Tailored design of Cu₂O nanocube/silicone composites as efficient foul-release coatings

Abstract

Environmental concerns about the use of toxic antifoulants have increased the demand to develop novel, environmentally-friendly antifouling materials. Silicone coatings are currently the most effective non-toxic alternatives. This study focused on developing a model for silicone foul-release nanocomposites that were successfully designed, fabricated, characterized, and tailored toward foul-release (FR) coatings. A series of elastomeric polydimethyl-siloxane (PDMS)/Cu₂O nanocube composites with different nanofiller concentrations was successfully synthesized, for the first time, as FR coatings via solution casting technique. Emphasis was given to the study of the physicomechanical and surface properties, as well as the easy release efficiency of the elastomer PDMS enriched with Cu₂O nanocubes. The bulk properties of the nanocomposites appeared unchanged after adding low amounts of nanofillers. By contrast, surface properties such as contact angle and surface free energy were improved, and the settlement resistance and easy release behavior of the nanocomposites were enhanced. The surfaces were further proven to have reversible tunable properties and are thus renewable in water. The antifouling property of the nanocomposites was investigated by laboratory assays involving microfoulants such as Gram-positive and Gram-negative bacteria, as well as yeast organisms, for 30 days. Exposure tests showed that lower surface energy and elastic modulus of coatings resulted in less adherence of marine microfouling. The most profound effect recorded was the reduction of fouling settlement with nanofiller loadings of up to 0.1% Cu₂O nanocubes. Thus, the good foul release and long-term durability confirmed that the present strategy was an attractive nontoxic and environmentally-friendly alternative to the existing antifouling systems.

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1. Introduction

Marine fouling is an extensive natural phenomenon that causes serious problems in the marine environment and for the shipping industry.^1,2 Shipping accounts for approximately 90% of global trade, and seaborne trade has nearly quadrupled over the past four decades.³ Once attached to the hull, fouling increases friction resistance because of surface roughness, thereby leading to an increase in hydrodynamic weight and subsequent top speed reduction and loss of maneuverability.⁴ Consequently, fouling increases fuel consumption, which in turn increases emissions of harmful compounds such as CO₂, NO_x, and SO_x to the atmosphere.⁵ The increase in fuel consumption can be up to 40%, and the overall voyage cost can increase by as much as 77%.⁶ The economic effects of hull fouling have accelerated the development of antifouling (AF) technologies, a global industry that has reached a worth of approximately US$ 4 billion annually.⁷ Traditionally, fouling is prevented through the application of AF paints that release biocides, which are toxic to marine organisms but may also affect non-target species. The wide-spread use of toxicants has raised concerns about their harmful effects on marine communities and led the International Maritime Organization in 2001 to the universal prohibition of further application of tributyltin compounds, which have been widely used before the complete phase-out of their use in 2008.⁸ Alternative tin-free AF coatings that employ copper and/or booster biocides are the principal replacement coatings. Unfortunately, their effects have been found to extend to non-target species and present potential ecological risk to 95% of organisms in the water column even at very low concentrations.⁹

The substantial environmental toxicity issues that surround the use of biocidal AF coatings have driven research in an environment-friendly direction with a particular focus on natural marine compounds and foul-release (FR) technology.¹⁰ Natural AF compounds also face regulatory hurdles with the estimated cost of assembling data packages on efficacy and environmental fate amounting to millions of dollars as well as with the timeline for the approval process.¹¹

Non-stick, silicone FR coatings present a feasible, cost-effective alternative to biocidal AF coatings. Silicone coatings rely on a technology that acts in two ways: inhibiting the settlement of colonizing species and weakening their adhesion strength. By providing low-friction ultra-smooth surfaces, organisms that stick can be easily removed hydrodynamically ideally by simply bringing the ship to speed. These coatings do not leach and could be more durable than tin-free AF paints for certain vessel applications.¹² Silicone polymers based on polydimethyl-siloxane (PDMS) have been the most promising FR coating system.¹³ The superior properties of PDMS FR coatings are due to their low surface energy, low surface roughness, low porosity, and high molecular mobility.¹⁴ The O–Si–O linkage, which presents water repellency, causes the good thermal stability, excellent resistance to oxygen, ozone, and UV light, anti-stickiness, and low chemical reactivity of the coating.¹⁵ Coatings based on silicone elastomers have inherently good FR properties; however, they require reinforcing additives (usually mineral fillers) to improve their specific properties and reduce the costs. This reinforcement can be achieved by incorporating inorganic nanoparticles (NPs) and conventional macro- and micro-scale composites because of the increased interaction at the polymer filler interface for the nanocomposites.¹⁶ The extent of nanocomposite property improvement depends on filler properties, concentration, morphology, degree of dispersion, and degree of adhesion with polymer chains.^15,17

The use of NP-based metal oxide coatings represents a promising approach for the development of non-toxic control technologies for micro-fouling organisms. Surfaces can be engineered with low-surface energy coatings that minimize biological adhesion strength and allow FR with modest brushing/water spray pressures or with coatings that can prevent fouling through their photocatalytic activity. Metal oxide NPs are stable during contact with microorganisms.¹⁸ TiO₂ NPs pose a greater potential than silica in minimizing biofouling on optical surfaces. In addition, techniques like nano-metal oxide coatings seem to be an effective method for combating fouling.¹⁹ Among them, Cu₂O NPs are relatively easy to make, safe, and inexpensive, and the natural abundance of its source materials favors the fundamental and practical research on Cu₂O.²⁰

