Fabrication of block copolymer brushes on hollow sphere surface via reverse iodine

This article appeared in a journal published by Elsevier.The attached copy is furnished to the author for internal non-commercial research and education use,including for instruction at the authors institutionand sharing with colleagues.Other uses,including reproduction and distribution,or selling or licensing copies,or posting to personal,institutional or third partywebsites are prohibited.In most cases authors are permitted to post their version of thearticle(e.g.in Word or Tex form)to their personal website orinstitutional repository.Authors requiring further informationregarding Elsevier’s archiving and manuscript policies areencouraged to visit:/copyrightFabrication of block copolymer brushes on hollow sphere surface via reverse iodine transfer polymerizationLi-Ping Wang a ,b ,Li-Hua Dong a ,c ,Jing-Cheng Hao a ,⇑,Xin-Hu Lv b ,Wen-Zhi Li b ,Yu-Chao Li b ,Jin-Ming Zhen b ,Yu-Cheng Hao b ,Fei Ma baKey Laboratory of Colloid and Interface Chemistry,Shandong University,Ministry of Education,Jinan 250100,China bCollege of Materials Science and Engineering,Liaocheng University,Liaocheng 252059,China cSchool of Chemistry and Chemical Engineering,Taishan Medical University,Taian,Shandong 271000,Chinaa r t i c l e i n f o Article history:Received 1March 2011Accepted 1May 2011Available online 9May 2011Keywords:Polymer brushes Hollow sphere RITP‘‘Living’’characteristica b s t r a c tThe block copolymer brushes grafted from hollow sphere surface via reverse iodine transfer polymeriza-tion (RITP)were investigated in this work.A sufficient amount of azo initiator was introduced onto hol-low sphere surface firstly.Then the monomer methyl methacrylate (MMA)was polymerized via surface-initiated reverse iodine transfer polymerization (RITP)using azo group modified hollow sphere as initi-ator.The microstructure of the samples was characterized by FT-IR,1H NMR,respectively.Results indi-cated that the poly(methyl methacrylate)(PMMA)with end functionality of alkyl iodine group had grafted from hollow sphere surface.TEM observations showed that the average diameter of hollow core was central at 1.3–1.4l m and the average wall thickness increased from 103nm to 138nm and 172nm after grafting polymerization of MMA and Tb complex,respectively.The closely linear plots of molecular weight (M n )versus conversion,linear kinetic plots for the free polymer formed in solution and the ability to extend the chains by sequential addition of monomer indicated that the RITP was a controlled process with a ‘‘living’’characteristic.Crown Copyright Ó2011Published by Elsevier Inc.All rights reserved.1.IntroductionControlled/‘‘living’’free radical polymerization is one of the most effective routes to prepare well-defined polymers (predeter-mined molecular weight,narrow distribution,and tailored archi-tectures).The most prominent living free radical polymerization techniques are iniferter (initiation-transfer-termination)method [1],nitroxide-mediated polymerization (NMP)[2],atom transfer radical polymerization (ATRP)[3,4],reversible addition-fragmen-tation chain transfer (RAFT)[5,6],iodine transfer polymerization (ITP)[7],and reverse iodine transfer polymerization (RITP)[8–10].Iodine transfer polymerization (ITP)was discovered by Tatemoto in late 1970s [11],and studied with much interest since the mid-1990s.By using an initiating radical,iodofluorocom-pounds could enter in a controlled process,based on a degenera-tive transfer (DT).The ITP process was first applied to the copolymerization of vinylidene fluoride with hexafluoropropene and was rapidly extended to other common nonhalogenated vinyl monomers.However,ITP exhibits two important limitations [12,13].Firstly,in the initiation system,the chain transfer