# Photo-switchable polyoxazoline additive for marine fouling release silicone coatings

Laura Vasilica Arsenie, Mama Aïssata Bangoura, Michel Ramonda, Rémi Merindol, Louise Hespel, Sébastien Blanquer, Fabrice Azemar, Vincent Lapinte

► To cite this version:

Laura Vasilica Arsenie, Mama Aïssata Bangoura, Michel Ramonda, Rémi Merindol, Louise Hespel, et al.. Photo-switchable polyoxazoline additive for marine fouling release silicone coatings. Progress in Organic Coatings, 2025, 200, pp.109096. 10.1016/j.porgcoat.2025.109096. hal-04964827

HAL Id: hal-04964827

https://hal.umontpellier.fr/hal-04964827v1

Submitted on 2 Mar 2025

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# Photo-switchable polyoxazoline additive for marine fouling release silicone coatings

Laura Vasilica Arsenie<sup>a</sup>, Mama Aissata Bangoura<sup>b</sup>, Michel Ramonda<sup>c</sup>, Rémi Merindol<sup>d</sup>, Louise Hespel<sup>e</sup>, Sébastien Blanquer<sup>a</sup>, Fabrice Azemar<sup>b</sup>, Vincent Lapinte<sup>a,*</sup>

## ARTICLE INFO

**Keywords:**

Polyoxazoline

Amphiphilic

Silicone

Photosensitive

Marine fouling release

## ABSTRACT

Marine fouling of shipping vessels induces ecological disasters with the transport of invasive species as well as economical costs for shipping due to frictional resistance on the surface vessels. To address this issue, fouling release coatings, without toxic biocide, gradually replaced bioactive antifouling coatings. However, the main candidates, the silicones, offer limited static fouling which can be improved by using amphiphilic additive. To render more attractive the incorporation of additive in the paint formulation, they can additionally be photo-sensitive allowing to repel the marine organisms under light and mitigate the fouling formation. This study presents silicone coatings using photosensitive azobenzene polyoxazoline (POx) additive able to reversibly switch from trans to cis configuration under UV/Visible (Vis) light. To avoid any additive-matrix incompatibility, that is a current issue specific to commercial hydrophilic additives, amphiphilic POx were designed. Di- and triblock POx containing hydrophobic poly(2-phenyl-2-oxazoline) blocks and hydrophilic poly(2-methyl-2-oxazoline) blocks were covalently incorporated in PDMS matrix by crosslinking via self-condensation. The impact of the macromolecular architecture of additive, UV irradiation and Vis light exposure on the surface and the mechanical properties of resulting coatings was investigated by contact angle, Atomic Force Microscopy (AFM) and nanoindentation. The reversibility of the photo-response was also evaluated. Finally, the macro-fouling properties of the coated surface mediated by UV/Vis light were monitored during 120 days by real-time immersion of coatings in Atlantic Ocean and macroscopic fouling evaluation of the immersed surfaces.

## 1. Introduction

Submerged structures within maritime industrial sector such as heat exchangers, off-shore wind turbines, shipping vessels or aquaculture enclosures are naturally susceptible to biofouling adhesion [1-4]. This multifaceted process is very complex and consists in the colonisation of these surfaces at the beginning by a biofilm (composed of micro-algae, bacteria, etc.), followed by macro-algae or sessile organisms and ultimately by the mobile species (worms, fish, crabs, etc.) [5,6]. In turn, the biofouling causes an alarming deterioration of these surfaces, resulting in powering efficiency losses of up to 86 % during cruising speeds [7,8].

The oldest strategy to prevent the marine biofouling involved the bioactive antifouling paints which mainly incorporate biocides

(tributyltin: TBT compounds or copper) released into the seawater and kill the micro-organisms. Despite their costs-effectiveness, the bioactive paints are very toxic not only to the microbiota, algae, or mollusks, but also to humans [6,9]. Fortunately, bioactive paints are gradually phased out as a result of legislations, as they have been found to provoke genetic modifications in some marine organisms [10]. This is a challenging bottleneck for the marine ecosystem because genetically modified micro-organisms may become invasive species for other regions of the planet, potentially causing ecological disasters [3,10,11].

A more efficient and ecofriendly strategy to combat the biofouling is to apply less toxic fouling release coatings, or more recently, biopassive antifouling coatings. The main advantage of the latter ones is their ability to mitigate fouling due to a repellent character without

<footnote><sup>a</sup> ICGM, University of Montpellier, CNRS, ENSCM, 34090 Montpellier, France</footnote>

<footnote><sup>b</sup> LBCM, Université Bretagne Sud, EMR CNRS 6076, IUEM, 56100 Lorient, France</footnote>

<footnote><sup>c</sup> Centrale de Technologie en Micro et Nanoélectronique, University of Montpellier, Campus Saint Priest, 34095 Montpellier, France</footnote>

<footnote><sup>d</sup> L2C, University of Montpellier, CNRS, UMR 5221, 34095 Montpellier, France</footnote>

<footnote><sup>e</sup> Univ Rouen Normandie, INSA Rouen Normandie, CNRS, Normandie Univ, PBS UMR 6270, Rouen F-76000, France</footnote>

<footnote>* Corresponding author at: ICGM, University of Montpellier, CNRS, ENSCM, Pole Recherche Balard, 1919 Route de Mende, 34293 Montpellier, France.
E-mail address: vincent.lapinte@umontpellier.fr (V. Lapinte).</footnote>
disrupting the proliferation cycle of the marine organisms, nor their
metabolism [10].

Traditionally, biopassive strategies include the fouling-resistant coatings. In general, they are made of hydrophilic polymers such as poly(ethylene glycol) PEG. However, fouling resistant coatings are usually swollen in marine environments, leading to very poor mechanical properties, which limits their applications.

Another class of biopassive strategies is represented by fouling release coatings (FRC). These coatings are characterized by a low surface energy and a low Young Modulus to avoid the interactions with the organisms. The coating surface (i.e. due to low surface energy) can easily remove the attached organisms by the water shear force during navigation. Moreover, the low Young Modulus develops a dynamic and elastic surface which is hard to be colonized by micro-organisms. Polydimethylsiloxane (PDMS) stands out as the most commonly used polymer developed in the marine industry for fouling release coatings [12]. Several advantages of PDMS include low toxicity, low surface energy and low elastic modulus, as well as minimal adhesion strength for fouling organisms [13,14]. In general, the fouling organisms weakly adhere to PDMS and they can be easily removed by the shear force of the seawater. However, PDMS coatings come with some limitations, such as a short lifetime service due to weak adhesion to the substrate, poor mechanical robustness and low static antifouling performance (i.e., they are effective only when the ship speed exceeds 15 knots) [15].

An approach to tackle the latter issue is the incorporation in PDMS matrix of hydrophilic polymeric additives to ensure an hydration layer in surface which inhibits bacterial attachment by diminishing electrostatic interactions [1,5,6,10,11,16]. Among these additives, PEG derivatives were extensively explored. A significant amount of work was dedicated to investigate these systems in terms of surface chemistry, and ability to avoid the fouling of bacteria, algae or diatoms [1,5,6,10,11,16]. Nonetheless, hybrid coatings may suffer of phase separation coming from structural incompatibility between hydrophobic silicone and hydropholic additive [17–20]. The undesired inhomogeneous coatings undermines the coating's lifespan, its surface properties and fouling repellent performances [20].