Cu₂O NPs exhibit excellent antibacterial activity against Gram-positive and Gram-negative bacteria.^18,21 A study has proven that surface hydrophobicity/super-hydrophobicity can be achieved on modified nano-Cu₂O films,²² which showed potential for our field of application. A variety of interesting Cu₂O nanostructures has been synthesized.^23,24 Nanocubes represent one of the most important structural types of Cu₂O because several other crystal morphologies of Cu₂O (e.g., nanocages, octahedral, and more complicated structures) can be prepared through the shape transformation of Cu₂O nanocubes.²⁵ Furthermore, the antibacterial activity of cubic Cu₂O NPs against Escherichia coli is superior to that of octahedral Cu₂O NPs. The polar properties of the {100} crystal planes of Cu₂O nanocubes are believed to perform an important function in the increased antibacterial activity.²⁶

In the present work, a series of hybrid PDMS/cubic Cu₂O nanocomposites was fabricated via solution casting technique for use as FR coatings. The surface properties were discussed based on the changes in water contact angle and surface free energy. New functions of Cu₂O nanocubes were introduced here based on increasing the easy cleaning phenomena through raising hydrophobicity and lowering surface free energy that result in ultra-smooth surfaces with a mechanism that involves physical anti-adhesion. This research highlights the significance of the extent of dispersion of CuO₂ nanofillers in determining the improvement in the physicomechanical and surface properties of the nanocomposites. Furthermore, AF performance was examined through biological assays to evaluate the nanocomposite FR behavior. The findings in this context are attractive for their merits such as simplicity, safety, environmental benignancy, commercial feasibility, and good potential for easy-cleaning systems.

2. Experimental section

2.1. Chemicals

Octamethylcyclotetrasiloxane (D₄, 98%), which was used as PDMS source, tetramethyldivinyldisiloxane (C₈H₁₈OSi₂, 97%), polymethylhydrosiloxane (PMHS; Mn = 1700–3200), and platinum catalyst commonly known as Karstedt catalyst (platinum(0) and divinyltetramethyl-disiloxane in solution to control catalyst concentration, stability, viscosity, and inhibition, as well as easy dosing and formulation; Pt content: 8–11%) were obtained from Sigma-Aldrich Company Ltd., USA. Copper sulfate (CuSO₄), which was used as copper source, and ascorbic acid were delivered from Acros Company (Belgium). Potassium hydroxide, sodium hydroxide, orthophosphoric acid, trichloroethylene, toluene, and all solvents are analytical reagent grade and were purchased from Merck, Mumbai, India and used as received.

2.2. Preparation of vinyl-terminated PDMS

In a three-neck round-bottom flask fitted with a condenser, a thermometer jacket, and a nitrogen inlet and outlet, a definite quantity of distilled D₄ was introduced to remove Si–H- and Si–OH-containing species. Finely grinded potassium hydroxide (0.55%), which has the alkali metal counter ion K⁺, was then added. The temperature was gradually increased to 145 ± 5 °C and was kept constant for 3 h, during which the viscosity of the material was tremendously increased. Afterwards, tetramethyldivinylsiloxane (2 × 10⁻⁴ mol) was added, and the reaction was continued for another 3 h. The temperature was then lowered gradually to RT with stirring for 8 h to stop the reaction and complete the chain termination. The prepared polymer was dissolved in toluene, and the unreacted KOH was neutralized by adding concentrated H₃PO₄ drop wise while stirring vigorously and detecting the pH of the resultant solution. The solution was stirred overnight for complete neutralization and precipitation of the salt generated and then subjected to filtration and toluene removal.

2.3. Preparation of cuprous oxide nanocubes

Cu₂O nanocubes were prepared with copper sulfate as starting material via a simple technique. Exactly 20 mL of NaOH aqueous solution (0.075 mol L⁻¹) was added into 10 mL of CuSO₄ aqueous solution (0.5 mol L⁻¹) with stirring (pH = 10.5). Then 25 mL of ascorbic acid aqueous solution (0.1 mol L⁻¹) was added dropwise into the above solution with vigorous stirring at RT. After 1 h, a yellow precipitate was obtained (pH = 4–4.5). The particles were separated from the solution by centrifugation (4233_EC+ laboratory centrifuge, Italy) at 2000 rpm for 30 min. The product was washed by distilled water and absolute ethanol. The final product was dried in vacuum at 60 °C for 8 h.

2.4. Curing of the prepared vinyl-terminated PDMS

The preparation of unfilled PDMS film was easily employed through the addition curing system. It was carried out by the addition reaction of the polyfunctional silicon hydride PMHS with the unsaturated groups in polysiloxane chains, and the bond-forming reaction is called hydrosilation curing. To carry out hydrosilation curing, 10 g of the prepared polymers was dissolved in 40 mL of toluene with continuous stirring until a homogenous solution was obtained. Exactly 0.035 g of Karstedt catalyst dissolved in trichloroethylene (10 mL) was then added and stirred for 30 min. A homogenous solution of 0.3 g of PMHS in 10 mL of toluene was added drop wise under stirring. The resulting solution was degassed and formed air bubbles for 15 min to remove any dissolved gases from the solution. The degassed solution was used to coat cleaned surfaces and slides, which, upon the evaporation of the solvent, gave a smooth sheet of cured PDMS with uniform thickness. The PDMS was completely cured at RT for 16 h.

