# Short-side-chain perfluorosulfonic acid ionomer dispersions in N-propanol / water: from nanostructures to viscosity

Jackie Burghart, Steven Meeker, Yaocihuatl Medina-Gonzalez, Guillaume Ovarlez

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Jackie Burghart, Steven Meeker, Yaocihuatl Medina-Gonzalez, Guillaume Ovarlez. Short-side-chain perfluorosulfonic acid ionomer dispersions in N-propanol / water: from nanostructures to viscosity. 2026. hal-05493414

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Short-side-chain perfluorosulfonic acid ionomer
dispersions in N-propanol / water: from
nanostructures to viscosity

Jackie Burghart, Steven Meeker, Yaocihuatl Medina-Gonzalez* and Guillaume Ovarlez*

Université de Bordeaux, CNRS, Syensqo, LOF, UMR 5258, 178 av. Schweitzer, Pessac
33600, France

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For Table of Contents use only

ABSTRACT

The multi-scale structural organization of a short-side-chain perfluorosulfonic acid (SSC PFSA) ionomer in N-propanol / water mixtures was elucidated through a combination of small-angle X-ray scattering (SAXS), dynamic light scattering (DLS), cryogenic transmission electron microscopy (cryo-TEM) and rheological analysis, to clarify its influence on catalyst ink processability for proton exchange membrane fuel cells. SAXS revealed solvent-dependent aggregate structures and polyelectrolyte-like behavior, while DLS tracked the evolution of agglomerate size upon aging. Cryo-TEM provided direct visualization of aggregate and agglomerate morphologies, supporting the findings from scattering techniques. Rheological measurements demonstrated that viscosity increases correlate with agglomerate growth and solvent composition, identifying distinct concentration regimes. These combined results enhance understanding of the relationship between ionomer structuring and rheological properties and offer new insights for optimizing ink formulations and catalyst layer quality.
# 1. INTRODUCTION

PFSA ionomers are primarily used in the fabrication of different constitutive parts of the Proton Exchange Membrane Fuel Cell (PEMFC)¹⁻³. Due to their high potential as a proton carrying network, short-side-chain perfluorosulfonic acid (SSC PFSA) ionomers are considered the principal current alternative to long-side-chain (LSC) PFSA ionomers such as Nafion, which has served as the benchmark ionomer for decades⁴⁻⁸. SSC PFSA ionomers have been the focus of numerous studies on the membrane⁹⁻¹¹ and on the catalyst layer¹,¹²⁻¹⁴ of PEMFCs. In the case of catalyst layer fabrication, the process involves catalyst ink preparation, deposition and drying¹⁵⁻¹⁹, with the ionomer structures and properties at each step critically influencing the final overall quality of the catalyst layer²⁰⁻²⁸. More particularly, the chemical structure of the ionomer can adopt very diverse morphologies and structures depending on the solvent²⁹⁻³¹, leading to variations in rheological behaviors and adsorption behaviors on the catalyst¹³,³²⁻³⁴, ultimately affecting the rest of the catalyst layer fabrication process³⁵.

Despite extensive research highlighting the effects of ionomer structures on catalyst ink processability and quality of the resulting catalyst layer, the links between structures and properties of ionomer dispersions remain poorly understood³,³⁶⁻³⁸. Notably, the use of alcohol / water solvent mixtures is common for catalyst ink formulation since they are known to tailor and enhance ink viscosity, which is sought after in some deposition processes¹⁶,²⁴,³⁹⁻⁴². While viscosity enhancement is often attributed to ionomer swelling⁴³⁻⁴⁶, studies report inconsistent results and divergent conclusions about the underlying ionomer structures²⁹,³⁰,³³,⁴³,⁴⁷. This lack of understanding hinders the rational design of ink formulations and the optimization of catalyst layer fabrication.

In this study, the structural organization of an experimental SSC PFSA ionomer supplied by Syensqo, is investigated by preparing dispersions in N-propanol / water mixtures at various concentrations. While most previous research typically focuses on either bulk rheology or nanoscale structure, the approach adopted here integrates findings across multiple length scales using three complementary techniques: small angle X-ray scattering (SAXS), dynamic light scattering (DLS) and cryogenic transmission electron microscopy (cryo-TEM).

This methodology enables the elucidation of the multi-scale organization of the ionomer and the clarification on how these structures influence the macroscopic rheology of the dispersions. Specifically, rheological measurements reveal that the flow behavior of ionomer dispersions is highly sensitive to formulation parameters and aging. They show that viscosity increases over time, that N-propanol-rich mixtures promote higher viscosity and that the ionomer exhibits polyelectrolyte-like behavior across all solvent mixtures. These findings are consistent with previous reports on similar systems, such as the work of Gupit et al.³¹ on Nafion dispersions in ethanol / water mixtures, or Khandavalli et al.⁴⁶ on a SSC PFSA ionomer in isopropanol / water mixtures.

Aggregate and agglomerate structures are then identified and characterized across the nano- and micrometric range. We define “aggregates” as the nanometric structures formed by the association of ionomer chains, and “agglomerates” refer to the larger assemblies of these aggregates. Both structures are found to be highly dependent on
formulation parameters and aging, directly influencing the rheological behavior of ionomer dispersions.

Flexible cylinder modeling is applied to SAXS data, which allows for direct quantification of the rigidity – via the Kuhn length parameter – of ionomer aggregates in various solvents. SAXS is used here to characterize nanoscopic aggregate structures and polyelectrolyte behavior. While prior studies have identified the characteristic ionomer peak in PFSA ionomer dispersions<sup>29,48,49</sup>, the wider range of scattering vectors explored in the experiments presented in this study also suggests the presence of larger agglomerate structures.

DLS is further employed to monitor the evolution of agglomerate size upon aging, with controlled oven heating revealing that ionomer dispersions are metastable systems. Such large structures can be found in some previous studies on PFSA dispersions<sup>49–51</sup>; however, no mention of agglomerate size sensitivity to oven heating or aging is found in the literature.

Finally, cryo-TEM provides direct visualization of both aggregates and agglomerates, supporting the structural hypotheses derived from scattering techniques. While the observation of aggregates by cryo-TEM is consistent with existing literature<sup>52,53</sup>, agglomerates are also successfully identified in this study. By correlating structural changes across nano- and micrometric scales with viscosity, our work offers insights that can inform the rational design and optimization of catalyst ink formulations.

This paper presents a multi-scale analysis of ionomer dispersions in N-propanol / water mixtures, combining rheological measurements, DLS, SAXS and cryo-TEM. First, the bulk rheological behavior and its dependence on solvent composition and ionomer concentration is examined. Next, the effects of thermal treatment and ambient aging on microstructure and viscosity using DLS and rheology is investigated. The nanostructural organization of ionomer aggregates via SAXS is then explored, followed by direct visualization and validation of structural hypotheses using cryo-TEM. By integrating these complementary techniques, the hierarchical organization of the ionomer is then elucidated and discussed.