Designing amphiphilic copolymers instead of hydrophilic polymeric additives holds many promises to avoid the phase separation. By attaching hydrophobic blocks to hydrophilic blocks, the compatibilization into silicon matrices can be significantly enhanced [21,22]. So far, poly(oxazolines) (POx) gathered a particular attention, primarily because they exhibit properties that are reminiscent as those of PEG, such as friction, viscosity, hydrodynamic diameter, etc. [23] Moreover, one of the main advantages of POx compared to PEG is that its pseudopeptide derived structure can be easily functionalised in order to obtain amphiphilic architectures. Additionally, POx structures are more stable at oxidation than PEG [24]. POx's resemblance to PEG makes it an interesting candidate for further exploration in various applications, particularly in the context of antifouling coatings and materials with desirable attributes similar to those of PEG [25]. Indeed, POx architectures with various shapes (linear, branched, brush or cyclic) were reported to have promising antifouling potential in the biomedical field (against bacteria and proteins) [23,25–30]. Despite the POx success in biomedicine, there were few reported studies stating the use of POx as additives for antifouling coatings in maritime industry [31–33]. Consequently, there is a considerable interest in investigating POx for combatting the marine fouling.

Not only the development of amphiphilic additives to enhance the surface properties and fouling repellence of silicone coatings, but also the use of stimuli-responsive polymers may be a promising strategy to prevent the fouling settlement, according to recent studies [34–38]. Photo-responsive polymers containing azobenzene units were distinguished in this area [38,40–43]. The particularity of the azobenzene segments is that they can undergo photo-mediated reversible isomerisation from *trans* to *cis* configuration [38]. For the most common azobenzene systems, UV irradiation induces the isomerization from *trans* to

cis, while visible light irradiation triggers the reverse transformation. As a result of switchable configuration, azobenzene systems were considered as on-demand fouling release platforms able to capture and release the biofouling by simply manipulating UV/Visible light sources. Despite their interesting mechanism of militating the fouling adhesion, the potential of these materials as additives in silicone matrices is still largely unexplored.

To address the challenges associated with heterogeneous surfaces when using hydrophilic additives into silicone, our strategy incorporates amphiphilic polyoxazoline (POx) containing UV-sensitive azobenzene units. Our aim is to enhance the compatibility with silicone matrix and thus to modulate the surface properties to reach effective fouling repellent properties. Amphiphilic di- and triblock copolymers were designed using hydrophobic poly(2-phenyl-2-oxazoline) block (P) and a hydrophilic poly(2-methyl-2-oxazoline) block (M) and a photosensitive azobenzene ring through two-steps synthesis: CROP and thiol-ene reaction. After detailing the synthesis of these POx additives, pure POx coatings and finally hybrid coatings were elaborated by crosslinking of POx additive bearing terminal trimethoxysilane groups with PDMS. The characterisation of the surfaces (wettability, roughness, stiffness) before and after UV-irradiation, as well as Vis light exposure will also be studied to illustrate the impact of photosensitive groups on the surface behavior. At the end, preliminary investigation on the efficiency of silicone coatings against macrofouling deposition will be presented during 120 days of immersion in Atlantic Ocean.

## 2. Experimental section

### 2.1. Materials

2-Methyl-2-oxazoline (M, 98 % purity) was bought from Sigma Aldrich (Germany) and 2-phenyl-2-oxazoline (P, 99 % purity) was acquired from Sigma Aldrich (India). N, N-dimethyl-4, 4'-azodianiline (Az, 97 % purity) and (3-mercaptopropyl) trimethoxysilane (95 % purity) were purchased from Sigma Aldrich (Germany). 4, 4'-azodianiline (Az₂, 95 % purity) was provided by Thermo Scientific (China). Allyl p-toluene sulfonate (95 % purity) was bought from Aldrich (Switzerland). 2,2'-azobis(2-methylpropionitrile) (AIBN, 98 % purity) was acquired from Sigma Aldrich (Germany) and recrystallised from methanol at 65 °C before use. Dry acetonitrile (99 % purity) and chloroform (95 % purity) were purchased from Carlo Erba (France) and stored under dry nitrogen atmosphere. Diethyl ether (99 % purity) was bought from Honeywell (Germany). Dimethylformamide (DMF, 99.8 % purity) was acquired from Fisher Scientific (Belgium). Deuterated chloroform (CDCl₃) was provided by Sigma Aldrich (Germany).

### 2.2. Instrumentation

¹H NMR spectra were recorded on NMR Bruker Avance 400-MHz spectrometer using CDCl₃ as deuterated solvent. DOSY experiments were obtained by using 600 MHz Bruker Spectrometer, by using CDCl₃ as deuterated solvent. The chemical shifts of protons were relative to tetramethylsilane (TMS) at δ = 0.

Size exclusion chromatography (SEC) was performed in DMF containing 0.1 wt% LiCl, with a flow rate of 0.8 mL min⁻¹ at 40 °C. Sample were filtered using TE36 Whatman PTFE-supported membrane filter paper (0.45 µm, 47 mm diameter) before the injection. The data were calibrated using polymethyl methacrylate (PMMA) standards.

UV-Vis absorption spectra were recorded by using a UV/Vis Perkin Elmer Lambda 365 spectrophotometer, equipped with a PTP-1 + 1 Peltier system in quartz cuvette as spectrophotometer cell (length = 1 cm).

Contact angle measurements were conducted at room temperature with a contact angle system (Digidrop GBX) equipped with a syringe and a video camera.

UV irradiation of polymer coatings were performed using a LITE-Box
G136 UV chamber (at 365 nm) purchased from NK-OPTIK, Germany.

The polymerisations were performed by using Anton Paar micro-wave synthesis reactor.

## 2.3. Methods

### 2.3.1. Synthesis of amphiphilic UV-sensitive POx

Amphiphilic UV-sensitive POx (mono and bi functional) were obtained in two steps: CROP co-polymerisation ended with UV-sensitive diazo-containing molecule (Step I) followed by the post-functionalisation with trimethoxysilane end groups (Step II).

#### 2.3.1.1. Step I- CROP polymerisation.
Allyl tosylate (for Az-P-M: 1 eq., 0.167 g, 0.78 mmol; for Az₂-P-M: 2 eq., 0.334 g, 1.57 mmol) and 2-phenyl-2-oxazoline (10 eq., 1 mL, 7.87 mmol or 30 eq., 3.1 mL, 23.6 mmol) were dissolved in dry acetonitrile at a concentration of 4 M. The solution was heated at 140 °C in a microwave synthesiser for 1 h (300 W). Then, the mixture was cooled at 40 °C and 2-methyl-2-oxazoline (30 eq., 2 mL, 23.6 mmol) was added, the solution being heated in the microwave reactor for another 30 min at 140 °C (300 W). After cooling to room temperature, an adequate amount of mono functional N, N-dimethyl-4, 4'-azidianiline (Az, 1 eq., 0.19 g, 0.78 mmol) or bi functional 4, 4'-azidianiline (Az₂, 1 eq., 0.167 g, 0.78 mmol) azo-derivative was added and the reaction was left overnight at 50 °C. The solution was concentrated under reduced pressure and then precipitated in cold diethyl ether. The obtained product was dried under vacuum to give a brown powder used in the next step of the synthesis. Weight yield: 91 %.

