Interface chemical mapping of sulfonated cellulose
nanocrystal/sPEEK nanocomposites

João Paulo Cosas Fernandes, Oussama Hamzah, Maxime Gondrexon, Padraic O'Reilly, Hakima Mendil-Jakani, Vincent Mareau, Laurent Gonon, Bruno Jean, Jean-Luc Putaux, Laurent Heux, et al.

► To cite this version:

João Paulo Cosas Fernandes, Oussama Hamzah, Maxime Gondrexon, Padraic O'Reilly, Hakima Mendil-Jakani, et al.. *Interface chemical mapping of sulfonated cellulose nanocrystalline sPEEK nanocomposites*. Materials Today Communications, 2025, 48, pp.113541. 10.1016/j.mtcomm.2025.113541. hal-05206697

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# Interface chemical mapping of sulfonated cellulose nanocrystal/sPEEK nanocomposites

João P. Cosas Fernandes<sup>a,*</sup>, Oussama Hamzah<sup>a</sup>, Maxime Gondrexon<sup>b</sup>, Padraic O'Reilly<sup>c</sup>, Hakima Mendil-Jakani<sup>b</sup>, Vincent Mareau<sup>b</sup>, Laurent Gonon<sup>b</sup>, Bruno Jean<sup>a</sup>, Jean-Luc Putaux<sup>a</sup>, Laurent Heux<sup>a</sup>, Yoshiharu Nishiyama<sup>a</sup>, Franck Dahlem<sup>a</sup>

<figure><img src="image_2.png" /></figure>

## ARTICLE INFO

### Keywords:

Nanocellulose

SPEEK

Polymer nanocomposite

Interface

Atomic force microscopy

Nanospectroscopy

## ABSTRACT

Nanocomposites of sulfonated poly(ether ether ketone) (sPEEK) reinforced with chemically modified tunicate cellulose nanocrystals (T-CNs) as partially biosourced proton-conducting membranes were prepared. Their morphology, mechanical properties, gas permeability, and chemical composition at macroscopic and microscopic scales were characterized. Aiming on the characterization of the filler-matrix interfaces, the Photoinduced Force Microscopy (PiFM) technique was used to collect spectroscopic information at the nanoscale. The PiFM allowed visualizing the improved dispersion of T-CNs by the surface sulfonation of the CNCs. The precise analysis of the chemical composition of the nanoscale interface by PiFM also highlights the likely heterogeneous nature of this modification. This provides insight into the quality of the chemical functionalization as well as the interfaces with the potential for optimizing the nanocomposite properties and manufacturing techniques, aiming to enhance both the mechanical properties and conductivity of membranes reinforced with nanocellulose.

## 1. Introduction

Perfluorinated sulfonic acid (PFSA) ionomers, such as Nafion, are ion-conducting polymers known for their proton conductivity and their chemical and mechanical stability.[1] On the one hand, among non-fluorinated alternatives, sulfonated poly(ether ether ketone) (sPEEK) is a promising option, offering improved thermomechanical resistance, higher thermal stability (higher glass transition temperature), superior gas impermeability, and cost-effectiveness compared to conventional Nafion membranes.[2,3] On the other hand, due to aromatic groups, the polyaromatic polymer backbone is less hydrophobic and less flexible than perfluorinated aliphatic polymers. In addition, the -SO₃H functions of sPEEK are less acidic (less labile proton) than those of Nafion. Therefore, the hydrophobic/hydrophilic phase separation is less pronounced for sPEEK than for Nafion, resulting in narrower ionic channels, branched with many "dead ends" (unconnected channels).[4] These factors reduce the ionic conductivity of sPEEK.[5] One can modify the membrane to improve the hydrophilic/hydrophobic phase separation and therefore water absorption, and conductivity of sPEEK.[6–8] To

improve the mechanical properties of ionomers, researchers tested polymer blending, partial cross-linking, and composite membranes. So far, these modifications have not led to the expected improvements and have often generated a critical loss of proton conduction.[9–13]

Cellulose, a linear crystalline biopolymer found in plant cell walls, microorganisms, and some aquatic animals like tunicates, is chemically stable and allows heterogeneous modification at the nanoscale.[14,15] While cellulose nanocrystals (CNCs) are now produced industrially, they remain practically non-existent in the development of versatile energy storage and production devices. In recent years, they have gained attention for enhancing the mechanical performance of PFSA membranes.[16–18] The addition of cellulose showed little loss in proton conductivity of composite membranes while offering mechanical reinforcement, although still based on PFSA ionomers. One limitation when using ionomers with a high cellulose content remains the intrinsic low ionic conductivity of the polysaccharide. However, CNCs offer a large specific surface area (up to hundreds of m²/g)[19] and tailorable surface chemistry, unique advantages that make them suitable for conversion into functional nanofillers. In addition, CNCs from tunicate have a high

<footnote><sup>a</sup> Univ. Grenoble Alpes, CNRS, CERMAV, Grenoble F-38000, France</footnote>

<footnote><sup>b</sup> Univ. Grenoble Alpes, CEA, CNRS, IRIG-SyMMES, Grenoble F-38000, France</footnote>

<footnote><sup>c</sup> Molecular Vista, San Jose, CA, USA</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: joao.cosas@cermav.cnrs.fr (J.P. Cosas Fernandes).</footnote>
form factor (L/D > 100), which can favor mechanical reinforcement in nanocomposites. Chemical modification, such as sulfonation, has been explored as an effective method to enhance cellulose proton conductivity.[20–23] Studies in the literature explored the use of microcystalline cellulose and different surface-modified celluloses in sPEEK composites, showing improvements in mechanical properties and proton conductivity performance.[24–27] Therefore, these materials are promising candidates for active nanofillers in the development of high-performance ionomeric polymer nanocomposites.

