【2010】PU表面接二氧化钛

G ModelPOC-2476;No.of Pages 5Progress in Organic Coatings xxx (2010) xxx–xxxContents lists available at ScienceDirectProgress in OrganicCoatingsj o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /p o r g c o atPreparation of nano-TiO 2/polyurethane emulsions via in situ RAFT polymerizationXiang-Chen Che a ,Yu-Zi Jin a ,∗,Youn-Sik Lee b ,∗∗a Department of Chemistry,College of science,Yan Bian University,Yanji 133-002,People’s Republic of ChinabDivision of Chemical Engineering,Nanomaterials Processing Research Center,Chonbuk National University,Chonju 561-756,South Koreaa r t i c l e i n f o Article history:Received 29April 2010Received in revised form 8August 2010Accepted 10September 2010Keywords:Polyurethane macromonomer TiO 2nanofillerTiO 2-RAFT agent hybrid RAFT polymerizationNano-TiO 2/polyurethane emulsiona b s t r a c tStable nano-TiO 2/polyurethane (PU)emulsions were prepared via in situ reversible addition-fragmentation chain transfer (RAFT)emulsion polymerization of 2-hydroxyethyl acrylate (HEA)-capped PU macromonomer,using azobisisobutyronitrile (AIBN)as a radical initiator and 2-{[(butylsulfanyl)carbonothioyl]sulfanyl }propanoic acid (BCSPA)anchored onto TiO 2nanoparticles (TiO 2-BCSPA)as a RAFT agent.When the molar ratio of AIBN to TiO 2-BCSPA was changed from 1:3to 1:10,the polydispersity index (PDI)of polymers in the emulsions decreased from 1.83to 1.06,due to more effective RAFT polymerization in the emulsions.The TiO 2nanofillers were well-dispersed through-out the polymer films.The tensile strengths of the nanocomposite films were significantly enhanced due to coordination bonding between the TiO 2nanofillers and the –COOH end groups of the polymers,as evidenced by the FT-IR spectral data.© 2010 Published by Elsevier B.V.1.IntroductionPolyurethane (PU)can be tailored to have a variety of use-ful characteristics such as wear-resistance,oil-resistance,chemical corrosion-resistance,and rigidity,depending on its functional groups and soft/hard segments [1].Waterborne PU can be pre-pared by adding hydrophilic groups to the polymer chains,which finds applications in coatings and adhesives,mainly as a result of environmental regulations which aim to reduce organic solvent pollution.Inorganic/polymer nanocomposites are a relatively new class of materials.In general,as compared to conventional composites,the nanocomposites exhibit improved physical properties such as ther-mal,mechanical and barrier,due to the much stronger interfacial interactions between the nanofillers and polymer matrices [2–5].Different types of nanofillers are used depending on the purpose of the resulting nanocomposites –common examples are silica,clay,carbon nanotubes,and titanium dioxide (TiO 2)[6].TiO 2is an inter-esting nanofiller because it is not only capable of photocatalytic degradation of organic matter,but also it can retard photo-aging of PU through ultraviolet light absorption [7,8].Mechanical strengths of nanocomposites can be reduced by the agglomeration of nanoparticles,which results from the par-∗Corresponding author.Tel.:+864332732295;fax:+864332733071.∗∗Corresponding author.Tel.:+82632702312;fax:+82632702306.E-mail addresses:jinyz@ (Y.-Z.Jin),yosklear@jbnu.ac.kr (Y.-S.Lee).ticles’large surface areas.Agglomeration can be minimized by modification of the nanoparticles’surfaces to strengthen their interfacial interactions.Charpentier et al.