Synthesis of monodisperse poly(methacrylic acid) microspheres by distillation–precipitation polymerization

EUROPEAN POLYMER JOURNAL

European Polymer Journal 43 (2007) 3923–3932

www.elsevier.com/locate/europolj

Synthesis of monodisperse poly(methacrylic acid) microspheres by distillation–precipitation polymerization Feng Bai, Bo Huang, Xinlin Yang *, Wenqiang Huang Key Laboratory of Functional Polymer Materials, The Ministry of Education, Institute of Polymer Chemistry, Nankai University, Tianjin 300071, China Received 14 March 2007; received in revised form 6 June 2007; accepted 14 June 2007 Available online 28 June 2007

Abstract Monodisperse poly(methacrylic acid) (PMAA) microspheres were prepared by distillation–precipitation polymerization in acetonitrile with 2,2 0 -azobisisobutyronitrile (AIBN) as initiator. The polymeric microspheres were formed simultaneously via a precipitation polymerization manner during the distillation of the solvent out of the reaction system in the absence of any surfactant and crosslinker. Monodisperse PMAA microspheres with spherical shape and smooth surface were synthesized with diameters ranging from 60 to 290 nm below the glass transition temperature of PMAA without any stabilizer. The particle size increased with increasing monomer concentration, which may be resulted from the higher molecular weight for the polymerization. To investigate the growth procedure of PMAA microspheres, the morphology of microspheres over the distillated acetonitrile volume was conducted by monitoring the morphologies with TEM. GPC and FTIR provide key insights into the particle growth mechanism. The PMAA microspheres may be formed by an internal contraction due to the marginal solvency of the continuous phase with the aid of the hydrogen-bonding interaction between the carboxylic acid unit, in which the particles were stabilized by the steric effect of the pendent chains and surface gel as well as the electrostatic repulsion from the carboxylic acid group.  2007 Elsevier Ltd. All rights reserved. Keywords: Poly(methacrylic acid) microsphere; Hydrogen-bonding interaction; Distillation–precipitation polymerization; Growth mechanism

1. Introduction Hydrophilic polymer particles are attractive for a wide number of applications, including dispersants, thickeners, flocculants, and superabsorbant polymer [1]. Methacrylic acid or acrylic acid is commonly

* Corresponding author. Tel.: +86 22 23502023; fax: +86 22 23503510. E-mail address: [email protected] (X. Yang).

polymerized in aqueous solution, which can also be heterogeneously polymerized in organic media [1]. Hydrophilic carboxylic monomers are often utilized for the stabilization during the polymer colloid formulations, freeze-thaw stability, and improved film forming properties as well as bonding agents in latex-base paper coatings [2]. The carboxylic acid commoners forms a major component of water-soluble chains on the surface of the latex particles referred as ‘hairy layer’ to provide both steric and electrostatic stabilization of the colloids [3,4].

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Most of the previous researches on the polymerization of methacrylic acid were carried out in batch reactors to afford the products as white, fluffy, and fine powders. The resultant polymer was a coagulum of primary particles with diameters around 100– 200 nm [5–7]. Roberts and his co-workers studied the kinetics to determine the locus of polymerization [8], the effects of the operating variables on the polymerization rate and on the polymer molecular weight [9], and the effects of the reaction conditions on the morphology of the polymer during continuous precipitation polymerization of methacrylic acid in supercritical carbon dioxide (scCO2). These precipitation polymerizations resulted in three morphologies: a coagulum of primary particles with diameters of 100–200 nm, irregular particles with diameters of 5–20 lm, and spheres with diameters of 10–100 lm [10]. The morphology of the resultant PMAA was a kind of coagulum of particles with various diameters in the range of 100 nm–100 lm having irregular shape and rough surface. In our previous work, we reported distillation– precipitation polymerization as a novel technique to prepare monodisperse polymer microspheres having different functional groups with either divinylbenzene (DVB) or ethyleneglycol dimethacrylate (EGDMA) as crosslinker [11–17]. It was reported that the hydrogen-bonding involved methacrylic acid monomer had much effect on the morphology of the resultant polymer network for precipitation polymerization [18] and distillation–precipitation polymerization [15,17,19] in the presence of either EGDMA or DVB as crosslinker. This paper described the preparation of monodisperse poly(methacrylic acid) (PMAA) microspheres by distillation–precipitation polymerization in neat acetonitrile with 2,2 0 -azobisisobutyronitrile (AIBN) as initiator in the absence of any crosslinker without any additive as stabilizer. 2. Experimental section 2.1. Chemicals Methacrylic acid (MAA) was purchased from Tianjin Chemical Reagent II Co. and purified by vacuum distillation. 2,2 0 -Azobisisobutyronitrile (AIBN) was analytical grade available from Chemical Factory of Nankai University, and was recrystallized from methanol. Acetonitrile (analytical grade, Tianjin Chemical Reagents II Co.) was dried ˚ molecular sieves and purified by distillaover 4 A