To develop and understand multiphase materials, especially nanocomposites where the dispersed phases and their interphases are at the nanometer scale, techniques to map properties, elemental and spectroscopic data at a high spatial resolution (chemical mapping) are of great interest. Combining atomic force microscopy (AFM) with chemical characterization techniques such as high-resolution secondary ion mass spectrometry (nanoSIMS), Raman and infrared (IR) spectroscopies proved to be powerful across various fields, including applications in multilayer films, polymer nanocomposites, thin films, pharmaceuticals, and polymer blends.[28–34] These studies highlight the techniques' capacity to provide comprehensive qualitative and quantitative chemical information at the nanoscale, facilitating the identification of phases [30,35,36], the monitoring of degradation processes[37,38], and precise chemical composition determination.[39] In particular, photoinduced force microscopy (PiFM) is a technique that involves using a tunable IR laser focused near the AFM tip, close to the sample surface. The IR laser is tuned to match the material absorption wavelengths while the AFM tip detects the photothermal interactions generated due to the molecular vibrations.[40,41] This produces a spectrum directly proportional to the material absorption coefficient, akin to traditional IR spectroscopy measurements. By fixing the laser at a specific wavelength and scanning the sample surface, chemical mapping images are created, revealing the spatial distribution of chemical components within the probed region, all with the nanoscale resolution capabilities of AFM.

In this study, we focused on the use of PiFM to investigate how sulfonation of tunicate CNCs (T-CNCs) influenced the morphology, dispersion, and interfaces of T-CNC/sPEEK nanocomposites. PiFM unveiled the internal structure of nanocomposite membranes cross-sectioned by cryo-ultramicrotomy, enabling the acquisition of localized spectroscopy data, particularly at the interfaces between the fillers and the matrix. The spectroscopic insights obtained for each phase at the nanoscale highlight the structure-properties relationships, revealing the potential heterogeneity of the functionalized surface and opening possibilities for optimizing the fabrication process and final properties of the material.

## 2. Experimental

### 2.1. Starting materials

Tunicin, the cellulosic mantle of tunicate (Halocynthia roretzi), a sea animal, was purified by alternating treatments[42] with NaClO₂ and KOH and treated with 48 % sulfuric acid (H₂SO₄) for 13 h at 55 °C under strong agitation using a Teflon blade. Acid-free T-CNC suspensions were obtained after repeated centrifugations, dialysis to neutrality against distilled water, and ultrasonication for 1 min. A sPEEK commercial polymer with an IEC of 1.34 meq.g⁻¹ (Fumapem® E-730) was purchased from Fumatech.

### 2.2. Periodate oxidation and sulfonation of nanocellulose

About 100 g of the nanocellulose water suspension (T-CNCs with concentration of 2.4 wt%) were inserted into a flask equipped with magnetic stirring. Sodium periodate (NaIO₄) in 0.45 equivalent (1.5 g) to the total amount of anhydroglucose units was dissolved in 20 mL of deionized water and added to the CNC suspension. Deionized water was further added to adjust the suspension concentration to 1 wt% of

cellulose. The flask was wrapped with aluminum foil to avoid UV-light degradation of sodium metaperiodate and the reaction was performed in darkness under stirring at 700 rpm for 96 h at room temperature. The suspension was centrifuged for 1 h at 20000 ×g. The pellets were then redispersed in deionized water using an Ultra-Turrax double-cylinder homogenizer. The centrifugation/redispersion process was repeated 3 times to remove any residual salts until the conductivity of the supernatant was below 40 µS/cm. The degree of oxidation (DO) of the diahydrode cellulose (DAC) was estimated as 0.12 (full oxidation DO = 2), using solid-state NMR (shown in Supporting Information).[43] The low DO allows for surface modification without affecting the bulk crystallinity of the T-CNCs. The suspension containing the DAC was used for further sulfonation. Sodium bisulfite was added in large excess, i.e., 7 g for 2 g of T-CNCs. The reaction was performed at room temperature under stirring at 700 rpm for 2 h. The centrifugation/redispersion process was carried out as previously described. The final sulfur content on the sulfonated T-CNCs (T-CNC-S) was measured by elemental analysis, giving a sulfur content of 2.72 ± 0.09 wt%, corresponding to a charge density of sulfonated groups of about 0.7 mmol/g.

### 2.3. Membrane preparation

The dry sPEEK polymer was dissolved in dimethylsulfoxide (DMSO, 95 wt%) for 1 h at room temperature under stirring. The suspensions of non-sulfonated or sulfonated CNCs were added to the mixture (10 wt% CNCs in the nanocomposite) and allowed to disperse for 2 h under stirring at room temperature. After degassing for 30 min under vacuum suction, the mixture was poured onto a polystyrene petri dish and allowed to dry at 60 °C for 24 h. A reference sPEEK membrane was prepared following the same procedure. The cast polymer membranes were peeled off from the petri dish, giving membranes with a thickness of 45 ± 5 µm. Finally, the membranes were re-acidified in a 1 M HCl solution at room temperature for 4 h and triple-rinsed with pure water.

### 2.4. Characterization

#### 2.4.1. Fourier-transform infrared (FTIR) spectroscopy

The CNC suspensions were freeze-dried and FTIR spectra were recorded with a Perkin Elmer 1720x spectrometer equipped with an ATR apparatus, within the 400–4000 cm⁻¹ wavenumber range, with a resolution of 2 cm⁻¹ and 10 accumulations. At least three analyses were performed at different locations for each sample. The resulting spectra were normalized with respect to the peak height at ~1030 cm⁻¹, taken as an invariant for the cellulose skeleton.

#### 2.4.2. Transmission electron microscopy (TEM)

Droplets of dilute aqueous suspensions of the T-CNCs were deposited onto carbon-coated TEM grids previously glow-discharged in a Pelco easiGlow system. After a few minutes, the liquid in excess was blotted with filter paper, and prior to drying, a droplet of 2 wt% uranyl acetate negative stain was deposited. The stain in excess was blotted and the specimens were allowed to dry. Images were recorded with a JEOL JEM-2100-Plus microscope operating at 200 kV and equipped with a Gatan Rio 16 digital camera.

