乳液聚合制备二氧化硅-PMMA核壳结构纳米粒子Silica-PMMA core-shell and hollow nanospheres

Colloids and Surfaces A:Physicochem.Eng.Aspects277 (2006) 145–150Silica-PMMA core-shell and hollow nanospheresKai Zhang,Linli Zheng,Xuehai Zhang,Xin Chen,Bai Yang ∗Key Lab of Supramolecular Structure and Materials,College of Chemistry 130012,Jilin University,2699#Qianjin Avenue,Changchun,PR ChinaReceived 8November 2005;received in revised form 15November 2005;accepted 17November 2005Available online 18 January 2006AbstractMonodispersed silica-polymer core-shell nanospheres (CSNs)were prepared by emulsion polymerization.To coat the cores by polymer,silica nanoparticles were modified by 3-(trimethoxysilyl)propyl methacrylate (MPS).The thicknesses of polymer shells were found to be dependent on the amount of monomer and grafted silica nanoparticles,the concentration of emulsifier and the sizes of grafted silica nanoparticles,and the morphologies of CSNs were affected by the kind of monomer.The formation mechanism of SiO 2-PMMA CSNs was speculated.In addition,we investigated the formation of hollow polymer nanospheres.© 2005 Elsevier B.V. All rights reserved.Keywords:Core-shell;Emulsion polymerization;Hollow;Nanospheres1.IntroductionDuring the past decades,there have been a lot of works devised with the preparation of core-shell nanospheres (CSNs)because of their excellent optical,electrical,thermal,mechan-ical,electro-optical,magnetic and catalytic properties [1–5].CSNs display interesting functions and characters,which come from different functional components in core and shell materials.Furthermore,CSNs provide a facile platform for the fabrication of hollow spheres or capsules by the removal of template cores.These CSNs and hollow capsules can open up potential applica-tions in catalysis,controlled delivery,artificial cells,light fillers,low dielectric constant materials and photonic crystals [6–9].Various facile or clever procedures have been developed to prepare these CSNs and hollow spheres,including tem-plate directed self-assembly [10,11],template directed living polymerization [12,13],template directed aggregations [14–16],copolymerization of hydrophobic and hydrophilic monomer [17]and core-shell emulsion polymerization [18].Using silica spheres as template cores to prepare silica-polymer CSNs is well known in the field of the inorganic–organic CSNs.Silica-polymer CSNs are broadly used in research on the physical performances of composite materials,pho-∗Corresponding author.Tel.:+864315168478;fax:+864315193423.E-mail address:yangbai@ (B.Yang).tonic crystals and antireflection film.Silica-polymer CSNs with various morphologies could be synthesized by physic-ochemical or chemical processes.After Bourgeat-Lami pre-pared silica-polystyrene (SiO 2-PS)core-shell microspheres by dispersion polymerization [19],other polymerization proce-dures involving emulsion,miniemulsion,and atom transfer radical polymerization have been tried to realize the coat-ing of silica spheres with polymer shells [20–24].Recently,Asher et al.have successfully synthesized monodispersed SiO 2-polymer core-shell spheres in submicrometer via the dis-persion polymerization,and then used them as the building blocks for the fabrication of photonic crystals [25].How-ever,the preparation of monodispersed SiO 2-polymer core-shell spheres in nanoscale is still an attractive work because of their potential applications in photonic crystals and antireflection film.In this paper,we report the preparation of 131–225nm silica-polymethyl methacrylate (SiO 2-PMMA)CSNs with 51–195nm cores by an emulsion polymerization.We have investigated the effects of several reaction parameters on the morphologies and sizes of SiO 2-polymer CSNs,such as,different monomers (MMA and St),concentration of emulsifier,amounts of grafted silica and monomer.And then,the formation mechanism of SiO 2-polymer CSNs was speculated and a suitable system for preparing silica-polymer CSNs was confirmed.Finally,hollow polymer nanospheres were obtained via an etching process using a solvent.0927-7757/$–see front matter © 2005 Elsevier B.V. All rights reserved.doi:10.1016/j.colsurfa.2005.11.049146K.Zhang et al./Colloids and Surfaces A:Physicochem.Eng.Aspects 277 (2006) 145–1502.Experimental part2.1.MaterialsTetraethyl orthosilicate(TEOS),absolute ethanol and ammo-nium hydroxide were analytical grade and used as received. 3-(trimethoxysilyl)propyl methacrylate(MPS),styrene(St)and methyl methacrylate(MMA)were distilled under reduced pres-sure before used.4,4 -isopropylidenediphenol bimethacrylate (BV A)was used as cross-linker and prepared in our laboratory. Potassium persulfate(K2S2O8)and other agents of analytical grade were utilized without further purification.Deionized water was applied for all polymerization and treatment processes. 