Simple Preparation of Polydimethylsiloxane and Polyurethane Blend Film for Marine Antibiofouling Application

Article Simple Preparation of Polydimethylsiloxane and Polyurethane Blend Film for Marine Antibiofouling Application

Jirasuta Chungprempree 1,2, Sutep Charoenpongpool 1, Jitima Preechawong 1 , Nithi Atthi 3 and Manit Nithitanakul 1,2,*

1 The Petroleum and Petrochemical College, Chulalongkorn University, Chula Soi 12, Wangmai Pathumwan, Bangkok 10330, Thailand; [email protected] (J.C.); [email protected] (S.C.); [email protected] (J.P.) 2 Center of Excellence on Petrochemical and Materials Technology, Chula Soi 12, Wangmai Pathumwan, Bangkok 10330, Thailand 3 Thai Microelectronics Center (TMEC), National Electronics and Computer Technology Center (NECTEC), Chachoengsao 24000, Thailand; [email protected] * Correspondence: [email protected]

Abstract: A key way to prevent undesirable fouling of any structure in the marine environment, without harming any microorganisms, is to use a polymer film with high hydrophobicity. The polymer film, which was simply prepared from a blend of hydrophobic polydimethylsiloxane elastomer and hydrophilic polyurethane, showed improved properties and economic viability for antifouling film for the marine industry. The field emission scanning electron microscope and energy dispersive X-ray spectrometer (FESEM and EDX) results from the polymer blend suggested a

homogenous morphology and good distribution of the polyurethane disperse phase. The PDMS:PU blend (95:5) film gave a water contact angle of 103.4◦ ± 3.8◦ and the PDMS film gave a water contact ◦ ± ◦ Citation: Chungprempree, J.; angle of 109.5 4.2 . Moreover, the PDMS:PU blend (95:5) film could also be modified with Charoenpongpool, S.; Preechawong, surface patterning by using soft lithography process to further increase the hydrophobicity. It was J.; Atthi, N.; Nithitanakul, M. Simple found that PDMS:PU blend (95:5) film with micro patterning from soft lithography process increased ◦ ◦ Preparation of Polydimethylsiloxane the contact angle to 128.8 ± 1.6 . The results from a field test in the Gulf of Thailand illustrated and Polyurethane Blend Film for that the bonding strength between the barnacles and the PDMS:PU blend (95:5) film (0.07 MPa) Marine Antibiofouling Application. were lower than the bonding strength between the barnacles and the carbon steel (1.16 MPa). The Polymers 2021, 13, 2242. https:// barnacles on the PDMS:PU blend (95:5) film were more easily removed from the surface. This doi.org/10.3390/polym13142242 indicated that the PDMS:PU blend (95:5) exhibited excellent antifouling properties and the results indicated that the PDMS:PU blend (95:5) film with micro patterning surface could be employed for Academic Editor: Ilker S. Bayer antifouling application.

Received: 1 June 2021 Keywords: antifouling; polymer blend; polydimethylsiloxane elastomer; polyurethane; hydropho- Accepted: 29 June 2021 bic ﬁlm Published: 8 July 2021

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in 1. Introduction published maps and institutional afﬁl- iations. The surfaces of buildings and boats in marine environments tend to accumulate high concentrations of fouling attachments of a micro–macro organism bioﬁlm. This is the major cause of damages to structures and equipment in a marine environment [1]. In general, biofouling is formed due to the attachment of micro-organisms (bacteria, algae and fungi) and macro-organisms (barnacles, sponges, seaweed, etc.) onto the structures’ surfaces in Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. water [2]. Especially on cargo ships, biofouling is a major problem because it increases This article is an open access article the weight, reduces the ship speed, leads to a rise in fuel consumption of up to 40%, and distributed under the terms and increases overall costs by up to 77% [3] and also carries non-native species to the other conditions of the Creative Commons marine environments. Moreover, medical equipment should also avoid the attachment Attribution (CC BY) license (https:// of microorganisms, which is the cause of middle ear infections, kidney infections, dental creativecommons.org/licenses/by/ caries, urinary tract infections and chronic prostatitis [4]. Biofouling is currently a major 4.0/). focus for the marine and medical industries.

Polymers 2021, 13, 2242. https://doi.org/10.3390/polym13142242 https://www.mdpi.com/journal/polymers Polymers 2021, 13, 2242 2 of 16

Tributyltin was one of the most widely used biocides for antifouling paints [5]. Many researchers have found negative effects of tributyltin due to its toxicity to other marine organisms [5] for example, it induces imposex and intersex conditions in marine snails via mechanisms of endocrine disruption. In 2003, the international maritime organization (IMO) banned tributyltin from any antifouling application [6]. Then, antifouling paint was developed using copper oxide pigment, thiocyanate, cuprous bromide, etc. Copper oxide was selected over the other pigments due to being inexpensive and soluble [7]; however, it is extremely toxic at elevated concentrations and negatively impacts mussels, fish, and crustaceans. Its mechanisms of toxicity include interference with osmoregulation due to enzyme inhibition, decreasing immune function and decreasing respiration [8]. In recent years, industry has focused on developing a non-toxic antifouling coating. There are several reports which studied the non-toxic antifouling properties of polymeric coatings such as polyethylene glycol (PEG) [9], fluoropolymers [9], and xerogels [10]. Polydimethylsiloxane (PDMS) is widely used for antifouling coating because it consists of -Si-O-Si- and a side chain of the -CH3 group, leading to good thermal stability, elasticity, and hydrophobicity [11]. The polymer main chain of siloxane bonds (-Si-O-Si-) has a high bond energy and bond angle that provide good thermal stability and elasticity for PDMS [12]. Side chain groups of PDMS are highly hydrophobic (water contact angle (WCA) 107–110◦)[13] and non-polar groups, which give a low surface energy as well as being excellent for releasing fouling materials [14]. PDMS is physiologically inactive, very low in toxicity, presents no health hazards, and is inexpensive [15]. Soft lithography is a simple, low cost and scalable way to fabricate micro- or nanostructures on a PDMS surface to make it become a superhydrophobic surface (WCA > 150◦) similar to a lotus-leaf surface [16]. PDMS is a material which suitable for soft lithography due to its low shrinkage rates and easy to penetrate to micropattern materials [17]. However, PDMS does not have high enough strength properties and when PDMS was fabricated with micropatterns of soft lithography the resultant micropatterns would collapse [15,18]. The stiffness of PDMS depends on the degree of crosslinking agent; the higher degree of PDMS network’s crosslinking, the higher its stiffness [18–20]. Furthermore, the mechanical properties of PDMS can also be improved using nanofiller; the tensile modulus of the PDMS will increase when increasing the amount of silica in the PDMS [21]. However, nanosilica were prepared via the solgel technique, which was a complicated process and involved many chemicals in its synthesis, and which modified silica surfaces such as tetraethyl orthosilicate, ammonium hydroxide solution, ethanol, silane coupling agent and toluene [21]. Polymer blending is a simple, inexpensive technique which is commercially feasible for preparing products with unique properties [22]. The combination of two or more polymers will give the obtained materials unique properties and also provide an eco- nomical way to produce new materials [23]. In previous work, neat PDMS fabricated on micro-patterning from soft lithography was easy to collapse under external forces (Van der Waals force) [24]. To solve this problem, in this paper, polyurethane with non-toxic, non-flammable, environmentally friendly, economic manufacturing and good mechanical properties [25] was blended with PDMS to produce an antifouling film. Moreover, the performance of the PDMS:PU blend on antifouling performance and its ability for use in soft lithography to achieve higher hydrophobicity were studied [26].

2. Materials and Methods 2.1. Material and Chemicals Polydimethylsiloxane elastomer (PDMS) used in this experiment had a density of 1.03, supplied by Dow Corning under the tradename of Sylgard 184. It had a two-part chemical (Part A as a base and Part B as a curing agent) containing the silicone base and a curing agent. Part A contained dimethyl siloxane, dimethylivinyl terminated, dimethylvinylated and trimethylated silica, tetra (trimethoxysiloxy) silane and ethyl benzene. The curing agent (Part B) contained dimethyl, methylhydrogen siloxane, dimethyl siloxane, dimethylvinyl terminated, dimethylvinylated and trimethylated silica, tetramethyl tetravinyl cyclotetra Polymers 2021, 13, x FOR PEER REVIEW 3 of 17

curing agent. Part A contained dimethyl siloxane, dimethylivinyl terminated, dimethylvinylated and trimethylated silica, tetra (trimethoxysiloxy) silane and ethyl benzene. The curing agent (Part B) contained dimethyl, methylhydrogen siloxane, dimethyl siloxane, Polymers 2021, 13, 2242 dimethylvinyl terminated, dimethylvinylated and trimethylated silica, tetramethyl 3tetra- of 16 vinyl cyclotetra siloxane, and ethyl benzene. Polydimethylsiloxane elastomer (PDMS) substrates were blended by completely mixing 10 parts base to 1 part curing agent. Poly- siloxane,urethane (PU) and ethylwith density benzene. 1.036 Polydimethylsiloxane was procured from elastomerSmooth-On, (PDMS) USA. It substrates had two-parts: were blendedPart A and by completelyPart B. Part mixingA is 4,4 10 ́ methylenedicyclohexyl parts base to 1 part curing diisocyanate agent. Polyurethane and part B (PU) is glycol with densityand phenylmercury. 1.036 was procured from Smooth-On, USA. It had two-parts: Part A and Part B. Part A is 4,40 methylenedicyclohexyl diisocyanate and part B is glycol and phenylmercury. 2.2. Preparation of the Polymer Blend 2.2. PreparationBlends of polydimethylsiloxane of the Polymer Blend elastomer and polyurethane were prepared using an overheadBlends stirrer of polydimethylsiloxane (IKA Overhead Stirrers elastomer RW 20 anddigital). polyurethane The ratios wereof PDMS:PU prepared used using in anthis overhead study were stirrer 100:0, (IKA 95:5, Overhead 90:10, 10:90 Stirrers 5:95, RW and 20 0:100. digital). A typical The ratios blending of PDMS:PU process used was incarried this studyout by were first preparing 100:0, 95:5, a 90:10,polyurethane 10:90 5:95, mixture and 0:100.with a Amixing typical ratio blending of 10A:9B. process The wasmixture carried of polyurethane out by first preparing was prepared a polyurethane using a mechanical mixture with stirrer a mixing at 250 ratiorpm. ofThen, 10A:9B. the Themixture mixture of polydimethylsiloxane of polyurethane was elastomer prepared was using added a mechanical to polyurethane stirrer at and 250 mixed rpm. at Then, 250 therpm mixture for 10 min. of polydimethylsiloxane After that, the polymer elastomer blend was was poured added into to polyurethanea mold. During and the mixed process, at 250air bubbles rpm for might 10 min. occur, After and that, a degassing the polymer step blend was wasrequired poured by intoplacing a mold. the samples During in the a process,vacuum airdesiccator bubbles for might 30 min occur, to andremove a degassing any small step and was large required bubbles by on placing the surface the samples of the insample. a vacuum Next, desiccator the sample for was 30 mincured to by remove heating any in small an oven and at large 60 °C bubbles for 3 h. on Finally, the surface after ◦ ofcooling the sample. the obtained Next, thesamples sample were was removed cured by from heating the mold. in an oven at 60 C for 3 h. Finally, after cooling the obtained samples were removed from the mold. 2.3. Characterization of the Polymer Blend 2.3. Characterization of the Polymer Blend 2.3.1. Scanning Electron Microscope (SEM) To investigate phase morphology of of the the cross-section cross-section of of the the polymer polymer blends’ blends’ films, films, a ascanning scanning electron electron microscope microscope (FE-SEM, (FE-SEM, Hitach Hitachii S-4800) S-4800) was was employed. employed. In this In thisstudy, study, the thepolymer polymer blend blend film film samples samples were were prepared prepared by immersing by immersing them them in liquid in liquid nitrogen nitrogen for 3–5 for 3–5min. min. Then, Then, the sample the sample was broken was broken between between two grips. two grips.Next, the Next, specimens the specimens were coated were coatedwith platinum with platinum under vacuum under vacuumbefore observation before observation by using byaccelerating using accelerating voltage of voltage 10 kV. ofThe 10 SEM/EDX kV. The SEM/EDX studies were studies also wereperformed also performed for the material for the identification material identification and phase anddis- phasepersion. dispersion.