#### 2.3.1.2. Step II- POx modification with a trimethoxysilane group.
Amphiphilic allyl-ended POx (1 eq.), (3-mercaptopropyl) trimethoxysilane (3 eq. per terminal C=C group) and AIBN (0.12 eq. per terminal C=C group) were introduced in a 50 mL round flask and dissolved in a CHCl₃ solution at a concentration of 0.4 M. Then, the mixture was heated at 50 °C for 72 h under a nitrogen atmosphere. The solution was concentrated under vacuum. The polymer was isolated by precipitation (two times) in cold diethyl ether, followed by product drying under vacuum. Weight yield: 85 %. ¹H NMR (400 MHz, CDCl₃) δ (ppm) = 0.73 (t, -Si-CH₂-CH₂, noted as l); 1.31 (m, -S-CH₂-CH₂-CH₂-N, noted as h); 1.70 (m, -Si-CH₂-CH₂-CH₂-S-, noted as k); 1.78-2.21 (s, CH₃ of poly(2-methyl-2-oxazoline) block M, noted as c); 2.53 (t, CH₂-CH₂-S-CH₂-CH₂, noted as j and i); 3.00 (n, CH₃ of Az fragment, noted as z); 3.16-3.89 (m, CH₂ of poly(2-methyl-2-oxazoline) block M and poly(2-phenyl-2-oxazoline) block P, noted as b; t, CH₂-N, noted as g; n, CH₃O-Si, noted as p); 6.87-7.55 (m, -CH=CH-, aromatic protons of poly (2-phenyl-2-oxazoline) block P, noted as f); 6.70-6.78 (d, -CH=CH-, aromatic protons of Az fragment, noted as e and d' or d and e when using Az₂ fragment); 7.65-7.74 (d, -CH=CH-, aromatic protons of Az fragment, noted as d and e').

### 2.3.2. Equations used to the calculation of experimental DP<sub>tot</sub>, M<sub>n</sub> and molar % of PheOx motifs

The total degree of polymerisation (DP<sub>tot</sub>) was calculated as a sum between the number of methyl oxazoline motifs (noted as DP<sub>M</sub>) and phenyl oxazoline motifs (noted as DP<sub>P</sub>), according to the equation (Eq. 1):

$$DP_{\text{tot}} = DP_{\text{M}} + DP_{\text{P}} \quad (1)$$

The number of methyl oxazoline motifs (DP<sub>M</sub>) was calculated considering the integral values of methyl protons of M block (noted as I<sub>c</sub>, between 1.98 and 2.2 ppm), compared to the integral of four aromatic protons of azo containing fragment (Az/Az₂) at 7.2-7.3 ppm (noted as I<sub>d/e'</sub> for Az or I<sub>d</sub> for Az₂), as it follows:

$$DP_{\text{M}} = \left( \frac{I_{c}/3}{I_{d/e'} \text{ or } I_{d}/4} \right)$$

The number of phenyl oxazoline motifs (DP<sub>P</sub>) was calculated by taking in account the integral values of phenyl protons of P block (noted as I<sub>f</sub>, between 7.5 and 7.7 ppm), compared to the integral of four aromatic protons of azo containing fragment (Az/Az₂) at 7.2-7.3 ppm (noted as I<sub>d/e'</sub> for Az or I<sub>d</sub> for Az₂), as it follows:

$$DP_{\text{P}} = \left( \frac{I_{f}/5}{I_{d/e} \text{ or } I_{d}/4} \right)$$

The molar % of PheOx motifs was calculated by dividing the number of 2-phenyl-2-oxazoline repeating motifs (DP<sub>P</sub>) by the total degree of polymerisation (DP<sub>tot</sub>), and expressing the result as a percentage, according to the equation (Eq. 2):

$$\text{Molar\% of PheOx} = \frac{DP_{\text{P}}}{DP_{\text{tot}}} \cdot 100 \quad (2)$$

Where DP<sub>P</sub> and DP<sub>tot</sub> were calculated in respect to the Eq. 1.

Experimental M<sub>n</sub> was calculated as a sum between the DP of P block (DP<sub>P</sub>) multiplied by molar mass of phenyl oxazoline unit (M<sub>P</sub>), the DP of M block (DP<sub>M</sub>) multiplied by molar mass of methyl oxazoline unit (M<sub>M</sub>), and considering the molar mass of chain extremities (M<sub>ext</sub>), according to the Eq. 3:

$$M_n = (DP_P \cdot M_P) + (DP_M \cdot M_M) + M_{\text{ext}} \quad (3)$$

Where DP<sub>P</sub> and DP<sub>M</sub> were calculated according to the Eq. 1, M<sub>P</sub> = 147.17 g/mol, M<sub>M</sub> = 85.10 g/mol, M<sub>ext</sub> = 476 g/mol (for multifunctional derivatives Az) or M<sub>ext</sub> = 684 g/mol (for bifunctional derivatives Az₂).

### 2.3.3. PDMS modification with trimethoxysilane end-groups

PDMS-vinyl (2 K) was kindly given by LBCM Lorient [39]. 2,2-dimethoxy 2-phenylacetophenone (0.4 g, eq), 3-mercaptopropyl(trimethoxy) silane (2.2 g, eq) and PDMS-vinyl (1 g, eq) were dissolved in chloroform (1:1 w/w). After 5 min of stirring at 1000 rpm, the mixture was placed in an UV chamber and irradiated at 365 nm for 1 h. Then, the product was dissolved in n-hexane and extracted three times with acetonitrile. The n-hexane containing phase was recuperated and concentrated under vacuum, resulting a yellow oil. Yield: 57 %

### 2.3.4. Investigation of UV-sensitive behavior of POx in solution

UV-Vis absorption spectroscopy was used to investigate the photo-responsive behavior of diazo containing POx in solution state. POx were dissolved in chloroform (0.1 mg/mL) and the kinetic UV-Vis measurements were performed after 5, 10 and 15 min by plotting the absorption intensity ~365-380 nm of samples irradiated by UV light (corresponding to trans-cis transition), as well as after exposure at visible light (specific to cis-trans transition).

### 2.3.5. Preparation of pure POx coatings and POx-PDMS hybrid coatings

The pure POx coatings were prepared by crosslinking of trimethoxysilane units of trimethoxysilyl modified-POx through a condensation reaction catalysed by a commercial catalyst (provided by an industrial partner) while the POx-PDMS hybrid coatings resulted from crosslinking between a trimethoxysilyl modified-PDMS (noted PDMS-TMS) and trimethoxysilyl modified-POx catalysed by the same catalyst.