2.2.Synthesis of SiO2-polymer CSNsThe monodispersed silica nanospheres grafted by MPS with average diameters ranging from51to195nm were prepared in ethanol according to the St¨o ber method[27]at ambient tem-perature,and the process was narrated in our prevenient paper [26].SiO2-polymer CSNs were synthesized using an emulsion polymerization.In the typical case of preparation for SiO2-PMMA CSNs,0.7g of grafted silica spheres(111nm in diame-ter)dispersed in5ml of ethanol by ultrasonic for30min,0.019g of sodium dodecyl benzene sulfonate(SDBS)as emulsifier, 0.24g of NaHCO3as buffer agent,100ml of water as the disper-sion medium,0.04g of KPS as initiator and4ml of MMA with 0.16g of BV A were added into a three-neckflask with a machine stirring(300rpm).The polymerization was then carried out in an atmosphere of nitrogen at80◦C for10h,and then the prod-uct was centrifuged by various speeds and time according to the different sizes to remove the PMMA nanospheres without SiO2 (for example,159nm SiO2-PMMA CSNs with111nm cores were centrifuged by15000rpm for6min).SiO2-PMMA CSNs were obtained.2.3.Preparation of hollow polymer nanospheresSolventfilled hollow polymer nanospheres were obtained after dissolving the SiO2cores using hydrofluoric acid(HF) as the etching solvent.Typically,Silica cores in SiO2-polymer CSNs were etched by a40wt.%HF aqueous solution at mod-erate stirring(about50rpm)within5min.The hollow polymer nanospheres can be obtained.2.4.Extraction of SiO2-polymer CSNsThe emulsion of SiO2-polymer CSNs were dried in vacuum oven under45◦C,then1.0g of the SiO2-polymer CSNs solid were dispersed in40.0ml solvent(toluene for PS and butyl acetate for PMMA)under vigorous magnetic stirring(about 200rpm)for1h at ambient temperature,and then they were centrifuged by various speeds and time according to the differ-ent sizes,and repeated the redispersion and centrifuged cycle until polymer remained in solvent after centrifuging could not be detected by FTIR.2.5.CharacterizationThe29Si solid NMR spectrum was characterized by Varian Infinity plus400spectrometer.FTIR spectra were measured in the wave number range4000–400cm−1at a resolution of 4cm−1using a Nicolet Avatar360FTIR spectrophotometer. The morphology of grafted silica,CSNs and hollow poly-mer nanospheres were investigated by transmission electron microscopy(TEM)using a JEOL JEM2010at an accelerator voltage of200kV.The diameter and size distribution of grafted silica and CSNs were determined by Zetasizer3000HS and TEM image.3.Results and discussion3.1.Effect of the monomerMonodispersed silica-polymer CSNs were obtained through emulsion polymerization of monomer and grafted silica parti-cles,as the preparation of SiO2-PS CSNs in our previous report [26].The silica particles were modified by C C bonds on their surfaces and used as templates in the polymerization process. Fig.1shows the TEM images of SiO2-PMMA(A)and SiO2-PS (B)CSNs.It is clearly seen from Fig.1that the morphologies of SiO2-PMMA and SiO2-PS CSNs are different.The SiO2core lies in the center of SiO2-PMMA CSN,while for SiO2-PS CSNs, the SiO2core dose not lie in the center of SiO2-PS CSN.Even if the silica cores with other diameters and cross-linking reagent with different amount were used,we could only get the similar results.We believed that the different morphologies of SiO2-polymer CSNs result from the different formation mechanisms of polymer shells.We have investigated the formation of the shells of SiO2-PS CSNs,which consists of two stages[26]:the polymerization of St with MPS on the grafted silica,and the physical cross-linking of twisted and adsorbed polystyrene polymerized in the further emulsion polymerization with polystyrene layer copoly-merized with MPS.In order to prove the formation of the shells of SiO2-PMMA CSNs,we characterized the content of SiO2 in SiO2-PMMA CSNs and SiO2-PMMA CSNs extracted with butyl acetate by element analysis,the PMMA chains adsorbed and twisted on SiO2-PMMA CSNs will be removed when the CSNs are extracted by solvent(here,the PMMA is not cross-linked).We found the weight proportion of polymer shell decreased32%(while for PS,the data is79%),which indicates that only a few of PMMA chains in shells are formed through adsorption and twist,and most of the PMMA chains in shells are linked on the SiO2surface.So we believe that the poly-mer shells of SiO2-PMMA CSNs were mostly formed by the copolymerization of MMA and MPS on the surfaces of grafted SiO2.In emulsion polymerization system,compared with St,the solubility of MMA in water is higher and the reaction activity of the carbon double bond of MMA is stronger.In our polymer-ization system,because of the addition of ethanol,the solubility of MMA became much higher and the polymerization of MMA in aqueous phase is easier than the copolymerization of