Dispersion Copolymerization of Styrene and Other Vinyl Monomers in Polar Solvents

Transcription

1 Dispersion Copolymerization of Styrene and Other Vinyl Monomers in Polar Solvents WULI YANG, 1 DONG YANG, 2 JIANHUA HU, 1 CHANGCHUN WANG, 1 SHOUKUAN FU 1 1 Department of Macromolecular Science and Key Laboratory of Molecular Engineering of Polymers, Educational Ministry, Fudan University, Shanghai , China 2 Department of Chemistry, Fudan University, Shanghai , China Received 2October 2000; accepted 8December 2000 ABSTRACT: Dispersion polymerization is avery attractive method for preparing micrometer-size monodisperse polymer particles. The applications of microspheres have been greatly expanded by the use of copolymers. Here, the dispersion copolymerization of styrene and seven other vinyl monomers was carried out in polar solvents. The effect of the different comonomers on the particle size was systematically investigated. The particle size first decreased and then increased with an increasing fraction of acrylamide in the monomer feed, and at ahigher fraction of such acomonomer, only a gel-like polymer was obtained. The particle size also increased with the increase in the contents of the hydrophilic comonomers in the monomer mixtures, and the copolymer molecular weight decreased meanwhile. Although the amount of the hydrophobic comonomer in the monomer mixture changed, the particle size was hardly affected John Wiley &Sons, Inc. JPolym Sci A: Polym Chem 39: , 2001 Keywords: dispersioncopolymerization;micrometer-size;polymerparticles;styrene; water-soluble monomer; hydrophilic monomer; hydrophobic monomer INTRODUCTION Micrometer-size monodisperse polymer particles have awide variety of scientific and technological applications, such as standard calibration, biomedical and clinical diagnosis, high-performance liquid chromatography (HPLC) fillers, catalyst carriers, coatings and ink additives, information storage materials, and colloidal crystals. 1 4 Such particles are difficult to obtain because their size is between the size of particles prepared by conventional emulsion polymerization ( m) and that of particles prepared by suspension polymerization ( m). Several techniques for preparing such micrometer-size monodisperse Correspondence to: S. Fu ( skfu@srcap.stc.sh.cn) Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 39, (2001) 2001 John Wiley &Sons, Inc. particles have been developed. Vanderhoff et al. 5 used asuccessive seed swelling method to produce such particles. Ugelstad et al. 6 synthesized particlesofasimilarsizewithatwo-stepswelling method. Okubo et al. 7 developed their own swelling techniques, and Omi et al. 8 recently reported aspecial process (a Shirasu Porous Glass emulsification technique) to obtain uniform micrometersize polymer microspheres. However, these approaches are time-consuming and require several reaction steps or need some special techniques. Dispersion polymerization is avery attractive method for preparing micrometer-size monodisperse polymer particles because of the inherent simplicity of its single-step process. It is most suitable for preparing beads with diameters in the range of 1 15 m The preparation of micrometer-size monodisperse homopolymer particles has been extensively studied, especially for 555

2 556 YANG ET AL. polystyrene (PS) and poly(methyl methacrylate) (PMMA) systems. However, only a few studies on unseeded batch dispersion copolymerization have been reported One reason is that dispersion polymerization is highly sensitive to small changes in the numerous reaction parameters involved in the process. The behavior of the polymerization may change in the presence of another monomer in the reaction system; even different comonomer ratios should be treated as new polymerization systems. Tseng et al. 14 sketchily studied dispersion copolymerization and synthesized PS spheres containing a small amount of functional groups such as hydroxyl, amines, and carboxyl groups. Ober and Lok 15 and Horak et al. 16 intensively studied the dispersion copolymerization of styrene (St) and n-butyl methacrylate in ethanol (EtOH)/water media. Saenz and Asua 17,18 investigated the kinetics of the dispersion copolymerization of St and butyl acrylate and the morphology of resultant particles in detail. Recently, Cao et al. 19 prepared a series of micrometer-size functional uniform PMMA particles by dispersion copolymerization. In our previous works, 20,21 we studied the synthesis of micrometer-size monodisperse St/glycidyl methacrylate (GMA) and St/acrylamide (Am) copolymer microspheres by