## 2. EXPERIMENTAL SECTION

### 2.1. Sample preparation.

An experimental grade of SSC PFSA ionomer (25 wt % dispersed in pure water, with an equivalent weight of 790 g/mol) was provided by Syensqo Specialty Polymers Italy SpA and added to various N-propanol/water mixtures (0/100, 20/80, 40/60, 60/40 and 80/20 (wt/wt)) at various concentrations (1-25 wt %) depending on the experiment. The general chemical structure of the SSC PFSA ionomer<sup>54,55</sup> is given in Figure 1.
<figure><img src="image_3.png" /></figure>

**Figure 1.** General chemical structure of the SSC PFSA ionomer provided by Syensqo, with n ≈ 5 and m ≈ 1000.

The ionomer was incorporated at room temperature (20 °C), homogenized and stored at ambient temperature for equilibration overnight prior to any measurement. All pristine samples were clear, homogeneous dispersions of various apparent viscosities. The impact of thermal treatment on the dispersions was studied by subjecting the samples to controlled oven heating, after overnight equilibration, at 80 °C for various time periods (15 min to 24 h).

**2.2. Rheological measurements.**

Rheological measurements were performed on a Netzsch Kinexus Ultra+ rotational rheometer, using a stainless-steel smooth 60 mm cone-and-plate geometry (1 ° cone angle), equipped with a solvent trap to prevent excessive solvent evaporation and concentration variation during the experiment. All samples were allowed a 2 min resting period prior to any measurement to ensure the constant temperature of 25 ± 0.1 °C to be reached over the entire sample volume. After temperature equilibration, prior to data collection, the samples were pre-sheared at 100 s⁻¹ for 3 min to eliminate sample history. Flow curves were obtained from steady shear measurements, averaging a double logarithmic shear-rate sweep between 100 and 0.1 s⁻¹, then back to 100 s⁻¹, taking 100 points of measurement over each decade. This ensured that acceleration inertia effects were smoothed out of the flow curves and measurement precision was sufficient for low-viscosity samples (a detailed explanation is available in the Supporting Information file).
## 2.3. Small Angle X-Ray Scattering (SAXS).

SAXS was used to investigate aggregate structures within ionomer dispersions over the ranges of solvent mixtures and ionomer concentrations. SAXS was carried out using a XENOCS Xeuss 2.0 operating at 50 kV and 0.6 mA under vacuum airlock to limit adsorption and diffusion from air. The beam line, operating at 8 KeV, is equipped with a GENIX-3D copper source, FOX-3D optics centered on the Kα ray (λ = 1.54 Å) and XENOCS “anti-scattering” motorized slits. 40 µL samples were transferred to 1.5 mm diameter glass capillaries, sealed and placed in a 27-positions-autosampler in a vacuum chamber. Solvents were also analyzed under the same conditions to subtract their scattering signal from those of the corresponding samples.

The bidimensional sample detector DECTRIS PILATUS-300K was placed at three different positions to explore *q* values from about *q* = 0.004 up to 0.7 Å⁻¹, where *q* is the momentum transfer *q* = $\frac{4\pi \sin\theta}{\lambda}$ and 2θ is the total scattering angle. Three modes were thus explored and the resulting scattered intensity I was measured:

- In SAXS mode: the detector was placed at 2490 mm from the samples, covering *q* range from 0.004 to 0.15 Å⁻¹. The incident beam diameter was 0.28 mm and samples were exposed for 3 h

- In intermediary mode: the detector was placed at 1200 mm from the samples, covering *q* range from 0.01 to 0.33 Å⁻¹. The incident beam diameter was 0.4 mm and samples were exposed for 2 h

- In Wide Angle X-Ray Scattering (WAXS) mode: the detector was placed at 158 mm from the samples, covering *q* range from 0.1 to 0.7 Å⁻¹. The incident beam diameter was 0.7 mm and samples were exposed for 30 min.

The images were analyzed and diffractograms were obtained with the FOXTROT software. The data processing software SasView was then used for diffractogram analysis and modeling, using the integrated ‘flexible cylinder’ modeling function (see Supporting Information file). This method gave rise to hypotheses on aggregate structures, which were then confirmed via cryo-TEM observations.

## 2.4. Cryogenic Transmission Electron Microscopy (cryo-TEM).

Cryo-TEM was used to confirm aggregate and agglomerate structures and sizes. Cryo-TEM observations were made using the Talos L120C TEM Bright Field. Samples were prepared using 1 wt % ionomer dispersions as is. Droplets of the dispersions were deposited onto holey carbon TEM grids, blotted with filter paper, and plunge-freezed in liquid ethane within a Leica EM GP freezing chamber, resulting in a vitrified microscopic sample layer.

To improve image quality and facilitate the identification of relevant structure, the acquired cryo-TEM images were further processed using a Python-based denoising and enhancement algorithm. This post-acquisition processing reduced background noise and increased contrast.
## 2.5. Dynamic Light Scattering (DLS).

A Malvern Zetasizer NanoSeries Nano - ZS was used to carry out DLS experiments to study the impact of thermal treatment and ambient aging of 1 wt % ionomer dispersions on the mean size of agglomerate structures. All measurements were performed at a fixed scattering angle of 173 °, at a constant temperature of 20 ± 0.1 °C, after a sample equilibration time of 60 s and repeated 10 x 3 times (10 scans on a given sample and 3 different samples from the same ionomer dispersion). The position of the probe along the quartz cuvette width and the incident intensity attenuator index were automatically optimized for each measurement. The instrument used here is equipped with a numerical autocorrelator for detected light intensity. Details on signal treatment can be found in the Supporting Information file. For the sample in this study, autocorrelation functions were found to be of sufficient quality for the 0/100 and 20/80 (wt/wt) N-propanol/water mixtures. The impact of thermal treatment on agglomerate size was only studied for the 20/80 (wt/wt) N-propanol / water mixture since the flow curves indicated no impact of thermal treatment on the relative viscosity of the 4 wt % ionomer in 0/100 (wt/wt) N-propanol / water sample.

# 3. RESULTS

## 3.1. Rheological behavior of ionomer dispersions.

### 3.1.1. Shear rheology in N-propanol / water.

In the context of PEMFC catalyst layer fabrication, typical ionomer concentrations used to formulate inks do not exceed 5 wt %⁵⁶⁻⁵⁸. Hence, the present study opens with the rheological investigation of 4 wt % ionomer dispersions in N-propanol / water mixtures. Their flow curves are presented in Figure 2 and given in terms of relative viscosity ηrel = η/ηs (with ηs being the solvent viscosity, values of ηs used are available in the Supporting Information file), which enables the direct impact of the ionomer on sample viscosity to be seen.

<figure><img src="image_4.png" /><figcaption>**Figure 2.** Flow curves of 4 wt % ionomer dispersions in various N-propanol / water mixtures for samples from pristine (a) and 1 h thermally treated (b) formulations.</figcaption></figure>
Pristine dispersions all exhibit Newtonian fluid behavior, with relative viscosities increasing upon N-propanol / water ratio increase. After 1 h of thermal treatment, all samples remain in the Newtonian-fluid-like flow domain, except for the 4 wt % ionomer in 80/20 (wt/wt) N-propanol / water dispersion which exhibits a shear thinning tendency at higher shear rates. This behavior is commonly observed in polymer solutions and dispersions⁵⁹⁻⁶¹. More particularly, shear thinning is observed in the case of ionomer dispersions — especially with Nafion - and is attributed to the progressive ordering of ionomer chains along the direction of the flow and the breakdown of weak microstructures within the sample which reduces resistance to shear upon shear rate increase³¹,⁶².

