EXAFS analysis of plasticized zinc-neutralized sulfonated polystyrene ionomers

The interaction of several plasticizers with the zinc cation in zinc-neutralized sulfonated polystyrene was examined with extended x-ray absorption fine structure (EXAFS) spectroscopy. Glycerol and water were found to interact strongly with the zinc cation, plasticizing the ionic aggregates. At full solvation by glycerol, the zinc atom was found to be coordinated by three glycerol molecules. Dioctylphthalate, acetonitrile, and toluene, which are unable to coordinate to the zinc cation, were found to have a minimal effect on the cation's local structure.     

Far-infrared studies of microphase separation in sulfonated ionomers

Far-infrared spectra of a series of un-neutralized and neutralized lightly sulfonated polystyrenes with varying sulfonation levels have been investigated to seek spectroscopic evidence for microphase separation known to control the physical properties of these polymers. Broad, strong absorbance bands, not found in the spectrum of unmodified polystyrene, are observed in the spectra of the sulfonated analogs. The effects on the far-infrared spectra both of sulfonation level and of the mass and charge of the neutralizing cation are discussed in terms of cation motion and the formation of ion-rich domains.     

Wetting behavior of lightly sulfonated polystyrene ionomers on silica surfaces

The wetting/dewetting behavior of thin films of lightly sulfonated low molecular weight polystyrene (SPS) ionomers spin-coated onto silica surfaces were studied using atomic force microscopy (AFM), contact angle measurements, and electron microscopy. The effects of the sulfonation level, the choice of the cation, the solvent used to spin-coat the films, and the molecular weight of the ionomer were investigated. Small angle X-ray scattering was used to determine the bulk microstructure of the films. The addition of the sulfonate groups suppressed the dewetting behavior of the PS above its glass transition temperature, e.g. no dewetting occurred even after 240 h of annealing at 120 degrees C. Increasing the sulfonation level led to more homogeneous and smoother surfaces. The choice of the cation used affected the wetting properties, but not in a predictable manner. When tetrahydrofuran (THF) or a THF/methanol mixed solvent was used for spin-casting, a submicron-textured surface morphology was produced, which may be a consequence of spinodal decomposition of the film surface during casting. Upon annealing for long times, the particles coalesced into a coherent, nonwetted film.     

Neutron study of self-organization in solutions of ionomers based on sulfonated polystyrene

Processes of self-organization of ionomers based on sulfonated PS containing ionogenic groups in the salt form (SO₃Na) in chloroform have been studied by small-angle neutron scattering. At a small content of ionogenic groups SO₃Na (1.35 mol %), the conformation of PS chains changes from coil-like to globular due to electrostatic interactions between them. An increase in the share of ionogenic groups to 2.6 mol % brings about the assembly of ionomer chains into a hollow spherical structure with the solvent inside. In the shell of a micelle, polar groups are densely packed and shielded from the solvent by nonpolar fragments of adjoining chains. At a low content of ionogenic groups in ionomers, two-thirds of macromolecules in solution are not incorporated in any structures. With an increase in the content of polar groups to 2.6 mol %, almost all chains are organized to small clusters—the stable pairs of macromolecules.     

Spherical and vesicular ionic aggregates in Zn‐neutralized sulfonated polystyrene ionomers

Ionic aggregates in a series of Zn‐neutralized poly(styrene‐co‐styrene sulfonate) (SPS) random ionomers have been imaged using scanning transmission electron microscopy. The Zn‐rich aggregates were found to have two shapes: solid spheres (Type I) and shells or vesicles (Type II). Type I aggregates range in a maximum diameter from 4 to 10 nm, whereas Type II aggregates range in a maximum diameter from 9 to 55 nm with a vesicle wall thickness of ∼ 3 nm. Lightly neutralized ionomers exhibited only Type I aggregates, whereas higher neutralization levels exhibited both Type I and II aggregates. Lightly neutralized ionomers also showed evidence of macrophase separation at the micron size scale. These direct observations of ionic aggregates contradict previous interpretations of small‐angle X‐ray scattering data with respect to size, size dispersity, shape, and spatial distribution. In addition, the aggregates observed in SPS differ markedly from the nearly monodisperse ∼ 2‐nm spherical aggregates observed in Zn‐neutralized poly(ethylene‐co‐methacrylic acid). The presence of vesicular aggregates encourages a re‐examination of the morphologies and properties of styrenic ionomers. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 477–483, 2001     

