Solvent Vapor Exposure Research Articles

Recent Progress in Solid-State Room Temperature Afterglow Based on Pure Organic Small Molecules.

Organic room temperature afterglow (ORTA) can be categorized into two key mechanisms: continuous thermally activated delayed fluorescence (TADF) and room-temperature phosphorescence (RTP), both of which involve a triplet excited state. However, triplet excited states are easily quenched by non-radiative transitions due to oxygen and molecular vibrations. Solid-phase systems provide a conducive environment for triplet excitons due to constrained molecular motion and limited oxygen permeation within closely packed molecules. The stimulated triplet state tends to release energy through radiative transitions. Despite numerous reports on RTP in solid-phase systems in recent years, the complexity of these systems precludes the formulation of a universal theory to elucidate the underlying principles. Several strategies for achieving ORTA luminescence in the solid phase have been developed, encompassing crystallization, polymer host-guest doping, and small molecule host-guest doping. Many of these systems exhibit luminescent responses to various physical stimuli, including light stimulation, mechanical stimuli, and solvent vapor exposure. The appearance of these intriguing luminescent phenomena in solid-phase systems underscores their significant potential applications in areas such as light sensing, biological imaging, and information security.

Effects of the Solvent Vapor Exposure on the Optical Properties and Photocatalytic Behavior of Cellulose Acetate/Perylene Free-Standing Films.

The search to deliver added value to industrialized biobased materials, such as cellulose derivatives, is a relevant aspect in the scientific, technological and innovation fields at present. To address these aspects, films of cellulose acetate (CA) and a perylene derivative (Pr) were fabricated using a solution-casting method with two different compositions. Consequently, these samples were exposed to dimethylformamide (DMF) solvent vapors so that its influence on the optical, wettability, and topographical properties of the films could be examined. The results demonstrated that solvent vapor could induce the apparent total or partial preferential orientation/migration of Pr toward the polymer-air interface. In addition, photocatalytic activities of the non-exposed and DMF vapor-exposed films against the degradation of methylene blue (MB) in an aqueous medium using light-emitting diode visible light irradiation were comparatively investigated. Apparently, the observed improvement in the performance of these materials in the MB photodegradation process is closely linked to the treatment with solvent vapor. Results from this study have allowed us to propose the fabrication and use of the improved photoactivity "all-organic" materials for potential applications in dye photodegradation in aqueous media.

Contact line motion in dewetted polymer droplets on anisotropic nanopatterned surfaces.

Thin polymer films can become unstable and dewet on a non-wettable substrate leading to the formation of an array of droplets. This instability-mediated drop formation lacks spatial order on flat substrates, but it can be ordered for better combinatorial studies using patterned substrates. In this work, we studied the process of dewetting polystyrene (PS) films on grating patterned substrates upon solvent vapor exposure. The PS film thickness was commensurate with the grating pattern height (hP). Our findings show that the initial dewetting stages follow the direction of the underlying grating pattern with the formation of directional holes in the film. During the later stages of dewetting, there was a lateral coalescence of polymer threads across the grating stripes. The final morphology comprised smaller droplets or threads confined within the pattern grooves and anisotropic large drops covering several pattern stripes. Furthermore, the larger drops show a unique behavior of shape change from anisotropic to isotropic as a function of solvent vapor concentration (Cn) inside the dewetting chamber. The drop regained its anisotropic shape with an increase in Cn, and this transition continued, with the movement of the three-phase contact line (TPCL). While the capillary flow of the polymer causes anisotropy during high Cn, the local orientation of the contact line and a mismatch in the value of the equilibrium contact angle can drive the drop back into an isotropic shape as Cn reduces and capillary forces weaken. We also observed that the extent of anisotropy quantified as droplet distortion ratio (Dr) not only depends on the Cn during dewetting but also on solvent type and hP. This new-found dynamic behavior of dewetted polymeric drops can be studied in greater detail and potentially leveraged for applications in sensing and microfluidics.

