Surface modification of monodisperse hydroxyl functionalized polymeric microspheres using ceric ammonium nitrate

European Polymer Journal 41 (2005) 2209–2215

EUROPEAN POLYMER JOURNAL www.elsevier.com/locate/europolj

Surface modification of monodisperse hydroxyl functionalized polymeric microspheres using ceric ammonium nitrate Seong-Heun Cho a, Jee-Hyun Ryu a, Jin-Gyu Park b, Kyung-Do Suh b

a,*

a Division of Chemical Engineering, College of Engineering, Hanyang University, Seoul 133-791, Republic of Korea Chemicals R&D Center, Cheil Industries Inc., 332-2 Gocheon-dong, Uiwang-si, Gyeonggi-do 437-711, Republic of Korea

Received 22 April 2005; accepted 29 April 2005 Available online 29 June 2005

Abstract The surface modification of monodisperse hydroxyl functionalized polymeric microspheres was carried out by utilizing a redox initiation system. Styrene, divinylbenzene and hydroxyethyl methacrylate were used as the second monomer in the seeded polymerization. An excessive amount of the second monomer emulsion was swollen into the polystyrene (PS) seed particles completely by controlling the medium solvency and swelling temperature. The hydroxyl functional groups were radicalized by the ceric ammonium nitrate in nitric acid solution, and the methyl methacrylate was reacted uniformly on the surface of microspheres. From the SEM, and FE-TEM measurements, highly monodisperse microspheres having a smooth surface, and polymethylmethacrylate (PMMA) coating layer were observed, respectively. The surface characteristics of the PS seed particles, hydroxyl functionalized and surface-modified polymeric microspheres were confirmed by utilizing FT-IR, XPS and thermal analysis.
2005 Elsevier Ltd. All rights reserved. Keywords: Surface modification; Monodisperse functionalized polymeric microspheres; Seeded polymerization; Ceric ammonium nitrate; PMMA grafting

1. Introduction Monodisperse and highly crosslinked polymeric microspheres were useful in a wide range of chemical applications including the standard kit, a catalyst, chromatography, supporter, or the ball-type spacer in the liquid crystal display (LCD) because of their dimen-

* Corresponding author. Tel.: +82 2 2220 0526; fax: +82 2 2295 2102. E-mail address: [email protected] (K.-D. Suh).

sional stability and durability [1,2]. The polymer particles can be prepared by using the heterogeneous polymerization techniques such as suspension, emulsion, dispersion and precipitation polymerization [3]. Suspension polymerization is a useful technique for making crosslinked polymer beads, but the size distribution of beads is too broad [4,5]. Therefore, an additional classification process must be followed to obtain the monodispersity. Spherical latex particles synthesized by the emulsion polymerization tend to fall in the nanometer size range [6], and a rough surface and a porous structure of polymer particles are obtained by using the

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2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2005.04.037

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precipitation polymerization [7,8]. The crosslinking of polymer particles by the dispersion polymerization is accompanied by a broad size distribution, odd shape or coagulum, despite of a small amount of crosslinker [9–11]. The heterogeneous polymerization techniques have the limitations for the preparation of monodisperse and highly crosslinked particles [12]. Monodisperse and highly crosslinked polymeric microspheres can be achieved by swelling the second monomer into the seed particles and consecutive polymerization. The seeded polymerization was extensively investigated by Ugelstad et al. [1,13,14] as an activated swelling procedure (ASP), El-Aasser et al. [2,15,16] as a successive seeded polymerization, Okubo and Nakagawa [17] as a dynamic swelling method (DSM), and others [18,19]. Once the monodisperse seed particles are utilized, the monodispersity of microspheres can be maintained during the seeded polymerization. Another merit of seeded polymerization is that the diameter, the matrix property or the functionality of polymer microspheres are easily manipulated by the nature of seed particles and the composition of the second monomer. In our earlier works [20–22], the monodisperse polymer particles having high crosslinking density and functionality were produced by using the seeded polymerization. In the recent day, the additional requirements of polymeric microspheres rise up for the specific application fields maintaining their monodispersity and high crosslinking density. For instance, an adhesive property of the ball-type LCD spacer is needed to prevent drop down by the gravity in the LCD [23,24]. Besides, the dispersion ability of hydrophobic microspheres in a hydrophilic medium, and the biocompatibility of polymer particles in a blood are also requested in the paint industry, and the medical treatments, respectively. The additional properties of polymeric microspheres can be provided by the surface modification utilizing the chemical modification [25], or the high energy radiation (ultraviolet, plasma, physical or chemical vapor deposition) methods [26,27]. Especially, the redox system by transition metal ions has been found to be effective in oxidizing the functional groups on the surface without changing the original properties of polymeric microspheres [25]. In the present study, the surface modification of monodisperse and highly crosslinked polymeric microspheres was performed by introducing hydroxyl groups on the surface, and utilizing the redox initiation system. Styrene (St), divinylbenzene (DVB) and hydroxyethyl methacrylate (HEMA) were chosen as the second monomer in the seeded polymerization, and swollen into the PS seed particles. The hydroxyl groups on the surface were initiated by ceric ammonium nitrate (CAN) in the nitric acid solution, then methyl methacrylate (MMA) was reacted on the microspheres uniformly.