agents (such as 2-iodo-perfluoropropane,1-iodo-1-chloroethane and 1-phenylethyl iodide)used in ITP are unstable which is due to the weak C–I bond and thus lead to inconvenience upon storage.Secondly,ITP of monomers involving tertiary propagating radicals (such as methacrylates with 1-phenylethyliodide as transfer agent)was not successful because it would require more activated iodoal-kyl compounds such as ethyl 2-iodo-2-methylpropionate,which are inherently even more unstable.To overcome these drawbacks,Tatemoto and Nakagawa [11]reported a new process called re-verse iodine transfer polymerization (RITP)which is based on a di-rect reaction of radicals with molecular iodine I 2.In this way,the reversible chain transfer agent is generated in situ in the reaction mixture,thus avoiding its preliminary synthesis and storage.At present,the study of RITP is relative few and mostly focuses on the investigations of the mechanism.In this work,we wish to report the first successful use of RITP technique to synthesize well-controlled polymer brushes from inorganic substrate.Hollow sphere is a kind of inorganic microspheres with abundant hydroxyl on its surface.It has large inner void,low density and high surface area which account for its actual and potential applications in imaging [14],wave absorber [15],acoustic [16],sensing devices [17],controlled drug-delivery carriers [18]and so on [19,20].Therefore,hollow sphere appears very attractive to be used as the inorganic substrate.The overall synthesis route is listed in Scheme 1.0021-9797/$-see front matter Crown Copyright Ó2011Published by Elsevier Inc.All rights reserved.doi:10.1016/j.jcis.2011.05.003⇑Corresponding author.E-mail address:wangliping5@ (J.-C.Hao).2.Experimental2.1.ReagentsUnless otherwise indicated,chemicals were obtained from commercial suppliers and used as received.The monomer methyl methacrylate(MMA)(AR,Shanghai Chemical Reagent Plant)was washed with10%NaOH and ion-free water,stirred over CaH2 and distilled under reduced pressure prior to use.4,40-azobis (4-cyanopentanoic acid)(ACPA)was purchased from Sigumas Co. The hollow sphere and the4,40-azobis(4-cyanopentanoic acid chloride)(ACPAC)were synthesized following a previously de-scribed procedure respectively[21,22].2.2.InstrumentationThe structure of samples was characterized by Nicolet-5700 Fourier transform infrared spectroscopy(FT-IR)from400cmÀ1to 4000cmÀ1by the KBr pellet methods.The molecular weights and polydispersities(PDI)of the polymers were determined via an alli-ance GPCV2000(Waters,USA)gel permeation chromatographer (GPC)using THF as the eluent at aflow rate of1.0mL minÀ1and operated at40°C.The compositions and the structures of the poly-mers obtained by RITP of MMA were determined by Varian Mer-cury Plus400MHz nuclear magnetic resonance instrument,using CDCl3as a solvent and tetramethylsilane(TMS)as internal refer-ence.Transmission electron microscopy(TEM)analysis was per-formed on a JEOL JEM-1400transmission election microscope. The elemental analyses(EA)for C,H,and N were performed on a GmbH VarioEL elemental analyse system.Fluorescence excitation and emission spectra were recorded on an Edinburgh Instruments FLS920spectrofluorimeter from a450W stable xenon lamp.The thermal decomposition behavior of the materials was examined by means of thermogravimetry(TG)with a heating rate of 10°C minÀ1in the nitrogen atmosphere on a model STA449C simultaneous DSC–TGA(Netzsch Instruments,Germany).L.-P.Wang et al./Journal of Colloid and Interface Science361(2011)400–4064012.3.Synthesis of azo groups immobilized hollow sphereThe target compound was synthesized according to our previ-ous work[23].Typically,hollow sphere(1g),(3-aminopropyl)tri-ethoxysilane(KH-550)(1mL)and toluene20mL were mixed and heated at80°C for11h under nitrogen atmosphere.The mixture wasfiltrated and thefiltrate was washed with toluene to remove excess compound KH-550to give amino-functionalized hollow sphere(Hollow sphere-NH2),which was dried at25°C in vacuum. Elem.Anal.Calcd.