Preparation of Polymer/Silica Composite Nanoparticles Bearing Carboxyl Groups on the Surface via
Emulsifier-Free Emulsion Copolymerization
ZHONG ZENG,JIAN YU,ZHAO-XIA GUO
Institute of Polymer Science and Engineering,Department of Chemical Engineering,
School of Materials Science and Engineering,Tsinghua University,Beijing100084,China
Received3November2004;accepted30January2005
DOI:10.1002/pola.20764
Published online in Wiley InterScience(www.interscience.wiley).
ABSTRACT:Polymer/silica organic/inorganic composite nanoparticles bearing carboxyl
groups on the surface were prepared via the emulsifier-free emulsion copolymeriza-
tion of methyl methacrylate and sodium methacrylate(NaMA).Carboxyl groups were
generated by the addition of hydrochloric acid at the end of the copolymerization.
Two methods of NaMA addition were studied:batch and two-stage procedures.The
batch procedure allowed only a limited number of carboxyl groups to effectively bond
to the composite nanoparticles.In contrast,the number of carboxyl groups could be
altered over a wide range with the two-stage procedure.Fourier transform infrared
spectroscopy and chemical titration were independently used to quantify the number
of carboxyl groups,giving values close to each other and to the feed.A kinetic study
indicated that the copolymerization followed a mechanism different than that found
earlier.The average size of the composite nanoparticles was approximately40nm,as
measured by both transmission electron microscopy(TEM)and laser scattering,and
their polydispersity index was close to1,indicating a fairly narrow size distribution.
TEM photographs of the composite nanoparticles showed a multilayered core–
shell structure with one silica bead as the core and with poly(methacrylate acid)
as the outmost shell.V C2005Wiley Periodicals,Inc.J Polym Sci Part A:Polym Chem43:
2826–2835,2005
Keywords:composite nanoparticles;core-shell polymers;emulsifier-free;emulsion;
copolymerization;functionalization of polymers
INTRODUCTION
Polymer/inorganic composite nanoparticles are of growing interest not least because of their extensive applications asfillers in polymer-based nanocomposites.1–3The advantages of such composite nanoparticles over pure inor-ganic nanoparticles are obvious:not only is the composite material strengthened by the inor-ganic moiety,but the compatibility between the filler and the matrix is improved by the polymer moiety.Such composite nanoparticles can be prepared mainly by the encapsulation or graft-ing of polymers onto the surface of inorganic nanoparticles.An enormous amoun
t of work has been reported,involving various inorganic par-ticles such as silica,4–6calcium carbonate,7tita-nium oxide,8–10and alumina11(among them, silica is the most widely studied).
Encapsulation can be roughly divided into two types,physical and chemical,according on whether there is any chemical bonding at the interface between the inorganic nanoparticles and
Correspondence to:J.Yu(E-mail:yujian03@mail. tsinghua.edu)
Journal of Polymer Science:Part A:Polymer Chemistry,Vol.43,2826–2835(2005) V C2005Wiley Periodicals,Inc.
2826
the polymer.Although physical encapsulation is simple,12,13chemical encapsulation is more inter-esting because of the stronger interfacial interac-tion provided by the covalent bonding at the inter-face.14,15There are mainly two approaches to achieve chemical encapsulation.One involves the use of inorganic nanoparticles pretreated with a silane coupling agent bearing a polymerizable dou-ble bond as a comonomer during the polymeriza-tion of vinyl monomers.16,17The other uses pre-treated silica as a macroinitiator for living polymerization.6,18,19For the former,the main polymerization techni
ques include emulsion and dispersion polymerization.Covalent bonds are formed between nanoparticles and polymer chains during encapsulation.For the latter,polymeriza-tion starts from the surface of inorganic nanopar-ticles,and encapsulation is formed as polymeriza-tion proceeds.
