Polynorbornene fluorosilica hybrids for hydrophobic and oleophobic coatings

Polym.Bull.(2013)70:619–630DOI10.1007/s00289-012-0882-zO R I G I N A L P A P E RPolynorbornene/fluorosilica hybrids for hydrophobicand oleophobic coatingsGiju Sung•Myeon-Cheon Choi•Saravanan Nagappan•Won-Ki Lee•Mijeong Han•Chang-Sik HaReceived:2March2012/Revised:17October2012/Accepted:13November2012/Published online:21November2012ÓSpringer-Verlag Berlin Heidelberg2012Abstract Polynorbornene dicarboxylic anhydride(PNA)/fluorosilica hybrid coating materials with good hydrophobicity,oleophobicity,transparency,thermal stability,and hardness were synthesised using a sol–gel method.The surface structure,transparency,hydrophobicity,oleophobicity,and surface free energy of the coating could be controlled by adjusting thefluorosilane ratio.The maximum static water and oil contact angles offilms were112°and87°,respectively.The PNA/fluorosilica hybridfilms exhibited good transparency and colourlessness.The marks written using water and oil-based pens on thefilms could be erased with a tissue even after eight times.In addition,the hardness of the hybridfilms was enhanced to4H with increasingfluorosilane content.Keywords Polynorbornene dicarboxyimideÁFluorosilicaÁHybridÁCoating IntroductionHydrophobic and oleophobic surfaces have important applicability in everyday life [1].Artificial amphiphobic surfaces are generally prepared using a two-step method. First,a surface with micro-or nano-sized roughness is created by lithography or by G.SungÁM.-C.ChoiÁS.NagappanÁC.-S.Ha(&)Department of Polymer Science and Engineering,Pusan National University,Busan609-735,South Koreae-mail:csha@W.-K.LeeDepartment of Polymer Engineering,Pukyong National University,Busan609-735,South KoreaM.HanAdvanced Materials Division,Korea Research Institute of Chemical Technology,100Jang-dong, Yuseong-gu,Daejeon305-600,South Koreadeposition of micro or nanomaterials.Subsequently,molecules with low surface energy are deposited to impart water-repelling properties[2,3].On the other hand,polymers are very promising coating materials owing to their transparency,flexibility,roughness,and lightweight in addition to low cost through mass-production using a roll-to-roll process.Therefore,colourless polymers with high thermal and mechanical stability as well as good optical transparency have been studied widely as coatingfilms for self-cleaning building products[4–6].In particular,a polymeric material for coating should be easy to process and must be capable of forming highly resistantfilms with good aging resistance[7]. Polynorbornene-based polymers show a high glass transition temperature(T g),good physical and mechanical properties,and high thermal resistance[8–12].For example,poly(N-adamantyl-norbornene-5,6-dicarboximide)exhibits a T g of271°C. Polynorbornenes easily react with a range of substituents,which allows the synthesis of a variety of functionalized polynorbornenes and composites with organic or inorganic materials[13–15].Coating with a low-surface-energy material,such as fluoroalkylsilane(FAS)is often used to obtain hydrophobic and oleophobic surfaces [16,17].FAS exhibits excellent water resistance,good release properties,weather resistance,good chemical and mechanical stability,low coefficient of friction,and low refractive index[18,19].FAS is also used mainly as a precursor or a cross-linker for silica coatings and creates Si–O–Si bonds in the backbone of these coatings in a sol–gel process[20,21].The precursor reacts with the polynorbornene backbone through hydrolysis and polycondensation reactions to form hybrid materials.These can be fabricated effectively by combining the appropriate surface roughness with materials of low surface energy[22–24].This paper reports a simple and inexpensive sol–gel coating method that facilitates the fabrication of large-area amphiphobic organic–inorganic composite nanocoatings based