2.5. Preparation of PDMS/Cu₂O nanocomposites

To prepare the PDMS/Cu₂O nanocomposites, Cu₂O NPs were dispersed in toluene by ultrasonication (Sonics & Materials, VCX-750, USA; at 20 kHz frequency and equipped with a 13 mm-diameter titanium probe) in an ice bath for 15 min. A solution of the prepared vinyl-terminated PDMS resin in toluene was then added with stirring for 10 min and sonicated for additional 10 min. The solution was subjected to hydrosilation curing as described above.

2.6. Apparatus

Certain characterization methods for the prepared polymer, metal oxide, and their nanocomposites are discussed; however, the bulk of these methods provide information on the physicochemical and surface properties of the nanocomposites.

The Fourier transform infrared (FTIR) spectra were recorded using a Nicolet iS10 (Thermo Scientific, USA) with 1 cm⁻¹ resolution and 4000–400 cm⁻¹ range. The samples were cast on potassium bromide (KBr) pellets (FTIR grade, Alfa Aesar, Karlsruhe, Germany). ¹H NMR spectra were recorded on a Varian Mercury VXR-300 NMR spectrometer at 300 MHz (Varian, Inc., Palo Alto, CA, USA) using tetramethylsilane Me₄Si (TMS) as internal standard and CDCl₃ as the main solvent.

Particle size measurement based on the principles of dynamic light scattering (DLS) was performed using a Brookhaven Instruments 90Plus model nanoparticle size/zeta potential analyzer (USA). The accurate sizes of the NPs were analyzed by TEM because DLS gives hydrodynamic nanoparticle size. High-resolution transmission electron microscopy (HRTEM) was conducted with an electron microscope (JEM2100 LaB6, Japan) at 200 kV accelerated voltage and with 0.14 nm point–point resolution. In HRTEM, the solid sample was dispersed in ethanol solution using an ultrasonicator and then dropped on a copper grid coated with carbon film. Prior to inserting the samples in the HRTEM column, the grid was vacuum dried for 10 min. The nanocomposite samples for TEM analysis were prepared by ultra-cryomicrotomy with a Leica Ultracut UCT (Leica Microsystems GmdH, Vienna, Austria). Freshly sharpened glass knives with 45° cutting edges were used to obtain cryosections with approximately 100–150 nm thickness at −150 °C. The cross sections were collected individually in sucrose solution and directly supported on a 300-mesh copper grid.

X-ray diffraction (XRD) is a versatile and non-destructive technique that reveals detailed information about the chemical composition and crystallographic structure of natural and synthetic materials. XRD patterns were measured using a Panlytical X'pent PRO (Netherlands) with monochromated CuKα radiation with scattering reflections recorded for 2θ angle between 10° and 80° corresponding to d-spacing between 1.47 and 3.26 Å. To confirm the resolution of the diffraction peaks with standard reproducibility in 2θ (±0.005), the sample measurement was recorded using a monochromator and detector, which were used to generate focusing beam geometry and parallel primary beam. The standard diffraction data were identified according to the International Centre for Diffraction Data (ICDD) software with PDF-4 release 2011 database.

The optical micrographs of the samples obtained by mechanical mixing were recorded with an Olympus BH-2 microscope (Japan) where the images were obtained using Image J software program. Scanning electron microscopy images were obtained by a scanning electron microscope (JEOL JSM530). Before insertion into the chamber, the disk-like monolith substrates were fixed on the SEM stage using carbon tapes. Gold (Au) films were deposited on the substrates at RT using an ion sputter (EDWARDS S150). The distance between the target and the disk-like monoliths substrate was 5.0 cm. The sputtering deposition system used for the experiments consists of a stainless steel chamber, which was evacuated down to 8 × 10⁻⁵ Pa with a turbo-molecular pump backed up by a rotary pump. Before sputtering deposition, the Au target (4 in. diameter, 99.95% purity) was sputter cleaned in pure Ar. The Ar working pressure (2.8 × 10⁻¹ Pa), the power supply (100 W), and the deposition rate were kept constant throughout the investigations. Moreover, to record the SEM images of the disk-like monoliths well, the SEM micrographs were operated at 20 keV.

2.7. Test methods for the cured polymer and its nanocomposites

2.7.1. Tensile modulus. The tensile properties of the model FR coatings were obtained in accordance with ASTM D₄₁₂ method. Dynamic mechanical analysis (DMA) was performed in tension mode using a TTDMA (UK) from TA instruments. Rectangular-shaped (30 mm × 5 mm), free standing samples were cast from the solution. The tensile modulus was assessed at RT from stress–strain at 1 Hz single frequency, 2 N preload, and 0.5–27 μm amplitude.

2.7.2. Swelling tests. For the swelling tests, rectangular pieces of the synthesized unfilled PDMS and PDMS/Cu₂O nanocomposites (1 cm (l) × 1 cm (w) × 0.5 cm (h)) were weighed and then immersed in 100 mL of heptane for 24 h. The solution was renewed three times during the test, and after the allotted time, the final swollen weight was determined. Each point recorded is the mean of three measurements. The swelling degree at equilibrium, SD (%), is expressed as a percentage and was calculated according to the literature²⁷ and by using eqn (1).

SD (%) = ((W_f − W_i)/W_i) × 100 (1)

where W_f was the final swollen weight of the sample at t and W_i is the initial weight of the dry sample. The sample measurements were determined at 25 °C.