(Invited) Bipolar Membranes with a 3D Junction of Interlocking Electrospun Fibers

Bipolar membranes (BPMs), typically laminated layers of anion-exchange and cation-exchange polymers, have the unique capability of splitting water at a potential near 0.83 V. Such membranes are used in electrodialysis membrane separation processes. They also have applications in water electrolyzers, CO2 electrolysis cells, and self-humidifying fuel cells. We report here on recent developments regarding BPMs with a high interfacial area, 3D nanofiber junction.Membranes were prepared by first creating a bipolar junction layer, by the simultaneous electrospinning of anion-exchange and cation-exchange polymers, with the addition of catalyst nanoparticles to facilitate water splitting. Solvent vapor exposure and hot-pressing closed all interfiber pores, to create a dense film of interpenetrating and interlocking nanofibers of positively and negatively charged polymers. In one fabrication method, dense films of solution cast anion and cation-exchange polymers were hot-pressed onto the opposing surfaces of the junction layer to create a tri-layer BPM. Membranes were also fabricated by electrospinning all three layers of the BPM: first spinning anion-exchange fibers, followed by dual fiber spinning with catalyst spraying, and then spinning only cation-exchange fibers. Solvent exposure and hot-pressing closed all interfiber voids.BPMs were made with a junction layer 12-15 mm thick, where the total membrane thickness was 50-80 mm. Membranes were prepared with both hydrocarbon and fluoropolymer ionomers, e.g., sulfonated poly(ether ether ketone) or perfluorosulfonic acid as the cation-exchange polymer and either quaternized poly(phenylene oxide), AEMIONä (an imidazolium-based polymer sold by Ionomr Innovations, Inc.), or PiperION (a poly(aryl piperidinium) from Versogen) as the anion-exchange polymer. A variety of different junction layer catalysts powders were examined, including Al(OH)3 and graphene oxide. Membranes were evaluated in an H-cell for the collection of steady-state current-voltage data and in a flow cell for long-term constant current water splitting (electrolysis) operation, typically with an aqueous Na2SO4 electrolyte. The 3D junction membranes performed exceptionally well: (i) the water splitting potential was low (near the expected value of 0.83 V), (ii) the transmembrane voltage drop was small at high currents (e.g., a voltage drop of 1.1 V up to 1.1 A/cm2), and (iii) the extended bipolar reaction zone for water splitting with interlocking fibers allowed for high current density operation with no evidence of membrane degradation.In this talk, the effect of membrane composition (the type/amount of anion and cation exchange polymers and choice of catalyst) and structure (the thickness of the layers) on short-term and long-term membrane performance will be discussed. Results will be presented for operating the bipolar membrane in water splitting and water generation modes.

Multistimuli-Responsive Squaraine Dyad Exhibiting Concentration-Controlled Vapochromic Luminescence.

Stimuli-responsive organic materials with controllable luminescence are of enormous importance because of their potential applications in sensing, data security, and display devices. In this study, a multistimuli-responsive squaraine dyad (SQ-d) composed of two rigid squaraine moieties and a flexible ethylene linker was rationally designed and synthesized. SQ-d exhibits polymorphic luminescence, which can be reversibly switched by various external stimuli, including solvent vapor exposure, heat, and shear force. Unexpectedly, the weakly luminescent phase (O1) of SQ-d exhibits concentration-controlled vapochromic behavior. Film O1 can convert to a highly green-emissive phase (G1) under a low concentration of CHCl3 vapor and convert to a highly yellow-emissive phase (Y) under a high concentration of CHCl3 vapor; these originate from two distinct crystallization-induced emission enhancement processes. To the best of our knowledge, this is the first investigation of the effect of vapor concentration on the phase transitions of organic vapochromic luminophores. By analyzing the single-crystal structures and photophysical properties of SQ-d, we concluded that the green and yellow emissions probably originated from a zigzag stacking mode and an H-type π-π stacking mode, respectively. Finally, two prototypes based on SQ-d for applications in information encryption and vapor sensing were successfully demonstrated.

