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Synthesis of Polydimethylsiloxane-Modified Polyurethane and the Structure and Properties of Its Antifouling Coatings Zhan-Ping Zhang, Xiao-Fei Song, Li-Ying Cui and Yu-Hong Qi * Department of Materials Science and Engineering, Dalian Maritime University, Dalian 116026, China; [email protected] (Z.-P.Z.); [email protected] (X.-F.S.); [email protected] (L.-Y.C.) * Correspondence: [email protected]; Tel.: +86-0411-8472-3556 Received: 24 March 2018; Accepted: 24 April 2018; Published: 26 April 2018

Abstract: Polydimethylsiloxane (PDMS) could be used to improve the antifouling properties of the fouling release coatings based on polyurethane (PU). A series of polydimethylsiloxane-modified polyurethane coatings were synthesized with various PDMS contents, using the solvent-free method. The effects of PDMS content and seawater immersion on the chain structure and surface morphology were investigated by confocal laser scanning microscopy (CLSM), atomic force microscopy (AFM), Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and X-ray diffraction (XRD). Based on the measurements of contact angles of deionized water and diiodomethane, surface free energies of the coatings were estimated according to the Owens two-liquid method. The PDMS-modified polyurethane exhibited lower surface free energy and a lower glass transition temperature than polyurethane. The presence of PDMS increased the degree of microphase separation, and enhanced the water resistance of the coatings. The optimum amount of PDMS reduced the elastic modulus and increased the ductility of the coating. The presence of PDMS benefited the removal of weakly attached organisms. Panel tests in the Yellow Sea demonstrated the antifouling activity of the PDMS-modified polyurethane. Keywords: polydimethylsiloxane-modified polyurethane; antifouling; coating; structure; morphology; in-site; seawater immersion

1. Introduction Biofouling is the undesirable growth of marine organisms on artificial structures immersed in the ocean. Ships hulls fouled by marine organisms experience frictional drag, increased fuel consumption, longer voyage times, higher dry-docking frequency, and increased fuel emissions [1,2]. Organisms may be transported between regions, harming local ecosystems and marine environments [3,4]. Antifouling coatings are applied on ships’ hulls to act against biofouling. Traditional antifouling coatings often contained tin and copper species, which are now or will be banned because of their environmental risks [5]. This has promoted the development of environmentally friendly antifouling coatings. Fouling-release coatings are a mature technology that has been commercialized. They do not necessarily inhibit the attachment of marine organisms, but allow only weak bonding between marine organisms and the surface. Organisms are then released with sufficient hydrodynamic force, which is applied as the ship moves through the water [6]. So far, most of investigations showed that there are currently two key types of fouling-release coatings, namely fluoropolymers and silicone-based polymer coatings [1,7]. Fluoropolymers exhibit interesting bulk and surface properties, including excellent environmental stability, water and oil repellency, low coefficients of friction, biocompatibility, excellent thermal

Coatings 2018, 8, 157; doi:10.3390/coatings8050157

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stability and chemical resistance, and low interfacial free energies [8–11]. Silicones exhibit low glass transition temperatures (Tg ), low surface energies, good biocompatibility, and high thermal and oxidative stability [12]. Fluoropolymers based on fluorocarbon backbone suffer from limited mobility because of rigidity of F–C bonds, which hinders rotation about backbone bonds. A higher critical stress is also required to sever the adhesive-substrate bond, because of their higher bulk modulus compared with elastomers. Thus, fouling that does accumulate on the surface is not easily released from rigid fluoropolymers [1,13]. Silicones exhibit better non-stick properties than fluoropolymers, because of their conformational mobile surface, low elastic modulus, and low Tg . Silicone-based coatings are the most widely used fouling-release coatings [1,9,14]. Studies have shown that the attachment of fouling organisms to such coatings can be related not only to the surface free energy, but also the elastic modulus of the coatings and other factors (e.g., thickness and smoothness) [14]. Specifically, it was experimentally confirmed that adhesion is proportional to the square root of the product of elastic modulus (E, MPa) and surface √ energy (γ, mJ/m2 ) [15,16]. Accordingly, the value of parameter γ· E can be used as a key reference for screening fouling-release coating. Silicone-based fouling-release coatings exhibit poor adhesion to substrates and mechanical properties, are easily damaged (e.g., by cutting, tearing, puncturing). Their recoatability also needs improvement. In addition, polyurethanes not only exhibit good toughness, flexibility, substrate adhesion, but also more favorable elastomeric properties than most polymers [17]. A wide variety of monomers including macrodiols, diisocyanates and chain extenders can be employed in their synthesis [18]. Polyurethanes can also be modified by many reagents [13]. In view of these, combining polyurethane with low surface energy polymers is expected to offer outstanding antifouling ability and excellent mechanical properties and adhesion [19]. The siloxane-polyurethane coating is a robust and durable fouling-release coating. Ekin et al. [20] explored the use of polydimethylsiloxane (PDMS) oligomers terminated with hydroxylalkyl carbamate and dihydroxyalkyl carbamate groups to increase the compatibility between the polyurethane underlayer and PDMS top layer. In addition, poly (ε-caprolactone) (PCL) blocks were added to previously functionalized PDMS oligomers to further increase the compatibility between the polyurethane and PDMS [21,22]. Pieper et al. [23] prepared siloxane-polyurethane coatings from 3-aminopropyl-terminated polydimethylsiloxane, the acrylic polyols and a polyisocyanate cross linker, and showed a low-force of release in the pseudo-barnacle pull-off adhesion test. In our previous article [24], PPG-TDI-BDO polyurethanes with different hard segment contents of 20, 30, 40, 50, 60, and 65 wt % was synthesized by polypropylene glycol (PPG), toluene diisocyanate (TDI) and 1,4-butanediol (BDO), their antifouling properties were studied. The results showed that PPG-TDI-BDO polyurethane with a hard segment content of 40 wt % displayed the best antifouling performance. We aimed to combine the advantages of siloxanes and polyurethanes to produce a favorable fouling-release coating for marine environments. Incorporating siloxane units into the polyurethane chain imparts the resulting siloxane-modified polyurethane with the properties of both polysiloxane and polyurethane. Based on the results of previous article [24], we will introduce polyether triol and PDMS in soft segment of PPG-TDI-BDO polyurethanes, control different content of PDMS and prepare a series of PDMS-modified PPG-TDI-BDO polyurethanes with a hard segment content of 25 wt %. The influence of PDMS content on the structure of the PDMS-modified PPG-TDI-BDO polyurethanes and antifouling properties of its coatings will be investigated in this article. 2. Materials and Methods 2.1. Raw Materials Careful design of the silicone-polyurethane copolymer structure is important for obtaining a coating with favorable surface, mechanical, and adhesive properties. Toluene-2,4-diisocyanate (2,4 TDI, Wanxiang Chemical Co., Ltd. Zhengzhou, China) was used in the current study. Macrodiols

