The Fabrication of Very Small Miniemulsion Latexes from N-Stearoylglutamate and Lauryl Methacrylate: Evidence for Droplet Budding

Transcription

1 1966 Macromol. Chem. Phys. 2003, 204, Full Paper: A new, biodegradable surfactant with high efficiency, N-stearoyl-di(sodium)glutamate (SDG), was used for miniemulsion polymerization of styrene and lauryl methacrylate (LMA). In the case of styrene, SDG produces latexes at least as small as those produced by sodium dodecyl sulfate (SDS), over a range of concentrations. In the case of LMA, a combination of SDG with an appropriate costabilizer allows one to obtain very small particles. With potassium persulfate (KPS) as initiator, the SDG/LMA system deviates significantly from the ideal miniemulsion, forming extremely small particles, which was speculatively attributed to a budding phenomenon. The new, biodegradable surfactant, N-stearoyl-di(sodium)- glutamate. The Fabrication of Very Small Miniemulsion Latexes from N-Stearoylglutamate and Lauryl Methacrylate: Evidence for Droplet Budding Ufuk Yildiz,* a Katharina Landfester, Markus Antonietti Max Planck Institute of Colloids and Interfaces, Research Campus Golm, Potsdam, Germany Fax: (þ49) ; uyildiz@kou.edu.tr Received: January 9, 2003; Revised: August 21, 2003; Accepted: August 25, 2003; DOI: /macp Keywords: biodegradable; emulsion polymerization; heterophase polymerization; lauryl methacrylate; surfactants Introduction Miniemulsion polymerization is a new way to synthesize polymer nanoparticles in the size range of nm with high morphological uniformity and chemical flexibility. [1,2] Focusing on the very small particles, miniemulsion polymerization already steps into the range of microemulsion polymerization, [3,4] however, with a minimum expense of surfactant which is up to a factor of 10 lower than in microemulsion formulations with similar particle size. The name miniemulsion refers to systems where small monomer droplets in a continuous phase are created by using high shear [5,6] and are stabilized by a combination of surfactant and an osmotic pressure agent. In a second step, these droplets are polymerized under conservation of the droplet number. The most effective systems with respect to particle size are based on the ionic surfactant sodium dodecyl sulfate (SDS). Usually, both miniemulsion droplets and polymer particles are just incompletely covered with surfactant a Kocaeli University, Department of Chemistry, Kocaeli, Turkey. (which is the basis of the high efficiency), which can be detected by the rather high surface tensions of the resulting polymer dispersions. [7] It was, however, observed that the coverage of the droplets by surfactant depends on the droplet size: the smaller the droplets, the higher the required surface coverage is in order to obtain stable droplets. This is why miniemulsion polymerization is somewhat selflimiting towards smaller particle sizes. Exchange of the SDS to other ionic or nonionic surfactants has only led to systems with lower performance. First exploitations of cationic surfactants such as octadecyl pyridinium bromide (however, with incomplete dispersion) showed only broad particle size distributions. [8,9] Recent work on steady state miniemulsions revealed that miniemulsions on the basis of cetyltrimethylammonium chloride give narrowly distributed stable cationic latex particles being slightly larger than those with the SDS system. [10] Employment of charged biosurfactants, such as cholic acids, lipids, and short polysaccharides, has indeed broadened the range of accessible surface structures, but did not lower the accessible particle size. [11] It is the purpose of the present contribution to show that another bio-based surfactant with a functional and Macromol. Chem. Phys. 2003, 204, No. 16 DOI: /macp ß 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