The formulations were developed as it follows: silylated POx (1 %, 5 % or 8 % w/w in addition to the PDMS-TMS) was dissolved in chloroform (at a stock concentration of 10 mg/mL, 50 mg/mL or 80 mg/mL) and the solution was stirred at room temperature (200 rpm) for 24 h. The stock solution at 10 mg/mL will be used for hybrid POx/PDMS copolymer with 1 % POx additive, the stock solution prepared at 50 mg/mL will be used for the hybrid formulations with 5 % of POx additive, while 80 mg/mL solution was used for the formulation prepared with 8 % of POx. PDMS-TMS, a commercial catalyst (2.5 % w/w in respect to the total mass of silylated POx. Then, PDMS-TMS is added in a vial and
then the stock solution of POx in chloroform (10 mg/mL or 50 mg/mL) was added in order to obtain 1 %, 5 % or 8 % (w/w) additive compared to PDMS-TMS. The vial was vigorously stirred (1000 rpm) for 8 h (for Az-PH-M/Az₂-PH-M additives) or for 24 h (for Az-Pl-M/Az₂-Pl-M additives) at room temperature. Then, the catalyst was added (2.5 % w/w in respect to the total mass of POx and PDMS-TMS), the mixture being stirred (1000 rpm) for 1 min. Then, the mixture was applied on glass slide (76 mm × 26 mm) with a bar coater and the coatings were dried for 5 days at room temperature to allow a complete cross-linking. UV irradiated coatings were prepared by applying the formulations described previously on glass slides followed by irradiation of coatings in the UV chamber (1 h, 365 nm, 10 mW/cm²) and drying the samples for 5 days at RT, while keeping the samples in a dark and dry place.

## 2.4. Characterisation of coatings

### 2.4.1. Static contact angle measurements

A constant volume (2 µL) of water droplet was applied on the coatings and the contact angle was measured 180 s after contact between the drop and the surface. The contact angle values were presented as an average of 15 measurements taken on different areas of the coatings. Non-coated glass, PDMS-glass, as well as POx-glass coated surfaces were used as controls. For the study of trans/cis conformational reversibility under UV/Vis cycles in the case of POx-PDMS coated glass surfaces, the measurements were taken for non-irradiated and UV-irradiated (1 h, 365 nm, 10 mW/cm²) samples, as well as for exposed UV-irradiated samples at visible light for 1 day. These exposure times were selected based on the kinetics of contact angle evolution, indicating that the contact angle value stabilized after this specific time duration. (Fig. S5., Fig. S6.).

### 2.4.2. Atomic Force Microscopy (AFM)

Atomic force microscopy (AFM) was performed in tapping© mode with a Dimension Nanoman V AFM microscope from Brücker Instruments equipped with a nanoscope V. The tip used was a silicon point probe-plus from Nanosensors. Images of sizes 1 × 1 µm² were recorded. Data were treated with the software Gwyddion to determine the mean square roughness, S(q).

### 2.4.3. Nanoindentation

Nanoindentation measurements were performed with a Piuma set-up from Optics11-life (Netherlands). The probes used had radii of 10 µm and stiffness of 50 N/m. The instrument was operated in indentation control mode which uses a feedback loop to control precisely the depth of indentation. The indentation depth was fixed to 1.8 µm and the load indentation curves were fitted with a JKR model up to 1.5 µm assuming a poison ration of 0.5, at a Pmax (%) equal to 100 and a Pmax (%) equal to 20 for contact point. The Young's moduli reported were the averages of 9 indentations at different positions.

### 2.4.4. Effectiveness of silicone coatings against macrofouling deposition

A PVC plate (41 × 12 × 0.4 cm) was pretreated with a commercial primer and left to dry for 7 days. Moreover, a commercial antifouling paint (provided by an industrial partner) containing biocides was applied on the PVC plate in order to protect it against fouling. After drying, the samples for investigation (POx/PDMS coatings deposited on microscope glass surfaces; uncoated surfaces used as controls; 25 × 75 mm × 1 mm) were applied to the PVC plate by using a commercial silicone adhesive. Each investigated sample was disposed in triplicate on the PVC plate. Then, the PVC plate is immersed vertically (at a depth < 1 m) into the Atlantic Ocean (Kernevel Port, Lorient). The immersion started in June 2024, over 120 days, in order to estimate the fouling release potential of the coatings in real time-conditions in a period providing the highest peak of marine colonisation. After pre-defined time points, the macrofouling deposition was macroscopically evaluated by visual inspection of the samples.

## 3. Results and discussion

### 3.1. Synthesis and characterisation of amphiphilic photosensitive POx copolymers

The synthesis in two steps of di- and triblock POx is illustrated in Fig. 1. The initial step involves the cationic ring-opening polymerisation (CROP) of the first hydrophobic P block, starting from an allyl-tosylate initiator. Subsequently, without isolating the propagating POx species, the polymerisation of the second hydrophilic M block occurs from the terminal reactive oxazolinium group of the hydrophobic P block. The polymerisation is terminated by using UV-sensitive molecules containing either a mono-amino diazo fragment (Az) to yield the diblock copolymer (Az-P-M) or a bis-amino diazo fragment (Az₂) to obtain the triblock copolymer (Az₂-P-M) in good yield (>90 %). The second step involves the thiol-ene reaction between the allyl terminal groups of Az-P-M/ Az₂-P-M and (3-mercaptopropyl)trimethoxysilane in a final yield ~85 %. The reaction efficiency was checked by ¹H NMR spectroscopy which showed the disappearance of the characteristic signal of allylic protons at δ 5.8–6.3 ppm. (Fig. S1.) ¹H NMR spectra of final POx products also show the existence of CH₂-CH₂ protons characteristic to the introduced trimethoxysilane containing reagent (k:1.8 ppm, l: 0.9 ppm) as well as those in the α position of sulphur atom around 2.5–2.8 ppm (j, i) (Fig. 2.). The introduction of trimethoxysilane ending groups in POx facilitates the cross-linking with PDMS through siloxane bond formation and enhances compatibility with PDMS during the incorporation of the additive in silicone matrix.

By employing this two steps synthetic route, four distinct block copolymer architectures were successfully synthesised with the same length of hydrophilic M block (DP<sub>M</sub> ~ 30) and different lengths of hydrophobic P block (DP<sub>P</sub> ~ 6 and DP<sub>P</sub> ~ 30) by using the monofunctional Az (with low DP<sub>P</sub>: Az-Pl-M and high DP<sub>P</sub>: Az-PH-M) and bifunctional Az₂ (with low DP<sub>P</sub>: Az₂-Pl-M and high DP<sub>P</sub>: Az₂-PH-M). The resulting copolymer structures were characterized by ¹H NMR, SEC (Fig. 2., Table 1) as well as DOSY experiments (Fig. S2.) ¹H NMR analysis revealed a good correlation between the experimental global degree of polymerisation (DP<sub>n tot, exp</sub>) and M<sub>n</sub> which varied between 4 and 7 kDa. This indicates the efficient occurrence of chemical modifications of POx backbone end groups. However, we note that for diblock POx Az-Pl-M, the precipitation step was limited (due to low M<sub>n</sub>), which might explain the discrepancy between total theoretical and experimental DP<sub>n</sub>.

Moreover, the low dispersity (D < 1.3) and the monomodal SEC traces confirm the absence of side reactions and the preservation of end-group integrity during the polymerisation process. Furthermore, DOSY-NMR experiments (Fig. S2.) were performed in CDCl₃ and exemplified in the case of Az-PH-M and Az₂-PH-M POx providing a closed total degree of polymerisation. Both di/triblock POx showed well-aligned spots belonging to the hydrogen atoms of polyoxazoline backbone, diazo segments as well as trimethoxy-silane ending groups, which was evidenced by the presence of a unique horizontal line corresponding to the logarithmic value of diffusion coefficient (logD), proving that the copolymerization successfully occurred, without secondary product. The diffusion coefficient (D) was around 3.16 × 10⁻¹⁰ m² s⁻¹, in the range according to the literature [44].