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can play an important role in improving the properties of polyurethanes. As a soft segment we chose polyether diol (HSH220, Jiangsu Haian Petrochemical Plant, Hai’an, China) and polyether triol (HSH330, Jiangsu Haian Petrochemical Plant). Hydroxypropyl-terminated polydimethylsiloxane (PDMS2200, Ark (Fogang) Chemical Materials Co., Ltd., Qingyuan, China) was used as the modifier. 1,4-Butylene glycol (BDO, Tianjin Kemiou Chemical Reagent Co., Ltd., Tianjin, China) is widely used as a low-molecular-weight chain extender, and was used as such in the current study. TDI and BDO constitute the hard segment of polyurethane. The characteristics of monomers are listed in Table 1. Before experiment, PDMS, HSH220 and HSH330 were dried in a vacuum oven at 120 ◦ C, to remove any absorbed water. Dimethylbenzene, butyl acetate, cyclohexanone and BDO were dried over 4 Å molecular sieves. Table 1. The characteristics of monomers. Monomer

HSH220

HSH330

PDMS2200

BDO

TDI100

Functional group Functionality Molecular weight

−OH 2 2000

−OH 3 3000

−OH 2 2000

−OH 2 90.121

−N=C=O 2 174.15

Epoxy anti-rust primer and iron oxide red epoxy intermediate coating were obtained from Sunrui Marine Coatings Co., Ltd. (Xiamen, China). Silicone paint was laboratory made for control panel, and composited with carbon nano-tube and polydimethylsiloxane. 2.2. Synthesis of Polyurethane Controlling the molar ratio of NCO to OH was equal to 1.36, the molar ratio of HSH330 to HSH220 + PDMS was equal to 0.1 and the content of hard segment (TDI + BDO) is 25 wt % in polyurethane prepolymers. The molar percentage of PDMS in HSH220 is respectively 0, 6, 13, 21, 29%, in other words, the material ratio with PDMS contents of 0, 4.2, 8.5, 12.8, and 16.9 wt %, a series of polyurethane prepolymers were prepared. They were respectively named as PU-Si0, PU-Si4, PU-Si9, PU-Si13, and PU-Si17. The formulations of five polyurethane prepolymers were listed in Table 2. Table 2. The formulation of five prepolymers. Amount of Monomer (wt %)

PU-Si0

PU-Si4

PU-Si9

PU-Si13

PU-Si17

HSH220 HSH330 PDMS2200 TDI BDO

67.7 7.6 0.0 20.3 4.4

64.1 7.1 4.2 20.2 4.4

59.9 6.7 8.5 20.5 4.4

56.1 6.3 12.8 20.4 4.4

52.5 5.8 16.9 20.4 4.4

The prepolymers were prepared as follows. PDMS, HSH220 and HSH330 were added to a dry vessel equipped with a reflux condenser, mechanical stirrer, and thermometer. TDI100 was then slowly added at 30 ◦ C under N2 atmosphere. After rapid stirring for 30 min, the temperature was maintained at 75 ◦ C for 2 h. The temperature was then lowered to 50 ◦ C, and BDO and DBTDL were added under stirring. The vessel was then heated at 70 ◦ C for 3 h. The reaction mixture was cooled to room temperature, and the PDMS-modified polyurethane was obtained, which was diluted twice with one solution mixed with dimethylbenzene, butyl acetate and cyclohexanone. Their ratio is 2:2:1 of dimethylbenzene:butyl acetate:cyclohexanone as weight. 2.3. Preparation of Coating Samples Some samples were spread onto 75 mm × 25 mm × 1 mm glass slides, which had been coated with epoxy paint, and some samples were poured into Teflon molds and solidified in an oven at 40 ◦ C for 24 h. Panel samples for antifouling test in sea were spread onto 350 mm × 250 mm × 3 mm steel

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plate, pre-coated with zinc rich epoxy anti-rust primer (the thickness of nominal dry film is about 30 µm) and iron oxide red epoxy intermediate coating (the thickness of nominal dry film is about 80 µm). Because the polyurethane coating is colorless and transparent, red color resulted from iron oxide red epoxy intermediate coating. Its thickness of nominal dry film was controlled as 100 µm. The slide samples were used for surface properties and immersion test in sterilized sea water, the casted ones were used for structural analysis. Except the coatings on field panels, other samples were cured for 7 days in the constant temperature and humidity test chamber of 80% relative humidity and 25 ◦ C. Then, the prepared samples were placed in aeration cabinet at 40 ◦ C for 24 h. The PU coatings on field panels are cured in ambient for 2 days before immersing into sea. 2.4. Characterization 2.4.1. Structural Analysis Attenuated total internal reflectance Fourier transform infrared (ATR-FTIR) spectra were acquired using a Frontier PerkinElmer infrared spectrometer (Thermo Fisher Scientific Inc., Waltham, MA, USA) in the range of 4000–650 cm−1 , at a resolution of 2 cm−1 from the average of 16 accumulated scans. Universal ATR sampling accessory and diamond crystal plate were used for ATR-FTIR. The effective path length of infrared beam is 2 µm. In the used configuration it is a single reflection ATR accessory with high IR throughput which makes it ideal for sample identification and quantitative analysis and/or quantitative calculation applications. Differential scanning calorimetry (DSC, NETZSCH DSC 200 F3, NETZSCH-Gerätebau GmbH, Selb, Germany) was used for studying the melting and glass transitions of polymers. Measurements were performed at a heating rate of 5 ◦ C/min from −90 ◦ C to 200 ◦ C. Thermogravimetric analysis (TGA) was performed using a NETZSCH STA 409C/CD (NETZSCH-Gerätebau GmbH) under argon at a heating rate of 5 ◦ C/min for sample weights between 10 and 15 mg. X-ray diffraction (XRD) patterns were recorded at 45 kV and 0.66 mA (30 W), using a Molmet/Rigaku diffractometer (Rigaku Corporation, Tokyo, Japan). A solid copper target was used in this study. The camera rotation speed was 2◦ /min. 2.4.2. Mechanical Properties Six specimens of each coating for the tensile test were prepared in strips (120 mm × 20 mm × 2 mm) and then stretched on a Labthink XLM Auto Tensile Tester (Ji’nan, China). The speed of stretching is 50mm/min. Stress–strain curves, tensile strength, and elongation at break were recorded. Elastic modulus was fitted with the data, which the strain was less than 0.2 mm/mm. Average value of three tests was reported in this paper. 2.4.3. Surface Properties Confocal laser scanning microscopy (CLSM, OLS4000, Olympus, Tokyo, Japan) was used to obtain images of the surface morphologies of the films in a non-destructive manner. A series of z-axis images were generated through optical sectioning, using a slice thickness of 0.01 µm. Each polyurethane sample was scanned at selected positions. Five confocal image stacks were acquired for five films. All images (view field: 128 µm × 128 µm) were stored and analyzed using LEXT image analysis software. Roughness is accomplished by the surface roughness measurement module in the software of LEXT (version 2.2.4, Olympus, Tokyo, Japan). The morphologies of the samples were determined by atomic force microscopy (AFM) under ambient conditions. Samples were scanned using a Nano Scope IV Multi-Mode SPM (Digital Instruments Veeco Metrology Group, Santa Barbara, CA, USA), operating in tapping mode with software version 5.30r3. The cantilevers were 235 mm long with a tip radius of 5–10 nm, spring

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constant of 55 N/m, and resonance frequency of 167.19 kHz. The tapping conditions were a tune tip of 3 V, drive amplitude of 150 mV, and tapping ratio of 0.80. Contact angle measurements were conducted using the sessile drop method on a JC2000C (Shanghai Zhongchen Co., Ltd., Shanghai, China) contact angle measuring system. A 3-µL droplet was placed on the surface using a syringe. Digital images of the droplet silhouette were captured with a charge-coupled device camera. Contact angles were evaluated using the measuring angle method. Reported contact angles are the mean of those measured at five points for each sample. The surface tension of deionized water and diiodomethane respectively are 72.8 mJ/m2 and 50.8 mJ/m2 at 20 ◦ C. The surface free energy was calculated using the Owens two-liquid method [25] based on the measurements values of contact angle of deionized water (WCA) and diiodomethane (DCA). It was p calculated as Equations (1)–(3), where σSd denotes the dispersive force and σs refers to the polar force of sample surface. p

2 137.5 + 256.1 × cos θ H2 O − 118.6 × cos θCH2 I2 /44.92 , 2 σSd = 139.9 + 181.4 × cos θCH2 I2 − 41.5 × cos θ H2 O /44.92 ,

σs =

p

σS = σs + σSd .