Residual changes and thickness effects in glass-forming polymer thin films after solvent vapor annealing

Residual solvent in polymer coatings caused by solvent vapor annealing (SVA) may be used as an indicator of the SVA effectivity for block-copolymer self-assembly and other applications. To analyze the residual changes, temperature-programmed desorption (TPD) technique has been applied to 6–200 nm thick glassy polymer coatings processed by SVA. This method is based on the film thickness contraction and expansion measurements by ellipsometry in vacuum as the temperature of the sample ramps up. The measurements are corrected for the polymer thermal expansion. The polymers used are polystyrene, poly (methyl methacrylate), and poly (2-vinyl pyridine). The TPD curves exhibit up to 2 distinct desorption peaks. One peak appears in the vicinity of the film's glass transition temperature Tg. This feature shows up only if the coating has reached a solvent-induced glass transition during the preceding vapor annealing. The other desorption event is a broad feature appearing at the temperatures below Tg. This peak is observed even for the coatings without previous solvent vapor exposure, although the SVA enhances the effect. The magnitude of the desorption effect corresponds to 1% of the residual solvent content or less for about 30 nm thick films. The desorption demonstrates a clear thickness effect and becomes more pronounced for thinner films, but not exceeding a few % of the residual solvent in 5–8 nm thick coatings. It is supposed that the desorption event in the Tg vicinity is mainly caused by the loss of the solvent or voids from the polymer due to the increased molecular mobility, while the low temperature feature is mostly a desorption of residual gases from the sample surface. Application of the TPD data and sorption-desorption isotherms to the optimization of the SVA procedure and creating a process window plot is demonstrated.

A facile approach for tuning optical and surface properties of novel biobased Alginate/POTE handleable films via solvent vapor exposure

Novel biobased films consisting of alginate blends with poly (octanoic acid 2-thiophen-3-yl-ethyl ester) (POTE), a conducting polymer, were prepared by solution casting, and their optical, morphological, thermal, and surface properties were studied. Using UV–visible spectroscopy, atomic force microscopy (AFM), and scanning electron microscopy (SEM), the effects of tetrahydrofuran solvent vapors on the optical properties and surface morphology of biobased films with different POTE contents were studied. Results indicate that morphological rearrangements of POTE take place during the process of solvent exposure. Specifically, the solvent vapor induced the formation of POTE small crystalline domains, which allows envisioning the potential of tuning UV–visible absorbance and wettability behavior of biobased films. Finally, theoretical electronic calculations (specifically frontier molecular orbitals analysis) provided consistent evidence on POTE's preferential orientation and selectivity toward the THF-vapor medium.

Feature Size Modulation in Dewetting of Nanoparticle-Containing Ultrathin Polymer Films

Ultrathin polymer films (thinner than ∼100 nm) undergo spontaneous rupture on a non-wetting surface with the appearance of random holes due to interfacial van der Waal’s interaction or disjoining pressure. These holes grow in size and coalesce into threadlike morphology, which subsequently disintegrate in an isotropic array of nearly equal-sized droplets, the size (dD) and periodicity (λD) of which scales with the initial film thickness (hF). We show that both λD and dD of the dewetted droplets can be modulated by adding trace amounts of nanoparticles (NPs) in the films. While addition of higher proportion of NPs is known to stabilize the films against dewetting, the presence of a lower amount of NPs leads to non-monotonic variation of λD and dD with hF, with the possible creation of miniaturized dewetted features under certain conditions. We also show that λD and dD of the dewetted films depend on whether dewetting is engendered by thermal annealing of the film beyond the glass transition temperature of the constituent polymer (TG) or by exposing the film to solvent vapor (SV). Our findings reveal that both λD and dD are much larger when the film (both with and without particles) is dewetted in the SV atmosphere, arguably due to penetration of the solvent into the film matrix, resulting in an increase in the effective thickness of the film during dewetting. We show that in SV-mediated dewetting, the much faster dynamics is attributed to solvent penetration through the film and its wetting the substrate, rather than a drastic drop in viscosity. We also observe that SV exposure not only leads to much faster dewetting dynamics, but it also successfully engenders dewetting in films with higher hF, which remain stable when annealed thermally.