2 The Fabrication of Very Small Miniemulsion Latexes Scheme 1. bivalent head group, N-stearoyl-di(sodium)glutamate (see Scheme 1), can be employed in miniemulsion polymerization with at least similar efficiency than that of SDS. As a model case, the dependence of the particle size of polystyrene particles on relative surfactant concentration will be discussed. In addition, we report on the miniemulsion polymerization of lauryl methacrylate, which is, because of its hydrophobicity, a critical monomer for heterophase polymerization in general and shows specific instabilities of formulated miniemulsions. Here, the combination of N- stearoyl-di(sodium)glutamate with a costabilizer with medium polarity, here either oligostyrene, poly(propylene glycol) (PPG), or cetyl alcohol (hexadecan-1-ol), helps to keep the original droplet size and to realize a clean mechanistic miniemulsion polymerization for this monomer, too. In addition, in some areas of recipe space, an unexpected lowering of particle size with increasing polymerization time and coupled extremely high surface tensions are found. Density experiments are performed to clarify the origin of this very special effect and can exclude secondary nucleation. Experimental Part N-stearoyl-di(sodium)glutamate. Styrene (Fluka) was distilled before use, lauryl methacrylate (Aldrich) was purified over a column. Sodium dodecyl sulfate (SDS), dodecanoic acid, hexadecane (HD), poly(propylene glycol) (PPG, M w ¼ g mol 1 ), oligostyrene (M w ¼ g mol 1 ) were purchased from Aldrich and used as received. Dodecyl alcohol (Fluka) and methacrylic acid (Röhm) were also used as received. We selected the nonionic initiator PEGA200 as it is inert with respect to stabilization and ph changes throughout the reaction and allows cleaner comparisons. PEGA200 was made according to reference [12]. A typical recipe is given in the following: six grams of the monomer (styrene or lauryl methacrylate), and 250 mg of hexadecane were mixed and added to a solution of the respective amount of N-stearoyl-di(sodium)glutamate (e.g., 72 mg or 1.2 wt.-% with respect to the disperse phase) in 24 g of water. After stirring for 1 h, the miniemulsion was generated by ultrasonication of the emulsion for 120 s at 90% amplitude with a Branson sonifier W450 Digital. To avoid any polymerization caused by heating of the sample, the mixture was ice-cooled during the homogenization. For polymerization, the temperature was increased to 72 8C and 115 mg of PEGA200 or 100 mg of potassium persulfate (KPS) was added to initiate the polymerization. In the case of using 2,2 0 -azoisobutyronitrile (AIBN) as initiator, 100 mg of AIBN was added to the monomer phase prior to miniemulsification. After 4 h, full conversion of the monomers was obtained as checked by NMR measurements. The particles sizes were measured using a Nicomp particle sizer (model 370, PSS Santa Barbara, U.S.A.) at a fixed scattering angle of 908. Electron microscopy was performed with a Zeiss 912 Omega electron microscope operating at 100 kv. The diluted colloidal solutions were applied to a 400-mesh carbon-coated copper grid and left to dry. No further contrasting was applied. All measurements regarding the surface tension were performed with the K10 processor-tensiometer from Krüss employing the DuNöuy Ring method. The radius of the Pt-Ir ring RI12 was 9.45 mm, and the wire had a radius of mm. The rt-dt experiments were performed on a modified Beckman Coulter XL-80K with turbidity optics as described in reference [13] applying a linear speed profile of 0 40 K. The measurements were carried out at 25 8CinH 2 O and D 2 O. A self-made Ti-monosector centerpiece was used. Results and Discussion Table 1 summarizes the results obtained for the miniemulsion polymerizations of styrene using N-stearoyl-di- (sodium)glutamate (SDG) as surfactant. Stable dispersions with a solid content of 20 wt.-% and particle diameters of 80 nm are already obtained at very small surfactant loads. All dispersions are quite monodisperse (about 15% standard deviation), as indicated by dynamic light scattering and transmission electron microscopy (see Figure 1) on selected samples. A systematic comparison of the performance of the N-stearoyl-di(sodium)glutamate to other surfactants in this reference situation is presented in Figure 2. It is observed Table 1. Characteristics of styrene latexes formed using SDG as surfactant and PEGA200 as initiator. Latex Surfactant Particle size Solid content Surface area per surfactant molecule a) % nm % nm 2 Sty Sty Sty Sty Sty Sty Sty Sty a) Calculated from the diameter and the amount of surfactant, for details see Reference [7]. ph

