Macroscopic and Microscopic Analyses of Hydrophobic Modification of

Article pubs.acs.org/JPCC

Macroscopic and Microscopic Analyses of Hydrophobic Modification of Rubbers with Silica Nanoparticles Yi Ye,†,‡ Chen Zhang,‡ Ming Tian,*,†,‡ Zhongjie Du,‡ and Jianguo Mi*,† †

State Key Laboratory of Organic−Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China

Downloaded by UNIV OF CAMBRIDGE on September 3, 2015 | http://pubs.acs.org Publication Date (Web): September 1, 2015 | doi: 10.1021/acs.jpcc.5b05865

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ABSTRACT: The wettability of rubbers with silica nanoparticle modification was investigated with theory and experiment. A simple coating technology was applied to generate the superhydrophobic styrene−butadiene rubber (SBR). Silica nanoparticles were covalently bonded with γ-methacryloxy propyl trimethoxysilane (γ-MPTMS), which was employed to improve coating durability through the thiol−ene click reaction with the SBR matrix. The contact angles of water droplets on the net and modified surfaces were then measured. The application of a three-dimensional density functional theory approach to predict the wetting contact angles showed that the chemical composition, chain conformation, and micro/nanostructure have different contributions to the hydrophobic behaviors. The theoretically predicted contact angles were partly validated by their experimental counterparts.

1. INTRODUCTION The use of superhydrophobic rubber composites as insulators is increasingly an alternative solution for polluted environments due to their self-cleaning properties.1−3 It is well-known that the key elements of natural or artificial superhydrophobic surfaces are extremely rough: roughness is indeed a key ingredient, because it favors the trapping of air or gas bubbles at the boundaries.4−6 Such roughness can be achieved by coating nanoparticles or a polymer nanocomposite. In particular, silica particles have been widely used as an additive to reinforce rubber matrix or surface hydrophobicity due to their remarkable physical and chemical properties. However, silica particles are usually rich in hydroxyl groups, resulting in their naturally hydrophilic characteristics. In order to formulate a hydrophobic roughness coating on rubber surface, it is necessary to tailor the hydroxyl groups with a silane coupling agent to reduce their surface energy. Another drawback existing in the ordinary coating technologies is their low washing and abrasion durability. To improve the durability, a simple strategy is to the incorporate nanoparticles into polymeric films.7−9 In this strategy, however, large amounts of particles are encased in the membrane bulk material, which limits the amount of particles on the membrane surface and decreases the modification efficiency. Another way is surface functionalization of polymer matrix by plasma-induced graft copolymerization to provide sufficient carboxyl groups as anchor sites for particle binding.10,11 As a consequence, the surface-modified particles are covalently bound to the grafted polymer matrix through the dip-coating technique.12,13 Graft copolymers provide new opportunities to control polymer composition and architecture, thus opening possibilities for new coating technology. However, the plasma treatment creates high surface roughness © XXXX American Chemical Society

by introducing pores, special patterns, or texture, which could inappropriately influence the boundary properties of coating. In the past decades, a large number of theoretical and computational studies have been performed to investigate the effect of surface characteristics on the wettability. The physics of a liquid drop on flat homogeneous surfaces has been relatively well understood;14,15 however, the effect of a liquid drop on heterogeneous surfaces remains in question because of its complexity arising from realistic situations: chemical contaminations, roughness, or inhomogeneity on the solid surface.16−18 In this regard, control of surface wettability of rubbers is of importance both from the point of view of fundamental understanding of interfacial properties and for engineering applications. On a macroscopic scale, wetting phenomena can be well described in terms of Cassie and Wenzel equations using the interface tensions between the different phases to determine the equilibrium contact angle. On a microscopic scale, however, interface phenomena are much richer and provide far more than simply a basis for macroscopic laws. The compressible liquid under nanoconfinement correlates not only Laplace’s pressure but also the oscillatory compression pressure.19 Due to lack of mesoscopic information, it is unclear to what extent these macroscopic models remain valid as the length scale of the substrate features approaches the characteristic size of fluid molecules. In recent years, increased emphasis has been placed on understanding and modeling the wetting behavior of fluids in the presence of rough substrates with nanoscale features. In Received: June 19, 2015 Revised: August 26, 2015

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DOI: 10.1021/acs.jpcc.5b05865 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

