European Polymer Journal 41 (2005)

Transcription

1 European Polymer Journal 41 (2005) Synthesis and properties of soap-free poly(methyl methacrylate-ethyl acrylate-methacrylic acid) latex particles prepared by seeded emulsion polymerization q Kai Kang, Chengyou Kan *, Yi Du, Deshan Liu Department of Chemical Engineering, School of Materials Science and Engineering, Tsinghua University, Beijing , People s Republic of China Received 7 September 2004; accepted 25 October 2004 Available online 24 December 2004 Abstract In order to obtain functional polymer latex particles with clean surface and with surface carboxyl groups, P(MMA- EA) seed particles with the diameter of 335 nm were first synthesized via soap-free batch emulsion polymerization of methyl methacrylate (MMA) and ethyl acrylate (EA), and then the seeded emulsion copolymerization of MMA, EA and MAA (methacrylic acid) onto the seed particles were performed in the absence of emulsifier. Influences of ingredients and conditions on polymerization, latex particle size (D p ) and its distribution were investigated. Results showed that most of the monomers polymerized onto the seed latex particles in the second step of polymerization by using drop-wise addition method, and D p increased from 483 nm to 829 nm with the mass ratio of core/shell monomers [C]/[S] decreased from 1:2 to 1:15. It was found that D p decreased with the increase of MAA and initiator amounts, and the size of the latex particles became uniform with the decrease of MAA amount and with the increase of [C]/ [S] value. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Soap-free latex particles; Seeded emulsion polymerization; Particle size and its distribution; Methyl methacrylate based copolymer 1. Introduction In recent years, functional polymer latices have been widely studied and used in special coating, information materials and biomaterials [1 4]. At present, how to synthesize latex particles with special particle morphology q Supported by National 863 Project of PR China. * Corresponding author. Tel.: ; fax: address: kancy@tsinghua.edu.cn (C. Kan). and with controllable size and functional surface is most important in the field of emulsion polymerization. Ugelstad et al. [5] developed a two-step microsuspension method for the production of large-uniform latex particles. Okubo and his coworkers synthesized monodispersed crosslinked polymer microspheres with surface vinyl groups by seeded emulsion polymerization of styrene (St) and divinylbenzene using dynamic swelling method [6], and they also prepared a series of submicron-sized polymer particles of St-butyl acrylate (BA)- methacrylic acid (MAA) and St-MAA with multihollow morphology by the stepwise alkali/acid method [2,7] /$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi: /j.eurpolymj 转载