tion before use. The other reagents were of analytical grade and used without any further purification. 2.2. Preparation of monodisperse PMAA microspheres by distillation–precipitation polymerization A typical procedure for the distillation–precipitation polymerization: MAA (2.0 mL, 2.0 g, 23.3 mmol, relative to 2.5 vol% of the whole reaction medium) and AIBN (0.04 g, 0.24 mmol, 2 wt% corresponding to the monomer) were dissolved in 80 mL of acetonitrile in a dried 100-mL two-necked flask attaching with a fractionating column, Liebig condenser and a receiver. The flask was immersed in a heating mantle and the reaction mixture was heated from ambient temperature to the boiling state within 20 min. Then the solvent began to be distilled from the reaction system. The initially homogeneous reaction mixture became opalescent and then deepened in color as a milky white dispersion after boiling for 10 min. The reaction was ended after 40 mL of acetonitrile was distilled from the reaction system within 1 h. The resultant PMAA microspheres were separated and purified by repeating ultra-centrifugation (10,000 rpm for 20 min), decanting, and resuspending in acetonitrile with ultrasonic bathing for three times. The procedures for the other distillation–precipitation polymerizations were similar to that of typical one, except for altering MAA loading in the range of 1.25–3.75 vol% relative to the whole reaction medium, while the AIBN initiator was fixed at 2 wt% comparing to MAA monomer. 2.3. Growth procedures of PMAA microspheres The growth procedure of PMAA microspheres was traced by transmission electron microscopy (TEM) observation, FTIR and gel permeation chromatography (GPC) characterization. Polymerization was carried out in a 2-L two-necked flask containing 800 mL of acetonitrile with 2.5 vol% of MAA monomer loading (relative to the reaction medium) and 2 wt% AIBN initiator (corresponding to the monomer). From the reaction system, 5 mL of reaction mixture was sampled each at different time and cooled immediately in liquid nitrogen to quit the polymerization. The PMAA microspheres for TEM and GPC characterization were separated by ultracentrifugation, and resuspended in acetonitrile with ultrasonic bathing for three times.

