ARTICLE pubs.acs.org/Langmuir
Formation of Superhydrophobic Microspheres of Poly(vinylidene fluoride hexafluoropropylene)/Graphene Composite via Gelation Li Zhang,† Dao-an Zha,† Tingting Du,† Shilin Mei,† Zujin Shi,‡ and Zhaoxia Jin*,† † ‡
Department of Chemistry, Renmin University of China, Beijing 100872, People’s Republic of China College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, People’s Republic of China
bS Supporting Information ABSTRACT: We report on the spontaneous formation of superhydrophobic poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP)/graphene composite microspheres with uniform size via gelation. When the suspension of PVDF-HFP/graphene (0.25 wt. % with respect to PVDF-HFP) in DMF adsorbs water vapor, it changes to a hybrid gel. A dried porous gel is obtained after solvent exchange and freeze drying. Morphology characterization shows that this hybrid gel is composed of PVDF-HFP/graphene microspheres with a diameter of 810 μm. In contrast, PVDF-HFP solution gives rise to a cellular microstructure following the same experimental procedures. We further elucidate the formation mechanism on the basis of the characterization by freeze fracture transmission electron microscopy, X-ray diffraction, and differential scanning calorimetry characterizations. Furthermore, contact angle measurements of water on PVDF-HFP/graphene indicates that the hydrophobic nature of PVDF-HFP combined with the micro/nanoscale hierarchical texture creates a superhydrophobic surface. Such superhydrophobic microspheres may have potential applications as water-repellent catalyst-supporting materials.
’ INTRODUCTION Polymer microspheres are widely exploited in electronics, photonics, medical imaging,1 immunoassay, drug delivery,2 and catalyst support.3,4 There is recently been growing interest in the applications of polymer microspheres as supports for heterogeneous catalysis.57 Immobilizing catalysts on polymer microspheres is verified via a useful strategy toward clean, rapid synthesis.5,8 Commercially available polymer microbeads for catalyst loading are based on polystyrene or its derivatives that are fabricated by an emulsion polymerization or phase separation (dispersion polymerization) method.1 Membrane emulsification is a widely used method of fabricating polymer microcapsules with a diameter ca. 50 μm.9 A number of polymer microspheres, such as polystyrenepoly(methyl methacrylate) composite microspheres, polylactide, and poly(lactide-co-glycolide) biodegradable microspheres, or microspheres containing magnetite (Fe3O4) have been successfully fabricated.10 It was observed that under harsh reactor conditions supported polymer microspheres should have good thermal and chemical stability and also are not sensitive to moisture. Thus, microspheres based on fluorinated polystyrene derivatives or poly(vinylidene fluoride) (PVDF) and its various copolymers are desirable in meeting these requirements.11 Recently, Buonomenna et al. have reported a method combining emulsion polymerization and phase inversion techniques to produce PVDF microcapsules with diameters ranging from 600 to 1200 μm,12 but smaller PVDF microcapsules have not been reported in the literature. However, graphene has attracted a lot of attention for its wide r 2011 American Chemical Society
applicability.1318 It is observed that the graphene surface is naturally hydrophobic.13 The surface functionalization14,15 or modification16 may change the surface energy of graphene, thus adjusting the surface properties of the composite materials.17 Furthermore, graphene can also improve the surface hydrophobicity through enhancing the surface roughness.17 It is known that the construction of surface roughness, especially hierarchically micro/ nanoscaled microstructures, may improve the water repellency of materials.19 In this study, we report a feasible method to produce superhydrophobic PVDF-HFP/graphene microspheres with nanoscaled surface roughness. The solution of PVDF-HFP/graphene (0.25 wt % with respect to PVDF-HFP) in DMF was changed to a gel by absorbing a nonsolvent (water). After solvent replacement and freeze drying, porous PVDF-HFP/graphene materials were obtained. SEM characterization showed that the PVDF-HFP/ graphene hybrid porous materials are composed of microspheres with diameters of 810 μm. In particular, the surfaces of these microspheres are covered with spikes and nets, forming nanoscale roughness. In contrast, pure PVDF-HFP gels showed a microcellular structure that is a typical morphology produced through liquidliquid demixing. To elucidate the formation mechanism of PVDF-HFP microspheres, we conducted freezefracture Received: March 16, 2011 Revised: June 8, 2011 Published: June 09, 2011 8943
dx.doi.org/10.1021/la200982n | Langmuir 2011, 27, 8943–8949
Langmuir
Figure 1. TEM image of graphene sheets produced by a dc arcdischarge method.
transmission electron microscopy to investigate the microstructure of wet gels. TEM micrographs revealed a dramatic morphology change in the PVDF-HFP gel state by the addition of a small amount of graphene (0.25 wt %). The formation mechanism of PVDF-HFP/graphene spherical microstructure has been proposed on the basis of the aforementioned morphology and crystallization characterizations. Furthermore, because the connection between PVDF-HFP/graphene microspheres is weak, isolated PVDF-HFP/ graphene microspheres with a narrow size distribution can be obtained by ultrasonication. Because of the superhydrophobic nature of PVDF-HFP/graphene microspheres, such uniformly sized microspheres may have potential applications as catalyst supporting materials in a humid environment.
’ EXPERIMENTAL AND MATERIALS PVDF-HFP (Mw = 400 000 g/mol) was purchased from SigmaAldrich and used as received. N,N-Dimethylformamide (DMF) of analytical grade was obtained from Sinopharm Chemical Reagent Beijing Co., Ltd. and redistilled before use. Graphene sheets were produced by an arc-discharge method as reported previously.20 The size of the graphene sheets is around 100200 nm, and there are mainly two to six layers according to HRTEM observation (Supporting Information Figure S1).20 Deionized (DI) water (Millipore Q; >18 MΩ cm) was used to leach and wash gels. PVDF-HFP was dissolved in DMF (20% w/w) to form a homogeneous solution by vigorous stirring. Graphene (0.25 wt % with respect to PVDF-HFP) was added to the PVDF-HFP/DMF solution and sonicated to get a stable suspension. For greater additions of graphene to PVDFHFP solution, a homogeneous dispersion of graphene in suspension was difficult to achieve. A bottle of PVDF-HFP/DMF solution or PVDFHFP/graphene/DMF suspension, along with a bottle of DI water, was then kept in a closed container. By adsorbing water vapor, the PVDFHFP/DMF solution or PVDF-HFP/graphene/DMF suspension gradually changed to gels. Normally, gels were aged for 3 days. To remove DMF in gels without damaging the gel structure, solvent exchange was carried out by immersing the gels in water for several days. The gel samples showed no significant shrinkage in volume during the solvent-leaching process (Supporting Information Figure S2). Finally, the gels were freeze dried at 60 °C (