[9]used a reversible addition-fragmentation transfer (RAFT)agent with a functional group ZC(S)SR,such as 2-{[butylsulfanyl]carbonothionyl }sulfanyl prpanoic acid (BCSPA),where Z and R are respectively an activa-tion of C S double bond and a leaving group.They modified TiO 2nanoparticles with BCSPA to obtaina hybrid where the –COOH group of BCSPA was linked to the TiO 2surface via three coor-dination modes,monodentate,chelating bidentate,and bridging bidentate [10].The use of the TiO 2-BCSPA hybrid along with a rad-ical initiator in the polymerization of acrylic acid allowed them to obtain TiO 2/poly(acrylic acid)nanocomposites where the polymers were grafted on well-dispersed TiO 2nanoparticles.The well-established RAFT polymerization mechanism is shown Scheme 1,where the Z and R groups of the ZC(S)SR type RAFT agent are eventually incorporated into the resulting polymer ends [11].In a previous study,emulsions of PU macromonomers end-capped with 2-hydroxyethyl acrylate (HEA)were successfully polymerized in the presence of a RAFT agent and a radical initiator [12].The results revealed that emulsion polymerization proceeded via a living polymerization mechanism.In this study,BCSPA was chosen as a bifunctional RAFT agent because it can coordinate to TiO 2nanoparticles and subsequently polymerize emulsions of PU macromonomers.TiO 2-BCSPA hybrids were first prepared by the method of Charpentier et al.[9],which were then used in in situ RAFT polymerization of PU macromonomer dispersions to obtain nano-TiO 2/PU emulsions.This paper describes the synthesis and brief characterization of such nano-TiO 2/PU emulsions.0300-9440/$–see front matter © 2010 Published by Elsevier B.V.doi:10.1016/j.porgcoat.2010.09.0132X.-C.Che et al./Progress in Organic Coatingsxxx (2010) xxx–xxxScheme 1.Proposed mechanism of the RAFT polymerization process,where M and P (P m ,P n ,P x and P y )denote a monomer and polymer chain,respectively.2.Experimental 2.1.MaterialsCarbon disulfide and triethylamine (TEA)were purchased from Tianjin Chemical Co.(China).2-{[(Butylsulfanyl)carbonothioyl]sulfanyl }propanoic acid (BCSPA),2-bromopropanoic acid,anatase nano-titanium dioxide (20–35nm),and other reagents were pur-chased from Aladdin Reagent Database Inc.(China).Anhydrous methanol,sodium hydroxide,and hydrochloric acid were pur-chased from Tianjin Shen Thai Chemical Co.(China).Poly(propylene glycol)-2000(PPG-2000),isophorone diisocyanate (IPDI),2-hydroxyethyl acrylate (HEA),2,2-dimethylolbutyric acid (DMBA),dibutyltin dilaurate (DBTDL),and azobisisobutyronitrile (AIBN)were obtained from Lookspoly Co.(South Korea).The TiO 2nanopar-ticles were functionalized with BCSPA by following the previously reported procedure [9].After filtration of the reaction mixture through a 0.05␮m polycarbonate membrane filter,the unreacted BCSPA was removed by washing with methanol,and unreacted2.2.Synthetic procedures of nano-TiO 2/PU emulsionsPU emulsions were prepared according to the previously used procedures shown in Scheme 2[12].Briefly,in a round-bottom flask equipped with a reflux condenser and a mechanical stir-rer,a mixture of IPDI (57.8g)and PPG-2000(200g)was heated to 80◦C for 3h,and cooled to 60◦C then added to it were DMBA (12.7g)and DBTDL (1.0g).The mixture was stirred for 2h,and then heated to 80◦C,followed by addition of HEA (6.6g).The resulting mixture was stirred further for 3h.Reac-tion progress was checked at 30min intervals by determining the concentration of free NCO groups [13].The PU prepolymer was neutralized with TEA (8.7g),followed by addition of deionized water and ethylene diamine (EDA,2.6g)to obtain a HEA-capped PU macromonomer (PUM,M n =4300)dispersion.The solid content ofScheme 2.Synthesis of nano-TiO 2/PU emulsions.the PUM dispersion was adjusted to 40wt%by addition of deionized water.AIBN (1.5g;0.01mol)and TiO 2-BCSPA (9.5g;0.03mol)were added to the PUM emulsion and mixed under ultrasonication.The resulting dispersion was mechanically stirred at 200rpm and 60◦C for 6h,to obtain the nano-TiO 2/PU emulsion (TiO 2/PU-3).For