Carboxyl carrying-large uniform latex particles

Colloids and Surfaces A: Physicochemical and Engineering Aspects 197 (2002) 79 – 94 www.elsevier.com/locate/colsurfa

Carboxyl carrying-large uniform latex particles A. Tuncel a,*, M. Tuncel b, B. Ergun a, C. Alago¨z a, T. Bahar a a

Chemical Engineering Department, Hacettepe Uni6ersity, Beytepe 06532, Ankara, Turkey b School of Medicine, Hacettepe Uni6ersity, 06532, Ankara, Turkey Received 15 March 2001; accepted 22 June 2001

Abstract Uniform polystyrene particles in the size range of 1.9– 6.2 mm were used as the seeds in a multistep polymerization, to produce compact or macroporous particles in the size range of 3 – 13 mm with reasonably narrow size distributions (i.e. CV B5%). The seed particles with different sizes and molecular weights could be achieved by the dispersion polymerization of styrene. In the synthesis of carboxyl carrying-particles, the seed latices were first swollen by a low molecular weight-organic agent (i.e., dibutylphthalate, DBP), then by styrene (S)– methacrylic acid (MAA)– divinylbenzene (DVB) mixture including an oil-soluble initiator (i.e. benzoyl peroxide, BPO). The final particles were obtained by the polymerization of monomer mixture in the swollen seed particles by keeping the particle uniformity. The average size, size distribution, surface morphology and internal structure of the final beads were evaluated both by scanning and transmission electron microscopy. The average particle size slightly increased with the increasing DBP/seed latex and monomer/seed latex ratios. By keeping the particle uniformity, the size of carboxyl carrying-particles could be adjusted in a relatively wider range (i.e. 3 – 13 mm) by the selection of seed latex size. Titratable MAA contents between 69 and 181 mg g − 1 were found for the particles produced under different conditions. Highly porous and carboxyl functionalized-uniform particles were obtained by starting from the polystyrene seed particles having number-average size and number-average molecular weight of 6.2 mm and 0.58 ×104, respectively. The smaller seed latices with higher molecular weights led to the synthesis of carboxyl carrying-particles with a non-porous surface and an internal part with crater-like porosity. In the case of porous particles, the surface porosity decreased significantly with the decreasing monomer/seed latex ratio and DVB feed concentration. Low monomer/seed latex ratios also resulted in a particle interior with crater-like pore structure. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Porous particles; Emulsion polymerization; Dispersion polymerization; Uniform latex particles; Divinylbenzene; Chromatographic packing; Activated swelling method

1. Introduction

* Corresponding author. E-mail address: [email protected] (A. Tuncel).

Compact or macroporous, large-uniform latex particles have been usually synthesized by multistep polymerization methods [1–10]. Ugelstad et al. developed a two-step microsuspension method

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for the production of large-uniform latex particles [1–3]. Uniform polymer particles were prepared in the macroporous form by El-Aasser et al. via seeded emulsion polymerization [8,9]. Frechet et al. proposed a multistage seeded polymerization method for the synthesis of uniform and macroporous poly(styrene-co-divinylbenzene) beads by using linear polystyrene as a component of the porogenic mixture [11– 13]. Functional groups on the surface of polymeric microspheres are especially important for the biotechnological and biomedical applications (i.e. attachment of biological ligands with specific recognition abilities and immobilization of biological molecules). Various emulsion polymerization procedures were proposed for the synthesis of uniform latex particles with different functionalities [14–44]. By applying these methods, the reactive functional groups like carboxyl, hydroxyl, amine, amide and chloromethyl could be incorporated onto the microspheres by protecting the monodispersity of the final product. These polymerization methods were developed based on the direct or multi-stage emulsion polymerization procedures involving the use of seed particles. Among them, the synthesis and characterization of carboxyl-functionalized latex particles have attracted significant attention since the carboxyl functionality on the particle surface can be easily activated for the covalent coupling of various ligands having interaction abilities with the biological molecules [14– 23,27 – 30]. Recently, the microporous glass membrane (MPG) emulsification technique was proposed as a new method for the synthesis of uniform and crosslinked polymeric microspheres up to 10 mm in size with the coefficients of variation being close to 10% [45– 47]. The first application of this method was performed on the BPO initiated-suspension polymerization of styrenic monomers [45]. The MPG method was also used for the preparation of carboxyl-carrying polystyrene and epoxypropylfunctionalized poly(methylmethacrylate) particles [46,47]. We studied the synthesis of uniform-porous particles with different average sizes and porosities [48–50]. In our earlier studies, the uniform polystyrene latices produced by a dispersion poly-

merization procedure were directly used as the seed particles. The size and porosity properties of the final particles could be controlled by changing size and molecular weight of the seed latex. In this study, we adopted the earlier method on the synthesis of carboxyl functionalized compact or macroporous latex particles. By applying the proposed procedure, methacrylic acid and styrene were copolymerized in the presence of divinylbenzene as the cross linking agent in the swollen polystyrene seed particles. The effects of diluent/ seed latex ratio, MAA and DVB concentrations in the repolymerization step, repolymerized monomer/seed latex ratio, the size and molecular weight of the seed latex on the size and morphology of final particles were investigated.