2.3.2. Fourier Transform InfraredInfrared SpectraSpectra (ATR-FTIR)(ATR-FTIR) FTIR analysis analysis was was performed performed with with a Thermo a Thermo Scientific/Nicolet Scientiﬁc/Nicolet Nexus Nexus IS5. FTIR IS5. FTIRspec- spectratra were were collected collected between between 4000 cm 4000−1 to cm 650−1 cmto− 6501 with cm a− signal-to-noise1 with a signal-to-noise ratio of 40,000:1 ratio ofat 40,000:1spectral atresolution spectral resolutionof 0.45 cm− of1. The 0.45 IR cm spectra−1. The were IR spectra used wereto characterize used to characterize the functional the functionalgroups and groups chemical and vibrational chemical vibrational frequencies frequencies of the polymers. of the polymers.

2.3.3. Contact Angle Measurement Static contact angle measurements were performed using a Krȕss (model DSA 10) contact angle measuring instrument at ambient temperature toto determinedetermine thethe wettabilitywettability change of thethe polymerpolymer blendblend surfaces.surfaces. A 10 µμL sessilesessile dropletdroplet ofof de-ionizedde-ionized waterwater waswas vertically dropped,dropped, with with a a microsyringe, microsyringe, onto onto the the surface surface of theof the samples. samples. The The images images of the of dropthe drop shape shape on theon the surface surface of theof the samples samples were were captured captured using using a camera a camera equipped equipped with with a magnifyinga magnifying lens. lens.

2.3.4. Mechanical Test (LLOYD)(LLOYD) Mechanical properties such as tensile strength,strength, modulus and elongation at breakbreak ofof the two polymers and the polymer blends were measured using universal testing machine the two polymers and the polymer blends were measured using universal testing machine with standard as per ASTM 882. The speed of stretching was 50 mm/min with dimensions with standard as per ASTM D 412-98. The speed of stretching was 50 mm/min. For each of 100 mm × 10 mm × 1 mm. For each sample, an average result obtained from ﬁve sample, an average result obtained from five individual specimens was reported. individual specimens was reported.

2.3.5. Atomic Force Microscopic (AFM) Atomic force microscopy (AFM) signiﬁcantly measures the local properties of materi-

als and intermaterial interactions with nanoscale spatial and piconewton force resolution. The spatial sensitivity of the AFM photodetector was calibrated against a clean silicon wafer. Thin ﬁlm sample of approximate 1 mm thickness was prepared from different ratio Polymers 2021, 13, 2242 4 of 16

of PDMS:PU. For each sample, the topology and elastic modulus were measured from ﬁve different places over sizes of 1 × 1 × 0.1 cm with the scanAsyst-Air probe using PeakForce QNM modes with 0.4 N/m spring constant. The Peak Force Quantitative Nanomechanical Mapping (PFQNM) mode, also known as quantitative nanomechanical mapping, is a semi- contact AFM mode, which constructed height and phase images simultaneously. Phase image acknowledged differences in modulus.

2.3.6. Swelling Test Swelling test was carried out to determine swelling properties of the samples when immersed in diverse mediums, following ASTM D3616. Swelling properties were calculated from variation in mass of the samples. The test specimens of weight, Mo, (g) were immersed in DI water and seawater in bottles at room temperature for 24 to 240 h. The seawater used in the analysis was collected from the Gulf of Thailand at Koh Sichang, Chonburi province, Thailand, and it has the average salinity of ocean water, around 33–34 ppt with pH 7.7–8.0. After the required immersion time, the samples were removed, gently wiped and dried to eliminate any excess liquid, and the swollen samples were reweighted, M, (g). The degree of swelling was calculated using Equation (1).

M − M Q = o × 100. (1) Mo where: Q is degree of swelling Mo is the initial weight of sample (g) M is the swollen weight of sample (g)

2.3.7. Microfouling Analysis The barnacle adhesion study was carried out to determine level of microfouling by immersing the samples in a marine environment and determined by visual inspection as described by ASTM D3623. Barnacles were collected from seawater in the Gulf of Thailand at Koh Sichang Marine Science Research Centre of Chulalongkorn University (13◦09010.600 N 100◦49002.600 E), which has high barnacle population because breeding Polymers 2021, 13, x FOR PEER REVIEWand broadcast-spawning frequently occur during this period, for 8 weeks during5 April of 16 to June [27]. The reproduction of barnacles increases because it is related to the warmer seawater temperatures. The reference specimen used in this study was SS400 carbon steel. SS400 steel sheets are typically used for general structures, and they are also used capacities of the force measuring device are between 0 and 150 N (0 to 34 lb) to an accuracy in marine ship structures, high-rise buildings, etc. [28]. For the field tests in a seawater of ± 0.5 N (±0.1125 lb). Furthermore, the adhesive strength, , (pascal, Pa) of the two foul- environment, all samples were set in Teflon frames with 12-channelτ (see Figure1). The sizeing release of the Teflon surfaces frame waswas evaluated 0.6 × 0.6 according m2. Then, to ASTM the frames D5618-94 were by set measuring on 2 different the sides:shear facingforce, F, the (newton, sea (uprush) N) required and facing to remove the shore the (backwash) barnacle by at the a depth base of area, 3 m A, from (square the seawater meters) levelof the ( Figurebarnacle1 )[ as24 in]. Subsequently,Equation (3): the number of barnacles on each sample surface were measured and captured by a digital still camera.= (3)

FigureFigure 1.1. Experiment setupsetup ofof microfoulingmicrofouling analysis.analysis.

3. Results 3.1. Characteristics of PDMS:PU Blend Film 3.1.1. FTIR Spectral of PDMS:PU Blend Film The chemical structures of the PDMS:PU blend film were verified using the FTIR spectra. To identify polydimethylsiloxane (PDMS), after polyurethane (PU) was added, attenuated total reflectance infrared spectroscopy was employed (Figure 2). The PDMS film displayed a characteristic Si-CH3 peak around 1250 cm−1, Si-O-Si at 1055 cm−1 (Si-O stretching), Si-CH3 at peak around 790 cm−1 (CH3 rocking and Si-C stretching) and asymmetric CH3 at 2962 cm−1. The FTIR spectrum of the polyurethane film showed NCO-termination at 2200 cm−1, -NH deformation at 1513.7 cm−1 and C-O-C ether group at 1160 cm−1: the C-H symmetric and asymmetric stretching vibrations of CH2 group were observed at 2923 cm−1 and 2850 cm−1, respectively. The PDMS:PU blend film with ratio 5:95 and 10:90 showed a strong peak of the N-H band at 3200–3400 cm−1 and a carbonyl peak (C=O) at 1700 cm−1, which represent PU’s characteristics in a polymer blend. On the other hand, the PDMS:PU blend film with ratio 90:10 and 95:5 showed a very weak peak of the N-H band at 3200–3400 cm−1 and of the carbonyl peak (C=O) at 1700 cm−1 due to the very small amount of PU in the blend. The obtained FTIR spectra support the proposed structure of the final PDMS:PU blend film.

Polymers 2021, 13, 2242 5 of 16

Barnacles of average dimensions, i.e., between 5 to 20 mm in diameter and at least 30 mm in height, were selected for this method. The base area of each barnacle, A, (square meters) was approximated from an average base diameter, do, (meters). The base area of each barnacle was calculated according to Equation (2).

1 A = πd2. (2) 4 o The adhesion force measurement for all samples when barnacles settled on the surface was carried out on a digital force gauge (Inspex IPX-808) at room temperature. The capacities of the force measuring device are between 0 and 150 N (0 to 34 lb) to an accuracy of ±0.5 N (±0.1125 lb). Furthermore, the adhesive strength, τ, (pascal, Pa) of the two fouling release surfaces was evaluated according to ASTM D5618-94 by measuring the shear force, F, (newton, N) required to remove the barnacle by the base area, A, (square meters) of the barnacle as in Equation (3):

F τ = (3) A

3. Results 3.1. Characteristics of PDMS:PU Blend Film 3.1.1. FTIR Spectral of PDMS:PU Blend Film The chemical structures of the PDMS:PU blend film were verified using the FTIR spectra. To identify polydimethylsiloxane (PDMS), after polyurethane (PU) was added, attenuated total reflectance infrared spectroscopy was employed (Figure2). The PDMS −1 −1 film displayed a characteristic Si-CH3 peak around 1250 cm , Si-O-Si at 1055 cm (Si- −1 O stretching), Si-CH3 at peak around 790 cm (CH3 rocking and Si-C stretching) and −1 asymmetric CH3 at 2962 cm . The FTIR spectrum of the polyurethane film showed NCO-termination at 2200 cm−1, -NH deformation at 1513.7 cm−1 and C-O-C ether group −1 at 1160 cm : the C-H symmetric and asymmetric stretching vibrations of CH2 group were observed at 2923 cm−1 and 2850 cm−1, respectively. The PDMS:PU blend film with ratio 5:95 and 10:90 showed a strong peak of the N-H band at 3200–3400 cm−1 and a carbonyl peak (C=O) at 1700 cm−1, which represent PU’s characteristics in a polymer blend. On the other hand, the PDMS:PU blend film with ratio 90:10 and 95:5 showed a very weak peak of the N-H band at 3200–3400 cm−1 and of the carbonyl peak (C=O) at 1700 cm−1 due to the very small amount of PU in the blend. The obtained FTIR spectra support the proposed structure of the final PDMS:PU blend film.