2. EXPERIMENTAL SECTION Modification of the Silica Particles. Silica particles (20 nm, 5g) were dispersed in 200 mL of methylbenzene in a flask by stirring for 3 h and with ultrasonic vibration for 1 h. Then a certain amount of γ-MPTMS was added to the suspension. The mixture was adjusted to pH 4.0 by ethanoic acid and subsequently stirred for 6 h in boiling state to obtain the modified particles with abundant CC groups. Preparation of the SBR Membrane. In the next step, a solution of 20 wt % SBR in tetrahydrofuran (THF) with 2 wt % 2-methyl-4-(methylthio)-2-morpholinopropiophenone (MMMP) and 0.5 wt % trimethylolpropane tris(3-mercaptopropionate) (TMPMP) for the thiol−ene click reaction was prepared. The mixture was poured into a glass dish and held in a ventilated oven at 40 °C until the solvent was completely evaporated. Grafting Modified Silica Particles on SBR. Premeasured quantities of modified silica particles and MMMP were dispersed in THF with stirring and ultrasonicating.25,26 The weight ratio of the three components is particle/MMMP/THF = 4:1:50. The solution was spray-cast onto the SBR film at a fixed distance of 20 cm using an airbrush atomizer, which was worked via siphon feeding. The coated substrate was dried for 3 h at 70 °C. Then the covalent bonds were formed between the CC groups and SBR by thiol−ene click reaction under a high pressure mercury lamp emitting light at 360 nm.27 Finally, the above samples were washed by ethanol for scouring off the ungrafted particles. Adhesion Test. Adhesion testing of modified SBR film was performed according to ASTM D 3359B-02. The cutting tool was used to make the cross-cut pattern at ca. 90° angles throughout the coating. LA-26 tape was applied to the cut surface and rubbed with the eraser end of pencil to ensure good contact with the film, and it was then removed after 90 s. Characterization of the Modified Silica Particles. Scanning electron microscopy (SEM) images were taken on a S4800 SEM. A thin gold film was sputtered on the sample before SEM images were taken. TGA data were taken on a TGA/DSC 1 instrument in nitrogen atmosphere at a rate of 20 °C/min from 30 to 1000 °C. To investigate the chemical interactions between γ-MPTMS and silica particles, NMR characterizations were performed. 29Si NMR spectra were collected using a Bruker AV400 spectrometer. 2,2-Dimethyl-2silapentane-5-sulfonic acid was used as the solvent for the NMR measurements. Contact Angle Measurement. The water contact angles measurements were carried out with an OCA 15141 goniometer under normal ambient conditions (25 °C and 30% relative humidity). The static contact angles were determined using drops of deionized water with a volume of 10 μL. Such contact angles were approximately applied to represent equilibrium values to examine our theoretical model. Because the interactions between water molecules and hydrophobic surface are relatively weak, which cause a very low adhesion, it is difficult to place a smaller drop on it. As such, these contact angles correspond to macroscopic values.

particular, wetting diagrams for nanoscale roughness surface corresponding to the Cassie−Wenzel impregnation wetting transition process have been well constructed.17,20 These wetting diagrams imply that the macroscopic wetting models are no longer reliable at nanoscale. The deviation within the wetting regime is attributed to corner effects and non-negligible line tension contribution.21−23 Difficulty is encountered when trying to understand the relationship between the two factors and their influence to the wettability of the nanoscale rough surface. In polymer nanocomposites, the size ratio of nanoparticle to polymer monomer or fluid molecule is exceedingly large. Description of such highly asymmetric interactions is the main difficulty in mechanism analysis. Nowadays, even in widely applied molecular simulation, particles are rarely more than 10 times the diameter of monomers due to equilibration difficulties and computational cost. How to correctly predict the wetting properties of these systems remains unsolved to date. It is important to construct a new theoretical model to clarify the relationship as it may determine its relevance to evaluate the wetting equations for microfluidic systems. In this work, we performed a comprehensive study of superhydrophobic rubber surfaces with experiment and theory. In the experimental part, we provided a simple and effective way to significantly improve the hydrophobicity of styrene− butadiene rubber (SBR) through click chemistry. Using an acrylic silane coupling agent of γ-methacryloxy propyl trimethoxysilane (γ-MPTMS), we first produced a solid connection of the CC functional groups on the organic segment with the rubber matrix. After condensation with hydroxyls on the surface of silica particles, γ-MPTMS chains were then covalently bonded to silica particles via traditional polycondensation reaction. Meanwhile, the CC groups in γMPTMS were employed to improve the coating durability, which involves cross-linking the coating layer and establishing covalent bonding between the coating and substrate via the thiol−ene click reaction which is a highly efficient, simple synthetic procedure and economical chemical reaction. In this way, we can improve the durability of the grafted surface with low dosage of particles and relatively simple technology. In the theoretical part, we attempted to interpret the hydrophobic mechanism of different rubber surfaces. Within the framework of our previously proposed three-dimensional density functional theory (3D-DFT),24 the asymmetric interactions between particle and water molecules were described by the Hamaker theory, and the net and rough surfaces of SBR, polymethyl-vinylsiloxane (PMVS), and natural rubber (NR) were systematically investigated to decipher the effects of chemical and geometric characteristics on hydrophobicity and superhydrophobicity. According to surface morphologies and the interactions of rubber-water and particle−water, different droplet nucleation behaviors were considered to evaluate the contribution of the corner effect and line tension. The contact angles were then subsequently extracted from the density profiles of nuclei, and thereby their wettabilities were theoretical predicted. In particular, the predicted wetting contact angles on SBR surfaces with different extents of silica particle modification were compared directly with the experimental values. As a result, this work provides macroscopic and microscopic viewpoint analyses of hydrophobic modification on difficult rubber surfaces.

3. THEORETICAL SECTION Within the theoretical framework, the interactions between different sites in water molecules were described by the TIP4P/ ice force field,28 the interactions between water molecules and surface sites were represented by its oxygen atom, and the B


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The Journal of Physical Chemistry C pairwise nonbonded interactions were governed by the Lennard-Jones (LJ) form. Polymers were modeled as the semiflexible chains, and the universal TraPPE-UA force field was applied to describe the topologic configuration of polymer chains and the intermolecular interactions.29,30 The united intermolecular potentials were calculated by taking a summation of the pair potentials


⎧ ⎡⎛ σij ⎞12 ⎛ σij ⎞6 ⎤ ⎪ ⎜ ⎟ 4 − ⎜ ⎟ ⎥ r ≥ σij ε ⎪ ∑ ij⎢⎝ ⎝r⎠⎦ U rubber(r) = ⎨ ⎣ r⎠ ⎪ ⎪ ∞ r < σij ⎩

