the film with 185 nm PSF layers had an aging rate slightly greater than the bulk value. The cooling rate dependence of the limiting fictive temperature (T_f′) in multilayered and bulk PSF samples was also characterized. Values of T_f′ were similar for all films at each cooling rate. The results of this work are in general agreement with our previous gas permeation aging study of multilayered PSF films aged at 35 °C, in which the effect of layer thickness on aging behavior was minimal. This stands in contrast to studies with thin, freestanding PSF films, which exhibit accelerated aging relative to bulk and have aging rates that depend strongly on film thickness.     

Characterization of the oxygen scavenging capacity and kinetics of SBS films

Butadiene-containing polymers such as styrene-butadiene-styrene (SBS) block copolymers are of a potential use as oxygen scavenging polymers (OSP) in barrier applications. To evaluate their use in such applications, oxygen uptake was measured for films made of an SBS copolymer and four cobalt catalyst loadings with various thicknesses. The oxygen uptake was found to be kinetically limited for thin films while diffusion controls the uptake at long times for thick films. The thickness of the oxidized region at long oxidation times is termed the critical thickness L_c and was quantified by various analyses. Thin films (i.e.,L ≤ 2L_c) oxidize fully and homogeneously, whereas heterogeneous oxidation typically occurs in thicker films (i.e., L > 2L_c). A thin film model was used to extract the reaction rate parameters and a stoichiometric oxidation coefficient that describe oxygen uptake in the absence of diffusion limitation. An approximate moving-boundary model was developed to describe thick film oxidation behavior at long times and was found to be in semi-quantitation agreement with the measured uptake.     

Al2O3–cyanate ester hybrid thick films through aerosol deposition and resin infiltration

Al₂O₃–cyanate ester hybrid thick films had high Al₂O₃ contents over 75 vol.% were fabricated as 3D integrated substrates. The Al₂O₃–cyanate ester hybrid thick films were completed by resin infiltration of the cyanate ester into the porous Al₂O₃ thick films deposited by aerosol deposition (AD). Al₂O₃ particles were packed as high-density layers in the porous Al₂O₃ thick films by controlling the carrier gas flow rate using AD at room temperature. As dielectric substrate materials, the Al₂O₃–cyanate ester hybrid thick films had dielectric constant of 7.6 and quality (Q) factor of 390, and both were nearly independent of the measuring frequency.     

Electrical properties of chemical solution deposited (Bi0.9RE0.1)(Fe0.975Cu0.025)O3−δ (RE=Ho and Tb) thin films

Pure BiFeO₃ (BFO) and (Bi_0.9RE_0.1)(Fe_0.975Cu_0.025)O_3?δ (RE=Ho and Tb, denoted by BHFCu and BTFCu) thin films were prepared on Pt(111)/Ti/SiO₂/Si(100) substrates by using a chemical solution deposition method. The BHFCu and BTFCu thin films showed improved electrical and ferroelectric properties compared to pure BFO thin film. Among them, the BTFCu thin film exhibited large remnant polarization (2P_r), low coercive field (2E_c) and reduced leakage current density, which are 89.15 C/cm² and 345 kV/cm at 1000 kV/cm and 5.38×10^?5 A/cm² at 100 kV/cm, respectively.     

Physical aging of polystyrene films tracked by gas permeability

Most studies using gas permeation to characterize physical aging in thin polymer films have focused on polymers of interest as membrane materials, such as polysulfone (PSF) and Matrimid. Many other physical aging studies, using techniques other than gas permeation, focus on polystyrene (PS). In this work, physical aging in bulk PS films and PDMS-coated thin PS films was studied using well-established gas permeation techniques. The ～400 nm PS films aged slightly faster than bulk PS. However, the difference between rates of aging in thin and thick films was much less than that reported in PSF and Matrimid films of similar thicknesses. The ～800 nm films aged in a manner generally similar to bulk PS. Comparison of the normalized oxygen permeability of ～400 nm films of PS, PSF, and Matrimid revealed that a ～400 nm PS film experiences a slower decline in relative permeability than a PSF or Matrimid film does. Unlike what has been observed previously in studies of PSF and Matrimid films, PS films do not appear to show aging behavior that is strongly dependent on film thickness or highly accelerated relative to bulk. Because it would be difficult to use the results of PS aging studies to predict the aging behavior of typical gas separation polymers, we suggest that PS is not a good model for the aging behavior of commercially useful gas separation membrane materials.