3.1.2. Water Contact Angle (WCA) of PDMS:PU Blend Film To achieve antifouling characteristics, the surface characteristics of the polymer film, measured by water contact angle (WCA), should be greater than 90◦, which is characteristic of hydrophobic surfaces. The contact angle from the water droplets on the PDMS film surface had a value of 109.5◦ ± 0.3◦ and PU film surface was 86.9◦ ± 3.2◦ (see Table1). The PDMS film showed hydrophobic properties because the WCA value was higher than 90◦ [29]. When PMDS was blended with PU, it was found that the PDMS:PU blend film showed a higher value of WCA than the PU film. However, the water contact angle of the PDMS:PU blend film decreased from 103.4◦ ± 3.8◦ to 91.4◦ ± 0.8◦ when the PU content was increased from 5 wt.% to 95 wt.% of PU. It seemed that the PDMS:PU blend film became more hydrophilic than the PDMS film due to the hydrophilic group (glycol) of the PU backbone [30]. Based on the results, it was concluded that the ratio of the PDMS:PU blend film at 95:5 exhibited high hydrophobic surface properties since it presented a water contact angle of larger than 100◦. PolymersPolymers 20212021,, 1313,, xx FORFOR PEERPEER REVIEWREVIEW 66 ofof 1616 Polymers 2021, 13, x FOR PEER REVIEW 6 of 16 PolymersPolymers 2021 2021, ,,13 13, ,,x xx FOR FORFOR PEER PEERPEER REVIEW REVIEWREVIEW 66 ofof 1616

Polymers 2021, 13, 2242 6 of 16 Polymers 2021, 13, x FOR PEER REVIEW 6 of 16

−−11 FigureFigure 2.2. FTIRFTIR spectraspectra ofof PDMS:PUPDMS:PU blendblend filmsfilms betweenbetween 35003500– ––500500 cmcm−1 .. Figure 2. FTIR spectra of PDMS:PU blend films between 3500 500 cm −.− 1 FigureFigure 2. 2. FTIR FTIR spectra spectra of of PDMS:PU PDMS:PU blend blend films films between between 3500 3500––500500 cm cm−1.1 .. 3.1.2.3.1.2.3.1.2. Water WaterWater Contact ContactContact Angle AngleAngle (WCA) (WCA)(WCA) of ofof PDMS:PU PDMS:PUPDMS:PU Blend BlendBlend Film FilmFilm 3.1.2.3.1.2. Water Water Contact Contact Angle Angle (WCA) (WCA) of of PDMS:PU PDMS:PU Blend Blend Film Film ToToTo achieve achieveachieve antifouling antifoulingantifouling characteristics, characteristics,characteristics, the thethe surface surfacesurface characteristics characteristicscharacteristics of ofof the thethe polymer polymerpolymer film, film,film, ToTo achieve achieve antifouling antifouling characteristics, characteristics, the the surface surface characteristics characteristics of of the the polymer polymer film, film, measuredmeasuredmeasuredTo achieve by byby water waterwater antifouling contact contactcontact angle characteristics,angleangle (WCA), (WCA),(WCA), should should shouldthe surface be bebe greater greatergreater characteristics than thanthan 90°, 90°,90°, which of whichwhich the ispolymer is ischaracter- character-character- film, measuredmeasured by by water water contact contact angle angle (WCA), (WCA), should should be be greater greater than than 90°, 90°, which which is is character- character- isticisticmeasuredistic of ofof hydrophobic hydrophobichydrophobic by water surfaces. contact surfaces.surfaces. angleThe TheThe contact contact(WCA),contact angle angle angleshould from fromfrom be the thegreaterthe water waterwater than droplets dropletsdroplets 90°, onwhich onon the thethe PDMSis PDMSPDMS character- film filmfilm isticistic of of hydrophobic hydrophobic surfaces. surfaces. The The contact contact angle angle from from the the water water droplets droplets on on the the PDMS PDMS film film surfacesurfaceisticsurface of had hydrophobic hadhad a avaluea valuevalue of ofofsurfaces. 109.5° 109.5°109.5° ± ± 0.3°±The 0.3°0.3° andcontact andand PU PUPU anglefilm filmfilm surface from surfacesurface the was was waswater 86.9° 86.9°86.9° droplets ± ±3.2°± 3.2°3.2° (see on(see(see theTable TableTable PDMS 1). 1).1). The film TheThe surfacesurface had had a a value value of of 109.5° 109.5° ± ± 0.3° 0.3° and and PU PU film film surface surface was was 86.9° 86.9° ± ± 3.2° 3.2° (see (see Table Table 1). 1). The The PDMSPDMSsurfacePDMS film film filmhad showed showedshoweda value hydrophobic ofhydrophobichydrophobic 109.5° ± 0.3° properties propertiesproperties and PU befilm bebecausecausecause surface the thethe WCAwas WCAWCA 86.9° value valuevalue ± 3.2°was waswas (seehigher higherhigher Table than thanthan 1). 90° The 90°90° PDMSPDMS film film showed showed hydrophobic hydrophobic properties properties be becausecause the the WCA WCA value value was was higher higher than than 90° 90° [29].[29].PDMS[29]. When WhenWhen film PMDS showedPMDSPMDS was waswashydrophobic blended blendedblended with withpropertieswith PU, PU,PU, it ititbewas waswascause found foundfound the WCAthat thatthat the valuethethe PDMS:PU PDMS:PUPDMS:PU was higher blend blendblend than film filmfilm 90° [29].[29]. WhenWhen PMDSPMDS waswas blendedblended withwith PU,PU, itit waswas foundfound thatthat thethe PDMS:PUPDMS:PU blendblend filmfilm showedshowed[29].showed When a ahighera higherhigher PMDS value valuevalue was of ofof WCAblended WCAWCA than thanthan with the thethe PUPU, PUPU fi itlm.fifi lm.lm.was However, However,However, found thatthe thethe water the waterwater PDMS:PU contact contactcontact angle angle angleblend of ofof thefilm thethe showedshowed a a higher higher value value of of WCA WCA than than the the PU PU fi film.lm. However, However, the the water water contact contact angle angle of of the the PDMS:PUPDMS:PUshowedPDMS:PU a blend higher blendblend film filmvaluefilm decreased decreaseddecreased of WCA from thanfromfrom 103.4° the 103.4°103.4° PU ± ±3.8°fi± 3.8°lm.3.8° to However, toto 91.4° 91.4°91.4° ± ±0.8°± 0.8°0.8°the when waterwhenwhen the contactthethe PU PUPU content contentanglecontent ofwas waswas the PDMS:PUPDMS:PU blend blend film film decreased decreased from from 103.4° 103.4° ± ± 3.8° 3.8° to to 91.4° 91.4° ± ± 0.8° 0.8° when when the the PU PU content content was was increasedincreasedPDMS:PUincreased from fromfromblend 5 55wt% film wt%wt% todecreased toto 95 9595 wt% wt%wt% fromof ofof PU. PU.PU. 103.4° It It Itseemed seemedseemed ± 3.8° thatto thatthat 91.4° the thethe ±PDMS:PU 0.8°PDMS:PUPDMS:PU when blend the blendblend PU film filmfilmcontent became becamebecame was increasedincreased from from 5 5 wt% wt% to to 95 95 wt% wt% of of PU. PU. It It seemed seemed that that the the PDMS:PU PDMS:PU blend blend film film became became moremoreincreasedmore hydrophilic hydrophilichydrophilic from 5 thanwt% thanthan theto thethe 95 PDMS PDMSPDMSwt% offilm filmfilm PU. due due dueIt seemedto toto the thethe hydrophilic thathydrophilichydrophilic the PDMS:PU group groupgroup (glycol) blend(glycol)(glycol) film of ofof the becamethethe PU PUPU moremore hydrophilic hydrophilic than than the the PDMS PDMS film film due due to to the the hydrophilic hydrophilic group group (glycol) (glycol) of of the the PU PU backbonebackbonemorebackbone hydrophilic [30]. [30].[30]. Based BasedBased than on onon the the thethe re PDMS reresults,sults,sults, itfilm ititwas waswas due concluded concludedconcluded to the hydrophilicthat thatthat the thethe ratio ratioratio group of ofof the thethe (glycol)PDMS:PU PDMS:PUPDMS:PU of blendthe blendblend PU backbonebackbone [30]. [30]. Based Based on on the the re results,sults, it it was was concluded concluded that that the the ratio ratio of of the the PDMS:PU PDMS:PU blend blend filmfilmbackbonefilm at atat 95:5 95:595:5 [30].exhibited exhibitedexhibited Based high onhighhigh the hydrophobic hydrophobichydrophobic results, it was surface surfacesurface concluded properties propertiesproperties that sincethe sincesince ratio it ititpresented presentedofpresented the PDMS:PU a awatera waterwater con-blend con-con- filmfilm at at 95:5 95:5 exhibited exhibited high high hydrophobic hydrophobic surface surface properties properties since since it it presented presented a a water water con- con- tacttactfilmtact angle angleangleat 95:5 of ofof largerexhibited largerlarger than thanthan high 100°. 100°.100°. hydrophobic surface properties since it presented a water con- −1 Figure 2. FTIR spectra tacttacttact angle angleangleof PDMS:PU of ofof larger largerlarger blend than thanthan films ﬁlms 100°. 100°.100°. between 3500–5003500–500 cm−1. . TableTableTable 1. 1. 1.Water WaterWater contact contactcontact angle angleangle on onon various variousvarious material materialmaterial su susurfacesrfacesrfaces polydimethylsiloxane; polydimethylsiloxane;polydimethylsiloxane; polyurethane; polyurethane;polyurethane; PDMS:PU PDMS:PUPDMS:PU blend blendblend (95:5, (95:5,(95:5, TableTable 1. 1. Water Water contact contact angle angle on on various various material material su surfacesrfaces polydimethylsiloxane; polydimethylsiloxane; polyurethane; polyurethane; PDMS:PU PDMS:PU blend blend (95:5, (95:5, 90:10,90:10,Table90:10, 10:90, 1. 10:90,10:90, Water3.1.2.Table 5:95). 5:95).5:95). contactWater 1. Water Contactangle contact on Anglevarious angle (WCA) onmaterial various of su PDMS:PUrfaces material polydimethylsiloxane; surfaces Blend Film polydimethylsiloxane; polyurethane; polyurethane; PDMS:PU blend (95:5, 90:10,90:10, 10:90, 10:90, 5:95). 5:95). 90:10, 10:90,PDMS:PU 5:95).To achieve blend (95:5, antifouling 90:10, 10:90, characteristics, 5:95). the surface characteristics of the polymer film, PolyurethanePolyurethanePolyurethane PDMS:PU PDMS:PU PDMS:PU blend blendblend (5:95) (5:95)(5:95) PDMS:PU PDMS:PU PDMS:PU blend blendblend (10:90) (10:90)(10:90) measured by water contact angle (WCA), should be greater than 90°, which is character- PolyurethanePolyurethanePolyurethane PDMS:PU PDMS:PU PDMS:PU blend blend (5:95) (5:95) PDMS:PU PDMS:PU PDMS:PU blendblend blend (10:90) (10:90) (10:90) istic of hydrophobic surfaces. The contact angle from the water droplets on the PDMS film surface had a value of 109.5° ± 0.3° and PU film surface was 86.9° ± 3.2° (see Table 1). The PDMS film showed hydrophobic properties because the WCA value was higher than 90°