As for the grafting of polymers onto inorganic nanoparticles,there are three main approaches that depend on the order of polymerization and grafting:grafting-from,20grafting-through,21and grafting-onto.22With the grafting-from approach, pretreated inorganic particles bearing initiating groups such as azo groups are used as initiators, and consequently polymerization starts from the surface of inorganic particles.The grafting-through approach uses pretreated inorganic nano-particles as comonomers of vinyl monomers.Graft-ing is achieved during polymerization.It may not necessarily form encapsulation;this depends on the polymerization conditions.As for the grafting-onto approach,prepolymers containing reactive groups are attached to inorganic particles.
Although composite nanoparticles often show better dispersion and compatibility when used asfillers for polymers,the interfacial interaction between the composite nanoparticles and the polymer matrix is based only on physical com-patibility offered by the polymer shell and can certainly be improved further by the formation of chemical bonds,just like in the case of reac-tive blending.23For this purpose,composite nanoparticles bearing functional groups such as carboxyl groups(called reactivefill
ers)are needed and can potentially be useful in reinforc-ing polymers having functional groups that can react with the functional groups of thefiller.
As part of an ongoing project aimed at the prep-aration of reactivefillers,we synthesized epoxy-functionalized polystyrene/silica composite nano-particles with a core–shell structure via emulsion polymerization.24A mixture of ionic and nonionic
emulsifiers was used to stabilize the particles, which could partly remain in the latices as impur-ities(it is known that the emulsifiers used in emulsion polymerization are very hard to remove completely)and consequently limit the applica-tions of the latices to some degree.
Emulsifier-free emulsion polymerization has been receiving considerably more attention in recent years because it can produce clean and monodisperse latices.25–28The emulsifier-free systems are often not truly free of an emulsifier in the strictest sense as the name indicates.The monomer or comonomer usually contains a part that resembles the structure of an emulsifier at one end of the molecular chain.Such a monomer or comonomer can play the role of an emulsifier while polymerizing.29Sodium methacrylate (NaMA)is one such comonomer.It is an ionic vinyl monomer with sodium carboxylate salt at one end of the molecule and a double bond at the other end,and it has bee
n used to conduct emulsifier-free emulsion copolymerization.30 In this study,the emulsifier-free emulsion copolymerization of methyl methacrylate(MMA) was carried out to prepare reactive polymer/silica composite nanoparticles with NaMA as a comono-mer.Carboxyl groups were generated after the copolymerization by neutralization with hydro-chloric acid(HCl);this made the composite nano-particles reactive.With their interesting charac-teristics(clean surface,reactive and ionizable properties,and core–shell structure),such func-tional composite nanoparticles are expected to be used not only as reactivefillers but also in a wide range of applications such as protein carriers, microcapsules,water purifiers,and polymer cata-lysts.The method and amount of NaMA addition, the copolymerization kinetics,the carboxyl con-tent,and the morphology of the composite nano-particles are investigated here in detail. EXPERIMENTAL
Materials
Nanometer silica1065nm in diameter was acquired from Zhoushan Mingri Nanomaterial, Ltd.(China),and used after drying in vacuo at 1058C for12h.The pretreatment of silica with 3-methacryloxypropyltrimethoxysilane(MPTMS) was carried out with a previously published pro-cedure.24The monomers,both MMA and metha-crylate acid(MAA),were distilled under reduced pressure before use.NaMA was obtained by the POLYMER/SILICA COMPOSITE NANOPARTICLES2
827
neutralization of MAA with an equal molar amount of NaOH at08C.Ammonium persulfate (APS)was freshly recrystallized from water.All other reagents were used as received. Copolymerization
The emulsifier-free emulsion copolymerization was carried out under a nitrogen atmosphere in a250-mL,four-neckedflask with a mechanical stirrer,thermometer,and condenser.For the whole process,the reaction was in a water bath, and the stirring rate wasfixed at150rpm. Batch Procedure
NaMA was introduced into a reactor charged with distilled water at408C.After20min of stirring at408C,the temperature was raised to 508C,and then a mixture of SiO2and MMA, which was treated by ultrasonic irradiation for 10min just before it was used,was added.After 10min of stirring at508C,the system was raised to608C,and a solution of APS in water was added to initiate the copolymerization.The reaction proceeded at808C for2h,and then it was raised to and held at908C for30min more. Excessive HCl was added to translate the acryl-ate into carboxyl groups;this also led to demul-sification.The product was washed thoroughly with hot water and then dried.