Molecular interpretation of miscibility in polyamide-6 blends with alkali ionomers of sulfonated polystyrene

Blends of polyamide-6 with lithium ionomers of 9.8 and 5.4 mole percent sulfonated polystyrene, formed by combining solutions of these polymers, are miscible over a wide compositional range, but those with the equivalent sodium ionomers are not. The molecular origin of this difference is addressed by studying the far infared and infrared spectra of the blends and pure materials to follow changes in the interactions between the cations and their surroundings, and changes in the interactions between functional groups. Based on analysis of these spectra, a molecular level interpretation of the blending is proposed. The initial step involves both the interaction of one amide carbonyl with an Li⁺ ion and simultaneous hydrogen bonding between an amide N? H and a sulfonate group. This eventually leads to formation of an Li(>CO)⁺_n(n ～ 4) entity while the sulfonates are converted to the acid form through hydrogen bonding to the amide N? H groups. The Na⁺ ion does not interact strongly enough with the amide groups to leave its sulfonate environment to a significant extent. © 1995 John Wiley & Sons, Inc.     

Viscoelastic properties of sulfonated styrene ionomers

The solid-state viscoelastic properties of polystyrene containing randomly distributed groups of styrene-p-sodium sulfonate are studied and compared with the corresponding properties of copolymers of styrene and sodium methacrylate (S-NaMA). The viscoelastic behavior in the primary transition region of these two ionomers is very similar. As for the S-NaMA copolymers, it is proposed that sulfonated polystyrene is composed of ion-rich regions (clusters) immersed in a matrix of low ion concentration. Two peaks are observed in the plot of mechanical loss tangent versus temperature for the sulfonated material. The lower peak is assigned to the glass transition of the ion-poor matrix and the upper to the glass transition of the clustered regions. As for some other ionomers, the presence of ions is found to slow down the stress relaxation rate, giving a broad distribution of relaxation times. Above a certain ion concentration, the sulfonated polystyrenes are thermorheologically complex owing to the onset of a secondary relaxation mechanism associated with the ion-rich regions.     

Self-organization of sulfonated polystyrene ionomers with various contents of ionogenic groups in toluene: Neutron scattering study

The structural organization of sulfonated polystyrene-based ionomers containing different amounts of sodium sulfonate groups (SO₃Na) in the concentration range from 0.5 to 5.8 mol % in deuterotoluene has been studied by cold neutron scattering. An analysis of molecular correlations shows that ionomer chains retain the Gaussian coil conformation even at a degree of sulfonation of 5.8 mol %, when the distance between ionogenic groups along a chain is comparable with the Kuhn segment length. At a content of SO₃Na groups of 1.35 mol %, the mutual penetration of polymer chains predominates and is accompanied by the formation of associates including up to seven macromolecules (effective chains) and functioning as main structural elements of the system. At a higher content of SO₃Na groups, the mutual penetration of macromolecules becomes weaker owing to the enhanced intramolecular association of ionogenic groups. As a result, the effective chains with sizes smaller than the size of the PS coil arise. These chains are composed of 1–2 ionomer macromolecules and are compacted structures in which the concentration of PS units is a factor of 4 higher than the concentration of units in the nonsulfonated PS coil. The self-organization of such particles in toluene yields secondary structures in the form of clusters composed of four to five effective chains.     