F hs[ρ] =

∫ d rΦhs[nm(r)]

(6)

and Φ [nm(r)] is the Helmholtz free-energy density which stems from the modified fundamental theory including both the scalar and vector contributions33 hs

⎡ n1n2 − n V1·n V2 Φhs[nm(r)] = ⎢ −n0 ln(1 − n3) + ⎢⎣ 1 − n3 +

⎞ n23 − 3n2 n V ·n V ⎤ n32 1 ⎛ 2 2⎥ ⎜n3 ln(1 − n3) + ⎟ 2 ⎥⎦ 36π ⎝ (1 − n3) ⎠ n33

(1)

(7)

where εij and σij are energy and size parameters, respectively; r is the distance between water molecule and a site located on interfacial rubber chains. The overall potential exerted by the solid surface can be written as

where nm(r) is the weighted densities with m = 0,1,2,3,V1,V2. The details of nm(r) have been given in the literature.34 For the attractive contribution, Fatt[ρ(r)] can be consistently and accurately expressed with the following equation

U (r) = U rubber(r) + U np(r)

F att[ρ(r)] = −

(2)

where the second term on the right side, Unp(r), refers to the summation of intermolecular potentials between water molecules and particles. Here the particles were represented with large spheres, for which only the outside layer has the influence to water molecules. As a result, the potential was calculated with Unp(r) = V(R)−V′(R), where V(R) is the force field of entire sphere, and V′(R) is for the particle center. V(R) can be represented with the Hamaker theory31 V (R) = V12(R) − V6(R)

∫ d r′ρ(r)ρ(r′)catt(|r‐r′|)

(8)

where catt(r) is the direct correlation function of the equilibrium interfacial density from attractive contribution. Here we used the attractive part of cOO(r) to represent water. The details to derive cOO(r) of water can be seen elsewhere.35 The contact angle was determined following previous procedure.36,37 Furthermore, the Young equation can broaden its validity and be a linear relationship between the contact angle and the radius of droplet that accounts for38,39 a cos θ = cos θ∞ − R droplet (9)

(3)

where Vn(R) = cnIb(n;R), Ib(6;R) = (4π/3)b3(R2−b2)−3, Ib(12;R) = (4π/45)b3[15R6 + 63R4b2 + 45R2b4 + 5b6](R2− b2)−9, cn = 4ε(ρσ)n, R is the particle radius, and b the distance from the particle center. In theoretical calculations, R was set to 10 nm to keep consistent with the experimental size. ε and σ are energy and size parameters for CH2 group,29 and ρ is the density of silica particle. We considered a single nucleus of liquid phase that is deposited onto various rough rubber surfaces and surrounded by the supersaturated gas phase. For heterogeneous nucleation, 3D-DFT was applied to obtain the spatial density distribution ρ(r) by minimizing the grand potential Ω[ρ(r)], which can be expressed as the following form

∫ drρ(r)[ln[ρ(r)] − 1] + F hs[ρ(r)] + F att[ρ(r)] + ∫ d r[ρ(r)(U (r) − μ)]

1 2

where θ is the equilibrium contact angle of the microscopic droplet, Rdroplet the radius of droplet in contact with the surface, θ∞ the equilibrium contact angle of a macroscopic drop, and a the constant correlated to interfacial tensions. For the theoretical description, different rubber surfaces were constructed using the molecular dynamics simulation method,40 including SBR, PMVS, NR, and their modifications with silica nanoparticles. In these simulations, polymer chains were treated as semiflexible with fixed bond lengths. Most importantly, the force field parameters for these semiflexible chains in simulations and theoretical calculations are exactly the same to ensure their consistency. The initial configurations were first constructed in a large periodic box to obtain a system of very low density. All molecule parameters were taken from the TraPPE-UA field.29,30 NPT ensemble was employed to compress the systems of low density to the desired density. The systems were equilibrated for at least 6 ns with NVT simulations under one bar pressure. Such equilibrated structures were used as initial configurations for a further production run of (10−20) × 106 MD steps by NPT simulations at 25 °C. During all simulation runs, a time step of 1.0 fs was used with a multiple time step integrator.41 Isothermal calculations were carried out with a Nose−Hoover thermostat using a coupling frequency of 0.02 fs−1, and the pressure was controlled by a Nose−Hoover barostat.42 The bond lengths were constrained using the SHAKE algorithm.43 Afterward, different modified rubber surfaces were constructed by distributing silica particles (R = 10 nm) randomly on net rubber surfaces at given particle density to keep consistent with the experiment condition. We constructed the model roughness surfaces by distributing silica particles randomly on rubber surfaces at given particle density. As such, the particle

Ω[ρ(r)] = kBT

(4)

hs

where F [ρ(r)] stands for the local Helmholtz free-energy of hard-sphere reference system, Fatt[ρ(r)] accounts for the local Helmholtz free-energy due to the attractive contribution, U(r) summarizes the interactions between water molecules and surface sites, and μ indicates the chemical potential of bulk gas. μ can be easily derived from ρ(r) = ρb, where ρb is the density of gas. Minimization of the grand potential functional leads to the Euler−Lagrange equation ⎛ ⎞ δ(F hs[ρ(r)] + F att[ρ(r)]) − βU (r)⎟ ρ(r) = exp⎜βμ − β δρ(r) ⎝ ⎠ (5)

where the contribution of hard-sphere repulsion is generally represented by the fundamental measure theory32 C