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2.4. Characterization The particle size and size distribution of PMAA microspheres were determined by transmission electron microscopy (TEM) using a transmission electron microscope (FEI TECNAI-20, Germany). All of the TEM size data reflect the averages about 20–100 particles each, which are calculated to the following formula: .X k Xk U ¼ Dw =Dn ; Dn ¼ ni Di ni ; i¼1 i¼1 Xk Xk Dw ¼ ni D4i = i¼1 ni D3i ; i¼1 where U is the polydispersity index, Dn is the number-average diameter, Dw is the weight-average diameter, N is the total number of the measured particles, and Di is the particle diameter of the determined microparticles. Gel permeation chromatography (GPC) was performed at 45 C using a Waters-510 with Waterts Ultrahydrogel-1000 column and Waters-410 refractive index detector, water was used as eluent and the calibration was accomplished at 45 C with a narrow molecular weight distribution (MWD) standard of polyethylene glycol (PEG) (Polymer Laboratories Ltd.). The sample solution in water was 1 g/L, 100 lL of which was injected the sample loop. Fourier transform infrared spectra were determined on a Bio-Rad FTS-135 FT-IR spectrometer over potassium bromide pellet and the diffuse reflectance spectra were scanned over the range of 400– 4000 cm1. 3. Results and discussion The formation of monodisperse polymer microspheres with various functional groups was reported in our previous works by distillation–precipitation polymerization [11–16]. In these cases, the crosslinkers such as DVB and EGDMA were necessary for the successful formation of monodisperse polymer microspheres with AIBN as initiator. Here, we intend to utilize this technique to afford monodisperse hydrophilic PMAA microspheres in the absence of crosslinker with AIBN as initiator in neat acetonitrile. 3.1. Effect of MAA concentration Distillation–precipitation polymerization is a heterogeneous process, in which the initiator, the

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monomer and the other comonomers initially form a homogeneous solution and then the polymer particles precipitate from the continuous liquid phase. Distillation–precipitation polymerization can be considered as a novel technique to synthesize high-purity polymers for sensitive applications, e.g., cosmetics and food additives, which are due to the absence of any either surfactant or additive required for the stabilization of the polymer particles. This is much different from that of suspension, emulsion and dispersion polymerizations. For the distillation–precipitation polymerization, low monomer loading, and a near h reaction condition were necessary to form narrow- or monodisperse polymer microspheres [11–17]. Apparently, acetonitrile meets the solvency conditions required for the formation of monodisperse particles, that is, it dissolves the monomer but precipitates the resultant polymer network. The effect of the solvent on the present distillation–precipitation polymerization could be interpreted by the three dimensional Hasen solubility parameters [18]. The interchain hydrogen-bonding interaction between the carboxylic acid groups in the polymer network promotes the formation of monodisperse microspheres during the distillation–precipitation polymerization [15,17,18]. However, the covalent crosslinkers such as DVB and EGDMA are still necessary for the successful formation of monodisperse polymer microspheres in these systems. It is expected to promote the formation of interchain hydrogen-bonding between the acid groups during the distillation–precipitation polymerization of hydrophilic monomer MAA in a non-protic polar solvent, acetonitrile. The interchain hydrogenbonding interaction between the carboxylic acid units should reduce the polymer–solvent interactions and aid the desolvation of the resultant polymer as a precipitate from the reaction medium during the polymerization and hence affect the morphology of the polymer network and particle formation. A series of experiments were initially designed to investigate the effect of the PMAA monomer concentration on the particle synthesis, in which MAA concentration was varied from 1.25 to 3.75 vol% (relative to the whole reaction medium) and the AIBN initiator level was maintained at 2 wt% comparing to the MAA monomer. The TEM micrographs of the resultant PMAA particles with different MAA concentrations are shown in Fig. 1 (only with A1, A3, and A5 as samples). All

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Fig. 1. TEM micrographs of PMAA microspheres with different MAA concentrations in the range of 1.25–3.75 vol% (relative to the whole reaction medium): (A1) 1.25 vol%; (A3) 2.50 vol%; (A5) 3.75 vol%.

the TEM images indicated that all PMAA microspheres had spherical shape with smooth surface. These results demonstrated that MAA in such range

of MAA concentration can be polymerized to afford stable microspheres in the absence of any stabilizer by distillation–precipitation polymerization.

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Table 1 Reaction conditions, size, size distribution, the molecular weight of PMAA microspheres, and monomer conversions with different MAA monomer concentrations Entry

Monomer feed (mL)

Dn (nm)

Dw/Dn (by TEM)

Mna (·103)

Mw/Mn

Monomer conversionb (%)

A1 A2 A3 A4 A5

1.0 1.5 2.0 2.5 3.0

126 175 179 276 288

1.011 – 1.004 1.009 –

3.84 9.15 17.1 14.4 21.0

2.19 2.00 2.62 2.38 2.81

67

a b

73 77

Molecular weight determined by GPC. Monomer conversions determined by the acid-base titration of the residual MAA in the system.