### 3.2. Photosensitivity of azo POx precursors and resulting azo POx coatings

#### 3.2.1. Photosensitivity of azo POx precursor in solution

Molecules containing azobenzene ring provide switchable UV–Vis Light behavior. These derivatives display two characteristic absorption bands corresponding to π-π* transitions: one band at high intensity in the UV region (~320–380 nm) and a second band with lower intensity in the visible spectra (above 450 nm). The first band is associated to the isomerisation of *trans* to *cis* azo isomer that occurs under UV irradiation, while the second band corresponds to the reversible transformation
<figure><img src="image_3.png" /><figcaption>**Fig. 1.** Two-steps synthesis of photosensitive Azo POx. Step A: CROP polymerisation performed by addition of 1) 2-phenyl-2-oxazoline (P) in anhydrous acetonitrile, microwave, 300 W, 140 °C, 1 h and 2) 2-methyl-2-oxazoline (M) in anhydrous acetonitrile, microwave, 300 W, 140 °C, 30 min, followed by termination by using an azo terminating agent containing one primary amine group (Az) or two primary amine groups (Az₂) (50 °C, overnight); Step B: thiol-ene reaction with (3-mercaptopropyl)trimethoxysilane in order to yield PDMS reactive diblock or triblock POx (AIBN, CHCl₃, 50 °C, 3 days). l: low content in P; H: high content in P.</figcaption></figure>

from *cis* to *trans* that takes place under Vis light exposure [38]. Thus, changes in the intensity of the above mentioned absorption bands can be monitored through UV–Vis spectroscopy and use to follow the *trans/cis* isomerization in solution. UV–Vis spectroscopy measurements were performed of amphiphilic Azo POx in CHCl₃ at a concentration of 0.1 mg/mL. Overall, UV–Vis spectra of POx additive were characterized by a very broad signal, emphasizing a peak maximum in UV region (~370–380 nm) (Fig. S3.). Starting from these, the maximum absorbance intensity ~370–380 nm was plotted as function of time (5 min-time intervals over 15 min), for the samples irradiated under UV light (Fig. 3., A., grey trace) and visible (Vis) light (Fig. 3., A., dark trace). Before being irradiated under UV, azo POx derivatives predominantly exist as *trans* isomer. Once subjected to UV irradiation, the absorbance decreases progressively, which indicates the occurrence of *trans* to *cis* isomerization (Fig. 3., A., grey curve). Oppositely, when the samples were exposed to Vis light, the absorbance increases (Fig. 3., A., dark curve). These results indicate that *cis* isomer underwent the reverse step

to *trans* isomer. Overall, the observed tendencies proved the occurrence of *trans-cis* isomerization in resulting derivatives, being consistent with those previously reported for other azo molecules [38,42,43].

The *trans-cis* azobenzene isomerization could also impact the thermal transition of azo-containing polymers and in particular the glass transition temperature (*T*<sub>g</sub>) which significantly varies depending on the chain mobility, as stated in the literature. Diblock and triblock POx powders were analysed in bulk by differential scanning calorimetry (DSC) before UV (non-irradiated samples) and after UV irradiation (Fig. S4.). First, DSC was assessed on the non-irradiated samples and the *T*<sub>g</sub> was depicted onto the second heating cycle. For a similar total DP<sub>n</sub> and length of P block, a slight increase of *T*<sub>g</sub> (from 76 °C Az-PH-M to 80 °C Az<sub>2</sub>-PH-M) was observed between di- and triblock POx (Fig. 3., B., dark contoured bars), which might be explained by an increase of chains crowding in triblocks compared to diblocks. Moreover, the samples were irradiated under UV (1 h, 10 mW/cm<sup>2</sup>), this longer irradiation time being chosen in order to maximize the isomerization efficiency in solid
<figure><img src="image_4.png" /><figcaption>Fig. 2. Characterisation of photosensitive diblock and triblock POx by ¹H NMR spectroscopy performed in CHCl₃ and SEC in DMF using PMMA calibration.</figcaption></figure>

<table><caption>**Table 1**
Summarized characteristics of diblock (Az-Pl/H-M) and triblock (Az₂-Pl/H-M) POx.</caption><thead><tr><th>Polymer</th><th>DP<sub>n tot,th</sub></th><th>DP<sub>n tot,exp</sub><sup>a</sup></th><th>%P<sub>exp</sub><sup>b</sup></th><th>M<sub>n,NMR</sub><br>(g/mol)<sup>c</sup></th><th>M<sub>n,SEC</sub><br>(g/mol)<sup>d</sup></th><th>D<sup>d</sup></th></tr></thead><tbody><tr><td>Az-Pl-M</td><td>50</td><td>35</td><td>15</td><td>3670</td><td>4400</td><td>1.25</td></tr><tr><td>Az-PH-M</td><td>50</td><td>54</td><td>48</td><td>6860</td><td>6500</td><td>1.20</td></tr><tr><td></td><td></td><td></td><td>Triblock POx</td><td></td><td></td><td></td></tr><tr><td>Az₂-Pl-M</td><td>50</td><td>42</td><td>15</td><td>4660</td><td>4100</td><td>1.23</td></tr><tr><td>Az₂-PH-M</td><td>50</td><td>60</td><td>50</td><td>7650</td><td>7800</td><td>1.30</td></tr></tbody></table>

a Calculated as a sum of DP<sub>P</sub> and DP<sub>M</sub>, by comparing the aromatic signals of Az or Az₂ moiety (7.2–7.3 ppm) with those of methyl region (1.98–2.2 ppm) for M block and with aromatic signals of phenyl ring (7.5–7.7 ppm) for P block, according to the equation Eq. 1; b Calculated by dividing DP<sub>P</sub> by DP<sub>tot</sub>, according to the equation Eq. 2; c Calculated as a sum of DP<sub>P</sub>·M<sub>P</sub> and DP<sub>M</sub>·M<sub>M</sub> and including the chain extremities (M<sub>ext</sub>), according to the equation Eq. 3; d Calculated by SEC in DMF, using PMMA standard calibration. l: low content in P; H: high content in P.

state (i.e. where the molecular mobility is more restricted in comparison to solution state). Since it is known that the heat could also induce a back isomerization from cis-to-trans configuration, for UV irradiated samples, the methodology consisted of two heating cycles performed successively, in order to compare the thermal behavior of the original cis-isomer obtained after UV (first heating cycle) and the trans-isomer (which might occur over the second heating cycle on the UV irradiated sample). For both di- and triblock POx, we noted that the T<sub>g</sub> slightly decreases after UV irradiation (Δ = 5 °C), which is explained by an increase of the mobility of the chain segments upon transformation of trans isomer into cis isomer, this tendency being consistent with previous works from the literature [34].