Pattern-Directed Phase Transitions and VOC Sensing of Liquid Crystal Films

Solvent vapor exposure could transform a crystalline or smectic liquid crystal (LC) film into nematic and isotropic phases under ambient conditions. The average time for such phase transitions is found to linearly reduce with an increase in vapor pressure and reduction in the molecular weight of solvents. Such responses of solvent vapor-annealed phase transitions of a nanoparticle-loaded LC droplet were then converted into an electrical signal, wherein the electrical resistance reduced (increased) with time upon destruction (restoration) of the orientational order of the LC matrix. Variation in the electrical response was used to identify the volatile organic vapors, phase transition of LCs, rate of diffusion–absorption of solvent into LCs, and rate of desorption–evaporation of solvent from LCs. Pattern-directed phase transitions on physically heterogeneous surfaces showed a faster (slower) kinetics on thinner (thicker) patterns. However, for chemically heterogeneous surfaces, weaker (stronger) anchoring of LCs on hydrophobic (hydrophilic) patches ensured a faster (slower) transition.

Tricolor fluorescence switching in a single component mechanochromic molecular material

Reversible tricolor switching in a mechanochromic α-dicyanodistyrylbenzene molecule has been developed. The tricolor switching (blue, orange, and green) is implemented by external stimuli of heat, solvent-vapor exposure, and mechanical force.

Creating Chameleon-like Smart Actuators

Inspired by how chameleons change their skin color, Du and co-workers have developed a general biomimetic strategy for preparing smart photonic crystal actuators with the fast and reversible colorimetric response and programmable shape transformation.

Screening a trace amount of pharmaceutical cocrystals by using an enhanced nano-spot method

Cocrystallization is an attractive and promising technology that can improve the physical properties of formulations of active pharmaceutical ingredients (APIs). We have developed a “nano-spot method” that can evaluate the crystalline form on the nanogram scale. In this study, the following studies were performed to obtain versatile and comprehensive improvements to the nano-spot method: modification of the sample solution, application of solvent vapor exposure to attempt the precipitation of various states of crystals, and adoption of low-frequency Raman spectroscopy. Carbamazepine was used as a model API and cocrystallization screening was examined with 12 cocrystal formers (coformers). In the case of combinations that are already known to form cocrystals, spectra similar to those of previously reported cocrystals or new spectra were obtained. It was considered that the reported cocrystals or new polymorphs were obtained. In contrast, in the case of the combination which has been reported not to form a cocrystal, the spectra were consistent with that for the physical mixture of API and coformer, suggesting that a cocrystal also did not form in this screening. In addition, the newly adopted low-frequency Raman spectroscopy enabled the high-sensitive detection of the crystalline form.

Effect of Solvent Vapor Treatment on Electrospun Nylon 6,6 Nanofiber Membrane

This paper highlights the effect of solvent vapor treatment on characterization of electrospun nanofiber membrane (ENM). Fouling issue in membrane separation has becoming a debate since it reduces the lifetime of the membrane. In order to solve this issue, ENM has been recommended due to its great performance in terms of filtration, permeability and stability. However, ENM has several drawbacks such as low mechanical strength and loosely packed layers of nanofibers. Thus, solvent vapor treatment was used to solve this issue. In this study, nylon 6, 6 was fabricated as ENM and was exposed to formic acid as solvent vapor treatment. Solvent vapor treatment helps to increase the bonding of fiber by promoting crosslinking at the junction of fiber thus making the membrane mechanically durable. By controlling the solvent vapor exposure time, excessive fusion between the fibers can be prevented as over-crosslinking would reduce membrane permeability. Solvent vapor treatment also reduces the number of layers of ENM, by making it more closely packed and reduces the risk of membrane delamination. All treated ENM were found to be more hydrophilic and has lower surface roughness, which would make them more resistant towards fouling than the untreated one.

Improving analyte selectivity by post-assembly modification of metal-organic framework based photonic crystal sensors.

