Sulfonated Polystyrene Ionomers Research Articles

Rapid Precipitation of Ionomers for Stabilization of Polymeric Colloids.

Polymeric colloids have shown potential as "building blocks" in applications ranging from formulations of Pickering emulsions and drug delivery systems to advanced materials, including colloidal crystals and composites. However, for applications requiring tunable properties of charged colloids, obstacles in fabrication can arise through limitations in process scalability and chemical versatility. In this work, the capabilities of flash nanoprecipitation (FNP), a scalable nanoparticle (NP) fabrication technology, are expanded to produce charged polystyrene colloids using sulfonated polystyrene ionomers as a new class of NP stabilizers. Through experimental exploration of formulation parameters, increases in the ionomer content are shown to reduce the particle size, mitigating a significant trade-off between the final particle size and inlet concentration; thus, expanding the processable material throughput of FNP. Further, the degree of sulfonation is found to impact stabilization with optimal performance achieved by selecting ionomers with intermediate (2.45-5.2 mol %) sulfonation. Simulations of single ionomer chains and their arrangement in multicomponent NPs provide molecular insights into the assembly and structure of NPs wherein the partitioning of ionomers to the particle surface depends on the polymer molecular weight and degree of sulfonation. By combining the insights from simulations with diffusion-limited growth kinetics and parametric fits to experimental data, a simple design formulation relation is proposed and validated. This work highlights the potential of ionomer-based stabilizers for controllably producing charged NP dispersions in a scalable manner.

Brittle-to-Ductile Transition of Sulfonated Polystyrene Ionomers.

This study examines the brittle-to-ductile transition of sulfonated polystyrene ionomers (SPS) with different counterions. The polystyrene precursor was unentangled and had two ionic groups per chain on average. Thus, its terminal relaxation time was comparable to the lifetime of the associating ionic groups. Three types of ionomer samples were used to tune the association lifetime: (1) fully neutralized SPS with different alkali-metal counterions, (2) fully neutralized SPS with mixed sodium and cesium counterions, and (3) partially neutralized SPS with sodium or cesium counterions. For all three systems, the brittle-to-ductile transition could be represented by a diagram of two Weissenberg numbers, Wi and WiR, defined with respect to the terminal and Rouse relaxation times, respectively. A flowable region existed at sufficiently low Wi, independent of WiR. At higher Wi, a brittle-to-ductile transition of the ionomer melt occurred above a critical value of WiR. To achieve ductility during the application of rapid elongational flow, the Rouse-type motions should be sufficiently slow relative to the rate of ion-dissociation, so that the strain-induced breakup of the ionic cross-links would not cause very strong chain retraction that may further lead to the macroscopic fracture.

Rheological properties of sulfonated polystyrene ionomers at high-ion contents

Structure and dynamics are examined for unentangled randomly-sulfonated polystyrene ionomer samples with cesium as counterion, using the X-ray scattering, DSC, and linear viscoelasticity (LVE) measurements. The large size of cesium cation leads to low association energy, enabling the dynamics to be examined for the high-ion-content samples, i.e., samples having a fraction of ionized monomers p up to 19%. X-ray scattering data shows an ionic peak, indicating the formation of ion aggregates. DSC reveals the glass transition process that shifts to higher T and broadens significantly when the ion content p is higher than 10%. In accordance with this broadening, the glassy and rubbery regimes seen in the LVE merge into one broad process with a wide relaxation distribution. We propose that the transition occurs when the number of ionic groups per chain becomes comparable to that of Kuhn segments per chain so that the polymer strands between ionic groups become non-flexible.