### 3.2.2. *Pure azo POx coatings*

After confirming the photoresponsive behavior of Azo POx precursors in solution and in solid state, the effect of UV irradiation of the pure Azo POx coatings on the glass surface was examined especially the surface wettability, by measuring the mean static contact angle (θ) in presence of water. This provides information about the coating's ability to repel water. Non-irradiated pure Azo POx coatings were prepared by applying a solution of POx additive in CHCl₃ onto glass surfaces, followed by drying to ensure the complete solvent removal from the coating (Fig. 3, C.). For UV irradiated samples, the pure POx coatings were deposited on the glass surfaces and then irradiated under UV (λ = 365 nm) for one hour before being dried at room temperature according to the general procedure described in the Materials and Methods section (Fig. 3, C.). Mean contact angle values were summarized in Fig. 3, D. Globally, POx coatings exhibited low θ (below 50°) which is consistent with the amphiphilic nature of the additives and with previous reports for polyoxazoline structures [27–29]. Before UV, as expected, θ increases with the length of P block (i.e. Az₂-Pl-M: 30° vs. Az₂-PH-M: 50°) and the copolymer hydrophobicity. In addition, the triblock POx coatings show significantly higher θ compared to diblock POx (i.e. Az-PH-M: 32° vs. Az₂-PH-M: 50°). Our hypothesis is related to the chain flexibility of diblock, which results in swollen M hydrophilic chains on the surface, being consistent with low θ. In contrast, triblocks provides two trime-thoxysilane end groups which can form cyclic or loop-like structures when anchored to the surface. According to the literature, cyclic architecture introduces steric hindrance in the coating, which could explain the observed increase of θ [45,46]. After UV exposure, the contact angle (θ) decreases significantly, whatever the POx used. This can be explained by the differences in the polarity of azo isomers reported in the literature: the trans isomer is nonpolar, while the cis isomer is polar. So, the increased polarity enhances affinity towards water, and decreases the θ. However, the shift of θ after UV exposure was more significant for POx with shorter hydrophobic P blocks. This is consistent with the positioning of azo segments near the hydrophilic M blocks. This
<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /><figcaption>**Fig. 3.** Effect of UV irradiation on Azo POx. (A) Photo-isomerisation of azo group in Az-PH-M solution in CHCl₃ (0.1 mg/mL) upon exposure to UV irradiation (grey trace, from *trans*- to *cis*-conformation) and upon exposure to Vis Light (black trace, from *cis*- to *trans*- conformation), as a function of exposure time (min); (B) Glass transition temperature (Tg) of diblock Az-PH-M and triblock Az₂-PH-M before and after UV irradiation (365 nm, 1 h, 10 mW/cm²); (C) Formulation scheme of non-irradiated and UV-irradiated POx coatings on glass surface; (D) Mean contact angle (towards water) of diblock and triblock POx coatings before and after UV irradiation (365 nm, 1 h, 10 mW/cm²). ND: not determined due to a too rapid spreading of the water drop on the surface; (E) AFM images (1 µm × 1 µm) of diblock and triblock POx coatings before and after UV irradiation (365 nm, 1 h, 10 mW/cm²) as well as the calculated root mean square roughness S(q).</figcaption></figure>
means that higher is the hydrophilicity, higher is the impact of UV irradiation on wettability properties.

The roughness of the surfaces was also determined to consider an eventual physical change of them (Fig. 3., E). Regardless the type of additive (di or triblock), the coatings were homogeneous, with a mean square roughness S(q) below 6 nm showing negligible surface roughness (i.e. Az-PH-M: 0.64 nm vs. Az₂-PH-M: 0.79 nm). A slight decrease in S(q) upon UV irradiation, which aligns with literature findings [40,42] and can be attributed to the coiling of azo segments into a more compact and uniform conformation (Fig. 3., E). Overall, UV irradiation increases the coating hydrophilicity and lowered the θ in uniform topography. This can be explained by the increased chain flexibility conformation due to azo segments bound to hydrophilic pendant blocks.

### 3.3. Hybrid POx-silicone coatings

As mentioned previously, the introduction of trimethoxysilane ending groups in POx enhance the coating stability by covalent bondings by the cross-linking via self-condensation of PDMS through siloxane bond formation, while the existence of an amphiphilic copolymer providing hydrophobic blocks enhances compatibility with PDMS (Fig. 4., A). Hybrid POx-silicone coatings were prepared by a similar methodology as Azo POx coatings, described in Materials and Methods section. POx/silicone formulations were prepared by using 1 and 5 % (w/w) of POx additive in the PDMS matrix. Higher amounts of additives (i.e. above 8 %) resulted in heterogeneous mixtures, being thus excluded from the present study. However, we noticed that the POx additives with high length of hydrophobic P blocks (i.e. Az-PH-M and Az₂-PH-M) were incorporated uniformly and quickly into PDMS, while the additives containing short P blocks (i.e. Az-Pl-M and Az₂-Pl-M) required longer time to get homogeneous mixtures. This is consistent with the expected contribution of hydrophobic block which improved the compatibility with silicone. Thus, further investigations were centered around the POx-silicone coatings (with 1 and 5 % POx) prepared with amphiphilic additives providing long hydrophobic P blocks.

#### 3.3.1. Effect of POx additive in silicone coatings

The hybrid coatings with 1 % and 5 % of POx additive were analysed before irradiation, to examine whether the addition of Azo POx additive in silicone formulation impacts the wettability and the mechanical properties of the resulting materials. Water static contact angle measurements were performed (Fig. 4., B. and Table 2). Compared to the silicone control (θ: 103°, green bar), the introduction of 1 % POx resulted in rather negligible change of θ (Δθ ~ 1–3°, dark bars), while 5 % of additive led to a slight decrease of θ (Δθ ~ 5–8°, dark bars) (Table 2). This indicates that the addition of amphiphilic POx slightly decreases the hydrophobicity of PDMS. However, these variations could be considered significant given the low amounts of additive introduced in the silicone matrix. A possible hypothesis is that such an increase in water affinity could be attributed to the presence of the hydrophilic segment, from the POx copolymer, on the surface of the hybrid material. The presence of additive also modified the stiffness of the surface as attested the measurement of Young Modulus by AFM (Fig. 4., E and F). The elastic modulus significantly reduced from 3 (control PDMS) to 1 (1 % POx) and 0.7 MPa (5 % POx) knowing that the literature reports a E value lower than 1 GPa for the hydrophobic polyoxazolines. [45]

#### 3.3.2. Effect of UV irradiation

The POx-silicone coatings were exposed to UV irradiation (1 h, 10 mW/cm²) by using the same protocol as pure POx coatings, and the θ was measured. As observed in Fig. 4., B. the UV irradiation led to a significant decrease of θ (grey bars) explained by the increase of polarity in cis azo isomer, as aforementioned in the article for POx precursors. We observed that the variation of the contact angle after UV irradiation was more pronounced for coatings prepared with diblock POx than those containing triblock POx: Δθ ~ 13°–15° for Az-PH-M and Δθ ~ 5–9° for

Az₂-PH-M coatings. An explanation of the limited effect on triblock Az₂-PH-M coatings could be that the POx chains (link and crosslink to PDMS by their both end groups) move in a more sterically restricted conformation like a loop or cyclic polymers which limits the change of conformation and the behavior towards a drop of water.