(%):C,18.6;H,2.9;N,2.8.Hollow sphere-NH2(1g)was dispersed in dichloromethane (10mL)and0.2g ACPAC was added.The reaction mixture was stir-red at room temperature for6h and stopped to remove the excess ACPAC byfiltration.The whitefiltrate was washed thoroughly with mixed solvent of ethanol and water(V/V=1:1),ethanol and ether to afford azo groups immobilized hollow sphere.Elem.Anal.Calcd. (%):C,19.6;H,3.1;N,4.5.The amount of surface immobilized azo group can be tenta-tively estimated(N@N/hollow sphere=I c)by the following relation:I c¼WN@NÀWN14Â2ð1Þwhere W N@N and W N is the nitrogen weight(per centum)of azo groups immobilized hollow sphere and amino-functionalized hol-low sphere resulted from the element analysis data,respectively. The estimated I c for hollow sphere is0.60mmol gÀ1.2.4.Synthesis of Tb complexSalicylic acid(1.10g,8mmol)and MMA(1.42ml,4mmol)were dissolved in anhydrous ethanol.Terbium nitrate(1.38g,4mmol) was added under stirring and refluxing for6h resulting in the Tb complex(Scheme2).2.5.PMMA brushes growing from hollow sphere surface by RITP methodA mixture of azo groups immobilized hollow sphere(0.11g,azo group content:$0.066mmol)and cyclohexanone(6mL)in the flask was ultrasonically agitated for30minutes and then iodine (0.008g,0.031mmol)and degassed MMA(6mL,0.056mol)were added.After three freeze–thaw-pump cycles,theflask was heated at70°C in an oil bath.The polymerization was conducted in the dark under nitrogen atmosphere with magnetic stirring.After reac-tion,the colloidal stability of the composite hollow spheres isfine because of the kind compatibility of PMMA grafted from hollow sphere surface and the PMMA formed in solution.Theflask was cooled in an ice bath and the mixture was diluted with tetrahydro-furan(THF).The hollow sphere substrate with grafted PMMA(hol-low sphere-g-PMMA)was separated by centrifugation.The crude product was extracted with THF and dried at40°C in vacuum to afford hollow sphere-g-PMMA.2.6.Diblock copolymer brushes on the hollow sphere surfaceAs shown in Scheme1,the synthesis of diblock copolymer brushes was carried out on the hollow sphere-g-PMMA substrate. The procedures were similar to those used for synthesis of the hol-low sphere-g-PMMA hybrid material,except using hollow sphere-g-PMMA hybrid particles as macro chain transfer agent and Tb complex as the second monomer via a conventional ITP process.A general procedure is as follows:The mixture of hollow sphere-g-PMMA hybrid particles(0.02g)and16mL ethanol inflask was ultrasonically agitated for30min and then the AIBN(0.0066g, 0.04mmol)and Tb complex(4mmol)were added.Theflask was subsequently evacuated andflushed with nitrogen.The polymeri-zation was carried out under nitrogen in a70°C oil bath for16h, stopped byfiltration and washing with excess THF to remove un-grafted Tb complex.Finally,the resulting brown powder was dried at40°C in vacuum.3.Results and discussion3.1.Surface-initiated RITP on the functionalized hollow sphere substrateThe hollow sphere-g-PMMA hybrid materials were prepared using RITP method.The chemical state and the topography of the402L.