2 440 K. Kang et al. / European Polymer Journal 41 (2005) Guillaume et al. [8] studied the influences of ingredients and conditions on the distribution of carboxyl groups in P(St-BA-MAA) and P(St-BA-potassium sulfopropyl methacrylate) latices, and found that most of the functional monomer was incorporated onto the particle surface during the last 30% conversion of the polymerization. Using azo-initiator containing carboxyl groups, Bastos et al. [9] prepared two carboxylated PSt latex particles and characterized their surface properties by conductimetric and potentiometric titration. After activated the carboxyl groups with carbodiimide, they bound antibodies onto these particles via covalent coupling. We have prepared a series of monodispersed core/shell latex particles containing carboxyl groups in the presence of emulsifier via seeded emulsion copolymerization, and the influences of unsaturated acid, ph value, swelling agent and crosslinking agent on the pore formation in seeded latex particles were also studied [10 12]. In order to avoid the disadvantages resulting from emulsifiers used in traditional emulsion polymerization, attempts have been made by some researchers to prepare soap-free latices in the absence of emulsifier. However, in most of the cases, some surfactant ingredients such as polymerizable emulsifier and surface-active initiator must be used to stablize the polymerization process and the resulted latices [13 15]. Moreover, most of the investigations were primarily focused on the system of St. Compared to St, methyl methacrylate (MMA) is more hydrophilic, and it is expected that PMMA particles with clean surface and with surface carboxyl groups may have many promising applications in biomedical and biochemical fields. In a previous article [16], P(MMA-EA-AA(acrylic acid)) latex particles with narrow size distribution and with surface carboxyl groups were synthesized by batch soap-free emulsion polymerization, and the D p of particles can be well controlled in the range from 300 nm to 600 nm. In this study, relative large P(MMA-EA-MAA) latex particles with the same properties were first synthesized by seeded emulsion polymerization in the absence of any surfactant materials, and the effects of monomer addition modes, MAA, initiator and the mass ratio of core/shell monomers([c]/[s]) used in the recipes on particle size and its distribution were investigated. 2. Experimental 2.1. Materials Methyl methacrylate (MMA), ethyl acrylate (EA) and methacrylic acid (MAA) (A.P. grades, First Chemical Reagent Factory, Tianjin, China) were purified by distillation under reduced pressure and kept in the refrigerator. Ammonium persulphate (APS) (A.P. grade, Aijian Modern Reagent Factory, Shanghai, China) was purified by recrystallization twice in water before use. NH 4 HCO 3 (A.P. grade, Beihua Fine Chemical Products Co., Ltd., Beijing, China) were used as received. Phosphotungstic acid (A.P. grade, Shenyang Reagent Factory, Shenyang, China) was diluted to 2% aqueous solution. Deionized and distilled water was employed Synthesis of P(MMA-EA-MAA) latex particles Preparation of seed latex [S]: P(MMA-EA) seed latex was prepared via batch emulsion polymerization at 80 C for 6 h using 120 g of H 2 O, 19 g of MMA, 1 g of EA, 0.24 g of APS and 0.2 g of NH 4 HCO 3. Seeded emulsion polymerization: Polymerization was carried out in nitrogen atmosphere at 80 C. The recipes were given in Table 1, in which MAA is expressed as the mole percentage to all monomers, and [C]/[S] denoted the mass ratio of core/shell monomers used in the two stages of polymerization. Two monomer addition methods were used as follows: Method A (Batch swelling process). Seed latex, water and the mixture of monomers were added into the flask in sequence. After stirring first at around 250 rpm for 2 3 h and then around 150 rpm for a prescribed time at ambient temperature, the system was heated to 80 C in a water bath, and APS solution was introduced to start reaction. Polymerization was completed at 4 h. Method B (Drop-wise addition process). Seed latex and water were charged into a flask, which was then dipped in a heated water bath. 2/3 of APS was introduced when the temperature reached to 80 C, and then the polymerization was conducted by drop-wise addition of the monomers into the system. The remnant APS was introduced at 4 h of polymerization, and the addition process lasted 8 h followed by an additional 3 h of aging Characterization Monomer conversion was determined by gravimetry. The diameter (D p ) and morphology of latex particles were characterized on Hitachi H-800 transmission electron microscope (TEM; Hitachi, Japan) using phosphotungstic acid as a staining agent. The theoretical value of D p was computed assuming that all of the monomers polymerized onto the seed particles with 100% conversion. The polydispersity of latex particle size was expressed as the quotient of the standard deviation (d) of the particle diameter and D p. The less the value of d/d p is, the more narrow the size distribution will be. The number of particles per liter (N p ) was calculated based on D p, monomer conversion and polymer density

