Journal of Colloid and Interface Science 368 (2012) 220–225

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Journal of Colloid and Interface Science

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Synthesis of porous poly(styrene-co-acrylic acid) microspheres through one-stepsoap-free emulsion polymerization: Whys and wherefores

Rui Yan, Yaoyao Zhang, Xiaohui Wang, Jianxiong Xu, Da Wang, Wangqing Zhang ⇑Key Laboratory of Functional Polymer Materials of Ministry of Education, Institute of Polymer Chemistry, Nankai University, Tianjin 300071, China

a r t i c l e i n f o

Article history:Received 8 September 2011Accepted 3 November 2011Available online 16 November 2011

Keywords:Emulsion polymerizationMicrospheresPhase separationPorous microspheresPoly(styrene-co-acrylic acid)

0021-9797/$ - see front matter � 2011 Elsevier Inc. Adoi:10.1016/j.jcis.2011.11.016

⇑ Corresponding author. Fax: +86 22 23503510.E-mail address: wqzhang@nankai.edu.cn (W. Zhan

a b s t r a c t

Synthesis of porous poly(styrene-co-acrylic acid) (PS-co-PAA) microspheres through one-step soap-freeemulsion polymerization is reported. Various porous PS-co-PAA microspheres with the particle size rang-ing from 150 to 240 nm and with the pore size ranging from 4 to 25 nm are fabricated. The porous struc-ture of the microspheres is confirmed by the transmission electron microscopy measurement andBrunauer–Emmett–Teller (BET) analysis. The reason for synthesis of the porous PS-co-PAA microspheresis discussed, and the phase separation between the encapsulated hydrophilic poly(acrylic acid) segmentand the hydrophobic polystyrene domain within the PS-co-PAA microspheres is ascribed to the poreformation. The present synthesis of the porous PS-co-PAA microspheres is anticipated to be a new andconvenient way to fabricate porous polymeric particles.

� 2011 Elsevier Inc. All rights reserved.

1. Introduction

In recent years, there has been great interest in the fabrication ofporous or hollow polymeric particles for their potential applica-tions including drug delivery, stationary phase for chromatography,supports for exchange of ionic compounds, carriers for catalysts,and so on [1–6]. The current approaches to the synthesis of porousmicrospheres are generally divided into two categories: (1) hardtemplate synthesis [7–16] and (2) soft template synthesis[17–29], each of which involves several well-established proce-dures, although some other methods such as emulsion/solventevaporation [30], spray drying [31], controlled phase separation[32,33], solvent swelling [34,35], incorporation of a blowing agent[36,37], and self-assembly [38] are also proposed. The hard tem-plate method, in which either the non-cross-linked polymericmicrospheres or the linear polymers are employed as the pore-forming agent, has the advantage of the controllable tuning in poresize [10–13]. However, the synthesis of porous microspheresthrough the hard template method involves multiple and complexprocedures. For example, the general method to fabricate hollow/porous microspheres by hard template polymerization includessynthesis of a suitable seed, polymerization of the shell-formingmonomer on the seed to form coated microspheres, and thenremoval of the seed by solvent etching [9–14]. In the soft templatesynthesis of porous microspheres, diluents [17–20] or surfactantmicelles [25–27] are usually employed as the pore-forming agent.With this method, template elimination is avoided, while the por-

ll rights reserved.

g).

ous structure is not easy to control. When diluents are adopted asthe pore-forming agent, the non-solvent is responsible for the for-mation of meso- and macro-pores in the internal structure of theresultant polymeric particles and the good solvent is responsiblefor the formation of microporous structure [17–20]. Porous micro-spheres are also prepared by the polymerization of the multiphaseemulsion stabilized by unitary or binary surfactants [25–27]. Forexample, the typical methods for making W/O/W emulsions in-clude a two-step process of the initial formation of an inversewater-in-oil emulsion and then emulsification in water by combi-nation of binary surfactants, and the subsequent controlled poly-merization of the W/O/W emulsion leads to the formation ofporous particles [25].

This work presents and discusses the convenient synthesis ofporous poly(styrene-co-acrylic acid) (PS-co-PAA) microspheres byone-stage soap-free emulsion polymerization. The present synthe-sis of the porous PS-co-PAA microspheres is much different fromthe general hard template synthesis and the soft template synthesisas introduced above. The reason for the synthesis of the porousmicrospheres is discussed and it is deemed that the phase separationbetween the encapsulated hydrophilic poly(acrylic acid) (PAA) seg-ment and the hydrophobic polystyrene (PS) domain within the ma-trix of the PS-co-PAA microspheres is ascribed to the pore formation.