single-step dispersion copolymerization. This study extends that investigation to the effect of the comonomer type on the particle size and size distribution. The major monomer was St, and the comonomers were different types of vinyl monomers. Among the comonomers were water-soluble monomers such as Am and acrylic acid (AA); hydrophilic monomers such as GMA, methyl methacrylate (MMA), ethyl methacrylate (EMA), and 3-(trimethoxysilyl)propyl methacrylate (TMSPMA); and the hydrophobic monomer N-vinyl carbazole (NVC). Dispersion polymerization generally focuses on the particle size; here, the influence of the St/ comonomer ratio on the particle size and size distribution was studied. EXPERIMENTAL Materials St, AA, GMA, MMA, and EMA were purified by distillation under reduced pressure before polymerization. NVC was obtained from Merck Corp. TMSPMA was supplied by TCI Corp. Am and 2,2-azobisisobytytronitrile (AIBN) were purified by recrystallization from EtOH. Polyvinylpyrolidone (PVP; weight-average molecular weight 360,000) was supplied by BASF Corp. Deionized water was used throughout this work. EtOH and toluene were used as received. Preparation of the Copolymer Microspheres by Dispersion Copolymerization Generally, the comonomer and PVP were dissolved in a mixture of EtOH and deionized water in a 250-mL, four-necked, round-bottom flask equipped with a reflux condenser, stainless steel stirrer, thermometer, and nitrogen gas inlet and outlet. Then, a solution of AIBN in St was added. After 30 min of stirring at room temperature under a nitrogen purge, the flask was immersed in a 70 C oil bath. Under a nitrogen atmosphere, the polymerization was carried at 70 C for 24 h. After centrifugal purification, the microspheres were dispersed into EtOH/water (3/1 v/v) for characterization. Characterization of the Particles The particle size and size distribution were measured on a Coulter LS230. Morphology analysis was carried out on a Hitachi S-520 scanning electron microscope (SEM). The molecular weight of the copolymers was analyzed on a Shimadzu LC-3A liquid chromatograph with chloroform as an eluent and a detector wavelength of 254 nm. The ultraviolet visible (UV vis) spectrum was recorded on a Shimadzu UV-240 spectrophotometer. The glass-transition temperature was determined on a Setaram DSC-92 differential scanning calorimeter under nitrogen with a heating rate of 10 C/min. RESULTS AND DISCUSSION Dispersion polymerization usually starts with a homogeneous solution of monomer, a radical initiator, and a polymeric steric stabilizer in organic solvents, such as hydrocarbons or EtOH. First, the polymerization in the solution forms oligomeric radicals, polymer, and grafted steric stabilizer. 22 The solubility of these polymers is a function of their molecular weight and composition. A polymer with a molecular weight larger than a certain critical value precipitates and perhaps aggregates to form colloidally unstable precursor particles (nuclei). These nuclei may further coa-

3 DISPERSION COPOLYMERIZATION OF STYRENE 557 Table I. Effect of the Monomer Component (Water-Soluble Comonomer) on the Particle Size and Size Distribution a Sample Comonomer St/Comonomer (g/g) D ( m) CV (%) d Remark A1 15/0 1.4 b 9 Sphere A2 14.5/ b 13 Sphere A3 Am 14/1 1.3 b 17 Sphere A4 13/2 2.2 b 26 Sphere A5 11/ b Particle formed during cooling A6 9/6 Gel-like polymer B1 15/0 1.5 c 12 Sphere B2 AA 14.5/ c 15 Sphere B3 14/1 2.3 c 19 Sphere B4 13/2 Gel-like polymer a EtOH/H 2 O, 80 g/5 g; AIBN, 0.3 g, 0.3 wt % based on total weight; PVP, 0.6 g, 0.6 wt % based on total weight; T, 70 C. b The diameter was measured by SEM; D is the volume-average diameter (D n). c The diameter was measured with a Coulter LS230; D is the volume-average diameter (D v). d The coefficient of the variation of the volume-average diameter, D CV % (standard deviation)/d n D i i 1 n n, standard deviation i 1 D i D 2 /n 1, lesce and adsorb enough stabilizer from the medium onto their surface to become sterically stable. At this point, particle nucleation ceases, and the total number of particles is fixed. Subsequently, the precipitated oligomers and precursor (nuclei) are captured by the existing particles before they can adsorb enough stabilizer to create a second generation of stable particles. This mechanism of particle nucleation could be applied for all kinds of stabilizers, including graft copolymers formed in situ. 10 Although the nucleation period in the dispersion polymerization is very short, 23 it is very critical for the resultant particle size and size distribution. Water-Soluble Comonomer The process of dispersion polymerization was quite complicated, especially when a water-soluble comonomer (e.g., AA and Am) was used, because polyacrylic acid (PAA) is a water-soluble polymer and generally is the stabilizer in dispersion polymerization. Polyacrylamide (PAm) also could dissolve in our system under the reaction conditions (EtOH/water medium at 70 C). Dispersion polymerization starts with a homogeneous solution, and the first stage is, in fact, a solution polymerization. The copolymer with an Am-rich segment and an St-rich segment was usually obtained when the copolymerization of Am and St was carried out in a strong polar solvent. 