onfluorosilica with polynorbornene carboxylic anhydride.The surface structure,hydrophobicity,oleophobicity,surface free energy,and transpar-ency of the coating can be controlled by adjusting the FAS.The coating exhibited not only excellent amphiphobicity but also high transparency.Moreover,the coating possessed good thermal stability.ExperimentalMaterials and instrumentsExo-norbornene-5,6-dicarboxylic anhydride(exo-NA)was prepared by thermal isomerisation of the corresponding endo isomer(Aldrich)according to procedure reported in the literature[25].3-Aminopropyl triethoxysilane(APTES)(99%), N,N-dimethylacetamide(DMAC),chlorobenzene,and chloroform were purchased from Aldrich.(Tridecafluoro-1,1,2,2,-tetrahydroctyl)triethoxy-silane(FAS)was obtained from Gelest,Inc.Bis(tricyclohexylphosphine)[(phenylthio)methylene] ruthenium(catalyst)was acquired from Strem Chemicals Inc.Vinyl ethyl ether was supplied by Fluka.All other chemicals were used as received without further purification.The water used in all syntheses was distilled and deionised.The number-average molecular weight(M n)and polydispersity index(PDI)of the poly(norbornenes)were measured by gel permeation chromatography(GPC)using a Waters515Differential Refractometer with a Waters410HPLC Pump and two Styragel HR5E columns in DMF(0.1mg/L)solvent at42°C,calibrated with polystyrene standards.1H-nuclear magnetic resonance(1H-NMR,Varian Unit Plus-300-300MHZ) spectroscopy was also performed.Elemental analysis was conducted using an elemental analyzer(EA,Vatio EL,Elemental Analysen System).Fourier transform infrared (FT-IR,Perkin Elmer)spectroscopy was performed in the frequency range 4,000–400cm-1.Scanning electron microscopy(SEM,JEOL6400microscope)at an operating at20kV was used to examine thefilm microstructure.Before the measurement,the samples were degassed at100°C for12h in a vacuum oven and coated with gold.The surface morphology,roughness,and thickness of the hybrid materials were analyzed by atomic force microscopy(AFM)in tapping mode using a Digital Instruments Nanoscope IIIa(USA)—Veeco metrology group at a scanning rate at1.969Hz with a200nm data scale.The sample size used for this study was 1cm91cm.X-ray photoelectron spectroscopy(XPS)was carried out using a Theta Probe XPS system(UK;Thermo Fisher Scientific Inc.)at a base pressure of4910-10 mbar(UHV)employing a monochromatic Al K a X-ray source(h m=1486.6eV)with charge compensation.The absorption and transmittance spectra were measured using an UV–Visible spectrophotometer(U-2010,HITACHI Co.)with a scan range and rate of transmission of350–800nm and200nm/min,respectively.Scratch resistance measurements(mechanical hardness)on the coated glass substrates were performed using a Yoshimitsu Pencil hardness tester221-D(Japan),according to the ASTM D-3363-74method[26].The water and oil contact angles were measured using a Kru¨ss Drop shape Analysis system,DSA100(Kru¨ss GmbH,Hamburg,Germany)according to the sessile drop method using a500l L syringe with a needle diameter and length at room temperature of0.5and38mm,respectively.The distilled water and hexadecane droplets used in each measurement for the static contact angles were3.0l L.The writing and erasing test on the coated glass substrates were tested using water/oil-based pens[27, 28].The marks written by the pens were erased by a soft tissue paper.The thermal stability was measured under nitrogenflow by a thermogravimetric analyzer(TGA,TA instruments Q50Q series)at a heating rate of10°C/min from50to800°C.The thermal transition of the polymers was measured under nitrogenflow by differential scanning calorimetry(DSC,TA instruments Q100)at a heating rate of10°C/min. Synthesis of polynorbornene dicarboxylic anhydride(PNA)As shown in Scheme1a,exo-NA(10g,61mmol)was dissolved in60mL of DMAc.Subsequently,0.052g(0.07mmol)of bis(tricyclohexylphosphine)[(phen-ylthio)methylene]ruthenium(II),as a catalyst,was added to the solution of exo-NA monomer with stirring.The reaction was maintained at room temperature for3h and then stirred at60°C for1h.0.5g(0.69mmol)of vinyl ethyl ether was added at room temperature and stirred for1h.The reaction was quenched by adding several drops of vinyl ethyl ether.The polymer,PNA,was dried in a vacuum oven at60°C for24h.The PNA was soluble in DMAc,chloroform,and chlorobenzene. 