2.8. Contact angle measurements

Static contact angle measurements were performed on the fabricated unfilled and filled PDMS/Cu₂O nanocomposites on coated microscopic slides using a Tantec line of contact angle meter apparatus (Germany) and the sessile drop technique. The hydrophobic/hydrophilic character of the PDMS layer was evaluated by measuring the contact angle between the surface of the coating and drops of test liquids. The results are the mean of the minimum of three determinations. The test liquids were water (JT Baker, HPLC grade), diiodomethane, and ethylene glycol (Aldrich products of the highest purity available).

2.9. Wetting behavior and surface tension measurements

The measured values of contact angles were used to extract the surface tension (γ^total_S) of the cured polymer films and nanocomposites following the VOCG thermodynamic approach.²⁸ It relies on the Fowkes's equation, which assumes the total surface energy to be the sum of different interaction components at the liquid–solid interface and postulates a geometric mean relationship for both the solid–liquid and liquid–liquid interfacial tensions.²⁹

The total surface tension of a solid γ^total_S is composed of three additive components: the Lifshitz–van der Waals dispersion component, γ^LW_S, the polar electron-donor (Lewis base) component, γ_S⁻, and the polar electron-acceptor (Lewis acid) component, γ_S⁺ (eqn (2) and (3)):

which results in the VOCG approach with the form

Utilizing the surface tension γ_L of at least three different liquids of known components, two polar and one nonpolar, are necessary to obtain the three equations that can be solved for the unknowns of the solid, γ^LW_S, γ_S⁺, and γ_S⁻. This research used diidomethane as the nonpolar liquid, and water and ethylene glycol were chosen as the polar liquids.

2.10. Biological assays

2.10.1. Microorganisms' details. Representatives of microorganisms that cause microbial fouling in cooling water systems, cooling towers, and ship's hull were tested. The tested organisms were the following:

(i) Gram positive bacteria: Staphylococcus aureus, NCTC-7447 (Gram +Ve 1) and Bacillus subtilis, NCTC-1040 (Gram +Ve 2); (ii) Gram negative bacteria: Pseudomonas aeruginosa, NCTC-10662 (Gram −Ve 1) and Escherichia coli, NCTC-10416 (Gram −Ve 2); and (iii) yeast: Candida albicans, IMRU 3669.

Nutrient broth media were used for the cultivation and maintenance of the tested microorganisms. The nutrient broth composition (g L⁻¹) was as follows: peptone, 5.0 g; NaCl, 5.0 g; yeast extract, 2.0 g; and beef extract, 1.0 g.³⁰ Basal salt media were used for the weight loss and biodegradability tests. The foregoing media broth composition (g L⁻¹) was as follows: potassium dihydrogen orthophosphate, 2.44 g; sodium dihydrogen orthophosphate, 5.57 g; ammonium chloride, 0.5 g; glycerol, 6.4 mL; magnesium chloride, 2.44 g; calcium chloride, 5.57 g; ferrous sulfate, 2.00 g; yeast extract, 0.1 g; and distilled water, 850 mL.³¹

2.10.2. Weight loss measurements. In weight loss experiments, 100 μL of fresh culture broth of each of the tested microorganisms was injected in 100 mL bottles that contain 30 mL of basal salt media broth. Coated samples were hung in the medium using nylon threads. Weight loss was calculated using eqn (4):³²

Weight loss (mg cm⁻²) = ((W_before − W_after)/time) (4)

where time is the duration of sample immersion in days.

2.10.3. Biodegradability test. The biodegradation study of the prepared PDMS compounds (as painted glass slides) was done in 100 mL batch flasks that contain 30 mL of basal salts medium with an initial pH of 7 prepared according to the literature.³³ The incubation period was 30 days at 30 °C in a shaking incubator (150 rpm). After the test period, the slides were removed from the medium, washed with distilled water, and dried. The amount of degradation was determined by studying the weight loss according to the literature.^34,35 The biodegradable percentage (BD) was determined from the weight loss measurements using eqn (5).

%BD = ([W_C − W_S]/W_S) × 100 (5)

where W_C and W_S are the weight loss of the sheets in grams in both control and sample conditions. Each value was the average of three separate experiments.

3. Results and discussion

3.1. Prepared PDMS design characterization and curing

PDMS belongs to the water-insoluble matrix class and has unique properties that distinguish it as a FR coating. PDMS has methyl (–CH₃) side chains that cause its low surface energy (20–24 mJ m⁻²) and a flexible inorganic –Si–O backbone linkage that causes its extremely low elastic modulus (≈1 MPa), which are both essential for the extremely low adhesion of fouling on silicone coating surfaces. Thus, biofilms can be easily removed from the surface by simple mechanical cleaning or during vessel movement.^36,37

In 2000, Wynne et al. evaluated two types of PDMS coatings, namely, the hydrosilation-cured and the condensation-cured PDMS, and found that the unfilled hydrosilation-cured PDMS has superior properties such as hydrophobicity, roughness, stability in water, non-shrinkage, and lower adhesion of barnacles compared with filled condensation-cured PDMS.³⁷

Vinyl-terminated PDMS was obtained via anionic ring opening polymerization of D₄ tetramer (because it is a less expensive and more readily available monomer) using a strong base catalyst (KOH), which is frequently used to ring open D₄ at common polymerization temperatures of 140–160 °C (ref. 14 and 38) (see ESI, Scheme S1†). Siloxane dimers (tetramethyldivinylsiloxane) are usually used as end-capping reagents to control the molecular weight.³⁹ After reaching equilibrium, the reaction is quenched by adding a strong acid (orthophosphoric acid). The conversion or polymerization rate of D₄ is high at the beginning and then flattens out with time because of the decrease in monomer concentration during polymerization and because the living centers are enclosed by polymer chains in bulk polymerization.