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The Journal of Physical Chemistry C distribution and aggregations on real surfaces were considered closely, but the multilayer packages were overlooked. Nucleation occurs under a supersaturated condition and the supersaturation ratio is defined as S = p/p0, where p denotes the pressure of the supersaturated vapor and p0 means the vapor− liquid coexistence pressure at a fixed temperature. In this work, all calculations were performed at 25 °C and S = 30.0. For the initial configurations of iteration calculations, a hemispherical cap region was placed at the top of the surface. The Cartesian coordinate was adopted with the initial density profile


⎧0 z ≤ z0 ⎪ ⎪ ρ(x , y , z) = ⎨ ρl* D ≤ D0 ⎪ ⎪ ρg* D > D0 ⎩

(10)

Figure 1. 29Si NMR spectra of the pristine silica particles and particles modified with γ-MPTMS.

for the iteration calculations. Here z0 is the original position of the solid substrate for the boundary effects, and D0 is the initial radius of the nucleus with D the distance to the center of the sphere cap. The initial density of the water molecules ρ*1 is 0.9, corresponding to the density at room temperature. The supersaturated gas density ρg* is 6.25 × 10−4. ρ* is dimensionless density with ρ* = ρσ3. It should be addressed that the supersaturation ratio can affect the size of nucleated droplet, but has little or no influence on contact angle. In the 3D calculations, the interactions of polymer−water and particle−water are a sum of pair potential functions, which is expressed in eq 2. The LJ parameters for unlike sites were calculated using the Lorentz−Berthelot mixing rules with εij = (εiiεjj)1/2 and σij = (σii + σjj)/2. The 3D density and free-energy distributions of water at the solid surfaces were calculated on a 3D grid of 512 × 512 × 512 points, which gives a grid resolution of 0.5 σO in each axis. The size was chosen so that the density and free-energy distributions are not affected by edge effects and thus more accurately represents the macroscopic case. The outmost site of the surfaces obtained by arranging the z coordinate of the polymer surfaces sites is located at z = 0. The traditional Picard iteration was applied to obtain the distribution of water molecules on a polymer surface to form the droplet. The iteration procedure was repeated until the average fractional difference over any 3D grid point between the old and the new density is less than 10−3.

Figure 2. TGA thermograms of the pristine and modified silica particles.

for the pristine ones. In Figure 2, one can see that the graft ratio of γ-MPTMS on the particles is about 10 wt %. The hydrophobic characteristics of the modified SBR surfaces were evaluated through the contact angle measurements. The results are shown in Figure 3. For comparison, Figure 3a displays the water droplet on the pure SBR surface. The contact angle is 93.8°, suggesting a slightly hydrophobic attribute of pure SBR. As the density of the grafted particles increase from 1.6 to 3.2 mg/cm2, the contact angle of water droplet on the modified SBR surfaces can be improved from 128.4° to 152.2°, as shown in Figure 3b,c. The results show clearly that a superhydrophobic surface was achieved for SBR by grafting silica nanoparticles. Further increase of particle density to 4.8 mg/cm2, however, leads to a slightly decrease of the contact angle to 144.8°. This drop could be induced by the aggregation of silica particles, leading to the reduction of roughness. In order to further illustrate the efficiency of current spraycoating method, we also performed another experiment by mixing the modified silica particles directly with the SBR matrix to cast different membranes for the hydrophobicity evaluation. Figure 4 shows that, as the weight ratio of particles increase to 8.0%, the contact angle of water droplet on the membrane surface is 110°. Further increase of the weight ratio up to 12.0% leads to only slight increase of the contact angle to 120.1°. Therefore, a significant increase of particle density cannot

4. RESULTS AND DISCUSSION Synthesized silica nanoparticles were analyzed with Fourier transform infrared (FTIR) spectroscopy and thermogravimetric analysis (TGA). Figure 1 shows the solid-state 29Si MAS NMR spectra for the pristine and modified silica particles, respectively. For the pristine particles, only two signals can be observed, corresponding to Q3 and Q4 silicate structures.44,45 The relative amount of the two silicate structures was estimated to be 70% for Q3 and 30% for Q4. For the modified particles, however, the relative amount of the Q4 silicate structures increases to 50%, and T2 and T3 silicate structures can be detected, suggesting that some SiOH groups were replaced with the structure of SiOSi. Figure 1 provides evidence for successful covalent linking of γ-MPTMS with silica particles. Figure 2 presents the TGA data for the pristine and modified silica particles. The results were obtained in the presence of air medium. The modified silica particles display a single step weight loss, which can be attributed to the decomposition of grafted γ-MPTMS. In contrast, there is a negligible mass loss D


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Figure 3. Contact angles of water droplets on SBR surfaces with different particle densities: (a) pure SBR, 93.9°; (b) 1.6 mg/cm2, 128.4°; (c) 3.2 mg/cm2, 152.2°; (d) 4.8 mg/cm2, 144.8°.

Figure 4. Contact angles of water droplets on modified SBR surfaces through the traditional blending technology with different weight ratios: (a) 8.0%, 110.0°; (b) 12.0%, 120.1°.