The microstructural topography of coatings with different ratios of POx was also investigated by AFM (Fig. 4., C and D). The results showed relatively homogeneous coatings, as evidenced by a mean S(q) below 6 nm. Moreover, a soft increase in S(q) was noticed for 5 % of POx and explained by the increased density of polymer chain additive into silicon matrix. From AFM measurements Young Modulus of hybrid coatings was also deduced (Fig. 4., E and F; grey bars). Regardless the type of POx (di/triblock), UV irradiation resulted in the increase of Young Modulus, especially with 5 % of additive. For diblock POx-silicone, Young Modulus increased of 36 % for 1 % additive and 266 % for 5 % additive whereas only 10 % for 1 % additive and 238 % for 5 % additive for triblock POx-silicone coatings (Table 2). While the larger differences observed with 5 % POx could be attributed to an increased density of UV-sensitive POx chains (this behavior being reported for other types of azo derivatives) [47], the overall increase in Young's modulus observed after UV contrasts, which typically show a decrease after UV exposure. It is important to underline that these studies involve coatings prepared from a single type of additive (hydrophilic or hydrophobic) unlike in our case amphiphilic additives was used in hydrophobic matrix.

In conclusion, diblock Azo POx behaviors very differently that triblock Azo POx. In the first case, the effect of the change of the polarity of azobenzene ring impacts the behavior towards water without change of the chain mobility. By contrast, for triblock, the photo-isomerization under UV changed the coating structure without modification of the water behavior.

#### 3.3.3. Effect of Vis light exposure

To investigate the reversibility of cis-trans isomerization of azobenzene dye in POx-silicone coatings, the UV-irradiated coatings were exposed to Vis light for 1 day. This protocol was chosen based on kinetic study, monitored via θ measurements, showing that the contact angle becomes constant after 1 day of exposure (Fig. S6.). In terms of wettability (Fig. 4., B), we observed an increase of θ after Vis Light exposure, which indicates the switch to the less polar trans isomer. 29 % decrease of Young Modulus for 1 % additive and 53 % for 5 % additive for diblock POx-silicone after Vis light exposure, whereas 4 % for 1 % additive and 54 % for 5 % additive were observed for triblock POx-silicone coatings (Table 2). This is consistent with the observed changes in Young's modulus, where visible light induced softness of the surface material while UV light created more stiffness coatings. The comparison of topographies observed by AFM (Fig. 4., C and D) of UV-irradiated coatings vs. those exposed back to Vis Light, revealed that UV exposure decreased the roughness, making the coatings more stiffness while visible light exposure increased S(q) again.

In addition, exposing the UV-irradiated coatings to the Vis light resulted in a decrease of Young Modulus (Fig. 4., E and F). To check the reproducibility of trans-cis isomerization, the Young Modulus variation between UV-irradiated samples and those exposed to Vis light, additional measurements were performed by nanoindentation (Table 2). First, nanoindentation measurements were performed in air/surface interface, as in the case of AFM measurements. The overall trend remained consistent with that observed by AFM: UV exposure increased the Young Modulus, while visible light exposure decreased it. However, it is important to note that the Young Moduli obtained by nanoindentation were generally approximately 2× higher than those obtained by AFM for the UV-irradiated samples (e.g. for 1 % POx in diblock POx-PDMS mixture, 1.9 MPa by AFM vs. 3.85 MPa by nanoindentation). Additionally, for the samples exposed to Vis light, the values obtained from nanoindentation were approximately 3.5× higher than those from AFM (e.g. for 1 % POx in diblock POx-PDMS mixture, 1.35 MPa by AFM vs. 3.67 MPa by nanoindentation). Nevertheless, when comparing the
<figure><img src="image_9.png" /></figure>

-Flexible-
(low Young Modulus
compared to control PDMS)

-Compact-
(Young Modulus increase)

-Less compact-
(Young Modulus decrease)

<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /></figure>

<figure><img src="image_14.png" /></figure>

<figure><img src="image_15.png" /></figure>

(caption on next page)
**Fig. 4.** Effect of the amount of POx additive on the properties of POx/PDMS coatings using diblock Az-PH-M and triblock Az₂-PH-M. (A) Scheme of supposed structural changes in POx/PDMS coatings induced by UV/Vis Light exposure; (B) Mean contact angle (towards water) of diblock and triblock POx/PDMS coatings (with 1 and 5 %w/w POx additive) before and after UV irradiation (365 nm, 1 h, 10 mW/cm²) as well as after Vis Light exposure (1 day), compared to control PDMS; (C, D) Measurement of the roughness of the surface S(q) from AFM images (section: 1 µm × 1 µm) of diblock and triblock POx/PDMS coatings (with 1 and 5 %w/w POx additive) after UV irradiation (365 nm, 1 h, 10 mW/cm²) and after Vis Light exposure (1 day); (E, F) Young Modulus (MPa) determined by AFM for diblock and triblock POx/PDMS coatings (with 1 and 5 %w/w POx additive) before and after UV irradiation (365 nm, 1 h, 10 mW/cm²) and after Vis Light exposure (1 day).

<table><caption>**Table 2**

Characterisation of POx-silicone hybrid coatings.</caption><thead><tr><th rowspan="2">POx % (w/w)</th><th colspan="2">Mean contact angle in water (°)</th><th colspan="2">S(q) (nm)</th><th colspan="6">Young Modulus (MPa)</th></tr><tr><th>Before UV</th><th>After UV</th><th>UV exposure</th><th>Vis Light exposure</th><th>AFM</th><th>Nanoindentation<br>Air</th><th>Nanoindentation<br>Water</th><th>AFM</th><th>Nanoindentation<br>Air</th><th>Nanoindentation<br>Water</th></tr></thead><tbody><tr><td>1 %</td><td>102</td><td>87</td><td>0.30</td><td>Diblock<br>(Az-PH-M)<br>0.60</td><td>1.90</td><td>3.85</td><td>3.71</td><td>1.35</td><td>3.67</td><td>3.45</td></tr><tr><td>5 %</td><td>98</td><td>85</td><td>4.60</td><td>5.30<br>Triblock<br>(Az₂-PH-M)<br>2.00</td><td>2.05</td><td>3.69</td><td>3.60</td><td>0.96</td><td>3.63</td><td>3.52</td></tr><tr><td>1 %</td><td>100</td><td>95</td><td>1.96</td><td>3.80</td><td>1.21</td><td>4.20</td><td>3.61</td><td>1.16</td><td>3.72</td><td>3.30</td></tr><tr><td>5 %</td><td>95</td><td>86</td><td>2.90</td><td></td><td>2.20</td><td>4.81</td><td>3.53</td><td>1.00</td><td>3.80</td><td>3.14</td></tr></tbody></table>

effect of UV or Vis light for the same sample, it was less pronounced in nanoindentation than in AFM (e.g. for 1 % POx in diblock POx-PDMS mixture, by AFM: 1.9 MPa after UV and 1.35 MPa after Vis light; by nanoindentation: 3.85 MPa after UV and 3.67 MPa after Vis light). This difference between the results obtained with AFM vs. nanoindentation techniques were already reported in the literature [48]. According to the literature [48], nanoindentation and AFM are both suitable at nanoscale resolution, the mechanical properties being measured by indenting the surface at a defined force, in order to deform it. Taking into account the indenter position relative to the tested surface, the deformation as a function of the applied force can be calculated, and the Young Modulus can be determined. However, one of the significant differences between AFM and indentation is that AFM is highly sensitive to surface-level effects, detecting changes on the nanometer scale (nm³), whereas indentation is more sensitive to deeper effects, in general displayed in the micrometer range (µm³). These differences influenced in consequence the resulting Young modulus.