The FTIR spectrum of the prepared vinyl-terminated PDMS sample revealed absorption bands at 2963 and 2905 cm⁻¹ ascribed to asymmetric –CH₃ stretching, at 1411 cm⁻¹ assigned to –CH₃ symmetric deformation, and at 1595 cm⁻¹ assigned to Si–CH=CH₂ stretching absorption. The band at 1261 cm⁻¹ corresponds to CH₃ symmetric deformation, that at 1096 cm⁻¹ to Si–O–Si asymmetric deformation, that at 866 cm⁻¹ to Si–O–Si skeletal stretching, and that at 699 cm⁻¹ to the symmetric stretching of the Si–C bond in –Si(CH₃) group. The absence of any absorption peak at 2060 cm⁻¹ and 3000–3500 cm⁻¹ indicates the absence of any hydrosilane (Si–H) or hydroxyl groups (Si–OH) in the prepared polymer (see ESI, Fig. S1†).^39a

The ¹H nuclear magnetic resonance (NMR) spectrum distinguishes the signals of the chemical shift at 1.00 ppm caused by (Si–CH₃) from those at 5.94–6.2 and 5.71–5.92 ppm by CH₂=CH–Si and CH₂=CH–Si. The absence of a chemical shift at 4.6 ppm indicates the absence of Si–H and Si–OH linkages. The DSC sample was super-cooled at −130 °C and then heated from −130 °C to 50 °C with a glass transition (T_g) at −122 °C, cold crystallization at −95 °C, and melting (T_m) at −46 °C. A crystallization exothermal peak is observed during the cooling step and a single melting endothermic peak during the second heating step. The low T_g of silicones as reflected in their molecular mobility may contribute to their superior FR characteristics.^39b

The curing of the prepared vinyl-terminated PDMS follows the hydrosilation curing mechanism where vinyl end-blocked polymers react with the SiH groups carried by functional oligomers. The addition occurs mainly on the terminal carbon and is catalyzed by organometallic compounds, preferably platinum metal complexes, to enhance their compatibility. This reaction has no by-product. Molded pieces made with a product from this curing mechanism are very accurate (no shrinkage). The mechanism of platinum hydrosilation (see ESI, Scheme S2†) was proposed by Chalk and Harrod,⁴⁰ and the catalytic cycle has also been reported before.¹⁴

3.2. Nanofiller morphology and characterization

To regulate the shape and size in wet-chemical techniques, most synthetic strategies in preparing Cu₂O NPs involve surfactants or template reagents. However, these additives are usually expensive, toxic, and hard to wash, and thus may affect the performance of the products. In this work, Cu₂O nanocubes were successfully prepared and controlled without using any template or surfactant at room temperature (RT) (Scheme 1). The dominant factor that influences the morphology and size of the particles is the concentration of the NaOH used. At low NaOH concentration, the Cu₂O crystal nuclei were ineffectively capped, remained in nanoscale, and grew randomly. In addition, OH⁻ ions affect the stability of {100}, leading to a cubic morphology.^25,41 However, continuous lowering of the NaOH concentration may result in other forms of copper nanocubic morphology.⁴² In this study, when the concentration of NaOH solution was decreased to 0.075 M, small nanocubes with an average side length of 70–110 nm were obtained. On the contrary, the nanocube size increased with increasing concentration of NaOH solution. At small nanoscale, the number of particles per unit area increases, and thus antibacterial effects can be maximized.⁴²

Scheme 1 Preparation of Cu₂O nanocubes (note: the picture on the right shows the crystal structure of Cu₂O oriented to show {100} plane).

The reaction mechanism can be summarized as follows: cupric sulfate could be dissolved in water and form a uniform ionic solution. When NaOH is added to the solution, Cu²⁺ reacts with OH⁻ and forms blue insoluble Cu(OH)₂. At RT, Cu(OH)₂ is decomposed into cupric oxide and water. Ascorbic acid is then added as reducer to reduce cupric oxide (CuO) into Cu₂O. In this process, NaOH serves not only as a reagent, but also for adjusting the pH of the solution.

The FTIR spectrum of the prepared Cu₂O NPs revealed a strong absorption band at 626 cm⁻¹ attributed to the Cu–O linkage of Cu₂O, which agrees with previous literature.⁴³ Therefore, the as-prepared products were pure Cu₂O because no infrared-active mode of CuO around 530 cm⁻¹ appeared (see ESI, Fig. S2†).

The high morphological uniformity of these Cu₂O crystals is reflected in their XRD patterns shown in Fig. 1. The strong and sharp diffraction peaks suggest that the resultant products were well crystallized. The characteristic peaks for Cu₂O (2θ = 29.69, 36.52, 42.41, 61.68, 73.61) marked by indices [(110), (111), (200), (220), (311)] showed that the resulting Cu₂O was essentially crystalline. All the peaks of the prepared Cu₂O NPs match well with that of standard Cu₂O, and no diffraction peaks from metal copper or cupric oxide appear in the XRD patterns.

Fig. 1 XRD pattern of the as-synthesized Cu₂O nanocubes, inside DLS of the as-synthesized Cu₂O nanocubes, and schematic of a liquid droplet on the surface of coated glass slide.

The size of the Cu₂O crystallites was estimated from the Debye–Scherrer eqn (6):

D = (Kƛ/β_1/2)cos θ (6)

where K is the Scherrer constant, which is related to the crystallite shape; ƛ and θ are the radiation wavelength and Bragg's angle, respectively; and β_1/2 is the full width at half maximum of the diffraction peak. The crystal sizes of the product were calculated and proven to be in the nanosize range.