Figure 5. SEM images of modified silica particles dispersed on the SBR surface: (a) 1.6 mg/cm2, (b) 3.2 mg/cm2, and (c) 4.8 mg/cm2.

generally homogeneous distribution (see Figure 5b below). In this case, a specific micro/nanostructure can be seen, and the roughness of surface can be extensively enhanced. When the particle density is further improved to 4.8 mg/cm2, the phenomenon of particle aggregation emerges, leading to the destruction of the micro/nanostructure. At the heterogeneous wetting state,46 or Fakir state47 where air is trapped between the surface and the droplet, a superhydrophobic surface can be easily obtained. In Figure 5a, the distances between particles are too big to contain air pockets on the surface, and the Fakir state cannot be built. In this case, the hydrophobic attribute of the

consequently result in the corresponding improvement of contact angle. Because most silica particles are packaged by the SBR matrix, there is little influence to the surface wettability. Therefore, it is extremely difficult to obtain a superhydrophobic SBR by the blending technology. In contrast, the spray-coating modification provides an efficient and environmentally friendly method to produce a superhydrophobic surface. Figure 5 shows the SEM images of SBR surfaces with different particle loadings. At low density (1.6 mg/cm2), particles are well dispersed on the surface, as shown in the Figure 5a. If the density increases to 3.2 mg/cm2, it also shows a E


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Figure 6. Digital photographs of the SBR surface filled with silica particles (3.2 mg/cm2) before (a) and after (b) the cross cut tape test.

Figure 7. Two-dimensional cuts of density distributions of droplets with a constant radius 8σ on (a) SBR, (b) PMVS, and (c) NR surfaces, respectively. White, liquid phase; blue, gas phase; rainbow line, gas−liquid interface; red and yellow, local density fluctuations at liquid/solid interface.

provides a comprehensive understanding of the wetting mechanism, where the thermodynamic properties are derived directly from the microscopic interfacial structure of the droplet on the roughness surface. In the theoretical calculations, we first considered water droplets formulated on different net rubber surfaces. The morphologies and sizes were determined by the interfacial density distributions arising from the interactions between water molecules and polymer chains. Figure 7 displays the twodimensional (2D) cuts of density distributions of droplets on the SBR, PMVS, and NR surfaces. From the subFigure 7a−c, one can see irregular liquid−solid interfaces with obvious density fluctuations. These irregular microrough interfaces are produced by the spatial conformations of polymer chains, and the fluctuation amplitudes are induced by the different water− polymer interaction strengths and the morphologies of surfaces. The fluctuation amplitude on SBR surface is similar to the one on NR surface, whereas a relatively weak fluctuation can be seen on PMVS surface. By comparing their topographical configurations, one can find that the backbones of SBR and NR chains make up of CH2 and CH sites, whereas the backbone of PMVS chain is filled with silica and oxygen atoms, which have larger size and weaker dispersive attractions to water molecules. According to the morphology of droplets, the contact angles can be easily determined at a given droplet size. Accordingly, different contact angles can also be obtained for different droplet sizes. Figure 8 plots the negative cosine of contact angle θ as a function of droplet base radius (1/Rdroplet). Extending these linear fits to the limit of infinitely large droplets (1/Rdroplet →0) can result in macroscopic contact angles (θ∞). The final wetting angles are 96°, 121°, and 93°, respectively, for the SBR,

surface is mainly determined by the gas−liquid, gas−solid, and liquid−solid interfacial tensions. However, in Figure 5b, the distances of the dispersed particles are very close. As a result, an air packet can be kept in the gaps between particles, which cause the reduction of liquid density at the interface. Such an air packet promotes the droplet mobility or hydrophobicity of surface. As the particle density continuingly increases, however, the particles start to aggregate, leading to the poor dispersion. In this case, the gaps between particles are too small to have an apparent influence on the adhesion of the droplet by the surface. Therefore, the hydrophobicity of SBR surface declines. Because the particle packing is high enough, the Fakir state has not been completely destroyed and thereby such a drop appears unobvious. When the results of the three panels of Figure 5 are analyzed, one can draw a conclusion: once the particle density is increased to form the Fakir state, an optimal superhydrophobic surface can be achieved. To investigate the adhesion of the modified silica particles on the SBR surface, ASTM standard D 3359-02 test was performed. Figure 6 displays digital photographs of the film surface with 3.2 mg/cm2 particle density before and after the cross cut tape test. One sees that the modified film shows good adhesion with no squares being removed by the tape. This proves that the covalent cross-linking between particles and SBR is solid. Apart from the experimental measurements, the hydrophobicity of SBR surfaces with and without silica particle modification were described with the 3D-DFT approach. In the theoretical calculations, all model parameters were taken from the TraPPE-UA force field,29,30 and therefore, the present theoretical model is predictive. Moreover, theoretical analysis F