A relative-viscosity increase is also observed after 1 h thermal treatment for all samples except for the 4 wt % ionomer in water. The relative viscosity increase is all the more pronounced as the N-propanol / water ratio increases.

### 3.1.2. Viscosity dependence on concentration.

To further investigate the rheology dependence on the solvent mixture, the specific viscosities $\eta_{sp} = \frac{\eta-\eta_s}{\eta_s}$ of the pristine samples displaying Newtonian behavior were studied at various ionomer concentrations. The results are reported in Figure 3. Upon primary observation, the scattered plots present two distinct scaling behaviors for all the solvent mixtures upon ionomer concentration variation ($\eta_{sp} \sim c^x$), switching from $x \sim 0.8$ to $x \sim 1.8$ or $x \sim 1.35$ depending on the solvent mixture, indicating a transition between two distinct concentration regimes, as commonly studied in the literature on polymer and ionomer solutions⁶³⁻⁶⁷. Theoretical scaling exponents for concentration regimes of neutral polymer and polyelectrolyte solutions are given in Table 1. From the predictions of scaling exponents, the transition from semi-dilute unentangled to semi-dilute entangled regime is visible on Figure 3 for all solvent mixtures.
<figure><img src="image_5.png" /><figcaption>**Figure 3.** Specific viscosities of ionomer dispersions in various N-propanol / water mixtures at various ionomer concentrations. The solid lines represent guides to the scaling behaviors ($\eta_{sp} \sim c^x$) in the dilute / semi-dilute unentangled ($x \sim 0.8$) and semi-dilute entangled ($x \sim 1.8$ or $x \sim 1.35$) concentration regimes.</figcaption></figure>

<table><caption>**Table 1.** Scaling exponents $x$ ($\eta_{sp} \sim c^x$) predicted from the theories of polymer and polyelectrolyte solutions in various concentration regimes.</caption><thead><tr><th>Concentration regime</th><th>Neutral polymers<sup>63,64</sup></th><th>Polyelectrolytes</th></tr></thead><tbody><tr><td>Dilute</td><td>~ 1</td><td>~ 0.5<sup>65</sup></td></tr><tr><td>Semi-dilute unentangled</td><td>~ 1.5</td><td>~ 0.5<sup>66,67</sup></td></tr><tr><td>Semi-dilute entangled</td><td>~ 4</td><td>~ 1.5<sup>66,67</sup></td></tr></tbody></table>

The ionomer concentration at which the regime transition appears is dependent on N-propanol / water ratio. Previous studies consistently report polyelectrolyte-like scaling in rheological studies of ionomers<sup>31,46,68</sup>, as summarized in Table 2. The experimental scaling exponents reported in Table 2 show that the behavior of the experimental SSC PFSA ionomer is more resembling that of the 3M PFSA than that of Nafion. This resemblance could, upon first examination, be interpreted as stemming from the difference in equivalent weight $EW = \frac{m_{polym}}{mol_{ionizable}}$ between SSC PFSA ionomers, such as 3M PFSA, and LSC PFSA ionomers, such as Nafion. Lower $EW$ is generally associated with higher charge density and more expanded coil conformations<sup>69</sup>, implying that the specific viscosity of the experimental SSC PFSA dispersions would increase faster than that of Nafion dispersions with concentration in similar solvent environments. However, the observed scaling exponents reported in Table 2 show the opposite trend, suggesting that the consideration of $EW$ alone is not sufficient to interpret these trends and that phenomena such as aggregation / agglomeration behaviors are probably also at play.
<table><caption>Table 2. Summary of scaling exponents found for SSC PFSA ionomer dispersions in the present study and comparison with the literature.</caption><thead><tr><th rowspan="2">Concentration regime</th><th colspan="3">SSC PFSA ionomer</th><th rowspan="2">Nafion<sup>31</sup><br>Water / ethanol</th><th rowspan="2">3M PFSA<sup>46</sup><br>Water / isopropanol</th></tr><tr><th>Water</th><th>Low-alcohol</th><th>High-alcohol</th></tr></thead><tbody><tr><td>Semi-dilute unentangled</td><td>0.8</td><td>0.8</td><td>0.8</td><td>1.25</td><td>0.4 – 0.5</td></tr><tr><td>Semi-dilute entangled</td><td>1.8</td><td>1.35</td><td>1.8</td><td>3.75</td><td>1.7 – 2.6</td></tr><tr><td>EW (g/mol)</td><td></td><td>790</td><td></td><td>1000</td><td>725</td></tr></tbody></table>

The scaling behavior of ionomer dispersions reported by Figure 3 is more similar to that of polyelectrolytes than of neutral polymers due to the presence of dissociating sulfonic acid groups on the side chains of the ionomer molecules. Deviation from theoretical ideal polyelectrolyte solutions behavior is likely due to phenomena arising from ionomer dispersion properties, such as aggregation, solvent quality or counterion effects affecting the effective concentration, chain overlap and heterogeneity within the samples.

It is likely that the dilute and semi-dilute unentangled regimes overlap in the concentration domains shown on Figure 3, as the scaling exponent alone does not allow for a clear distinction between these two regimes, as reported in Table 1. However, Rubinstein et al.⁶⁷ and Dobrynin et al.⁶⁶ explain that, in the semi-dilute regime, in contrast to neutral polymers, the reduced viscosity of salt-free polyelectrolyte solutions may decrease with increasing concentration. Thus, plotting the reduced viscosity $\eta_{red} = \frac{\eta_{sp}}{c}$ of samples against ionomer concentration c should reveal the transition between dilute and semi-dilute unentangled regimes, as seen on Figure 4. The dilute regime is characterized by a slope ~ 0, whereas the semi-dilute unentangled regime presents a negative slope.

<figure><img src="image_6.png" /><figcaption>**Figure 4.** Reduced viscosities of ionomer dispersions in various N-propanol / water mixtures at various ionomer concentrations. The dotted lines are guides to the eye.</figcaption></figure>
The concentration regimes at 4 wt % ionomer can now be evaluated for each N-propanol / water solvent mixture, as reported in Table 3, and provide insights for the discussion on the previously presented flow curves, which is developed in section 4 of this paper.

<table><caption>Table 3. Concentration regimes at 4 wt % ionomer for each N-propanol / water mixture estimated from the reduced viscosities.</caption><thead><tr><th>N-propanol / water (wt/wt)</th><th>Concentration regime at 4 wt % ionomer</th></tr></thead><tbody><tr><td>0/100</td><td>Dilute</td></tr><tr><td>20/80</td><td>Semi-dilute unentangled</td></tr><tr><td>40/60</td><td>Semi-dilute unentangled</td></tr><tr><td>60/40</td><td>Semi-dilute entangled</td></tr><tr><td>80/20</td><td>Semi-dilute entangled</td></tr></tbody></table>

## 3.2. Impact of thermal treatment on microstructure and viscosity.

### 3.2.1. DLS analysis of agglomerate size evolution.

The results of the DLS study on 1 wt % ionomer dispersion in 20/80 (wt/wt) N-propanol / water are presented in Figure 5 (a). A comparison of agglomerate size evolution with a pristine sample aged under ambient conditions is also made in Figure 5 (b).