DLS is a non-invasive technique that measures the size and size distribution of NPs dispersed in a liquid, as shown in Fig. 1. The size distribution profile of the synthesized NPs showed an average particle size of 90 nm. The polydispersity index of Cu₂O NPs was 0.236, which indicates that the particles are polydispersed in nature. These results matched with the results from the XRD analysis.

The transmission electron microscopy (TEM) analysis (Fig. 2A–C) and SEM analysis (Fig. 2D) of the prepared Cu₂O NPs showed that they have approximately uniform size, cubic shape, clean surface, and particle sizes between 70 and 110 nm. The inset shows the selected area electron diffraction (SAED) patterns (Fig. 2E) obtained by directing the electron beam perpendicular to the square faces of the cube. The square symmetry of this pattern indicates that each Cu₂O nanocube was a single crystal bounded mainly by {100} facets. Furthermore, it exhibits individual NPs and clear lattice fringes with d spacing of 0.25 and 0.30 nm, corresponding to the {111} and {110} reflections of the cubic Cu₂O structure, respectively (Fig. 2F).

Fig. 2 (A), (B), and (C) are the TEM images of the prepared Cu₂O nanocubes at low and high magnifications; (D) corresponding SEM images of the as-synthesized Cu₂O nanocubes; (E) corresponding SAED patterns of the as-synthesized nanocubic Cu₂O; (F) corresponding crystal lattice, which is consitent with XRD results; and (G) and (H) are the TEM images of the PDMS/Cu₂O nanocomposites (0.1% nanofillers) at low and high magnification powers.

3.3. Nanocomposite design and physicomechanical characterization

For the fabrication of PDMS/Cu₂O nanocomposites appropriate for marine easy-release coatings, investigating various concentrations of Cu₂O nanocubes to be embedded into the PDMS matrix is crucial. The detailed fabrication process of the PDMS/Cu₂O nanocomposites is shown in Scheme 2. The TEM observations of the PDMS/Cu₂O nanocomposites (Fig. 2G and H) demonstrate that a complete disagglomeration of low concentrations of Cu₂O nanocubes is achieved in the nanocomposites, whereas high concentrations show a different trend. The bright background shows the polymer matrix, whereas the dark cubic structures are the structure of Cu₂O NPs, and their diameter is approximately 90 nm. In terms of concentration (0.1% nanofillers), individual cubes are well dispersed and separated from one another. Thus, the finest extent of dispersion is achieved, and the samples exhibit high-quality dispersion without any remaining aggregate, thereby causing significant improvement in the nanocomposite properties.

Scheme 2 Synthesis of PDMS/Cu₂O nanocomposites.

The FTIR spectra of the PDMS/Cu₂O nanocomposites provide evidence for the interaction between the polymer and the NPs. The shifts in the absorptions for Si–O–Si asymmetric deformation and Si–O–Si skeletal deformation are less vivid for low concentrations but are well observed when the nanofiller content was increased up to 5% loading. The peak for Si–O–Si asymmetric deformations shifts from 1034 cm⁻¹ for vinyl-terminated PDMS to 1020 cm⁻¹ for the nanocomposites, and the peak for Si–O–Si skeletal stretching shifts from 805 cm⁻¹ to 792 cm⁻¹ for the 5% loading sample. The increase in the intensity of the peak at 3500 cm⁻¹, which is due to the OH stretching of the H-bond of the adsorbed water on the surface of Cu₂O, did not appear at low concentrations but was observed on high filler loadings (see ESI, Fig. S3†).

The tensile modulus of the prepared nanocomposites is illustrated in Fig. 3. It is not affected by the incorporation of a small amount of Cu₂O NPs (up to 0.1%) in the nanocomposites with an average value remaining at 3.6 ± 0.5 MPa. In other words, the stiffness of the silicone-based materials remains constant. A progressive increase of up to 8 MPa is observed for high concentrations (5% nanofiller loadings), which may be explained by aggregation and agglomerations that cause the increase in stiffness. For the PDMS/Cu₂O nanocomposites and within the limited content of nanofillers (less than 0.5 wt%), the dynamic stress was unchanged with an average value of 1300 ± 185 Pa and then increased up to 3315.5 Pa (for the high content in Cu₂O at 5 wt% nanofillers), which may be caused by the presence of aggregates at high concentrations.

Fig. 3 Tensile modulus of the unfilled and filled PDMS/Cu₂O nanocomposites with different loadings.