The porous nature and structural diversity of metal-organic frameworks (MOFs) provide a versatile platform for specific and selective sorption behavior. When integrated as functional layers into photonic crystals (PCs), loading of the porous network with organic solvent vapors translates into an optical response, allowing analyte discrimination according to the specific host-guest interactions and, hence, framework affinity to the analytes. However, the optical response of PCs is critically influenced by the overall PC architecture, leading to batch-to-batch variations, thus rendering unequivocal analyte assignment challenging. To circumvent these problems, we have developed a straightforward and mild "post-assembly" modification strategy to impart differences in chemical selectivity to the MOF layers whilst keeping the overall PC backbone constant. To this end, one-dimensional photonic crystal (1D PC) sensors based on CAU-1 and TiO2 layers were fabricated to obtain a generic platform for post-assembly modification, targeting either the secondary building unit (SBU) or the linker unit of the as-assembled MOF nanoparticle layers. The optical response to solvent vapor exposure was investigated with the pristine CAU-1 based sensor as well as its modifications, showing enhanced analyte selectivity for the post-modified systems.

Membranes from Blended Ionomer/PVDF Nanofibers: I. PFSA/PVDF and PFIA/PVDF Fiber Spinning and Membrane Fabrication

Blends and composites of various perfluoroionomers and uncharged polymers, e.g., poly(vinylidene fluoride) (PVDF), have been widely studied as the proton conducting membrane material for direct methanol and hydrogen/air fuel cells. The general idea of blending was to decouple the mechanical and swelling properties of a membrane from its proton conductivity. Thus, for a hydrogen/air fuel cell, the goal was to fabricate a membrane with low sheet resistance, low areal swelling, and good mechanical properties. For a direct methanol fuel cell membrane, blended were made to lower methanol crossover methanol while maintaining a high proton conductivity. In 2008, Pintauro and co-workers [1] showed how nanofiber electrospinning can be used to fabricate composite fuel cell membranes. The method was an alternative to traditional blending. These early membranes were made by electrospinning an ionomer fiber mat followed by the impregnation of an uncharged, reinforcing polymer into the inter-fiber voids; inverse structures where an uncharged polymer was electrospun and the voids were filled with an ionomer were also studied [2]. Dual-fiber electrospinning was introduced in 2011 by Ballengee and Pintauro [3] as a means to eliminate a separate interfiber void-filling impregnation step during nanofiber composite membrane fabrication. The ionomer and uncharged polymers were simultaneously electrospun as separate fibers that co-deposited as a well-mixed mat on the collector surface. Subsequent processing via hot-pressing, and either annealing or solvent vapor exposure, induced the flow of one of the polymer components into the interfiber void space while retaining the nanofiber morphology of the second polymer. Two distinct morphologies were produced from such dual fiber mats: (i) a network of ionomer nanofibers embedded in an uncharged reinforcing polymer matrix and (ii) an ionomer matrix reinforced by a network of uncharged nanofibers. Membranes were prepared via dual fiber electrospinning using various polymer combinations, including Nafion, Aquivion and 3M Company PFSA ionomers, and polyphenylsulfone, poly(vinylidene fluoride), and polyamide-imide as the uncharged components. We have recently developed a new electrospinning strategy for membrane preparation, were a solution of a binary polymer mixture is electrospun and then the single fiber mat is hot-pressed into a dense blended membrane. Here, we exploit the electrospinning process to insure blending of seemingly incompatible polymers (i.e., thorough mixing of the polymer components as the electrospinning solution emerges from the spinneret tip followed by rapid solvent evaporation and the “freezing in” of a blended morphology within a 100-500 nm diameter fiber. In this presentation the methods used to electrospin various single fibers blends of perfluoroionomers with KynarÒ HSV 900 PVDF will be presented, with a focus on NafionÒperflourosulfonic acid ionomer and low equivalent weight PFIA ionomer from 3M Company. Procedures for converting the nanofiber mats into dense and defect-free membranes will be described. Preliminary characterization results will be summarized (water swelling, proton conductivity, and mechanical properties) and contrasted with dual fiber membranes and/or conventional solution cast blended films. REFERENCES [1] J. Choi, K. M. Lee, R. Wycisk, P. N. Pintauro and P. T. Mather, Macromolecules, 41, 4569, 2008. [2] M. Gummalla, Z. Yang, P. Pintauro, K.M. Lee, R.Wycisk, US Patent Application, US 13/995,580, 2011 [3] J. B. Ballengee and P. N. Pintauro, Macromolecules , 44, 7307, 2011.