-P.Wang et al./Journal of Colloid and Interface Science361(2011)400–406hollow sphere-g-PMMA hybrid materials were probed by FT-IR,1H NMR and TEM,respectively.Fig.1shows the infrared spectrum of hollow sphere before(Fig.1a)and after(Fig.1b and c)grafting polymerization.In Fig.1a,the peak at3414cmÀ1was attributed to the O–H stretching vibrations of hydroxyl group in hollow sphere,the peak at1087cmÀ1was assigned to the Si–O–Si stretch-ing vibration and the peak at962cmÀ1was a result of Si–OH vibra-tion.The peaks at3000–2800cmÀ1resulted from the C–H stretching vibrations and the peak at1619cmÀ1came from C@C stretching vibrations of benzene ring,which was a result of the small quantity of undissolved polystyrene in hollow sphere.As for Fig.1b,the disappeared peaks at3000–2800cmÀ1and 1619cmÀ1was due to the polystyrene in hollow sphere dissolved in cyclohexanone during reaction.The absorption peak at 2957cmÀ1came from C–H stretching vibration and the peak at 1730cmÀ1represented the C@O stretching vibration of PMMA which indicated the successful grafting between hollow sphere and PMMA.After grafting copolymerization for16h on the hollow spheres-g-PMMA substrate(Fig.1c),several changes were ob-served,the new peak at1615cmÀ1belonged to C@C stretching vibrations of benzene ring in salicylic acid and the strong new peak at1399cmÀ1means COOÀof salicylic acid complexes with Tb3+. The existence of benzene ring and COOÀgives the direct proof for successful grafting of Tb complex on hollow spheres-g-PMMA substrate.The ability to extend the chains by sequential addition of monomer indicated that the process of grafting polymerization on hollow spheres surface is a controlled/‘‘living’’polymerization.In order to get more information about the microstructures of hollow spheres,TEM was performed.The hollow nature of the microspheres was observed in the contrast between the thin dark edge and pale center in TEM observations,as shown in Fig.2.From the TEM images,we can alsofind that the dark edge become thick-er with the process of grafting polymerization.The average diam-eter of the hollow spheres before grafting polymerization centered at1.3–1.4l m with an average wall thickness of103nm indicated by the size distribution histogram.After grafting polymerization of MMA and Tb complex(Fig.2b and c),the average wall thickness in-creased to138nm and172nm,respectively,according to the size distribution histogram,which corresponded to the successful grafting process.L.-P.Wang et al./Journal of Colloid and Interface Science361(2011)400–4064031H NMR spectrum of PMMA formed in solution was performed to verify the reverse iodine transfer polymerization mechanism re-ported previously(Fig.3)[12,13].The signal at2.9ppm is assignedto the methylene(–CH2–C(CH3)(CO2CH3)–I)in the b position of io-dine(c)at the chain end.The signals centered at0.84,1.81,and 3.60ppm are attributed to the methyl–CH3(b),the methylene –CH2–(a),and the methoxy–OCH3(d)groups of the monomer units in PMMA chain respectively.The1H NMR results are consis-tent with chemical shifts of[–CH2–C]nÀ1–(CH3)(CO2CH3)–CH2–C (CH3)(CO2CH3)–I given in the literature[13].3.2.Controlled graft polymerization of PMMA on hollow sphere surfaceA living polymerization is characterized by narrow polydisper-sity products,linear increase in molecular weight with conversion, and the ability to extend the chains by sequential addition of monomer.Table1summarizes the results of all polymerization under various experimental conditions.They offer the narrowTable1Polymerization of methyl methacrylate(MMA)by reverse iodine transfer polymerization(RITP)performed at70°C.Sample Time(h)M n a(Â10À4g molÀ1)M w a(Â10À4g molÀ1)PDI b Conv.(%)Polymer content(wt%)HS-g-PMMA-4h49.7214.78 1.5229.5713.4 HS-g-PMMA-6h612.3516.41 1.3248.0216.2 HS-g-PMMA-8h812.7516.29 1.2451.2119.3 HS-g-PMMA-12h1214.6317.88 1.2280.6920.1 HS-g-PMMA-b-Tb complex-16h c16 5.75 6.44 1.1248.0428.4a Free polymer formed in the solution,obtained by GPC with standard polystyrene as reference.b PDI:molecular weight distribution is calculated from Mw/M n.c HS-g-PMMA-b-complex-16h:using HS-g-PMMA-12h as macroinitiator.polydispersity,the molecular weight increased with the polymeri-zation time.To prove the living nature of reverse iodine transfer polymerization fatherly,the relationship