MMAK.Zhang et al./Colloids and Surfaces A:Physicochem.Eng.Aspects 277 (2006) 145–150147Fig.1.TEM images of SiO2-PMMA CSNs(A)and SiO2-PS CSNs(B).with MPS grafting on silica spheres.In the polymerization,part of MMA dissolved in aqueous phase whereas others formed monomer minidroplets,and then MMA molecules were released from the monomer minidroplets when the polymerization was processing.A few of MMA dissolving in aqueous phase copoly-merized with MPS on the surfaces of silica cores to form PMMA shells and coat silica cores,and most of MMA dissolving in aqueous phase rapidly polymerized to form PMMA oligomers. Most of PMMA oligomers entered the emulsifier micelles and formed the PMMA colloids without silica cores,whereas a few of them were adsorbed and twisted on the shells of SiO2-PMMA CSNs.Finally,a mixture of the PMMA colloids without silica (the weight proportion to MMA could vary from5%to87%) and SiO2-PMMA CSNs was obtained.We separated the SiO2-PMMA CSNs from the mixture via centrifugation by various speeds and time according to the different sizes to remove the PMMA nanospheres without SiO2.SiO2-PMMA CSNs with symmetrical structures were syn-thesized for the formation mechanism of shells of SiO2-PMMA CSNs.While for SiO2-PS CSNs,because the polarity of St is weak,so the wetting between the surfaces of grafted silica and polystyrene is weak,SiO2-PS CSNs with dissymmetrical struc-tures were obtained.At the same time,because the shells of SiO2-PS CSNs were mostly formed by adsorption and twist of PS chains(about79%),the morphologies of SiO2-PS CSNs were also affected by the shearing stress coming from the agitation in an emulsion polymerization.Further experiments also proved the formation of shells of SiO2-polymer CSNs through changing the amount of emulsifier, monomer and the size of cores,etc.3.2.Effect of the concentration of emulsifierIt is well known that the content of emulsifier plays an impor-tant role in the emulsion polymerization.Tofind an appropriate concentration of emulsifier to prepare the SiO2-polymer CSNs, we tried a number of experiments and observed the results by the characterization of TEM.As shown in Fig.2,the sizes of SiO2-polymer CSNs decrease with the increase of emulsifier, but the affection of emulsifier for SiO2-PMMA and SiO2-PS CSNs is different in detail.The shells thicknesses of the SiO2-PMMA CSNs decreased from30to9nm when emulsifier SDBS increased from0.0002 to0.0018g/ml(Fig.2A).By TEM,we noted that the num-ber of PMMA nanospheres without silica cores increased with the increase of emulsifier,but most of SiO2-PMMA CSNs had a single silica core,and the number of colloids with two or more cores did not increase.This result was coincident with the formation mechanism of SiO2-PMMA CSNs spec-ulated by us.The increase of emulsifier lead to form more new PMMA micelles in system,because the polymerization of MMA in water is easier than the copolymerization of MMA with MPS grafting on silica spheres,more MMA will polymer-ize and form PMMA colloids without SiO2,and the monomer copolymerizing with MPS on the surface of grafted silica will decrease,and then the shells of SiO2-PMMA CSNs will become thinner.Here,the diameter and amount of grafted silica nanoparticles are80nm and0.7g,respectively,and the amount of MMA is 4.0ml.So,we can obtain SiO2-PMMA CSNs with different thicknesses shells when we change the concentration of SDBS148K.Zhang et al./Colloids and Surfaces A:Physicochem.Eng.Aspects277 (2006) 145–150Fig.2.The relation plot between the content of emulsifier (SDBS)and the sizes of SiO 2-polymer CSNs (the data from TEM images).Here,the polymer is PMMA (A)and PS (B),respectively.in a broad range.However,it is difficult to do this for SiO 2-PS CSNs.Considering the SiO 2-PS CSNs,it can be found by TEM that the colloid particles without silica increase with the increase of emulsifier because the empty micelles increase,and the col-loid particles with several cores increase and the stability of the emulsion synthesized decreased with the decrease of emulsifier.We find 0.00024–0.00032g/ml SDBS is the appropriate amount of emulsifier in the synthesis system in order to achieve col-loid particles with a single core,and it is lower than the critical micelle concentration (CMC)of SDBS (the CMC of SDBS is 0.00042g/ml in 60◦C in water.).The result is accordance with the formation mechanism of SiO 2-PS CSNs.The concentration of emulsifier has an obvious affection for the PS shells formed by adsorption and twist,SiO 2-PS CSNs can be obtained in a narrow concentration range of emulsifier.3.3.Effect of the amount of monomerFig.3shows the relation plot between the amount of monomer and the sizes of polymer shell.It indicates a linear rela-tion between the thickness of polymer shell and the amount of monomer in a definite