Preparation of microsized crosslinked microspheres by dispersion copolymerization of polystyrene/pol

Journal of University of Science and Technology BeijingMaterialsVolume 13, Number 4, August 2006, Page 380Preparation ofmicrosizedcrosslinkedmicrospheresbydispersioncopolymerization of polystyrene/polyethylene glycol 200-dimethacrylaeWenli Guo, Juan Wang, and Tongxiang LiangInstitute of Nuclear Energy Technology, Tsinghua University, Bejing 100084. ChinaReceived (2005- 10-13)Abstract: Dispersion copolymerization of styrene with polyethylene glycol 200-dimethacrylae as the cross-linking agent wascompleted by using poly(N-vinyl pyrolidone) and 2.2-azo-bisisobutyronitrile as the steric stabilier and initiator, respectivelyCoslinked copolymeric microspheres were prepared directly by the one step method of dispersion copolymerization. The efetse ofthe content of polyethyene glycol 200 dimethacrylae on the particle morphology and the copolymerization rat were investigated. Itshows that the crossinking agent plays an important role in the particle morphology and the system stabilit. When the content ofcrosslinking reached 2.5wt%, the floriated particles were obtained.Key words: polystyrene microsphere; dispersion polymerization; croslinking agent1. Introductionhydrocarbons (i.e. hexane and heptane) and polarsolvents (ie. alcohol and alcohol/water).Recently, considerable attention has been focusedcrosslinker is usually divinylbenzene (DVB). There areon the development of polymer microspheres bythree problems in the preparation of crosslinkeddispersion polymerization, owing mainly to their manypolystyrene microspheres. First, only a very smalladvantages, such as small size, high specific surfaceamount of the crosslinker DVB could be incorporatedareas, spherical shape and highly monodisperse sizeto form stable latex in the dispersion copolymerizationdistribution as well as strong absorbability andwithout coagulum [8]. Second, because of the shortfunctional groups enrichment on the surface oflength of the chain between the two double bonds, it isparticles and so on. These special properties ardifficult for DVB to react completely during theessential in their application in various fields, e.g.polymerization. Third, the morphology of the particleschemical industry, medicine, biotechnology andis sensitive not only to the absolute level of theelectronics [1-3].crosslinker, but also to the time and duration of theTraditionally, polymer microspheres are prepared bydelivery addition of reactive materials. With batchsuspension polymerization. This technique is usuallypolymerization, the radical mode of particle growthused to produce particles with the size of 50- 100 μm.precludes the direct synthesis of macroporous particles.Polymer particles with uniform size in the submicron-For insoluble microspheres, it is clear that a uniformsize range have been produced by emulsiondistribution of crosslinkers is a desirable trait, and itpolymerization [4]. Lovelace et al. prepared polymericcan improve the mechanical strength. However, thisparticles in micron-size range in the space shuttle withgoal is difficult to obtain because the formation ofmicrogravity [5]. The“swollen emulsion polymerimicrospheres in dispersion copolymerization involveszation developed by Ugelstad is the first successfulthree stages: nucleation, stabilization, and growth [9-technique to produce monodisperse polymeric particles10]，which make the formation mechanism quiteof micron-size in the range of 0.2-1.5 μm [6]. However,complicated. Thomson suggests that the incorporationall these methods are inconvenient and expensive.of DVB makes the reaction of lining the PVP-PStDispersion copolymerization is a simple method for the(polthe PSt-DVB phase ofsynthesis of crosslinked polymer particles. Thisbulk中国煤化工article, polyethyleneapproach was discovered in the 1970s and developed inglycY片CNMHG0DMA)wasusedasthe 1980s [7]. Two main kinds of dispersion media,the crosslinker to prepare crossinked microspheres ofwhich have been extensively investigated, arestyrene (St). The results showed that when the contentCoreserirtor: Tongxiang Liang. E-mail: xliang@o tinghua edu.cnW.L Guo et al, Preparation of microsized coslinked microspheres by dispersion copolymerization of..381of the crosslinker was less than 2.5wt%, smooth and2.4. Characterizationspherical monodisperse microspheres were obtained.The morphology and size of the crosslinkedThe more interesting fact is that flower shapedmicrospheres were directly observed using JSM-6301Fmicrospheres were obtained when the content of thescanning electron microscopy (SEM). The samplescrosslinker increased.were sputter-coated with Au film prior to theexamination. The particle dispersion was obtained2. Experimentalfrom SEM micrograph and the average particle size(d )2.1. Materialsand dispersion coefficient (E) were calculated asfollows [12]:St and PEG200DMA were washed using 10vol%aqueous sodium hydroxide solution, then ditiledl. d= Zd1n(2)under reduced pressure to remove the inhibitor anddried with Na2SO4. 