Solvent-vapor-assisted dewetting of prepatterned thin polymer films: control of morphology, order, and pattern miniaturization.

Ultrathin (<100 nm) unstable polymer films exposed to a solvent vapor dewet by the growth of surface instability, the wavelength (λ) of which depends on the film thickness (h(f)). While the dewetting of a flat polymer thin film results in random structures, we show that the dewetting of a prepatterned film results in myriad ordered mesoscale morphologies under specific conditions. Such a film undergoes rupture over the thinnest parts when the initial local thickness of these zones (h(rm)) is lower than a limiting thickness h(lim) ≈ 10 nm. Additionally, the width of the pattern grooves (l(s)) must be wider than λ(s) corresponding to a flat film having a thickness of h(rm) for pattern-directed dewetting to take place over surface-tension-induced flattening. We first present an experimentally obtained morphology phase diagram that captures the conditions where a transition from surface-tension-induced flattening to pattern-directed-rupture takes place. Subsequently, we show the versatility of this technique in achieving a variety of aligned mesopatterns starting from a prepatterned film with simple grating geometry. The morphology of the evolving patterns depends on several parameters such as the initial film thickness (h(f)), prepattern amplitude (h(st)), duration of solvent vapor exposure (SVE), and wettability of the stamp used for patterning. Periodic rupture of the film at regular intervals imposes directionality on the evolving patterns, resulting in isolated long threads/cylindrical ridges of polymers, which subsequently disintegrate into an aligned array of droplets due to Rayleigh-Plateau instability under specific conditions. Other patterns such as a double periodic array of droplets and an array of holes are also possible to obtain. The evolution can be interrupted at any intermediate stage by terminating the solvent vapor annealing, allowing the creation of pattern morphology on demand. The created patterns are significantly miniaturized in size as compared to features obtained from dewetting a flat film with the same hf.

High-contrast red-green-blue tricolor fluorescence switching in bicomponent molecular film.

Highly efficient red-green-blue (RGB) tricolor luminescence switching was demonstrated in a bicomponent solid film consisting of (2Z,2'Z)-2,2'-(1,4-phenylene)bis(3-(4-butoxyphenyl)acrylonitrile) (DBDCS) and (2Z,2'Z)-3,3'-(2,5-bis(6-(9H-carbazol-9-yl)hexyloxy)-1,4-phenylene)bis(2-(3,5-bis(trifluoromethyl)phenyl)acrylonitrile) (m-BHCDCS). Reversible RGB luminescence switching with a high ratiometric color contrast (λ(em)=594, 527, 458 nm for red, green, and blue, respectively) was realized by different external stimuli such as heat, solvent vapor exposure, and mechanical force. It was shown that Förster resonance energy transfer in the bicomponent mixture could be efficiently switched on and off through supramolecular control.

High‐Contrast Red–Green–Blue Tricolor Fluorescence Switching in Bicomponent Molecular Film

AbstractHighly efficient red–green–blue (RGB) tricolor luminescence switching was demonstrated in a bicomponent solid film consisting of (2Z,2′Z)‐2,2′‐(1,4‐phenylene)bis(3‐(4‐butoxyphenyl)acrylonitrile) (DBDCS) and (2Z,2′Z)‐3,3′‐(2,5‐bis(6‐(9H‐carbazol‐9‐yl)hexyloxy)‐1,4‐phenylene)bis(2‐(3,5‐bis(trifluoromethyl)phenyl)acrylonitrile) (m‐BHCDCS). Reversible RGB luminescence switching with a high ratiometric color contrast (λem=594, 527, 458 nm for red, green, and blue, respectively) was realized by different external stimuli such as heat, solvent vapor exposure, and mechanical force. It was shown that Förster resonance energy transfer in the bicomponent mixture could be efficiently switched on and off through supramolecular control.