1H-NMR spectroscopy was used to determine the structure and composition of thepolymers.The signal at d(ppm)=5.70demonstrated typical trans double bonds of the polymer,whereas the corresponding cis signal was observed at approximately 5ppm.Many Ru-based catalysts provide mainly trans double bonds in ring opening metathesis polymerization(ROMP)of other norbornenes.Yield:82%, M n=7.59105,M w=12.99105,M w/M n=1.72,1H-NMR(DMSO-d6): d(ppm)=5.70(2H,s), 3.35(2H,s), 2.83(2H,m), 2.06(2H,m)1.53(3H,m). Elemental Anal.Calcd.for C9O3:C69.23,O30.77.Found:C71.92,O26.79.Synthesis of3-aminopropyl triethoxysilane and polynorbornene anhydride (precursor)PNA(0.25g,1.52mmol)was dissolved in10mL of DMAC.Subsequently,0.23g (1.52mmol)of APTES was added to the solution,as shown in Scheme1b.The amine groups of APTES reacted with the anhydride group of PNA at room temperature.The synthesis of carboxylic amides by ring opening of anhydride was carried out in glass vials.After the reaction,the secondary amide was confirmed by a FT-IR spectrum.The reaction mixture was stirred at room temperature for30min.The PNA/aminosilane precursor exhibited characteristic peaks,such as CH2(Cyclopen-tane)(*2,900cm-1),NH2?(*2,000cm-1),–CH2COOH(*1,200cm-1),Si–O–Si(625–480cm-1),Si–OH(929cm-1),carboxylic acid salts(1,440–1,335cm-1), C=O(1,680–1,630cm-1),and secondary amides(amide N–H)(3,460–3,420cm-1), respectively.Elemental Anal.Calcd.for C18O6NSi:C61.02,O27.19,N3.95,Si7.90. Found:C69.71,O23.86,N1.86,Si3.57.Sol–gel reactionFive types of PNA/FAS(PNF)hybrids were prepared using a sol–gel reaction,as shown in Scheme1b.The PNFs were synthesised by the addition of different amounts of FAS in DMAc,as listed in Table1.The solutions were stirred at room temperature for15–30min.For this reaction,0.001mL of water was added. Polycondensation occurred between the hydrolyzed APTES molecules and hydroxyl groups on FAS.Results and discussionPNF hybrids were prepared by two steps(Scheme1)and were confirmed by spectroscopic analysis.Figure1shows the transmission FT-IR spectra of the precursor(PNA with aminosilane)and PNF10hybridfilm.The strongest absorption bands relative to the cyclopentane host structure appeared at2,900cm-1from both the precursor and PNFfilm.The precursor showed a peak at1,440–1,335cm-1for carboxylic acid salts,and3,460–3,420cm-1for secondary amides(amide N–H) due to the ROMP reactions.After modification of the precursors by FAS,new peaks were observed at745–730cm-1for–CF3CF2and625–480cm-1for the Si–O–Si stretching vibration.A weak alkylfluorine stretching peak was appeared at 1,200cm-1.On the other hand,the929cm-1peak for the silanol group of modified PNF hybrids had been disappeared.This was attributed to the formation of a Si–O–Si linkage between PNA andfluorosilica via sol–gel reactions.Almost all the Si–OH surface groups were modified byfluorosilica.The FT-IR spectra strongly support the successful modification of the PNA precursor by FAS.Table1Composition of PNAfilm and PNF hybridfilms with various FAS contentsPNA PNF02PNF04PNF06PNF08PNF10DMAc101010101010 PNA(g)0.250.250.250.250.250.25 APES(g)–0.350.350.350.350.35 FAS(g)–0.0050.010.0150.020.025Figure 2shows SEM images of PNA and PNF hybrid films.Pure PNA film exhibited a flat morphology and the PNF films exhibited some fluorosilica nanoparticles in the PNA matrix.The roughness was increased by the presence of nanoparticles.Increasing the amounts of FAS induces a better morphology for the coating.This can explain why some silica particles agglomerate upon mixing