Preparation of Core-Shell Polystyrene-Polyimide Particles by Dispersion Polymerization of Styrene Using Poly(Amic Acid) As a Stabilizer

Macromol. Rapid Commun. 2000, 21, 1323–1326 1323

Communication: Dispersion polymerization of styrene (S) and vinylbenzyltrimethylammonium chloride (VBA) was conducted in an ethanol-water medium using an aromatic poly(amic acid) (PAA) as the stabilizer. When equimolar amounts of VBA and the carboxylic acid of PAA were used, monodisperse particles with high PAA content were obtained quantitatively. The imidization of PAA on the particles proceeded with acetic anhydride and N,N- dimethylaminopyridine to form core-shell PS-polyimide particles.

Scanning electron micrograph of PS-PI particles.

Preparation of core-shell polystyrene-polyimide particles by dispersion polymerization of styrene using poly(amic acid) as a stabilizer

Shinji Watanabe,* Kenji Ueno, Kazuhiko Kudoh, Miki Murata, Yuzuru Masuda Department of Materials Science, Kitami Institute of Technology, Kitami, Hokkaido 090-8507, Japan Fax: +81-157-26-4973; E-mail: WATANABE-Shinji/[email protected]

Introduction PAA incorporated into the particles can be converted into Dispersion polymerization is a versatile method for the pre- polyimide (PI) to yield core-shell PS-PI particles paration of monodisperse polymer particles in the 0.5– (Scheme 1). Thus PAA could play two important roles, i.e., 10 lm size range. Especially the polymerization of styrene as a steric stabilizer during polymerization, and to improve and methyl methacrylate in polar media has been exten- mechanical and thermal properties of particles by means of sively studied. To disperse particles stably, a suitable steric imidization of PAA after the polymerization. For this pur- stabilizer was used, such as polyethylene oxide,[1] poly(vi- pose, PAA must be incorporated into the particles effec- nylpyrrolidone),[2] poly(2-ethyl-2-oxazoline),[3] hydroxy- tively. In general, most stabilizers for dispersion polymeri- propyl cellulose[4] and poly(acrylic acid)[5]. In this study, zation are not incorporated into the particle and a large we employed an aromatic poly(amic acid) (PAA) as the sta- amount of stabilizer is needed to disperse particles stably. bilizer in the dispersion polymerization of styrene (S). PAA For an effective incorporation into the particles, reactive is soluble in alkaline ethanol-water mixtures due to the pen- stabilizers bearing polymerizable groups[7] or radical trans- dant carboxylic acid groups and works as a steric stabilizer fer agents (mercapto group)[8] were used in the polymeriza- for dispersion polymerization. Furthermore, PAA is easily tion. These compounds stabilized particles even in low converted to polyimide (PI), which is one of the most concentration because they react with the monomer and get important high performance polymers providing high ther- incorporated into the particles. In this study, the cationic mal stability, excellent mechanical properties, high soften- comonomer 4-vinylbenzyltrimethylammonium chloride ing temperatures and good electric properties, and has been (VBA) was employed in the dispersion polymerization. utilized for aerospace materials and dielectric insulator in VBA might copolymerize with S, and absorb an anionic the semiconductor industry.[6] After the polymerization, the PAA effectively via electrostatic attraction.

Macromol. Rapid Commun. 2000, 21, No. 18 i WILEY-VCH Verlag GmbH, D-69451 Weinheim 2000 1022-1336/2000/1812–1323$17.50+.50/0 1324 S. Watanabe, K. Ueno, K. Kudoh, M. Murata, Y. Masuda

Scheme 1.

Experimental part (2 cm62 cm) was completely subjected to combustion under oxygen atmosphere in a combustion flask containing Measurements deionized water (20 mL). The evolved fluorine was adsorbed into the water. Into the fluoride solution (1.6 mL), 1 mL of The diameter of polymer particles was determined by a scan- 5% aqueous alfusonem solution (DOJINDO), 0.4 mL of ning electron microscope (JEOL JSM-588). X-ray photoelec- 0.1 M acetate buffer (pH 5.0), 2 mL of acetone, and 5 ml of tron spectroscopic data were obtained on a RIGAKU XPS- deionized water were added and the solution was allowed to 7000 instrument with AlKa X-ray source (10 kV, 30 mA) at stand at room temperature for 1 h. The absorption of the a take-off angle of 908. solution was measured at 610 nm.