Effect of zinc stearate on the properties of a sulfonated polystyrene ionomer

The properties of compounds of zinc stearate (ZnSt) and the zinc salt of a lightly sulfonated atactic polystyrene ionomer are described. ZnSt was miscible with the ionomer glass at concentration <5 wt. %. The presence of molten ZnSt significantly lowered the terminal relaxation time and the melt viscosity of the ionomer, which greatly improved its melt processability. The effects of the ZnSt addition on the mechanical properties of the ionomer were uncertain. The addition of ZnSt lowered the tensile modulus of the ionomer but improved the tensile strength for compounds with ≤10 wt. % ZnSt. The addition of ZnSt embrittled the compound due to the phase separation of crystalline ZnSt. However, the general conclusion of the effect of ZnSt on the mechanical properties was clouded by the fact that less than half of ZnSt crystallized due to the very slow crystallization kinetics of the fatty acid in the compound.The properties of compounds of zinc stearate (ZnSt) and the zinc salt of a lightly sulfonated atactic polystyrene ionomer are described. ZnSt was miscible with the ionomer glass at concentration <5 wt. %. The presence of molten ZnSt significantly lowered the terminal relaxation time and the melt viscosity of the ionomer, which greatly improved its melt processability. The effects of the ZnSt addition on the mechanical properties of the ionomer were uncertain. The addition of ZnSt lowered the tensile modulus of the ionomer but improved the tensile strength for compounds with ≤10 wt. % ZnSt. The addition of ZnSt embrittled the compound due to the phase separation of crystalline ZnSt. However, the general conclusion of the effect of ZnSt on the mechanical properties was clouded by the fact that less than half of ZnSt crystallized due to the very slow crystallization kinetics of the fatty acid in the compound.

Rheological Behavior of Partially Neutralized Oligomeric Sulfonated Polystyrene Ionomers

The linear viscoelastic (LVE) behavior of partially neutralized oligomeric sulfonated polystyrene (SPS) ionomers with different degrees of sulfonation (p) and degrees of neutralization (x) was investigated. The ionic dissociation time, τs, obtained from the reversible gelation model [Chen Macromolecules 2015, 48, 1221−1230] is mainly controlled by the neutralization degree, x, rather than the functional group (i.e., sulfonic acid and metal sulfonate) concentration, p. For a fixed p, increasing x significantly increases τs and the zero shear viscosity, η0, especially near complete neutralization. These results explain the observations reported by Lundberg et al. [Ions in Polymers; American Chemical Society: 1980; Vol. 187, pp 67−76] that the increase of the viscosity of SPS ionomers with neutralization undergoes a substantial increase between 90% and 100% neutralization of the sulfonic acid groups to metal salts. This rapid increase of τs and η0 is probably related to the decrease of sulfonic acid groups i...

Nonlinear Rheology of Random Sulfonated Polystyrene Ionomers: The Role of the Sol–Gel Transition

The linear and nonlinear rheological behaviors of nonentangled sulfonated polystyrene (SPS) ionomers near the sol–gel transition were studied. When the degree of sulfonation, p, was below the gel point, the ionomer exhibited sol-like linear viscoelastic (LVE) behavior, and shear thinning was observed for steady shear flow. For p close to the gel point, the ionomer showed power-law-like LVE behavior over a wide frequency range. Strain hardening and shear thickening behavior were observed, and their magnitudes depended on the temperature, molecular weight of the PS precursor, and the Coulomb energy of the ion pair. Above the gel point, a distinct rubbery plateau was observed in the dynamic modulus. Melt fracture occurred upon start-up shear, which prevented quantitative examination of the nonlinear rheology. The possible mechanisms for strain hardening and shear thickening near the gel point are discussed with respect to formation of large clusters that nearly percolate in space.

Reversible Gelation Model Predictions of the Linear Viscoelasticity of Oligomeric Sulfonated Polystyrene Ionomer Blends

The linear viscoelastic (LVE) behavior of oligomeric sulfonated polystyrene ionomers (SPS) and binary blends of two SPS ionomers with different sulfonation levels and cations was compared to the predictions of the reversible gelation model for the rheology of ionomers [Macromolecules 2015, 48, 1221−1230]. Binary blends had the same gel point as the neat ionomer components if a linear mixing rule was used to calculate an average sulfonation level for the blend. The binary blends, however, exhibited a broader relaxation time distribution than the neat ionomers having the same number density of ions. A linear mixing rule for the ionic dissociation frequency of the blend was proposed, and when incorporated into the reversible gelation model, reasonable predictions of the terminal relaxation time of the blends were achieved.

Nonlinear Rheology of Lightly Sulfonated Polystyrene Ionomers

The effects of nonlinear deformation on the melt rheology of low molecular weight lightly sulfonated polystyrene ionomers (SPS) were investigated with dynamic, steady shear, and transient shear exp...