The morphologies of polymer particles were monitored by an optical, scanning and transmission electron microscope. The thermal analysis of surface-modified polymeric microspheres was also carried out. 2. Experimental 2.1. Materials Styrene (St, Kanto), methyl methacrylate (MMA, Junsei), azobis(isobutyronitrile) (AIBN, Junsei), polyvinylpyrrolidone (PVP, Mw = 4.0 · 104 g/mol, Sigma), aerosol-OT (AOT, Sigma) and ethanol (Carlo) were all reagent grades. Divinylbenzene (DVB, Fluka), 2hydroxyethyl methacrylate (HEMA, Aldrich), sodium lauryl sulfate (SLS, Yakuri), ceric ammonium nitrate (CAN, Aldrich) and nitric acid (Aldrich) were also used without further purification. 2.2. PS seed particles crosslinked with PPGDA Polypropylene diacrylate (PPGDA, crosslinker) was synthesized by the reaction of polypropylene glycol (PPG, Mw = 2.0 · 103 g/mol, Polyol) with acryloyl chloride (AC, Sigma Chemicals) in tetrahydrofuran (THF, Mallinckrodt) [28]. The crosslinked PS seed particles were produced by the dispersion polymerization. AOT (0.2 g), PVP (1.8 g) and ethanol (85 g) were weighed into a 250 ml four-necked round flask equipped with a reflux condenser, nitrogen inlet apparatus and a mechanical stirrer. Then St (10 g), PPGDA (2.5 g), AIBN (0.125 g) mixture was poured into the reactor at room temperature. After 30 min of vigorous stirring, the homogeneous mixture was reacted at 70 C for 24 h with 40 rpm stirring. The product was purified three times through centrifugation at 2500 rpm for 10 min, and washed with ethanol to remove the surface-anchored PVP molecules. Final particles were dried at room temperature. 2.3. Hydroxyl functionalized polymeric microspheres Polymer particles containing hydroxyl functional groups were prepared by the seeded polymerization. The PS seed particles were redispersed in 0.25 wt% SLS of EtOH/water (1/5, g/g) solution (SE solution) by sonification. The emulsion of the second monomer mixture (St/DVB/HEMA) in a SE solution was poured into the reactor. The swelling was continued at 30 C until emulsion droplets disappeared completely. The swollen particles were stabilized with 5% PVP aqueous solution, and polymerized at 70 C for 10 h. The particles washed repeatedly with water and dried in vacuum oven. A standard recipe of the seeded polymerization was summarized in Table 1.

