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 (<i>p</i>) and degrees of neutralization (<i>x</i>) was investigated. The ionic dissociation time, τ_s, obtained from the reversible gelation model [Chen Macromolecules 2015, 48, 1221−1230] is mainly controlled by the neutralization degree, <i>x</i>, rather than the functional group (i.e., sulfonic acid and metal sulfonate) concentration, <i>p</i>. For a fixed <i>p</i>, increasing <i>x</i> significantly increases τ_s 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 τ_s and η₀ is probably related to the decrease of sulfonic acid groups in the ionic aggregates with increasing <i>x</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, <i>p</i>, was below the gel point, the ionomer exhibited sol-like linear viscoelastic (LVE) behavior, and shear thinning was observed for steady shear flow. For <i>p</i> 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

Viscoelasticity of Reversible Gelation for Ionomers

Linear viscoelasticity (LVE) of low-ion-content and low-molecular-weight (nonentangled) randomly sulfonated polystyrene shows a sol–gel transition when the average number of ionic groups per chain approaches unity. This transition can be well understood by regarding the number of ionizable sites over a chain as the relevant functionality for cross-linking. For ionomers below but very close to the gel point, the LVE shows power law relaxation similar to gelation of chemical cross-linking. Nevertheless, ionomers near and beyond the gel point also show terminal relaxation not seen in chemically cross-linking systems, which is controlled by ionic dissociation. Careful analysis of the power law region of the frequency dependence of complex modulus close to the gel point shows a change in exponent from ∼1 at high frequency to ∼0.67 at low frequency, which strongly suggests a transition from mean-field to critical percolation known as the Ginzburg point. A mean-field percolation theory by Rubinstein and Semenov for gelation with effective breakup has been modified to include critical percolation close to the gel point and predicts well the observed LVE of lightly sulfonated polystyrene oligomers

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

Efficient Polymer Solar Cells by Lithium Sulfonated Polystyrene as a Charge Transport Interfacial Layer

In this paper, we report the highly efficient bulk heterojunction (BHJ) polymer solar cells (PSCs) with an inverted device structure via utilizing an ultrathin layer of lithium sulfonated polystyrene (LiSPS) ionomer to reengineer the surface of the solution-processed zinc oxide (ZnO) electron extraction layer (EEL). The unique lithium-ionic conductive LiSPS contributes to enhanced electrical conductivity of the ZnO/LiSPS EEL, which not only facilitates charge extraction from the BHJ active layer but also minimizes the energy loss within the charge transport processes. In addition, the organic–inorganic LiSPS ionomer well circumvents the coherence issue of the organic BHJ photoactive layer on the ZnO EEL. Consequently, the enhanced charge transport and the lowered internal resistance between the BHJ photoactive layer and the ZnO/LiSPS EEL give rise to a dramatically reduced dark saturation current density and significantly minimized charge carrier recombination. As a result, the inverted BHJ PSCs with the ZnO/LiSPS EEL exhibit an approximatively 25% increase in power conversion efficiency. These results indicate our strategy provides an easy, but effective, approach to reach high performance inverted PSCs