Sulfonation Distribution in Sulfonated Polystyrene Ionomers Measured by MALDI-ToF MS

Matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-ToF MS) was used to quantify the sulfonation level and sulfonation distribution of sulfonated polystyrene ionomers prepared by homogeneous solution sulfonation. The sulfonation levels obtained by MALDI-ToF MS and acid-base titration were compared, and the sulfonate distributions determined by MALDI-ToF MS were compared with theoretical random distributions. The results indicate that the sulfonation reaction used produces a sample with a random sulfonate distribution.

Effects of the cation valence on the mechanical properties of sulfonated polystyrene ionomers containing dicarboxylate salts

The dynamic mechanical properties of Ba(II)-neutralized poly(styrene-co-styrene sulfonate) ionomers-containing Ba salt of aliphatic diacid were investigated and compared with those of Na(I)-neutralized ionomers-containing Na diacid salt. It was found that the addition of diacid salts led to a negligible change in the matrix and cluster Tgs of the Ba and Na ionomers regardless of the chain length of the aliphatic diacid salts. The log E′ of Ba ionomers, however, increased linearly with increasing chain length of Ba diacid salt, which was different from that of Na ionomers. On increasing wt% of the salts in the ionomers, the matrix Tgs of Ba and Na ionomers did not change, but the cluster Tgs increased slightly for Ba and Na ionomers, except for the Na ionomer-containing C16 diacid salt. In addition, the log E′ increased linearly with the wt% of diacid salts, but the increasing rate was higher for the Ba ionomers, compared to Na ionomers. Thus, it was suggested that the Ba diacid salts would face more difficulty in the formation of multiplet with the ionic groups of the ionomers, compared to Na ionomer system; thus, more Ba diacid salts were phase-separated, which showed a stronger filler effect. In addition, since the Ba ionomers did not flow easily above the cluster Tg of the ionomers, it was proposed that the multiplets of Ba ionomers acted still as effective physical cross-links above cluster Tg, to some extent. Finally, it was concluded that the difference in anionic groups, the increasing amount of Ba salts, and the divalent cation enhanced the filler effects strongly.

A Facile Method for Preparing Sticky, Hydrophobic Polymer Surfaces

Textured surfaces consisting of nanometer- to micrometer-sized lightly sulfonated polystyrene ionomer (SPS) particles were prepared by rapid evaporation of the solvent from a dilute polymer solution-cast onto silica. The particle textured ionomer surfaces were prepared by either spin-coating or solution-casting ionomer solutions at controlled evaporation rates. The effects of the solvent used to spin-coat the film, the molecular weight of the ionomer, and the rate of solvent evaporation on the surface morphology of cast films were investigated. The surface morphologies were consistent with a spinodal decomposition mechanism, where the surface first existed as a percolated-like structure and then ripened into droplets if molecular mobility was retained for sufficient time. The SPS particles or particle aggregates were robust and resisted separation from the surface even after annealing at 120 °C for 1 week. The water contact angles on as-prepared surfaces were relatively low, ∼90°, due to the polar groups in the ionomer, but when the surface was modified by chemical vapor deposition of 1H,1H,2H,2H-perfluorooctyltrichlorosilane, the surface contact angles increased to ∼109° on smooth surfaces and up to ∼140° on the textured surfaces. Although the surfaces were hydrophobic, the contact angle hysteresis was relatively high and water droplets stuck to these surfaces even when the surface was turned upside down.

Linear Viscoelastic and Uniaxial Extensional Rheology of Alkali Metal Neutralized Sulfonated Oligostyrene Ionomer Melts