24 The monomer reactivity ratios of Am and St in EtOH at 70 C were r 1 (Am) 0.3 and r 2 (St) Ohtsuka et al. 25 investigated the emulsifier-free emulsion polymerization of Am/St. They found that the whole polymerization process could be divided into three steps: (1) Am was initiated, and PAm fragment proliferated; (2) the PS fragments mainly proliferated until almost all the St monomer had been consumed; and (3) the PAm fragment proliferated again. They also found that the particle size increased with a decrease in the amount of Am, and they speculated that the PAm fragment just served as a stabilizer and was not incorporated into particles. As shown in Table I, with the increase in the weight fraction of Am in the monomer feed, the particle size first decreased and then increased. In the dispersion polymerization of Am, Ray and Mandal 26 thought that PAm-grafted poly(vinyl methyl ether) (PVME), not just PVME itself, was the real stabilizer in aqueous tert-butyl alcohol media. In the dispersion copolymerization of St/ Am, an increase in the Am fraction generally leads to a copolymer [P(St-Am)] rich in PAm fragments. The copolymer could act as a costabilizer, and more efficient costabilizers would be obtained with an increase in the Am fraction, thus leading to a decreasing trend for the particle size. 10,16,17,20 At the same time, the copolymer rich in PAm was

4 558 YANG ET AL. Figure 1. Effect of the St/AA ratio on the particle size and size distribution: (a) B1, (b) B2, and (c) B3. more soluble in the system because PAm could dissolve in the EtOH/water medium at the reaction temperature (70 C). Then, the critical chain length of the copolymer would increase, and the rate of adsorption of the stabilizer-grafted copolymer [PVP-g-P(St-Am)] or the costabilizer onto the nuclei would decrease, so both the rate of nuclei formation and the amount of the adsorption of the grafted stabilizer would decrease, resulting in a larger particle size. 10,12,15,16,20 The two opposite effects on the particle size with an increase in the Am fraction lead to the minimum particle size (run A2). In other words, when the Am fraction is low, the effect of an increase in the stabilizer is dominant, so the particle size decrease with an increasing Am fraction. At a higher Am fraction, the effect of increasing critical chain length and the decrease of the rate adsorption of the stabilizer become dominant, and the particle size increases with an increase in the Am fraction. In addition, the particle size distribution became wider with an increase in the Am fraction in the monomer feed because the copolymer richer in PAm was more soluble in the reaction system; the critical molecular weight of the copolymers increased, and the nucleation period was prolonged. Before the polymerization, all of the runs were homogeneous solutions. In run A5, when the fraction of Am was 4/15, the system was half-muddy after polymerization and turned milky when the temperature went from 70 C to room temperature, meaning that the particle formed during cooling. However, in run A6 the fraction of Am increased to 0.4, and during the whole reaction, the system was transparent and experienced a solution polymerization. When the system was cooled, only gel-like copolymers were obtained. Ray and Mandal 26 also only obtained a translu- Table II. Effect of the Monomer Component (Hydrophilic Comonomers) on the Particle Size and Size Distribution a Sample Comonomer St/Comonomer (g/g) D v ( m) c CV (%) G1 b 15/ G2 14/ G3 GMA 13/ G4 10/ M2 14/ M3 MMA 13/ M4 12/ M5 10/ E2 14/ E3 EMA 13/ E4 12/ E5 10/ T2 14/ T3 TMSPMA 13/ T4 12/ T5 10/ a Fifteen percent (w/w) monomer relative to the total mixture; EtOH/H 2 O (w/w) 80/5; 1% (w/w) AIBN relative to the monomers; 4.5% (w/w) PVP relative to the monomers. b G1 is also named M1, E1, and T1. c The diameter was measured with a Coulter LS230; D v is the volume-average diameter.

