Article pubs.acs.org/Langmuir
Heat-Resistant Crack-Free Superhydrophobic Polydivinylbenzene Colloidal Films Zefeng Wang, Weiwei Ye, Xinran Luo, and Zhonggang Wang* Department of Polymer Science and Materials, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, China S Supporting Information *
ABSTRACT: Highly cross-linked poly(divinylbenzene) (PDVB) spherical colloidal particles with nano-, submicron-, and micron-sizes of 157.2 nm, 602.1 nm, and 5.1 μm were synthesized through emulsion and dispersion polymerization methods. The influences of particle size on the surface morphology, roughness, superhydrophobicity, and critical cracking thickness of colloidal films were studied in detail. The results show that PDVB colloidal films possess large water contact angle (CA) over 151°, belonging to superhydrophobic materials. Moreover, it is interesting to observe that the highly cross-linked network structure leads to PDVB film’s excellent heat-resistance. The CA and rough surface morphology remain nearly unchanged after thermal-treatment of films at 150 °C for 24 h. In addition, no cracks were observed in films with thicknesses up to 8.1 μm, exceeding most of polymer and inorganic particle films reported in the literature. The simple and scalable preparation method, low-cost, superhydrophobicity, and excellent thermal stability endow the PDVB colloidal films with promising applications in advanced coating fields, especially when employed in the high-temperature service environment.
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modified silica films prepared via one-step spin-coating method. The film thickness is larger than 0.7 μm without cracking.30 Prosser et al. prepared the crack-free silica films with thickness up to 1.7 μm by multistep deposition technique.31 Kozuka et al. found that the modification of inorganic particles with organic polymers could increase the critical cracking thickness (CCT, a film thickness below which films do not crack).32 For example, the CCT of BaTiO3 films was only 0.2 μm, but it could increase to 2 μm after the particles were modified with polyvinylpyrrolidone. Recently, Hatton and co-workers reported the preparation of poly(methyl methacrylate)/silica coassembled colloidal films. No cracks were observed in films with thicknesses up to approximately 5 μm.33 The reported results reveal that the inherent viscoelasticity of polymers can release the interfacial stress of particles and therefore effectively avoid the crack-formation. Nevertheless, the low glass transition temperature (Tg) of polymers makes the particles deform and coalesce at the elevated temperature, resulting in the decrease of roughness so that the films partially or completely lose the superhydrophobic nature. One feasible strategy to solve this problem is to enhance the elastic modulus of polymer particles by chemical cross-linking. Compared to linear polymers, in addition to the increased Tg, mechanical modulus, rigidity, and dimensional stability due to the restricted
INTRODUCTION Superhydrophobic coating materials with water contact angle larger than 150° have attracted tremendous interest in both academic and industrial fields1−7 owing to their potential applications in self-cleaning,8,9 anti-icing,10,11 antibacterial,12,13 drag reduction,14,15 anticorrosion,16,17 and so on. The superhydrophobic surface can be obtained by modifying low-surface energy materials such as fluorine-containing polymers but the expensive price limits their wide application. In this aspect, fluorine-free polyolefins including polystyrene completely composed of hydrogen and carbon atoms are ideal materials because of their low-cost, nontoxicity, and ease of production. Moreover, most of the nonpolar polyolefins also have low surface energy. Besides surface energy, another essential factor for achieving superhydrophobic surface is suitably large roughness.18−20 In recent years, researchers have utilized various techniques such as lithography,21,22 vapor deposition,5 and template method23 to create rough surfaces. However, these methods usually need expensive equipment and multiple processing steps. Comparatively, the rough surface derived from directly casting hard colloidal particles onto substrate is more economical and simple.24,25 On the other hand, the major barrier encountered in the fabrication of superhydrophobic surfaces from colloidal particles is that the films are liable to crack due to the capillary forces generated under drying, and the crack phenomenon is particularly serious for the thick films.26−29 To solve this problem, Yang and co-workers reported the fluorosilane© XXXX American Chemical Society
Received: January 28, 2016 Revised: March 8, 2016
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min−1. Contact angles of water were measured by a JY-82 contact angle goniometer (Dataphysics, Germany) at room temperature. For each sample, two films were used for contact angle measurements. Four microliter droplets were tested on 4−5 different positions and the obtained values were averaged. The experimental errors of the contact angle are within ±2°. Field-emission scanning electron microscopy (FE-SEM, FEI NOVA NanoSEM 450) was utilized to observe the morphology of the PDVB and PS film surface. The thickness of film was obtained by observing the cross-section through SEM.
polymer segmental motion, the three-dimensional cross-linked polymers also have many other merits such as excellent resistances to chemical corrosion and high temperature. With the above considerations in mind, herein, highly crosslinked poly(divinylbenzene) (PDVB) spherical colloidal particles were synthesized in an effort to obtain fluorine-free and crack-free superhydrophobic polymer films with excellent stability against high temperature. The resultant particles have varied sizes over a wide range from nanometer to micrometer in order to investigate the influence of particle size on the critical cracking thickness of polymer films. In addition, the comparative studies of thermal and hydrophobic stabilities, surface morphologies and glass transition temperatures between cross-linked PDVB and non-cross-linked polystyrene colloidal films were conducted.