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higher nucleation free-energy barriers and larger contact angles. Therefore, nanoscale roughness plays a predominate role to extensively enhance hydrophobic properties. On the other hand, although the wetting properties of the three pure rubbers are different, there are nonperceivable differences that can be seen when the particle density is larger than 3.6 mg/cm2. Under the condition of dense particle-packing, rubber surfaces have been almost fully occupied by particles and thereby the contributions of specifically chemical compositions and chain architectures of rubber matrixes have been completely screened out. In particular, the calculated contact angle of droplet on the Langmuir−Blodgett monolayer48 surface is 154.8°, indicating the screen effect is reasonable. Moreover, some particles aggregate to form different clusters. In this case, the micro/ nanostructure on the roughness surfaces is no longer regular, resulting in the slightly reduced contact angles. In particular, a general agreement between theoretical predictions and experimental measurements has been achieved on the modified SBR surfaces, indicating that the present 3D-DFT approach is suitable for description of the wetting behaviors and the underlying mechanisms on nanoscale roughness surfaces. The deviation between the theoretical predictions and experimental data at dense density is caused by the particle overlaps in the theoretical description. The wetting transition on nanoscale roughness surface can be also illustrated by the density distributions, as shown in Figure 10. At low density (2.0 mg/cm2), a few liquid molecules penetrate into the grooves, leading to the partial wetting and the weak hydrophobicity (Figure 10a). As the particle density increases to 3.2 mg/cm2, an ordered micro/nanostructure has been clearly displayed on the roughness surface, and liquid molecules in the grooves have been replaced by gas molecules. As such, the surface becomes superhydrophobic (Figure 10a). In Figures 10a,b, one can see that the wetting transition from Wenzel to Cassie state emerges. However, a further increase of particle density to 6.8 mg/cm2 leads to an unordered micro/ nanostructure, which is unhelpful to further improve the hydrophobicity. Figure 11 shows the morphologies of droplets on the modified SBR surfaces with different roughness scales. With low particle loadings, the droplets are in a homogeneous wetting state, where the entire surface beneath the drop can be fully wetted (Figure 11a,b). As the particle densities increase to 3.2 or 4.4 mg/cm2, a heterogeneous wetting state can be observed, where gas packets can exist under the drops (Figure 11c,d). A further increase of the particle density leads to another homogeneous wetting state (Figure 11e,f). In these cases, the contact angles show a slight drop. The results demonstrate that the wetting transition with increasing particle loading fulfills via the following sequence: homogeneous wetting → heterogeneous wetting → homogeneous wetting.

Figure 8. Negative cosine of the contact angles vs inverse of base radii of droplets on the SBR, PMVS, and NR surfaces, respectively.

PMVS, and NR surfaces. According to Figure 7, the relatively larger contact angle on PMVS surface is mainly induced by the lower attractive forces or surface energy, whereas the effect of microrough is insignificant. Figure 9 summarizes the theoretically predicted and experimentally measured contact angles on the modified SBR,

Figure 9. Theoretical and experimental contact angles of droplets as a function of the number of silica particles on the SBR, PMVS, and NR surfaces, respectively.

PMVS, and NR surfaces with different silica particle loadings. One can see similar variation tendency of wetting behaviors. At the low density region, the contact angles can be quickly improved with the increasing particle density. Subsequently, the values rise slowly. As the particle density increases up to 3.6 mg/cm2, the contact angles approach the maximum values, respectively. Further increase of particle density, however, leads to the values declining gradually. Obviously, such variation tendency is coherently related to the surface roughness. Due to the relatively weak attractions at the inner corners of roughness, liquid molecules tend to be repelled by the solid surfaces to maintain lower density distributions in the corners than the density in the bulk. Because the high density regions have been largely reduced, the interfacial free-energies as well as the threephase contact line tensions have been improved, leading to

5. CONCLUSION We presented a detailed investigation of the wetting properties of net and modified rubber surfaces via the combination of experimental and theoretical approaches including NMR, TGA, SEM, and 3D-DFT calculations. Through the covalent crosslink and thiol−ene click reaction, silica nanoparticles were solidly adhered to the SBR matrix. With a suitable particle loading, the contact angle of water droplet on the modified SBR surface can be improved up to 152.2°, showing a superhydrophobic attribute. On the other hand, the mechanisms of hydrophobicity and superhydrophobicity were interpreted G


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Figure 10. Morphologies of droplets on the modified SBR surfaces with different nanoscale roughness. The particle densities are (a) 2.0 mg/cm2, (b) 3.2 mg/cm2, (c) 6.8 mg/cm2, respectively. Red, liquid phase; blue, gas phase; rainbow line, gas−liquid interface; white, solid surface.

Figure 11. Morphologies of droplets on the modified SBR surfaces with different particle loadings: (a) 0.8 mg/cm2; (b) 2.0 mg/cm2; (c) 3.2 mg/ cm2; (d) 4.4 mg/cm2; (e) 5.6 mg/cm2 ; (f) 6.8 mg/cm2.

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theoretically. By correlating the asymmetric interactions among water molecules, polymer sites, nanoparticles, an accurate spatial free-energy functional for the confined water molecules was constructed, where the liquid−solid, gas−solid, and gas− liquid interfacial tensions, as well as the gas−liquid−solid threephase contact line tension were included through the droplet nucleation. In particular, the previous proposed corner effect was fully addressed. For pure rubber matrixes, their chemical compositions and chain conformations play important roles in wettability. After modified by silica particles, these materials display enhanced hydrophobic capabilities. When the surfaces are covered by silica particles to some extent, highly ordered arrays of particles can be constructed. Due to the weak attraction, the grooves can be partially or completely filled with gas molecules, the wetting behavior can be transferred from Wenzel to Cassie state. As such, nanoscale roughness makes the essential contribution to the superhydrophobicity. If the surface is overloaded by particles, an approximately homogeneous wetting behavior can be seen again, and the corresponding wettability starts to decline. In summary, this work provides a sample way to prepare macroscopically superhydrophobic rubber surfaces, and a microscopic view of the hydrophobic mechanism of the surfaces with silica nanoparticle arrays.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Nos. 21276010 and 21476007) and by Chemcloudcomputing of Beijing University of Chemical Technology.