Two-Stage Procedure
Thefirst stage was similar to the batch proce-dure.After1h of stirring at808C,the second stage of copolymerization was started by the feeding of aqueous NaMA(at a rate of10mL/h)dropwise to the system.After the completion of aqueous NaMA addition,it was stirred at808C for30min and then was raised to and held at 908C for30min more.Excessive HCl was added to translate the acrylate into carboxyl groups; this also led to demulsification.
The recipes of all runs are listed in Table1.The yield and conversion of the monomers were deter-mined by the gravimetric method as follows: Yieldð%Þ¼
Total product(g)
Total monomer(g)and SiO2ðgÞ
Â100 Conversionð%Þ¼
Polymer former(g)
Monomer used(g)
Â100
To estimate the strength of the interaction between either PMMA and silica or PMMA and poly(methacrylate acid)(PMAA),the sample was extracted with chloroform for12h with a Soxh-let apparatus,and then the binding efficiency was calculated as follows:
Binding efficiency(%)¼
Polymer grafted(g)
Polymer formed(g)
Â100
Characterization
A latex sample was used directly for the mor-phology observation and particle size and size distribution determination with a JEOL200CX transmission electron microscope(dyed by RuO4 for20min)or a Zetaparticle HS3000laser scat-tering(LS)particle size and z-potential analyzer. Na was used to denote the number of silica beads per particle,and it was be calculated with a formula reported by Bourgeat-Lami and
Table1.Recipes of Emulsifier-Free Emulsion Copolymerization
Run No.
NaMA(mmol)
MMA
(mL)
APS
(mmol)
H2O(mL)
HCI
(mmol) Portion1Portion2A a B b
Batch series B1  2.1—15.00.2850.0—  5.0 B2  2.4—15.00.2850.0—  5.0
B3  2.7—15.00.2850.0—  5.0
B4  3.0—15.00.2850.0—  6.0
B5  3.3—15.00.2850.0—  6.0 Two-stage series T1  2.7  5.515.00.2850.0  5.010.0 T2  2.713.715.00.2850.0  5.020.0
T3  2.730.215.00.2850.010.050.0
T4  2.746.715.00.2850.015.0100.0
T5  2.763.215.00.2850.020.0100.0
a Added before thefirst part of NaMA was introduced.
b Used to dissolve the second part of NaMA.
2828ZENG,YU,AND GUO
Lang.31The core–shell structure of composite nanoparticles can be considered well-defined,that is,only one silica bead per particle,when the value of Na ranges from 0.95to 1.05.