2. Experimental

2.1. Materials Styrene monomer (Yarpet A.S.,Turkey) was distilled under vacuum and stored in the refrigerator until use. Ethanol (absolute) was supplied from Merck AG, Darmstad, Germany. 2methoxyethanol (HPLC grade) was purchased from Aldrich Chem. Co, Milwaukee, WI, USA. Polyacrylic acid obtained by a solution polymerization (PAA, M v: 1.4× 103) and polyvinylpyrrolidone-40 (PVP-40, Mr: 40 000) supplied from Sigma Chemical Co, St. Louis, Misourri, USA were used as steric stabilizers in the preparation of seed latices. 2,2%-Azobisizobutyronitrile (AIBN, BDH Chemicals Ltd, Poole, UK) was freshly crystallized and used as the initiator in the preparation of seed latices. The cross linking agent, divinylbenzene (DVB, including 55% DVB isomers, Aldrich Chem. Co) was treated with aqueous NaOH solution (5% w/v) to remove the inhibitor. Methacrylic acid (MAA, Merck AG), benzoyl peroxide (BPO, including 97% of active compound, Aldrich Chem. Co) and sodium dodecyl sulfate (SDS, Sigma Chemical Co) were used as received. The low molecular weight-organic compound, dibutyl phthalate (DBP, Polisan A.S., Turkey) was used for the swelling of seed particles without further purifica-

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tion. Distilled deionized water was used in all polymerizations.

ture. The preparation conditions, the size distribution properties and the molecular weights of different seed latices are given in Table 1.

2.2. Preparation of uniform polystyrene seed particles

2.3. Preparation of carboxyl functionalized particles

The uniform seed particles with three different sizes (i.e. with different molecular weights) were prepared by the dispersion polymerization of styrene. Typically, the dispersion medium was prepared by mixing the prescribed amounts of ethanol–water or ethanol –methoxyethanol. Then the steric stabilizer (PAA or PVP-40) was dissolved in the dispersion medium. After the addition of styrene, a certain amount of initiator (AIBN) was dissolved in the homogeneous medium by sonication. The dispersion polymerization was performed in the sealed pyrex cylindrical reactor (total volume: 125 ml) shaken with 120 cpm in a thermostatted water-bath at 70 °C for 24 h. The produced latex was washed extensively with distilled water by applying a centrifugation-decantation procedure and used as the seed for the synthesis of carboxyl-carrying particles. The number average-molecular weights of seed latices were determined by HPLC (Waters, USA), using methylene chloride as the eluent in an Ultrasytragel column operated at ambient tempera-

A two-step seeded polymerization procedure was employed for the synthesis of carboxyl carrying-uniform latex particles. In the development of our polymerization procedure, the principles of seeded polymerization established by Wang et al. were considered [13]. Typically, dibutylphthalate (DBP) (0.175 ml) was emulsified in the aqueous medium (25 ml) including 0.25% (w/w) sodium dodecyl sulfate (SDS) as the emulsifier. For emulsification, the mixture of DBP-aqueous SDS solution was sonicated for 30 min in an ultrasonic water bath (Bransonic 200, USA) at the room temperature. An aqueous dispersion of seed particles (: 2.0 ml) including 0.175 g polystyrene (PS) seed particles was added into the DBP emulsion. The resulting dispersion was stirred at room temperature for 24 h with 400 rpm for the absorption of DBP by the polystyrene seed particles. In the next step, a monomer phase comprised of styrene (0.4 ml), DVB (0.4 ml), MAA (0.4 ml) and BPO (60 mg) was emulsified in the aqueous medium (25 ml) including 0.25% (w/w) SDS by sonication for 5 min at room temperature. The monomer emulsion was then mixed with the aqueous dispersion of DBP swollen-seed particles. The obtained emulsion was stirred at the room temperature for 24 h with 400 rpm stirring rate for the absorption of monomer phase by the DBP swollenpolystyrene seed particles. At the end of this period, an aqueous solution (3 ml) containing 10% (wt.) PVA was added into the resulting emulsion and the medium was purged with nitrogen for 5 min. The polymerization of monomer phase in the swollen seed particles was performed at 70 °C for 24 h with 120 cpm shaking rate in a sealed reactor. Polymerization step conducted in the presence of PVA as the stabilizer (5 mg ml − 1) provided large-uniform and carboxyl functionalized-latex particles. The low-sized particles formed in the repolymerization as a by-product (i.e. approximately 1 mm in size) were removed by apply-