plot between molecularweight and monomer conversion and the kinetic plots are drawed according to Table1.In Fig.4,the linear relationship between the molecular weight of the free polymer formed in the solution and the monomer con-version was observed.Although the exact molecular weight of polymer grafted on the inorganic substrate is not known,its molec-ular weight is expected to be proportional to that of the polymer formed in the solution[24].The closely linear increase in molecu-lar weight with monomer conversion indicated that the process of surface-initiated reverse iodine transfer polymerization of MMA is controlled.The kinetic plots of Ln[M]0/[M](where[M]is the concentration of monomer)versus polymerization time was shown in Fig.5.The linearfirst-order kinetics plot suggested that the radical concentra-tion was constant throughout the polymerization.However,there exist about a two hours induction period.3.3.The ability to form block copolymer brushes on hollow sphere substrateAccording to the mechanism of RITP,the polymer prepared via the RITP-mediated process has an end functionality of alkyl iodine group.Thus,the graft chains prepared in this way on hollow sphere substrate can be used as macroinitiator for subsequent block polymerization or further functionalization.The Tb complex was chosen for the block copolymerization as the Tb complex repeat unit contains MMA group,which can serve as a monomer in the following iodine transfer polymerization(ITP).After the graft copolymerization proceeded for16h on the hol-low sphere-g-PMMA substrate in an ethanol medium,the diblock polymer brushes grafted from hollow sphere surface(hollow sphere-g-PMMA-b-Tb)came into being.Thefluorescence proper-ties of the sample were determined by spectrofluorimeter.Fig.6 shows the excitation spectra of hollow sphere-g-PMMA-b-Tb com-plex.The excitation spectrum of the resulting hybrid materials was obtained by monitoring the emission of Tb3+ions at548nm.The excitation spectrum is dominated by a broad band from290to 330nm in narrow region and the maximum peak is at310nm, which can be attributed to the characteristic absorption of the lan-thanide complexes arising from the efficient transition based on the conjugated double bonds of the aromatic cycle of salicylic acid ligand.These excitation spectra bands are the effective absorption for the luminescence of Tb3+.As a result,the strong green lumines-cence was observed(see Fig.7),indicating that the effective energy transfer took place between the aromatic ligand and the chelated Tb3+ions.In the emission spectrum,there are four characteristic fluorescence emission bands associated with Tb3+.The band at 489nm is assigned to the5D4?7F6electron transition of Tb3+.L.-P.Wang et al./Journal of Colloid and Interface Science361(2011)400–406405The band at around544nm is associated with the5D4?7F5tran-sition of Tb3+,and the bands at about583nm and621nm corre-sponded to the5D4?7F4and5D4?7F3electron transition of Tb3+respectively.Thefluorescence properties analysis and the EDS results conformed that the Tb complex has grafted from the hollow sphere-g-PMMA substrate successfully,which indicated that most of the PMMA chains were living and took part in the de-sired graft copolymerization.3.4.The grafted polymer content of the hybrid materialsFig.8displays the TGA curves of hollow sphere,HS-g-PMMA and HS-g-PMMA-b-Tb complex.The data show three loss zones associated with the evaporation of excess H2O molecules under 100°C(zone I),organic groups on hollow sphere(zone II)and the inorganic salts is remained(zone III).The organic content can be determined from the difference in weight loss between100 and500°C.The polymer contents calculated for the hybrid mate-rial are also summarized in Table1.The value obtained are rela-tively