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REFERENCES

(1) Seyedmehdi, S. A.; Zhang, H.; Zhu, J. Superhydrophobic RTV silicone rubber insulator coatings. Appl. Surf. Sci. 2012, 258, 2972− 2976. (2) Boinovich, L. B.; Domantovskiy, A. G.; Emelyanenko, A. M.; Pashinin, A. S.; Ionin, A. A.; Kudryashov, S. I.; Saltuganov, P. N. Femtosecond laser treatment for the design of electro-insulating superhydrophobic coatings with enhanced wear resistance on glass. ACS Appl. Mater. Interfaces 2014, 6, 2080−2085. (3) Peng, C.-W.; Chang, K.-C.; Weng, C.-J.; Lai, M.-C.; Hsu, C.-H.; Hsu, S.-C.; Li, S.-Y.; Wei, Y.; Yeh, J.-M. UV-curable nanocasting technique to prepare bio-mimetic super-hydrophobic non-fluorinated H


Article


The Journal of Physical Chemistry C polymeric surfaces for advanced anticorrosive coatings. Polym. Chem. 2013, 4, 926−932. (4) Biben, T.; Joly, L. Wetting on nanorough surfaces. Phys. Rev. Lett. 2008, 100, 186103. (5) Lafuma, A.; Quere, D. Superhydrophobic states. Nat. Mater. 2003, 2, 457−460. (6) Tuteja, A.; Choi, W.; Ma, M.; Mabry, J. M.; Mazzella, S. A.; Rutledge, G. C.; McKinley, G. H.; Cohen, R. E. Designing superoleophobic surfaces. Science 2007, 318, 1618−1622. (7) Liang, S.; Xiao, K.; Mo, Y.; Huang, X. A novel ZnO nanoparticle blended polyvinylidene fluoride membrane for anti-irreversible fouling. J. Membr. Sci. 2012, 394-395, 184−192. (8) Zhou, H.; Wang, H.; Niu, H.; Gestos, A.; Wang, X.; Lin, T. Fluoroalkyl silane modified silicone rubber/nanoparticle composite: A super durable, robust superhydrophobic fabric coating. Adv. Mater. 2012, 24, 2409−2412. (9) Wang, H.; Xue, Y.; Ding, J.; Feng, L.; Wang, X.; Lin, T. Durable, self-healing superhydrophobic and superoleophobic surfaces from fluorinated-decyl polyhedral oligomeric silsesquioxane and hydrolyzed fluorinated alkyl silane. Angew. Chem., Int. Ed. 2011, 50, 11433−11436. (10) Liang, S.; Kang, Y.; Tiraferri, A.; Giannelis, E. P.; Huang, X.; Elimelech, M. Highly hydrophilic polyvinylidene fluoride (PVDF) ultrafiltration membranes via postfabrication grafting of surfacetailored silica nanoparticles. ACS Appl. Mater. Interfaces 2013, 5, 6694−6703. (11) Roth, J.; Albrecht, V.; Nitschke, M.; Bellmann, C.; Simon, F.; Zschoche, S.; Michel, S.; Luhmann, C.; Grundke, K.; Voit, B. Surface functionalization of silicone rubber for permanent adhesion improvement. Langmuir 2008, 24, 12603−12611. (12) Dugay, J.; Tan, P.; Loubat, A.; Lacroix, L.-M.; Carrey, J.; Fazzini, P. F.; Blon, T.; Mayoral, A.; Chaudret, B.; Respaud, M. Tuning deposition of magnetic metallic nanoparticles from periodic pattern to thin film entrainment by dip coating method. Langmuir 2014, 30, 9028−9035. (13) Faustini, M.; Capobianchi, A.; Varvaro, G.; Grosso, D. Highly controlled dip-coating deposition of fct FePt nanoparticles from layered salt precursor into nanostructured thin films: An easy way to tune magnetic and optical properties. Chem. Mater. 2012, 24, 1072− 1079. (14) Kang, H. C.; Jacobi, A. M. Equilibrium contact angles of liquid droplets on ideal rough solids. Langmuir 2011, 27, 14910−14918. (15) Borcia, R.; Borcia, I. D.; Bestehorn, M. Drops on an arbitrarily wetting substrate: A phase field description. Phys. Rev. E 2008, 78, 066307. (16) Kumar, V.; Errington, J. R. Impact of small-scale geometric roughness on wetting behavior. Langmuir 2013, 29, 11815−11820. (17) Leroy, F.; Müller-Plathe, F. Can continuum thermodynamics characterize wenzel wetting states of water at the nanometer scale? J. Chem. Theory Comput. 2012, 8, 3724−3732. (18) Malanoski, A. P.; Johnson, B. J.; Erickson, J. S. Contact angles on surfaces using mean field theory: nanodroplets vs. nanoroughness. Nanoscale 2014, 6, 5260−5269. (19) Chen, Y.; Wetzel, T.; Aranovich, G. L.; Donohue, M. D. Generalization of Kelvin’s equation for compressible liquids in nanoconfinement. J. Colloid Interface Sci. 2006, 300, 45−51. (20) Joly, L.; Biben, T. Wetting and friction on superoleophobic surfaces. Soft Matter 2009, 5, 2549−2557. (21) Kumar, V.; Errington, J. R. Impact of small-scale geometric roughness on wetting behavior. Langmuir 2013, 29, 11815−11820. (22) Chen, X.; Ma, R.; Li, J.; Hao, C.; Guo, W.; Luk, B. L.; Li, S. C.; Yao, S.; Wang, Z. Evaporation of droplets on superhydrophobic surfaces: Surface roughness and small droplet size effects. Phys. Rev. Lett. 2012, 109, 116101. (23) McBride, S. P.; Law, B. M. Influence of line tension on spherical colloidal particles at liquid-vapor interfaces. Phys. Rev. Lett. 2012, 109, 196101. (24) Wang, Y.; Wang, X.; Du, Z.; Zhang, C.; Tian, M.; Mi, J. Evaluation of macroscale wetting equations on a microrough surface. Langmuir 2015, 31, 2342−2350.