Table 1 The production conditions and properties of the seed latex particles Ingredients

Seed latex type L1

Ethanol (ml) Water (ml) Methoxyethanol (ml) Styrene (ml) AIBN (g) PAA (g) PVP-40 (g) Common conditions Dn (mm) CV (%) Mn×10−4

90 10 – 10 0.1 1.0 –

1.9 B1 1.89

L2

L3 18 – 12

5 0.05 – 0.788 70 °C, 24 h, 120 cpm 4.1 2.50 1.56

18 – 12 5.0 0.11 – 0.525

6.2 2.64 0.58

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ing a centrifugation-decantation procedure. The large particles were washed with tetrahydrofuran and ethanol extensively to remove the diluent and the linear polymer. Finally, the particles were washed with water and redispersed in distilleddeionized water.

conditions were included as a reference material. Before recording the related spectra, the particles and KBr were first dried in vacuum at 40 °C for 2 days and then over anhydrous CaCl2 for another 2 days.

2.4. Characterization of carboxyl functionalized-particles

3. Results and discussion

The average size, size distribution and surface morphology of the carboxyl-carrying particles were determined by Scanning Electron Microscope (SEM). A small amount of aqueous dispersion of the cleaned latex particles (about 0.1 ml) was spread onto a copper disc and the water was evaporated at room temperature. The dried particles were coated with a thin layer of gold (about 100 A) in vacuum. The specimens were examined and photographed in a SEM (JEOL, JEM 1200 EX, Japan) with 1000 and 3000× magnifications to observe the uniformity and the surface morphology of particles, respectively. The SEM photographs obtained with 1000× magnification were printed in 14× 10 cm dimensions and evaluated for the determination of number- and weight-average diameters (Dn, Dw) and coefficient of variations for size distribution (CV). The internal structure of the particles was examined in Transmission Electron Microscope (TEM). The dried particles (100– 200 mg) were fixed in 1% aqueous OsO4 solution and dehydrated in a graded series of alcohols, then embedded in Araldit CY 212. Thin sections were cut serially (60–90 nm) by Ultrathom (LKB, UK) and mounted on 100 mesh grids and examined under TEM (JEOL, JEM 1200 EX, Japan). After washing with ethanol and water, a certain volume of particle suspension (i.e. 40 ml in most cases) with a known particle concentration was titrated potentiometrically with 0.1 N NaOH solution. The MAA contents of cleaned particles were calculated by using the related inflection point in the titration curve. FTIR and FTIR-DRS spectra of carboxyl-functionalized particles were recorded by using KBr tablets and KBr powder, respectively. Here, styrene-divinylbenzene copolymer particles prepared in the absence of MAA, under identical

The functional groups on the surface of the particles play an important role in the synthesis of chromatographic-packing materials carrying covalently bound ligands with the specific-recognition abilities against to different biological molecules. Carboxyl group is one of the easily derivatizable groups by applying an activation procedure (i.e. water soluble carbodimide activation). The introduction of carboxyl functionality onto the uniform macroporous particles can be achieved by the use of carboxyl-carrying comonomers during the production. Methacrylic acid (MAA) is one of the well-known acrylic monomers carrying carboxyl functionality. The lower water solubility of MAA relative to acrylic acid makes easier the incorporation of this agent into the forming particles in a polymerization process involving the use of both aqueous and organic phases. However, the presence of MAA in the multi-step microsuspension polymerization significantly affects on the formation of macroporous structure of final particles.

3.1. DBP/seed latex ratio To test the effect of DBP/seed latex ratio on the average size and morphology of the carboxyl carrying-particles, two different seed latices were used. The first set was performed with the seed latex coded as L2. In this set, DBP/seed latex ratio was varied between 0.5 and 2.0 ml g − 1. Monomer phase/seed latex ratio was fixed at 1.2/ 0.175 ml g − 1. St, DVB and MAA concentrations in the monomer phase were 33.3% based on the total volume of the monomer. The BPO concentration in the monomer phase was 50 mg ml − 1. An aqueous SDS solution (0.25% (w/v)) was used as the continuous medium in both swelling steps. The repolymerizations were conducted at 70 °C