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RESULTS AND DISCUSSION The spherical PDVB nano-, submicron- and micron-particles were synthesized through emulsion polymerization, seeded emulsion polymerization, and dispersion polymerization methods. The dynamic light scattering (DLS) curves (Figure 1) show that the resultant PDVB particles have quite narrow
EXPERIMENTAL SECTION
Materials. Divinylbenzene and styrene purchased from Aladdin Chemistry Co., Ltd. were vacuum-distilled to remove the inhibitor prior to use. Potassium peroxydisulfate (KPS) purchased from Tianjin Kemion Chemical Reagent Co., Ltd. was recrystallized from deionized water (40 °C) prior to use. 1-Hexadecanol (HD) and sodium dodeyl sulfate (SLS) were purchased from Aladdin Chemistry Co., Ltd and used as received. Preparation of PDVB Colloidal Nanoparticles. The PDVB colloidal nanoparticles were prepared through emulsion polymerization method. A 0.25 g sample of SLS, 0.84 g of HD, and 85 mL of deionized water were introduced into a three-necked flask under vigorously stirring at 70 °C. After stirring for 30 min, the mixture was cooled to room temperature and then 13 g of DVB was added dropwise under nitrogen atmosphere. Subsequently, the KPS aqueous solution (0.05 g of KPS in 5 mL water) was added, and the mixture was continually stirred at 70 °C for 2 h to obtain the PDVB nanoparticle colloidal. Preparation of PDVB Colloidal Submicron Particles. Using the resultant PDVB nanoparticles as seeds, the submicron colloidal particles were prepared by means of seeded emulsion polymerization technique. Specifically, the 85 mL seed latex was introduced into a 500 mL three-neck flask. The system was heated to 80 °C and purged with nitrogen. Then the monomer emulsion composed of 35 g of DVB, 0.9 g of SLS and KPS aqueous solution (0.25 g of KPS in 15 mL water) were added dropwise into the reactor. Finally, the system was reacted at 85 °C for additional 1.5 h. The polystyrene (PS) nano- and submicron-particles were prepared in the similar procedure except that the monomer used is styrene rather than divinylbenzene. Preparation of PDVB Colloidal Micron Particles. Thirty grams of DVB, 1.8 g of polyvinylpyrrolidone, 0.3 g of azobis(isobutyronitrile), and 67 g of acetonitrile were added into a 250 mL three-neck flask equipped with mechanical stirrer, nitrogen inlet, and thermometer. The system was heated to 70 °C and polymerized at this temperature for 6 h in nitrogen atmosphere. Preparation of Colloidal Films. The colloids prepared were centrifuged and the resultant particles were redispersed in ethanol to obtain colloidal suspension with different concentrations. The colloidal films were prepared by directly casting the colloidal suspensions onto precleaned slide glasses. The films were allowed to dry under ambient conditions (25 °C and relative humidity ∼40%) and then transferred to a vacuum oven and further dried for additional 24 h. Instrumentation. Dynamic light scatterings of colloidal particles were performed on a Malvern Zetasizer Nano-ZS90 instrument at room temperature. DSC measurements were carried out on a DSC TQ Q500 with indium metal as a standard. All samples (about 10 mg in weight) were scanned from 40 to 250 °C at a heating rate of 10 °C· min−1 in nitrogen atmosphere. Thermal gravimetric analysis (TGA) measurements were performed on a NETZSCH TG 209 thermal analyzer in nitrogen atmosphere over a temperature range of 25−800 °C at a heating rate of 10 °C·min−1 and a gas flow rate of 60 mL·
Figure 1. Sizes and size distributions for (a) PDVB nanoparticles, (b) PDVB submicron-particles, and (c) PDVB microparticle with zaverage diameters of 157.2 nm, 602.1 nm, and 5.1 μm, respectively, measured through DLS method.