[29]. When PMDS was blended with PU, it was found that the PDMS:PU blend film 86.9°86.9°86.9° ± ±3.2°± 3.2°3.2° 91.4°91.4°91.4° ± ±0.8°± 0.8°0.8° 92.2°92.2° ±± 2.5°2.5° ◦ ◦ 91.4° ± 0.8° 92.2° ± 2.5° showed a86.9°86.9°86.9 higher ± ±± 3.2° 3.2° 3.2value of WCA than the91.4°91.4° PU◦ ± fi±± 0.8°lm. 0.8° ◦ However, the water 92.2°contact92.2°◦ ± ± 2.5° angle2.5°◦ of the PDMS:PUPDMS:PUPDMS:PU blend blendblend (90:10) (90:10)(90:10) PDMS:PU PDMS:PU PDMS:PU91.4 blen blenblen0.8d dd(95:5) (95:5)(95:5) Polydimethylsiloxane Polydimethylsiloxane Polydimethylsiloxane92.292.2°± ±2.5 2.5° PDMS:PUPDMS:PU blend blend film (90:10) decreased from PDMS:PU 103.4° ±blen 3.8°d to (95:5) 91.4° ± 0.8° when Polydimethylsiloxane the PU content was PDMS:PUPDMS:PUPDMS:PU blend blend blend (90:10) (90:10)(90:10) PDMS:PU PDMS:PU PDMS:PU blen blen blenddd (95:5) (95:5) Polydimethylsiloxane Polydimethylsiloxane Polydimethylsiloxane increased from 5 wt% to 95 wt% of PU. It seemed that the PDMS:PU blend film became

more hydrophilic than the PDMS film due to the hydrophilic group (glycol) of the PU

backbone [30]. Based on the results, it was concluded that the ratio of the PDMS:PU blend

film at 95:5 exhibited high hydrophobic surface properties since it presented a water con-

tact angle of larger than 100°. ◦ ◦ 103.4°◦ ± 3.8°◦ 109.5°109.5° ±± 4.2°4.2° 94.6°94.6°94.694.6° ± ± 0.8°± 0.8°0.8°0.8 103.4°103.4103.4° ±± 3.8°± 3.83.8° 109.5°109.5◦ ±± 4.24.2°◦ 94.6° ± 0.8° 103.4°103.4° ± ± 3.8° 3.8° 109.5°109.5° ± ± 4.2° 4.2° Table 1. Water contact angle on various94.6°94.6° material ± ± 0.8° 0.8° su rfaces polydimethylsiloxane;103.4° ± 3.8° polyurethane; PDMS:PU109.5° blend ± 4.2° (95:5, 3.1.3. Morphology of the Polymer Blend Film 90:10, 10:90, 5:95). 3.1.3. Morphology3.1.3.3.1.3. of Morphology theMorphology Polymer of Blend of the the Polymer Film Polymer Blend Blend Film Film 3.1.3.3.1.3. Morphology Morphology of of the the Polymer Polymer Blend Blend Film Film PolyurethaneThe morphology PDMS:PU of the blend polymer (5:95) blends between PDMS:PU PDMS blend and (10:90) PU were observed by using SEM. The neat PDMS ﬁlm cross-section showed a rough surface (Figure3a) which was caused by the ductile material fracture, but on the other hand, the morphology of polyurethane (Figure3b) showed a smooth surface due to brittle material fracture. The

brittle properties of a material can normally be identiﬁed by the smoothness of the fracture

86.9° ± surface3.2° of the cross-section,91.4° such± 0.8° as observed in polyurethane,92.2° ± 2.5° and the ductile properties PDMS:PU blendcan (90:10) be observed by PDMS:PU the roughness blend (95:5) of the fracture Polydimethylsiloxane surface cross-section, such as observed in polydimethylsiloxane [31]. The ratio of the polymer blends of PDMS:PU; 95:5 and 5:95 showed spherical particles of polyurethane, as observed in Figure3c,d. The SEM

image of PDMS:PU blend (95:5) ﬁlm (Figure3c) clearly illustrates good distribution of PU

particles in PDMS matrix, with the average diameter of the dispersed PU particles at about

8.3 ± 5.6 µm. Energy-dispersive X-ray spectroscopy (EDX) was carried out to study the elemental 109.5° ± 4.2° 94.6° ± distribution0.8° of PDMS,103.4° PU and ± 3.8° the PDMS-PU blends (see Figure4). It could be observed that the silicon (Si) weight percentage of the PDMS:PU blend decreased from 57.35 wt.% 3.1.3.to 32.29 Morphology wt.% while of more the Polymer carbon atomsBlend wereFilm detected with the addition of polyurethane into the system. It also illustrated that the carbon (C) weight percentage of the surface rose

Polymers 2021, 13, x FOR PEER REVIEW 7 of 16

The morphology of the polymer blends between PDMS and PU were observed by using SEM. The neat PDMS film cross-section showed a rough surface (Figure 3a) which was caused by the ductile material fracture, but on the other hand, the morphology of polyurethane (Figure 3b) showed a smooth surface due to brittle material fracture. The brittle properties of a material can normally be identified by the smoothness of the fracture surface of the cross-section, such as observed in polyurethane, and the ductile properties can be observed by the roughness of the fracture surface cross-section, such as observed in polydimethylsiloxane [31]. The ratio of the polymer blends of PDMS:PU; 95:5 and 5:95 showed spherical particles of polyurethane, as observed in Figure 3c,d. The SEM image of PDMS:PU blend (95:5) film (Figure 3c) clearly illustrates good distribution of PU particles in PDMS matrix, with the average diameter of the dispersed PU particles at about 8.3 ± 5.6 μm. Energy-dispersive X-ray spectroscopy (EDX) was carried out to study the elemental Polymers 2021, 13, 2242 distribution of PDMS, PU and the PDMS-PU blends (see Figure 4). It could be observed7 of 16 that the silicon (Si) weight percentage of the PDMS:PU blend decreased from 57.35 wt.% to 32.29 wt.% while more carbon atoms were detected with the addition of polyurethane into the system. It also illustrated that the carbon (C) weight percentage of the surface rose fromfrom 24.50 24.50 wt.% wt.% to to 53.47 53.47 wt.% wt.% with with the the increase increase of of polyurethane polyurethane up up to to 95%. 95%. The The results results conﬁrmedconfirmed that that polyurethane polyurethane was was distributed distributed in in PDMS PDMS matrix matrix (Table (Table2)[ 2)32 [32].].

(a) (b)

(c) (d) Polymers 2021, 13, x FOR PEER REVIEW 8 of 16 FigureFigure 3.3. SEMSEM image image of of cross-section cross-section view view of of (a ()a )polydimethylsiloxane; polydimethylsiloxane; (b) ( bpolyurethane;) polyurethane; (c) (cPDMS:PU) PDMS:PU blend blend (95:5); (95:5); and and (d ()d PDMS:PU) PDMS:PU blend blend (5:95). (5:95).

Table 2. EDX results (weight percentage) obtained on each type of materials. (a) Polydimethylsiloxane PDMS:PU Blend (95:5) Polyurethane Element Weight% Atomic Weight% Atomic Weight% Atomic C 24.30 38.82 53.47 68.58 66.05 72.16 O 18.35 22.00 14.24 13.72 33.95 27.84 Si 57.35 39.18 32.29 17.71 0 0

(b)

(c)

FigureFigure 4. 4.SEM-EDXSEM-EDX image image of: of: (a) ( apolydimethylsiloxane;) polydimethylsiloxane; (b) ( bpolyurethane;) polyurethane; (c) ( cPDMS:PU) PDMS:PU blend blend (95:5). (95:5). The The graphs graphs represent represent thethe relative relative atomic atomic fractions fractions of ofthe the three three major major elements. elements.

3.1.4. Mechanical Properties Studies of the PDMS:PU Blend Film The PDMS film showed an elastic property with a tensile strength and Young’s modulus of 1.14 ± 0.29 MPa and 1.50 ± 0.18 MPa, respectively. After blending PDMS with PU, the mechanical properties of the PDMS:PU blend films were increased due to the introduction of the more rigid polyurethane. As shown in Figure 5, for the PDMS:PU blend film when PU content increased from 0, 5, 95, and 100, Young’s modulus increased 1.50 ± 0.18 MPa (PDMS film), 1.75 ± 0.35 MPa, 863.78 ± 46.49 MPa, and 1559.84 ± 266.16 MPa, respectively, and the PDMS:PU (95:5) blend film achieved an acceptable mechanical properties and still retained good deformation resistance [33] which was suitable for micropattern fabricating using the soft lithography process.

Polymers 2021, 13, 2242 8 of 16

Table 2. EDX results (weight percentage) obtained on each type of materials.

Polydimethylsiloxane PDMS:PU Blend (95:5) Polyurethane Element Weight% Atomic Weight% Atomic Weight% Atomic C 24.30 38.82 53.47 68.58 66.05 72.16 O 18.35 22.00 14.24 13.72 33.95 27.84 Si 57.35 39.18 32.29 17.71 0 0

3.1.4. Mechanical Properties Studies of the PDMS:PU Blend Film The PDMS film showed an elastic property with a tensile strength and Young’s modulus of 1.14 ± 0.29 MPa and 1.50 ± 0.18 MPa, respectively. After blending PDMS with PU, the mechanical properties of the PDMS:PU blend films were increased due to the introduction of the more rigid polyurethane. As shown in Figure5, for the PDMS:PU blend film when PU content increased from 0, 5, 95, and 100, Young’s modulus increased 1.50 ± 0.18 MPa (PDMS film), 1.75 ± 0.35 MPa, 863.78 ± 46.49 MPa, and 1559.84 ± 266.16 MPa, respectively, Polymers 2021, 13, x FOR PEER REVIEWand the PDMS:PU (95:5) blend film achieved an acceptable mechanical properties and9 of still16 retained good deformation resistance [33] which was suitable for micropattern fabricating using the soft lithography process.

FigureFigure 5. 5.Stress–strainStress–strain curves curves of polydimethylsiloxa of polydimethylsiloxane,ne, polyurethane, polyurethane, PDMS:PU PDMS:PU blend (at blend room (at temperature).room temperature).

3.1.5.3.1.5. Atomic Atomic Force Force Microscopic Microscopic (AFM) (AFM) Studies Studies AtomicAtomic force force microscope microscope (AFM) (AFM) was was employed employed to to collect collect topography topography images images and and measuremeasure the the roughness roughness of of the the sample sample scanned. scanned. The The AFM AFM height sensor imagesimages displayeddisplayedin inFigure Figure6 a,c,e6a,c,e represent represent the the PDMS:PU PDMS:PU blend blend (95:5), (95:5), PDMS:PU PDMS:PU blend blend (5:95) (5:95) and and polyurethane polyure- thanefilm, film, respectively. respectively. The polyurethaneThe polyurethane film showedfilm showed a relatively a relatively smooth smooth and uniform and uniform surface. surface.On the On other the hand,other hand, the PDMS:PU the PDMS:PU blend blend film showedfilm showed a rough a rough surface. surface. Thus, Thus, different dif- ferenttopographical topographical features features (both nanoscale (both nano andscale macroscale) and macroscale) were observed were on PDMS:PUobserved blendon PDMS:PUfilms with blend different films mixing with different ratios. mixing ratios. The AFM DMT images displayed in Figure 6b,d,f represent the PDMS:PU blend (95:5), PDMS:PU blend (5:95) and polyurethane, respectively. The indentation modulus among the samples was calculated from the QNMtm mode in PFQNM. In the AFM topography images of the film, the surface showed both a brighter phase and a darker phase, with the brighter phase showing a higher modulus than darker phase [34]. The AFM topography images of the PDMS:PU blend film presented the dark areas as polydimethylsiloxane and the bright areas as polyurethane. The relative modulus of the material surfaces of PDMS:PU (95:5) blend was 123 ± 13 mArb. However, for PDMS:PU blend (5:95) film, the modulus increased to 160 ± 34 mArb, which was a 30% higher modulus than PDMS:PU blend (95:5). The relative modulus of the PU film was 180 ± 30 mArb, and the modulus tended to increase with increase PU ratio [35]. Therefore, the blending of PDMS and PU could improve the surface modulus of the PDMS:PU blend.