Materials Imidization of PAA on the polymer particle S was washed with aqueous NaOH and distilled under reduced pressure. 2,29-Azobisisobutyronitrile (AIBN) was Lyophilized polymer particles (see Tab. 1, run 5; 1.0 g) were recrystallized from methanol. Poly(amic acid) (PAA) was suspended in a solution of 2.0 g of N,N-dimethylaminopyri- prepared by conventional polycondensation[9] of 4,49-(hexa- dine (DMAP) in 15 mL of acetic anhydride (Ac2O) and stir- fluoroisopropylidene)diphthalic anhydride with 4,49-oxydia- red at room temperature for 1 d. The suspension was poured niline and purified by reprecipitation in water. Its inherent into 50 mL of water and the particles were centrifuged, viscosity was 0.76 dL/g in N,N-dimethylacetamide at 308C. washed twice with ethanol, and dried under reduced pres- 4-Vinylbenzyltrimethylammonium chloride (VBA) was sure. PS-PI particles were recovered quantitatively (98%). synthesized by the reaction of 4-vinylbenzyl chloride with To separate the PI shell, the resulting particles were sus- trimethylamine[10] and recrystallized from a mixture of aceto- pended in 30 mL of toluene, stirred at room temperature for nitrile and acetone (1/1 v/v). Other reagents were obtained 3 h and centrifuged twice with toluene. commercially and used as received.

Dispersion polymerization Results and discussion A typical dispersion polymerization was conducted as fol- The dispersion polymerization of S and VBA was carried lows. PAA (0.266 g; monomer unit: 0.50 mmol) and potas- out using PAA as a steric stabilizer in ethanol-water med- sium carbonate (0.104 g; 0.75 mmol) were dissolved in ium (80/20 v/v) at 708C for 20 h. The results of the poly- 100 mL of ethanol-water (80/20 v/v). To the solution, S merization using varying amounts of VBA are summar- (8.0 g; 76.8 mmol), VBA (0.202 g; 1.0 mmol), and AIBN ized in Tab. 1. The incorporation ratio of PAA was (0.16 g; 1.0 mmol) were added, and the mixture was stirred increased by adding the cationic comonomer. Without with mechanical stirrer at 708C for 20 h under a nitrogen VBA, only 15% of PAA were incorporated into the parti- atmosphere. After removal of the coagulated polymer by fil- cles (run 1). When equimolar amounts of cationic como- tration, the resulting particles were purified by centrifugation nomer and carboxylic acid of PAA were used, 80% of three times using ethanol (18500 g). PAA were incorporated, and monodisperse particles were obtained in high yield (run 5). The polymerization with Fluorine content of the particles excess comonomer generated coagulated polymer (run 6 To determine the incorporation ratio of PAA into the parti- and 7). In the absence of PAA, the polymer completely cles, the fluorine content of the particles was measured by coagulated and no particle was obtained (run 4), indicat- the lanthanum alizarin complexone (La-ALC) method.[11] ing that PAA worked as a stabilizer. When VBA was used Lyophilized polymer (10 mg) wrapped with filter paper in amounts of 0.50 to 1.50 mmol, monodisperse particles Preparation of core-shell polystyrene-polyimide particles by dispersion polymerization ... 1325

Tab. 1. Dispersion polymerization of S and VBA in the presence of PAA (S: 76.8 mmol, PAA: 0.50 mmol, K2CO3 : 0.75 mmol, AIBN: 1.0 mmol, ethanol: 80 mL, water: 20 mL, 708C, 24 h).

a) b) b) c) Run No. VBA Yield Coagulated polymer Dn Dw /Dn Incorporated PAA mmol % % lm %

1 0 7 80 2.09 1.10 15 2 0.50 68 20 1.39 1.01 30 3 0.75 69 20 1.12 1.01 50 4d) 1.00 0 100 – – – 5 1.00 80 0 0.74 1.01 80 6 1.50 47 42 1.37 1.01 93 7 2.00 0 100 – – – a) Yield of coagulated polymer during polymerization. b) Determined by scanning electron microscopy. c) Ratio of incorporated PAA determined by fluorine content of the particle. d) Polymerization was carried out in the absence of PAA.

Fig. 1. Scanning electron micrographs of (a) PS-PAA particles, (b) PS-PI particles and (c) PI particles. were obtained as can be seen in Fig. 1a. The particle size effect of VBA on the particle size must be a little compli- decreased from 2.09 to 0.74 lm dependent on the concen- cated. Before polymerization, ion pairs of PAA and VBA tration of VBA and increased to 1.37 lm using excess must be formed in the medium. As the copolymerization VBA (run 6). Since VBA was copolymerized with S and of S and VBA-PAA proceeded, physically crosslinked bonded the polymer particles with the steric stabilizer, the polymers as nuclei may be formed. When the amount of 1326 S. Watanabe, K. Ueno, K. Kudoh, M. Murata, Y. Masuda

