2013-71-CPS-Preparation of

SHORT COMMUNICATIONPreparation of hemispherical polymer particles via phase separation induced by microsuspension polymerizationTomoe Yamagami &Takuya Tanaka &Toyoko Suzuki &Masayoshi OkuboReceived:28November 2011/Revised:28February 2012/Accepted:5March 2012/Published online:12April 2012#Springer-Verlag 2012Abstract Micrometer-sized,hemispherical polystyrene (PS)particles were successfully prepared by microsuspension po-lymerization of homogeneous styrene/hexadecane (HD)drop-lets dispersed in polyoxyethylene nonylphenyl ether (Emulgen 931)aqueous solution,followed by rapid removal of HD from formed PS/HD particles with a “Janus ”structure.It was im-portant for the formation of the morphology of Janus particles in thermodynamically stable state to carry out the polymeriza-tion slowly.The formation of by-product small PS particles by emulsion polymerization was suppressed by the additions of CuCl 2as a water-soluble inhibitor and NaCl to decrease the solubility of styrene in the aqueous phase.Keywords Nonspherical particles .Hemisphere .Microsuspension polymerization .Phase separation .Micrometer size .PolystyreneIntroductionNonspherical polymer particles have attracted much attention because they can be utilized to synthesize materials with unique crystal structures [1–3],light scattering properties[4],and properties responsive to external fields,e.g.,shear field [5]and electric field [6].Generally,polymer particles synthesized by heterogeneous systems such as suspension,emulsion,and dispersion polymerization under thermody-namic control have normally a spherical shape due to the minimization of the interfacial free energy between the par-ticles and medium.However,nonspherical composite poly-mer particles consisting of two kinds of polymers have been synthesized under kinetic control utilizing various seeded polymerization methods [7–25],microfluidic technique [26–30],deformation of spherical polymer particles by exter-nal force [31–34],the self-organized precipitation method [35],and heating and cooling method in the presence of hydrocarbon droplets [36].In previous works,we have proposed a novel approach for the preparation of nonspherical polymer particles by slow evaporation of toluene as a common good solvent from ho-mogeneous polystyrene (PS)/poly(methyl methacrylate)(PMMA)/toluene droplets dispersed in various kinds of emul-sifier aqueous solutions at room temperature (solvent evapo-ration method)[37–39].When polyoxyethylene nonylphenyl ether with an average ethylene oxide chain length of 30.8units (Emulgen 931)was used as nonionic emulsifier,and the PMMA was replaced with hexadecane (HD),PS/HD particles having a Janus structure were prepared.Finally,by rapid removal of HD from the PS/HD particles with methanol,hemispherical PS particles with flat surface,which are,for example,expected to be applied to a microlens [40,41],were successfully obtained [42].However,a multistep process was required for the preparations of the hemispherical PS particles with flat surface:(1)synthesizing of PS by solution polymer-ization;(2)dissolving of PS and HD in toluene;(3)dispersing in Emulgen 931aqueous solution;(4)slowly releasing tolu-ene;and (5)removing of HD by methanol.In this article,in order to simplify it,microsuspension polymerization ofThis paper was part CCCLI of the series “Studies on Suspension and Emulsion.”T.Yamagami :T.Tanaka :T.Suzuki :M.Okubo Graduate School of Engineering,Kobe University,Rokko,Nada,Kobe 657-8501,JapanM.Okubo (*)Smart Spheres Workshop Co.,Ltd.,2-1-214-122,Koyo,Naka,Higashi-Nada,Kobe 658-0032,Japane-mail:okubo@kobe-u.ac.jpColloid Polym Sci (2013)291:71–76DOI 10.1007/s00396-012-2625-ystyrene/HD droplets dispersed in Emulgen 931aqueous solu-tion will be applied in place of 1–4processes.Experimental section MaterialsStyrene was distilled under reduced pressure in a nitrogen mercial-grade polyoxyethylene nonyl-phenyl ether with average ethylene oxide chain lengths of 30.8units (Emulgen 931,nonionic surfactants)was supplied by Kao Co.,Japan.Reagent-grade lauroyl peroxide (LPO),HD,NaNO 2,CuCl 2,NaCl,and methanol (Nacalai Tesque Inc.,Kyoto,Japan)were used as received.Water was purified using an Elix UV system (Nihon Millipore