Polymers 2021, 13, 2242 9 of 16 Polymers 2021, 13, x FOR PEER REVIEW 10 of 16

(a) (c) (e)

(b) (d) (f)

FigureFigure 6. 6. AFMAFM topology topology images images (a,b) PDMS:PU (a,b) PDMS:PU (95:5) blend; (95:5) ( blend;c,d) PDMS:PU (c,d) PDMS:PU (5:95) blend; (5:95) (e,f blend;) polyurethane.(e,f) polyurethane.

3.1.6. TheEffect AFM of Swelling DMT images displayed in Figure6b,d,f represent the PDMS:PU blend (95:5), PDMS:PUThe applicability blend (5:95) of and polymer polyurethane, films for respectively. antifouling Theproducts indentation is based modulus on the among bond tm breakagethe samples of samples was calculated when immersed from the QNM in differentmode testing in PFQNM. mediums, In the which AFM can topography be pre- dictedimages by of swelling the film, behavior the surface and showed water resistance both a brighter in a natural phase andenvironment. a darker phase, In this with paper, the polydimethylsiloxanebrighter phase showing elastomer, a higher polyurethane modulus than and darker the PDMS:PU phase [34 blend]. The at AFM 95:5 topographyblend ratio wereimages studied of the for PDMS:PU their swelling blend behaviors. film presented The mechanism the dark areas of swelling as polydimethylsiloxane is represented by and the bright areas as polyurethane. The relative modulus of the material surfaces of two properties; the addition of weight and the reduction of sample weight. The final sam- PDMS:PU (95:5) blend was 123 ± 13 mArb. However, for PDMS:PU blend (5:95) film, the ple would shrink or damage for reduction conditions [36]. The penetration of DI water modulus increased to 160 ± 34 mArb, which was a 30% higher modulus than PDMS:PU and seawater (SW) through the polymer was observed. After immersion for 72 h, each blend (95:5). The relative modulus of the PU film was 180 ± 30 mArb, and the modulus type of sample began to lose weight, illustrating hydrolysis degradation [37]. However, tended to increase with increase PU ratio [35]. Therefore, the blending of PDMS and PU seawater immersion is a complicated medium due to the microorganisms and elements could improve the surface modulus of the PDMS:PU blend. contained in sea water. The PDMS film immersed in seawater showed an increase in the rate3.1.6. of Effectmass loss of Swelling by enzymatic biodegradation [38] when compared with tests carried out in DI water. This was because the seawater contained microorganisms such as bacteria, The applicability of polymer films for antifouling products is based on the bond fungi, algae and plankton. The highest degree of swelling was 2.12 in 72 h and over 5 days breakage of samples when immersed in different testing mediums, which can be predicted the mass loss was slightly reduced. On the other hand, the mass loss of PU in the two by swelling behavior and water resistance in a natural environment. In this paper, poly- mediums showed that the degree of swelling of PU slightly increased. Then, the steadi- dimethylsiloxane elastomer, polyurethane and the PDMS:PU blend at 95:5 blend ratio were nessstudied degree for of their swelling swelling was behaviors. accomplished The mechanismafter approximately of swelling 72 h is in represented DI water and by twosea water.properties; Mass theloss addition for the PDMS:PU of weight andblend the (95:5) reduction increased of sample with an weight. increase The in final immersion sample timewould and shrink then it or remained damage forsteady. reduction However, conditions the polymer [36]. The blend penetration of PDMS:PU of DI blend water (95:5) and exhibitsseawater a degree (SW) through of swelling the polymer close to wasPDMS observed. at 2.36 ( AfterFigure immersion 7). for 72 h, each type of sample began to lose weight, illustrating hydrolysis degradation [37]. However, seawater immersion is a complicated medium due to the microorganisms and elements contained in sea water. The PDMS film immersed in seawater showed an increase in the rate of mass loss by enzymatic biodegradation [38] when compared with tests carried out in DI water. This was because the seawater contained microorganisms such as bacteria, fungi, algae and plankton. The highest degree of swelling was 2.12 in 72 h and over 5 days the mass loss was slightly reduced. On the other hand, the mass loss of PU in the two mediums showed that the degree of swelling of PU slightly increased. Then, the steadiness degree of swelling was accomplished after approximately 72 h in DI water and sea water. Mass loss for the PDMS:PU blend (95:5) increased with an increase in immersion time and then it remained steady. However, the polymer blend of PDMS:PU blend (95:5) exhibits a degree of swelling close to PDMS at 2.36 (Figure7).

Polymers 2021, 13, x FOR PEER REVIEW 10 of 16

(a) (c) (e)

(b) (d) (f)

Figure 6. AFM topology images (a,b) PDMS:PU (95:5) blend; (c,d) PDMS:PU (5:95) blend; (e,f) polyurethane.

3.1.6. Effect of Swelling The applicability of polymer films for antifouling products is based on the bond breakage of samples when immersed in different testing mediums, which can be predicted by swelling behavior and water resistance in a natural environment. In this paper, polydimethylsiloxane elastomer, polyurethane and the PDMS:PU blend at 95:5 blend ratio were studied for their swelling behaviors. The mechanism of swelling is represented by two properties; the addition of weight and the reduction of sample weight. The final sample would shrink or damage for reduction conditions [36]. The penetration of DI water and seawater (SW) through the polymer was observed. After immersion for 72 h, each type of sample began to lose weight, illustrating hydrolysis degradation [37]. However, seawater immersion is a complicated medium due to the microorganisms and elements contained in sea water. The PDMS film immersed in seawater showed an increase in the rate of mass loss by enzymatic biodegradation [38] when compared with tests carried out in DI water. This was because the seawater contained microorganisms such as bacteria, fungi, algae and plankton. The highest degree of swelling was 2.12 in 72 h and over 5 days the mass loss was slightly reduced. On the other hand, the mass loss of PU in the two mediums showed that the degree of swelling of PU slightly increased. Then, the steadiness degree of swelling was accomplished after approximately 72 h in DI water and sea water. Mass loss for the PDMS:PU blend (95:5) increased with an increase in immersion time and then it remained steady. However, the polymer blend of PDMS:PU blend (95:5) Polymers 2021, 13, 2242 exhibits a degree of swelling close to PDMS at 2.36 (Figure 7). 10 of 16

Polymers 2021, 13, x FOR PEER REVIEW 11 of 16

Figure 7. Time dependent mass loss in DI water and seawater of (a) polydimethylsiloxane, Figure 7. Time dependent mass loss in DI water and seawater of (a) polydimethylsiloxane, (b) pol- (b) polyurethane and (c) PDMS:PU blend (95:5). Polymers 2021,yurethane 13, x FOR PEER and (REVIEWc) PDMS:PU blend (95:5). 12 of 16

3.1.7. Barnacle Measurements 3.1.7. Barnacle Measurements The short-term antifouling measurement of the polymer film was evaluated in seawater The short-term antifouling measurement of the polymer film was evaluated in sea- ◦ 0 00 atadhesive Koh Sichang force Marine by the Science barnacle Research base Centrearea. The of Chulalongkorn results indicated University that PDMS (13 09 and10.6 theN water at Koh Sichang◦ Marine0 00 Science Research Centre of Chulalongkorn University 100PDMS:PU49 02.6 blendE) during (95:5) film April showed to June. good The antifo antifoulinguling properties behaviors because of polydimethylsiloxane, of the low attach- (13°09′10.6′′ N 100°49′02.6′′ E) during April to June. The antifouling behaviors of polydi- polyurethanement number and and thelow PDMS:PU adhesive strength blend (95:5) of the were barnacles compared on the with surface carbon of steel these (CS) materials. as the methylsiloxane, polyurethane and the PDMS:PU blend (95:5) were compared with carbon referenceThis therefore material, means with that an the area surface of 6 cm of× th6e cm,PDMS using and a digitalthe PDMS:PU microscope blend as (95:5) shown film in steel (CS) as the reference material, with an area of 6 cm × 6 cm, using a digital microscope Figurehave excellent8 . The reference antifouling carbon properties. steel started Theref toore, corrode the PDMS and rustingfilm and was the observedPDMS:PU on poly- the as shown in Figure 8. The reference carbon steel started to corrode and rusting was ob- surfacemer blend after exhibited being immersed lower adhesive in seawater strength for 2 weeks. than carbon The growth steel, rate by ofaround the barnacles 80–90%, on after the served on the surface after being immersed in seawater for 2 weeks. The growth rate of surfaceimmersion of the in carbon seawater steel for was 4 and much 8 weeks higher (Figure than the 11). other Therefore, studied from surfaces. the study To improve we could the the barnacles on the surface of the carbon steel was much higher than the other studied antifoulingconclude that properties barnacles of the favored carbon attachment steel, a blend to hydrophilic of PU and PDMS surfaces, was studiedi.e., carbon as a steel polymer and surfaces. To improve the antifouling properties of the carbon steel, a blend of PU and film.PU film. From Our the results study, correspond it was clearly to observedthe data from that surfaceFinlay ofet al., the [40] PDMS which film suggested and PDM:PU that PDMS was studiedblend as a polymer (95:5) film film. clearly From improved the study, the it resistance was clearly to foulingobserved of that the surfacesurface (Figure 9). hydrophilic surfaces or higher energy surfaces increased barnacle attachment. of the PDMS film and PDM:PU blend (95:5) film clearly improved the resistance to fouling of the surface (Figure 9). After 2 weeks, it(a) was clearly observed that barnacles(b) on the surface of the carbon steel had rapidly increased and partially covered all surfaces at approximately 2.5 ± 0.4 marine barnacles/cm2 (see Figure 10). The marine organisms on the carbon steel samples, which were placed in the sea facing the shore and facing away from the shore, decreased from 2.5 ± 0.4 to 0.7 ± 0.2 marine barnacles/cm2 and 2.7 ± 0.8 to 1.0 ± 0.2 marine barnacles/cm2 after 4 and 8 weeks, respectively, which was due to the increase in the size of the barnacles. After 2 weeks of immersion of the PDMS film in sea water, the number of barnacles was rather low initially; biofouling on the surface of samples facing the shore side was approximately 0.5 ± 0.1 marine barnacles/cm2, while that for the samples facing away from the shore was approximately(c) 0.9 ± 0.1 marine barnacles/cm(d) 2. When immersed longer, some biofouling disappeared from the surface of the PDMS film since the fouling was released more easily from the surface of the silicone samples due to weak adhesion between the surfaces [39]. Due to its more hydrophilic properties, the surface of the PU film had more barnacles attached than the PDMS, but it had slightly fewer barnacles attached than the carbon steel. The marine organisms collected after 2 weeks were 1.2 ± 0.7 marine barnacles/cm2. The PDMS:PU blend (95:5) samples exhibited a lower number of barnacles than the carbon steel and the PU film. After 2 weeks of immersion, the number of marine barnacles on the PDMS:PU blend (95:5) film facing the shore side was 0.5 ± 0.2 marine barna- FigureFigure 8.8. DigitalDigital microscopemicroscope imagesimages ofof marinemarine biofoulingbiofouling onon variousvarious materialmaterial surfacesurface inin seawaterseawater cles/cm2 and the PDMS:PU(a) carbon blend steel, ((95:5)b) polydimethylsiloxane, film facing away from(c) PDMS:PU the shore blend was (95:5) 1.1 ±and 0.3 ( ma-d) polyurethane. (a) carbon steel, (b) polydimethylsiloxane, (c) PDMS:PU blend (95:5) and (d) polyurethane. rine barnacles/cm2, which were close to the PDMS surfaces. For 4–8 weeks, the polymer blend samples showed the best result for the number of barnacles on the surface, and the different of antifouling characteristics between the PDMS:PU blend (95:5) samples and the carbon steel were clearly noticeable. Moreover, since the study focused on antifouling surfaces, which is how easily adhered marine organisms were removed from the surface without toxicity to marine organisms, barnacle adhesion strength measurements were also calculated by dividing the

Figure 9. Marine biofouling on various sample surfaces after immersed in seawater environment (a–e) immersed for 2 weeks, (f–j) immersed for 4 weeks and (k–o) immersed for 8 weeks (in April- June).