#### 2.4.3. Dynamic mechanical analysis (DMA)

The viscoelastic properties of the nanocomposite membranes were characterized using a Metravib DMA 50 in tension mode. Rectangular samples were cut with dimensions of about 35 × 10 × 0.04 mm³. The distance between clamps was 15 mm. A normal force of 0.4 N was applied, and then the samples were subjected to a controlled dynamic force with an amplitude of 0.1 N and a frequency of 10 Hz. The temperature was varied from 50 to 250 °C with a heating rate of 5 °C/min.

#### 2.4.4. Gas permeability

The oxygen transmission rate (OTR) was determined using a Fibox 4
trace equipment equipped with SP-PSt6-NAU sensor (Presens). The method was adapted from ASTM F2714 standard in the following conditions: 23 °C / 50 % relative humidity (RH). The apparatus consisted of two chambers with inlets and outlets for gas flow. A tested piece was sealed between them to prevent leaks. The lower chamber was filled with ambient air at 23 °C / 50 % RH, while the upper chamber was flushed with nitrogen. An optical fiber sensor monitored the increase in oxygen concentration in the upper chamber, indicating permeation through the tested sample. The OTR was calculated using the following formula:

$$OTR = \frac{P_{O2} * V_{cell}}{P_{std} * A * \Delta P_{O2}} * \frac{T_{std}}{T_{meas}} - OTR_{zero} \quad (1)$$

with OTR expressed in cm³/(m².day.bar), ΔP₀₂ the difference in oxygen partial pressure between the two sides of the film, P₀₂ the oxygen partial pressure on the top side of the cell (hPa.day⁻¹), Vcell the volume of the top side of the cell (cm³), Pstd = 1013 hPa, Tstd = 273 K, Tmeas = 296 K, A the permeation area (m²) and OTRzero the oxygen transmission rate with only nitrogen in both cells in cm³/(m².day.bar).

### 2.4.5. Proton conductivity

The ionic conductivity was measured using a BT-110 four-probe in-plane membrane conductivity clamp (Scribner Inc., USA). A strip of the membrane was cut with dimensions of 1 × 3 cm² and blocked between the two platinum-gauze force leads (distance of 0.425 cm). The membrane was hydrated with H₂O (18.2 MOhm.cm⁻¹). The thickness of the swollen membrane was verified by micrometer measurement after the conductivity measurements. The conductivity was determined by linear-sweep voltammetry (LSV) from -1-1 V at a sweep rate of 10 mV/ s using a potentiostat (Biologic Science Instrument). The cell resistance R was determined from the slope of the LSV trace and the conductivity was calculated using the formula:

$$R = \rho \frac{L}{w.e} \quad (2)$$

where ρ is the resistivity (Ohm/cm), L is the distance between the counter- and working-sense leads (0.425 cm), R is the cell resistance, and w and e are the width and thickness of the membrane, respectively. The conductivity σ (S/cm) is calculated as 1/ρ.

### 2.4.6. Morphological characterization by AFM

Images of the topography and nanomechanical properties of the samples were acquired using the Peak Force QNM mode of the Dimension Icon AFM from Bruker. Images of 2 × 2 µm² areas were taken with a resolution of 256 × 256 pixels at a scan rate of 1 Hz and a piezo scanner frequency of 1 kHz. The cantilevers used in this study were the RTESPA-150. The deflection sensitivity (in the range of 50 nm/V) was determined by probing a hard sapphire sample, and the cantilever spring constant was around 2 N/m as determined by the Thermal Tune method. A tip check sample (model TGT1 from NT-MDT) composed of an array of sharp tips was used to determine the tip radius by blind reconstruction (~10 nm). For AFM observation of the CNCs, droplets of dilute CNC suspensions were deposited onto plasma-treated silicon wafers and allowed to dry for about 30 min and then imaged using ScanAsyst-Air probes (nominal radius ~5 nm).

### 2.4.7. Ultramicrotomy and photoinduced force microscopy (PiFM)

The membranes were trimmed and sectioned with a Leica EM UC7 cryo-ultramicrotome at -120 °C, producing a flat surface of about 500 × 50 µm². Membranes were cut without epoxy embedding, secured in a vice, to prevent potential chemical contamination.[44] It was imperative to ensure that the sample surface did not protrude more than 50-100 µm above the vice's top surface to prevent any bending of the membrane during AFM analysis. Therefore, extra caution was needed during the cutting process to prevent any contact between the knife and

the vice.

PiFM was performed and analyzed using a VistaScope photo-induced force microscope from Molecular Vista (San Jose, CA, USA). This instrument was equipped with a quantum cascade laser (QCL) unit and IR spectra were recorded in the 775-1950 cm⁻¹ range. Topography images (3 × 3 or 1 × 1 µm²) with a resolution of 256 × 256 pixels were recorded in non-contact mode (scan rate 0.2 Hz) with PiFM maps at the wave-numbers of interest and local IR spectra.

## 3. Results and discussion

### 3.1. Morphology of the tunicate nanocrystals

The morphology of the T-CNCs before and after chemical functionalization was observed by TEM and AFM, as shown in Fig. 1. The T-CNCs appeared with a whisker-like morphology, several micrometer-long and about 20 nm in diameter. Some nanocrystal aggregation was observed, and the negative staining TEM images revealed that most particles were composed of a few parallel subunits not separated during the preparation (Fig. 1(b)). Line profiles on several isolated crystals in the AFM images gave an average width of 47 ± 8 nm. Longitudinal line profiles on images of isolated CNCs showed that the height typically varied from 6 to 14 nm.

The chemical modification did not significantly change the isolated nanocrystals' length or width observed by TEM and AFM. However, in the images of the T-CNC-S (Fig. 1(c), (d), (g) and (h)), an apparent increase in the number of individual nanocrystals was observed, possibly due to the repulsive forces arising from sulfonated groups added to the surface.