(1) (2) (3)

Interface contact angles (ICA) of diiodomethane between seawater and the samples were also measured in situ during seawater immersion test. Every slide sample was put in a polymethyl methacrylate trough with dimension 90 mm × 35 mm × 20 mm, and immersed in 40 mL sterilized seawater. After immersion for a predetermined time, a 3-µL droplet of diiodomethane was dripped at the interface, the images were captured at set intervals and then contact angles measured. 2.4.4. Antifouling Property To test the antifouling properties of the PDMS-modified polyurethane samples, coatings panels were immersed in the Yellow Sea at depths from 0.5 to 2 m in the Lvshun Port, China, between April and June 2014. The samples were checked regularly to monitor the progress of the fouling process. The salinity was 3.2%, and the average temperature was 22 ◦ C. A control sample containing a silicone resin fouling-release coating was tested under the same conditions for comparison. The surface state of panels immersed were checked regularly and taken pictures by camera. After the images were deducted the invalid area of the edge, transformed by gray scale and selected manually the range of gray scale, the objects were counted and the area of each object was measured by using the software Image-Pro Plus (version 5.1.0.20, Media Cybernetics, Rockville, MD, USA), the coverages of marine organism were evaluated. The coverage of marine organisms means that the area covered by the attached organisms accounts for the percentage of the effective area of the panel. 3. Results and Discussions 3.1. Influence of Polydimethylsiloxane (PDMS) 3.1.1. Molecular Structure The synthesis of PU-Sis involved reaction between the isocyanate and hydroxyl groups to form a urethane group. Figure 1a,b shows the FTIR spectra of the PU-Sis samples. Characteristic peaks of hydrogen-bound N−H at 3300 cm−1 and urethane carbonyl groups at 1727 cm−1 were observed. No isocyanate peak at 2270 cm−1 was observed in any of the spectra, indicating that all −NCO groups were consumed during reaction. The characteristic peaks of siloxane groups at 1259, 1017, and 801 cm−1 indicated that all PDMS was incorporated into the copolymer [26].

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the intensity of the peaks decrease, the order degree in the polyurethanes decrease. Adding PDMS resulted in the characteristic diffraction peak at 21.28° shifting to 20.63°, because of the narrower Coatings 2018, 8, 157 6 of 18 molecular width of PDMS compared with pure polyurethane [27]. 70

(a)

1017

60

PU-Si0 PU-Si4 PU-Si9 PU-Si13 PU-Si17

Absorbance (%)

50 40

801

1259

30

1727

20 3300

10 0 4000

3500

3000

2500

2000

1500

1000

500

-1

Wavenumber (cm )

(b) >C=O -N-H

-Si-CH3

-C-O-C- -Si-O-Si-

-Si-CH3

Absorbance (a.u.)


2000

1800

1600

1400

1200

1000

800

600

-1

Wavenumber (cm ) Figure1.1.Fourier Fouriertransform transforminfrared infrared(FTIR) (FTIR)spectra spectraofofthe thePU-Sis PU-Sissamples: samples:(a) (a)wave wavenumber numberfrom from4000 4000 Figure to 600, (b) wave number from 2000 to 600 local amplification of (a). to 600, (b) wave number from 2000 to 600 local amplification of (a). 300

Intensity (a.u.)

Peaks at 3300 cm−1 (hydrogen-bound N−H), 1727 cm−1 (free urethane carbonyl), 1705 cm−1 (hydrogen-bound carbonyl) and 1600 cm−1 (benzene) weakenedPU-Si0 obviously with the addition of 250 PU-Si4 PDMS. The intensities of siloxane group peaks at 1259 cm−1 (Si−CH bending), 1017 cm−1 (Si−O−Si 3 PU-Si9 − 1 PU-Si13 stretching), and 801 cm (Si−CH3 rocking) increased with increasing PDMS content. This indicated 200 PU-Si17 that more and more PDMS present in PU-Sis samples with increasing PDMS content. XRD is a bulk characterization technique that can provide direct evidence for crystalline or amorphous in segmented 150 and block copolymers, and was used to investigate the microstructures of the PU-Sis films. The XRD patterns of the samples100 (Figure 2) were similar, with intense and weak broad diffractions at 2θ = 20◦ ◦ and 42 , respectively. The former shows that there is micro crystalline domain in the hard segment of polyurethanes. The latter is typical of amorphous polymers. In addition, weaker peak observed at 50 2θ = 12◦ appears only in the samples containing PDMS. With the increase of the content of the PDMS, the intensity of the peaks0 decrease, the order degree in the polyurethanes decrease. Adding PDMS 10 diffraction 20 30 40 21.28◦ 50shifting 60 to 20.63 70 ◦ , because 80 resulted in the characteristic peak at of the narrower o 2( ) molecular width of PDMS compared with pure polyurethane [27].

2000

1800

1600

1400

1200

1000

800

600

-1

Wavenumber (cm ) Figure 1. Fourier transform infrared (FTIR) spectra of the PU-Sis samples: (a) wave number from 4000 7 of 18 to 600, (b) wave number from 2000 to 600 local amplification of (a).

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300 PU-Si0 PU-Si4 PU-Si9 PU-Si13 PU-Si17

Intensity (a.u.)

250 200 150 100 50 0 10


20

30

40

o

2( )

50

60

70

80

7 of 17

Figure Figure2.2.X-ray X-raydiffraction diffraction(XRD) (XRD)patterns patternsofofthe thePU-Sis PU-Sissamples. samples.