S.-H. Cho et al. / European Polymer Journal 41 (2005) 2209–2215 Table 1 Standard recipe of the seeded polymerizationa Stage

Ingredient

Quantity (g)

Seed dispersion

PS seed particles SE solutionb

0.10 40.0

The second monomer

St DVB HEMA BPO SE solutionb

0.50/1.0 3.00 1.5/1.0c 0.05 21.00

Stabilization

PVP solutiond

40.00

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mined. The existence of hydroxyl groups and the PMMA grafted on the surface of polymer particles were confirmed by using a fourier transform infrared spectroscope (FT-IR, Niclolet, Mahgna IR-550), a X-ray photoelectron spectroscope (XPS, SIGMA PROBE, Thermo VG, UK), and a transmission electron microscope (FE-TEM, JEM 2100F, JEOL). Thermal stability and glass transition temperature (Tg) of polymeric microspheres were determined by using a thermogravimetric analysis (TGA, TA Instruments) and a differential scanning calorimeter (DSC, DSC-7, Perkin Elmer), respectively.

a

80 C, 10 h, 125 rpm. b EtOH/water = 1/5 (w/w), 0.25 wt% SLS in the solution. c 20 wt% and 30 wt% of HEMA based on the total weight of the second monomer. d 5% PVP aqueous solution.

2.4. Graft polymerization of MMA using hydroxyl functional groups Hydroxyl functionalized polymer particles were dispersed in nitric acid solution. The MMA in nitric acid solution was poured into the reactor. Nitrogen atmosphere was maintained throughout the reaction period. An amount of ceric ammonium nitrate in nitric acid solution was added slowly. Then, graft polymerization was carried out at 45 C for 4 h. The surface coated particles were filtered, and washed with water repeatedly. The recipe of surface graft polymerization was shown in Table 2. 2.5. Characterization Swelling procedure and morphology of polymeric microspheres were monitored by a optical microscope (OM, Olympus BH-2) and a scanning electron microscope (SEM, JSM-6330F JEOL). About a hundred individual particle sizes were measured from SEM photographs, the average diameter of microspheres was taken, and the polydispersity index (PDI) was deterTable 2 Standard recipe of the graft polymerization on the surface of microspheresa Ingredient

Quantity (g)

OH functionalized microspheres HNO3 solutionb MMA HNO3 solution Ceric ammonium nitratec HNO3 solution

0.20 40.0 0.20 5.00 0.10 5.00

a b c

45 C, 4 h, 250 rpm. 0.188 mol/l. CAN, 3.16 · 10 3mol/l, with dropping funnel.

3. Results and discussion 3.1. Morphology The monodispersity of seed particles plays an important role in the preparation of monodisperse polymeric microspheres in the seeded polymerization. Therefore, monodisperse PS particles crosslinked with the polypropylene diacrylate (PPGDA) were prepared by using the dispersion polymerization. As shown in Fig. 1(a), highly monodisperse (PDI = 1.01) and 1.3 lm in diameter were observed and a smooth surface was confirmed even 2 wt% of PPGDA was copolymerized. The flexible backbone and the diacrylate structure of PPGDA in the PS particles helped the complete swelling of the second monomer, and prevented the deswelling to the medium during the seeded polymerization stage. The 50-fold of the second monomer relative to the weight of PS seed particles was chosen to incorporate high content of DVB for the durability of microspheres. SLS and ethanol was mixed with the aqueous solution (0.25 wt% SLS in EtOH/water = 1/5, g/g) to adjust the medium solvency, which facilitated the migration of emulsions to the PS seed particles. In addition, much faster swelling was achieved when the swelling temperature was set at 30 C than at room temperature. High ethanol content and temperature caused that the emulsions were dissolved into the medium or the polydisperse polymeric microspheres. In case of the opposite conditions, a lot of the emulsions were still observed in the medium in spite of 72 h of swelling. Under the conditions, an excessive amount of the second monomer was completely and uniformly swollen into the PS seed particles, and the deswelling or phase separation in the morphology was not appeared after the polymerization as shown in Fig. 1(b) (4.2 lm, PDI = 1.01). The hydroxyl functional groups were required to achieve the surface modification of monodisperse and highly crosslinked polymeric microspheres by using the chemical initiation system. The radicalized hydroxyl groups, which were initiated by the ceric ions, reacted with an acrylic monomer on the surface, consequently,