3.4. Surface and AF investigation of the nanocomposite films

Biofouling is a dynamic process that spans numerous length scales and involves a complex variety of molecules and organisms. Surface chemistry is a significant factor in the formation, stability, and release of the fouling organisms' adhesion to surfaces.⁴⁴ The hydrophobicity of the surface of hybrid nanocomposites was evaluated using static contact angle measurements, as shown in Fig. 4. The measurements were performed before and after immersion in demineralized water for 7 days.^15,45 The unfilled PDMS contact angle obtained before immersion was 102 ± 2°, indicating a hydrophobic character, whereas the contact angle after immersion dropped to 90°, indicating the decrease in surface hydrophobicity. With different loading concentrations of Cu₂O nanofillers before and after immersion, the wettability of the coatings shows an increase in the hydrophobic behavior with 0.1% nanofillers. The water contact angle analysis (i.e., at 130°) shows a film surface that is simultaneously hydrophobic and lipophobic due to the well dispersion of filler NPs, leading to (1) increased surface area and chemical bonding, and (2) reduced the surface roughness of polymer-NPs patterns, as shown in Table S1.† Results shows a decrease in the hydrophobicity at high NPs concentrations for both the un-immersed and immersed samples, indicating the effect of agglomeration and aggregation of NP nanofillers. As a result, the absence of the chemical boding interactions between the polymer and agglomerated filler NPs onto surface pattern may occur with the high-concentration of NP fillers.⁴⁶ Table S1† shows that with high-concentrations of NPs used, the fine Cu₂O NPs may tend to combine together and form strongly bonded aggregates onto surfaces, leading to drastically decrease in surface area and hydrophobic character. The enhanced surface hydrophobicity increases the surface roughness and facilitates the fouling adhesion. Our finding also shows that the contact angle reaches to a value close to that obtained before immersion under drying condition. Therefore, the unfilled and filled silicone surfaces appear to have reversibly tunable properties.⁴⁶

Fig. 4 Water contact angle of the unfilled and filled PDMS/Cu₂O nanocomposites.

Surface energy measurements on the PDMS/Cu₂O nanocomposites were performed according to the van Oss–Chaudhury–Good (VOCG) model by measuring the surface contact angle with both polar (water, as in Fig. 4, and ethylene glycol) and nonpolar (diiodomethane) liquids, as shown in Table 1. The calculated surface free energy values of the unfilled and filled PDMS are summarized in Table 1. The results illustrate low surface free energy for the 0.1% Cu₂O nanofillers and thus low adhesion of microorganisms. By contrast, the surface free energy gradually increases with increasing filler loadings of up to 5% because of the roughness caused by agglomerations and aggregation.

Table 1 Contact angle (θ) measurements for various solvents (ethylene glycol and diiodomethane); total surface tensions (γ^total_S) of the chemically synthesized PDMS and filled PDMS/Cu₂O nanocomposites controls before (dry) and after (wet) immersion in distilled water for 7 days in various solvents according to VOCG equation; and the percentage degree of swelling SD (%) in n-heptane as a good solvent^a

Sample design	θ Ethylene glycol		θ Diiodomethane		γ^totalS (mN m^−1)		Swelling degree in n-heptane (SD (%))
	Dry	Wet	Dry	Wet	Dry	Wet
a Note: γ^totalS calculated with van oss–Chaudhury–Good approach, Lifshitz–van der Waals component γ^LWS, lewise base component γS^− and lewise acid component γS^+.
PDMS blank	84° ± 2°	78° ± 2°	75° ± 1°	70° ± 2°	20.23	23.06	93.6 ± 1%
PDMS/Cu2O (0.01%)	95° ± 2°	88° ± 3°	80° ± 2°	75° ± 3°	17.85	21.32	90.3 ± 2%
PDMS/Cu2O (0.05%)	104° ± 3°	97° ± 3°	84° ± 2°	79° ± 2°	16.22	19.6	88.5 ± 1.5%
PDMS/Cu2O (0.1%)	120° ± 2°	112° ± 2°	89° ± 1°	85° ± 2°	14.096	16.39	89.9 ± 1%
PDMS/Cu2O (0.5%)	106° ± 1°	96° ± 3°	85° ± 1°	80° ± 1°	15.47	18.55	88.6 ± 2.6%
PDMS/Cu2O (1%)	98° ± 3°	89° ± 2°	81° ± 2°	76° ± 2°	17.37	20.2	88.4 ± 1.8%
PDMS/Cu2O (3%)	91° ± 2°	87° ± 2°	76° ± 2°	72° ± 1°	20.38	23.11	87.5 ± 1.2%
PDMS/Cu2O (5%)	83° ± 4°	79° ± 4°	71° ± 1°	67° ± 2°	23.31	31.31	85.9 ± 2.5%

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Swelling measurements represent a technique of choice for the characterization of the polymer network. This test consists of immersing a piece of composite in a good solvent (heptane) and monitoring the evolution of the swelling mass at regular intervals. The swelling degree of the filled PDMS was lower than that of the unfilled, as shown also in Table 1. A low swelling degree is typical of a more important crosslinking density. Thus, Cu₂O NPs can be seen as additional (physical) cross-linking points, acting positively during network formation because of the excellent affinity between components.

Biodegradation can be defined as the process in which substances are broken down by the action of microorganisms. The microorganism's growth in the material increases the size of pores and induces cracks. As a result, the structure of the material is destabilized.⁴⁷ The biodegradation of PDMS in natural or living organisms has been poorly examined.³⁹ PDMS has been treated as nonbiodegradable and inert. At the turn of 1970s, the possibility of their biodegradation has been proven.^48,49 Only few studies that focus on the biodegradation of various siloxanes have been reported. Nevertheless, the lack of studies did not affect the general assessment that polysiloxanes are a group of polymers that is difficult to biodegrade.⁴⁹ In this work, AF and biodegradation analysis was performed on the prepared PDMS/Cu₂O nanocomposites as follows.