DSCcurves curves PU-Si0 and PU-Si17 shown in Figure 3, and show transitions. The DSC ofof PU-Si0 and PU-Si17 areare shown in Figure 3, and show clearclear glassglass transitions. The T g ◦ Tg values of PU-Si0 PU-Si17 were −37.9 −46.6C,°C, respectively. The lower g of PU-Si17was was values of PU-Si0 and and PU-Si17 were −37.9 andand −46.6 respectively. The lower TgTof PU-Si17 duetotothe thePDMS PDMShas haslower lower glass transition.TgTof g of PDMSis is 2000 g/molshould shouldbebearound around−−120 °C. due glass transition. PDMS 2000 g/mol 120 ◦ C. With the increase of PDMS content, the glass transition temperature of silicone modified With the increase of PDMS content, the glass transition temperature of silicone modified polyurethane polyurethane decreases. Thebedecrease may be due toofpartial of the soft segment of urethane decreases. The decrease may due to partial mixing the softmixing segment of urethane with the PDMS. with the PDMS. Poly(ether-urethanes) exhibit additional interactions between urethane N−H and Poly(ether-urethanes) exhibit additional interactions between urethane N−H and ether groups [28]. ether groupsin[28]. The decrease polyether contentbonding reducedbetween hydrogen between the hard The decrease polyether contentin reduced hydrogen thebonding hard and soft segments, and soft segments, which increased phase separation the Tg. The peak of which increased phase separation and lowered the Tg .and Thelowered crystallization peakcrystallization of the soft segment ◦ the soft segment at −17.4 °C for PU-Si0 was identical to that for PU-Si17. The melting peaks of in the at −17.4 C for PU-Si0 was identical to that for PU-Si17. The melting peaks of the hard segments ◦ hard segments in PU-Si0 PU-Sis17 at 50 and 86.6 °C,increased respectively. This increased PU-Si0 and PU-Sis17 were and observed at 50were and observed 86.6 C, respectively. This temperature was temperature also caused byThe phase separation. hard contained domain inless PU-Si17 contained less soft also caused bywas phase separation. hard domain inThe PU-Si17 soft segment, and thus segment, and thus exhibited a higher melting temperature. exhibited a higher melting temperature.

Figure3.3.Differential Differentialscanning scanningcalorimetry calorimetry(DSC) (DSC)curves curvesofofPU-Si0 PU-Si0and andPU-Si17. PU-Si17. Figure

TGA scans of the PU-Sis series were collected under argon at a heating rate of 5 °C/min to detail TGA scans of the PU-Sis series were collected under argon at a heating rate of 5 ◦ C/min to detail the thermal degradation (Figure 4). All samples showed a 5 wt % loss at 238–272 °C. the thermal degradation (Figure 4). All samples showed a 5 wt % loss at 238–272 ◦ C. 110 100 90 eight loss (%)

80 70 60 50 40


Figure 3. Differential scanning calorimetry (DSC) curves of PU-Si0 and PU-Si17.

TGA scans of the PU-Sis series were collected under argon at a heating rate of 5 °C/min to detail 8 of 18 the thermal degradation (Figure 4). All samples showed a 5 wt % loss at 238–272 °C.

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110 PU-Si0 PU-Si4 PU-Si9 PU-Si13 PU-Si17

100 90 Weight loss (%)

80 70 60 50 40 30 20 10 0 0

100

200

300

400

500

600

Temperature (C)

Figure 4. Thermogravimetric analysis (TGA) curves of the PU-Sis samples. Figure 4. Thermogravimetric analysis (TGA) curves of the PU-Sis samples.

3.1.2. Surface Topography 3.1.2. Surface Topography Figure 5 shows trends in surface roughness PU-Sis samples, determined field roughness of of thethe PU-Sis samples, as as determined in in thethe field of of 8 of 17 × 128 μm CLSM. This trend could explained phase separation, and tendency 128128 µmμm × 128 µm byby CLSM. This trend could bebe explained byby phase separation, and thethe tendency of of PDMS PDMSto toenrich enrichon onthe thecopolymer copolymersurface. surface.Introducing IntroducingPDMS PDMSin inPU PUincreased increasedthe thephase phaseseparation, separation, so rougher than that of of PU-Si0. Increasing the the PDMS ratioratio alsoalso enriched the sothe thesurface surfaceofofPU-Si4 PU-Si4was was rougher than that PU-Si0. Increasing PDMS enriched PDMS content on the copolymer surface, which led to the lower surface roughness observed for PUthe PDMS content on the copolymer surface, which led to the lower surface roughness observed for Si9. ThisThis enrichment in PDMS hadhad reached saturation ininPU-Si9. PU-Si9. enrichment in PDMS reached saturation PU-Si9.Phase Phaseseparation separationcontinued continuedto to increase increasewith withincreasing increasingPDMS PDMScontent, content,which whichincreased increasedthe theroughness roughnessfrom fromPU-Si9 PU-Si9to toPU-Si13 PU-Si13and and then thento toPU-Si17. PU-Si17. Figure trends in surface Coatings 2018, 8,5xshows FOR PEER REVIEW

0.12 0.024 Ra Rz

0.022

0.11

0.020 0.09

Rz (m)

Ra (m)

0.10

0.018 0.08 0.016 0.07 0

2

4

6

8

10

12

14

16

Content of PDMS (wt.%)

Figure Figure5.5.Surface Surfaceroughness roughnessof ofthe thePU-Sis PU-Sissamples samplesas asdetermined determinedby byCLSM. CLSM.

AFM was used to observe the phase-separated microstructures of the PU-Sis samples in the field AFM was used to observe the phase-separated microstructures of the PU-Sis samples in the field of 500 nm × 500 nm. The regular texture is shown by the AFM height and phase maps of PU-Si0 in of 500 nm × 500 nm. The regular texture is shown by the AFM height and phase maps of PU-Si0 Figure 6. During AFM scanning, smaller phase angles indicate harder or more rigid materials, in Figure 6. During AFM scanning, smaller phase angles indicate harder or more rigid materials, whereas higher phase angles indicate the presence of softer material such as PDMS. In view of the whereas higher phase angles indicate the presence of softer material such as PDMS. In view of the above, the bright phase should be soft PDMS domains, as shown in Figure 6b AFM phase (right) above, the bright phase should be soft PDMS domains, as shown in Figure 6b AFM phase (right) maps. Based on this conclusion, we can conclude that the bright phase in Figure 6b AFM height (left) maps. Based on this conclusion, we can conclude that the bright phase in Figure 6b AFM height (left) maps is soft PDMS domains. The fact indicates that the soft PDMS domains do not appear as dips, maps is soft PDMS domains. The fact indicates that the soft PDMS domains do not appear as dips, but as bulges on the surface of polyurethane. Very prominent surface features were observed for the coatings with high content of PDMS. This is similar to the results of the reference [1]. PU-Si0 contained alternating small soft and hard regions, which indicated phase separation. In the height images, soft regions are represented by lighter coloring. Urethanes self-associate by forming intermolecular hydrogen bonds between the N−H and carbonyl groups. Poly(ether-urethanes) contains additional

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but as bulges on the surface of polyurethane. Very prominent surface features were observed for the coatings with high content of PDMS. This is similar to the results of the reference [1]. PU-Si0 contained alternating small soft and hard regions, which indicated phase separation. In the height images, soft regions are represented by lighter coloring. Urethanes self-associate by forming intermolecular hydrogen bonds between the N−H and carbonyl groups. Poly(ether-urethanes) contains additional interactions between urethane N−H and ether groups. The phase boundary was not clearly defined, because of hydrogen bonding between urethane N−H and ether groups. Light regions in the phase map of PU-Si17 corresponded to PDMS domains. PDMS domains tended to aggregate during film formation. A sufficiently high surface PDMS concentration resulted in the PDMS chains beginning to aggregate. phase-separated Coatings 2018,The 8, x FOR PEER REVIEW microstructure is similar to that showed in reference [21]. 9 of 17

Figure 6. Atomic force microscopy (AFM) images showing height (left) and phase (right) maps of (a) Figure 6. Atomic force microscopy (AFM) images showing height (left) and phase (right) maps of (a) PU-Si0 and (b) PU-Si17. PU-Si0 and (b) PU-Si17.