com-parison,PU-3was also prepared using AIBN and BCSPA in a molar ratio of 1:3,where only BCSPA was used as the RAFT agent instead of TiO 2-BCSPA ing the same method,TiO 2/PU-5,TiO 2/PU-7,and TiO 2/PU-10were prepared.The different numbers in PU-3,PU-5,PU-7,and PU-10indicate the molar ratios of AIBN to TiO 2-BCSPA (or BCSPA in the case of PU-3).The nano-TiO 2/PU emulsions were cast on silicon plates,dried at room temperature for 1week.The resulting polymer were further dried in a vacuum oven at 60◦C overnight.The TiO 2/PU films were washed with acetone in Soxhlet apparatus for 60h for FT-IR analysis and %conversion calculation.Neglecting the weight of AIBN,the %conversion of PUM to TiO 2/PU nanocomposite was calculated as follows:Conversion (%)=[W film /W emulsion ×0.4]×100where W film and W emulsion represent the weight of the dried film and of the emulsion used for the preparation of the film,respec-tively.X.-C.Che et al./Progress in Organic Coatings xxx (2010) xxx–xxx32.3.Cleaving grafted polymers from TiO 2nanoparticlesIn order to determine molecular weights of the polymers grafted on the TiO 2nanoparticles,the polymers were cleaved from the TiO 2nanoparticles,following a modified previously reported method [9].Briefly,500mL of each sample was mixed with 150mL 2M HCl and was stirred under reflux overnight,cooled to room tempera-ture,neutralized with NaHCO 3,and finally filtered.The resulting mixture of polymer and nano-TiO 2in tetrahydrofuran (THF)was refluxed for 3h,and filtered to remove the TiO 2nanoparticles,and dried in a vacuum oven.The dried polymer samples were used for NMR and GPC experiments.2.4.CharacterizationProton NMR spectra were obtained from a Swiss Bruker AV-300MHz NMR spectrometer,using TMS as an internal standard.FT-IR spectra were recorded using KBr pellets on a Shimadzu Cor-poration FI-IR1730instrument.Molecular weights of polymers were measured by gel permeation chromatography (GPC)with a Shimadzu GPC (LC-10Avp),using a RI detector referenced to monodisperse polystyrene standards and THF as the eluent at a flow rate of 1.0mL/min.Scanning electron microscopy (SEM)images were recorded using a Hitachi S3500N instrument.The tensile strengths and elongations at breaking of the TiO 2/PU nanocompos-ite films were measured with a universal test machine (Shimadzu AGS-J SES1000)at 25◦C with a crosshead speed of 50mm/min.Five specimens were tested for each nanocomposite,and an average value was taken for each data point.3.Results and discussion 3.1.SynthesisThe TiO 2nanoparticles were functionalized by following the reported procedure [9].The untreated TiO 2nanoparticles and yel-low TiO 2-BCSPA powders were dispersed in a mixture of ethyl acetate and water (1:1,v/v)under ultrasonication,and the resulting mixtures were allowed to stand for 2weeks.The TiO 2nanoparticles were suspended in the aqueous lower phase,and the TiO 2-BCSPA hybrid powders were well-dispersed in the ethyl acetate upper phase,as shown in Fig.1.This indicated that the hydrophilic TiO 2nanoparticles were successfully functionalized by BCSPA and becamehydrophobic.Fig.1.Photographs of (left)TiO 2-BCSPA and (right)TiO 2both dispersed in ethyl acetate (top phase)/water (bottom phase).T r a n s m i t t a n c e (%)Wavenumber (cm -1)Fig.2.FT-IR spectra of (a)PUM,(b)TiO 2-BT 3P,and (c)TiO 2/PU-3.The PUMs were prepared by a previously described method [12].The yellow solid TiO 2-BCSPA was added to the PUM emulsion and mixed under ultrasonication to form core-shell typed emulsion droplets of TiO 2-BCSPA coated with PUM.Consequently,AIBN,a hydrophobic radical initiator,was added and the resulting emul-sion was heated to 60◦C to obtain the nano-TiO 2/PU emulsions.The FT-IR spectra of TiO 2-BCSPA,PUM