Polymers 2021, 13, x FOR PEER REVIEW 12 of 16

adhesive force by the barnacle base area. The results indicated that PDMS and the PDMS:PU blend (95:5) film showed good antifouling properties because of the low attachment number and low adhesive strength of the barnacles on the surface of these materials. This therefore means that the surface of the PDMS and the PDMS:PU blend (95:5) film have excellent antifouling properties. Therefore, the PDMS film and the PDMS:PU polymer blend exhibited lower adhesive strength than carbon steel, by around 80–90%, after immersion in seawater for 4 and 8 weeks (Figure 11). Therefore, from the study we could conclude that barnacles favored attachment to hydrophilic surfaces, i.e., carbon steel and PU film. Our results correspond to the data from Finlay et al., [40] which suggested that hydrophilic surfaces or higher energy surfaces increased barnacle attachment.

(a) (b)

Polymers 2021, 13, 2242 11 of 16 Figure 8. Digital microscope images of marine biofouling on various material surface in seawater (a) carbon steel, (b) polydimethylsiloxane, (c) PDMS:PU blend (95:5) and (d) polyurethane.

Figure 9. Marine biofouling on various sample surfaces after immersed in seawater environment (a–e) Figure 9. Marine biofouling on various sample surfaces after immersed in seawater environment (aimmersed–e) immersed for 2 for weeks, 2 weeks, (f–j) immersed(f–j) immersed for 4 for weeks 4 weeks and ( kand–o) ( immersedk–o) immersed for 8 weeksfor 8 weeks (in April-June). (in April- June). After 2 weeks, it was clearly observed that barnacles on the surface of the carbon steel had rapidly increased and partially covered all surfaces at approximately 2.5 ± 0.4 marine barnacles/cm2 (see Figure 10). The marine organisms on the carbon steel samples, which were placed in the sea facing the shore and facing away from the shore, decreased from 2.5 ± 0.4 to 0.7 ± 0.2 marine barnacles/cm2 and 2.7 ± 0.8 to 1.0 ± 0.2 marine barnacles/cm2 after 4 and 8 weeks, respectively, which was due to the increase in the size of the barnacles. After 2 weeks of immersion of the PDMS film in sea water, the number of barnacles was rather low initially; biofouling on the surface of samples facing the shore side was approximately 0.5 ± 0.1 marine barnacles/cm2, while that for the samples facing away from the shore was approximately 0.9 ± 0.1 marine barnacles/cm2. When immersed longer, some biofouling disappeared from the surface of the PDMS film since the fouling was released more easily from the surface of the silicone samples due to weak adhesion between the surfaces [39]. Due to its more hydrophilic properties, the surface of the PU film had more barnacles attached than the PDMS, but it had slightly fewer barnacles attached than the carbon steel. The marine organisms collected after 2 weeks were 1.2 ± 0.7 marine barnacles/cm2. The PDMS:PU blend (95:5) samples exhibited a lower number of barnacles than the carbon steel and the PU film. After 2 weeks of immersion, the number of marine barnacles on the PDMS:PU blend (95:5) film facing the shore side was 0.5 ± 0.2 marine barnacles/cm2 and the PDMS:PU blend (95:5) film facing away from the shore was 1.1 ± 0.3 marine barnacles/cm2, which were close to the PDMS surfaces. For 4–8 weeks, the polymer blend samples showed the best result for the number of barnacles on the surface, and the different of antifouling characteristics between the PDMS:PU blend (95:5) samples and the carbon steel were clearly noticeable. Polymers 2021, 13, x FOR PEER REVIEW 13 of 16

Polymers 2021, 13, 2242 12 of 16

2 (a) 2 4.0 4.0 (b) 3.5 3.5 3.0 3.0 2.5 2.5 2.0 2.0 1.5 1.5 Polymers 2021, 13, x FOR PEER REVIEW 13 of 16 1.0 1.0 0.5 0.5 0.0 0.0 Number of Barnacle counts/ cm Number of Barnacle counts/ cm Number of Barnacle 14 days 28 days 56 days 14 days 28 days 56 days 2 CS PDMS PDMS:PU (95:5) blend PU(a) 2 CS PDMS PDMS:PU (95:5) blend PU 4.0 4.0 (b)

Figure 10.3.5Barnacle counts after 2 weeks, 4 weeks and 8 weeks in seawater3.5 with different immersion duration: (a) facing the Figure 10. Barnacle counts after 2 weeks, 4 weeks and 8 weeks in seawater with different immersion duration: (a) facing shore and3.0 (b) facing away from the shore. 3.0 the shore and (b) facing away from the shore. 2.5 Moreover, since the study focused2.5 on antifouling surfaces, which is how easily adhered 2.0 marine organisms were removed from2.0 the surface without toxicity to marine organisms, 2.50 (a) 2.50 1.5 barnacle adhesion strength measurements1.5 were also calculated by dividing the(b) adhesive 1.02.00 force by the barnacle base area. The1.0 results2.00 indicated that PDMS and the PDMS:PU blend (95:5) film showed good antifouling properties because of the low attachment number and 0.5 0.5 low adhesive strength of the barnacles on the surface of these materials. This therefore 1.50 1.50 0.0 means that the surface of the PDMS0.0 and the PDMS:PU blend (95:5) film have excellent Number of Barnacle counts/ cm Number of Barnacle Number of Barnacle counts/ cm Number of Barnacle 14 daysantifouling 28 days properties. 56 days Therefore, the PDMS14 filmdays and the 28 days PDMS:PU 56 polymer days blend ex- 1.00 hibited lower adhesive strength than1.00 carbon steel, by around 80–90%, after immersion CS PDMS PDMS:PUin seawater (95:5) forblend 4 andPU 8 weeks (FigureCS 11).PDMS Therefore,PDMS:PU from the (95:5) study blend we couldPU conclude Adhesive strength Adhesive strength (MPa) 0.50 that barnacles favored attachment Adhesive strength (MPa) to0.50 hydrophilic surfaces, i.e., carbon steel and PU film. Figure 10. Barnacle counts afterOur 2 resultsweeks, correspond4 weeks and to8 weeks the data in seawater from Finlay with et different al. [40] immersion which suggested duration: that (a) hydrophilicfacing the shore and0.00 (b) facing awaysurfaces from the or shore. higher energy surfaces increased0.00 barnacle attachment. 28 days 56 days 28 days 56 days 2.50 (a) 2.50 CS PDMS PDMS:PU (95:5) blend PU CS PDMS PDMS:PU (95:5) blend (b)PU

2.00 2.00 Figure 11. Adhesive strength (MPa) of barnacle on surface after 2 weeks, 4 weeks and 8 weeks in seawater: (a) facing the shore and (b) facing away from the shore. 1.50 1.50 3.2. Characteristics of PDMS:PU Blend Film with Micro Patterning Fabricated by Soft 1.00 Lithography 1.00 3.2.1. Morphology of PDMS:PU Blend Film with Micro Pattering Fabricated by Soft Li- Adhesive strength Adhesive strength (MPa) 0.50 thography Adhesive strength (MPa) 0.50 Microstructures or micro patterning were fabricated on the surface of the PDMS:PU 0.00 blend (95:5) by the soft lithography process0.00 to further increase the hydrophobic properties 28 daysof the polymer 56 daysblend film. The microstructures 28or days micropatterns on 56 daysthe PDMS:PU blend (95:5) surface were observed by using SEM (Figure 12). The side view using of the CS PDMS PDMS:PU (95:5) blend PU CS PDMS PDMS:PU (95:5) blend PU PDMS:PU blend (95:5) film (see Figure 12a) clearly illustrated uniform sharklet structures Figure 11. Adhesive strengthon (MPa) the surface, of barnacle suggesting on surface that after the 2 PDMS:PU weeks, 4 weeks blend and (95:5) 8 weeks was in able seawater: to be further (a) facing modified the Figure 11. Adhesive strength (MPa) of barnacle on surface after 2 weeks, 4 weeks and 8 weeks in seawater: (a) facing the by the soft lithography process. shoreshore and and ( (bb)) facing facing away away fr fromom the the shore. shore.

3.2. Characteristics of PDMS:PU Blend Film with Micro Patterning Fabricated by Soft Lithography 3.2.1. Morphology of PDMS:PU Blend Film with Micro Pattering Fabricated by Soft Li- thography Microstructures or micro patterning were fabricated on the surface of the PDMS:PU blend (95:5) by the soft lithography process to further increase the hydrophobic properties of the polymer blend film. The microstructures or micropatterns on the PDMS:PU blend (95:5) surface were observed by using SEM (Figure 12). The side view using of the PDMS:PU blend (95:5) film (see Figure 12a) clearly illustrated uniform sharklet structures on the surface, suggesting that the PDMS:PU blend (95:5) was able to be further modified by the soft lithography process.

Polymers 2021, 13, 2242 13 of 16

3.2. Characteristics of PDMS:PU Blend Film with Micro Patterning Fabricated by Soft Lithography 3.2.1. Morphology of PDMS:PU Blend Film with Micro Pattering Fabricated by Soft Lithography Microstructures or micro patterning were fabricated on the surface of the PDMS:PU blend (95:5) by the soft lithography process to further increase the hydrophobic properties of the polymer blend film. The microstructures or micropatterns on the PDMS:PU blend (95:5) surface were observed by using SEM (Figure 12). The side view using of the PDMS:PU Polymers 2021, 13, x FOR PEER REVIEWblend (95:5) film (see Figure 12a) clearly illustrated uniform sharklet structures14 on of the 16 Polymers 2021, 13, x FOR PEER REVIEW 14 of 16 Polymers 2021, 13, x FOR PEER REVIEW 14 of 16 surface, suggesting that the PDMS:PU blend (95:5) was able to be further modified by the soft lithography process.