<figure><img src="image_7.png" /><figcaption>**Figure 5.** Mean hydrodynamic diameters of agglomerates in 1 wt % ionomer dispersions in 20/80 (wt/wt) N-propanol / water over (a) various heating times for thermal treatment and (b) for pristine samples upon aging under ambient conditions. The black curve represents an exponential asymptotic fit of the scattered plot.</figcaption></figure>
As seen in Figure 5, the mean agglomerate diameter increases over time, whether it be under heating or under ambient conditions. More precisely, under controlled oven-heating conditions, as presented in Figure 5 (a), a rapid initial growth is seen, followed by a plateau indicating the reach of an equilibrium state. The data was modelled with an exponential asymptotic equation $Y = Y_0 + A \cdot e^{-\frac{t}{t_1}}$. The fit parameters are given in the Supporting Information file. An exponential asymptotic behavior of particle size upon aging in DLS is typically interpreted as an illustration of aggregation phenomena⁶⁴,⁷⁰,⁷¹. The results presented here thus suggest that ionomer agglomerates undergo further aggregation during heating until an equilibrium size is reached, which may be dependent on the chosen experimental conditions such as the ionomer concentration. The same equilibrium size seems to be attained after 75 weeks of ambient aging, suggesting that thermal treatment plays the role of accelerated aging. Previous DLS studies mention the existence of similarly sized agglomerates in PFSA ionomer dispersions⁴⁹⁻⁵¹. For example, El Kaddouri et al.⁵⁰ identified the presence of two distinct scales of Fumion ionomer structures in water, which they named “primary” and “secondary” aggregates. However, no report of agglomerate size sensitivity to heating or aging could be found in the literature. The present study therefore highlights the metastable nature of SSC PFSA ionomer dispersions and the importance of considering aging effects in the formulation and storage of catalyst inks.

### 3.2.2. Effects of thermal treatment on viscosity.

The rheological behavior of two sets of thermally treated samples - 1 and 4 wt % ionomer in 20/80 (wt/wt) N-propanol / water - was studied to examine the impact of heating time on the viscosity of ionomer dispersions under two distinct concentration regimes. As stated in section 3.1.2, 1 wt % ionomer in 20/80 (wt/wt) N-propanol / water is in the dilute regime, whereas 4 wt % ionomer in 20/80 (wt/wt) N-propanol / water is in the semi-dilute unentangled regime.

Over the various heating times, all the samples exhibited Newtonian behavior. Their relative viscosities upon heating time are given in Figure 6.
<figure><img src="image_8.png" /></figure>

**Figure 6.** Evolution of the relative viscosities of 1 and 4 wt % ionomer dispersions in 20/80 (wt/wt) N-propanol / water mixtures for increasing heating times. The error bars have been estimated from the sensitivity of the rheological measurement.

The observed behavior is radically different for the two sets of data points. At 1 wt %, a slight decrease of relative viscosity is seen upon aging followed by stabilization, whereas at 4 wt %, an initial rapid increase is observed followed by stabilization around the same heating times as for 1 wt %.

According to the DLS study, the 1 wt % ionomer dispersion in 20/80 (wt/wt) N-propanol / water contains agglomerates which undergo exponential growth upon heating followed by stabilization of the mean agglomerate size after around 400 min heating. This characteristic time is coherent with the time at which the relative viscosity reported for both series in Figure 6 stabilizes. Variations in relative viscosity are thus likely to be linked to agglomerate size variations. The trends are discussed in more detail in section 4 of this paper.

### 3.3. SAXS analysis of nanostructures.

During the SAXS experiments, the beam line enabled coverage of the scattering intensity for objects in the dispersion samples over a size range $d$ from 0.2 to 180 nm approximately, according to Bragg's law $d = \frac{2\pi}{q}$. The software used to model structures from SAXS plots is SasView, which offers a variety of ready-to-apply models, ranging from spheres, to lamellae, to custom plugin models. Based on the literature on structure determination of PFSA ionomers in solvents, the flexible cylinder was chosen to model data in this study³¹,⁵¹,⁷²,⁷³. Importantly, the use of the flexible cylinder model allows us to extract the Kuhn length of ionomer aggregates, providing direct insight into their rigidity under different solvent conditions (detailed equations for the flexible cylinder model and SasView fitting parameters are available in the Supporting Information file).
### 3.3.1. Influence of solvent ratio on aggregate structure.

The results obtained over the solvent mixtures range for 4 wt % ionomer dispersions for both pristine samples and after 1 h thermal treatment are presented in Figure 7.

<figure><img src="image_9.png" /><figcaption>**Figure 7.** SAXS profiles of 4 wt % ionomer dispersions (a) pristine and (b) after 1 h thermal treatment in various N-propanol / water mixtures. Plots have been shifted to avoid overlapping. The arrow indicates shifting of $q_{peak}$.</figcaption></figure>

Upon first observation, aqueous samples differed from N-propanol / water samples. In fact, for ionomer dispersions in water, the q-dependence of scattering intensity $I(q)$ shifted from $I \sim q^{-1}$ to $I \sim q^{-4}$ upon q increase, indicating cylindrical rod-like morphology with a well-defined structure and sharp solvent / object interfaces$^{74,75}$. In contrast, for all ionomer dispersions in N-propanol / water mixtures, $I(q)$ shifted from $I \sim q^{-1}$ to $I \sim q^{-2.5}$ upon q increase, indicating a transition from simple rods to more complex aggregated or branched structures at lower length scales$^{74}$, as similarly observed by Loppinet et al.$^{48}$.

A peak is visible on the plots at around $0.04 \AA^{-1}$, clearly identified on the 0/100 sample and fading in intensity as the N-propanol / water ratio increases. This peak is usually called polyelectrolyte peak since it is most commonly found for SAXS experiments on polyelectrolyte solutions$^{65,76,77}$. It is characteristic of charged polymer solutions and the presence of electrostatic repulsions. In the case of PFSA ionomer dispersions, this peak is called the ionomer peak and it is representative of the state of dissociation of the sulfonic acid on the end of the ionomer molecule side chains within a given solvent$^{78,79}$.

The apparent ionomer peak intensity variation seen in Figure 7 indicates that the sulfonic acid dissociation is stronger in water than in N-propanol / water mixtures, which can - at least in part - be attributed to the variations of polarity of the solvent mixtures, as reported in previous studies on the same types of systems$^{29,30}$. The same behavior is reported by Welch et al.$^{29}$ in a small angle neutron scattering study of Nafion in 2-propanol / water mixtures, or by Hoffman et al.$^{30}$ in a study of Aquivion in diacetone alcohol / water mixtures. The position of the ionomer peak $q_{peak}$ can be used to estimate the distance between ionic clusters $d_{ion}$ formed by the dissociation and ordering of the sulfonic acid
using Bragg's law: $d_{ion} = \frac{2\pi}{q_{peak}}$ and is discussed in more details in section 3.3.2 of this paper in the case of 0/100 and 20/80 (wt/wt) N-propanol / water solvent mixtures. It can primarily be noted that $d_{ion}$ is sensitive to the N-propanol / water ratio; in fact the shift of $q_{peak}$ to lower $q$ in N-propanol / water mixtures with increasing N-propanol content was attributed to swelling of the ionomer molecules, as swelling is commonly observed in PFSA dispersions$^{73,80-82}$.

A low-$q$ up-turn at $q < 0.01 \AA^{-1}$ is clearly visible on all the SAXS profiles, indicating the presence of larger structures in every sample, of sizes 100 nm and above. These structures are identified as agglomerates in this paper.