monomodel distribution and their z-average diameters are 157.2 nm, 602.1 nm, and 5.1 μm, respectively. Moreover, the observed particles by FE-SEM are spherical and uniform (Figure 2) and sizes increase sequentially in line with the DLS data. For comparison, the nano- and submicron-sized PS particles with z-average diameters of 76.8 and 362.3 nm were also synthesized (Figure S1, Supporting Information). The PDVB prepared cannot dissolve in any common organic solvents such as benzene, toluene, cyclohexane, cyclohexanone, tetrahydrofuran, chlorinated hydrocarbons, dimethyl sulfoxide, and so forth, indicative of the highly cross-linked network structure. Thermal stabilities of PDVB and PS were evaluated by DSC and TGA in nitrogen atmosphere. As shown in Figure 3a, the linear PS exhibits a glass transition at around 105 °C.34,35 In contrast, for PDVB film no glass transition can be detected. The reason is attributed to that the densely crosslinked network inhibits the mobility of polymer segment, leading to a dramatically increased Tg beyond the measurement range. In addition, relative to the linear PS, the cross-linked PDVB displays an enhanced resistance to thermal decomposition as illustrated in TGA curves (Figure 3b). For PS sample, the initial decomposition temperature (Ti) and the B
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Figure 2. FE-SEM images of PDVB films derived from (a) nanoparticles, (b) submicro-particles, and (c) microparticles.
Figure 3. Comparison of thermal stability for PS and PDVB films through the measurements of (a) DSC and (b) TGA curves.
positively affect the hydrophobic property of the film. The similar results have also been observed by other groups. Tuvshindorj recently reported that the nanostructure of surface with 11 nm roughness showed a CA of 145°, whereas the microstructure of surface had a roughness value of 218 nm and therefore exhibited a larger CA of 162°.36 On the other hand, the dynamic advancing/receding contact angles of the three PDVB films composed of different colloidal particle sizes were measured. The results in Figure 4 show that the dynamic advancing angles for nano-, submicron-, and microparticle PDVB films are 137.6°, 151.5°, and 149.2°, respectively, which are consistent with their static contact angles. In fact, from the viewpoint of self-cleaning application, relative to the advancing angles, large receding angles with small hysteresis (the difference between advancing and receding angles) are more desirable. It is interesting to see that submicron- and microparticle PDVB films exhibit larger receding angles of 145.6° and 143.7° with quite small hysteresis values of only 4.9° and 5.5°, respectively. The nonadhesion of the superhydrophobic films was further verified by putting a 4.0 μL water droplet on the surfaces of glass and submicron-particle PDVB film, respectively. As shown in Figure 5, because of the highly hydrophilic nature of glass, upon approaching onto the surface the droplet easily departs from the syringe needle and rapidly spreads over the surface. In contrast, after coating the glass with submicron-particle PDVB film the superhydrophobic surface exhibits a strong repellence against water, and the droplet can be pulled off the film surface with ease. The superhydrophobic stability of PDVB colloidal films against high temperature was evaluated by heating the films at 60, 100, and 150 °C for 24 h in an oven, respectively. For comparison, the above experiments were also conducted for the linear PS colloidal films under the same condition. After
temperatures at maximum degradation rate (Tmax) are 366 and 406 °C, whereas the Ti and Tmax values for PDVB are 375 and 435 °C, respectively. Besides, it is found that the cross-linked PDVB exhibits the apparently higher residual weight (16 wt %) than PS (almost 0 wt %), indicating that the cross-linked PDVB has a significantly improved resistance to thermal decomposition. PDVD films composed of particles with particle sizes of 157.2 nm, 602.1 nm, and 5.1 μm, respectively, were obtained by casting the colloidal suspensions onto glass slides, respectively. The casting method was selected instead of spin-coating or dipcoating because its process is simple and the thickness can be conveniently controlled over a wide range through adjusting the concentrations of colloidal particles. In addition, the solvent of the colloidal was exchanged from water or acetonitrile with ethanol prior to the film preparations. Relative to water, ethanol has an improved affinity toward organic PDVB particles and therefore can act as a binder to enhance the strength of the close-packed arrays of particles, which is advantageous for the increase of critical cracking thickness of films. Moreover, ethanol has a lower boiling point and much decreased toxicity compared to acetonitrile and many other organic solvents. The hydrophobicity of PDVB films were examined by the static contact angles of water (CA). It is observed that the CA values of submicron- and microparticle films are 151.8° and 150.2°, respectively, much larger than that of nanoparticle film (136°). The similar situation happens to the PS films. For example, when the sizes of PS particles change from 76.8 to 362.3 nm, the CA values remarkably increase from 125° to 148°. The reason is because the films composed of larger spherical particles are beneficial for the increase of roughness. The roughness values of submicron- and microparticle films are 136 nm and 1.27 μm, respectively, much larger than that of nanoparticle film (49 nm). The large roughness values C
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nearly constant after successively heating for 24 h up at 150 °C. As a comparison, for the linear PS film, when the temperature surpasses 60 °C, the CA starts to decrease. The CA value is 112.7° at 100 °C, then rapidly decreases to 86° when the temperature rises to 150 °C. The remarkable difference in hydrophobic stability between PDVB and PS is owing to their glass transition temperatures. The cross-linked network structure results in PDVB spheres increased rigidity that makes the surface morphology of PDVB film remain almost unchanged after heat-treatment at high temperature. However, upon raising temperature the increased flexibility of the linear PS chains makes PS particles soften and coalesce each other. The film becomes quite smooth at 150 °C (Figure 7) and completely loses the superhydrophobic characteristic.37
Figure 4. Dynamic advancing/receding angles for (a) nano-, (b) submicron-, and (c) microparticle PDVB films.