3 K. Kang et al. / European Polymer Journal 41 (2005) Table 1 Recipe of seeded emulsion polymerization Latices [S] (g) MMA (g) EA (g) MAA (mol%) APS (g) H 2 O (g) [C]/[S] C 1 S :2 C 1 S :5 C 1 S a Variable 113 1:7 C 1 S :9 C 1 S :11 C 1 S :13 C 1 S :15 a In some experiments (listed in Table 4), the mole percentage of MAA was variable as the total amount of monomers was fixed at 20.0 g and the mass ratio of MMA to EA was 19:1. [17]. The f potential was determined by Zeta potential instrument (Malvern Zetasizer 3000HS, UK) to express the stability of the latices. 3. Results and discussion To perform seeded emulsion polymerization, seed latex with narrow size distribution must be synthesized [18]. One can see, from Table 2 and Fig. 1, that monodispersed microspheres with diameter of 335 nm were obtained in the seed latex preparation Influence of monomer addition modes In seeded emulsion polymerization, the solubility of monomers have a great influence on the polymer composition, particle size and its distribution by affecting the proportion of polymerization site of monomers in water and on latex particles [19 21]. In the experiments, the system with 1/7 of [C]/[S] was selected to test the effect of monomer addition modes on polymerization and latex properties, and the results were showed in Table 3. The subscript data marked to Method A in Table 3 represent swelling time at ambient temperature. It is clear, in Method A, that the swelling time has a little influence on the polymerization and the properties of the resulted latex for there were a lot of new particles generated during any one of the three polymerization process. In contrast, when Method B was employed, the values of experimental D p and N p are almost the same as their corresponding theoretical values, indicating that few new particles were produced during the second step of polymerization. TEM photographs of P(MMA-EA-MAA) latex particles prepared by different Table 2 Properties of seed latex Conv. (wt%) D p (nm) N p (L 1 ) d/d p f (mv) 97.74% Fig. 1. TEM photographs of P(MMA-EA) seed latex particles. methods were given in Fig. 2, which showed that narrow dispersed latex particles can be obtained in both methods. The above phenomenon was disagree with the results obtained in the process of polymerization with emulsifier, in which the more the new particles were, the wider the particle size distribution would be [10]. In Method A, since the equilibrium swelling ratio (the mass of swollen monomers/ the mass of seed particles) was definite, excessive monomers would result in the formation of new particles via homogeneous nucleation in polymerization process. Because the generated new particles in the initial stage of polymerization were unstable, they will tend to agglomerate with each other to form relative large particles. This new kind of particles will compete with the swelled seed particles for monomers, and as a result, the size of these two kinds of particles will increase simultaneously, which lead to the uniformity of all particle size in system. Homogeneous nucleation oc-

4 442 K. Kang et al. / European Polymer Journal 41 (2005) Table 3 Influences of monomer addition modes on latex properties a Adding modes of monomers Conv. (wt%) D p (nm) N p (L 1 ) d/d p f (mv) A A A B a C 1 S 7 : APS, g; MAA, 8.0 mol%. Fig. 2. TEM photographs of P(MMA-EA-MAA) latex particles prepared via (a) batch swelling process (Method A, sample A 24 ) and (b) drop-wise addition process (Method B). curred in these experiments not only make the experimental value of D p less than the theory value of it by far, but also resulted in the diversification of particle composition [22]. When Method B was used, relative large particles with uniform composition can be obtained. According to results reported by Misra et al. [23] and Pichot et al. [24], homogeneous copolymer compositions can also be obtained by the semicontinuous polymerization process under a starved condition, though during emulsion copolymerizations the homogeneity of the copolymer composition is expected to be a strong function of reactivity ratios and monomer solubility in both water and copolymer. In this work, when the monomer addition time was controlled at 6 h, 7 h and 8 h respectively, the corresponding experimental value of N p was L 1, L 1 and L 1. The theoretical value of N p is L 1, that was to say, very few new particles were generated in the seeded emulsion polymerization if monomer addition time was adjusted to 8 h. In drop-wise addition process, because the monomer concentration was always high in the locus where the monomers were added and the diffuseness of monomers into seed particles need some time, if the addition speed of monomers was too fast, the concentration of EA and MAA in aqueous phase must be high, which will certainly lead to the homogenous nucleation and as a consequence, more new particles would be produced. Results showed that the shorter the addition time was, the more the new particles were. However, too long addition time would result in the decrease of monomer conversion in polymerization. For example, the monomer conversion decreased to 90.7% when the addition time was prolonged for 10 h. Table 4 Influences of MAA on latex properties a MAA (mol%) ph Conv. (wt%) D p (nm) N p (L 1 ) d/d p f (mv) a C 1 S 7 : APS, g.