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S.-H. Cho et al. / European Polymer Journal 41 (2005) 2209–2215

the total weight of the second monomer by fixing the DVB content at 60 wt%, as shown in Table 1. When the HEMA was used over 30 wt%, the phase separation was occurred due to the difference of interfacial tension between the polymeric constituents [21]. Fig. 1(c) shows the SEM photographic of PMMA coated polymeric microspheres by using the CAN in the nitric acid solution as a redox initiator. The difference in the surface of particles was observed comparing with Fig. 1(b) and (c). Hydroxyl functionalized polymeric microspheres showed a smooth surface, while Fig. 1(c) seen that something was overspread the whole surface of particles. For the clear confirmation, the FE-TEM analysis of particles after the MMA surface grafting was carried out. Uniform 70 nm in diameter of another layer was clearly ascertained as shown in Fig. 2. From the results, it is established that the MMA was reacted with the radicalized hydroxyl groups then uniformly coated on the surface of polymeric microspheres. 3.2. Surface modification The acetate groups were introduced on the polymer particles, and converted to the hydroxyl functional groups by the saponification in a methanol solvent in our previous studies [29,30]. However, it was well known that 100% of hydrolysis cannot take place, thus it was supposed that the amount of hydroxyl groups on the particles were relatively small. From the point, the HEMA was used instead of the vinyl acetate (VAc) in the seeded polymerization to get hydroxyl functional groups directly, simultaneously, the saponification procedure was eliminated. The contents of HEMA were

Fig. 1. SEM photographs of the PS seed particle (a); polymeric microspheres having hydroxyl functional groups (b); PMMA coated polymeric microspheres (c).

an additional property of polymeric microspheres was supplied without affecting the original property. In the seeded polymerization, the hydroxyl functional groups were derived by incorporating the HEMA in the second monomer mixture. The other compositions of the second monomer are St and DVB of which provide the affinity with the PS seed particles and crosslinking structure of microspheres, respectively. The amount of HEMA was manipulated as 20 and 30 wt% based on

Fig. 2. FE-TEM photographic of polymeric microspheres after MMA surface coating procedure.

S.-H. Cho et al. / European Polymer Journal 41 (2005) 2209–2215

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(a)

(b)

Tranmittance

Intensity

(b)

(a)

525

530

535

540

(c)

3000

Binding Energy (eV) Fig. 3. XPS spectra: O1s peaks of hydroxyl functionalized polymeric microspheres containing 20 wt% (a), and 30 wt% (b) of HEMA in the second monomer.

controlled at 20 and 30 wt% in the second monomer, and the hydroxyl groups on the surface were investigated by the XPS analysis. Fig. 3 shows the O1s spectra of poly(DVB-co-HEMA) particles with different contents of HMEA. It was confirmed that the hydroxyl groups were existed on the surface of particles, in addition, the intensity of OH peak at 532 eV [31] was increased with increasing the HEMA content. Surface graft polymerization is that the hydroxyl functional groups were radicalized by the redox initiator, and polymerized with acrylic monomer on the surface of polymer particles. Ceric ammonium nitrate in the nitric acid, which was chosen in this experiment, was well known redox initiation system as fast reaction and good yield in the surface grafting of acrylic monomer [25]. The reaction recipe and condition of grafting polymerization, as shown in Table 2, were optimized by our preliminary studies. Depending on the nature of the acrylic monomer, an additional property could be introduced to the monodisperse and highly crosslinked polymeric microspheres. In this paper, PMMA was grafted on the surface to give the adhesive property of monodisperse and highly crosslinked polymeric microspheres by using the difference of glass transition temperature (Tg). PMMA coated polymeric microspheres having another low Tg would be stuck strongly on substrates after appropriate thermal treatment sustaining their dimensional stability and durability. The whole process of dispersion, seeded and surface graft polymerization were monitored by using a FT-IR measurement as depicted in Fig. 4. A broad hydroxyl peak around 3600–3100 cm 1 and a carbonyl peak at 1730 cm 1 were appeared after the seeded polymeriza-

2000

1000 -1

Wavenumber (cm ) Fig. 4. FT-IR spectra of the seed particles (a); hydroxyl functionalized (b); surface-modified polymeric microspheres (c).