Figure 5. Comparison of a water droplet on the surfaces of (a) glass and (b) submicron-particle PDVB film to verify the nonadhesion of PDVB film.
Figure 7. FE-SEM images of surface morphology for PS and PDVB films before and after thermal-treatment at 150 °C for 24 h. (a) PS film (room temperature), (b) PS film (150 °C/24 h), (c) submicronparticle PDVB film (room temperature), and (d) submicron-particle PDVB film (150 °C/24 h).
thermal-treatments, the CA of films were measured again and the data are presented in Figure 6. It is surprising to find that the highly cross-linked PDVB film exhibits excellent superhydrophoibic stability. The CA values of PDVB sample are
On the other hand, Figure 8 shows that the CA values for nano-, submicron-, and micron-particle films are closely related to the thickness. Taking the submicron-particle film as an example, when the thickness of the submicron-particle film is smaller than 5.5 μm, the CA values are over 151°, belonging to superhydrophobic materials. Nevertheless, the CA value rapidly decreases with the further increase of thickness. The CA is 62° with the film thickness of 8.1 μm, and final value is about 15° after the film thickness surpasses 11.0 μm. Moreover, it was observed that the critical cracking thicknesses (CCT) of the colloidal films vary with the particle sizes, which are ranked in the following order: microfilm (8.1 μm) > submicron-film (5.1 μm) > nanofilm (1.6 μm). The real mechanism for the crackformation of film is not clear yet, but it is acknowledged that the chemical structure, physicochemical nature, van der Waals interaction, elastic modulus of the particles, drying condition, and the substrate utilized are the major affecting factors for the CCT of films.26−29,38,39 In this work, the three kinds of polymer particles have the same chemical structure, and the filmpreparation process as well as solvent and temperature are also the same, so the reason for their different CCT values can only be attributed to the geometrical configuration of the particle
Figure 6. Superhydrophobic stability of PS and PDVB films against high temperature by observing the variations of contact angles of water of films after thermal-treatment at different temperatures for 24 h. D
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PDVB particles and film-formation process are simple, and the raw materials are commercially available and cheap compared to the fluorine- and silicon-containing polymers. The combination of excellent superhydrophobicity, thermal stability, and large critical cracking thickness as well as convenient operation, low toxicity, and low cost makes the PDVB colloidal films candidates in many advanced nonadhesion coating applications.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b00328. DLS curves of polystyrene nano- and submicronparticles. (PDF)
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Figure 8. Influence of water contact angles of PDVB films on film thickness for (a) nanoparticle film, (b) submicro-particle film, and (c) microparticle film.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected].
packing. Relative to submicron- and micron-particle films, if given the same thickness the colloidal film derived from nanoparticles will consist of more layer numbers. It is rational that multilayered packing of particles is more easily to collapse compared to the mono- or fewer-layered particle films during evaporation of solvent. In addition, the observation of submicron-particle PDVB films using an Olympus IL70 optical microscope reveals that the colloidal film with thickness less than 5 μm is uniform but after the thickness surpasses 5 μm, many microcracks start to appear on the film, and the cracks further develop with the increase of thickness (Figure 9a−c). The exposed hydrophilic glass substrate results in the decrease of contact angle of water.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the National Science Foundation of China (Nos. 51273031, 51473026, and U1462125) and the Program for New Century Excellent Talents in University of China (No. NCET-06-0280) for financial support of this research.
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CONCLUSIONS Superhydrophobic films with large contact angle over 151° were successfully prepared from spherical highly cross-linked PDVB particles. Surprisingly, the spherical shape of particles, the rough surface morphology, and superhydrophobic property of films can maintain almost unchanged even after thermaltreating at 150 °C for 24 h, showing significant advantage over the linear polymer colloidal films such as polystyrene. The excellent thermal stability of PDVB films arises from enhanced rigidity of particles because of the dense network structure. Relative to nanosized particles, the PDVB films fabricated with submicron- and micron-particles exhibit the remarkably increased thickness without crack. The critical cracking thickness of micron-sized PDVB film is up to 8.1 μm, which exceeds most of the reported films derived from both polymeric and inorganic particles. Moreover, the preparation method of
Figure 9. Optical microscopy images of submicron-particle PDVB films with different thicknesses. (a) Thickness of 5 μm, (b) thickness of 8 μm, and (c) thickness of 11 μm. E
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