### 3.2. ATR-FTIR spectroscopy analysis of sPEEK and T-CNCs

Fig. 2(a) displays the resulting spectra for both the reference T-CNC and the T-CNC-S. The spectrum after sulfonation is altered within the 1150-1250 cm⁻¹ range, particularly at 1250, 1230 and 1200 cm⁻¹, which can be assigned to the asymmetric and symmetric O=S=O stretching [45-48], confirming the successful introduction of sulfonated groups (SO₃H) onto the nanocrystal surface. In Fig. 2(b), sPEEK and T-CNC samples showed distinct regions where the absorbance was dominated by one component or the other. The peaks 1594 cm⁻¹, characteristic of sPEEK, and 1055 cm⁻¹, characteristic of T-CNCs were chosen to distinguish the two, due to their low overlap with the other material. Detailed information regarding the IR bands and their corresponding chemical assignments can be found in Table S1 in Supporting Information.

### 3.3. Thermomechanical properties, gas permeability, and proton conductivity

The thermomechanical properties of a nanocomposite material were assessed using DMA, revealing enhancements in mechanical performance (Fig. 3(a)). Compared to the pure sPEEK membrane, introducing 10 wt% T-CNCs increased the storage modulus from 1.2 ± 0.1 GPa to 2.2 ± 0.2 GPa. This improvement was observed across a broad temperature range, extending up to 200 °C, indicating that the nanocomposite maintained its rigidity even at high temperature. Furthermore, the presence of nanocrystals shifted the glass transition temperature (Tg) from 208 to 224 °C. Literature studies have shown that cellulose nanocrystals could affect the thermal properties of nanocomposites and increase the Tg of polymers, particularly when strong interfacial interactions were present.[49-51] These results suggested that the material not only performed well at higher temperatures but also exhibited improved mechanical stability, making it a candidate for applications requiring prolonged exposure to high temperatures, at least in dry conditions. The incorporation of T-CNC-S yielded a comparable effect on the mechanical properties of the nanocomposite, but notably
<figure><img src="image_3.png" /><figcaption>Fig. 1. TEM images (a) to (d) of negatively stained preparations and AFM topography images (e) to (h) images of T-CNCs and sulfonated T-CNC-S samples.</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>Fig. 2. ATR-FTIR spectra of (a) T-CNCs and T-CNC-S and (b) sPEEK and T-CNC samples. Blue and green bars on the bottom indicate possible zones for differentiation of each phase of the nanocomposite. The chosen peaks characteristic of each material for PiFM imaging are indicated by the arrows.</figcaption></figure>

enhanced the storage modulus to 2.8 ± 0.2 GPa, beyond what was achieved with unmodified T-CNCs. This suggests that the chemical alterations applied to the T-CNCs resulted in a more effective dispersion of the nanocrystals within the material and a better quality of the interface interactions.

The OTR measurements conducted to assess the barrier properties of the samples are shown on (Fig. 3(b)). Additionally, a sample composed solely of T-CNCs was cast as a film for comparison (shown in *Supporting Information*). Notably, the sPEEK reference sample displayed oxygen transmission properties approximately one order of magnitude higher than the reference T-CNC sample. However, adding T-CNCs to the nanocomposite substantially improved barrier properties, resulting in a one-order-of-magnitude decrease in OTR values. The impact of sulfonation on the proton conductivity of T-CNC was assessed using films cast from their suspensions. Films made with unmodified T-CNC were not mechanically stable under wet conditions, making measurements impossible. The surface chemical modification enhanced the wet mechanical strength of the T-CNC-S films and conferred proton conductivity (Fig. 3(c)). Although the value remains low (0.46 ± 0.06 mS/cm), this further confirms the success of the surface modification and suggests potential effects on the properties of the nanocomposites. The introduction of unmodified T-CNCs had no discernible impact on the proton conductivity of the sPEEK matrix, resulting in similar levels of conductivity for both, as shown in Fig. 3(d)). However, the sulfonated T-CNC-S slightly enhanced bulk proton conductivity from 16.5 ± 0.5–18.6 ± 1.4 mS/cm (hydrated state at 25°C). This modest yet noteworthy enhancement may be attributed to the introduction of T-CNC-S, which contributed to increased charge density at the polymer-filler interfaces. This addition potentially aided in proton transport mechanisms while preserving the membrane mechanical and barrier properties. To address this hypothesis, further focus was placed on the characterization of the interfaces between the T-CNCs and the sPEEK matrix through local analysis by AFM techniques.

### 3.4. Surface morphology and nanomechanical properties of nanocomposite membranes

The surface of the casted nanocomposite membranes containing 10 wt% T-CNCs without chemical modification was characterized by AFM in Peak Force QNM mode (Fig. 4). The T-CNCs were oriented parallel to the membrane surface (red arrow in Fig. 4(b)). Given their length, these crystals came into contact with one another, forming a percolated network. This can explain the positive influence on the material mechanical properties observed by DMA.[52–54] The adhesion contrast revealed by the PeakForce QNM technique indicated a lower
<figure><img src="image_5.png" /><figcaption>Fig. 3. (a) DMA of the sPEEK matrix and nanocomposite samples sPEEK/T-CNC and sPEEK/T-CNC-S. (b) Comparison of OTR and proton conductivity among the sPEEK matrix, sPEEK/T-CNC nanocomposite, and a membrane composed of T-CNCs. (c) Proton conductivity measurements of membranes cast using T-CNC and T-CNC-S (d) Proton conductivity measurements of the sPEEK matrix and the nanocomposites containing 10 wt% of unmodified and sulfonated tunicate nanocrystals (T-CNCs and T-CNC-S).</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Fig. 4. 2 × 2 µm² topographic AFM images of the surface of samples (a) sPEEK and (b) sPEEK/T-CNC. (c) Adhesion mapping overlaid on topography alongside profiles taken at the red line indicated on the image.</figcaption></figure>

adhesion of the tip to the T-CNCs compared to the matrix, resulting in a darker contrast for T-CNCs (Fig. 4(c)). This observation was corroborated by examining line profiles across T-CNCs, which consistently

showed that the T-CNCs exhibited a higher elevation but a lower adhesion.