The size and size distribution of the final PMAA particles for the polymerization with different MAA concentrations in the range of 1.25–3.75 vol% were summarized in Table 1. The results demonstrated that all the resultant PMAA microspheres were monodisperse with a distribution index around 1.009–1.011. The narrowest distribution of PMAA particles was obtained at MAA of 2.5 vol% with the diameter of 179 nm and the polydispersity index of 1.004 (Entry A3). The size of the PMAA particles increased from 126 nm for MAA of 1.25 vol% (relative to the whole reaction medium) (Entry A1) to 288 nm for MAA of 3.75 vol% (Entry A5). The low initiating efficiencies at low monomer concentrations led to the lower yield. The yields of the polymeric particles increased considerably from 67% at 1.25 vol % of MAA (Entry A1) to 77% at 3.75 vol % of MAA (Entry A5). The molecular weight data and corresponding MAA concentrations for these polymerizations were given in Table 1. Note that the relatively low monomer concentrations and high initiator level were used for all of the reactions in the present work. Therefore, the resultant deviations in molecular weight due to the chain transfer from polymer to the solvent during the polymerization should not be obvious with acetonitrile as solvent. Table 1 illustrated that the MAA concentration had much effect on the molecular weight of the resultant PMAA products. The molecular weight (Mn) of PMAA increased considerably from 3.8 · 103 with MAA of 1.25 vol% (Entry A1) to 2.1 · 104 with MAA of 3.75 vol% (Entry A5) in the reaction medium. At lower monomer concentrations, a substantial portion of the initiator radicals may self-terminate before escaping from the solvent cage and resulting in the low initiating efficiencies, which led to the much lower molecular weight of PMAA. As a result, much small particles (125 nm) were afforded in the case of low MAA concentrations (1.25 vol%, Entry A1).

3.2. Growth mechanism of PMAA microspheres As a sample, the growth processes of PMAA microspheres with MAA loading of 2.5 vol% (relative to the whole reaction medium) were traced with TEM observation as shown in Fig. 2 (only with B1, B3, B7 as samples) and GPC characterization as summarized in Table 2, respectively. The TEM micrographs of PMAA microspheres at different reaction times showed that the PMAA particles had spherical shape and smooth surface during all the procedures of the distillation–precipitation polymerization. The size and size distribution of the PMAA microspheres were summarized in Table 2. The results indicated that the PMAA microspheres grew significantly and continuously from 50 nm at the beginning of the polymerization to 176 nm of the final polymer microspheres, while the polydispersity index (U) remained monodispersion of below 1.010 without any either second-initiated or coagulated particles. In other words, no new particles were formed and the resultant PMAA particles were well stabilized during the further polymerization to enlarge the particle size after the formation of nuclei. At the same time, the monomer conversion was increased from 2.8% to 71.2% with the distillation of solvent. These results indicated that the PMAA particles grew stably and simultaneously with the amount of MAA monomer consumed through the capture of the oligomers and the residual monomer from the reaction system during the polymerization. The molecular weights of PMAA microspheres at different reaction times were listed in Table 2. The results indicated that the molecular weight (Mn) of PMAA increased considerably from 7.5 · 103 at the beginning of the reaction to 2.2 · 104 at the end of the polymerization, although the molecular weight distribution was broad with polydispersity

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Fig. 2. TEM micrographs of PMAA microspheres while distilling different volumes of acetonitrile during the distillation–precipitation polymerization: (B1) 20 mL; (B3) 100 mL; (B7) 400 mL.

index (PDI) higher than 1.6 owing to the nature of the radical polymerization. In general the particle formation consists of two stages: nucleation and growth in the typical disper-

sion, precipitation or distillation–precipitation polymerization. Based on the previous reports and our experiment results, we considered that the particle formation in this method also consists of these