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Fig. 1. SEM photographs of the carboxyl carrying-particles produced by using L2 seed latex with different DBP/seed latex ratios. DBP/seed latex ratio (ml g − 1): (A) 1/2, Mag: 1000 ×; (B) 1/2; (C) 1/1; (D) 2/1, Mag: 6000 × for (B); (C); and (D).

for 24 h with 120 cpm shaking rate. Unless stated otherwise, these common conditions were utilized in the all experiments. A representative SEM photograph showing the monodispersity of carboxylcarrying particles produced by using L2 seed latex with the DBP/seed latex ratio of 1/2 is given in Fig. 1(A). The SEM photographs showing the detailed surface morphology of the carboxyl carrying-particles obtained from L2, by using different DBP/seed latex ratios are given in Fig. 1(B–D). As seen here, all tried DBP/seed latex ratios provided final particles with reasonably smooth (i.e. non-porous) surfaces. Another set of polymerizations were performed by using the seed latex of L3. Here, DBP/seed latex ratio was changed between 1 and 2.5 ml g − 1. The size distribution of the particles produced by using L3 with the DBP/seed latex ratio of 1/1 is shown in Fig. 2(A). The SEM photographs showing the detailed surface morphology of particles obtained with different DBP/seed latex ratios in the presence of L3 are given in Fig. 2(B–D). As seen here, all DBP/seed latex ratios

provided particles having a macroporous surface. Note that the surface structure was completely different from that obtained by L2 (i.e. Fig. 1(B– D)). Although the surface morphology of the final particles was strongly influenced by the selected seed latex type, DBP/seed latex ratio was not an effective parameter for controlling the surface morphology. The properties of carboxyl carrying particles obtained with different DBP/seed latex ratios are presented in Table 2. As expected, the average particle size increased with increasing DBP/seed latex ratio in the presence of both seed latices. Note that lower CV values were obtained with the higher DBP/seed latex ratios. This result probably indicated that the monodispersity of seed particles during the monomer-swelling step could be protected more effectively in the case of higher DBP/seed latex ratio. In other words, the monomer phase was distributed more equally among the seed particles swollen with the higher amount of DBP. In the activated swelling process, the low molecular weight organic agents like DBP have been introduced for increasing the monomer

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adsorption capacity of seed particles [2–4,12,13]. More efficient monomer adsorption probably led to the more equal distribution of monomer phase among the seed particles. An example TEM micrograph showing the internal structure of the particles produced by starting from the seed latex of L2 with the DBP/seed latex ratio of 1/1 is given in Fig. 3(A). As seen here, there were large voids within the particle

interior although the particle surface was reasonably smooth. This structure was also characteristic for the other particles obtained from the seed latex of L2 with different DBP/seed latex ratios. The TEM photographs indicating the internal structures of particles produced by using L3, with different DBP/seed latex ratios are given in Fig. 3(B–D). As seen here, a sponge-like pore structure could be achieved with all DBP/seed latex

Fig. 2. SEM photographs of the carboxyl carrying-particles produced by using L3 seed latex with different DBP/seed latex ratios. DBP/seed latex ratio (ml g − 1), (A) 1/1, Mag: 800 ×; (B) 1/1; (C) 2/1; (D) 2.5/1; Mag: 3000 × for (B); (C); and (D). Table 2 Properties of carboxylated particles produced with different dibutylphthalate/seed latex (DBP/SL) feed ratios Seed latex

DBP/SL (ml g−1)

Dn (mm)

Dw (mm)

CV (%)

MAA content (mg g−1 particles)

L2 L2 L2 L3 L3 L3

1/2 1/1 2/1 1/1 2/1 2.5/1

8.66 8.65 9.52 11.90 12.53 13.11

8.73 8.68 9.54 11.99 12.60 13.17

5.61 3.81 2.97 5.11 4.25 3.87

118.0 129.0 186.0 127.0 132.0 166.0

DBP swelling conditions, SL, 0.175 g; SDS, 0.25% w/v; aqueous medium, 27 ml. Monomer swelling conditions: monomer phase, 1.2 ml; BPO, 0.06 g; S, 0.4 ml; DVB, 0.4 ml; MAA, 0.4 ml; SDS, 0.25% w/v; aqueous medium, 52 ml. Repolymerization conditions, 70 °C; 24 h; 120 cpm.