Although the nanomechanical properties are of interest, they only
provide limited information about the physicochemical interactions between each phase and the tip. Additionally, while it is possible to characterize the interfaces between particles and the matrix from a mechanical property point of view, this approach does not provide in-sights into the local chemistry. Therefore, PiFM has been used to gain a deeper understanding of local chemistry. Moreover, while surface characterization offers valuable insights, it does not comprehensively represent the material overall morphology and particle distribution throughout its bulk volume. To gain a deeper insight into the nano-crystal distribution throughout the membrane thickness, cryo-ultramicrotomy has been used to section the material. This technique allows us to access the complete cross-section, facilitating a thorough examination of CNC morphology, distribution, and interfaces within the nanocomposite membranes.

### 3.5. Cross-section characterization by photoinduced force microscopy (PiFM)

Fig. 5(a) shows a scheme and an optical image of the membrane cross-section obtained through cryo-ultramicrotomy, captured using the optical microscope of the AFM. The inset shows an AFM image (3 × 3 µm²) of the resulting topography. These images demonstrate that the cryo-ultramicrotomy process yielded samples characterized by low roughness and minimal topographical artifacts. This roughness is related to the differential thermal expansion (or swelling due to water absorption) of T-CNCs and the sPEEK matrix when warming up after cryo-microtomy. IR spectra were acquired from three distinct positions at the surface of the cross-section using the PiFM technique, aiming for regions in the matrix and the T-CNCs. Fig. 5(b) presents three spectra representative of the observed differences. The red spectrum exhibits its strongest peak at approximately 1594 cm⁻¹, characteristic of C=C in the aromatic ring of sPEEK. The blue spectrum closely resembles the red one, suggesting the presence of sPEEK as well, but with notable peak variations, particularly at around 1480 and 1250 cm⁻¹, with an inversion of the ratio between these peaks and the adjacent ones. Interestingly, this spectrum also shows higher absorption at 1150, 1100, and 700 cm⁻¹. However, a significant peak increase is observed at

approximately 1055 cm⁻¹ in the green spectrum, unmistakably indicative of T-CNC.

Specific PiFM images were captured at distinct wavenumbers, namely 1594 cm⁻¹ characteristic of sPEEK, 1250 cm⁻¹ indicative of S=O vibrations, and 1055 cm⁻¹ representing T-CNCs (Fig. 6(b) to (d)) for 2 different image sizes: 3 × 3 µm² and 1 × 1 µm². The 1055 cm⁻¹ image reveals the precise location of T-CNCs, appearing predominantly as spherical objects and aggregates. These are views of the T-CNC cross-sections since they align perpendicularly to the cutting direction (parallel to the membrane surface), as previously observed in Fig. 4. This is further confirmed by the presence of elliptical particles, indicating some degree of misorientations of CNCs (supplementary images shown in Supporting Information). Interestingly, the image corresponding to the 1594 cm⁻¹ sPEEK absorption wavelength (Fig. 6(b)) does not perfectly complement the T-CNC distribution (Fig. 6(d)). Indeed, while the 1250 cm⁻¹ image in Fig. 6(c) perfectly complements the 1055 cm⁻¹ image, there are regions with reduced intensity in the 1594 cm⁻¹ absorption, which would contradictorily indicate regions with no sPEEK but still presenting S=O vibrations. To provide a comprehensive illustration, a composite image was generated incorporating the distribution of 1594 cm⁻¹ in red, 1250 cm⁻¹ in blue, and 1055 cm⁻¹ in green (Fig. 6(e)). Since the matrix exhibits both red and blue characteristics, the resulting average color appears pink in numerous regions, effectively highlighting distinct zones within the polymer matrix, featuring circular blue areas measuring approximately 50–100 nm in diameter within the sPEEK matrix.

To further explore these distinctions within the matrix, 24 spectroscopic measurements were conducted at a 10 nm interval across a single T-CNC feature, as shown in green Fig. 7. The spectra included regions of the sPEEK matrix (pink) and a smaller blue section, in an attempt to acquire a clear spectrum in the blue region for identification. In Fig. 7(c), spectra from 1 to 5 exclusively depict sPEEK, presenting a single peak at approximately 1594 cm⁻¹. Spectra 6–9, inside one small blue region, exhibit a growing shoulder around 1577 cm⁻¹, accompanied by an increase in the peak at around 1150 cm⁻¹, together with a slight increase around 1250 cm⁻¹. This suggests that another component is mixed with sPEEK inside these regions. Spectra 11–14 exclusively represent T-CNCs,

<figure><img src="image_7.png" /><figcaption>Fig. 5. (a) Scheme of the cross-section analysis, with optical image of the cryo-ultramicrotomed SPEEK/T-CNC membrane surface alongside a PiFM topography image captured from a central 3 × 3 µm² region. (b) PiFM spectra recorded at the designated locations indicated by red, blue, and green triangles.</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>**Fig. 6.** 3 × 3 µm² and 1 × 1 µm² PiFM image at the cryo-ultramicrotomed sample sPEEK/T-CNC. (a) Topography and spatial mapping of (b) 1594 cm⁻¹ (sPEEK), (c) 1250 cm⁻¹ (S=O) and (d) 1055 cm⁻¹ (T-CNC) and (e) combined PiFM image of 1594 cm⁻¹ (red), 1250 cm⁻¹ (blue), and 1055 cm⁻¹ (green).</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>**Fig. 7.** 1 × 1 µm² PiFM image of the cryo-ultramicrotomed sPEEK/T-CNC sample. (a) Topography of the cryo-ultramicrotomed surface, (b) combined PiFM image of 1594 cm⁻¹ (red), 1250 cm⁻¹ (blue), and 1055 cm⁻¹ (green). Spectral analyses were acquired at 10 nm intervals along the line marked on the image.</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>**Fig. 8.** 3 × 3 µm² and 1 × 1 µm² PiFM image at the cryo-ultramicrotomed sample sPEEK/T-CNC-S. (a) Topography and spatial mapping of (b) 1594 cm⁻¹, (c) 1250 cm⁻¹ (d) 1055 cm⁻¹, and (e) combined PiFM image of 1594 cm⁻¹ (red), 1250 cm⁻¹ (blue), and 1055 cm⁻¹ (green).</figcaption></figure>
characterized by a pronounced intensity at 1055 and 1030 cm⁻¹. Subsequently, spectra 15–24 once again exclusively display sPEEK.