shrink and change in shape. The atomic ratio of fluorine to carbon of the particles (run 5) determined by XPS (surface) is 75.4610 – 3, which is much larger than that determined by La-ALC method (F/C: 3.56610–3) (bulk). This means that the PAA having fluorine atoms is mainly located at the particle surface, and core-shell PS-PI particles were obtained by imidization of the particles. Even after thermal treatment at 2408C for 1 h, the diameter of most PS-PI particles did not change, whereas PS particles obtained by conventional dispersion polymerization were completely destroyed by thermal treatment, as observed by SEM. The PI shell protects the particles at such a high temperature due to the very high glass transition temperature of PI (about 3018C, as determined by DSC). We reported the novel preparation method of core-shell PS-PI particles by dispersion polymerization using PAA as the stabilizer and achieving the imidization of PAA on the particles. PAA was incorporated into the particles effectively by using the cationic comonomer VBA. Imidi-

zation of PAA on the particle was achieved by Ac2O and Fig. 2. IR spectra of (a) PS-PAAparticles, (b) PI particles, and DMAP to yield PS-PI particles. Further investigation, (c) PI film prepared with conventional thermal dehydration of such as regulation of the particle size and polymerization PAA. using other PAAs are now under way.

VBA was increased, a large amount of PAA was adsorbed into the nuclei, which stabilizes the particles and inhibits Received: July 27, 2000 coalescence between particles during the particle growth Revised: August 29, 2000 stage, leading to the formation of small particles. On the other hand, an excess amount of VBA (run 6 and 7) desta- bilized the particles probably due to being too hydrophilic for the dispersion polymerization, resulting in coales- [1] (a) P. Lacroix-Desmazes, A. Guyot, Macromolecules 1996, 29, 4508; (b) I. Capek, M. Riza, M. Akashi, J. Polym. Sci., cence of the particles, a decreasing number of particles Part A: Polym. Chem. 1997, 35, 3131. and the formation of large particles. [2] (a) H. Bamnolker, S. Margel, J. Polym. Sci., Part A: Polym. The imidization of PAA on the polymer particle was Chem. 1996, 34, 1857; (b) A. J. Paine, W. Luymes, J.

carried out by means of Ac2O and DMAP at room tem- McNulty, Macromolecules 1990, 23, 3104; (c) C. M. perature for one day. To quantify the extent of imidiza- Tseng, Y. Y. Lu, M. S. El-Aasser, J. W. Vanderhoff, J. Polym. Sci., Part A: Polym. Chem. 1986, 24, 2995. tion, IR spectroscopy was used after removing the PS of [3] H. Uyama, S. Kobayashi, Polym. Int. 1994, 34, 339. the particles by washing with toluene (Scheme 1). IR [4] (a) Y. Chen, H. Yang, J. Polym. Sci., Part A: Polym. Chem. spectra of the particles before and after treatment with 1992, 30, 2765; (b) K. P. Lok, C. K. Ober, Can. J. Chem. toluene are shown in Fig. 2. The characteristic absorp- 1985, 63, 209; (c) A. J. Paine, J. Colloid Interface Sci. tions for polystyrene at 1601, 1493, 1452, and 697 cm – 1 1990, 138, 157. [5] (a) A. Tuncel, R. Kahraman, E. Piskin, J. Appl. Polym. Sci. disappeared after washing with toluene, and the resulting 1993, 50, 303; (b) T. Corner, Colloids Surfaces 1981, 3, IR profile of the PI particleswas identical with that of an 119. aromatic PI film prepared by conventional thermal-imidi- [6] W. Volksen, “High Performance Polymers”, P. M. Her- zation of PAA,[9] indicating that PAA was converted into genrother, Eds., Springer-Verlag, Berlin, Heidelberg 1994, PI. A scanning electron micrograph of PS-PI particles is p. 111–164. [7] (a) S. Kobayashi, H. Uyama, S. W. Lee, Y. Matsumoto, J. shown in Fig. 1b. The particle size did not change with Polym. Sci., Part A: Polym. Chem. 1993, 31, 3133; (b) P. imidization, however, the particle surface became a little Lacroix-Desmazes, A. Guyot, Polym. Adv. Technol. 1997, rough. On the other hand, the particles after removing PS, 8, 608. i.e., PI, were observed to be smaller than PS-PI, and to be [8] E. Bourgeat-Lami, A. Guyot, Polym. Bull. 1995, 35, 691. nonspherical (Fig. 1c). The polyimide may have swelled [9] S. R. Sandler, W. Karo, “Polymer Syntheses” Academic Press, Inc., San Diego, CA 1992, p. 257–265. a little by treatment with toluene, and polymer chains [10] G. D. Jones, S. J. Goetz, J. Polym. Sci. 1957, 25, 201. might have been reoriented to release the chain stress at [11] R. Belcher, M. A. Leonard, T. S. West, J. Chem. Soc. 1958, the imidization stage. As a consequence, particles might 2390.