Weight loss tests were carried out on the unfilled and filled PDMS nanocomposites from 0.1% PDMS/Cu₂O (which was proven to enhance characteristics with low nanofiller loading of 0.01% and 0.05%, as described in the aforementioned tests) and up to 5% fillers. A comparison of the weight loss results of the unfilled and filled PDMS nanocomposites at various loadings were carried out and shown in Fig. 5A, which suggests that the weight loss is higher for the unfilled PDMS and decreased to nearly zero with 0.1% loadings in PDMS/Cu₂O. As shown in Fig. 5B, the biodegradability percentages for 30 days were calculated, where the blends with low nanofiller concentrations degrade slowly, whereas those with high nanofiller concentrations degrade rapidly in 30 days. This finding indicated that the significant FR characteristics were found with design patterns that have well dispersion of Cu₂O NP fillers at loading amount up to 0.1%. In such patterns, the Cu₂O NPs may lead to increase surface area, chemical bonding, and surface smoothness of polymer-NPs surfaces (see Table S1†). Consequently, the enhancement in the surface properties and the adhesion resistance of microorganisms is achieved, thereby preventing surface deterioration. On the contrary, with the surface design patterns fabricated at high concentrations of up to 5% filler NPs, the gradual increase in the fouling adhesion is due to the agglomeration and aggregation of the NPs at high filler concentrations.

Fig. 5 (A) and (B) present the weight loss and biodegradability measurements, respectively, of the unfilled and filled PDMS/Cu₂O nanocomposites with different microorganisms [Gram (+Ve 1), Gram (+Ve 2), Gram (−Ve 1), and Gram (−Ve 2) bacteria and yeast].

Generally, the resistance of Gram-negative bacteria toward antibacterial substances is related to the hydrophobic surface of their outer membrane rich in lipopolysaccharide molecules. The membrane acts as barrier to the penetration of numerous antibiotic molecules associated with the enzymes in the periplasmic space, which are capable of breaking down the molecules introduced from outside.⁵⁰

The biofilm coverage on the surface of silicone was roughly determined through the biofilm formation images in the optical microscope for the unfilled and filled PDMS specimens before and after immersion in the used microorganisms, as shown in Fig. 6. For the unfilled PDMS, given that the silicone and glass slide are transparent, the picture background was white if no bacteria are attached to the silicone surface. The surface coverage of the dark area (fouled area) relative to the total area was assumed to be the surface coverage of the bacteria. The well-dispersed NPs observed, which are related to the low concentrations of Cu₂O NPs up to 0.1%, were the reason for the homogeneity and immunity of the surface. By contrast, the non-homogeneity observed for the blank samples were due to the fouling settlement on its surface, which is observed for the characteristics achieved from the failure mechanism technique. With increasing nanofiller concentrations of up to 5%, the specimens were also densely fouled. The SEM micrographs for the unfilled and filled polymer nanocomposites (see ESI, Fig. S4†) were obtained and showed Cu₂O NPs as white spots. A good dispersion and homogeneity of NP distributions is observed for concentrations up to 0.1%, thus affords improved surface characteristics and enhanced immunity against microorganisms. By contrast, additional loadings up to 5% of Cu₂O NPs lead to aggregation and agglomerations and consequently increased surface roughness. Thus, air voids may be trapped between the agglomerates and reduce the surface properties and immunity of the coatings against fouling.

Fig. 6 Optical microscope images (A), (B), (C), and (D) of the unfilled PDMS; (E), (F), (G), and (H) of the 0.01% nanofillers in the PDMS/Cu₂O nanocomposites; (I), (J), (K), and (L) of the 0.05% nanofillers in the PDMS/Cu₂O nanocomposites; (M), (N), (O), and (P) of the 0.1% nanofillers in the PDMS/Cu₂O nanocomposites; (Q), (R), (S), and (T) of the 0.5% nanofillers in the PDMS/Cu₂O nanocomposites; (U), (V), (W), and (X) of the 1% nanofillers in the PDMS/Cu₂O nanocomposites; (Y), (Z), (AA), and (AB) of the 3% nanofillers in the PDMS/Cu₂O nanocomposites; (AC), (AD), (AE), and (AF) of the 5% nanofillers in the PDMS/Cu₂O nanocomposites; all images were recorded before and after immersion in microorganisms for 30 days.

4. Conclusions and outlook

A series of PDMS/cubic Cu₂O nanocomposites with advanced surface properties was successfully fabricated for use as FR coatings. The XRD and TEM results of the prepared Cu₂O NPs exhibit truncated nanocubes bounded by {100} facets with an average diameter of 90 nm. These nanocomposites were synthesized with various filler concentrations, and conventional hydrosilation curing mechanism was employed using platinum catalyst and Si–H functional cross-linker. The inclusion of small concentrations (0.1%) of NPs can significantly enhance the surface properties and, to a lesser degree, the physicomechanical performance of the nanocomposites because of well dispersion. However, high NP concentrations (5%) tend to strongly agglomerate (particle clustering) because the intermolecular attraction forces of the NPs are high given their large surface area, thus reducing the nanocomposite properties. The surface characteristics of the prepared nanocomposites were studied from a mathematical viewpoint using appropriate mathematical tools. At low levels, an increase in hydrophobic character and a decrease in surface tension were achieved because of the well dispersion of nanofillers. The surface is homogenous such that sessile drops of test liquids assume a hemispherical shape with maximum increase in contact angle for 0.1% nanofillers, which enhance the easy cleaning phenomena. The results obtained from the present investigation revealed that the AF potential of PDMS/Cu₂O gradually increased with increasing filler loadings up to 0.1% but decreased at high loading levels because of agglomerations. The presence of Cu₂O nanocubes in the PDMS matrix endows it with various properties that make the polymer nanocomposite a promising candidate as an environment-friendly FR coating.

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Footnote

† Electronic supplementary information (ESI) available. CCDC 73304. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra01597a

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