The same analytical approach was applied to the nanocomposite containing sulfonated T-CNC-S. Fig. 8 shows the topographic and PiFM images (again for 3 × 3 µm² and 1 × 1 µm² dimensions) corresponding to wavenumbers 1594 cm⁻¹, 1250 cm⁻¹, and 1055 cm⁻¹. Notably, the 1055 cm⁻¹ image reveals that sulfonated CNCs exhibit a more homogeneous dispersion within the nanocomposite, likely due to the presence of SO₃⁻ groups on the surface of T-CNC-S. These negatively charged groups promote electrostatic repulsion between individual nanocrystals, which reduces aggregation in suspension and improves dispersion during membrane casting. Furthermore, these sulfonic acid groups can enhance compatibility with the sulfonated polymer matrix, likely through favorable electrostatic interactions and potential hydrogen bonding, contributing to improved dispersion. While the sulfonated CNC sample appears more homogeneously distributed in the regions analyzed, the authors acknowledge this as a qualitative observation. This apparent better dispersion is exemplified by the presence of individual crystals measuring less than 20 nm in width (cross-section). In contrast, unmodified T-CNCs tend to aggregate, forming bundles with widths exceeding 500 nm, as previously observed (Fig. 6 and Fig. S3 in Supporting Information). The blue regions present in the previous sample are also observed, and, in this particular image, appear on a larger scale, including phases exceeding a micrometer in length. The intrinsic heterogeneous distribution of the blue phase can be inconvenient, yet useful as this allows for the acquisition of IR spectra within these extensive domains, eliminating potential contributions from adjacent phases or the material volume. It is worth noting that PiFM offers exceptional surface sensitivity and can probe a surface depth of less than 10 nm, making it particularly suitable for detecting subtle surface variations without subsurface interferences.[41,55]

IR spectra were collected from each phase, as shown in Fig. 9. The red spectrum prominently exhibits the characteristic signature of sPEEK. In the blue spectrum, similarities to the previous sample are evident (Fig. 5). However, distinctions between this spectrum and the red one are more pronounced than in the previous sample, probably due to the bigger size of the probed phase. For instance, the peak at 1575 cm⁻¹, which appeared as part of a doublet in Fig. 5, is now distinctly a single peak. The carbonyl peak at 1646 cm⁻¹ is completely absent, while there is an increase at 1147 cm⁻¹ (symmetrical stretching vibration of S=O), and new peaks are observed at 1103 cm⁻¹ (S=O stretching) and 716 cm⁻¹, attributed to ring vibrations. Once again, the signal is stronger at 1250 cm⁻¹, characteristic of the sulfonated groups, and it is not associated with sulfonated T-CNC-S, as it was acquired in a zone far from the crystals.

On the one hand, one possible explanation for these differences could be the presence of regions where the sPEEK phase has undergone thermal crosslinking, forming –SO₂– bridges between chains. Indeed,

solvent-assisted thermal treatments are known to affect the thermal stability, hydration and mechanical properties of sPEEK membranes. [56–58] On the other hand, the cross-link reaction occurs only on membranes cast from DMSO and at temperatures higher than 150 °C for extended time exposure. Below this temperature and using different solvents, the membranes are unchanged.[59] Di Vona et al.[59] have observed variations in 1:2:4-substituted phenyl rings at 1225 and 1080 cm⁻¹, aromatic sulfone moieties at 1210 cm⁻¹ (shoulder) and 1065 cm⁻¹ (tail), which are not observed in the blue spectrum. Furthermore, the carbonyl peak at 1646 cm⁻¹ is usually unaffected after thermal treatments and its absence could only be achieved by the reduction of the carbonyl groups.[60] Considering these conditions, it is unlikely that the samples were crosslinked during the nanocomposite elaboration, and the observed peak shifts are not consistent with the literature.

In further analysis, the blue spectrum was compared to FTIR spectra from a database, and the spectrum (detailed in the Supporting Information) with the highest quality index match suggests a striking resemblance to polyethersulfone (PES). The recorded spectra are also coherent with the literature.[61–63] PES shares a similar structure with sPEEK and may have been employed as an additive in the commercial product used, to modify properties such as membrane formation and thermal hydrolytic stability in both hot and moist environments.[64]

Although this finding was not initially within the scope of the study, the authors believe it is highly relevant. This unexpected result underlines the analytical power of the technique in discerning distinct phases within an intricate multiphase polymer system and, importantly, may help prevent misinterpretation by other researchers using the same material in future studies.

### 3.6. Spectroscopic analysis at the interfaces by PiFM

The cross-sections were analyzed along the T-CNCs in the nanocomposite samples (Fig. 10). When comparing the spectra obtained across particles with a pronounced signal at 1055 cm⁻¹ (indicative of T-CNCs), it becomes apparent that the sulfonated sample demonstrates a higher degree of intermixing of IR features originating from T-CNC-S and sPEEK across the particle-matrix interface. This is exemplified by the increasing intensities of characteristic sPEEK peaks (1594, 1487, 1466 cm⁻¹) in conjunction with T-CNC peaks (1055, 1033 cm⁻¹) at the particle boundary.