The linear viscoelastic and nonlinear extensional behavior of melts of alkali metal salts of oligomeric sulfonated polystyrene (SPS) ionomers were characterized by dynamic shear and nonlinear, uniaxial extensional flow experiments. The oligomeric SPS had a weight-average molecular weight of 4000 g/mol, a polydispersity index of 1.06, and a degree of sulfonation of 6.5 mol %. The molecular weight was below the entanglement molecular weight of PS, so all rheological effects were due to association of the ionic dipoles and nanophase separation of the ionic species that provides a transient elastic network. The SPS salts exhibited linear viscoelastic properties similar to well-entangled polystyrene (PS) melts, with a distinct rubbery region that had a shear modulus comparable to that of high molecular weight PS. Time–temperature superposition failed as a consequence of overlapping relaxations for the terminal response of the chain and ion hopping of the ionic dipoles. Unlike entangled PS melts, the modulus of the ionomer increased with increasing extensional strain rate, and a maximum in the stress occurred at a relatively low Hencky strain that was nearly independent of strain rate. The maximum in the stress during stretching was attributed to a catastrophic failure of the physical ionic network. At sufficiently high stress, the chains can pull the ionic groups out of nanophase-separated ionic domains, which significantly disrupts the network microstructure.

New Insights into Ionic Aggregate Morphology in Zn-Neutralized Sulfonated Polystyrene Ionomers by Transmission Electron Tomography

The three-dimensional microstructure of ionic aggregates in Zn-neutralized sulfonated polystyrene ionomers was characterized, for the first time, by transmission electron tomography. Two methods were used in order to prepare ionomer films: the hot-pressing and solvent-casting. With the first method, spherical aggregates with a mean diameter of about 5 nm are observed within the material. They are heterogeneously dispersed within the matrix and tend to form a network. When the second processing method is used, a nanoplatelet shape (with a thickness of about 6 nm) is unambiguously observed for the ionic aggregates. A preferential alignment of these aggregates parallel to the film plane is induced by this method. The solvent-casting process is a very slow method where the medium viscosity is gradually increasing; such an aggregate morphology is assumed to represent a complete aggregation state for the ionic groups.

Influence of Cation Type on Structure and Dynamics in Sulfonated Polystyrene Ionomers

The structure and dynamics of Na-, Cs-, and Zn-neutralized sulfonated polystyrene (SPS) ionomers were investigated using scanning transmission electron microscopy (STEM), X-ray scattering, and dielectric relaxation spectroscopy. Aggregates ∼2 nm in diameter were revealed by STEM and X-ray scattering. Additional information on aggregate diameter, radius of closest approach, and number density were determined by fitting the scattering data with a modified hard sphere model. A single broad segmental relaxation was identified in dielectric loss spectra for the ionomers neutralized with monovalent ions Cs and Na. The relaxation time of the segmental process was found to slow down systematically with increasing degree of sulfonation but is not significantly affected by monovalent ion type. For Cs-neutralized SPS, two high temperature relaxations associated with Maxwell–Wagner–Sillars and ion association/disassociation dynamics were identified.

Structure and Dynamics of Zinc-Neutralized Sulfonated Polystyrene Ionomers

Scanning transmission electron microscopy (STEM), X-ray scattering, and dielectric relaxation spectroscopy were used to investigate the structure and dynamics of Zn-neutralized sulfonated polystyre...

Self-organization of sulfonated polystyrene ionomers in low-polarity solvents

The self-organization of ionomers based on sulfonated polystyrene containing sodium sulfonate groups SO3Na in chloroform, deuterated toluene, and deuterated benzene is studied by the method of small-angle cold neutron scattering. It is shown that in chloroform, at a content of ionogenic groups of 1.35 mol %, chains undergo coil-to-globule conformation transformations owing to their electrostatic interactions. An increase in the amount of SO3Na groups to 2.6 mol % leads to the assembly of ionomer chains into a hollow spherical structure with the solvent inside it (vesicle). Ionomer chains in chloroform join and form stable pairs. The structural organization of ionomers containing different amounts of SO3Na groups (0.5–5.8 mol %) in deuterated toluene manifests itself as the formation of effective chains comprising up to seven macromolecules and functioning as the main structural elements of the system. Chains preserve the conformation of the Gaussian coil even at a degree of sulfonation of 5.8 mol %, when the distance between ionogenic groups along the chain is comparable with the length of the Kuhn segment. The effective chains of ionomers with a high content of ionogenic groups are characterized by sizes smaller than PS coils; they are composed of one to two macromolecules, and their density is increased. The self-organization of such species in deuterated toluene facilitates formation of secondary structures, namely, clusters consisting of four to five effective chains. In deuterated benzene, ionomer chains form supramolecular structures (associates) that behave as Gaussian chains and include 13–18 macromolecules at moderate and high degrees of sulfonation of PS. When the centers of gravity approach each other at a distance of the diameter (40–70 nm), associates come into contact and give rise to clusters. It is found that, at the scale of segments within the chain segment, the rigidity of chains is increased due to formation of intermolecular physical crosslinks between chains arising from electrostatic dipolar interactions of ionogenic groups.