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Table 2 Reaction conditions, size, size distribution and molecular weight of PMAA microspheres at different reaction timesa Entry

Volume of acetonitrile (mL)b

Dn (nm)

Dw/Dn

Mnc (·103)

Mw/Mn

Monomer conversiond (%)

B1 B2 B3 B4 B5 B6 B7

20 50 100 150 250 350 400

52 122 136 145 154 161 176

– 1.011 – – – 1.001 1.006

7.48 – 8.29 – 10.3 – 22.2

1.61 – 2.48 – 2.41 – 2.99

3 20 32 43 56 66 71

a Five-milliliter reaction mixture was sampled from the reaction system each at different reaction time and cooled immediately in liquid nitrogen to quit the polymerization. b The volume of acetonitrile referred as the distilled volume of acetonitrile out of the reaction system at each sampled point. c Molecular weight determined by GPC. d Monomer conversions determined by the acid-base titration of the residual MAA in the system.

tration for the precipitation polymerization [20,21] and distillation–precipitation polymerization [10,22] permitted the further growth of the particle by capture of oligomer radical and monomer. For the present polymerization, there were not any residual vinyl groups on the surface of PMAA particles due to the absence of any crosslinker. But in the present work, the residual MAA can be adsorbed on the PMAA nuclei by the interaction hydrogen-bonding, which provide the incorporated vinyl groups on the particle surface as shown in Scheme 1A. The adsorption of MAA monomer on PMAA surface was proven further by FT-IR spectra. The FT-IR spectra of PMAA particles decanting from the reaction system without further washing in Fig. 3a had a strong peak at 1636 cm1 corresponding to the typical vinyl group of adsorbed monomer. On the other hand, the FT-IR spectra of PMAA particles in Fig. 3b, in which the sample was obtained from the reaction system with repeated washing with acetone, was absence of the peak at

two stages: nucleation and growth. The system was homogenous prior to the polymerization. As soon as the polymerization began, primary radicals, generated by decomposition of the initiator, grow in the continuous phase by the addition of monomer units until they reach their critical chain length where they precipitated to form nuclei. These nuclei are unstable and aggregate with each other to form the mature particles. In this experiment the initial homogeneous reaction mixture became milky white after boiling for 10 min, which indicated the formation of mature particles of PMAA particles. Then the PMAA microspheres were formed by the capture of the oligomer species and the residual monomer from the reaction medium during distilling the solvent out of the reaction system through two main growth motions as illustrated in Scheme 1. Here we mainly discussed on the growth process of the particles. In the previous reports, the residual double bonds on the particle surface owing to the poor solvent medium and much low monomer concen-

H O O

.

O

Residual monomer

Oligomer radical

. Monomer

C O H Capture

H O C O H PMAA nuclei

Growth

Captured radical

PMAA nuclei

PMAA microsphere

. . Monomer

O

Growth

O Oligomer radical

Captured radical

PMAA microsphere

Scheme 1. The growth mechanism of PMAA microspheres.

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Fig. 3. FT-IR spectra of PMAA seeds: (a) with adsorbed MAA monomer; (b) in the absence of adsorbed MAA monomer.

1636 cm1. In the other words, some of the residual MAA monomer can be adsorbed on the surface of PMAA nuclei during the distillation–precipitation polymerization, which incorporated the reactive vinyl group on the surface of PMAA particles. The particle growth may be through the capture of the soluble oligomers or residual MAA monomers by the adsorbed vinyl groups on the surface during the polymerization via a reactive and entropic capture mechanism. In this method, the initiator may react with the monomer in the solution to form the oligomer radical through the experiment. When the oligomer grew longer than the critical chain length, these oligomer will precipitated from the homogenous reaction system, and were captured by the original PMAA seeds through the hydrogen-bonding interaction between the carboxylic acids on the surface of PMAA seeds and the carboxylic acid units on the oligomer radicals, which was shown as Scheme 1B. In other words, the polymerization was initiated in the homogeneous medium, which was much similar to the typical solution polymerization. Then the oligomer radicals were precipitated out of the reaction system to the surface of the PMAA seeds with the aid of hydrogen-bonding interaction between the seed particles and oligomer radicals in the presence of carboxylic acid units in acetonitrile. The hydrogen-bonding interaction has played an active role for the formation of raspberry-like polymer composite through the heterocoagulate self-assemble [23,24] and the layer-by-layer deposition [25]. The essence of the hydrogen-bonding interaction