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Fig. 3. TEM photographs showing the internal structure of carboxyl carrying-particles produced by using L2 and L3 seed latices with different DBP/seed latex ratios. Seed latex type and DBP/seed latex ratio (ml g − 1), (A) L2, 1/1, Mag: 15 000 ×; (B) L3, 1/1, Mag: 3000 × ; (C) L3, 2/1, Mag: 4000 × ; (D) L3, 2.5/1, Mag: 4000 ×.

ratios in the presence of L3. However, no significant effect of DBP/seed latex ratio on the pore volume could be observed in these photographs. As seen in Table 2, the MAA content of the final particles increased with increasing DBP/seed latex ratio. It should be noted that the total volume (i.e. the total surface area) of DBP-swollen seed particles increased as the DBP/seed latex ratio increased. This case should be considered as a factor causing an increase in the diffusion rate of MAA into the DBP-swollen seed particles. Based on the Fick’s first law, the diffusion rate of MAA from the aqueous phase is directly proportional to the available surface area of DBP-swollen particles. More solvent (i.e. DPB) in the seed particles should also involve an increase in the solubility of MAA in the swollen particle-structure before the repolymerization. Additionally, the viscosity of medium in the DBP-swollen particles decreases with increasing DBP content. Then higher effective diffusion coefficient of MAA should be expected in the swollen particles with

lower viscosity. For all these reasons, an increase should be probably observed in the final MAA content of the carboxylated particles.

3.2. MAA concentration These runs were performed with the seed latex encoded as L2. The MAA concentration in the monomer phase were changed 11.1–52.9% by volume. DBP/seed latex ratio was fixed at 1.0 ml g − 1. The monodispersity of particles produced with 11.1% MAA feed concentration was exemplified in Fig. 4(A). The SEM photographs showing the surface morphology of carboxylated particles produced with different MAA concentrations are given in Fig. 4(B–D). As seen here, reasonably smooth surfaces carrying some craters were obtained with the all MAA concentrations. The properties of the same particles are given in Table 3. As seen here, no significant change was observed in the average size with the MAA concentration. However, high MAA concentration in

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the monomer phase resulted in a meaningful increase in the CV. As expected a significant increase was observed in the MAA content of the final particles with increasing MAA feed concentration.

3.3. Seed latex type Three different seed latices were used in this set. In addition to the seed latices of L2 and L3, a seed latex coded as L1 was included. DBP/seed latex ratio was fixed at 1.0 ml g − 1. The unifor-

mity of carboxylated particles produced by starting from different seed latices was exemplified in Fig. 5(A, C and E). The properties of the same particles are presented in Table 4. As seen here, the average size of carboxylated particles clearly increased with increasing seed latex size. While the carboxylated particles produced from the smallest seed latex (i.e. L1) were highly monodisperse, larger seed latices provided particles with a certain size distribution (i.e. with higher CV values). Note that the seed latices with higher average sizes also had higher CV values (Table 1). In the case of

Fig. 4. SEM photographs of carboxyl carrying-particles produced with different MAA concentrations. MAA concentration (% v/v), (A) 11.1, Mag: 1000 × ; (B) 11.1; (C) 33.3; (D) 52.9, Mag: 6000 × (B); (C) and (D). Table 3 Properties of carboxylated particles produced with different MAA feed concentrations Seed latex

MAA (% v/v)

Dn (mm)

Dw (mm)

CV (%)

MAA content (mg g−1 particles)

L2 L2 L2

11.1 33.3 52.9

8.32 8.65 8.45

8.35 8.68 8.58

3.03 3.81 6.23

69.0 129.0 191.0

DBP swelling conditions, SL, 0.175 g; DBP/SL, 1 ml g−1; SDS, 0.25% w/v; aqueous medium, 27 ml. Monomer swelling conditions: monomer phase, 1.2 ml; BPO, 0.06 g; MAA, Variable; DVB, 33.3% v/v; SDS, 0.25% w/v; aqueous medium, 52 ml. Repolymerization conditions, 70 °C; 24 h; 120 cpm.

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Fig. 5. SEM photographs of carboxyl carrying-particles produced by using different seed latices. Seed latex code, (A) L1, 2000 ×; (B) L1, 8000 × ; (C) L2, 1000 × ; (D) L2, 6000 × ; (E) L3, 800 × ; (F) L3, 3000 × . Table 4 Properties of carboxylated particles produced with different seed latices Seed latex

Dn (mm)

Dw(mm)

CV (%)

MAA content (mg g−1 particles)

L1 L2 L3

3.21 8.65 11.90

3.22 8.68 11.99

2.06 3.81 5.11

134.0 129.0 127.0

DBP swelling conditions, SL, 0.175 g; DBP/SL, 1 ml g−1, SDS, 0.25% w/v; aqueous medium, 27 ml. Monomer swelling conditions: monomer phase, 1.2 ml, BPO, 0.06 g; S, 0.4 ml; DVB, 0.4 ml; MAA, 0.4 ml; SDS, 0.25% w/v; aqueous medium, 52 ml. Repolymerization conditions, 70 °C; 24 h; 120 cpm.