high(13.5–20.1wt%)which accords with the relatively high grafting rate.The grafted polymer content(calculated from TGA data)vs polymerization time was plotted as illustrated in Fig.9.The grafted polymer content exhibited a nearly linear in-crease of polymerization pared with the weight loss curves of the HS-g-PMMA,the loss ratio of HS-g-PMMA-b-Tb com-plex at zone II is larger and the quantity of last remains is smaller than that of the HS-g-PMMA.The calculated organic content of HS-g-PMMA-b-Tb complex(using HS-g-PMMA-12h as macroinitiator) is28.4wt%which is larger than that of HS-g-PMMA-12h,20.1wt%. That is,the Tb complex grafted on hollow sphere is about8.3wt%. The Tb3+content in the hollow spheres can be estimated by the following relation:Tb3þwt%¼M TbTb complexÂ8:3wt%¼159Â8:3wt%¼2:5wt%ð2Þwhere M Tb and M Tb complex is the molecular weight of Tb and Tb complex respectively.The calculated Tb3+wt%in the hollow spheres is about2.5wt%.4.ConclusionThe optical-functional diblock copolymer brushes grafted from hollow sphere surface were synthesized by surface-initiated re-verse iodine transfer polymerization(RITP).After immobilization of the azo initiator,PMMA chains were successfully grafted from the surface of hollow sphere.Moreover,the PMMA grafted from hollow sphere surface had the ability to extend the chains by sequential addition of Tb complex,illustrating the living nature of the polymerization process.TEM images clearly revealed the hollow structure of the hollow spheres and the increase of the average wall thickness with the grafting polymerization.Fluores-cence spectra confirmed that the PMMA-b-Tb complex grafted hol-low sphere hybrid materials exhibited strongfluorescence properties.This work presented a new method to synthesize opti-cal-functional hybrid material with controlled molecular weights and‘‘well-defined’’structures,which may extend potential appli-cations of hollow sphere and RITP.AcknowledgmentThis research was supported by scientific research start up fund for doctor of Liao-Cheng University.References[1]T.Sato,S.Tsuji,H.Kawagauchi,Ind.Eng.Chem.Res.47(2008)6358–6361.[2]Y.K.Chong(bill),F.Ercole,G.Moad,E.Rizzardo,S.H.Thang,A.G.Anderson,Macromolecules32(1999)6895–6903.[3]K.Matyjaszewski,J.Xia,Chem.Rev.101(2001)2921–2990.[4]C.Y.Lin,M.L.Coote,A.Gennaro,K.Matyjaszewski,J.Am.Chem.Soc.130(2008)12762–12774.[5]S.Liu,K.D.Hermanson,E.W.Kaler,Macromolecules39(2006)4345–4350.[6]P.He,L.He,Biomacromolecules10(2009)1804–1809.[7]C.Boyer,D.Valade,L.Sauguet,B.Ameduri,B.Boutevin,Macromolecules38(2005)10353–10362.[8]J.Tonnar,croix-Desmazes,B.Boutevin,Macromolecules40(2007)186–190.[9]J.Tonnar,croix-Desmazes,B.Boutevin,Macromolecules40(2007)6076–6081.[10]B.N.Patra,D.Rayeroux,croix-Desmazes,React.Funct.Polym.70(2010)408–413.[11]M.Tatemoto,T.Nakagawa,Daikin Kogyo Co.,Ltd.,Japan.DE.2729671,1978.[12]croix-Desmazes,R.Severac,B.Boutevin,Macromolecules38(2005)6299–6309.[13]C.Boyer,croix-Desmazes,J.J.Robin, B.Boutevin,Macromolecules39(2006)4044–4053.[14]Q.Peng,Y.Dong,Y.Li,Angew.Chem.Int.Ed.42(2003)3027–3033.[15]M.Han,Y.Ou,L.Deng,J.Magn.Magn.Mater.321(2009)1125–1129.[16]M.Y.Shatalov,S.V.Joubert,C.E.Coetzee,I.A.Fedotov,J.Sound Vib.322(2009)1038–1047.[17]Z.Wu,M.Zhang,K.Yu,S.Zhang,Y.Xie,Chem.Eur.J.14(2008)5346–5352.[18]S.H.Im,U.Jeong,Y.N.Xia,Nat.Mater.4(2005)671–675.[19]X.He,W.Yang,L.Yuan,X.Pei,J.Gao,Mater.Lett.63(2009)1138–1140.[20]C.Oh,Y.G.Lee,C.U.Jon,S.G.Oh,Colloid Surface A337(2009)208–212.[21]M.Chen,L.Wu,S.Zhou,B.You,Adv.Mater.18(2006)801–806.[22]G.Zhai,W.H.Yu,E.T.Kang,K.G.Neoh,C.C.Huang,D.J.Liaw,Ind.Eng.Chem.Res.43(2004)1673–1680.[23]L.P.Wang,W.Z.Li,L.M.Zhao,C.J.Zhang,Y.D.Wang,L.L.Kong,L.L.Li,Mater.Res.Bull.45(2010)1314–1318.[24]W.H.Yu,E.T.Kang,K.G.Neoh,Ind.Eng.Chem.Res.43(2004)5194–5202.406L.-P.Wang et al./Journal of Colloid and Interface Science361(2011)400–406。