3 1968 U. Yildiz, K. Landfester, M. Antonietti Figure 1. TEM picture of a typical PLMA latex (sample Sty-13). that under standard conditions in the range of 1 wt.-% of surfactant, N-stearoyl-di(sodium)glutamate indeed lowers the particle size of the polystyrene particles. Just at very high surfactant concentrations comparable to microemulsion recipes, the curve bends over, and apparently larger particles are obtained. This is presumably because of surfactant-mediated aggregation of primary particles, as it was discussed for polymerization in microemulsions of multivalent surfactants. [14] The high efficiency is also seen in the measurement characterizing the surface tension of the latexes. The optimal systems show surface tensions g of 68 mn m 1, that is, are very close to pure water (72.4 mn m 1 ). This is Figure 2. Variation of the particle size obtained by changing the amount of N-stearoyl-di(sodium)glutamate used for the preparation of styrene miniemulsions. For comparison, the corresponding particle size curves using SDS and nonionic Lutensol AT50 are also shown. explained by a low surface coverage of the polystyrene particles with surfactant and a related very small equilibrium concentration of surfactant molecules in the continuous phase (please note that the critical micelle concentration (cmc) is reached at 280 mg L 1, the surface tension is then 43.2 mn m 1 ). On the other hand, at higher surfactant concentrations, we reach g values that are typical for saturated surfaces and large amounts of free surfactant, thus we indeed have left the composition range where a clean miniemulsion polymerization can be performed. Based on the stoichiometry of the reactions, we have also calculated the apparent surface area stabilized per surfactant molecule in these dispersions. For example, at 1.2 wt.-% and 2.4 wt.-% of surfactant, values of 4.1 and 2.1 nm 2 molecule 1 are obtained respectively, which are well below the SDS case. [7] In a second set of experiments, the very hydrophobic monomer lauryl methacrylate was polymerized, which is known to result even with miniemulsion polymerization only in very large particles. These results are summarized in Table 2 and Figure 3. It is also seen that with N-stearoyl-di(sodium)glutamate and with PEGA as initiator, quite large particles, stable with a low polydispersity, are obtained. In addition, surface tension measurements prior and after polymerizations (data included in Table 2) indicate that the miniemulsions become unstable throughout polymerization, and that the latex particles grow. The measure of the apparent droplet size prior to polymerization should be quite accurate as the solubility of lauryl methacrylate is so low that the emulsions can presumably be diluted without change, and indeed the primary particle size corresponds to the measured surface tension and is much lower than the one of the final latex particles. This destabilization is speculatively attributed to changes in polarity, as the partly polar methacrylate group is consumed throughout polymerization, and the resulting polymer only exposes the very hydrophobic tail ends to the outside phase. In miniemulsion polymerization, it is in principle possible to counterbalance this effect either by addition of minor amounts of a polar comonomer or by addition of an inert costabilizer with higher polarity, which mediates the adherence of the surfactant layer onto the particles. For this purpose, we have chosen poly(propylene glycol) (PPG), oligostyrene, cetyl alcohol (hexadecan-1-ol), and 5 wt.-% of poly(lauryl methacrylate) (PLMA) which have been added to the monomer prior to miniemulsification. It has been mentioned that the two polymers act at the same time as the osmotic agent of the miniemulsion so that no hexadecane was added in those cases. The resulting data were also added in Table 2. Using AIBN as initiator allows the maintenance of the droplet size throughout the polymerization (see Table 2), and much smaller particle sizes are obtained as those with