The development of conductive composite surfaces by a diffusion-limited in situ polymerization of pyrrole in sulfonated polystyrene ionomers

Electrically conductive composite surfaces were prepared by a diffusion-controlled in situ polymerization of pyrrole in the surface layer of sulfonated polystyrene ionomer films. Premolded films of the ionomer sulfonic acid derivatives were sequentially immersed in aqueous solutions of pyrrole and FeCl₃, and polymerization occurred only where both the monomer and the oxidant were present. The penetration of the polypyrrole (PPy) into the film was controlled by varying the immersion time in the monomer solution. The amount of PPy produced depended on the immersion time of the film in the monomer and the degree of sulfonation of the ionomer. Surface conductivities of 10⁻⁴-10⁻¹ S/cm were achieved with PPy concentrations from 2 to 22 wt % and composite layers as thin as 15 μm. Intermolecular interactions occurred between PPy and the ionomer by proton transfer. Incorporation of PPy also increased the tensile strength of the ionomer film, significantly increased its modulus above T_g, and inhibited melt flow. © 1997 John Wiley & Sons, Inc.     

Fluorescence and light-scattering studies on the formation of stable colloidal nanoparticles made of sodium sulfonated polystyrene ionomers

The lightly sulfonated polystyrene ionomer is only soluble in some organic solvents, such as toluene and tetrahydrofurnan (THF). The mixture of its organic solution with water normally leads to macroscopic phase separation, namely precipitation. In this study, using the steady-state fluorescence, the nonradiative energy transfer and dynamic laser light scattering, we demonstrate that the sulfonated polystyrene ionomers can form stable colloidal nanoparticles if the THF solution of the ionomers is dropwisely added into an excessive amount of water, or vice verse, water is added in a dropwise fashion into the dilute ionomer THF solution under ultrasonification or fast stirring. The hydrophobic core made of the polystyrene backbone chains is stabilized by the ionic groups on the particle surface. Such formed stable nanoparticles have a relatively narrow size distribution with an average diameter in the range of 5–12 nm, depending on the degree of sulfonation, the initial concentration of the ionomer THF solution, and the mixing order. This study shows another way to prepare surfactant-free polystyrene nanoparticles. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35 : 1593–1599, 1997     

Glass transition in partially sulfonated polystyrene

The glass transition temperature T_g of partially sulfonated polystyrene has been measured dilatometrically as a function of degree of sulfonation. A semitheoretical relationship between T_g and degree of sulfonation has been derived by treating the strong-acid polymer as a highly polar copolymer of styrene and styrenesulfonic acid. The T_g of copolymer has been found to increase linearly up to 0.15 weight fraction of styrene-sulfonic acid w_A as given by: where T_gB is the glass transition temperature of loosely crosslined (1%) polystyrene matrix. Our experimental results agree well with theoretical relations developed on the basis of the iso-free-volume state of glass transition applied to sulfonated polystyrene. The marked linear increase in copolymer T_g with the styrenesulfonic acid is accounted for by the effect of progressively higher intermolecular forces due to the highly polar sulfonic acid substituents.     

Microphase separation transition in block copolymer melts

The microphase separation transition (MST) in block copolymer melts has been studied using synchrotron SAXS. The results indicate that the MST occurs in the range of molecular weights and Flory-Huggins interaction parameters x predicted by the theory of Leibler (Ref. 5). Studies of the MST as a function of molecular weight for constant composition can be used to determine the temperature dependence of x. The observed change of macrolattice constants with temperature is somewhat different from theoretically predicted values.     

Rheology of water-in-water suspensions formed with lightly sulfonated ionomers

The formation and rheological properties of water-in-water suspensions formed by mixing a dilute nonaqueous solution containing a lightly sulfonated ionomer with an aqueous solution are described. The spheres formed via this process are composed of a very thin (approximately 2000 Å), ionically crosslinked gel membrane which separates the continuous aqueous phase from the encapsulated aqueous phase. The membrane itself is composed of the lightly sulfonated ionomer (i. e., sulfonated polystyrene) swollen with the nonaqueous solvent. Interestingly, surface tension measurements indicate that the sulfonated ionomer in this nonaqueous solvent has no significant interfacial activity. Viscometric measurements confirm that aqueous solvents containing these spheres have considerably enhanced viscosity even in the presence of high concentrations of a salt or acid. Thixotropic behavior is observed at low shear rates, whereas Newtonian behavior is observed at higher rates of shear (> 40 sec^–1). Cessation of stress reverts the viscosity to its initial value. Dilution studies show that the streamlines in the flowing exterior aqueous phase cause circulation of the aqueous fluid within the sphere. These results point to the fluid-like characteristics of the gel membrane, since transmission of the stress across the membrane is not dramatically inhibited. Comparison of the low shear rate data with the Mooney equation support these conclusion.     