Electrospinning PFSA + PVDF Nanofibers for Fuel Cell Membrane Fabrication

In 2008, Pintauro and co-workers [1] showed how nanofiber electrospinning can be used to fabricate composite fuel cell membranes. The method of membrane fabrication and the membrane morphology were alternatives to traditional membranes based on polymer blends and copolymers. These early membranes were made by electrospinning an ionomer fiber mat followed by the impregnation of an uncharged, reinforcing polymer into the inter-fiber voids.A series ofpost-electrospinning processing steps were required to convert a porous mat into a dense and defect-free fuel cell membrane, namely (a) mat compression, (b) fiber welding, (c) ionomer annealing, and (d) inert polymer impregnation. The resultant nanofiber composite membrane morphology decoupled the proton conduction function of the ionomeric nanofibers from the mechanical support and swelling control functions of the uncharged matrix polymer. Dual-fiber electrospinning was introduced in 2011 by Ballengee and Pintauro [2] as a means to eliminate a separate interfiber void-filling impregnation step during nanofiber composite membrane fabrication. Here, the ionomer and uncharged polymers were simultaneously electrospun as separate fibers that co-deposited as a well-mixed mat on the rotating drum collector surface. Subsequent processing via hotpressing and either annealing or solvent vapor exposure, induced the flow of one of the polymer components into the interfiber void space while retaining the nanofiber morphology of the second polymer. Two membrane morphologies were produced from a single dual fiber mat: (i) an interconnected network of ionomer nanofibers embedded in an uncharged polymer reinforcing polymer matrix and (b) a network of uncharged reinforcing polymer nanofibers surrounded by an ionomer matrix. In the dual fiber membrane work, the ionomer was NafionÒ perfluorosulfonic acid polymer and the uncharged polymer was polyphenylsulfone.Recently our group has begun fabricating dual-fiber Nafion-PVDF and AquivionÒ-PVDF composite proton conducting membranes (Aquivion is a short side chain 825 EW PFSA polymer from Solvay Solexis). These membranes were designed for use in regenerative hydrogen-bromide and conventional hydrogen/air fuel cells. Two general membrane structures were prepared: (1) membranes with “melted” PVDF fibers that fill the voids between PFSA fibers, and (2) membranes with “melted” PFSA which fills the voids between PVDF nanofibers. These two membrane types were obtained from the same as-spun two-component mats using the procedures described in the preceding paragraph. The PFSA content of membranes was typically in the range 30-60% PVDF for morphology #1 and 20-50% for morphology #2.This presentation will provide additional details regarding the fabrication and properties of PFSA/PVDF fuel cell membranes. Specific topics to be covered in the talk are:New ways of electrospinning Nafion and Aquivion nanofibersNanofiber composite membranes where an interconnecting network of PFSA nanofibers is embedded in a PVDF matrix.Nanofiber composite membranes where PVDF nanofibers are embedded in a PFSA matrixPFSA/PVDF blended membranes with no nanofiber morphology. The various membranes in items 2-4 will be contrasted with one another and with commercial Nafion 212 films in terms of the following fuel cell properties: proton conductivity, gravimetric and areal water swelling, wet/dry mechanical properties, and gas permeability. Physical property differences will be linked with changes in membrane morphology.

Structural transitions in asymmetric poly(styrene)-block-poly(lactide) thin films induced by solvent vapor exposure.

Successive structural transitions in thin films of asymmetric poly(styrene)-block-poly(lactide) (PS-PLA) block copolymer samples upon exposure to tetrahydrofuran (THF) vapors have been monitored using atomic force microscopy (AFM) and both in situ and ex situ grazing incidence small-angle X-ray scattering (GISAXS). A direct link was established between the structure in the swollen state and the morphology formed in the dried state post solvent evaporation. This was related to the high incompatibility between the constituting blocks of the copolymer that thwarted the system from reaching the homogeneous disordered state in the swollen state under the specific conditions utilized in this study. Upon rapid solvent removal, the morphologies formed in the swollen state were trapped due the fast evaporation kinetics. This work provides a better understanding of the mechanisms associated with block copolymer thin film morphology changes induced by solvent vapor annealing.