The number-average diameter (D n )and the weight-average diameter (D w )were calculated with the f
ollowing equations:
D n ¼
X
i
N i D i .X i
N i :D w ¼
X
i
N i D 4i
.X
i
N i D 3i :
where N i (i ¼1,2,...)is the number of the par-ticles with the size of D i (i ¼1,2,...).Both N i and
D i are given by LS measurement.At least 105par-ticles (i.e.,P
i N i ,measured by LS)were counted for each calculation.The polydispersity index (PDI)of the particle size was expressed as D w /D n .A value ranging from 1.00to 1.05can be regarded as a monodisperse distribution of the particle size.Determination of the Contents of Carboxyl Groups The contents of carboxyl groups in the final car-boxyl-functional composite nanoparticles were determined by both Fourier transform infrared (FTIR)and chemical titration methods.In the for-mer,products bearing various contents of carboxyl groups after Soxhlet extraction were analyzed by FTIR with a Nicolet 560FTIR spectrometer.The vibration peak of the carbonyl group (1738cm À1)was taken as the reference peak.Thus,the area ratio of the carboxyl peak (1703cm À1)to the refer-
ence peak could be calculated.The quantifications of the carboxyl contents (with respect to PMMA)were determined,with a calibration curve obtained from mixtures of MAA and MMA with different ratios.In the latter (i.e.,the titration method),a sample containing a product with a known mass was swollen in 1,4-dioxane for 24h.Then,a standardized NaOH solution with a known volume was intro
duced to neutralize the carboxyl group of the sample.The excess NaOH was then titrated by a standardized solution of an HCl–dioxane reagent with cresol red as an indica-tor.Experimental error due to dissolved CO 2was minimized by the performance of the titration under a nitrogen atmosphere.The difference between the number of moles of NaOH originally added and that neutralized by HCl equaled the number of moles of carboxyl groups.32Data from a minimum of three sets of these analyses were averaged.
RESULTS AND DISCUSSION
Batch Procedure
In this emulsifier-free emulsion copolymeriza-tion system,the hydrophilic ionic acrylate end and the hydrophobic carbon–carbon double-bond end made NaMA amphiphilic,so the role of NaMA was twofold:emulsifier and comonomer.Because silica was pretreated with MPTMS,it was hydrophobic and could be well dispersed in MMA after an ultrasonic treatment.When
a
Scheme 1.Preparation of polymer/silica composite nanoparticles bearing carboxyl groups on the surface.
POLYMER/SILICA COMPOSITE NANOPARTICLES 2829
mixture of MPTMS-treated silica and MMA was introduced into the system containing the emul-sifier(N
aMA),it broke and formed small drop-lets under the drive of emulsification and shear, and MMA absorbed onto the hydrophobic sur-face of MPTMS-treated silica.As an emulsifier, NaMA molecules covered the surfaces of the droplets and stabilized them.The hydrophilic ionic acrylate ends made NaMA molecules approach a water phase rather than mix with MMA homogeneously and go inside the droplets. Meanwhile,the hydrophobic double-bond ends admixed with the oil droplets and were oriented toward the center of the droplets,which could copolymerize with MMA and act as a comono-mer(Scheme1).Our previous work on the encapsulation of MPTMS-treated silica by poly-mers via emulsion polymerization has shown that the amount of the emulsifier is very impor-tant with respect to the binding efficiency.24 When excess emulsifiers were used,free latices formed,and this led to decreased binding effi-ciency.When the amount of the emulsifiers was not sufficient,partial demulsification easily hap-pened,especially in the presence of polar como-nomers,and this also led to decreased binding efficiency.Therefore,a series of copolymeriza-tions with different amounts of NaMA were per-formed,as shown in Table1.
Figure1shows the effects of the amount of NaMA on the yield and binding efficiency.The trends of these two parameters are similar to those observed in our previous work mentioned previously in which sodium dodecyl sulfonate (SDS)was used as the emulsifier.It is clear that the optimal amount of
NaMA was around 2.7mmol,at which both the yield and binding efficiency were higher than90%.With either less or more NaMA,the binding efficiency decreased, and partial demulsification or the formation of free latices occurred,respectively.It seems that NaMA behaves like the normal emulsifier SDS.
The average size(D n)and size distribution (PDI)of the composite nanoparticles obtained with different recipes are listed in Table2.With an increasing amount of NaMA,the average particle size decreased gradually.Again,NaMA showed a typical behavior of a normal emulsi-fier.Only when the amount of NaMA was in the range of the optimal value did the obtained com-posite nanoparticles have a narrow size distribu-tion,and they could be considered monodisperse because the PDI was equal to1.04.With less or more NaMA,either the agglomeration of par-ticles(due to less emulsifier)or the formation of free latices without a silica core widened the size distribution.