tion (Fig. 4(b)). It means that the HEMA was copolymerized during the seeded polymerization and polar hydroxyl groups were incorporated on the surface of microspheres, also, this is in a good agreement with the XPS analysis. After surface grafting polymerization, the diminution of OH peak was confirmed as shown in Fig. 4(c). It can be explained that the hydroxyl groups were consumed during the grafting polymerization because of the radicalization by the CAN. More detailed investigations were discussed by using the thermal analysis. 3.3. Thermal analysis An additional property of surface-modified polymeric microspheres was evaluated by utilizing thermal analysis. Although Tg of PS seed particles were detected at 108 C in the DSC measurements, only an exothermal peak appeared in a range of 150–160 C in case of the hydroxyl functionalized and the surface-modified poly(DVB-co-HEMA) microspheres. This is similar to the trend observed in our previous work [30]. It can be elucidated by a reaction of residual double bond in the polydivinylbenzene. It has been reported that some second double bonds should be kept free of polymerization in the formed polymers and no Tg of polymer is shown, when high content of DVB was polymerized [32]. The high amount of DVB in the seeded polymerization resulted in the disappearance of Tg, the composition ratio of St and DVB was adjusted until the Tg was detected. When the DVB content was decreased to 10 wt% (St/DVB/HEMA = 60/10/30), an endothermal peak was emerged at 135 C, and continuously appeared even after the surface modification as shown in Fig. 5.

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S.-H. Cho et al. / European Polymer Journal 41 (2005) 2209–2215 Table 3 Thermal stability of seed, hydroxyl functionalized and surfaced modified polymeric microspheres

Exo

(a)

↑

Heat flow

(b)

Tda (C)

Residual quantity (%)

Seed particles OH functionalized microspheres PMMA grafted microspheres

422.6 447.8 440.8

8.08 19.3 18.9

a

Thermal decomposition temperature by TGA measurement.

(c)

Endo

↑

50

100

150

200

250

Temperature (ºC) Fig. 5. DSC curves of the seed particles (a); hydroxyl functionalized (b); PMMA coated polymeric microspheres (c) with 10 wt% of DVB content in the second monomer.

Another Tg was appeared at 113 C after the MMA was polymerized on the surface of microspheres. It means that MMA was polymerized on the monodisperse and highly crosslinked polymeric microspheres. It is difficult to directly evaluate the surface modification of highly crosslinked polymeric microspheres by using the difference of Tg, therefore, thermal stability by a TGA measurement was examined. Fig. 6 and Table 3 show the TG curves and the thermal decomposition temperature (Td) of seed, hydroxyl functionalized and

100

80

Weight (%)

Produced particles

60

40

20

0

100

200

300

400

500

surface-modified microspheres. The hydroxyl functionalized and surface-modified microspheres have much higher Td and the residue than these of seed particles. The Td and the residue of hydroxyl functionalized microspheres are slightly higher than that of PMMA coated microspheres though the same DVB content was used. Because of the low thermal stability of PMMA layer, the surface-modified microspheres decomposed more early and remained a smaller residue than the hydroxyl functionalized microspheres. To conclusion, the surface modification of monodisperse and highly crosslinked polymeric microspheres achieved by using the redox initiation system, and it could be applied in industry usefully.

4. Conclusion The surface of monodisperse and highly crosslinked polymeric microspheres was modified by using the chemical initiation system to provide the additional property maintaining original dimensional stability and durability. By incorporating the HEMA in the second monomer, the hydroxyl functional groups were introduced, and then PMMA was uniformly coated on the surface of microspheres. From the XPS, FT-IR, and FE-TEM analysis, the hydroxyl groups, and PMMA coating layer were confirmed, respectively. Monodispersity and a smooth surface of highly crosslinked polymeric microspheres were observed at the SEM measurements. After the MMA surface grafting, another Tg was appeared when the DVB content was decreased to 10 wt%. Although the same amount of DVB was used in the seeded polymerization, Td was occurred early in case of the surface-modified polymeric microspheres. Depending on the nature of acrylic monomer, an additional property would be provided to the polymeric microspheres sustaining their own property.

600

Temperature (ºC) Fig. 6. TG curves of the seed particles (solid); hydroxyl functionalized (dash); PMMA coated polymeric microspheres (dot) with 60 wt% of DVB content in the second monomer.

Acknowledgement This study was supported by a grant of the Korea Health 21 R&D Project, Ministry of Health & Welfare, Republic of Korea. (03-PJ1-PG1-CH14-0001).

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