5 DISPERSION COPOLYMERIZATION OF STYRENE 559 Figure 2. Effect of the St/GMA ratio on the particle size and size distribution: (a) G1, (b) G2, (c) G3, and (d) G4. cent polymer solution when tert-butyl alcohol was beyond 40% in the dispersion polymerization of Am in aqueous tert-butyl alcohol media. In the St/AA system, the particle size increased with an increase in the weight fraction of AA in the monomer feed (Table I and Fig. 1). It was also found that the stage in which the system turned blue and then turned milky (nucleated) from transparency lengthened when the fraction of AA increased. When the fraction of AA was 2/15, the system was always transparent during the whole reaction, and only gel-like polymers were obtained after the system cooled. In other words, the critical fraction of water-soluble monomer, beyond which only gel-like polymers were obtained, in the AA/St system (2/15) is lower than that in the Am/St system (4/15). The reason is that PAA is more soluble in the EtOH/water medium than PAm. Hydrophilic Comonomer The effect of the hydrophilic comonomers on the system was studied through changes in the ratio of St and the comonomer, as shown in Table II. Figure 2 presents an SEM picture of the St/GMA system as an example. It can be seen from Table II that the particle size increased with the contents of the hydrophilic comonomers in the monomer mixtures, and all the trends in St/GMA, St/ MMA, St/EMA, and St/TMSPMA, are similar. Ober et al. 15 also observed that the particle size

6 560 YANG ET AL. Table III. Molecular Weight of the Copolymers with Different St/GMA Ratio Sample St/GMA a ( 10 4 ) M n M w ( 10 4 ) M w /M n G1 15/ G2 14/ G3 13/ G4 10/ a In the monomer feed. increased when the St/n-butyl methacrylate ratio decreased in the dispersion copolymerization of these two monomers in an ethanlol water medium. Another similar result was obtained by Saenz and Asua 17 in an St/butyl acrylate system. The difference in particle size might result from the hydrophilicity and polarity of the copolymer 15 because GMA, MMA, EMA, and TMSPMA have higher hydrophilicity and polarity than St. For example, in the St/GMA system the polarity and hydrophilicity of the resultant copolymer rich in poly(glycidyl methacrylate) (PGMA) are higher than those of the copolymer rich in PS. The copolymer richer in PGMA is more soluble in EtOH water, which results in a longer critical chain length and a larger particle size. Because the copolymer richer in PGMA is more soluble in EtOH water, the reaction is more like a solution polymerization, and the molecular weight will be lower than the weight of the copolymer rich in PS. The molecular weight also confirmed the explanation (Table III). An inverse correlation between particle size and molecular weight was noted: 10 larger particles often had lower molecular weights (see Tables II and III). Hydrophobic Comonomer Here the hydrophobic monomer is NVC, and the medium is EtOH toluene, in which NVC has a Table IV. Effect of the Monomer Component (Hydrophobic Monomer) on the Particle Size and Size Distribution a Sample St/NVC (g/g) D v ( m) CV (%) N1 20/ N2 19/ N3 18/ N4 17/ N5 15/ a Twenty percent (w/w) monomers relative to the total mixture; EtOH/toluene (w/w) 78/2; 2% (w/w) AIBN relative to the monomers; 5% (w/w) PVP relative to the monomers. higher solubility. The effect of the St/NVC ratio on the particle size is shown in Table IV. When the amount of NVC in the monomer mixture changed, the particle size was hardly affected. Because the polarity and hydrophilicity of poly(nvinyl carbazole) (PVK) are near those of PS, the critical chain length of the copolymer richer in PVK may be close to the length of the copolymer rich in PS, and the critical chain length of the copolymers is also similar to that of the pure PS system; therefore, the particle size would not change when the St/NVC ratio changed. The molecular weights and molecular weight distribution of the polymers (N1 N5) are described in Table V. When the NVC amount in the feed changed, there was no great difference in the molecular weights. Generally, low molecular weights and broad molecular weight distributions resulted, similar to those in solution polymerization. The amount of PVK increased from 0 (N1) to 20.4% (N5) when the fraction of NVC in the monomer feed increased (Table V). By comparing the amount of PVK in the copolymers with the amount of NVC in the monomer feed, we found relatively low PVK contents in the copolymers. This happened because of the different reactivity ratios of St (monomer 1) and NVC (monomer 2; r 1 1, r 2 1). 27 Table V. Characterization of the Copolymers with Different St/NVC Ratios Sample NVC in Monomers (%) PVK in Copolymer (%) a M n ( 10 3 ) M w ( 10 3 ) M w /M n N N N N N a Calculated on UV vis, the work wavelength was 340 nm, and dichloromethane was used as a solvent and reference.