K.K.,Japan).Preparation of PS particles utilizing microsuspension polymerizationFirst,PS/HD particles were prepared as follows:A homo-geneous solution of styrene,HD,and LPO was dispersed asdroplets in a 1.0-wt%Emulgen 931aqueous solution in the absence or presence of CuCl 2and NaCl under the recipes listed in Table 1using a NISSEI ABM-2homogenizer at 4,000rpm for 2min in a glass vial.Microsuspension poly-merization of the monomer dispersions was carried out at 70°C for 24h under nitrogen atmosphere in a Schlenk flask.The dispersions of PS/HD particles were observed with a Nikon Eclipse 80i optical microscope.Dried PS particles were observed with a JEOL JSM-6510scanning electron microscope (SEM)at an acceleration voltage of 10kV .Confirmation of thermodynamically stable particle morphology utilizing the solvent evaporation method PS was previously synthesized by microsuspension poly-merization as described above.A homogeneous toluene solution (0.643g)of PS and HD (PS/HD/toluene 01/1/29,v /v /v )was mixed with 0.33wt%(relative to water)Emulgen 931aqueous solution (15g).The mixture was stirred vigor-ously using a NISSEI ABM-2homogenizer at 4,000rpm for 2min in a glass vial,and subsequently toluene was slowly released from droplets via aqueous phase by evaporation during gentle stirring at room temperature for 24h in an uncovered glass vial (interfacial area between dispersion and air was 8cm 2).Resulting particles were washed with methanol using centrifugation to remove HD and excess emulsifier and subsequently dried under vacuum at room temperature.MeasurementsStyrene conversion was determined by gas chromatography (Shimadzu Corporation,GC-2014)with helium as the car-rier gas,tetrahydrofuran as a solvent,and p -xylene as an internal standard.Number-and weight-average molecular weight (M n ,M w ,respectively)and molecular weight distri-bution were measured by gel permeation chromatography with two styrene/divinylbenzene gel columns (TOSOH Cor-poration,TSK gel GMH HR -H,7.8mm i.d.×30cm)usingTable 1Preparation of PS/HD particles by microsuspension polymerizations Ingredients No.1No.2Styrene a (g)0.500.50HD a (g)0.360.36LPO(mg)50 6.5–50Emulgen 931(g)0.150.15CuCl 2(mg)–50NaCl (mg)–130Water(g)1515Microsuspension polymerizations in Schlenk flasks;N 2,70°C,24h,stirring rate at 400rpmPS polystyrene,HD hexadecane,LPO lauroyl peroxide,Emulgen 931polyoxyethylene nonylphenyl etheraPS/HD 01/1(v /v )at 100%conversionFig.1a Optical micrograph of PS/HD particles prepared by microsuspensionpolymerization of styrene/HD/LPO (LPO concentration,0.23mol/L styrene)droplets dispersed in 1.0wt%Emulgen 931aqueous solution and b SEM photograph of PSparticles after removal of HD from the PS/HD particles by washing with methanoltetrahydrofuran as the eluent at 40°C at a flow rate of 1.0mL min −1employing refractive index (TOSOH RI-8020/21)and ultraviolet detectors (TOYO SODA UV-8ΙΙ).The columns were calibrated with six standard PS samples (M w 01.05×103–5.48×106,M w /M n 01.01–1.15).The amount and diameter of by-product PS particles were,re-spectively,measured by gravimetry and a dynamic light scattering (FPAR-1000RK,Fiberoptics particle analyzer,Photal Otsuka electronics,Osaka,Japan)at the light scat-tering angle 90°at room temperature using the Contin analysis routine.One to two droplets of emulsion samples withdrawn from the reactor were diluted with approximately 8mL of distilled water before measurement in the dilution mode.Results and discussionFigure 1shows an optical micrograph of PS/HD particles prepared by microsuspension polymerization under theconditions of no.1listed in Table 1and a SEM photograph of PS particles after removal of HD from the PS/HD par-ticles by washing with methanol.Micrometer-sized,two-phase-separated PS/HD particles were observed (Fig.1a ).In addition,a large number of nanometer-sized PS particles less than 100nm were observed in the SEM photograph (Fig.1b ),which suggests that emulsion polymerization took place as well as the microsuspension polymerization.This point will be clarified later by the measurement of molecular weight distributions (MWDs)of the nanometer-sized and micrometer-sized PS particles by GPC.The dispersion (the polymerization product)was sepa-rated