Globally, our results emphasized that the difference of Young Modulus obtained after UV vs. Vis Light exposure for a same sample was proportionally higher by AFM, compared to nanoindentation. This confirms that the exposure at UV and Vis light have a strong interfacial effect and weaker bulk effect, which correlates with this trend. Additionally, when comparing the type of POx copolymer architecture (i.e. diblock vs. triblock) and its response with UV and Vis light exposure, the variations are more significant for diblock by AFM, while nanoindentation showed stronger effects for triblock based formulations. These results are in line with the assumption that diblock POx impact mostly the surface of the coatings (being easily detected via AFM) whereas triblock POx can impact both the surface and the bulk of the coating (being more evident in nanoindentation).

Additional Young's Modulus measurements were conducted at the water/surface interface by nanoindentation, allowing for a closer simulation of real experimental conditions (Table 2). In general, as observed for the measurements taken at the air/surface interface and by AFM, Young's Modulus followed the same trend: exposure to UV light increases the modulus, while exposure to visible light decreases it. However, when comparing the Young's Modulus obtained by nanoindentation in air versus in water, a difference (ΔE) of approximately 0.5 MPa was observed, with the Young Modulus values in water being lower than those in air. These differences confirm that the mechanical properties change where the coatings are immersed in water, then they are exposed only to the air.

Lastly, two consecutive cycles of UV/ Vis light exposure were performed on POx-silicone coatings, and their surface and the mechanical

properties were investigated. For example, the properties of coatings containing 1 % additive are shown in Fig. 5. Generally, regardless of the type of additive used (di- or triblock POx), the variation in contact angle, Young Modulus, and S(q) under consecutive UV and Vis light exposure was similar. This indicates a reversible switching of the surface and mechanical behavior of the coatings mediated by light.

### 3.4. Marine macrofouling on POx-silicone coatings

In-situ tests to evaluate the fouling release activity of the coatings were performed during 120 days during summer period in the Atlantic Ocean (Kernevel Port, Lorient). This time was selected since it aligns with a period when fouling development is at its highest peak, a phenomenon specific to temperate climate in Lorient area. Briefly, the coatings prior deposited to glass surfaces were attached to a PVC plate which was immersed in the seawater. PVC necked support, as well as silicone coated glass surfaces were used as control. The visual inspection of coatings during immersion in seawater at different time points allowed the evaluation of fouling release efficiency against the macrofouling (Fig. 6.). This test aligns with the relevance of the chosen immersion site during the boom season. The uncoated control substrates were quickly covered by macrofouling after only 14 days, while PDMS coatings without additive start to be less effective after 21 days.

Distinct aspects were observed when analyzing POx-PDMS coatings. In the first 60 days, these coatings demonstrated improved performance compared to the PVC control, as shown by the presence of homogeneous surfaces without areas settled by macrofouling. It is important to underline that throughout the 60-day period, no significant differences were observed, regardless of the type of POx (di/triblock), the POx content (1 %, 5 %, or 8 %), or the specific UV or Vis light treatment applied to the coating. This performance might be attributed to the PDMS performance and is consistent with previous work [39]. Moreover, these results confirm that the addition of POx additives does not have a negative effect on the coatings surface. Uniform coatings providing low-roughness topology (S<sub>q</sub> < 6 nm) were obtained which is according to the literature [10–12], more effective to avoid macrofouling adhesion compared to rougher surfaces. However, a color change was observed for PDMS coatings prepared with low amount of POx additive (1 % w/w) (see columns 3 and 9 in Fig. 6.). This might indicate a consequent release of the POx additive from the matrix, but without impacting the macrofouling anti-adhesion efficacy.

After 60 days, some differences became apparent during visual inspection of the samples. While no differences were related to the type of POx structure or amount, the exposure to the UV irradiation or to the Vis
<figure><img src="image_16.png" /><figcaption>A</figcaption></figure>

<figure><img src="image_17.png" /><figcaption>B</figcaption></figure>

<figure><img src="image_18.png" /></figure>

<figure><img src="image_19.png" /></figure>

**Fig. 5.** Investigation of UV/Vis Light reversible behavior after two cycles of POx/silicone coatings, by using Az-PH-M or Az₂-PH-M as POx additives. (A) Mean contact angle measurements (towards water) after two consecutive cycles of UV/Vis light exposure; (B) Young Modulus (MPa) and root mean square roughness S(q) determined by AFM (section: 1 µm × 1 µm) under consecutive UV/Vis Light exposure. UV exposure: irradiation at 365 nm, 1 h, 10 mW/cm². Vis light exposure: 1 day. The UV/Vis light reversible properties were tested for POx/PDMS formulations containing 1 % w/w of POx additive.

light strongly impacted the macrofouling adhesion properties. In particular, we noticed that the samples prior treated by Vis light exhibited less macrofouling adhesion compared to those prior exposed to UV light. This behavior may be correlated with the Young's modulus results, which showed that Vis light-exposed coatings had Young modulus 2× lower compared to the UV-irradiated samples. This finding aligns with the literature, which suggests that surfaces with a lower Young's modulus are generally more effective at mitigating fouling adhesion.

Overall, macrofouling adhesion was observed for all coatings after 120 days, however POx-silicone coatings showed better resistance to fouling adhesion compared to the PDMS reference sample. These results confirm the interest of the POx additive for marine applications. To improve performance, it seems important to limit the release of the additives.

## 4. Conclusions

This study presented photo-sensitive hybrid silicone coatings with potential fouling release properties for maritime industry. The silicone coating formulations integrated functional POx additives that combine amphiphilicity and photo-responsive azobenzene groups. These

additives own a good compatibility with PDMS precursor resulting in homogeneous coatings with S(q) values below 6 nm. Compared to pure silicone coatings, the presence of POx additives resulted in softer surfaces, providing Young modulus below 2 MPa. Such soft materials are attractive alternatives for fouling release applications avoiding the settlement of organisms like shells and macroalgae.

Notably, the surface and the mechanical properties of the coatings changed under UV and Vis light irradiation. UV irradiation significantly increases the stiffness of the surface consistent with the structural isomerization from *trans* to *cis* form of azobenzene units. Oppositely, the exposure of the coatings to Vis light switched the *cis* azo isomer back to the *trans* form, resulting in a more flexible network with a decreased Young Modulus and higher elasticity. Reversible cycles of the modulation of surface properties were observed according to under alternative UV/Vis light irradiation.

By this study the potential contribution of the polyoxazolines for marine antifouling was shown. The versatile chemistry of this class of polymers allows the design of smart additives for coatings decorated by photosensitive building block to modulate the surface under stimulation.
<figure><img src="image_20.png" /><figcaption>**Fig. 6.** Investigation of the covering of POx-silicone surfaces by a macrofouling observed during the in-situ immersion in Atlantic Ocean, over 120 days during summer period. Uncoated PVC substrates as well as silicone coated substrates were used as controls.</figcaption></figure>

**CRediT authorship contribution statement**

LVA: conceptualization, performed experiments, writing manuscript and reviewing-editing; MAB: performed experiments, writing manuscript; MR: performed experiments; RM: manuscript correction, funding; FA: manuscript correction, funding; LH and SB: manuscript correction; VL: conceptualization, supervision, funding, manuscript correction, reviewing-editing. All authors have given approval to the final version of

the manuscript.

**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.
# Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.porgcoat.2025.109096.

## Data availability

Data will be made available on request.

## References

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