with the precursors in the solvent with some forming micro/nanoparticles on the surface,which would increase the hydrophobicity and oleophobicity of the resulting hybrids.Figure 3presents AFM images of the surface structures of the coatings.PNF10was used as a typical PNF hybrid.Figure 3a shows a smooth surface in the coating prepared from PNA,which was in accordance with that observed in the SEM image of PNA.For the coating prepared from the PNF film (Fig.3b),the silica colloid particles on the surface were constructed in a micro/nanostructure.The roughness maximum (R max )\10nm indicates a smooth surface [29].In this view,the prepared PNA films exhibited a smooth surface on the substrate.The R max of the PNA film was 10nm but the R max of the PNF film was 50nm.This showed that the surface of the PNF film had been coated with silica colloid particles.The micro/nano-aggregation of silica particles on the coated surface would enhance the roughness values.The roughness values may vary by the amount of FAS.Increasing the amount of FAS may increase the roughness on the coated substrate [30,31].XPS was used to obtain surface chemical information of PNF film,as shown in Fig.4.The XPS survey spectrum of the hybrid film demonstrated the atomic information of the PNF hybrid film.C1s,F1s,N1s,O1s,and Si2p were observed at binding energies of 283eV (54.82at.%),687eV (12.67at.%),398eV (3.84at.%),530eV (22.16at.%),and 101eV (6.5at.%),respectively.The obtained result confirmed that the silicone and fluorine atoms present in the PNF hybrid film surface.The FAS content in the PNF hybrid film reduces the wettability and increase the contact angle (CA)of the coated surface.The hydrophobic natureof Fig.1FT-IR spectra of precursor (PNA with aminosilane)and PNF10hybrid filmfluorine atom may reduce the wettability of the coated substrate with increasing the amount of FAS.PNF10hybrid film showed 12.67at.%of fluorine atoms on the surface.The higher fluorine content in the hybrid film may responsible to reduce the wettability of the surface [30].Transparency and colorlessness are very important for producing coating materials.PNA and PNF hybrid films were highly transparent.As shown in Fig.5,the transmittances at 400–800nm of the PNA,PNF02,PNF04,PNF06,PNF08,and PNF10hybrid films were [90%.Therefore,the hybrid films were transparent in the visible wavelength range.The scratch hardness of PNA film was measured using the 1H pencil hardness (Table 2).PNF hybrid films with fluorosilica contents of 2to 4wt%showed a 2H pencil hardness,whereas PNF hybrid films with a 6to 8wt%FAS content exhibited a 3H pencil hardness.PNF hybrid films containing up to 10wt %of fluorosilica showed a 4H pencil hardness.The hardness increased with increasing FAS content.This suggests that the hardness was improved by increasing the FAScontents,Fig.2SEM images of a PNA,b PNF02,c PNF04,d PNF06,e PNF08,and fPNF10Fig.3AFM images of sample surfaces:a PNA and b PNF10filmswhich is not surprising because inorganic materials normally help improve the mechanical properties,such as the surface hardness.Figure 6shows the static contact angle (SCA)data of PNA and PNF hybrids.The hydrophobicity or oleophobicity of a solid surface was determined using water or oil (hexadecane)[32].Wenzel and Cassie–Baxter explained the CA from wettability of homogeneous and heterogeneous surface,respectively [33–35].The smooth PNA film surface showed homogeneous surface wetting with almost hydrophobic (water CA,84°)and oleophilic (hexadecane CA,35°)nature.PNF10hybrid film showed rough surface morphology by the introduction offluorinated Fig.5Optical transmittance spectra of the PNA and PNF hybrid films with various fluorosilica contents.The inset photograph displays the transparent PNF hybrid (PNF10)film which contains 10wt%FASsilica groups on the surface.The fluorinated rough surface also showed homoge-neous surface wetting as for the Wenzel