into two layers by centrifuging at 10,000rpm for 10min.In the upper layer,PS/HD particles were floated because HD had low density (ρHD 00.77cm 3/g).After removing HD from them with methanol,micrometer-sized nonspherical PS particles were obtained as shown in the SEM photograph (Fig.2a ).On the other hand,in the lower layer,nanometer-sized spherical particles having approximately 30nm in diameter (Fig.2b ),in which HD was not contained,were dispersed.The nanometer-sized particles as by-product could be easily eliminated from the dispersion by centrifugation.Figure 3shows MWDs of the submicrometer-sized PS particles and the nanometer-sized PS particles,which were separated by the centrifugation.Before centrifugation,the MWD of total PS was bimodal (M n ,3.4×104;PDI,21.8),and after centrifugation,both MWDs were monomodal and M n values in the upper and lower layers were,respectively,1.2×104and 5.8×105.The high molecular weight of nanometer-sized PS particles in the lower layer indicates that they were prepared by emulsion polymerization.The weight fraction of the by-product PS particles was 22wt%of the initial weight of styrene,which affected the morphol-ogy of the main PS/HD particles.In fact,the volume ratio of the PS phase to the HD phase in the obtained PS/HD particles calculated from optical photographs was signifi-cantly small (PS/HD ≈0.6/1(v /v ))even though the conver-sion of styrene was almost 100%.Fig.2a SEM and b TEM photographs of PS particles in the upper and lower layers,respectively,after the PS/HD dispersion was centrifuged,where PS/HD particles in the upper layer were washed with Emulgen 931aqueous solution and methanolMicronmeter-sized PS particlesOriginal PS particlesNanometer-sized PS particlesFig.3Molecular weight distributions of total PS (solid line )in the PS/HD dispersion prepared by the microsuspension polymerization of styrene/HD/LPO droplets dispersed in 1.0wt%Emulgen 931aqueous solution and of those in the upper (broken line )and lower (dotted line )layers separated by centrifugation of the dispersionIn order to suppress the emulsion polymerization in the microsuspension polymerization system,firstly,the concen-tration of Emulgen 931was decreased to less than critical micelle concentration (cmc),but by-product PS particles were still prepared.Even when NaNO 2was added as a water-soluble inhibitor into the aqueous medium [43],they were unsuppressed.However,when both CuCl 2as a water-soluble inhibitor and NaCl,which decreases the solubility of styrene in water phase [44],were added into the aqueous medium as listed in no.2of Table 1,the number of the by-product PS particles was drastically decreased though they were still observed in a SEM photograph of PS particles after washing PS/HD particles with methanol (see Fig.4b ).A lower layer appeared by centrifuging the obtained PS/HD dispersion at 10,000rpm for 10min was almost transparent.The weight fraction of by-product PS particles was ~0,and the molecular weight distribution of total PS before the centrifugation was monomodal (M n ,1.6×104;PDI 2.0).These results indicate that the generation of by-product PS particles was almost suppressed.A very small amount of the by-product PS particles was easily eliminated by washing with Emulgen 931aqueous solution,followed by washing with methanol (see Fig.4c ).According to a reference [45],cmc of emulsifier was decreased with increasing of the concentration of salt.It has the potential to promote the emulsion polymerization.Therefore,the obtained result indicates that the positive effect based on the decrease in the solubility of styrene in water operated larger than the negative effect based on the decrease in the solubility of Emulgen 931.The volume ratio of the PS phase to the HD phase in the obtained PS/HD particles calculated from the optical photographs (Fig.4a )increased to 0.9/1compared with that in no.1listed in Table 1,but the surface of PS phase that contacted with HD was convex.That is,the shape of PS particles was not hemispherical butacorn.Fig.4a Optical micrograph of PS/HD particles prepared by micro-suspension polymerization of styrene/HD/LPO (LPO concentration,0.23mol/L styrene)droplets dispersed in 1.0wt%Emulgen 931aqueous solution