for the growth of the polymer microspheres was confirmed further by the following experiment. When PMAA particles were used as seeds for the second-stage polymerization of divinylbenzene (DVB), only inter-connected particles with irregular shape were obtained as shown in Fig. 4(C2), which was due to the absence of hydrogen-bonding interaction between the PMAA seeds and the newly formed PDVB oligomers. The reactive radical on the PMAA surfaces during the growth processes according to the above manners may not instantaneously terminate to further react with the residual MAA monomers, which enlarges the PMAA microspheres. These reactive radical on the PMAA surface may terminate via the coupling with the other radical from solution or via atom transference to the other groups in the reaction system. The probability of the termination via the radical coupling decreased with the increasing size of PMAA particles. Therefore, the molecular weight increased significantly with increasing size of PMAA microspheres. A slightly crosslinked outer layer surface gel layer was suggested as steric stabilizer layer in the presence of a solvent better than h solvent to prevent the aggregation of the colloidal gels [26,27]. Generally, the polymer microspheres were sterically stabilized by the pendent chains and surface gels during the precipitation polymerization [27] and distillation–precipitation polymerization [10,11]. The amphiphilic copolymers with active carboxylic acid group were reported as stabilizers for emulsifier polymerization [28–31] via electrostatic stabilization. For the present distillation–precipitation polymerization of MAA, the microparticles should be co-stabilized by electrostatic and steric effects. The formation of PMAA microspheres may be attributed to the electrostatic stabilization through the interchain hydrogen-bonding interaction between the carboxylic acid units as well as the solvent swollen out-layer of the particles. These driving forces led to the desolvation during the growth processes and resulted in the collapse of the polymer network. In other words, the formation of monodisperse PMAA microspheres occurred through an entropic distillation–precipitation manner, during which the soluble oligomer species reacted with the adsorbed vinyl groups on the surface of PMAA particles to enlarge the PMAA microspheres. The glass transition temperature of PMAA is 156 C [29], which is much higher than the reaction temperature of around 82 C. On

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Fig. 4. TEM micrographs of the seeded polymerization of PMAA microspheres: (C0) Seed PMAA microspheres; (C1) grown microspheres with PMAA seeds; (C2) grown PDVB with PMAA seeds.

the one hand, the captured PMAA was difficult to move below glass transition temperature during the growth of PMAA microspheres, which resulted in the polymer microspheres with non-segmented spheres. On the other hand, the PMAA precipitated around the nuclei randomly during the growth procedure, which afforded the resultant PMAA microspheres with smooth surface. Furthermore, the solvency of the continuous phase with acetonitrile as solvent was below h condition for the polymer, which caused the separation with the aid of inter-

chain hydrogen-bonding to result in monodisperse PMAA microspheres. 4. Conclusion A simple and effective synthesis of hydrophilic PMAA microspheres with spherical shape and smooth surface in the range of 50–288 nm by distillation–precipitation polymerization in neat acetonitrile with AIBN as initiator without any crosslinker was described. Both the size and the molecular