(25) Ogihara, H.; Okagaki, J.; Saji, T. Facile fabrication of colored superhydrophobic coationgs by spraying a pigment nanoparticle suspension. Langmuir 2011, 27, 9069−9072. (26) Ogihara, H.; Xie, J.; Okagaki, J.; Saji, T. Simple method for preparing superhydrophobic paper: Spray-deposited hydrophobic silica nanoparticle coatings exhibit high water-repellency and transparency. Langmuir 2012, 28, 4605−4608. (27) Qi, Y.; Li, L.; Fang, Z.; Zhong, J.; Dong, Q. Effects of small molecular weight silicon-containing acrylate on kinetics, morphologies, and properties of free-radical/cationic hybrid UV-cured coatings. J. Appl. Polym. Sci. 2014, 131, 40655. (28) Abascal, J.; Sanz, E.; Fernández, R.; Vega, C. A potential model for the study of ices and amorphous water: TIP4P/Ice. J. Chem. Phys. 2005, 122, 234511. (29) Wick, C. D.; Martin, M. G.; Siepmann, J. I. Transferable potentials for phase equilibria. 4. United-atom description of linear and branched alkenes and alkylbenzenes. J. Phys. Chem. B 2000, 104, 8008−8016. (30) Stubbs, J. M.; Potoff, J. J.; Siepmann, J. I. Transferable potentials for phase equilibria. 6. United-atom description for ethers, glycols, ketones, and aldehydes. J. Phys. Chem. B 2004, 108, 17596−17605. (31) Henderson, D.; Duh, D.; Chu, X.; Wasan, D. An expression for the dispersion force between colloidal particles. J. Colloid Interface Sci. 1997, 185, 265−268. (32) Rosenfeld, Y. Free-energy model for the inhomogeneous hardsphere fluid mixture and density-functional theory of freezing. Phys. Rev. Lett. 1989, 63, 980−983. (33) Yu, Y.-X.; Wu, J. Structures of hard-sphere fluids from a modified fundamental-measure theory. J. Chem. Phys. 2002, 117, 10156−10164. (34) Alts, T.; Nielaba, P.; D’Aguanno, B.; Forstmann, F. A local density functional approximation for the ion distribution near a charged electrode in the restricted primitive model electrolyte. Chem. Phys. 1987, 111, 223−240. (35) Wang, X.; Wang, S.; Xu, Q.; Mi, J. Thermodynamics of ice nucleation in liquid water. J. Phys. Chem. B 2015, 119, 1660−1668. (36) Tang, Y.; Wu, J. Modeling inhomogeneous van der Waals fluids using an analytical direct correlation function. Phys. Rev. E 2004, 70, 011201. (37) Werder, T.; Walther, J. H.; Jaffe, R. L.; Halicioglu, T.; Koumoutsakos, P. On the water−carbon interaction for use in molecular dynamics simulations of graphite and carbon nanotubes. J. Phys. Chem. B 2003, 107, 1345−1352. (38) Wei, N.; Lv, C.; Xu, Z. Wetting of graphene oxide: A molecular dynamics study. Langmuir 2014, 30, 3572−3578. (39) Marmur, A.; Krasovitski, B. Line tension on curved surfaces: Liquid drops on solid micro- and nanospheres. Langmuir 2002, 18, 8919−8923. (40) Plimpton, S. J. Fast parallel algorithms for short-rang molecular dynamics. J. Comput. Phys. 1995, 117, 1−19. (41) Curro, J. G.; Frischknecht, A. L. The structure of poly(ethylene oxide) liquids: Comparison of integral equation theory with molecular dynamics and neutron scattering. Polymer 2005, 46, 6500−6506. (42) Martyna, G. J.; Tuckerman, M. E.; Tobias, D. J.; Klein, M. L. Explicit reversible integrators for extended systems dynamics. Mol. Phys. 1996, 87, 1117−1157. (43) Argyris, D.; Tummala, N. R.; Striolo, A.; Cole, D. R. Molecular structure and dynamics in thin water films at the silica and graphite surfaces. J. Phys. Chem. C 2008, 112, 13587−13599. (44) Glaser, R. H.; Wilkes, G. L.; Bronnimann, C. Solid-state 29 Si NMR of TEOS-based multifunctional sol-gel materials. J. Non-Cryst. Solids 1989, 113, 73−87. (45) Lippmaa, E.; Mäegi, M.; Samoson, A.; Engelhardt, G.; Grimmer, A. R. Structural studies of silicates by solid-state high-resolution silicon-29 NMR. J. Am. Chem. Soc. 1980, 102, 4889−4893. (46) Cassie, A. B. D.; Baxter, S. Wettability of porous surfaces. Trans. Faraday Soc. 1944, 40, 546−551. (47) Quéré, D. Surface chemistry: Fakir droplets. Nat. Mater. 2002, 1, 14−15. I


Article



(48) Detrich, Á .; Nyári, M.; Volentiru, E.; Hórvölgyi, Z. Estimation of contact angle for hydrophobic silica nanoparticles in their hexagonally ordered layer. Mater. Chem. Phys. 2013, 140, 602−609.

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