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constant amount of seed latex, the total surface area available for the diffusion of swelling agent (i.e. DBP) or the diffusion of monomer phase to the particles should be smaller with increasing seed latex size. Therefore, the equal distribution

Fig. 6. TEM photographs showing the internal structure of carboxyl carrying-particles produced by using different seed latices. Seed latex code, (A) L1, Mag: 5000 ×; (B) L2, Mag: 4000 × ; (C) L3, Mag: 3000 × .

of swelling agent and monomer phase among the particles should be more difficult with smaller surface area which in turn probably leads to the formation of some defects in the monodisperse character of the resulting particles produced from seed latices with higher average sizes. The SEM photographs indicating the detailed surface morphology of these particles are given in Fig. 5(B, D and F). While the particles with non-porous surface were obtained with the seed latices having lower sizes and higher molecular weights (i.e. L1 and L2), the largest seed latex with the lowest molecular weight (i.e. L3) provided particles with a porous surface. To show the internal structure of particles, the TEM photographs of the thin sections of the particles produced with different seed latices are included in Fig. 6. While there were non-homogeneously distributed large voids in the particles produced by starting from L1 to L2, an internal structure including relatively small pores with a reasonably narrow pore size distribution (i.e. spongelike pore structure) could be achieved by the seed latex of L3. In our procedure, polymeric part of the porogen solution directly comes from polystyrene seed particles. A decrease in the molecular weight of the seed should involve a decrease in the viscosity of porogen solution. Our results indicated that the porogen solution with a relatively lower viscosity (i.e. due to the lower molecular weight of L3) provided a macroporous structure with smaller pore size. A similar tendency was also reported by Wang et al. [13]. By considering the mechanism for the pore formation process in the uniform latex particles and our results, it should be concluded that the larger microspheres or agglomerates are probably produced when the viscosity of porogen solution including DBP and linear polystyrene is higher. For this reason, the voids between the fixed larger agglomerates or microspheres are larger in the forming particles. Hence the average pore size increases with increasing viscosity of porogen solution. Note that no meaningful change was observed in the titratable MAA content of the particles by the seed latex type.

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Table 5 Properties of carboxylated particles produced with different monomer phase/seed latex ratios Seed latex

Monomer/SL (ml g−1)

Dn (mm)

Dw(mm)

CV (%)

MAA content (mg g−1 particles)

L3 L3 L3 L3a

1.2/0.175 1.2/0.225 1.2/0.325 1.2/0.325

12.53 12.20 9.63 9.89

12.60 12.29 9.68 9.92

4.25 5.15 3.96 3.25

132.0 155.0 143.0 –

DBP swelling conditions, SL, 0.175 g; DBP/SL, 2 ml g−1; SDS, 0.25% w/v; aqueous medium, 27 ml. Monomer swelling conditions: monomer phase, 1.2 ml; BPO, 0.06 g; S, 0.4 ml; DVB, 0.4 ml; MAA, 0.4 ml; SDS, 0.25% w/v; aqueous medium, 52 ml. Repolymerization conditions, 70 °C; 24 h; 120 cpm. a This polymerization was performed in the absence of MAA for obtaining poly(S–DVB) particles as a reference.

Fig. 7. SEM photographs of carboxyl carrying-and plain poly(S – DVB) particles produced with different monomer/seed latex ratios. Monomer/seed latex ratio (ml g − 1), (A) 1.2/0.175; (B) 1.2/0.225; (C) 1.2/0.325; (D) plain poly(S – DVB) particles produced with 1.2/0.325 ml g − 1, Mag: 3000 × for (A) and (B); 4000 × for (C) and (D).

3.4. Monomer/seed latex ratio The monomer/seed latex ratio was varied between 1.2/0.175 and 1.2/0.325 ml g − 1 by using the seed latex of L3 and by fixing the DBP/seed latex ratio at 2.0 ml g − 1. The properties of particles produced with different monomer/seed latex ratios are given in Table 5. The average size decreased with decreasing monomer/seed latex ratio.

