PREPARATION OF SUPER-HYDROPHOBIC SURFACES BY USING ELASTOMER TEMPLATES AND UV-CURABLE RESIN

Abstract

Super-hydrophobic surfaces have attracted great research interest due to their novel performance as self-cleaning,anti-fog,and anti-oxidation.In this work,a new approach,based on elastomeric templates and UV-curable resins,has been developed to prepare super-hydrophobic surfaces.Poly(dimethylsiloxane) (PDMS,Sylgard 184) was purchased from Dow Corning and used to replicate lotus-leaf surfaces.In the replication step,PDMS blocks were prepared through replica molding against lotus-leaf surfaces.PDMS prepolymer was prepared by mixing the elastomer base and curing agent in a ratio of 10∶1. After 20 min,the PDMS prepolymer was poured into molds within fresh lotus-leaf surfaces as masters.Then the prepolymer was cured in 40℃ oven for 4 h.After curing and peeling off,the PDMS blocks with negative lotus-leaf surfaces' structures were obtained.Microscope observation indicates that the PDMS blocks' surfaces were featured with hollow cavities of 7～12 microns in diameter.The PDMS blocks could be used as stamps in the following curing step of ultraviolet (UV)-curable prepolymers,which consist of oligomer,cross-linking agent,reactive monomer,and photo-initiator.In the experiment,isobornyl acrylate,siliconized urethane acrylate,benzoin dimethyl ether and dipentaerythritol pentaacrylate were used as monomer,oligomer,photoinitiator and crosslinking agent,respectively.In a typical process,UV-curable prepolymer was prepared by mixing monomer,oligomer,photoinitiator and crosslinking agent in a ratio of 10∶68∶2∶20. Then the prepolymer was put into a 0.085 MPa vacuum oven to remove air bubble.The prepolymer was spread on substrates (silicon wafer or glass slide),pressed by the PDMS stamp with replicated lotus-leaf surface.Finally,the UV-curable prepolymer covered with the PDMS stamp was exposed to UV-light (500 μW/cm~2).After curing for 15 min,the PDMS stamp was peeled off,and the cured sample was store in 40℃ oven for 5 h before further experiments.Measured by scanning electron microscopy (SEM,Hitachi S-4500),a large amount of papillary microstructures with an average diameter of about 7～10 microns and average height of 10～18 microns could be seen on the surfaces.These papillary microstructures are replicas of the surface patterns of the lotus leaves.The obtained surfaces had water contact angles larger than 150° and small sliding angles,exhibiting fascinating super-hydrophobicity like lotus-leaf surfaces.Obviously,the replicated papillary microstructures played important role in the increase of surface hydrophobicity from about 105° (smooth film surface) to 157°.This result can be explained with the Wenzel theory.Factors influencing the wettability of resulted surfaces,such as content of monomer,cross-linking agent,initiator and light irradiation time,were also studied in detail.Experiments showed that the surface contact angles increase with the content of photoinitiator before 0.7%,and decrease with the content of crosslinking agent when larger than 20%.A monomer content of about 10% and irradiation time longer than 10 min were also preferable for achieving high hydrophobicity.Combining the advantages of soft-lithographic and UV-curing techniques,this approach can be potentially applied in fabrication of large-area super-hydrophobic surfaces.