(a) (b) (a) (b) Figure 12. SEM image(a) of (a) side view of the PDMS:PU blend(b (9) 5:5) ridge sharklet pattern and (b) Figure 12. SEM image of (a) side view of the PDMS:PU blend (95:5) ridge sharklet pattern and (b) top-viewFigureFigure 12.12. SEM SEM image image of ofPDMS:PU of ( (aa)) side side viewblen viewd of (95:5) of the the PDMS:PUridge PDMS:PU sharklet blend blend pattern (9 (95:5)5:5) fabricated ridge ridge sharklet sharklet by soft pattern patternlithography and and (b ) top-view SEM image of PDMS:PU blend (95:5) ridge sharklet pattern fabricated by soft lithography process.top-view SEM image of PDMS:PU blend (95:5) ridge sharklet pattern fabricated by soft lithography process.(b) top-view SEM image of PDMS:PU blend (95:5) ridge sharklet pattern fabricated by soft lithogra- phyprocess. process. 3.2.2. Water Contact Angle (WCA) of Polymer Blend with Micro Patterning Fabricated 3.2.2.by3.2.2.3.2.2. Soft Water WaterWater Lithography Contact ContactContact Angle Angle Angle (WCA) (WCA)(WCA) of of Polymer Polymer Blend Blend with with Micro Micro Patterning Patterning Fabricated Fabricated by by Soft Lithography Softby SoftThe Lithography Lithography hydrophobicity of the PDMS:PU blend (95:5) film with micro patterning by the soft lithographyTheTheThe hydrophobicity hydrophobicityhydrophobicity process was of ofof the thedeterminedthe PDMS:PU PDMS:PUPDMS:PU using blend blendblend the (9 (95:5)(9 water5:5)5:5) film filmfilm contact with withwith anglemicro micromicro measurement. patterning patterningpatterning by byby Thethe thethe softcontactsoftsoft lithography lithographylithography angle from process processprocess the water was was was dropletsdetermined determineddetermined on th using using usinge PDMS:PU the the the water water water blend contact contact contact (95:5) angle angle angle surface measurement. measurement. measurement. without micro- The The The contactstructurescontactcontact angle angleangle has from from froma value the thethe water waterwaterof 103.4° droplets dropletsdroplets ± 3.8° on onon and th thethee PDMS:PUthe PDMS:PUPDMS:PU water blendcontact blendblend (95:5) (95:5)(95:5) angle surface surfacesurface on PDMS:PU without withoutwithout micromicro-blendmicro- ◦ ◦ structures(95:5)structuresstructures film has withhashas a a a microvalue value patternsof of 103.4103.4° 103.4° on ± ± 3.8° the3.83.8° surfaceand andand the thethe showed water waterwater contact contactimprovedcontact angle angleangle hydrophobicity on onon PDMS:PU PDMS:PU withblend blend a (95:5)water(95:5)(95:5) filmcontact filmfilm with withwith angle micro micromicro (WCA) patterns patternspatterns of 128.8° on onon the thethe ± surface 1.6°.surface The showed showed result indicatedimproved improved improved that hydrophobicity hydrophobicityhydrophobicity the hydrophobicity with withwith a aa ◦ ◦ waterofwaterwater the contactPDMS:PU contact angle angle blend (WCA)(WCA) (WCA) (95:5) ofof off 128.8 ilm128.8° 128.8° could±± ± 1.61.6°. 1.6°.be. further TheThe The resultresult result improved indicated indicated indicated by that thethat that thesoft the the hydrophobicity lithography hydrophobicity hydrophobicity pro- of ofcesstheof the the PDMS:PU(Table PDMS:PU PDMS:PU 3). blend blend blend (95:5) (95:5) (95:5) film f ilmfilm could could could be be furtherbe further further improved improved improved by by theby the softthe soft soft lithography lithography lithography process pro- pro- cess(Tablecess (Table (Table3). 3). 3). Table 3. Water contact angle on various material surfaces PDMS:PU blend (95:5) and PDMS:PU TableTable 3. Water contactcontact angle angle on on various various material material surfaces surfaces PDMS:PU PDMS:PU blend blend (95:5) (95:5) and PDMS:PUand PDMS:PU blend blendTable (95:5) 3. Water with contactsharklet angle pattern. on various material surfaces PDMS:PU blend (95:5) and PDMS:PU blend (95:5) with sharklet pattern. (95:5)blend with (95:5) sharklet with sharklet pattern. pattern. PDMS:PU blend (95:5) PDMS:PU blend (95:5) PDMS:PUPDMS:PUsharklet blend blendpattern (95:5)(95:5) (95:5) PDMS:PUPDMS:PU blend blend (95:5)(95:5) (95:5) sharkletsharkletsharklet pattern pattern

◦ ◦ 103.4° ±± 3.8° 128.8° ± 1.6° 103.4° ± 3.8° ◦ ◦ 103.4° ± 3.8° 128.8°128.8128.8° ±± ± 1.6°1.6 1.6° 4. Conclusions 4. Conclusions 4.4. ConclusionsConclusionsA nontoxic material with simple preparation process and economic viability with an- tifoulingAAA nontoxic nontoxicnontoxic film performance material material with with with was simple simple simpleprepared preparation preparation preparation from a process polymerprocess process and and blend and economic economic economicwith different viability viability viability withratios with with an- an-of tifoulingPDMSantifoulingtifouling and film film ﬁlmPU. performance performance performanceBased on wasthe was was hydrophobicityprepared prepared prepared from from from a ofa polymer polymer athe polymer film, blend blend the blend PDMS:PUwith with with different different different blend ratios ratios ratios(95:5) of of PDMSshowedofPDMS PDMS and theand and bestPU. PU. PU. resultsBased Based Based aton on 103.4° onthe the the hydrophobicity ±hydrophobicity hydrophobicity3.8°. When PU of wasof of the the theintroduced film, film, ﬁlm, the the toPDMS:PU PDMS:PUPDMS:PU PDMS, the blend blendblend phase (95:5) (95:5)(95:5) and ◦ ◦ showedmorphologyshowedshowed the the the best best bestof theresults results results polymer at at 103.4° 103.4 103.4° blend ± ± 3.8°. 3.83.8°. clearly .Wh Wh Whenen enillu PU PUstrated was was introduced introduceda good distribution to toto PDMS, PDMS,PDMS, the theofthe phasethe phasephase PU and andand in morphologyPDMS.morphologymorphology In addition, of of thethe the polymer thepolymer polymer mechanical blend blend blend clearly propertiesclearly clearly illustrated illu illu resultingstratedstrated a good a a fromgood good distribution stress–strain distribution distribution of the curve of PUof the the in and PDMS.PU PU PF- in in PDMS.QNMPDMS. revealed In In addition, addition, the stiffnessthe the mechanical mechanical differences properties properties among resulting resultingthe samples from from with stress–strain stress–strain different curve mixingcurve and and ratios PF- PF- QNMsinceQNM polyurethane revealed revealed the the hasstiffness stiffness been direcognized differencesfferences toamong among increase the the hardnesssamples samples withand with modulus different different when mixing mixing blended ratios ratios sincewithsince polyurethanePDMS. polyurethane From has thehas been results,been recognized recognized the antifou to to increase increaseling performance hardness hardness and andof modulusPDMS:PU modulus when when blend blended blended (95:5) withshowedwith PDMS. PDMS. a low From numberFrom the the of results, barnaclesresults, the the on antifou antifouthe surfacelingling performance andperformance a low adhesive of of PDMS:PU PDMS:PU strength blend ofblend 0.07 (95:5) MPa(95:5) showedforshowed 8 weeks, a a low low or number numbera 80–90% of of barnacles decreasebarnacles onwhen on the the comparedsurface surface and and to a a thelow low reference adhesive adhesive carbonstrength strength steel of of 0.07 0.07sample, MPa MPa forandfor 8 the8 weeks, weeks, biofouling or or a a 80–90% 80–90% could easilydecrease decrease be removedwhen when compared compared from the to to surface. the the reference reference The result carbon carbon showed steel steel sample,that sample, the andPDMS:PUand the the biofouling biofouling blend (95:5) could could exhibited easily easily be beexcellent removed removed antifo from fromuling the the surface. properties,surface. The The making result result showed itshowed suitable that that to the bethe PDMS:PUappliedPDMS:PU as blenda blend polymer (95:5) (95:5) coating exhibited exhibited in the excellent excellent marine antifo environment.antifoulinguling properties, properties, In this study, making making it was it it suitable suitablealso demon- to to be be appliedstratedapplied that as as a a softpolymer polymer lithography coating coating process in in the the marine marinecould beenvironment. environment. employed to In Infurther this this study, study, improve it it was was the also also hydropho- demon- demon- stratedstrated that that soft soft lithography lithography process process could could be be employed employed to to further further improve improve the the hydropho- hydropho-

Polymers 2021, 13, 2242 14 of 16

In addition, the mechanical properties resulting from stress–strain curve and PF-QNM revealed the stiffness differences among the samples with different mixing ratios since polyurethane has been recognized to increase hardness and modulus when blended with PDMS. From the results, the antifouling performance of PDMS:PU blend (95:5) showed a low number of barnacles on the surface and a low adhesive strength of 0.07 MPa for 8 weeks, or a 80–90% decrease when compared to the reference carbon steel sample, and the biofouling could easily be removed from the surface. The result showed that the PDMS:PU blend (95:5) exhibited excellent antifouling properties, making it suitable to be applied as a polymer coating in the marine environment. In this study, it was also demonstrated that soft lithography process could be employed to further improve the hydrophobicity of the ﬁlm. The PDMS:PU blend (95:5) showed small spherical droplets of polyurethane which could penetrate into a 3 µm width sharklet pattern on the soft lithography mold and further improve the hydrophobicity of the polymer blend ﬁlm, with an improved water contact angle (WCA) of 128.8◦ ± 1.6◦.

Author Contributions: Conceptualization, J.C., S.C., J.P., N.A. and M.N.; methodology, J.C., S.C., J.P. and M.N.; formal analysis, J.C., S.C., J.P., N.A. and M.N.; investigation, J.C., S.C., J.P., N.A. and M.N.; data curation, J.C., S.C., J.P., N.A. and M.N.; writing—original draft preparation, J.C., S.C., J.P. and M.N.; writing—review and editing, J.C., S.C., J.P. and M.N.; visualization, J.C., S.C., J.P., N.A. and M.N.; supervision, M.N.; project administration, M.N.; funding acquisition, J.C., M.N. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by Ratchadaphiseksomphot Endowment Fund, grant number CU_GR_63_48_63_03 and the Center of Excellence on Petrochemical and Materials Technology, Chulalongkorn University. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Acknowledgments: This research was funded by Ratchadaphiseksomphot Endowment Fund, Chu- lalongkorn University (CU_GR_63_48_63_03) and the Center of Excellence on Petrochemical and Materials Technology, Chulalongkorn University. Conﬂicts of Interest: The authors declare that this work is original and has not been published elsewhere nor is it currently under consideration for publication elsewhere. All authors have approved the manuscript and agree with its submission to this journal.