3.1.3. Mechanical Properties 3.1.3. Mechanical Properties The tensile properties of the polyurethane samples were examined. The results showed that the The tensile properties of the polyurethane samples were examined. The results listed in Table 3 tensile strength of all PU-Sis samples is higher than that of the PU samples, and increases with showed that the tensile strength of all PU-Sis samples is higher than that of the PU samples, increasing PDMS contents. Except of the PU-Si9 samples, the elongation at break of other PU-Sis and increases with increasing PDMS contents. Except of the PU-Si9 samples, the elongation at samples is lower than that of the PU samples. The PU-Si9 samples represent not only the highest break of other PU-Sis samples is lower than that of the PU samples. The PU-Si9 samples represent elongation at break, but also the lowest elastic modulus. It means that the PU-Si9 samples have not only the highest elongation at break, but also the lowest elastic modulus. It means that the PU-Si9 excellent ductility. samples have excellent ductility. Table 3. The mechanical properties of five coatings. Table 3. The mechanical properties of five coatings. Mechanical Property PU-Si0 PU-Si4 PU-Si9 PU-Si13 PU-Si17 Mechanical PU-Si4 1.05 PU-Si9 TensileProperty strength (MPa) PU-Si0 0.51 1.09 1.37 PU-Si13 2.09 Elongation at break (%) 2070 955 2442 1258 1315 Tensile strength (MPa) 0.51 1.09 1.05 1.37 Elastic modulus 0.69 0.79955 0.40 24421.10 1.13 Elongation at break (%) (MPa) 2070 1258 Elastic modulus (MPa) 0.69 0.79 0.40 1.10

PU-Si17 2.09 1315 1.13

3.1.4. Interface Characterization 3.1.4.The Interface Characterization change in molecular chain structure and topography of the surface layer affected the interfacial properties of the PU-Sis samples. The interface properties were characterized by water The change in molecular chain structure and topography of the surface layer affected the interfacial contact angle (WCA) andsamples. diiodomethane contact angle (DCA) surface free energy properties of the PU-Sis The interface properties weremeasurements characterizedand by water contact angle calculations. Figure 7 shows that the WCAs of all the PU-Sis samples were >100°, which indicated (WCA) and diiodomethane contact angle (DCA) measurements and surface free energy calculations. hydrophobicity. The DCA of PU-Si0 was smaller than those of PU-Si4, PU-Si9, PU-Si13 and PU-Si17, which resulted from the enrichment of PDMS on the surface. The WCAs of PU-Si13 and PU-Si17 are lower than that of PU-Si4 and PU-Si9, which may have resulted from surface roughness. Figure 8 shows the surface free energies and their corresponding polar and dispersive components of the PU-Sis samples. The surface free energies and components were calculated from

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Figure 7 shows that the WCAs of all the PU-Sis samples were >100◦ , which indicated hydrophobicity. The DCA of PU-Si0 was smaller than those of PU-Si4, PU-Si9, PU-Si13 and PU-Si17, which resulted from the enrichment of PDMS on the surface. The WCAs of PU-Si13 and PU-Si17 are lower than that Coatings 2018,and 8, x FOR PEERwhich REVIEW 10 of 17 of PU-Si4 PU-Si9, may have resulted from surface roughness. 75

110

Water contact angle ()

65 60

100


Water contact angle ()

Diiodomethane contact angle () Diiodomethane contact angle ()

70 105

10 of 17

55 75

110 95

Deionized water Diiodomethane 105 90 0

2

4

6

8

10

12

14

16

50 70 45 18 65

Content of PDMS (wt%) 60

100

Figure 7. 7. Water Water contact angle (DCA) measured results of the Figure contact angle angle(WCA) (WCA)and anddiiodomethane diiodomethanecontact contact angle (DCA) measured results of PU-Sis samples. 55 the PU-Sis samples. 95

Deionized water

50

Surface free energy (mJ/m )

2

40 0 3

2

4

6

8

Content of PDMS (wt%)

2416 18 22

Figure 8. Surface free energies of the PU-Sis samples. 2

20

3.2. Influence of Seawater Immersion 1

18



2618 Dispersive component Polar component Surface free energy 10 12 14 16

2

2

51


5 free energies and their corresponding Figure 8 shows the surface Diiodomethane polar26and dispersive components Dispersive component of the PU-Sis samples. The surface free energies and components were calculated from contact angle 45 90 Polar component 4 0Owens 24 2 two-liquid 4 6 8 method 10 12[25]. 16 test 18liquids measurements according to the The were deionized water Surface free14energy Content of PDMSof (wt%) and diiodomethane. The total surface free energies PU-Si4, PU-Si9, PU-Si13 and PU-Si17 were 3 22 approximately 5 mJ/m2 lower than that of PU-Si0. PU-Si0 had the highest polar component and Figure 7. Water contact angle (WCA) and diiodomethane contact angle (DCA) results ofsimilar lowest dispersive component of the surface free energy. Samples containing measured PDMS exhibited the PU-Sis samples. 2 20 dispersive components.

2

Seawater immersion experiments were carried out to understand the water resistance of the PUSis samples. Following immersion, the properties of the samples were investigated by FTIR 0 16 spectroscopy, DSC, AFM, CA0measurements, calculations. 2 4 6 and 8 surface 10 12 free 14 energy 16 18 Content of PDMS (wt%)

3.2.1. Fourier Transform Infrared (FTIR) Analysis

Figure free energies Figure8.8.Surface Surface free energiesofofthe thePU-Sis PU-Sissamples. samples.

Figure 9 shows the FTIR spectra of PU-Si0 before and after immersion in seawater. The 3.2. Influence ofofpeaks Seawater intensities of at Immersion 876 cm−1 (out-of-plane flexural vibration of benzene C−H) and 1413 cm−1 3.2. Influence Seawater Immersion (vibration of benzene ring) were significantly enhanced after immersion in seawater. Figure 10 shows Seawater immersion experiments were carried out out to understand the water resistance of theof PUSeawater immersion were carried understand water resistance the the FTIR spectra of PU-Si4experiments before and after immersion intoseawater. The the intensities of peaks at 801 Sis samples. Following immersion, the properties of the samples were investigated by FTIR PU-Sis samples. Following immersion, the properties of the samples were investigated by FTIR −1 (Si−CH cm 3 rocking vibration) and 1259 cm−1 (Si−CH3 symmetric bending) decreased after spectroscopy, DSC, AFM, CA and free calculations. spectroscopy, DSC, AFM, CAmeasurements, measurements, andsurface surface freeenergy energy immersion in seawater. This may have been caused by polar groupscalculations. migrating toward the surface during seawater immersion, effectively decreasing the content of nonpolar PDMS. 3.2.1. Fourier Transform Infrared (FTIR) Analysis −1 Figure 11 shows the absorption reduction at 801 and 1259 cm of samples containing PDMS after Figure 9The shows the FTIRreduction spectra of PU-Si0 and with after increasing immersionPDMS in seawater. immersion. absorptivity at 801 cm−1before decreased content. The This −1 −1 intensities of peaks at 876 cm (out-of-plane flexural vibration of benzene C−H) and 1413 cm indicated that the water resistance of PU-Sis increased with increasing PDMS content. (vibration of benzene ring) were significantly enhanced after immersion in seawater. Figure 10 shows the FTIR spectra of PU-Si4 before and after immersion in seawater. The intensities of peaks at 801 cm−1 (Si−CH3 rocking vibration) and 1259 cm−1 (Si−CH3 symmetric bending) decreased after