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weight of the resultant PMAA microspheres increased significantly with increasing MAA monomer concentration in the reaction medium and with increasing reaction time, while the size distribution of PMAA microspheres was kept at monodispersion with polydispersity index lower than 1.010. The growth mechanism for monodisperse PMAA microspheres occurred through an entropic distillation–precipitation polymerization manner. The capture of the oligomer radicals either by the adsorbed reactive vinyl group on the surface of PMAA nuclei or by the hydrogen-bonding interaction between the carboxylic acid on PMAA nuclei and carboxylic acid on oligomer radical in the solution may result in the growth of PMAA microspheres. Both the steric stabilization from the pendent chains and surface gel layer, and the electrostatic repulsion from the carboxyl acid groups prevent the particle coagulating leading to mono- or narrow-disperse PMAA microspheres. Acknowledgements This work was supported in part by the National Science Foundation of China (Project No.: 20504015) and the Opening Research Fund from the State Key Laboratory of Polymer Chemistry and Physics, Chinese Academy of Sciences (Project No.: 200613). References [1] Swift G. In: Bailey J, Koschwitz JI, editors. Encyclopedia of polymer science and technology, 3rd ed., vol. 10. Hoboken, NJ: John Wiley & Sons; 2003. [2] Blackley DC. Polymer lattices, 2nd ed., vol. 2. London: Chapman & Hall; 1997. [3] Bassett DR, Hoy KL. In: Fitch RM, editor. Polymer colloids II, vol. 1. New York: Plenum; 1980.

[4] Napper DH. Polymeric stabilization of colloidal dispersions. London: Academic Press; 1983. [5] Romack TJ, Maury EE, DeSimone JM. Macromolecules 1995;28:912. [6] Xu Q, Han BX, Yan HK. Polymer 2001;42:1369. [7] Xu Q, Han BX, Yan HK. J Appl Polym Sci 2003;88:1876. [8] Liu T, DeSimone JM, Roberts GN. Chem Eng Sci 2006;61:3129. [9] Liu T, DeSimone JM, Roberts GW. J Polym Sci Part A Polym Chem 2005;43:2546. [10] Liu T, Garner P, DeSimone JM, Roberts GW, Nothun GD. Macromolecules 2006;39:6489. [11] Bai F, Yang XL, Huang WQ. Macromolecules 2004;37: 9746. [12] Bai F, Yang XL, Huang WQ. Eur Polym J 2006;42:2088. [13] Li SF, Yang XL, Huang WQ. Chin J Polym Sci 2005;23:197. [14] Lu XY, Huang D, Yang XL, Huang WQ. Polym Bull 2006;56:171–8. [15] Bai F, Yang XL, Li R, Huang B, Huang WQ. Polymer 2006;47:5775. [16] Bai F, Li R, Yang XL, Li SN, Huang WQ. Polym Int 2006;55:319. [17] Dai Z, Yang XL, Huang WQ. Chin J Polym Sci 2007;25:303. [18] Bai F, Yang XL, Huang WQ. Chin J Polym Sci 2006;24:163. [19] Dai Z, Yang XL, Huang WQ. Polym Int 2007;56:224. [20] Li WH, Sto¨ver HDH. Macromolecules 2000;33:4354. [21] Downey JS, Frank RS, Li WH, Sto¨ver HDH. Macromolecules 1999;32:2838. [22] Qi DL, Ba F, Yang XL, Huang WQ. Eur Polym J 2005;41: 2320. [23] Li R, Yang XL, Li GL, Li SN, Huang WQ. Langmuir 2006;22:8127. [24] Li GL, Song YY, Yang XL, Huang WQ. J Appl Polym Sci 2007;104:1350. [25] Wang LY, Wang ZQ, Zhang X, Shen JC, Chi LF, Funds H. Macromol Rapid Commun 1997;18:509. [26] Graham NB, Hayes MH. Macromol Symp 1995;93:293. [27] Graham NB, Mao J. Colloids Surf A Physicochem Eng Aspects 1996;118:211. [28] Lefay C, Charleux B, Save M, Chassenieux C, Guerret O, Magnet S. Polymer 2005;46:1935. [29] Beal C, Chevalier Y. Polymer 2005;46:1395. [30] Vorwerg L, Gilbert RG. Macromolecules 2000;33:6693. [31] Zhou X, Goh SH, Lee SY, Tan KL. Polymer 1997;39:3631.