Two-Stage Procedure
As mentioned previously,in the batch procedure, only with the optimal amount of NaMA feeding could the composite materials be obtained with a high yield and binding efficiency.Therefore, the number of functional groups that could be effectively bound to the composite nanoparticles was limited.To obtain composite nanoparticles with a wide range of functional groups,a two-stage procedure in view of NaM
A addition was investigated.In thefirst stage,an optimal amount of NaMA was copolymerized with MMA (as recipe B3)so that latex seeds could be formed in a high yield and binding efficiency.In the second stage,NaMA was added dropwise to the system to keep NaMA under starved condi-tions to avoid the formation of free
latices.
Table2.Evaluation of the Encapsulation with
Different Recipes
Recipe D n(nm)PDI Na
reactive toB164.6  1.13—
B253.8  1.14—
B338.5  1.04  1.08
B437.4  1.11—
B535.2  1.17—
T139.6  1.04  1.14
T237.9  1.030.95
T340.1  1.07  1.04
T441.5  1.05  1.06
T540.8  1.080.94 2830ZENG,YU,AND GUO
Figure2shows the yield and binding effi-ciency versus the total amount of NaMA feed-ing.As expected,good encapsulation was obtained when the amount of NaMA varied over a wide range(from10to70mmol).The yields were all higher than90%,and the binding effi-ciencies were superior to85%.The high effi-ciency can be explained by the following points. First,the latex seeds obtained during thefirst stage had a very high binding efficiency(95%). Second,when NaMA was added to the emulsion system during the second stage,thefirst-stage copolymerization of MMA and MAA had justfin-ished,as indicated by the kinetic curve(shown later in Fig.5),the terminal free radicals of the copolymer chains were still alive,and the newly added NaMA could still copolymerize to the existing copolymer chains.Third,NaMA added during the second stage was adsorbed onto the latex particle and copolymerized almost instan-taneously because it was kept under starved conditions.33There was no formation of extra micelles,and this prevented the formation of free latices[pure poly(sodium methacrylate) (PNaMA)].Fourth,as the copolymerization of NaMA proceeded,the particles grew,the sur
fa-ces of the particles became larger,and more NaMA could be adsorbed and accommodated on the surface without saturation ever being reached.
As shown in Table2,the average particle sizes were all around40nm with different amounts of NaMA feeding,and the particle size distributions were all narrow.In this two-stage procedure,the latex seed that formed during the first stage had a fairly narrow particle size dis-tribution(PDI¼1.04).During the second stage, the polymerization of NaMA occurred on the surface of the seeds,and little new latices formed because NaMA was kept under starved conditions.Therefore,thefinal composite par-ticles had a narrow size distribution. Determination of Carboxyl Groups
The carboxyl contents of the composite nanopar-ticles were quantified independently with two methods:a physical method(FTIR)and a chemi-cal method(titration).Figure3shows the FTIR spectra of the products with different recipes. The band at1703cmÀ1in the spectra can be attributed to the C¼¼O stretching vibration of the carboxyl group,proving that the carboxyl group was bound onto the composite particles. In comparison with the spectrum of PMMA/ silica composite nanoparticles prepared previ-ously,the extra characteristic peaks of PNaMA at1566cmÀ1can hardly be observed,and this suggests that almost all of the salt was trans-lated into carboxyl groups.The intensity of the C¼¼O band increased with an increasing amount of NaMA feeding.
The titration method was calibrated with known concentrations of acrylic acid,acetic acid, and HCl.As shown in Figure4,the average error in a set of(at least three)duplicate analy-ses for one compound at one carboxyl concentra-tion was63.2%.The correlation between the numbers of moles of carboxyl groups initially in each sample and that measured was linear with a slope of1.0027and an intercept of3.2Â10À5
. Figure3.FTIR spectra of composite nanoparticles with different MAA feed concentrations:(a)0,(b)10, (c)20,(d)30,and(e)
40%.

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