and TiO 2/PU-3films are shown in Fig.2.Several characteristic absorption peaks appear for each sample:peaks at 3332,2970,1716,and 1539cm −1resulted from the N–H,C–H,C O,and CO–NH groups,respectively [14].Of particular note is the absorption peak of PUM at 657cm −1which was shifted to 669cm −1in the FT-IR spectra of the TiO 2-BCSPA and TiO 2/PU-3films,indicating the presence of bridging or chelating coordination between the carboxyl groups of BCSPA and TiO 2[9].1H NMR spectra of PUM and TiO 2/PU-3are shown in Fig.3.InFig.3a,the proton resonance peaks in the range of 5.8–6.5ppm were assigned to vinyl protons (–CH CH 2)at the end of each PUM.This compares with the NMR spectrum of TiO 2/PU-3(Fig.3b)which did not show any vinyl proton peak in the same spectral range.This indicates that any possible unreacted HEA end-capped PUMs were almost completely removed by the purification steps.3.2.Molecular weights of cleaved polymersDuring the polymerization of TiO 2/PU-10,5mL aliquots were withdrawn from the reaction mixture at different times for GPCFig.3.1H NMR spectra of (a)PUM and (b)TiO 2/PU-3in CDCl 3.4X.-C.Che et al./Progress in Organic Coatings xxx (2010) xxx–xxxTable1Molecular weights and PDIs(from GPC)of polymers cleaved from TiO2/PU-10atdifferent reaction times.Reaction time(h)Conversion(%)M n(g/mol)M w(g/mol)PDI(M w/M n)12313,50019,100 1.4123827,20035,600 1.3147266,40084,900 1.28 692110,000125,000 1.14 892354,000376,000 1.06analysis.Polymers grafted on the TiO2nanoparticles were cleaved from them under acidic conditions,as described in Section2.The results of the GPC measurements are listed in Table1.The number-averaged molecular weight(M n)of the cleaved polymers increased linearly with reaction time,but the PDI value decreased from1.83 to1.06.Considering the PDI of PUM is2.14,the molecular weight distribution range narrowed considerably,clearly indicating that the polymerization of HEA end-capped PUM followed the living polymerization mechanism in the presence of the RAFT agent and AIBN.The GPC estimated molecular weights of the polymers in the various nano-TiO2/PU emulsions formed via RAFT polymerization for8h are listed in Table2.The PDI values decreased gradually to 1.06when the molar ratio of TiO2-BCSPA to AIBN increased from1:3 to1:10.This indicates that the molecular weight of the polymers became more uniform as the ratio increased,due to more efficient RAFT processes at higher molar ratios.The proposed RAFT polymerization mechanism(Scheme1)and the present experimental results led speculation that the TiO2-BCSPA hybrid was consumed by propagating radicals by a RAFT mechanism.Then the fragmented radicals reinitiated polymeriza-tion,resulting in new propagating radicals,subsequently taking part in the equilibrium established between the dormant poly-mer and active chains.The equilibration process allowed all the polymer chains with TiO2at their ends to grow in a uniform manner.Table3Tensile properties of PU-3and variousTiO2/PU nanocompositefilms.Sample Tensile strength(MPa)Elongation at break(%)PU-32190TiO2/PU-311180TiO2/PU-52293TiO2/PU-718330TiO2/PU-10112303.3.Mechanical properties of TiO2/PU nanocompositefilmsThe tensile strengths and elongations at breaking of the TiO2/PU nanocompositefilms were measured and listed in Table3.The ten-sile strengths of all the TiO2/PU nanocompositefilms are much greater than that of the PU-3film,except for elongation at breaking of the TiO2/PU-5film whose data are not at present fully understood and require more experiment.The significantly enhanced tensile strength of the nanocompositefilms is attributable to the strong adhesion between TiO2nanofillers and polymer matrix.These arise from the wrapping of the nanofillers with the polymer