This case is an expected result since less monomer volume is available for the swelling of each seed particle in the presence of lower monomer/seed latex ratio. No meaningful change was observed in the polydispersity of resulting particles in the studied range. SEM photographs of carboxylated particles produced with different monomer phase/seed latex ratios are given in Fig. 7(A– C). As seen in

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Fig. 7(A), carboxylated particles with a macroporous surface could be achieved with the monomer/ seed latex ratio of 1.2/0.175 g ml − 1. However, further increase in the monomer/seed latex ratio resulted in disappearance of the surface porosity. As seen in Fig. 7(B and C), carboxylated particles obtained with the monomer/seed latex ratios of 1.2/0.225 and 1.2/0.325 ml g − 1 had smooth and non-porous surfaces. Here, a control experiment was done with the monomer/seed latex ratio of 1.2/0.325 g ml − 1, by excluding MAA from the polymerization recipe. All other production conditions were the same with those of carboxyl-functionalized particles produced with the same monomer/seed latex ratio. The SEM photograph showing the surface morphology of poly(S–DVB) particles produced with the monomer/seed latex ratio of 1.2/0.325 g ml − 1 is given in Fig. 7(D). As seen here, excluding of MAA from the polymerization recipe resulted in the formation of a macroporous particle surface under the identical polymerization conditions. To compare the internal structures, TEM pho-

tographs of thin sections of the particles produced with different monomer/seed latex ratios are given in Fig. 8. As seen in Fig. 8(A), a macroporous interior including regularly distributed small pores in the whole cross-section could be achieved with the carboxyl-functionalized particles produced with the highest monomer/seed latex ratio (i.e. 1.2/0.175 ml g − 1). The particle interior obtained with the monomer/seed latex ratio of 1.2/0.225 ml g − 1 had slightly lower porosity with a larger pore size (Fig. 8(B)). Finally, the lowest monomer/seed latex ratio (i.e. 1.2/0.325 ml g − 1) led to a particle interior including only large voids (Fig. 8(C)). Note that a highly porous particle interior including homogeneously distributed pores in the whole cross-section was observed for the particles produced in the absence of MAA (Fig. 8(D)). The comparison of Fig. 8(C and D) showed that the introduction of MAA into the monomer phase resulted in a significant decrease both in the porosity and specific surface area of the particles produced with the monomer/seed latex ratio of 1.2/0.325 ml g − 1. A similar conclusion was also

Fig. 8. TEM photographs of carboxyl carrying- and plain poly(S – DVB) particles produced with different monomer/seed latex ratios. Monomer/seed latex ratio (ml g − 1), (A) 1.2/0.175, Mag: 4000 ×; (B) 1.2/0.225, Mag: 3000 × ; (C) 1.2/0.325, Mag: 5000 × ; (D) plain poly(S –DVB) particles produced with 1.2/0.325 ml g − 1, Mag: 6000 × .

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Fig. 9. FTIR and FTIR-DRS spectra of poly(S –DVB) and poly(S– MAA–DVB) particles.

valid for the particle surfaces obtained in the absence and existence of MAA under identical conditions (i.e. Fig. 7(C and D)). According to the mechanism proposed by Cheng et al., the first step in the pore formation was defined as the production and agglomeration of low-energy and highly crosslinked-gel microspheres by the phase separation taking place between the crosslinked copolymer and the porogen phase including linear polystyrene + nonsolvent in the forming particle structure [9]. The fixation and binding of microspheres and agglomerates took place in the second stage and the voids between the fixed microspheres filled with the linear polymer and nonsolvent [9]. Based on this description, we came to the conclusion that the presence of MAA makes difficult either the formation of rigid microspheres in the crosslinked form or the clear separation of these microspheres from the diluent mixture. More sticky gel microspheres probably occur in the existence of MAA relative to those generated

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in a forming particle containing only S and DVB as the monomers. The adhesion tendency of these microspheres should be higher. This can make easier the formation of larger blocks by the excessive aggregation of sticky gel microspheres containing MAA chains. Then larger voids occur between the larger blocks fixed in the particles. Note that carboxyl-functionalized particles produced with higher monomer phase/seed latex ratios did not have a particle interior including extremely large pores like those in Fig. 8(C). The higher viscosity of porogen solution originated from the lower monomer/seed latex ratio is probably another factor affecting on the formation of larger voids in the existence of MAA. Since, an increase in the average pore size with the decreasing monomer/seed latex ratio was shown for uniform poly(S–DVB) particles elsewhere [48]. To show the bulk and surface characteristics, FTIR and FTIR-DRS spectra of poly(S–DVB) and carboxyl-functionalized particles (i.e. poly(S– MAA –DVB)) produced with the monomer/seed latex ratio of 1.2/0.325 ml g − 1 are given in Fig. 9. As seen here, no significant difference was observed between the FTIR and FTIR-DRS spectra of poly(S–DVB) particles. However, FTIR-DRS spectrum showing the surface structure of poly(S–DVB) particles included a hydroxyl band at 3500 cm − 1 probably coming from the strongly entrapped or covalently bound PVA chains on the particle surface. The hydroxyl band was almost absent in the FTIR spectrum of the same particles. This finding indicated that concentration of PVA on the particle surface was higher than its bulk concentration. The weak carbonyl bands at 1740 cm − 1, both in the FTIR and FTIR-DRS spectra of the poly(S–DVB) particles probably originated from the initiator (BPO) used in the repolymerization. The strong carbonyl bands appeared in either FTIR or FTIR-DRS spectra of poly(S–MAA –DVB) particles at 1740 cm − 1 showed existence of MAA both in the bulk structure and on the particle surface. However, the relative intensity of carbonyl band in the FTIRDRS spectrum of poly(S–MAA –DVB) particles was higher than that in the FTIR spectrum of the same particles. FTIR-DRS spectrum of poly(S– MAA –DVB) particles also included a stronger