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3.2.1. Fourier Transform Infrared (FTIR) Analysis Figure 9 shows the FTIR spectra of PU-Si0 before and after immersion in seawater. The intensities of peaks at 876 cm−1 (out-of-plane flexural vibration of benzene C−H) and 1413 cm−1 (vibration of benzene ring) were significantly enhanced after immersion in seawater. Figure 10 shows the FTIR spectra of PU-Si4 before and after immersion in seawater. The intensities of peaks at 801 cm−1 (Si−CH3 rocking vibration) and 1259 cm−1 (Si−CH3 symmetric bending) decreased after immersion in seawater. This may have been caused by polar groups migrating toward the surface during seawater immersion, Coatings2018, 2018, FORPEER PEER REVIEW Coatings 8,8,x xFOR REVIEW 1111ofof1717 effectively decreasing the content of nonpolar PDMS. 1413 1413

Absorbance (a.u.) Absorbance (a.u.)

876 876

Immersed Immersed Initial Initial

2000 1800 1800 1600 1600 1400 1400 1200 1200 1000 1000 2000

800 800

600 600

-1

-1 Wavenumber(cm (cm Wavenumber ))

Figure9. FTIRspectra spectraof PU-Si0before beforeand andafter afterimmersion immersionin seawater. Figure Figure 9.9.FTIR FTIR spectra ofofPU-Si0 PU-Si0 before and after immersion ininseawater. seawater.

Absorbance (a.u.) Absorbance (a.u.)

1413 1413

876 801 876 801

1259 1259

Immersed Immersed Initial Initial

2000 1800 1800 1600 1600 1400 1400 1200 1200 1000 1000 2000

800 800

600 600

-1

-1 Wavenumber(cm (cm Wavenumber ))

Figure10. 10.FTIR FTIRspectra spectraof PU-Si4before beforeand andafter afterimmersion immersionin seawater. Figure Figure 10. FTIR spectra ofofPU-Si4 PU-Si4 before and after immersion ininseawater. seawater. 6060

Absorptivity reduction (%) Absorptivity reduction (%)

Figure 11 shows the absorption reduction at 801 and 1259 cm−-11-1of samples containing PDMS 800cm cm 800 after immersion. The absorptivity reduction at 801 cm−1 decreased with increasing PDMS content. -1 -1 1260cm cm 5555 1260 This indicated that the water resistance of PU-Sis increased with increasing PDMS content. 5050 4545 4040 3535 3030

44

55

66

77

8 8 9 9 1010 1111 1212 1313 1414 1515 1616 1717 ContentofofPDMS PDMS(wt%) (wt%) Content

−1−1of Figure11. 11.Infrared Infraredabsorptivity absorptivityreduction reductionatat800 800and and1260 1260cm cm ofsamples samplescontaining containingPDMS PDMSafter after Figure

2000

1800

1600

1400

1200

1000

800

600

-1

Wavenumber (cm )

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Figure 10. FTIR spectra of PU-Si4 before and after immersion in seawater. 60 -1

800 cm -1 1260 cm

Absorptivity reduction (%)

55 50 45 40 35 30 4

5

6

7

8 9 10 11 12 13 Content of PDMS (wt%)

14

15

16

17

−−1 1 of Figure11. 11.Infrared Infraredabsorptivity absorptivityreduction reductionatat800 800and and1260 1260cm cm of samples samples containing containing PDMS PDMSafter after Figure immersionin inseawater. seawater. immersion

3.2.2.Atomic AtomicForce ForceMicroscopy Microscopy(AFM) (AFM)Analysis Analysis 3.2.2. Figure 12 12 shows shows AFM height nm × 500 nmnm of PU-Si0 andand PUFigure height and and phase phasemaps mapsininthe thefield fieldofof500 500 nm × 500 of PU-Si0 Si17 afterafter immersion in seawater. The texture of the height phase maps of PU-Si0 after immersion PU-Si17 immersion in seawater. The texture of theand height and phase maps of PU-Si0 after Coatings 2018, 8, x FOR PEER REVIEW 12 of 17 was not regular. was This attributed to seawater influencing H-bonding interactions and phase immersion was notThis regular. was attributed to seawater influencing H-bonding interactions and phase separation. The height of PU-Si0 contained small bulges, and exhibited an increase in separation. The height map ofmap PU-Si0 contained small bulges, and exhibited an increase in surface surface roughness. The surface of PU-Si17 contained bulges that were larger and smoother. This was roughness. The surface of PU-Si17 contained bulges that were larger and smoother. This was attributed attributed to to hard hard domains domains under under the the surface surface migrating migrating toward toward the the surface surfaceduring duringimmersion. immersion.

Figure 12. AFM images showing height (left) and phase (right) maps of (a) PU-Si0 and (b) PU-Si17 Figure 12. AFM images showing height (left) and phase (right) maps of (a) PU-Si0 and (b) PU-Si17 after immersion immersion in in seawater. seawater. after

3.2.3. Water Water Contact Contact Angle Angle (WCA) (WCA) and and Surface Surface Free Free Energy Energy 3.2.3. The WCA WCA of of PU-Si0 PU-Si0 decreased decreased from from 104 104° to 74 74° (hydrophilicity) upon upon immersion. immersion. Figure Figure 13 13 ◦ to ◦ (hydrophilicity) The shows the WCA reductions of the PU-Sis samples after immersion in seawater. The WCA reductions shows the WCA reductions of the PU-Sis samples after immersion in seawater. The WCA reductions of PU-Si4, PU-Si4, PU-Si9, PU-Si9, PU-Si13 PU-Si13 and and PU-Si17 PU-Si17 were were small, small, and and decreased decreased slightly slightly with with increasing increasing PDMS PDMS of 2 after content. Figure free energy of of PU-Si0 increased from 25.125.1 to 38.7 mJ/m 2 content. Figure14 14shows showsthat thatthe thesurface surface free energy PU-Si0 increased from to 38.7 mJ/m immersion. The polar and dispersive components of PU-Si0 both significantly increased after immersion. The surface free energies of PU-Si4, PU-Si9, PU-Si13 and PU-Si17 also increased after immersion. The magnitudes of these increases were similar to that of PU-Si0. This was attributed to hard segments migrating toward the surface during immersion. The dispersive components of the four