The scattering vector range between the position of the peak $q_{peak}$ and $q = 0.7 \AA^{-1}$ is representative of nanometric ionomer aggregates and is modelled using SasView with the method described in the Supporting Information file. An example of SasView flexible cylinder modelling is given in Figure 8.

<figure><img src="image_10.png" /><figcaption>**Figure 8.** SAXS profile of pristine 4 wt % ionomer in water (black) and flexible cylinder fitting (red) with set values of 20 % radius polydispersity and 400 Å contour length, fitted radius of $17.4 \pm 2.3 \AA$, fitted Kuhn length of 400 Å</figcaption></figure>

The values of radii and Kuhn lengths obtained from the modelling are represented in Figure 9. A comparison between pristine samples and samples which underwent 1 h of thermal treatment is presented.
<figure><img src="image_11.png" /><figcaption>**Figure 9.** Values of fitted radii (a) and Kuhn lengths (b) from flexible cylinder modelling with set values of polydispersity (20 %) and contour length (400 Å) on SAXS profiles of 4 wt % ionomer samples from pristine and 1 h thermally treated formulations in various N-propanol / water mixtures. For Kuhn lengths, in the case of pristine aqueous sample, the error is not computable due to the fitted value being approximately equal to the contour length.</figcaption></figure>

For the radii, several observations can be made. The aqueous sample is clearly different, exhibiting a radius approximately twice as large as the radius found in N-propanol / water samples (17 Å versus 8 – 10 Å). This suggests that, in water, ionomer aggregates remain in a more grouped form, likely due to the strong hydrophobicity of the backbone and the high degree of sulfonic acid dissociation – consistent with the pronounced ionomer peak observed in the SAXS profiles. This result is in good agreement with the findings of Loppinet et al.<sup>48</sup>, who reported that short-side-chain PFSA ionomers form rod-like colloids with a radius of 17 Å in water.

In contrast, lower radii in N-propanol / water mixtures are interpreted as the consequence of a better solvation of ionomer aggregates leading to more dispersed structures. Interestingly, other SAXS studies on similar systems do not always draw the same conclusions. For example, in alcohol / water mixtures, several studies on Nafion dispersions report aggregate radii ranging between 12.5 and 22.5 Å<sup>48,49,83</sup>, which is notably higher than those observed for in the present study under similar conditions. These differences are attributed to higher equivalent weight and longer side chains of Nafion compared to the experimental SSC PFSA ionomer, which can promote the formation of thicker aggregates.

Nonetheless, some trends appear consistent across the different systems. For instance, Yamaguchi *et al.*<sup>49</sup> found that the fitted radius of Nafion aggregates increases with higher N-propanol fraction. This trend is also observed in our samples containing N-propanol. More precisely, both studies report a minimal aggregate radius at low N-propanol mass fraction – 0.2 in this study versus 0.25 in the work of Yamaguchi *et al.*<sup>49</sup> – with a slight increase at the highest N-propanol / water ratios. This increase is attributed to the swelling of ionomer molecules, as evidenced by the shift of the ionomer peak in the SAXS data with increase of N-propanol / water ratios on Figure 7.
As for the Kuhn length, the aqueous samples are also different from the rest. Specifically, the fitted Kuhn length of the pristine aqueous sample is approximately equal to the set contour length value (400 Å), indicating that the aggregates behave as very rigid cylinders⁶⁴. In this rigid limit, the model becomes insensitive to further increases in Kuhn length, making precise estimation difficult and resulting in extremely large fitting errors which could not be plotted. For samples in N-propanol / water mixtures, the fitted radii and Kuhn length values were comparable between pristine and thermally treated samples (10 – 20 Å), indicating that the aggregate structure remains relatively stable over time in these solvent environments. Interestingly for the thermally treated aqueous sample, the fitted value of Kuhn length dropped to 100 ± 50 Å, indicating loosening of the aggregate structure. Notably, this change in rigidity was not accompanied by a decrease in aggregate radius, suggesting that the aggregates do not undergo significant fragmentation upon aging. Instead, these results point to a scenario where the aggregates maintain their overall size but become more flexible, which could possibly be due to internal structural rearrangement, increased chain mobility, or gradual plasticization by water, and highlights the metastable nature of the system.

Finally, a schematic structure was drawn from these results and is presented in Figure 10.

<figure><img src="image_12.png" /><figcaption>**Figure 10.** Schematic representation of SSC PFSA ionomer aggregate structures in water and N-propanol / water mixtures, with $l_b$ the Kuhn length, L the length of the aggregate and d the diameter of the aggregate.</figcaption></figure>
### 3.3.2. Concentration effects on aggregate structure.

This section focuses on the ionomer peak, which can be clearly identified in samples made from 0/100 and 20/80 (wt/wt) N-propanol / water mixtures, at around a scattering angle of 0.04 Å⁻¹. More particularly, the peak position $q_{peak}$ was in fact found to shift depending on the ionomer concentration. The position of the peak could thus be followed upon ionomer concentration variation. The results are shown in Figure 11 for pristine samples. The peak position upon ionomer concentration variation in both solvent mixtures was modelled using power law equations $Y = a \cdot X^b$. The fit parameters are given in the supplementary materials document.

<figure><img src="image_13.png" /><figcaption>**Figure 11.** SAXS profiles of ionomer dispersions at various concentrations c in (a) water, and (b) in 20/80 (wt/wt) N-propanol / water; and (c) effect of c on ionomer peak position $q_{peak}$ in 0/100 and in 20/80 (wt/wt) N-propanol / water. The solid lines indicate power law fittings.</figcaption></figure>
In both cases, the ionomer peak position $q_{peak}$ was found to shift towards higher $q$ upon ionomer concentration increase. This solvent-dependent shift of the ionomer peak with concentration is consistent with previous observations in PFSA ionomer systems$^{31,46,49}$. Notably, $q_{peak}$ follows a power law evolution, with scaling exponents of 0.41 and 0.47 for 0/100 and 20/80 (wt/wt) N-propanol / water mixtures respectively. These values are close to, but slightly lower than, the theoretical exponent of 0.5 expected for ideal polyelectrolyte solutions in the dilute and semi-dilute regimes$^{77}$. In fact, this difference in exponents between the two solvent mixtures – and compared to the theory – may reflect variations in solvation, aggregation, or effective charge screening. In 20/80 (wt/wt) N-propanol / water mixtures, better solvation and reduced aggregation, as discussed in the previous section of this paper, could lead to behavior closer to the ideal polyelectrolyte case, whereas in pure water, stronger aggregation or ion pairing may slightly reduce the scaling exponent.

The ionic cluster spacing $d_{ion}$, directly derived from the peak position using Bragg's law $d_{ion} = \frac{2\pi}{q_{peak}}$, also follows a power law with respect to ionomer concentration, with a negative scaling exponent and values approximately ranging from 290 to 80 Å for 0/100 and from 250 to 70 Å for 20/80 (wt/wt) N-propanol / water mixtures with increasing concentration. The cluster spacing decrease with increase of the volume fraction of ionomer is consistent with the expected densification of ionic domains at higher concentrations. Moreover, at equal concentration, it is observed that $d_{ion}$ for aqueous samples is consistently larger than for 20/80 (wt/wt) N-propanol / water samples. This indicates that ionic clusters are either farther away from one another or larger in water than in 20/80 (wt/wt) N-propanol / water. This observation aligns with the conclusions of Malek et al.$^{80}$ who, through numerical simulations of SSC PFSA ionomer structures in alcohol / water mixtures, reported that the higher diffusion coefficient of SSC PFSA ionomer in water leads to hydronium ions being solvated farther from the sulfonate groups in water than in alcohol / water mixtures, resulting in wider ionic clusters in water.