7 DISPERSION COPOLYMERIZATION OF STYRENE 561 CONCLUSIONS The dispersion copolymerization of St and other vinyl monomers was carried out in polar solvents. St was taken as the major monomer, and different types of vinyl monomers were taken as comonomers. The effects of the different comonomers on the particle size was investigated. When the contents of Am increased in the monomer feed, the particle size first decreased then increased, and only a gel-like polymer was obtained at a high comonomer fraction. The particle size also increased with an increase in the contents of the hydrophilic comonomers in the monomer mixtures, such as the St/GMA, St/MMA, St/EMA, and St/TMSPMA systems. When the amount of the hydrophobic comonomer (NVC) in the monomer mixture changed, the particle size was hardly affected. The authors thank the National Natural Science Foundation of China for its financial support (Grant No ). REFERENCES AND NOTES 1. Ugelstad, J.; Berge, A.; Ellingsen, T.; Schmid, R.; Nilsen, T. N.; Mork, P. C.; Stenstad, P.; Hornes, E.; Olsvik, O. Prog Polym Sci 1992, 17, Pichot, C.; Delair, T.; Elaissari, A. In Polymeric Dispersions: Principles and Applications; Asua, J. M., Ed.; Kluwer Academic: Dordrecht, the Netherlands, 1997; p Arnold, S.; Liu, C. T.; Whitten, W. B. Opt Lett 1991, 16, Velev, O. D.; Kaler, E. W. Adv Mater 2000, 12, Vanderhoff, J. W.; El-Aasser, M. S.; Micale, F. J.; Sudol, E. D.; Tseng, C. M.; Silwanowicz, A.; Sheu, H. R.; Kornfeld, D. M. Polym Mater Sci Eng 1986, 54, Ugelstad, J.; Mork, P. C.; Kaggerud, K. H.; Ellingsen, T.; Berge, A. Adv Colloid Interface Sci 1980, 13, Okubo, M.; Shiozaki, M.; Tsujihiro, M.; Tsukuda, Y. Colloid Polym Sci 1991, 269, Omi, S.; Katami, K.; Yamamoto, A.; Iso, M. J Appl Polym Sci 1994, 51, Lok, K. P.; Ober, C. K. Can J Chem 1985, 63, Paine, A. J.; Luymes, W.; McNulty, J. Macromolecules 1990, 23, Shen, S.; Sudol, E. D.; El-Aasser, M. S. J Polym Sci Part A: Polym Chem 1993, 31, Shen, S.; Sudol, E. D.; El-Aasser, M. S. J Polym Sci Part A: Polym Chem 1994, 32, Takahashi, K.; Miyamori, S.; Uyama, H.; Kobayashi, S. J Polym Sci Part A: Polym Chem 1996, 34, Tseng, C. M.; Lu, Y. Y.; El-Aasser, M. S.; Vanderhoff, J. W. J Polym Sci Part A: Polym Chem 1986, 24, Ober, C. K.; Lok, K. P. Macromolecules 1987, 20, Horak, D.; Svec, F.; Frechet, J. M. J. J Polym Sci Part A: Polym Chem 1995, 33, Saenz, J. M.; Asua, J. M. J Polym Sci Part A: Polym Chem 1996, 34, Saenz, J. M.; Asua, J. M. Macromolecules 1998, 31, Cao, K.; Li, B.; Pan, Z. Macromol Symp 2000, 150, Yang, W.; Hu, J.; Tao, Z.; Li, L.; Wang, C.; Fu, S. Colloid Polym Sci 1999, 277, Tao, Z.; Yang, W.; Zhou, H.; Wang, C.; Fu, S. Colloid Polym Sci 2000, 278, Paine, A. J. J Colloid Interface Sci 1990, 138, Thomson, B.; Rudin, A.; Lajoie, G. J Polym Sci Part A: Polym Chem 1995, 33, Minsk, L. M.; Kotlarchik C.; Darlak, R. S. J Polym Sci Polym Chem Ed 1973, 11, Ohtsuka, Y.; Kawaguchi, H.; Sugi, Y. J Appl Polym Sci 1981, 26, Ray, B.; Mandal, B. M. Langmuir 1997, 13, Alfrey, T.; Kapur, S. J Polym Sci 1949, 4, 217.