2,2'-azobisisobutyronitrile (AIBN)was purified by ethanol. Poly(N-vinylpyrrolidone)(PVP, M = 40000) and ethanol were commerciallyε=之(d,-)2 (n-)日(3)available and used without further purification.Deionized water was applied in all polymerization andwhere d, and n are the particles' diameter and number,treatment processes.respectively.FTIR spectra were measured in the wavenumber2.2. Preparation of microspheresrange of 500-4000 cm-1 at a resolution of 4 cm-1 usingUsing PVP and AIBN as the steric stabilizer anda Nicolet Avatar 360 FTIR spectrophotometer.initiator respectively, dispersion copolymerization ofSt with PEG200DMA as the crosslinking agent was3. Results and discussioncarried out. A four- necked round-bottom flask (250 mL)was used as the container for polymerization, whichWhen DVB was used as the crosslinker in thewas fllel with the solvent mixture of ethanol/PVP andpreparation of crosslinked polymer microspheres, it isethanol used as the dispersion media. Under an argondifficult for DVB to react completely in theatmosphere, the dispersion copolymerization of styrenepolymerization because of the short length of the chainand PEG200DMA was performed. The mixed solutionbetween the two double bonds. Here， we usedof St, AIBN, and PEG200DMA was added slowly intoPEG200DMA, a longer chain crosslinking agent. Thethe flask under stirring with a constant agitation speedFTIR spectra of the St/PEG200DMA are shown in Fig.of 120 r/min. The flask was suspended in a1. It can be seen that the peaks at 31033000 cm-temperature-contolled water bath at 70°C. Meanwhile,represent C-H on benzene ring, the peaks at 3000 2800the zero-reaction time was defined and allowed tccm-1 represent C-H on CH2 or CH, the peaks at 2000-proceed for 18 h. The polymer particles were separated1680 cm^ 1 represent a series of peculiar weak spectra ofby centrifugation, washed with ethanol and deionizedPS, the peaks at 1452, 1493, and 1601 cm-' representwater for five times and dried at 50°C under vacuum.benzene ring, and the peaks at 697 and 756 cmrepresent C-H on benzene ring. The spectra contain2.3. Determination of conversionapparent absorptions in the C=O (1728 cm~) andThe conversions were determined by a gravimetricC- 0-C (1112 cm~l) stretching region which are notmethod. The sample (about 1-2 g) was extracted afterpresent in those of original polystyrene. Based on thethe onset of the reaction at fixed time intervals and aspectra, it can be proved that PEG200DMAdrop of 5vol% 1,4-hydroquinone solution wacopolymerized with styrene and formed copolymericpromptly added to terminate the polymerization. Eachparticles. In addition, Fig. 1 shows that the IRsample was dried at 50°C under vacuum until aabsorbance of the C=C double bonds at 1630 cm~ I canconstant weight was reached. The conversion washardly be detected, which indicates that almost all thecalculated as follows [11]:double bonds including that of the crosslinker havebeen reacted completely.C=[M2- M.x(W2+ W3)]/(M;W)1)where C is the monomer conversion, M the mass of the3.1. El中国煤化工0DMAsample before drying, M2 the mass of the sample afterTheCNMHCG200DMA on thedrying, W. the mass fraction of the monomers, W2 theparticle sicC，sic" upcIDIUll allumorphology weremass fraction of PVP, and W3 the mass fraction ofinvestigated. The relation between the diameter and theAIBN in the reaction.content of the crosslinking agent is shown in Table 1.382J. Unir. Sci. Technol. Beijing, VoL.13, No.4, Aug 200610040distribution (E < 0.04 indicates that the particles aremonodisperse) when polymerization is carried out with0-1942/different contents of the crosslinking agent. The0r18011728112particle size increases as the crosslinking agent content308230601 284916011028539increases, but the variation is very small, whichindicates that the content of the crosslinking agent has2029221493 i452697little effect on the particle size.0L100030002000Fig. 2. shows the morphology of the particles. TheWavenumber/ cm-'basic recipe (not including crosslinking agent) is St, 25Fig.1. FTIR spectra of SUPEG200DMA microspheres (St,g; EtOH, 75 g; and PVP, 2.5 g. The content of the25 g; AIBN, 0.3 g; PVP, 2.5 g; PEG200DMA, 0.625 g; EtOH,crosslinking agent increases gradually from (a) to (c) in75 g).Fig.2, which are 2.0wt%, 2.5wt%, 3.0wt% of St,Table 1. Effect of the PEG200DMA content on particle sizerespectively. The particle morphology is sensitive toand dispersion coefficientthe content of the crosslinking agent and changes fromw(PEG200DMA)/ wt%_d/μm_E"spherical" particles with smooth surface to“floriated"3.020.021ones. When the content of PEG200DMA increases to.03.150.025or over 2.5wt%, the shape of the particles