Microphase separation in melts of double comb copolymers

Within the random-phase approximation, the conditions providing instability of a spatially homogeneous state of melts of double comb copolymers relative to microphase separation have been studied. As compared with the case of diblock copolymer melt connectivity between branch points of linear diblock copolymers and a backbone (densely grafted comb copolymer) is expected to assist the transition from the spatially homogeneous state to the microphase separated state; in other words, this factor decreases the incompatibility parameter at the transition point. Two characteristic types of spinodal behavior as a function of the number of repeating units have been found: one type is observed when the fragment of backbone between neighboring branch points is long; another type of behavior takes place when this distance is small. In one of the cases under study, the period of microstructure abruptly changes with variation of the interaction parameters of the system.     

Microphase separation in solutions of crystallizable segmented polyurethane

At present time the level of microphase separation together with polymer nture is considered to be a main factor in determining the physico‐mechanical properties of polyurethane materials. The researches show that realization of a certain polymer structure and properties of polymer are defined by microstructure of polyurethane chain. The purpose of this work was to study physico‐chemical regularities between nucleation and growth of microphase aggregations in crystallizable segmented polyurethane compstions as a function of the phase state of the system during transferring from comon solvent to a selective solvent. Besides we investigated the architecture of macromolecules, associates and phase particles of segmented polyurethane (SPU) formed in different regions of the phase diagram.     

Microphase separation in two-dimensional systems with competing interactions

The formation of clusters in condition of thermodynamic equilibrium can be easily observed both in two and three dimensions. In two dimensions relevant cases include pattern formation in Langmuir monolayers and ferrofluids, while in three dimensions cluster phases have been observed in colloids and in protein solutions. We have analyzed the problem within the scenario of competing interactions: typically, a short-range attractive interaction against a long-range repulsive one. This simplified approach is suggested by the fact that the forces, governing self-organization, act on a length scale which is larger than the molecular size; as a consequence many specific details of the molecules of interest are not necessary for studying the general features of microphases. We have tackled the microphase formation by simulations in bidimensional fluids, exploiting the parallel tempering scheme. In particular, we have analyzed the density range in which the particles arrange in circular domains (droplets), and the temperature range in which the system goes from microphases to the homogeneous fluid phase. As the density increases, the droplet size increases as well, but above a certain density the morphology changes and stripes are formed. Moreover at low density, we observe the formation of a liquidlike phase of disordered droplets; at higher densities, instead, the droplets tend to arrange onto a triangular superlattice. Such a change affects the features of the static structure factor, which gives well defined signatures of the microphase morphology. In each case, the specific heat exhibits a peak close to the transition from microphases to the homogeneous fluid phase, which is due to the breaking up of the clusters. The saturation of the height of the specific heat peak, with the increasing of the system size, suggests the possibility of a Kosterlitz-Thouless transition.     

The ionic conductive property of sulfonated polyethylene oxide polyurethane ionomers

A series of lithium and sodium salts of sulfonated polyethylene oxide (PEO) polyurethane ionomers in different levels of ionization were prepared. It has been found that this material is a new type of ionic conductive polymer, characterized by a single-ion transport mechanism and good mechanical properties. The ionization level significantly influences the ionic conductivity of the samples. When the mole ratio of the metal ion and ether oxygen atom is about 0.05, the ionomers exhibit maximum cationic conductivity. An optimal cationic conductivity of 1.0 × 10^?5 S/cm is obtained at about 70°C without any addition of organic plasticizer. The conductivity increases apparently when propylene carbonate and low MW PEO are added to the polyurethane ionomer. © 1995 John Wiley & Sons, Inc.     

Polybenzimidazole block sulfonated poly(arylene ether sulfone) ionomers

Novel ionomers based on polybenzimidazole block sulfonated poly(arylene ether sulfone) show excellent thermal properties. The ionic aggregation of sulfonic acid groups leads to well-developed phase separated morphology and thus high proton conductivity at wide humidity range, up to 65 mS cm(-1) at 90% relative humidity.