2. Experimental

2.1. Materials

The monomers of styrene (St, >98%, Tianjin Chemical Company),acrylic acid (AA, >99%, Tianjin Chemical Company), and methyl

R. Yan et al. / Journal of Colloid and Interface Science 368 (2012) 220–225 221

acrylic acid (MAA, >99%, Tianjin Chemical Company) were distilledunder vacuum before being used. The initiator of K2S2O8 (>99.5%,Tianjin Chemical Company) and other analytical reagents wereused as received. Deionized water was used in the present study.

2.2. Synthesis of porous and solid PS-co-PAA microspheres

To prepare the porous PS-co-PAA microspheres, neutral water(120.0 mL) and a given amount of AA were initially added into aglass flask. After the feeding AA being fully dissolved, a givenamount of styrene was added. The total amount of the feedingmonomers of AA and styrene was kept at 6.00 g. The flask contentwas vigorously stirred at about 300 rpm for about 30 min at roomtemperature, and then a given weight of the K2S2O8 initiator(0.030–0.120 g) was added. The flask content was degassed undernitrogen purge, and the polymerization was performed with vigor-ously stirring at 80 �C for 24 h under nitrogen atmosphere. Afterthe completion of the polymerization, the resultant colloid waswashed with water (20 mL � 3), collected by centrifugation, anddried under vacuum at 40 �C to obtain the porous PS-co-PAAmicrospheres. The solid PS-co-PAA microspheres were preparedwith the feeding monomers of styrene (4.80 g, 0.0461 mol) andAA (1.20 g, 0.0167 mol) as similar as the PS-co-PAA microspheresexcept NaOH (0.667 g, 0.0167 mol, which is equimolar amount rel-ative to AA) being added into the solvent of water (120.0 mL)before addition of the monomers, and therefore the polymerizationwas performed in basic aqueous solution (pH = 13). Herein, itshould be noted that solid PS-co-PAA microspheres are named,although the solid microspheres synthesized in basic aqueous solu-tion should be poly(styrene-co-sodium acrylate) microspheres.

2.3. Synthesis of the reference solid polystyrene microspheres and thesolid poly(styrene-co-methyl acrylic acid) microspheres

The solid poly(styrene-co-methyl acrylic acid) (PS-co-PMAA)microspheres and the solid polystyrene (PS) microspheres were syn-thesized in neutral water with a similar way for the porous PS-co-PAA microspheres. For the synthesis of the solid PS-co-PMAA micro-spheres, neutral water (120.0 mL) and MAA (1.20 g, 0.0139 mol)were initially added into a flask and followed addition of styrene(4.80 g, 0.0461 mol). The flask content was vigorously stirred atroom temperature for 30 min and then K2S2O8 (0.060 g, 0.22 mmol)was added. The flask content was degassed under nitrogen purge,and polymerization was performed with vigorously stirring at80 �C for 24 h under nitrogen atmosphere. The resultant PS-co-PMAA microspheres were collected by centrifugation, washed withwater (20 mL � 3), and dried under vacuum at 40 �C to obtain the so-lid PS-co-PMAA microspheres. The solid PS microspheres were pre-pared and collected with the similar procedures except thepolymerization that was performed in the absence of the hydro-philic monomer of MAA.

2.4. Characterization

Transmission electron microscopy (TEM) measurement wasconducted by using a Philips T20ST electron microscope at an accel-eration voltage of 200 kV, whereby a small drop of the dispersion ofthe synthesized microspheres was deposited onto a piece of coppergrid and then dried at room temperature under vacuum. The nitro-gen adsorption–desorption analysis was carried out at 77 K on aMicromeritics Gemini V system. The specific surface area wasdetermined by the Brunauer–Emmett–Teller (BET) method, andthe mesopore size distribution was calculated from the adsorptionisotherms using the Barret–Joyner–Halenda method. The 1H NMRspectra were recorded on a UNITYPLUS-400 spectrometer usingCDCl3 as solvent. Dynamic laser scattering (DLS) measurements

were taken on Nano-ZS90 (Malvern) laser light scattering spec-trometer with He–Ne laser at a wavelength of 633 nm.