Flexible cylinder fittings in SasView were used to model the aggregate structures of ionomer dispersions in 0/100 and 20/80 (wt/wt) N-propanol / water solvent mixtures upon ionomer concentration variation, using the same method as described in section 3.1.1 and detailed in the Supporting Information file. The values of the radii and Kuhn lengths obtained from the modelling are given on Figure 12.
<figure><img src="image_14.png" /></figure>

**Figure 12.** Values of fitted radii (a) and Kuhn lengths (b) from flexible cylinder modelling with set values of polydispersity (20 %) and contour length (400 Å) on SAXS profiles of ionomer dispersions in 0/100 and 20/80 (wt/wt) N-propanol / water at various ionomer concentrations c. For Kuhn lengths, in the case of aqueous samples, errors are not computable due to the fitted value being approximately equal to the contour length. Poor data quality on the 1 wt % 20/80 (wt/wt) N-propanol / water sample did not allow for a reasonable fit.

These fittings reveal that the aggregate structure remains largely constant across the studied concentration range for both solvent mixtures, thus suggesting that increasing ionomer concentration does not significantly alter the internal structure of the aggregates. This trend has been frequently reported in the literature³¹,⁴⁸,⁸⁴⁻⁸⁶. Notably in the present case, the fitted radius is consistently larger for aqueous samples than for 20/80 (wt/wt) N-propanol / water mixtures, with values of approximately 17 Å and 9 Å respectively, as already discussed in section 3.3.1 of this paper.

For samples in 20/80 (wt/wt) N-propanol / water, Kuhn length is about 10 Å over the whole concentration range, indicating high flexibility of the ionomer rods in this solvent mixture. For the aqueous samples, the Kuhn length remains close to the set contour length, confirming the high rigidity of the aggregates, as previously discussed. Consistently, the rigid cylinder model yields fitted radii similar to those from the flexible cylinder model, as displayed by Figure 13, further supporting the interpretation of rigid rod-like ionomer structures in water.
<figure><img src="image_15.png" /><figcaption>**Figure 13.** Values of fitted radii from flexible and rigid cylinder models with set values of radius polydispersity (20 %) and contour length (400 Å) on SAXS profiles of ionomer dispersions in water at various ionomer concentrations c.</figcaption></figure>

Overall, these findings are consistent with the notion that solvent quality strongly influences aggregate flexibility, with water favoring rigid, rod-like structures and N-propanol / water mixtures promoting greater solvation and flexibility.
### 3.4. Cryo-TEM validation of SAXS and DLS-derived structural models.

#### 3.4.1. Identification of structures in cryo-TEM images.

To identify ionomer structures within cryo-TEM images of ionomer dispersions samples, pure solvent was analyzed and compared to a sample containing ionomer, as illustrated by Figure 14 for the case of 20/80 (wt/wt) N-propanol / water solvent. On each image, the darker phase was attributed to the carbon grid, while the lighter phase was attributed to the vitrified solvent phase, whose thickness gives rise to a contrasted interface between carbon grid and solvent phase. Some contrasted darker spots can be seen on several occasions on the images, which were attributed to residual ethane impurities from the freezing step.

<figure><img src="image_16.png" /><figcaption>**Figure 14.** Cryo-TEM images obtained at 45000 x magnification from (a) pure 20/80 (wt/wt) N-propanol / water solvent and (b) 1 wt % ionomer dispersion in 20/80 (wt/wt) N-propanol / water. White arrows indicate ionomer aggregate structures and the white circle indicates an ionomer agglomerate structure.</figcaption></figure>

As seen on Figure 14 (a), pure solvent appears smooth under cryo-TEM observation, whereas on Figure 14 (b), darker filaments can be distinguished under two specific forms. The first structure is a worm-like filament estimated to be of 1-5 nm diameter and 20-50 nm length when observed parallel to the sample surface, which is called ‘aggregate’ in this paper. An example of an aggregate is indicated by a white arrow on the image. The same type of structure was observed in a number of studies⁵²,⁵³. The second type of structure is called ‘agglomerate’ in this paper, resembling a star brush structure on the image, estimated to vary over a few hundred nm diameter. An example of an agglomerate has been highlighted in a white circle on the image. These images provide direct evidence of the multi-scale structural organization of ionomer dispersions.
### 3.4.2. Effect of solvent ratio on morphologies.

Cryo-TEM images at high magnification obtained from 1 wt % ionomer pristine samples in five different solvent mixtures (0/100, 20/80, 40/60, 60/40, 80/20 (wt/wt) N-propanol / water) are presented in Figure 15. For each of the solvent mixtures, the image is presented at 45000 x magnification.

<figure><img src="image_17.png" /><figcaption>**Figure 15.** Cryo-TEM images at 45000 x magnification of 1 wt % ionomer dispersions in various N-propanol / water mixtures: (a) 0/100, (b) 20/80, (c) 40/60, (d) 60/40, (e) 80/20 (wt/wt).</figcaption></figure>

As seen in section 3.1.1 of this paper, as the N-propanol / water ratio increases, the viscosity of the samples also increases, resulting in a thickening of the vitrified cryo-TEM sample layer under observation. As a result, progressive opacification of the cryo-TEM images background is seen with increase of the N-propanol / water ratio, resulting in a loss of contrast between the solvent phase and the carbon grid phase, especially at the highest N-propanol contents.

As introduced in the previous section, both aggregates and agglomerates structures can be identified on cryo-TEM images in this study. Aggregates can be seen dispersed homogeneously over the vitrified layer surface of images (a), (b) and (c) of Figure 15, whereas they can no longer be distinguished on images (d) and (e).

Agglomerate structures can be seen in all five images. They appear as particularly well-defined in images (a), (b) and (c) from low-alcohol samples, characterized by a star brush circular structure. In images (d) and (e), they appear as darker circular areas. The loss of definition could arise from the thickening of the vitrified layer. Table 4 provides a summary of the agglomerate structures size estimations over the N-propanol / water solvent range.
<table><caption>Table 4. Summary of observations from cryo-TEM images of 1 wt % ionomer dispersions in various N-propanol / water solvent mixtures.</caption><thead><tr><td>N-propanol / water (wt/wt)</td><td>Estimated agglomerate diameter (nm)</td></tr></thead><tbody><tr><td>0/100</td><td>60 - 90</td></tr><tr><td>20/80</td><td>100 - 150</td></tr><tr><td>40/60</td><td>150 - 200</td></tr><tr><td>60/40</td><td>300 - 400</td></tr><tr><td>80/20</td><td>300 - 400</td></tr></tbody></table>

The observations summarized in Table 4 reveal a clear increase in agglomerate size as the N-propanol / water ratio increases. This effect could arise from swelling or aggregation. Additionally, the overall increase in agglomerate diameter with increase of N-propanol content correlates with the observed increase in sample viscosity, as seen in section 3.1 of this paper, and may also reflect changes in the balance between hydrophobic and hydrophilic interactions.