chains via bridging or chelating coordination between titanium atoms and the end groups of polymers,resulting in efficient transfer of the external force to the nanofillers from the polymer matrix.This method of fabrication producedfilms with superior mechanical properties when higher molar ratios of AIBN to TiO2-BCSPA were used.The nano-TiO2/PU emulsions were stored at room tem-perature for more than3months without precipitate being observed,indicating that the emulsions may be stable enough for practical applications.The TiO2/PU nanocompositefilms were semi-transparent,suggesting relatively good dispersion of TiO2 nanoparticles in the polymer matrices.SEM micrographs of the TiO2/PU-3and TiO2/PU-10film surfaces are shown in Fig.4,where average diameters of the nanoparticles observed in TiO2/PU-3 (Fig.4a)and TiO2/PU-10(Fig.4b)films are in a range of20–500nm. The nanoparticles appeared to be polymer chains grown from the TiO2-BCSPA hybrid surfaces.There was no significantly largeTable2Molecular weights and PDIs of PUM,PU-3,and polymers cleaved from various nano-TiO2/PU emulsions.Sample code Reaction time(h)Conversion(%)M n(g/mol)M w(g/mol)PDI(M w/M n)PUM8934,3009,200 2.14PU-3891466,000573,000 1.23TiO2/PU-389083,100152,000 1.83TiO2/PU-5891271,000344,000 1.27TiO2/PU-7892237,000279,000 1.18TiO2/PU-10892354,000376,0001.06Fig.4.SEM micrographs of(a)PU-3and(b)TiO2/PU-10films.X.-C.Che et al./Progress in Organic Coatings xxx (2010) xxx–xxx5aggregate in either image.This observation confirms that the TiO2 nanoparticles were well dispersed in the PU matrix,due to the coordination bonds between the TiO2particles and the polymer chains.4.ConclusionNano-TiO2/PU emulsions were successfully prepared via in situ RAFT polymerization of HEA-end capped PUM emulsions,using both AIBN and TiO2-BCSPA.With changing molar ratio of AIBN to TiO2-BCSPA from1:3to1:10,the PDI of the polymers cleaved from the TiO2nanoparticles decreased from1.83to1.06,which is smaller than that of PUM(2.14),due to the living polymerization of the macromonomers.SEM confirmed good dispersion of the modified TiO2nanoparticles in the polymer matrices.The tensile strengths of the nanocompositefilms were significantly enhanced,due to the coordination bonds between the TiO2and the PU chains.The nano-TiO2/PU emulsions were stable enough not to precipitate after at least3months.This synthetic method of nano-TiO2/PU emulsions appears excellent for controlling the molecular weight and distri-bution of PU as well as the dispersion of TiO2nanoparticles in the PU matrix.AcknowledgementsThis research was supported by the funds from Jilin Prov.S&T Department R&D Program20080307and Jilin Prov.S&T Depart-ment R&D Program200905100.References[1]G.Oertel,Polyurethane Handbook,Hanser,Munich,Germany,1985.[2]P.Pramanik,Bull.Mater.Sci.18(1995)819.[3]V.Sambhy,M.M.MacBride,B.R.Peterson,A.Sen,J.Am.Chem.Soc.128(2006)9798.[4]C.Artelt,H.J.Schmid,W.J.Peukert,Aerosol Sci.36(2005)147.[5]J.Israelachvili,Intermolecular and Surface Forces,Academic Press,London,2000.[6]J.Kasanen,M.Suvanto,T.T.Pakkanen,J.Appl.Polym.Sci.111(2009)2597.[7]D.K.Chattopadhyay,K.V.S.N.Raju,Prog.Polym.Sci.32(2007)352.[8]M.S.M.Mirabedini,J.Zohuriaan-Mehr,M.Atai,.Coat.65(2009)222.[9]B.Hojjati,R.Sui,P.A.Charpentier,Polymer48(2007)5850.[10]F.P.Rotzinger,J.M.Kesselman-Truttmann,S.J.Hug,V.Shkover,M.Gratzel,J.Phys.Chem.B108(2004)5004.[11]R.Bussels,C.Bergman-Göttgens,J.Meuldijk,C.Koning,Macromolecules37(2004)9299.[12]Y.-Z.Jin,Y.B.Hahn,K.S.Nahm,Y.-S.Lee,Polymer46(2005)11294.[13]HG/T2409-92,Determination of the concentration of free NCO groups in thepolyurethane prepolymer,The People’s Republic of China Chemical Standard.[14]M.L.Hsaing,C.H.Chang,M.H.Chan,D.Y.Chao,J.Appl.Polym.Sci.96(2005)103.。