becomes1.53.260.026“floriated", but the“floriated" particles have even.73.650.028diameter and narrow size dispersion. It can be seen2.53.740.031from Fig. 2(d) that the“petals”on the particle surface3.780.035are regular, which differ considerably from usualAs shown in Table 1, there is a narrow sizesmooth spherical particles.aFig. 2. Morphology of microspheres by SEM(scale of (a)-(c) is 10 um, scale of (d) is 2 um), The content of crosslinking agent is:(a) 2.0wt%; (b) 2.5wt%; (c) 3.0wt%; (d) 3.0wt%.When DVB crosslinking agent is used, there are twostabilizer to bury before exchanging outward, and thismain reasons for low stability of the polymerizationwill lower the PVP stabilizing efficiency. Secondly,system. Firstly， during the early period ofthere“ 中国煤化工dicals formned on thepolymerization, DVB leads to forming crosslinkedpartiwo double bonds irnuclei with high degree of crosslink, which hinderDVE|YHC NM H Gtes (Fig. 3). Thesenuclei from being swelled by monomers. Accordingly,unreacted vinyl radicals can react with newly formedthis could prevent oligomer chains from moving andoligomeric radicals [12]. In this case, the movement ofadjusting conformation. As a result, it can cause thecopolymer chains is limited, and the existing particlesw.L Guo et al, Preparation of microsized crosslinked microspheres by dispersion copolymerization of..383begin to destabilize. However, when PEG200DMA iscompositions are obtained as shown in Fig. 5. At theused as the crosslinking agent, the stability of theinitial stage, there is no polymerization inductiondispersion system is improved. The chain length ofperiod whether the crosslinking agent content is high orPEG200DMA is longer than that of DVB (Fig. 4). Thelow. This indicates that the period of forming nuclei islonger chain length helps to form the looser crosslinkedshort enough to provide a precondition for thenetwork structure, which makes monomers swell morepreparation of monodisperse particles. When theeffectively. At the same time, the oligomer chains arePEG200DMA content increases, the polymerizationable to adjust conformation freely even when the nucleirate is faster and the final monomer conversion is muchabsorb oligomeric radicals, initiator and monomers.higher. PEG200DMA has a higher reactivity thanstyrene. It helps to decrease the activation energy ofCH=CH2copolymerization and to increase the reaction rate. Ashe polymerization proceeds, the polymerization rateslows down. In the last stage, the number of nuclei doesCH-CH2not rise any more, and the amount of monomers andcrosslinking agent decreases. At the same time, theFig. 3. Structure of DVB.crosslinking density of nuclei becomes higher. Thisresults in lowering of the swellability of monomers inethanol solution.foN fm1000-Fig. 4. Structure of PEG200DMA.In addition, at a low content of PEG200DMA, it can6 60be considered as the costabilizer because of the40|structural dispersion -medium soluble groups in itsmolecule. This could be confirmed by particles'20maintaining monodisperse condition with no coagulumeven when they change from“spherical" particles to10“floriated”ones. When the content of PEG200DMATime/hreaches its critical amount, the reaction ofPEG200DMA and PVP becomes dominant instead ofFig.5. Efect of PEG200DMA content on the conversion.its costability becoming dominant. PVP loses itsw(PEG200DMA), wt%: a,0; b,1.0; C, 1.5; and d, 2.5.protection function to nuclei. Moreover, the4. Conclusionsnonswellability of highly crosslinked nuclei canimpede the movement and adjustment of theThe dispersion copolymerization of St andconformation of oligomer chains.PEG200DMA was successfully carried out in ethanolAccording to the different structures of DVB andmedium with AIBN as an initiator and PVP as aPEG200DMA, the change in particles' morphology canstabilizer. Different from the usual crosslinking agentsbe explained as follows: when DVB is used as thesuch as DVB， PEG200DMA crosslinking agentcrosslinker, in view of its short chains, it can make thecontributes to improving the stability of thecrosslinked network structure rigid to keep particlespolymerization system. In addition, it can bespherical. In contrast, PEG200DMA possesses longerconsidered as a costabilizer when its content is low,and softer chains than DVB. When PEG200DMAthereby facilitating the formation of monodispersecontent is high, flower-shaped particles can beparticles. Floriated particles were obtained when theobserved while shrinkage of particles is caused by thePEG200DMA content reached 2.5wt% (St). Thecrosslinking reaction. In fact, the shrinkage of particlescrosslinking agent content has lttle effect on thecan result in forming depressed surface, which makesparticle size, but has an apparent influence on thepolymerization rate.the particles look like flowers.中国煤化工3.2. 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