### 3.4.3. Thermo- and aging-induced structural changes.

Cryo-TEM images at 45000 x magnification obtained from 1 wt % ionomer dispersions in 20/80 (wt/wt) N-propanol / water after 1 h thermal treatment and 6 months ambient aging are presented in Figure 16.

<figure><img src="image_18.png" /><figcaption>**Figure 16.** Cryo-TEM images at 45000 x magnification of 1 wt % ionomer dispersions in 20/80 (wt/wt) N-propanol / water, after 1 h thermal treatment (a) and after 6 months ambient aging (b).</figcaption></figure>

For the sample after 1 h thermal treatment, as seen in Figure 16 (a), the observed morphologies are very similar to those of the pristine sample presented in the previous
section, which is consistent with the SAXS results discussed previously, although the
aggregates appear to be slightly agglomerated. This suggests that short-term accelerated
aging under the conditions used here does not significantly alter the local structure of
ionomer agglomerates for this solvent mixture, at least within the resolution and field of
view of cryo-TEM.

For the sample aged 6 months under ambient conditions, as seen in Figure 16 (b), the aggregate structures appear to have mostly merged into agglomerates, with diameters ranging from 20 to 150 nm.

These findings indicate that prolonged aging can lead to the formation of more complex hierarchical structures in ionomer dispersions. This correlates with the general trend of increasing agglomeration observed via DLS and the increase in viscosity observed via rheology. In contrast, Yamaguchi et al.⁸⁶ reported that subjecting aqueous Nafion dispersions to high-temperature treatment (240°C) resulted in the breakdown of aggregates into monodisperse short cylinders, with initial aggregate sizes comparable to those observed in our study. While our relatively mild aging conditions (80°C or ambient) appear to promote aggregation and the development of more hierarchical structures, more extreme conditions could have the opposite effect, breaking down aggregates. This suggests a dual role for thermal treatment which, under mild conditions may facilitate aggregation and hierarchical structuring, whereas under more harsh conditions may disrupt and reduce aggregate size.

4. DISCUSSION

To interpret the rheological behavior of SSC PFSA ionomer dispersions, it is essential to distinguish between two levels of structural organization observed in this study. We define “aggregates” as the nanometric structures formed by the association of ionomer chains, typically characterized by SAXS. “Agglomerates” refer to larger assemblies of these aggregates, which are visualized by cryo-TEM and tracked by DLS. Both aggregates and agglomerates contribute to the macroscopic properties of the dispersions, but their roles and evolution depend on solvent composition and thermal history.

The rheological study of pristine ionomer dispersions shows that the scaling exponents
in the dilute / semi-dilute unentangled regime are equivalent for all N-propanol / water
mixtures (0.8). These results indicate that below the overlap concentration, the ionomer
molecules behave similarly under shear regardless of the N-propanol / water ratio. The
results from the structural study of ionomer dispersions however, indicate that both
aggregate and agglomerate structures are dependent on the N-propanol / water ratio.
The interactions between these structures may thus be too weak for them to impact the
specific viscosity of ionomer dispersions below overlap concentration<sup>63,64,87</sup>.
Consequently, only the hydrodynamic volume of the ionomer is at play in governing the
rheological properties of ionomer dispersions in dilute / semi-dilute unentangled samples.
It is important to note, however, that while scaling behaviors are similar, the absolute
values of the relative and specific viscosity do vary significantly with solvent composition.
This observation suggests that the hydrodynamic volume of the ionomers increases
markedly as the N-propanol content rises. The enhanced hydrodynamic volume likely
reflects changes in polymer-solvent interactions and chain conformation, leading to greater expansion of the polymer coils in mixtures with higher N-propanol fractions. Thus, even in the absence of strong inter-aggregate interactions below the overlap concentration, solvent composition exerts a pronounced influence on the effective size and rheological response of the dispersed ionomer molecules.

Above the entanglement concentration, in the semi-dilute entangled regime, the rheological study shows a dependence of the scaling exponent on the N-propanol / water ratio. Specifically, it is observed that intermediate N-propanol / water mixtures (20/80 and 40/60 (wt/wt)) display a lower scaling exponent (1.35), while both pure water and high-alcohol mixtures (60/40 and 80/20 (wt/wt) N-propanol / water) exhibit a higher exponent (1.8). This variation can be rationalized by considering the interplay between aggregate structure, solvation and polyelectrolyte effects in these different solvent environments.

Molecular dynamics simulations$^{81,88}$ provide further insight into how solvent polarity and dielectric constant modulate PFSA ionomer aggregate morphology. For example, a study by Tarokh et al.$^{81}$ shows that in low-dielectric solvents such as alcohols, PFSA ionomer chains form loose, physically cross-linked aggregates, while in high-dielectric solvents like water, aggregates become more elongated and tightly packed. In mixed solvents, microphase segregation can occur, with water preferentially localizing within ionic domains and alcohols remaining in the outer regions. These findings support our experimental observation that aggregate swelling and agglomerate formation are strongly influenced by solvent composition and highlight the molecular mechanisms underlying the macroscopic rheological trends observed in our ionomer dispersions.

In pure water, the ionomer behaves as a strong polyelectrolyte: SAXS data reveal a pronounced ionomer peak and larger ionic cluster spacing, indicating high ion dissociation and strong electrostatic repulsion between chains. This leads to more extended aggregates with a larger effective hydrodynamic volume. At the next level, these aggregates can assemble into agglomerates, but in pristine aqueous dispersions, cryo-TEM shows that agglomerate sizes remain relatively modest (see Table 5). The dominance of expanded aggregates with high hydrodynamic volume enhances the viscosity increase with concentration, reflected in the higher scaling exponent.

In high-alcohol mixtures, although ion dissociation is reduced compared to water, as evidenced by the diminished ionomer peak in SAXS, the aggregates experience significant swelling due to improved solvation of the hydrophobic backbone$^{46,89}$. These swollen aggregates further assemble into large agglomerates, as confirmed by cryo-TEM images showing agglomerate diameters reaching 300-400 nm at high N-propanol content (see Table 5). The presence of such large agglomerates, with their substantial hydrodynamic volume, again promotes a steep increase in viscosity with concentration, similar to the behavior observed in pure water, albeit via a different mechanism.

By contrast, in intermediate N-propanol / water mixtures, the balance between solvation and electrostatic interactions leads to better-solvated aggregates. Here, the aggregates are smaller and more likely to overlap or interpenetrate, and the formation of large agglomerates is less pronounced, as seen in cryo-TEM. This structural arrangement correlates with the lower scaling exponent observed in these mixtures, as the viscosity increases less sharply with concentration.
The role of electrostatic interactions in stabilizing PFSA dispersions has recently been revisited by Srivastav *et al.*⁹⁰, who developed a theoretical framework for the “single-ion” regime relevant to PFSA systems without added electrolyte. Their work demonstrates that the electrostatic repulsion between aggregates, governed by protons as the sole counterions, is much stronger than predicted by classical Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. This enhanced repulsion can stabilize aggregates at low concentrations but as concentration or alcohol content increases, the balance between repulsive and attractive forces shifts, favoring aggregation or agglomeration. Furthermore, ion-pairing between sulfonate groups and counterions becomes more favorable in low-dielectric solvents, reducing effective charge and electrostatic stabilization, and thus promoting closer association of the polymer chains. This mechanism provides a molecular-level explanation for the solvent-dependent aggregation phenomena observed in our ionomer dispersions.