with dissolved CuCl 2and NaCl and b ,c SEMphotographs of PS particles after removal of HD from PS/HD particles by washing b with methanol and c with Emulgen 931aqueous solu-tion,followed by withmethanolFig.5a Optical micrograph of PS/HD particles prepared by release of toluene from PS/HD/toluene (1/1/29,v /v /v )droplets dispersed in 0.33wt%Emulgen 931aqueous solution and b SEM photograph of PS particles after removal of HD with methanol from PS/HD particles.PS used in this experiment was prepared by microsuspensionpolymerization of styrene/HD/LPO (LPO concentration,0.23mol/L styrene)droplets dispersed in 1.0wt%Emulgen 931aqueous solution with dissolved CuCl 2and NaCl under the conditions of no.2listed in Table 1The nonspherical PS particles and HD were dissolved with toluene and dispersed as droplets in Emulgen 931aqueous solution,and then the toluene was slowly re-leased from the droplets by evaporation.Figure 5a shows an optical micrograph of PS/HD particles after the evap-oration of the toluene,and Fig.5b shows a SEM photo-graph of PS particles after removal of the HD with methanol from the PS/HD particles.The PS particles had a hemispherical shape with flat interface.On the other hand,when the toluene was rapidly evaporated,obtained PS particles had an acorn shape.These indicate that the PS/HD particles prepared by the slow toluene evaporation had a thermodynamically stable morphology [42].In other words,the acorn shape (without flat interface)of the PS/HD particles prepared by the microsuspension polymerization was not thermodynamically controlled but kinetically con-trolled.On the basis of this idea,next,we decreased LPO concentration to decrease the rate of the microsuspension polymerization of styrene/HD/LPO droplets (see Fig.6a ),which should cause smoothly the phase separation of formed PS and HD in styrene droplets throughout the polymerization.(a)(b)Fig.6a Conversion –time plots for microsuspension polymerizations of styrene/HD/LPO droplets dispersed in 1.0wt%Emulgen 931aqueous solution with dissolved CuCl 2and NaCl and b molecular weight distri-butions of the obtained PS.LPO concentration (in moles per liter styrene):(red open circle ,red solid line )0.03(green triangle ,green broken line (one broken line and a dot))0.06;(blue square ,blue broken line )0.12;(black circle ,black dotted line )0.23Fig.7a –d Optical micrographs of PS/HD particles prepared by microsuspension polymerizations for styrene/HD/LPO droplets dis-persed in 1.0wt%Emulgen 931aqueous solution with dissolved CuCl 2and NaCl and a ’–d ’SEM photographs of PS particles after removal of HD from the PS/HD particles by washing with Emulgen 931aqueous solution,followed by washing with methanol.LPO con-centrations (in moles per liter styrene):(a ,a ’)0.03;(b ,b ’)0.06;(c ,c ’)0.12;(d ,d ’)0.23Figure7shows optical micrographs of PS/HD particles prepared by the microsuspension polymerization at various initiator concentrations under the condition of no.2listed in Table1and SEM photographs of PS particles removal of HD by washing the PS/HD particles with Emulgen931 aqueous solution,followed by with methanol.All polymer-izations were completed.As expected,the degree of convex interface between PS and HD phases decreased with a decrease in the LPO concentration,resulting in a flat surface at0.03mol/L styrene.It was confirmed that the similar hemispherical PS particles having flat interface were obtained by the solvent evaporation method using the PS particles prepared by the microsuspension polymerization at high LPO concentrations(the photograph omitted).The decrease in the initiator concentration increased mo-lecular weight at the same time(see Fig.6b),which should make the phase separation slow because of higher viscosity in the droplet.Therefore,the obtained result indicates the positive effect for the purpose based on the decrease in the polymerization rate operated larger than the negative effect based on the increase in the molecular weight.From above results,it is concluded that micrometer-sized,hemispherical PS particles having flat surface can be prepared by micro-suspension polymerization of styrene/HD droplets under conditions that phase 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