range.PS shell of 140nm can be obtained when the content of St is 13.0ml (Fig.3B).Beyond 13.0ml,although we continued adding St,the diameter of spheres no longer varied linearly and a great more coagulation appeared because of the lack of emulsifier [26].The change of PMMA shell thickness shows a similar result (Fig.3A),the thickness of PMMA shell initially increased from 0to 43nm as the MMA content increased from 0to 15ml.However,the maximal size of PMMA shell is much smaller than that of PS shell although we increased the content of MMA much more than that of St.The better polymerization activity and bigger solubility of MMA than St limited the growth of PMMA shells,and most of MMA molecules were polymerized to form PMMA colloids.The fact further proves our speculations for the formation of PMMA shells or PS shells on SiO 2.3.4.Effect of the size of coresWhen the amount of monomer and SiO 2,and the concen-tration of emulsifier were constant,the thickness changesofFig.3.The relation plot between the amount of monomer and the sizes of polymer shell (the data from TEM images).Here,the polymer is PMMA (A)and PS (B),respectively.K.Zhang et al./Colloids and Surfaces A:Physicochem.Eng.Aspects 277 (2006) 145–150149Fig.4.TEM images of the SiO 2-PMMA CSNs.the size of SiO 2is 51,126,165,195nm,respectively (from left to right).polymer shell are similar for PMMA and PS following the sizes altering of grafted silica nanoparticles.As shown in Fig.4,the PMMA shell of SiO 2-PMMA CSNs is thinner when the sizes of SiO 2are bigger.For the grafted silica cores of 51,126,165and 195nm,the average thickness of shell is 40,19,17and 15nm,respectively.For the grafted silica nanospheres,when the amount of SiO 2is constant,the total superficial extent of nanospheres decreases linearly with the increase of colloid diameter,and the total amount of MPS on surface of colloids decreases,then the amount of MMA copolymerized with MPS on the surface of grafted silica will decrease,so that much more monomers will form polymer colloids without cores,as a result,the polymer shells of SiO 2-MMA CSNs became thinner.At the same time,we calculated the amount of MMA to form SiO 2-PMMA CSNs according to the sizes of cores and shells of CSNs,and found there were less than 21%of total MMA to form the shells of SiO 2-PMMA CSNs,which further proved the formation mechanism for SiO 2-PMMA CSNs.In conclusion,we found the appropriate system to prepared monodispersed SiO 2-polymer CSNs.The SiO 2-polymer CSNs with different sizes and morphologies can be obtained accord-ing to need,via controlling the concentration of emulsifier,the amount of cores and monomer and changing different monomer.3.5.Hollow nanospheresHollow polymer nanospheres were obtained via dissolving the SiO 2cores using hydrofluoric acid.Fig.5shows TEM micrographs of hollow PS nanospheres (A;insert,SiO 2-PS CSNs)and PMMA nanospheres (B;insert,SiO 2-PMMA CSNs).Compared the hollow PS nanospheres with SiO 2-PS CSNs,it can be clearly seen that the surfaces of hollow nanospheres are as smooth as those of SiO 2-PS CSNs,but some creases and folds appear at the thin parts of shells.We think the PS shells without cross-linking cannot support the cavities so that the thin parts of shells shrank and deformed.This can be further proved by the cross-linking hollow PMMA nanospheres (Fig.5B;the cross-linking degree is 4%),which holds perfect spherical shape.From Fig.5B and the inset,we also note the thickness of hollow PMMA shell increased and the cavity shrank in a certain extent than that of original SiO 2-PMMA CSNs,which is the result of melting of PMMA under electronic beam.Hollow polymer nanospheres with dif-ferent sizes and thickness can be obtained,when we choose appropriate SiO 2-polymer CSNs.The hollow polymer can be applied in the research of photonic crystals and antireflectionfilm.Fig.5.TEM images of hollow polymer nanospheres,hollow PS nanospheres (A;insert:TEM image of SiO 2-PS CSNs);hollow PMMA nanospheres (B;insert:TEM image of SiO 2-PMMA CSNs).The scale bar is 200nm.150K.Zhang et al./Colloids and Surfaces A:Physicochem.Eng.Aspects 277 (2006) 145–1504.ConclusionIn summary,the monodispersed SiO2-polymer CSNs and hollow polymer nanospheres were prepared by versatile pro-cesses.We investigated the effects on the morphologies and sizes of SiO2-polymer CSNs coming from the monomer,emulsifier and the grafted silica,and demonstrated the thickness of shells and the size of cores could be adjusted in a wide range.At the same time,we investigated the synthesis mechanisms of dif-ferent monodispersed SiO2-polymer CSNs.Then,we prepared hollow polymer nanospheres that would display large potential application in photonic crystals and antireflectionfilm.AcknowledgementsThis work was 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