Overall, these findings highlight that the scaling behavior of SSC PFSA ionomer dispersions is governed by the combined effects of ion dissociation, aggregate swelling, agglomerate formation, and the interplay of electrostatic and ion-pairing interactions - all of which are tunable by the solvent composition. The observed trends in viscosity scaling exponents are thus a direct manifestation of the underlying multi-scale organization. Moreover, a direct correlation is observed between agglomerate size and viscosity: cryo-TEM images show that agglomerate diameters increase from approximately 100 nm in water to about 300 nm in 80/20 (wt/wt) N-propanol / water, corresponding to nearly a ninefold increase in effective volume. Consistently, rheological measurements reveal that the viscosity of 1 wt % ionomer dispersions increases by about an order of magnitude under the same solvent conditions. This correlation supports the interpretation that agglomerate growth is a key driver of viscosity enhancement.

Building on this understanding of the multi-scale structure in pristine dispersions, the impact of thermal treatment and aging on agglomerate formation and rheological properties can next be examined. The range of mean hydrodynamic diameters measured in the DLS studies is consistent with agglomerate sizes observed on cryo-TEM images of the corresponding samples, as seen in section 3.4 of this paper. Thus, the objects detected by DLS have been interpreted to be ionomer agglomerates.

The rheological study of thermally treated ionomer dispersions shows a powerful effect of heating on the flow behavior of samples with increasing N-propanol / water ratio at 4 wt % ionomer concentration. As identified in section 3.1.2, increasing the N-propanol / ratio induces a shift in the overlap concentration to lower values, which correlates with reports of swelling effect of alcohol in ionomer dispersions⁴³,⁴⁴. As a result, in N-propanol-rich mixtures, the system more readily enters the semi-dilute unentangled and entangled regimes, where aggregates and agglomerates are close enough to interact significantly. Therefore, the impact of heating on ionomer dispersion viscosity becomes more prominent as the concentration regime switches from dilute to semi-dilute unentangled and entangled, implying strengthening of interactions between ionomer structures. This effect is enhanced by the structural changes induced with thermal treatment, as shown via DLS and cryo-TEM.
It is plausible that thermal treatment further modifies the solvation environment and the balance of electrostatic and attractive interactions described by Srivastav et al.⁹⁰, potentially enhancing ion-pairing and promoting agglomeration. Upon heating, changes such as partial dehydration or altered solvent organization can reduce the dielectric constant of the medium and disrupt the solvation shells around ionic groups. This favors closer association between sulfonate groups and counterions, which decreases the effective charge on the polymer chains and weakens electrostatic repulsion. As a result, aggregates are more likely to come together and form larger agglomerates, consistent with the observed increase in viscosity and agglomerate size in thermally treated samples in our study.

The variations in relative viscosity upon heating are thought to be a consequence of ionomer aggregation, since the characteristic times are coherent between mean size and relative viscosity measurements, as seen in section 3.2. Based on the observed trends it can be hypothesized that the process of agglomeration may result in structures that occupy a smaller hydrodynamic volume than the sum of their constituent individual aggregates. This densification could explain why, for samples in the dilute regime, such as 1 wt % ionomer in 20/80 (wt/wt) N-propanol / water, the total hydrodynamic volume decreases as aggregation proceeds upon heating, leading to a decrease in relative viscosity as observed in Figure 6. In contrast, at higher concentrations, such as 4 wt % ionomer in 20/80 (wt/wt) N-propanol / water, the system enters the semi-dilute unentangled regime. Here, even if agglomeration leads to some densification, the larger agglomerates are now sufficiently close that their effective hydrodynamic volumes overlap and their collective interactions become significant. The increased crowding and the possibility of transient network formation or partial jamming restrict the mobility of the dispersion, resulting in an overall viscosity increase. In summary, at low concentrations, densification upon agglomeration can reduce viscosity by decreasing the effective hydrodynamic volume. However, as concentration increases, the dominant factor shifts to collective interactions and crowding among agglomerates, which leads to a rise in viscosity despite any densification within individual structures. While this interpretation remains a hypothesis, it is supported by cryo-TEM images, which show that agglomerates appear denser and more compact than individual aggregates.

Interestingly, while DLS measurements revealed changes in mean agglomerate size upon aging, cryo-TEM did not allow for property distinction between pristine and thermally treated samples. These observations underline the fact that the local observation of individual objects and their morphologies via cryo-TEM is not sufficient to explain the bulk behavior and microscopic structures. The apparent discrepancy between agglomerate bulk, aggregate bulk and local observations highlights the importance of multi-scale characterization. It is possible that subtle changes in agglomerate number or size distribution are not captured in the limited field of view of cryo-TEM under the current imaging conditions. Future studies could employ cryo-TEM in screening mode to obtain statistical data on agglomerate number and size, providing a more comprehensive link between bulk and local structural evolution. Unfortunately, due to time constraints, cryo-TEM observations could not be performed across the entire range of thermally treated samples studied by DLS, which would have further strengthened this analysis.
## 5. CONCLUSIONS

In this study, we established a multi-scale understanding of an experimental SSC PFSA ionomer's structural organization in N-propanol / water mixtures by integrating rheological measurements, SAXS, DLS and cryo-TEM. This comprehensive approach revealed that increases in viscosity with solvent composition and aging are directly linked to agglomerate growth and aggregate formation, as demonstrated by the complementary scattering and imaging techniques.

Our findings show that both solvent ratio and aging induce hierarchical structuring, with evidence for the emergence of larger agglomerates upon prolonged aging. This hierarchical organization, spanning from nanometric aggregates to micrometric agglomerates, governs the rheological behavior critical for catalyst ink processability.

Notably, we provide evidence for the sensitivity of aggregate rigidity and agglomerate size to environmental conditions, highlighting the complex interplay between molecular structure, dispersing solvent and macroscopic properties. The integration of recent theoretical and simulation advances further clarifies the roles of solvent polarity, electrostatic interaction and ion-pairing in driving these structural transitions.

These integrative insights bring new elements for optimizing ink formulations and improving the performance of fuel cell components. Future studies could extend this approach to operational or device-relevant conditions, explore the influence of counterion type, or employ advanced *in situ* imaging and spectroscopy to unravel the dynamic mechanisms by which thermal treatment and aging modulate aggregate structure and properties in PFSA ionomer dispersions.

## ACKNOWLEDGMENTS

This article is based entirely on work carried out as part of the doctoral thesis of Jackie Burghart. The authors gratefully acknowledge Syensqo (formerly Solvay) for financial support of this PhD project and for providing the SSC PFSA ionomer. We thank Ahmed Bentaleb (CRPP) for access to SAXS facilities, for performing the SAXS measurements and providing the initial diffractograms, Olivier Lambert and Sisareuth Tan (CBMN) for access to cryo-TEM facilities and sample preparation, and Frédéric Nallet (CRPP) for fruitful discussions on SAXS experiments. This work benefited from the use of the SasView application, originally developed under NSF Award DMR-0520547. SasView also contains code developed with funding from the EU Horizon 2020 programme under the SINE2020 project Grant No 654000.
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