3. Results and discussion

3.1. Synthesis of the porous PS-co-PAA microspheres

The present synthesis of the porous PS-co-PAA microspheresthrough one-step soap-free emulsion polymerization in neutralwater is as similar as those of the core–shell microspheres dis-cussed in the previous manuscript [39]. Fig. 1 shows the TEMimages of the porous PS-co-PAA microspheres synthesized withdifferent molar ratio of I/M, in which I represents the K2S2O8 initi-ator and M represents the feeding monomers of St/AA (4/1 byweight). Generally, the PS-co-PAA microspheres have core–shellstructure, which are somewhat as similar as the core–shell micro-spheres synthesized by one-stage soap-free emulsion polymeriza-tion as discussed elsewhere [39]. It is deemed that the core ismainly composed of the hydrophobic PS segment, and the shellis mainly composed of the hydrophilic PAA segment. However, dif-ferent from the general core–shell microspheres [39], 4–10 nmpores can be discerned in all the PS-co-PAA microspheres as shownin Fig. 1. Typically as shown in Fig. 1A, dispersed microsphereswith average size at about 165 nm are fabricated at I/M = 1. Fromthe high-magnification TEM image shown in Fig. 1B, various pores,some of which are indicated by cycles, with size ranging from4–10 nm have been discerned, confirming the synthesis of porousmicrospheres. DLS analysis shows that the hydrodynamic diameterDh of the porous microspheres centered at 180 nm is uniformly dis-tributed (Fig. 2A). It is found that the feeding monomers are almostquantitatively converted into the porous PS-co-PAA microspheres.Based on the weight ratio of the feeding monomers of St/AA andthe average size of the porous PS-co-PAA microspheres as shownin Fig. 1A, the shell thickness of the porous PS-co-PAA micro-spheres, 32 nm, is approximately estimated, assuming the core ofthe microspheres being composed of the sole PS segment and theshell being composed of the sole PAA segment. Clearly, the calcu-lated value is as similar as those by the TEM image shown inFig. 1B (32 nm vs. �34 nm). The slight difference between the cal-culated and the measured values is discussed. It is believed that theshell should be composed of the copolymer of styrene and acrylicacid but not the sole PAA segment, which will make a thick shelland will be further discussed subsequently. Furthermore, it isfound that porous PS-co-PAA microspheres with pore size at4–10 nm can be fabricated within a wide I/M range, although thesize of the porous PS-co-PAA microspheres is somewhat different.For example, 238 nm porous PS-co-PAA microspheres (Fig. 1C)are synthesized at I/M = 0.5, and 222 nm porous PS-co-PAA micro-spheres (Fig. 1D) are fabricated at I/M = 2. The DLS analysis showsthat the hydrodynamic diameter Dh of these both microspheres isuniformly dispersed (Fig. 2B and C).

The monomer of MAA is somewhat as similar as AA. For example,both MAA and AA are hydrophilic and contain the carboxyl group. Itis expected that porous PS-co-PMAA microspheres should also besynthesized with the similar method as the porous PS-co-PAAmicrospheres. However, the speculation is not the fact. As shownin Fig. 3A, 180 nm solid PS-co-PMAA microspheres instead of porousmicrospheres are fabricated at I/M = 1.0% with the weight ratio ofthe feeding monomer of St/MAA = 4/1. The solid PS-co-PMAA micro-spheres are just as similar as the 330 nm solid PS microspheres syn-thesized at I/M = 1.0% (Fig. 3B). In comparison, both the solid PS-co-PMAA microspheres and the solid PS microspheres show no porouscharacter, which are much different from the porous PS-co-PAAmicrospheres as shown in Fig. 1, although these three microspheresare synthesized under the same conditions.

A B

C D

Fig. 1. TEM images of the porous PS-co-PAA microspheres synthesized at I/M = 1.0% (A and B), 0.50% (C), and 2.0% (D) with the weight ratio of the feeding monomer St/AA = 4/1.

10 50 100 500 1000

f (D

h)

Diameter (nm)

A BC

Fig. 2. The hydrodynamic diameter (Dh) distribution of the porous PS-co-PAAmicrospheres synthesized at I/M = 1.0% (A), 0.50% (B), and 2.0% (C) with the weightratio of the feeding monomer St/AA = 4/1.