Peak ratios in FTIR are usually employed to normalize signal intensities, allowing for comparing different samples, by mitigating artifacts that would arise from uncontrolled topographic or instrumental variations. This strategy can also be applied to images taken at different wavenumbers, and it is particularly effective in highlighting matrix-filler interfaces. A more detailed understanding of the interfaces can be achieved by dividing the images captured at 1250 cm⁻¹ (Fig. 11(a)

<figure><img src="image_11.png" /><figcaption>Fig. 9. 3 × 3 µm² combined PiFM image at the cryo-ultramicrotomed sample sPEEK/T-CNC-S: spatial mapping of 1594 cm⁻¹ (red), 1250 cm⁻¹ (blue), and 1055 cm⁻¹ (green) signals and the PiFM spectra acquired at the marked red and blue phase locations (indicated by triangles).</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>**Fig. 10.** 500 × 500 nm² combined PiFM images of (a) sPEEK/T-CNC and (b) sPEEK/T-CNC-S cryo-ultramicrotomed samples showing the distribution of 1594 cm⁻¹ (red), 1250 cm⁻¹ (blue), and 1055 cm⁻¹ (green) signals. (c) Spectral analyses acquired at 10 nm intervals along the line marked on the images of each sample.</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>**Fig. 11.** 2 × 2 µm² PiFM images at the cryo-ultramicrotomed sample sPEEK/T-CNC at (a) 1055 cm⁻¹ (b) 1250 cm⁻¹ and (c) the ratio I₁₂₅₀/I₁₀₅₅ between the two images to highlight the interfaces. Additionally, for sample sPEEK/T-CNC-S, PiFM images at (d) 1055 cm⁻¹ (e) 1250 cm⁻¹, and (f) I₁₂₅₀/I₁₀₅₅ ratio.</figcaption></figure>

and (d)) by those at 1055 cm⁻¹ (Fig. 11(b) and (e)), as depicted in Fig. 11 (c) and (f). The improved dispersion of the CNCs due to the sulfonation, accompanied by reduced agglomeration, results in an increased presence of interfaces (appearing in green) in the nano-composite. This enhanced dispersion and accessible interfaces can explain the increase in mechanical strength and proton conductivity of this sample. The use of T-CNC-S can mechanically stabilize the nano-composite structure, and at the same time limit the loss of conductivity by the insertion of a non-conductive phase within the matrix, since the

sulfonated crystals can promote surface proton transport mechanisms at the polymer-filler interface.

Cross-sectional analyses were performed to examine the chemical distributions of bands at 1055 cm⁻¹ and 1250 cm⁻¹ in both sets of samples. Fig. 12 illustrates the profiles obtained from different directions within a unique CNC and for different CNCs within the nano-composite. The interface length was defined from where an increase of the 1055 cm⁻¹ intensity was observed compared to the matrix, up to the average intensity of this band inside the particle. The average length of
<figure><img src="image_14.png" /><figcaption>**Fig. 12.** PiFM cross-section chemical analyses of the interfaces in samples sPEEK/T-CNC at (a) and (b), and sPEEK/T-CNC-S at (c) and (d). Intensity profile of 1055 cm⁻¹ (solid line) and 1250 cm⁻¹ signals (dashed) along the indicated lines across the CNCs inside the matrix.</figcaption></figure>

the interfaces measured in sample sPEEK/T-CNC was 18 ± 3 nm, which means at least 4 pixels (one pixel every ~4 nm). When analyzing the profiles across the interfaces between the T-CNCs and the sPEEK matrix (Fig. 12 (a) and (b)), one can observe a distinct and abrupt decrease in the signal of the 1250 cm⁻¹ band as it approached the outer surface of the nanocrystal. This decrease typically occurred at the point where the CNC characteristic peak at 1055 cm⁻¹ began to rise, which is consistent with our earlier findings (Fig. 10). This effect is particularly pronounced in the profiles shown in Fig. 12 (a), as highlighted in red zones. In Fig. 12 (b), while some interfaces appear slightly broader, they still exhibited a signal loss of more than 50 % inside the interfaces. This variation may be attributed to the orientation of the nanocrystal in that specific location, where the 1055 cm⁻¹ signal originates from the subsurface (approximately 5–10 nm deep) with a thin layer of sPEEK on top.

In contrast, besides the same average length of the interfaces measured as 20 ± 3 nm, the nanocomposite containing T-CNC-S exhibited a more complex interface composition (Fig. 12 (c) and (d)). A sharp decrease is observed in the 1250 cm⁻¹ signal (highlighted in red) at some interfaces, but one can also notice boundaries with an extended high 1250 cm⁻¹ signal across the interface or even a plateau of the signal with an intermediate intensity (highlighted in blue in the Fig. 12 (d)). This is probably related to the sulfonated groups at the surface of the crystals and intermixing of the sPEEK at the interface. In some instances, a combination of both types of interfaces was observed within the same

profile (e.g., the blue and green profiles in Fig. 12 (c)), indicating a potential non-uniform distribution of sulfonated groups on the surface of the modified T-CNCs. Complementary interface characterization analyses in other particles are shown in *Supporting Information*. From 37 profiles taken in the sample sPEEK/T-CNC-S, 47 % of them presented high 1250 cm⁻¹ signal across the interface (classified as blue), while 95 % of the same number of interfaces characterized in sample sPEEK/T-CNC were classified as red (a sharp decrease in the 1250 cm⁻¹ signals).

This methodology offers a means to elucidate the interfaces existing between the nanocrystals and the polymer matrix. Various scenarios can be hypothesized given the potential non-uniform distribution of sulfonated groups on the nanocrystal surfaces. When examining the interface between a non-modified nanocrystal surface and the sPEEK matrix, the resulting profile would exhibit a distinct boundary, characterized by a decline in signals associated with sulfonated groups at 1250 cm⁻¹ (and sPEEK characteristic peaks) when transitioning from the matrix to the nanocrystal, as depicted by the red line in Fig. 13. Conversely, in cases where the profile traverses a sulfonated surface, a more gradual interface is expected, with the signal diminishing gradually from the sPEEK matrix towards the inner regions of the nanocrystal, given that sulfonated groups are predominantly located at the surface (illustrated by the blue line). In situations where the distribution is heterogeneous, a third possibility appears, where one surface exhibits a well-defined boundary, while the opposite side demonstrates a gradual signal decrease (as
<figure><img src="image_15.png" /><figcaption>Fig. 13. Scheme of the potential nanocrystal-polymer matrix interfaces considering non-uniform sulfonated group distribution on nanocrystal surfaces.</figcaption></figure>

indicated by the green profile). In light of the findings from our cross-section analysis of the interfaces between the surface-modified CNCs and the polymer matrix, the distribution of functional groups on the surface of sulfonated T-CNCs using the PiFM technique is under investigation to confirm these hypotheses.