Conformational properties of chains and supramolecular structures of sulfonated polystyrene ionomers in D-benzene

The method of small-angle cold-neutron scattering is used to study the conformational properties of chains of sulfonated polystyrene-based ionomers containing different amounts of sodium sulfonate groups SO3Na (0.5, 1.35, and 2.6 mol %) and the character of their self-organization in D-benzene. An analysis of the momentum transfer dependence of scattering intensity shows that the conformation of ionomer chains remains close to the Gaussian conformation, regardless of the content of ionogenic groups. It is found that, in local portions within a segment, the rigidity of chains increases owing to formation of physical crosslinks between chains arising from electrostatic interactions of ionogenic groups. Interchain interactions involving multiplets of ionogenic groups lead to the self-organization of ionomers into supramolecular structures (associates) that manifest themselves as Gaussian chains and include 13–18 macromolecules at moderate and high degrees of sulfonation of PS. When the centers of gravity approach each other at the distance of a diameter (40–70 nm), the associates come into contact and give rise to clusters.

Enhanced crystallization of bisphenol-A polycarbonate by organoclay in the presence of sulfonated polystyrene ionomers

Polycarbonate (PC)/sulfonated polystyrene (SPS) ionomer/organoclay nanocomposites were prepared by a solution intercalation process using the SPS ionomer as a compatibilizer. The effect of an organoclay on the melt crystallization behavior of the ionomer compatibilized PC were examined by differential scanning calorimetry (DSC). The melt crystallization behavior of PC was dependent on the extent of organoclay dispersion. The effect of the ionomer loading and cation size on intercalation/exfoliation efficiency of the organoclay in PC/SPS ionomer matrix was also studied using wide angle X-ray diffraction (WAXD) and transmission electron microscopy (TEM). Dispersion of the organically modified clay in the polymer matrix improved with increasing ionomer compatibilizer loadings and cation size. The SPS ionomer compatibilized PC/organoclay nanocomposite showed enhanced melt crystallization compared to the SPS ionomer/PC blend. Well dispersed organoclay nanocomposites showed better crystallization than the poorly dispersed clay nanocomposites. These nanocomposites also showed better thermal stability than the SPS ionomer/PC blend.

Dynamic mechanical and morphological studies of styrene-co-methacrylate and sulfonated polystyrene ionomers containing aliphatic dicarboxylate salts

This study examined the effects of the sodium salts of aliphatic dicarboxylic acids (DCAs) on the dynamic mechanical properties and morphology of two sets of poly(styrene-co-sodium methacrylate) (MNa) and poly(styrene-co-sodium styrenesulfonate) (SNa) ionomers. When the DCA content was relatively low, the ionic moduli of the MNa and SNa ionomers increased but the matrix and cluster glass transition temperature (T g ) did not change significantly. The increasing ionic modulus was almost independent of the type of the ionic groups of the ionomer, and the chain length of DCAs. When a large amount of the sodium succinate (DCA4) was added to the MNa and SNa ionomers, the ionic moduli of the two ionomers increased strongly but the matrix and cluster T g ’s increased slightly and significantly, respectively. In the case of sodium hexadecanedioate (DCA16), DCA16 increased the ionic moduli of the two ionomers. The addition of DCA16 changed the matrix and cluster T g ’s of the MNa ionomer slightly, but decreased the cluster T g of the SNa ionomer significantly with no change in the matrix T g . In addition, the DCA-containing ionomers showed an X-ray diffraction peak indicating the presence of ordered domains of DCAs in the ionomers. Hence, DCA4 acts mainly as a reinforcing filler in MNa and SNa systems. In the case of DCA16, it initially behaved like a filler but also functioned as a preferential plasticizer for the clusters at high content.

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.