222 R. Yan et al. / Journal of Colloid and Interface Science 368 (2012) 220–225

The BET analysis is usually utilized to characterize porous mate-rials [40]. To diagnose the porosity, the porous PS-co-PAA micro-spheres synthesized at different I/M values and the typicalreference sample of the solid PS microspheres are characterizedby BET nitrogen adsorption analysis, and the results are shown inTable 1. Herein, the porous PS-co-PAA microspheres synthesizedat I/M = 0.50, 1.0, and 2.0 are called porous PS-co-PAA-0.5, PS-co-PAA-1.0, and PS-co-PAA-2.0 microspheres, respectively. From theN2 adsorption–desorption isotherms shown in Fig. 4, the hysteresis

loop at P/P0 > 0.2 is observed for the porous PS-co-PAA-0.5 and PS-co-PAA-2.0 microspheres (Fig. 4A and C). The BET surface area andpore volume are 13.4 m2/g and 0.27 cm3/g for the porous PS-co-PAA-0.5 microspheres and 16.2 m2/g and 0.29 cm3/g for the porousPS-co-PAA-2.0 microspheres, respectively. The hysteresis loop forthe porous PS-co-PAA-1.0 microspheres is observed at higher P/P0

value of 0.4 than those for the porous PS-co-PAA-0.5 andPS-co-PAA-2.0 microspheres at P/P0 > 0.2 (Fig. 4B), and the BET sur-face area of 13.7 m2/g and the pore volume of 0.16 cm3/g arecalculated. The hysteresis loop at high P/P0 > 0.4 for the porousPS-co-PAA-1.0 microspheres is possibly due to their smallest sizein all the three porous particles (see in Table 1). As a comparison,the reference sample of the solid PS microspheres exhibits type Iisotherms (Fig. 4D), and the corresponding BET surface area andthe pore volume are 10.6 m2/g and 0.067 cm3/g, respectively.Clearly, all the porous PS-co-PAA microspheres have a much largerpore volume than the solid PS microspheres, confirming the porouscharacter of the PS-co-PAA microspheres [40]. Based on the porevolume of the porous PS-co-PAA microspheres and the solid PSmicrospheres, the pore volume percent in the three PS-co-PAAmicrospheres is estimated to be about 9–22%.

Now, the possible reason on synthesis of the porous PS-co-PAAmicrospheres is discussed. As introduced in the experimental sec-tion, no surfactant is employed in the present synthesis of theporous PS-co-PAA microspheres, thus the synthesis of the porousPS-co-PAA microspheres should be different from those of W/O/Wemulsion employing surfactants [25–27]. We suppose that thepore formation within the matrix of the porous PS-co-PAA micro-spheres is due to the phase separation between the encapsulated

Fig. 3. TEM images of the reference solid PS-co-PMAA microspheres synthesized at I/M = 1.0% with the weight ratio of the feeding monomer St/MAA = 4/1 (A) and the solid PSmicrospheres synthesized at I/M = 1.0% (B).

Table 1The parameters of the porous PS-co-PAA microspheres and the reference solid PS microspheres.

Entry Microspheres Diameter(nm)

Pore volume(cm3/g)

BET surface area(m2/g)

Pore size(nm)

1 PS-co-PAA-0.5 238 0.27 13.4 5.72 PS-co-PAA-1.0 165 0.16 13.7 6.83 PS-co-PAA-2.0 222 0.29 16.2 8.44 PS 330 0.067 10.6 –

0.0 0.2 0.4 0.6 0.8 1.00

100

200

300

400

500

C

D

B

A

Volu

me

adso

rbed

(cm

3 /g)

Relative Pressure (P/P0)

Fig. 4. The N2 adsorption–desorption isotherms of the porous PS-co-PAA-0.5microspheres (A), PS-co-PAA-1.0 microspheres (B), PS-co-PAA-2.0 microspheres(C), and the reference sample of solid PS microspheres (D). The isotherms of (A), (B),and (C) were offset vertically by 300, 200, and 100 cm3/g.