References 1. Farkas, A.; Song, S.; Degiuli, N.; Martić,I.; Demirel, Y.K. Impact of biofilm on the ship propulsion characteristics and the speed reduction. Ocean Eng. 2020, 199, 107033. [CrossRef] 2. Dharmlingam, S. Biofouling-Bioadhesion of Micro-Organisms and Its Prevention: A Review. IJERR 2018, 11–21. 3. Yebra, D.M.; Kiil, S.; Dam-Johansen, K. Antifouling technology—Past, present and future steps towards efficient and environmentally friendly antifouling coatings. Prog. Org. Coat. 2004, 50, 75–104. [CrossRef] 4. Simões, M. Antimicrobial strategies effective against infectious bacterial biofilms. Curr. Med. Chem. 2011, 18, 2129–2145. [CrossRef] 5. Yi, J.; Huang, C.; Zhuang, H.; Gong, H.; Zhang, C.; Ren, R.; Ma, Y. Degradable polyurethane based on star-shaped polyester polyols (trimethylolpropane and ε-caprolactone) for marine antifouling. Prog. Org. Coat. 2015, 87, 161–170. [CrossRef] 6. Anthony Yesudass, S.; Mohanty, S.; Nayak, S.K.; Rath, C.C. Zwitterionic–polyurethane coatings for non-specific marine bacterial inhibition: A nontoxic approach for marine application. Eur. Polym. J. 2017, 96, 304–315. [CrossRef] 7. Hu, W.; Culloty, S.; Darmody, G.; Lynch, S.; Davenport, J.; Ramirez-Garcia, S.; Dawson, K.A.; Lynch, I.; Blasco, J.; Sheehan, D. Toxicity of copper oxide nanoparticles in the blue mussel, Mytilus edulis: A redox proteomic investigation. Chemosphere 2014, 108, 289–299. [CrossRef] 8. Gomes, T.; Pereira, C.G.; Cardoso, C.; Pinheiro, J.P.; Cancio, I.; Bebianno, M.J. Accumulation and toxicity of copper oxide nanoparticles in the digestive gland of Mytilus galloprovincialis. Aquat. Toxicol. 2012, 118–119, 72–79. [CrossRef][PubMed] 9. Nurioglu, A.G.; Esteves, A.C.C.; de With, G. Non-toxic, non-biocide-release antifouling coatings based on molecular structure design for marine applications. J. Mater. Chem. B 2015, 3, 6547–6570. [CrossRef] 10. Sokolova, A.; Cilz, N.; Daniels, J.; Stafslien, S.J.; Brewer, L.H.; Wendt, D.E.; Bright, F.V.; Detty, M.R. A comparison of the antifouling/foul-release characteristics of non-biocidal xerogel and commercial coatings toward micro- and macrofouling organisms. Biofouling 2012, 28, 511–523. [CrossRef] Polymers 2021, 13, 2242 15 of 16

11. Zalewski, K.; Chyłek, Z.; Trzciński,W.A. A Review of Polysiloxanes in Terms of Their Application in Explosives. Polymers 2021, 13, 1080. [CrossRef] 12. Pérez, E.; Contreras-Lopez, D.; Vallejo, J.; Gonzalez-Calderon, J.A. Polysiloxanes as polymer matrices in biomedical engineering: Their interesting properties as the reason for the use in medical sciences. Polym. Bull. 2020, 77.[CrossRef] 13. Vinckier, I.; Moldenaers, P.; Mewis, J. Relationship between Rheology and Morphology of Model Blends in Steady Shear Flow. J. Rheol. 1996, 40, 613–631. [CrossRef] 14. Cui, X.; Zhu, G.; Pan, Y.; Shao, Q.; Zhao, C.; Dong, M.; Zhang, Y.; Guo, Z. Polydimethylsiloxane-titania nanocomposite coating: Fabrication and corrosion resistance. Polymer 2018, 138, 203–210. [CrossRef] 15. Thanakhun, K.; Puttapitukporn, T. PDMS Material Models for Anti-fouling Surfaces Using Finite Element Method. Eng. J. 2019, 23, 381–398. [CrossRef] 16. Yun, X.; Xiong, Z.; He, Y.; Wang, X. Superhydrophobic lotus-leaf-like surface made from reduced graphene oxide through soft-lithographic duplication. RSC Adv. 2020, 10, 5478–5486. [CrossRef] 17. Placet, V.; Delobelle, P. Mechanical properties of bulk polydimethylsiloxane for microfluidics over a large range of frequencies and aging times. J. Micromech. Microeng. 2015, 25, 035009. [CrossRef] 18. Weibel, D.B.; Diluzio, W.R.; Whitesides, G.M. Microfabrication meets microbiology. Nat. Rev. Microbiol. 2007, 5, 209–218. [CrossRef][PubMed] 19. Kim, M.; Moon, B.-U.; Hidrovo, C.H. Enhancement of the thermo-mechanical properties of PDMS molds for the hot embossing of PMMA microfluidic devices. J. Micromech. Microeng. 2013, 23, 095024. [CrossRef] 20. Wang, Z.; Volinsky, A.A.; Gallant, N.D. Crosslinking effect on polydimethylsiloxane elastic modulus measured by custom-built compression instrument. J. Appl. Polym. Sci. 2014, 131.[CrossRef] 21. Ibrahim, I.A.M.; Zikry, A.A.F.; Sharaf, M.A.; Mark, J.E.; Jacob, K.; Jasiuk, I.M.; Tannenbaumn, R. Elastic behavior of silica/poly(dimethylsiloxane) nanocomposites: Nano-size effects. IOP Conf. Ser. Mater. Sci. Eng. 2012, 40, 012008. [CrossRef] 22. Hasan, A.M.A.; Abdel-Raouf, M.E.-S. Cellulose-Based Superabsorbent Hydrogels. In Cellulose-Based Superabsorbent Hydrogels; Mondal, M.I.H., Ed.; Springer International Publishing: Cham, Switzerland, 2019; pp. 245–267. 23. Damrongsakkul, S.; Sinweeruthai, R.; Higgins, J. Processability and chemical resistance of the polymer blend of thermoplastic polyurethane and polydimethylsiloxane. Macromol. Symp. 2003, 198, 411–420. [CrossRef] 24. Atthi, N.; Sripumkhai, W.; Pattamang, P.; Thongsook, O.; Srihapat, A.; Meananeatra, R.; Supadech, J.; Klunngien, N.; Jeamsaksiri, W. Fabrication of robust PDMS micro-structure with hydrophobic properties. Microelectron. Eng. 2020, 224, 111255. [CrossRef] 25. Wu, Z.; Wang, H.; Tian, X.; Cui, P.; Ding, X.; Ye, X. The effects of polydimethylsiloxane on transparent and hydrophobic waterborne polyurethane coatings containing polydimethylsiloxane. Phys. Chem. Chem. Phys. 2014, 16, 6787–6794. [CrossRef] 26. Boya, R.; Kulkarni, G. Micromolding—A Soft Lithography Technique; Micromanufacturing Processes: Boca Raton, FL, USA, 2012; pp. 329–347. 27. Vinagre, P.A.; Simas, T.; Cruz, E.; Pinori, E.; Svenson, J. Marine Biofouling: A European Database for the Marine Renewable Energy Sector. J. Mar. Sci. Eng. 2020, 8. 495. [CrossRef] 28. Luo, S.-J.; Su, Y.-H.; Lu, M.-J.; Kuo, J.-C. EBSD analysis of magnesium addition on inclusion formation in SS400 structural steel. Mater. Charact. 2013, 82, 103–112. [CrossRef] 29. Chieng, B.W.; Ibrahim, N.A.; Ahmad Daud, N.; Talib, Z.A. Chapter 8—Functionalization of Graphene Oxide via Gamma-Ray Irradiation for Hydrophobic Materials. In Synthesis, Technology and Applications of Carbon Nanomaterials; Elsevier: Amsterdam, The Netherlands, 2019; pp. 177–203. [CrossRef] 30. Kojio, K.; Mitsui, Y.; Furukawa, M. Synthesis and properties of highly hydrophilic polyurethane based on diisocyanate with ether group. Polymer 2009, 50.[CrossRef] 31. Mouritz, A.P. (Ed.) 1—Introduction to aerospace materials: In Introduction to Aerospace Materials; Woodhead Publishing: Cambridge, UK, 2012; pp. 1–14. [CrossRef] 32. Dai, Z.; Yang, K.; Dong, Q. Mechanical, Thermal and Morphology Properties of Thermoplastic Polyurethane Copolymers Incor- porating α,ω-Dihydroxy-[poly(propyleneoxide)-poly (dimethylsiloxane)-poly(propyleneoxide)] of Varying Poly(propyleneoxide) Molecular Weight. Open J. Synth. Theory Appl. 2015, 4, 41–57. [CrossRef] 33. Zhu, R.; Wang, X.; Yang, J.; Wang, Y.; Zhang, Z.; Hou, Y.; Lin, F. Influence of hydroxyl-terminated polydimethylsiloxane on high-strength biocompatible polycarbonate urethane films. Biomed. Mater. 2016, 12, 015011. [CrossRef] 34. Magonov, S.N. Atomic Force Microscopy in Analysis of Polymers. Encycl. Anal. Chem. Appl. Theory Instrum. 2006.[CrossRef] 35. Ouyang, L.; Kuo, C.-c.; Farrell, B.; Pathak, S.; Wei, B.; Qu, J.; Martin, D.C. Poly[3,4-ethylene dioxythiophene (EDOT)-co-1,3,5-tri[2- (3,4-ethylene dioxythienyl)]-benzene (EPh)] copolymers (PEDOT-co-EPh): Optical, electrochemical and mechanical properties. J. Mater. Chem. B 2015, 3, 5010–5020. [CrossRef] 36. Mayasari, H.E.; Setyadewi, N.M. Thermogravimetry and swelling characteristics af NBR/EPDM blends with some compatibilizers. AIP Conf. Proc. 2018, 2049, 020042. [CrossRef] 37. Ma, C.; Zhang, W.; Zhang, G.; Qian, P.-Y. Environmentally Friendly Antifouling Coatings Based on Biodegradable Polymer and Natural Antifoulant. ACS Sustain. Chem. Eng. 2017, 5, 6304–6309. [CrossRef] 38. Pathak, V.M.; Navneet. Review on the current status of polymer degradation. A microbial approach. Bioresour. Bioprocess. 2017, 4, 15. [CrossRef] Polymers 2021, 13, 2242 16 of 16

39. Ba, M.; Zhang, Z.; Qi, Y. Fouling Release Coatings Based on Polydimethylsiloxane with the Incorporation of Phenylmethylsilicone Oil. Coatings 2018, 8, 153. [CrossRef] 40. Finlay, J.A.; Bennett, S.M.; Brewer, L.H.; Sokolova, A.; Clay, G.; Gunari, N.; Meyer, A.E.; Walker, G.C.; Wendt, D.E.; Callow, M.E.; et al. Barnacle settlement and the adhesion of protein and diatom microfouling to xerogel ﬁlms with varying surface energy and water wettability. Biofouling 2010, 26, 657–666. [CrossRef]