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hydroxyl band (i.e. coming from the hydroxyl parts of carboxyl groups) at 3500 cm − 1 relative to that in the FTIR spectrum. Both of these findings indicated that the concentration of MAA on the particle surface was higher than that in the bulk structure. Note that the relative intensity of hydroxyl band in the FTIR-DRS spectrum of poly(S –MAA – DVB) particles was also higher than that in the FTIR-DRS spectrum of poly(S– DVB) particles.

3.5. DVB feed concentration The effect of DVB concentration was examined by using L3 as the seed latex. In these experiments, DVB concentration was changed between 25 and 50% (by volume) based on the monomer phase. DBP/seed latex ratio was fixed at 2.0 ml g − 1. An example SEM photograph indicating the size and size distribution of the carboxyl-functionalized particles produced with 25% DVB concen-

Fig. 10. SEM photographs of carboxyl carrying-particles produced with different DVB concentrations. DVB concentration (% volume), (A) 25.0, Mag: 800 ×; (B) 25.0, Mag: 3000 ×; (C) 33.3, Mag: 3000 ×; (D) 50.0, Mag: 3000 × . Table 6 Properties of carboxylated particles produced with different DVB concentrations Seed latex

DVB (% v/v)

Dn (mm)

Dw(mm)

CV (%)

MAA content (mg g−1 particles)

L3 L3 L3

25.0 33.0 50.0

13.09 12.53 12.68

13.13 12.60 12.77

2.92 4.25 5.01

140.0 132.0 156.0

DBP swelling conditions, SL, 0.175 g; DBP/SL, 2 ml g−1; SDS, 0.25% w/v; aqueous medium, 27 ml. Monomer swelling conditions: monomer phase, 1.2 ml; BPO, 0.06 g; DVB, variable, MAA, 33.3% v/v; SDS, 0.25% w/v; aqueous medium, 52 ml. Repolymerization conditions, 70 °C; 24 h; 120 cpm.

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Fig. 11. TEM photographs showing the internal structure of carboxyl carrying-particles produced with different DVB concentrations. DVB concentration (% volume), (A) 25.0, Mag: 4000 ×; (B) 33.0, Mag: 4000 × ; (C) 50.0, Mag: 5000 × .

tration is given in Fig. 10(A). The properties of the particles produced with different DVB concentrations are given in Table 6. Although no

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significant change was observed in the average size, the particles obtained with higher DVB concentrations exhibited slightly wider size distribution. The surface morphology of particles are shown in Fig. 10(B–D). The TEM photographs showing the internal structures of the particles are given in Fig. 11. As seen in Fig. 10 and Fig. 11, more porous surfaces and particle interiors could be achieved with the higher DVB concentrations relative to those obtained by the lowest DVB concentration (i.e. 25%). Higher particle porosity should be explained by a more efficient phase separation between the fixed gel microspheres and porogen solution in the forming particles produced with the higher DVB feed concentration. The mechanism proposed by Cheng et al. involved the production and agglomeration of highly crosslinked gel microspheres in the forming particles and the binding of these formations for the fixation of the network [9]. During the particle formation process, some part of porogen solution is utilized for the swelling of fixed gel microspheres/agglomerates. So, the other part of porogen should remain in the voids located between the fixed gel microspheres (i.e. in the macropores). Crosslinking degree of the fixed gel microspheres is expected to be higher as the DVB concentration increased. Hence, the porogen absorption capacity of the gel microspheres decreases. This makes easier the separation of these microspheres from the porogen solution. Owing to the rigid and stable character of the formed gel-microspheres, the microspheres do not combine and dissolve in each other during the fixation process. Instead of larger agglomerates leading to a crater-like pore structure, relatively smaller agglomerates clearly separated from each other take place by the fixation of these rigid microspheres. The volume of voids between the fixed gel microspheres should be probably higher in this case. That means higher porosity in the final particles. This conclusion explains the structural differences between Fig. 10(B and D), or Fig. 11(A and C). On the other hand, it should be noted that no significant change occurred in the titratable MAA content of the particles by the DVB feed concentration.

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