R. Yan et al. / Journal of Colloid and Interface Science 368 (2012) 220–225 223

hydrophilic PAA segment and the hydrophobic PS domain. As dis-cussed above, the porous PS-co-PAA microspheres have a PS-richcore and a PAA-rich shell. The hydrophilic PAA segment encapsu-lated within the PS-rich core is incompatible with the hydrophobicPS domain; and therefore, phase separation between the encapsu-lated PAA segment (possibly as well as some concomitant water)and the PS domain occurs. It should be noted that the possiblephase separation in the PAA-rich shell of the PS-co-PAA micro-spheres also occurs, but it is not discussed for the sake of concise-ness. It is speculated that the phase separation produces thepores within the matrix of the PS-co-PAA microspheres. To confirmthe speculation, the following experiments are made.

First, the exact amount of the hydrophilic monomer of AA dis-solved in the hydrophobic monomer of styrene in neutral water

before polymerization is measured. Clearly, to diagnose the phaseseparation between the encapsulated PAA segment and the PS do-main, the amount of the encapsulated PAA segment within the PSdomain should be determined. However, it is very difficult to knowthe exact amount of the encapsulated PAA within the PS-rich core ofthe PS-co-PAA microspheres. To overcome this difficulty, theamount of the hydrophilic monomer of AA dissolved in the hydro-phobic monomer of styrene in neutral water is measured. We thinkthat the dissolved AA in the hydrophobic monomer of styreneapproximately corresponds to the encapsulated PAA segment with-in the PS domain. Thus, after the addition of the monomer mixtureof AA/styrene (1.20 g/4.80 g) into neutral water (120.0 mL) inwhich the feeding monomer is just as same as those in the synthesisof the porous PS-co-PAA microspheres discussed in the Experimen-tal section, the mixture is magnetically stirred for 1 h, kept at roomtemperature for 1 h without stirring, and then the supernatant oillayer is analyzed by 1H NMR. It is found that the supernatant oillayer contains 99 wt% styrene and 1 wt% AA. Thus, it is deemed thatthe PS-rich core of the PS-co-PAA porous microspheres is composedof 99 wt% PS segment and 1 wt% PAA segment, although the specu-lation is very approximate.

Second, the synthesis of PS-co-PAA microspheres in basic aque-ous solution is explored. As discussed above, the formation of PS-co-PAA porous microspheres in neutral water is ascribed to thephase separation between the encapsulated PAA segment and thePS domain. Thus, if the feeding hydrophilic monomer is fully solu-ble in the aqueous phase but not in the oil (styrene) phase, theamount of the encapsulated PAA segment in the PS domain shouldbe very less, and therefore solid PS-co-PAA microspheres should besynthesized. This is found to be the just case. When a given amountof NaOH, which is equimolar to the AA monomer, is initially addedin water to neutralize the AA monomer and then the polymeriza-tion is performed under other similar conditions. It is found that220 nm solid microspheres (Fig. 5) are fabricated, which is muchdifferent from those of the porous microspheres synthesized inneutral water as shown in Fig. 1. These results confirm that the

Fig. 6. TEM images of the porous PS-co-PAA microspheres synthesized in neutral water at I/M = 1.0% with the weight ratio of the feeding monomer St/AA = 1/1 (A and B)and 2/1 (C and D).

Fig. 5. TEM images of the solid PS-co-PAA microspheres synthesized in basic aqueous solution (pH = 13.0) at I/M = 1.0% with the weight ratio of the feeding monomerSt/AA = 4/1.

224 R. Yan et al. / Journal of Colloid and Interface Science 368 (2012) 220–225

synthesis of PS-co-PAA porous microspheres in neutral water isjust ascribed to the phase separation between the encapsulatedPAA segment and the PS domain within the matrix of thePS-co-PAA microspheres.

Based on the above-mentioned discussion, the synthesis of theporous PS-co-PAA microspheres through one-step soap-free emul-sion polymerization in neutral water is further illuminated. Afteraddition of the monomers of AA and styrene in neutral water, the

hydrophilic monomer of AA is soluble in water. Besides the solubleAA in aqueous phase, the concentration of AA at the monomer drop-let (which is composed of �99 wt% styrene and �1 wt% AA) inter-face is high because AA somewhat acts as stabilizer for themonomer droplets in aqueous phase. The AA mass at the monomerdroplet interface depends on the surface and/or size of the droplet.The AA concentration in aqueous phase depends on the numberand size of the monomer droplets and the feeding AA, which is high