3.2.3. Water Contact Angle (WCA) and Surface Free Energy The WCA of PU-Si0 decreased from 104° to 74° (hydrophilicity) upon immersion. Figure 13 shows the WCA reductions of the PU-Sis samples after immersion in seawater. The WCA reductions Coatings 2018, 8, 157 13 of 18 of PU-Si4, PU-Si9, PU-Si13 and PU-Si17 were small, and decreased slightly with increasing PDMS content. Figure 14 shows that the surface free energy of PU-Si0 increased from 25.1 to 38.7 mJ/m2 after immersion. The polar and dispersive components of PU-Si0 both both significantly increased after after immersion. The polar and dispersive components of PU-Si0 significantly increased immersion. after immersion. Thesurface surfacefree freeenergies energiesof of PU-Si4, PU-Si4, PU-Si9, PU-Si9, PU-Si13 PU-Si13 and and PU-Si17 PU-Si17 also also increased increased after after immersion. immersion. The The magnitudes magnitudes of of these these increases increases were were similar similar totothat thatofofPU-Si0. PU-Si0. This This was was attributed attributed to to hard hard The segments migrating migrating toward toward the the surface surface during duringimmersion. immersion. The The dispersive dispersive components components of of the the four four segments PDMS-containing samples showed little change upon immersion, which was attributed to PDMS PDMS-containing samples showed little change upon immersion, which was attributed to PDMS segmentson onthe thesurface. surface.These ThesePDMS PDMSchains chainsmigrated migratedtotothe thesurface surfaceofof the copolymer film, forming segments the copolymer film, forming a a uniform hydrophobic layer with a low surface energy. uniform hydrophobic layer with a low surface energy. 45 40

o

WCA reductions ( )

35 30 25 20 15 10 5 0 0

2

4

6

8

10

12

14

16


Figure 13. WCA WCA reductions reductions of of the the PU-Sis PU-Sissamples samplesafter afterimmersion immersionin inseawater. seawater. Coatings 2018, 8, xFigure FOR PEER REVIEW 13.

13 of 17

35

polar component before seawater immersion nonpolar component before seawater immersion polar component after seawater immersion nonpolar component after seawater immersion

2


30

25

20

15

10

5

0 0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17


Figure14. 14.Surface Surfaceenergies energiesof ofthe thePU-Sis PU-Sissamples samplesbefore beforeand andafter afterimmersion immersionininseawater. seawater. Figure

3.2.4.Interface InterfaceContact ContactAngles Angles(ICA) (ICA)In-Situ In-SituMeasurements Measurementsduring duringImmersion ImmersionininSeawater Seawater 3.2.4. Asmentioned mentionedearlier, earlier,the thechemical chemicalproperties propertiesofofthe thecoating coatingsurface surfacechange changeupon uponimmersion immersionin in As seawateras asthe thesurface surfacemolecules moleculesof ofthe thecoating coatingrearrange rearrangeand andthe thepolar polarhard hardsegments segmentsmove moveto tothe the seawater surface of the coating. Hence, to better evaluate the interface behavior of the polyurethane coatings surface of the coating. Hence, to better evaluate the interface behavior of the polyurethane coatings during seawater seawater immersion, immersion, diiodomethane diiodomethane was was selected selected for for in-situ in-situ measuring measuring ICA. ICA. Due Due to tothat that during diiodomethanehas has significant solubility in water (1.2 With g/L).increasing With increasing time immersed in diiodomethane significant solubility in water (1.2 g/L). time immersed in seawater seawater of diiodomethane droplet, we are afraid that in case of the diiodomethane dissolved in of diiodomethane droplet, we are afraid that in case of the diiodomethane dissolved in water, it might water, it mightmeasurements. lead to smaller To measurements. To evaluate its effect, out a long time inlead to smaller evaluate its effect, we carried out a we longcarried time in-situ observation situ observation of diiodomethane droplets on the interface between seawater and coating. The of diiodomethane droplets on the interface between seawater and coating. The results of PU-Si17 results of PU-Si17 coating respectively immersed one day and five days were showed in Figure coating respectively immersed one day and five days were showed in Figure 15. With increasing 15. of With increasing of immersion time, both of the droplet and ICA changed smaller. However, as immersion time, both of the droplet and ICA changed smaller. However, as showed in Figure 16, showed in Figure 16, in which Figure 16a,b for coating immersed one day in sterilized seawater, Figure 16c,d for coating immersed five days. Figure 16a,c for dripped diiodomethane droplet in 10 s, and Figure 16b,d at 60 s. Quite evidently, the dissolution of diiodomethane was not obvious at the early stage of immersion in seawater, and the change of shape of droplets and interface contact angle were not obvious. In order to ensure the validity and comparability of the measurement results

surface of the coating. Hence, to better evaluate the interface behavior of the polyurethane coatings during seawater immersion, diiodomethane was selected for in-situ measuring ICA. Due to that diiodomethane has significant solubility in water (1.2 g/L). With increasing time immersed in seawater of diiodomethane droplet, we are afraid that in case of the diiodomethane dissolved in water, it might lead to smaller measurements. To evaluate its effect, we carried out a long time inCoatings 2018, 8, 157 14 of 18 situ observation of diiodomethane droplets on the interface between seawater and coating. The results of PU-Si17 coating respectively immersed one day and five days were showed in Figure 15. With increasing of immersion time, both of the droplet and ICA changed smaller. However, as in which Figure 16a,b for coating immersed one day in sterilized seawater, Figure 16c,d for coating showed in Figure 16, in which Figure 16a,b for coating immersed one day in sterilized seawater, immersed five days. Figure 16a,c for dripped diiodomethane droplet in 10 s, and Figure 16b,d at 60 s. Figure 16c,d for coating immersed five days. Figure 16a,c for dripped diiodomethane droplet in 10 s, Quite and evidently, the dissolution of diiodomethane was not of obvious at the early of immersion Figure 16b,d at 60 s. Quite evidently, the dissolution diiodomethane wasstage not obvious at the in seawater, and the change of shape of droplets and interface contact angle were not obvious. In order to early stage of immersion in seawater, and the change of shape of droplets and interface contact angle ensurewere the validity and comparability of the measurement results during the experiment, the images not obvious. In order to ensure the validity and comparability of the measurement results were frozen after dripped in seconds. on these images, measured ICA of all during the experiment, theten images were Based frozen after dripped in tenthe seconds. Basedresults on theseofimages, the were measured results ICA of17. all samples were reported in Figure 17. samples reported inofFigure

15. Long interface contact angles (ICA) in-situ measurementsofofdiiodomethane diiodomethane for for PU-Si17 PUFigureFigure 15. Long time time interface contact angles (ICA) in-situ measurements Si17 sample immersed in seawater. sample immersed in seawater. Coatings 2018, 8, x FOR PEER REVIEW 14 of 17 Coatings 2018, 8, x FOR PEER REVIEW

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Figure 16. ICA in-situ measurements of diiodomethane at initial period for PU-Si17 sample immersed Figure 16. ICA in-situ measurements of diiodomethane at initial period for PU-Si17 sample immersed Figure 16. in ICA in-situ measurements of diiodomethane at initial period for PU-Si17 sample immersed in seawater. seawater. in seawater. PU-Si0

140

PU-Si4 PU-Si0 PU-Si9 PU-Si4 PU-Si13 PU-Si9 PU-Si17 PU-Si13 PU-Si17

140 120 ICA ()

ICA ()

120 100

100 80

80

1

2

3

4

5

Immersion time in seawater (Day)

1

2

3

4

5

Figure ICA thePU-Sis PU-Sis samples samples during immersion in seawater. Figure 17.17. ICA ofofthe during immersion Immersion time in seawater (Day) in seawater.