5 K. Kang et al. / European Polymer Journal 41 (2005) Fig. 3. TEM photographs of P(MMA-EA-MAA) latex particles using different amount of MAA: (a) 0 mol%, (b) 8 mol% and (c) 16 mol%. Table 5 Influences of initiator on latex properties a APS (g) Conv. (wt%) D p (nm) N p (L 1 ) d/d p f (mv) a C 1 S 7 : MAA, 8.0 mol% Influence of MAA amount We have prepared soap-free P(MMA-EA-AA) latex particles by batch emulsion polymerization, and found that the monomer conversion increased and D p decreased with the increase of AA amount [25]. In this study, seeded emulsion polymerization process was used and most of monomers polymerized onto the seed particles in the second step of polymerization, so the influences of ingredients on polymerization and latex properties should be different from those in the batch polymerization process. It can be seen, from Table 4, that the monomer conversion and the particle size and its distribution were almost unchanged when the mole ratio of MAA was below 8.0%; when MAA was more than 10.0%, the monomer conversion had a trend to increase, and D p decreased and the particle size distribution widened with the increasing amount of MAA. The same results can also be observed from TEM photographs of latices in Fig. 3. MAA is soluble in water, the more it was used, the severer the homogeneous nucleation was, and therefore the more new particles would be produced. While during drop-wise addition process, since the monomers were added slowly, the concentration of MAA throughout reaction process was low enough to limit the homogeneous nucleation when MAA amount was less than 8.0 mol% Influence of initiator In conventional seeded emulsion polymerization with emulsifier, relative more amount of initiator was usually used in the second step of polymerization to enhance the monomer conversion [11]. However, in a soap-free system, stabilization is purely electrostatic and depends on two parameters: the surface charge density of the particles and the ionic strength of the aqueous medium. Here, APS can act as a source of surface charge and ionic strength, and thus has both stabilizing and destabilizing effects. Results in Table 5 showed clearly that when its amount was below g, the monomer conversion increased, the particle size had a trend to decrease slightly, and particle size distribution widened with the increase of APS amount. Once the amount of APS was increased to g, although the polymerization can be carried out smoothly, there was some coagulation appeared during polymerization because of more HSO 4 existed in the system, and D p decreased and particle size distribution widened significantly.

6 444 K. Kang et al. / European Polymer Journal 41 (2005) Fig. 4. TEM photographs of P(MMA-EA-MAA) latex particles using different amount of APS: (a) g, (b) g and (c) g. Table 6 Influences of [C]/[S] on latex properties [C]/[S] Conv. (wt%) D p (nm) N p (L 1 ) d/d p f (mv) 1: : :7 a : : : : a APS, g; MAA, 8.0 mol%. Fig. 5. TEM photographs of P(MMA-EA-MAA) latex particles with different mass ratio of [C]/[S]: (a) 1:2, (b) 1:5 and (c) 1:15. TEM photographs of P(MMA-EA-MAA) latex particles using different amount of APS were showed in Fig. 4. It is clear that large amount of initiator was not favorable in this study, and monodispersed polymer particles can be obtained when g APS was used Influence of [C]/[S] The ratio of [C]/[S] had also influences on the polymerization and latex properties though seeded emulsion polymerization with different [C]/[S] can be performed smoothly with high monomer conversion. Results were

7 K. Kang et al. / European Polymer Journal 41 (2005) showed in Table 6 and Fig. 5. Since the concentration of monomers in aqueous phase increased obviously with the decrease of [C]/[S] ratio, the number of new particles generated via homogeneous nucleation would certainly increase, thus D p increased and the particle size distribution widened. When the value of [C]/[S] decreased to 1:15, about 5.9% of new particles were generated. 4. Conclusions Soap-free P(MMA-EA-MAA) latex particles with carboxyl groups on their surface were synthesized by seeded emulsion polymerization. Drop-wise addition process was better than batch swelling process to produce large particles with narrow size distribution. When drop-wise addition process was employed, few new particles were generated in the seeded emulsion polymerization, and monodispersed seeded latex particles could be obtained at optimum recipes of less than 8.0 mol% of MAA and g of APS. 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