R. Yan et al. / Journal of Colloid and Interface Science 368 (2012) 220–225 225

enough in the present study. The polymerization initiator of K2S2O8

is very hydrophilic and primary radicals form in water phase, andtherefore the initial polymerization occurs in two fields: the aque-ous phase and the monomer droplet interface to produce the hydro-philic oligomer of AA. With the progress of the polymerization, theoligomer of AA can substitute the hydrophilic monomer of AA atthe monomer droplet interface, and therefore copolymer of AA withstyrene preferably forms at the drop interface due to the suitableconstants of copolymerization of r1 = 0.15–0.45 (AA) and r2 = 0.15–0.25 (styrene) and the high concentration of styrene and AA in themonomer droplet interface. Most of this copolymer locates on thedroplet interface to construct the shell of the core–shell micro-spheres. Besides, some of the copolymer may diffuse into the mono-mer droplets especially at the initial polymerization due to the lowviscosity of the monomer droplets and be encapsulated within theresultant microspheres after completion of the polymerization.Due to the incompatibility between the encapsulated hydrophilicPAA segment, which is possibly accompanied with water, and thehydrophobic PS domain, formation of cores occurs and thereforeporous PS-co-PAA microspheres are produced. As to the synthesisof the solid PS-co-PMAA microspheres, the polymerization in neu-tral water runs similarly with those for the porous PS-co-PAA micro-spheres. However, since the PMAA segment is not as hydrophilic asthe PAA segment, the phase separation between the encapsulatedPMAA segment and the hydrophobic PS domain is not as strong asthose between PAA and PS, thus solid PS-co-PMAA microspheres in-stead of porous particles are resulted.

3.2. Synthesis of porous PS-co-PAA microspheres under different ratiosof feeding monomer of St/AA

As discussed above, the synthesis of the porous PS-co-PAAmicrospheres in neutral water with the feeding monomer of St/AA = 4/1 is performed and the reason is discussed. Herein, the syn-thesis of porous PS-co-PAA microspheres with a wide ratio range ofthe feeding monomer of St/AA is further introduced. Fig. 6 showsthe TEM images of the PS-co-PAA porous microspheres synthesizedat I/M = 1.0% with the feeding monomer of St/AA equal to 1/1 and2/1, respectively. As indicated by the TEM images, 153 nm porousmicrospheres (Fig. 6A and B) and 192 nm porous microspheres(Fig. 6C and D) are fabricated under the both cases. The pores sizeranges from 4 to 25 nm, which is slightly larger than those of thePS-co-PAA porous microspheres synthesized with the feedingmonomer of St/AA equal to 4/1 as shown in Fig. 1. Interestingly,the particles are generally non-spherical especially for those syn-thesized at low ratio of the feeding monomer of St/AA (Fig. 6Aand B). It is found the polymerization runs very fast, and gel forma-tion can be optically observed at low ratio of the feeding monomerof St/AA such as at 1/3. Thus, the formation of the non-sphericalparticles of the porous PS-co-PAA microspheres as shown inFig. 6 is possibly due to the fast polymerization at high concentra-tion of the hydrophilic AA monomer.

4. Conclusions

Synthesis of porous PS-co-PAA microspheres through one-stepsoap-free emulsion polymerization is reported. Various PS-co-PAAporous microspheres with the particles size ranging from 153 to238 nm and the pore size ranging from 4 to 25 nm are fabricatedby tuning the concentration of the K2S2O8 initiator or the weight

ratio of the feeding monomer of St/AA. To diagnose the reason forthe synthesis of the porous PS-co-PAA microspheres, synthesis ofreference solid PS-co-PMAA microspheres and solid PS micro-spheres is checked and the encapsulation of the hydrophilic PAAsegment within the hydrophobic PS domain is explored. The resultsdemonstrate that the synthesis of porous PS-co-PAA microspheresis ascribed to the phase separation between the encapsulatedhydrophilic PAA segment and the hydrophobic PS domain withinthe PS-co-PAA microspheres. The present synthesis of porous PS-co-PAA microspheres is anticipated to broad a new and convenientway to fabricate porous polymeric particles through phase separa-tion within the particle matrix.

Acknowledgments

The financial support by National Science Foundation of China(Nos. 20974051 and 21074059) and Tianjin Natural ScienceFoundation (No. 09JCYBJC02800) is gratefully acknowledged.

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