The ICA increases with increasing immersion time, the ICA tends to be stable after four days Figure 17. ICA of the PU-Sis samples during immersion in seawater. immersion. It increases from around 100° at early immersion to 135° after five days immersion. Similarly phenomenon used to observe for the PPG-TDI-BDO polyurethane coatings in previous The ICA increases with increasing immersion time, the ICA tends to be stable after four days research [24]. When immersed in seawater, polyurethane coatings will exhibit different immersion. It increases from around 100° at early immersion to 135° after five days immersion. physicochemical properties from those exposed in air. Because polar hard segments tended to Similarly phenomenon used to observe forimmersion, the PPG-TDI-BDO polyurethane coatings in previous migrate toward the coating surface during it tends to form a hydrophilic and oleophobic

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The ICA increases with increasing immersion time, the ICA tends to be stable after four days immersion. It increases from around 100◦ at early immersion to 135◦ after five days immersion. Similarly phenomenon used to observe for the PPG-TDI-BDO polyurethane coatings in previous research [24]. When immersed in seawater, polyurethane coatings will exhibit different physicochemical properties from those exposed in air. Because polar hard segments tended to migrate toward the coating surface during immersion, it tends to form a hydrophilic and oleophobic surface. The enhanced oleophobicity further inhibited the adhesion of marine organisms. The increase in the contact angle may also have some contribution from an increase in surface roughness caused by the collapse of surface PDMS chains underwater. 3.3. Antifouling Properties of the PU-Sis Samples The antifouling properties of the PU-Sis coatings were investigated by immersing coated panels in the Yellow Sea, China. A PDMS fouling-release coating was also tested as a control. Its surface free energy is 26.07 mJ/m2 , its elastic modulus is 0.31 MPa. The panels were coated with PDMS-modified polyurethane. Because the polyurethane coating is transparent, the red color resulted from the intermediate coat. Because of the uncontrollable factors, the panels were regretfully found having fallen into the sea at the time of three months of inspection. Therefore, this article can only give the photographs until immersion for 77 days. The photographs of the panels after immersion for 25 and 77 days are shown in Figure 18. The calculated coverages are shown in Table 4. Only a few green algae were adhered to the PU-Sis panels after 77 days. Although the effect of PDMS content is not obvious on the antifouling properties of the PU-Sis coatings, it can be seen that the difference between the PDMS-modified polyurethane and the polyurethane coating. The PU-Si0 panel was partly coveredCoatings by bacterial biofilm. The control silicone panel was not only covered wholly15by bacterial 2018, 8, x FOR PEER REVIEW of 17 biofilm, but also by macroalgae. This demonstrated obviously the enhanced antifouling activity of the biofilm, butpolyurethane, also by macroalgae. This demonstrated obviously the enhanced antifouling activity of PDMS-modified compared with the control sample and non-modified polyurethane. the PDMS-modified polyurethane, compared with the control sample and non-modified However, it does not enhance with increasing PDMS content, nor with decreasing the surface free polyurethane. However, it does not enhance with increasing PDMS content, nor with decreasing the energy surface of the PDMS-modified polyurethane. free energy of the PDMS-modified polyurethane. PU-Si0

PU-Si4

PU-Si9

PU-Si13

PU-Si17

Control

Immersed 77 days

Immersed 25 days

Before immersion

Sample

Figure 18. Photographs of panels immersed in the Yellow Sea, China.

Figure 18. Photographs of panels immersed in the Yellow Sea, China. The relative adhesion factor  values of the coatings with different PDMS contents are also listed in Table 4. As observed, the relative adhesion factor decreases significantly with increasing PDMS contents, it reaches the lowest at PU-Si9 sample, then it increases with increasing PDMS contents. For the PDMS-modified polyurethane coatings, it seems that the bigger the relative adhesion factor is, the more the coverage of marine organism is. However, the values obtained therein were inconsistent well with the panel test results, thereby indicating that the relative adhesion factor is inappropriate for characterizing the adhesion properties (or antifouling performance) of the PDMS-

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Table 4. The Specimen Coverage √(%) γ· E

√

γ· E values and coverage of marine organism studied of the coatings.

PU-Si0

PU-Si4

PU-Si9

PU-Si13

PU-Si17

Silicone Coating

26.7

8.2

1.6

2.9

22.1

100

4.15

3.68

2.67

4.41

4.72

2.86

√ The relative adhesion factor γ· E values of the coatings with different PDMS contents are also listed in Table 4. As observed, the relative adhesion factor decreases significantly with increasing PDMS contents, it reaches the lowest at PU-Si9 sample, then it increases with increasing PDMS contents. For the PDMS-modified polyurethane coatings, it seems that the bigger the relative adhesion factor is, the more the coverage of marine organism is. However, the values obtained therein were inconsistent well with the panel test results, thereby indicating that the relative adhesion factor is inappropriate for characterizing the adhesion properties (or antifouling performance) of the PDMS-modified polyurethane coatings examined herein. In other words, the antifouling performance is dependent not only on mechanical properties and surface free energy, but also on the properties of the coating such as topography and surface structure stability in seawater. 4. Conclusions This study investigated the effect of incorporating PDMS on the structure and morphology of polyurethane, and the effect of immersing the resulting PU-Sis materials in seawater. PDMS-modified polyurethane samples of different PDMS contents were prepared. The structure and morphology of the segmented polyurethanes were analyzed by various methods. (1)

(2) (3)

(4)

Clear microphase separation was observed in polyurethane, but decreased or even disappeared after immersion in seawater. Incorporating PDMS weakened H-bonding between the soft and hard phases, and increased phase separation. The size of the soft and hard domains also increased. Phase separation was not observed on the surface of samples containing PDMS, because the surface layer was enriched in PDMS. The flexible PDMS segments decreased the Tg of polyurethane. This improved the flexibility and elasticity of polyurethane, so fouling organisms could be more readily removed. PDMS chains migrated toward the film surface, forming a uniform hydrophobic layer with a low surface free energy. This surface enrichment of PDMS increased with increasing PDMS content. Because of this enrichment, PU-Si4, PU-Si9, PU-Si13 and PU-Si17 exhibited low surface energies, and were stable toward immersion in seawater. During seawater immersion, polar groups have an upward tendency. Immersed panels tests in seawater demonstrated the antifouling activity of the PDMS-modified polyurethane. However, it was not the case that the higher the PDMS content, the better the antifouling performance of the PDMS-modified polyurethane. The best modified polyurethane is with 8.5 wt % PDMS. It is a promising material for fouling-release coatings.

Author Contributions: Y.-H.Q., X.-F.S. and Z.-P.Z. conceived and designed the experiments; X.-F.S. and L.-Y.C. performed the experiments; X.-F.S. and Z.-P.Z. analyzed the data; Z.-P.Z., Y.-H.Q. and L.-Y.C. contributed reagents/materials/analysis tools; Z.-P.Z., X.-F.S. and Y.-H.Q. wrote the paper. Acknowledgments: This work was supported by National nature science foundation of China (51079011, 51179018), The Defense Funds. The authors gratefully acknowledge for financial support. Conflicts of Interest: The authors declare no conflict of interest.

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