state of wetting.The homogeneous wetting was confirmed from the contact angle images as indicated in the inset images of Fig.6.The surface wettability may enhance the water and oil CA by the increase of FAS [36,37].The enhancement of CA was due to the presence of pendant alkyl fluorine group on the surface of PNF film.The increase of FAS contents may reduce the surface energy and enhance the surface roughness (Fig.3)[38,39].The rough surface reduces the wettability of the coated substrate with the increase of CA.As a result,the 10wt%of FAS showed the maximum water (112°)and oil CA (87°).The hybrids maintained the Wenzel state even with the increase in the surface roughness by the increase of FAS (up to 10wt%).The results suggest that PNF films exhibited good hydrophobicity and oleophobicity by the increase of fluorosilane.Moreover,it should be noted that the PNF10film underwent writing and erasing test with water-and oil-based pens eight times (Fig.7).The marks in the hybrid films were erased using a common tissue paper.Even after eight repeated writing and erasing cycles,the marks were removed easily,meaning that the coating hybrids showed excellent anti-stain properties [27,28].The thermal stability of the PNF hybrid films were determined by TGA,as shown in Fig.8.In TGA analysis,the PNA and hybrid films exhibited 20and 15%weight loss,respectively,due to the degradation of alicyclic norbornene moieties at 150–300°C.The PNA and PNF hybrid films showed a degradation temperature of approximately *400and *510°C,respectively.The enhanced thermal stabilityofFig.6Static contact angles of the PNA and PNF hybrid coating films using a water and b hexadecane.The inset displays optical images of water droplets on the hybrid coating surfacesTable 2Pencil hardness of PNA film and PNF hybrid filmsPencil hardnessPNA PNF02PNF04PNF06PNF08PNF101HPASS PASS PASS PASS PASS PASS 2HFAIL PASS PASS PASS PASS PASS 3HFAIL FAIL FAIL PASS PASS PASS 4H FAIL FAIL FAIL FAIL FAIL PASSthe PNF hybrid films was attributed to the presence of inorganic FAS.From DSC analysis,the glass transition temperature of the PNA film was approximately 300°C.In addition,the PNF hybrid film containing 2wt%FAS was approximately 320°C.On the other hand,the glass transitions did not appear when the FAS composition was increased to 4wt%.ConclusionsPNA/fluorosilica hybrid coating materials with good hydrophobicity,oleophobicity,transparency,thermal stability,and hardness was synthesised using a sol–gel method.The surface morphology,amphiphobicity and transparent was controlled by the FAS content.The maximum static water contact angle (CA)and oil CA were 112°and 87°,respectively.The PNA/fluorosilica hybrid films exhibitedgood Fig.7Write and erase test of PNF10hybrid films on glasssubstratesFig.8TGA curves of the PNA and PNF hybrid films with various FAS contentstransparent and colourlessness.The transmittance of the coatings reached[90%. Thefilms written using water-and oil-based pens could be erased using a tissue eight times.The hardness of the hybridfilms was enhanced to4H by increasing the FAS content.The hybrids have advantages for easy fabrication as well as amphiphobicity,colourlessness,transparency,and thermal stability that would be suitable for large-scale coatings.Acknowledgments The work was supported by the National Research Foundation of Korea(NRF) Grant funded by the Ministry of Education,Science and Technology,Korea(MEST)(Acceleration Research Program(No.2012-0000108),Pioneer Research Center Program(No.2012-0000421/2012-0000422),and WCU program),a grant from the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy,Korea,and the Brain Korea21Project of the MEST.References1.Nakajima A,Hashimoto K,Watanabe T,Takai K,Yamauchi G,Fujishima A(2000)Transparentsuperhydrophobic thinfilms with self-cleaning ngmuir16:7044–70472.Li XM,Reinhoudt D,Crego-Calama 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