The surface functionalization of the T-CNCs with sulfonated groups has improved the dispersion of the fillers, increased the mechanical properties of the nanocomposites, and modified the local chemistry of the polymer-filler interfaces, with a potential for improvement of the proton conductivity properties.[26,27,65] It is worth emphasizing that the process parameters significantly influence the membrane's final properties and morphology. Additionally, thermal post-treatments can be used to condition and optimize the functional aspects of membranes, such as proton conductivity, membrane swelling, and thermomechanical properties.[8,66] In future research, our goal is to comprehensively investigate the impact of key membrane fabrication stages on nanocomposite morphology, nanofiller distribution, and their influence on the hydrophobic/hydrophilic separation within the matrix. Furthermore, the effect of the charge density of the CNCs on the conduction mechanism will be explored. To achieve this, the nanofiller content, morphology, and sulfonation degree will be varied, enabling a detailed study of their influence on the matrix intrinsic properties and the final performance of the nanocomposite.

## 4. Conclusions

Functionalized tunicate cellulose nanocrystals (T-CNCs) were used in the production of nanocomposite membranes based on sPEEK. Our study assessed the thermomechanical, barrier properties, and proton conductivity of sPEEK nanocomposites containing T-CNCs. DMA results revealed improvements in the thermomechanical properties of the nanocomposite membranes compared to the pure sPEEK matrix. The introduction of 10 wt% T-CNCs increased the storage modulus, demonstrating enhanced mechanical performance across a broad temperature range up to 200 °C. The T-CNCs significantly improved the barrier properties, leading to a notable reduction in oxygen transmission rates. While unmodified T-CNCs had no significant impact on proton conductivity, sulfonated T-CNC-S slightly enhanced bulk proton conductivity, likely due to increased surface proton transport mechanisms at the surface of the functionalized fillers. To address this hypothesis, a detailed characterization of the local chemistry of the samples was performed.

Photoinduced force microscopy was used to investigate the nanocomposite chemical composition, tracking molecular signatures for

distinguishing between sPEEK and T-CNCs at the nanoscale. It revealed a considerable heterogeneity within the sPEEK matrix, with specific regions displaying reduced absorption at around $1594 \text{ cm}^{-1}$ in PiFM imaging. Within one of these regions, a spectral analysis yielded a compelling match for polyethersulfone (PES), which is employed as an additive in the commercial sPEEK product. This result highlights the technique's remarkable sensitivity in discerning distinct phases within intricate multiphase polymer systems. Furthermore, local IR spectra and chemical profiles at the polymer-filler interfaces of the nanocomposite containing sulfonated T-CNC-S exhibited a different interface composition, related to the sulfonated groups at the surface of the CNCs and a probable intermixing of the sPEEK at the interface. The authors assumed that these changes were responsible for the better bulk properties of the compatibilized nanocomposites. A hypothesis based on a non-uniform distribution of sulfonated groups on the nanocrystal surfaces was presented to explain the observed interfaces and will be explored in future work.

In summary, the sulfonation of CNCs can be a suitable strategy to improve the functional properties of nanocomposites based on sPEEK. The utilization of PiFM holds significant promise in the study of energy-related polymer materials, enabling the characterization of phase-specific molecular structures and chemical distributions, especially interesting at the polymer-filler interfaces. Expanding the application of this technique to more complex systems offers the potential to investigate the spatial distribution of chemical modifications, including polymer grafting, silanization, and crosslinking, on the surfaces of CNCs. This can give insights into the material's intricate structure-property relationships, facilitating the design of materials with specific macro-mechanical behaviors.

### CRediT authorship contribution statement

**João P. Cosas Fernandes:** Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Visualization, Writing – original draft, Writing – review & editing, Project administration. **Oussama Hamzah:** Formal analysis, Investigation, Methodology, Resources, Writing – review & editing. **Maxime Gon-drexon:** Formal analysis, Investigation, Methodology, Resources, Writing – review & editing. **Padraic O'Reilly:** Investigation, Methodology, Resources, Writing – review & editing. **Hakima Mendil-Jakani:** Conceptualization, Methodology, Resources, Writing – review & editing. **Vincent Mareau:** Conceptualization, Methodology, Resources, Writing – review & editing. **Laurent Gonon:** Conceptualization, Methodology, Resources, Writing – review & editing. **Bruno Jean:** Conceptualization,
**Funding acquisition, Methodology, Resources, Writing – review & editing.**
**Jean-Luc Putaux:** Formal analysis, Investigation, Methodology, Writing – review & editing. **Laurent Heux:** Funding acquisition, Methodology, Resources, Writing – review & editing. **Yoshiharu Nishiyama:** Conceptualization, Funding acquisition, Methodology, Project administration, Resources, Writing – review & editing. **Franck Dahlem:** Conceptualization, Funding acquisition, Project administration, Resources, Writing – review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors acknowledge the NanoBio-ICMG platform (UAR 2607, Grenoble) for granting access to the electron microscopy facilities, the PCI-ICMG platform for the nanomechanical AFM measurements, and the Cermav@Sciences foundation for the seed funding for the project (FC-MeTa/2022). The authors also acknowledge the financial support from Institut Carnot PolyNat ANR N°22-CARN-025-01 (MCellion/2023). We would like to thank Pierre Sailler (CERMAV) for the DMA measurements, and Bruno Carré (Centre Technique du Papier, Grenoble) for his help with the permeability measurements. CERMAV is a member of Institut Carnot PolyNat (Investissements d'Avenir #ANR-11-CARN-030-01) and participant to the Glyco@Alps program (Investissements d'Avenir #ANR-15-IDEX-02).

Appendix A. Supporting information

Supplementary data associated with this article can be found in the online version at doi:10.1016/j.mtcomm.2025.113541.

Data availability

Data will be made available on request.

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