4 The Fabrication of Very Small Miniemulsion Latexes Table 2. Characteristics of LMA latexes formed with different amounts of SDG and SDS as surfactants, different initiators, and varying cosurfactants. Latex Surfactant Cosurfactant Initiator Particle size nm Solid content ph Surface tension of the final latex Type wt.-% Before polymerization After polymerization mn m 1 LMA-1 SDG 1.2 PEGA a) LMA-2 SDG 2.4 PEGA a) LMA-3 SDG 10.2 PEGA a) LMA-16 SDG mg oligostyrene c) PEGA a) 72, LMA-17 SDG mg cetyl alcohol PEGA a) LMA-18 SDG mg dodecanoic acid PEGA a) LMA-19 SDG mg PPG d,f) PEGA a) LMA-20 SDG mg methacrylic acid PEGA a) LMA-21 SDG mg PPG d) PEGA a) LMA-22 SDG mg methacrylic acid þ PEGA a) LMA-23 SDG 2.4 AIBN b) LMA-24 SDG 7.2 AIBN b) LMA-25 SDG mg methacrylic acid þ AIBN b) LMA-26 SDG mg methacrylic acid þ AIBN b) LMA-27 SDG mg PLMA AIBN b) LMA-10 SDG 0.6 KPS e) LMA-11 SDG 1.2 KPS e) LMA-6 SDG 2.4 KPS e) LMA-7 SDG 3.0 KPS e) LMA-8 SDG 4.8 KPS e) LMA-4 SDG 7.2 KPS e) LMA-9 SDG 10.2 KPS e) LMA-31 SDS 7.2 AIBN LMA-28 SDS 1.2 KPS e) LMA-29 SDS 4.8 KPS e) LMA-30 SDS 10 KPS e) a) 115 mg PEGA200. b) 100 mg AIBN. c) M w ¼ g mol 1. d) Poly(propylene glycol) M w ¼ g mol 1. e) 100 mg KPS. f) No hexadecane was used. PEGA200. Here, employment of SDG enables the formulation of smaller latexes as with SDS. At 7.2 wt.-% of SDG, a particle size of 109 nm (LMA-24) is reached instead of about 150 nm in the case of corresponding amounts of SDS (LMA-31). The use of KPS as initiator leads to unexpectedly small particle sizes. These go from 155 nm (0.6 wt.-% of the surfactant SDG) down to particle sizes as small as about 40 nm at 10.2 wt.-% of surfactant. This size is even considerably smaller than for the comparable LMA/SDS/KPS system (56 nm) which shows, however, a similar anomaly in the diameter surfactant load curve and even beats the more hydrophilic Sty/SDS/KPS system (see Figure 3). One possible explanation for the small particle size using KPS is that potential hydrolysis products (i.e., sodium methacrylate and dodecyl alcohol) could occur, leading to a decrease of particle size by a cosurfactant effect. However, control experiments with the hydrolysis products and AIBN as initiator did not result in much smaller particles. A second explanation is that new particles are formed by secondary nucleation. Besides the experience-based observation that this is practically excluded for the very hydrophobic monomer, it is also believed that such a procedure would result in two types of particles, as the ultrahydrophobic hexadecane would only stay in the original droplets leading to particles with decreased density compared to the pure polymer, whereas the new droplets would only consist of poly(lauryl methacrylate). In ultracentrifuge rt-dt experiments performed in H 2 O and D 2 O, one

5 1970 U. Yildiz, K. Landfester, M. Antonietti Figure 3. Variation of the particle size obtained by changing the amount of N-stearoyl-di(sodium)glutamate (SDG) used for the preparation of LMA miniemulsions using different initiators. For comparison, the corresponding particle size curves using SDS are also shown. can see that all particles show the same density of g cm 3 indicating that the hexadecane is homogeneously distributed in the particles. The only explanation left is that the new particles are obtained by budding, that is, that the primary latexes split throughout polymerization, presumably due to surface bending effects. Budding is well known for vesicles, [15 17] but up to now has not been observed for liquid droplets or latexes. The surface tensions of those dispersions are indeed extraordinarily high, as indicated in Table 2 (i.e., 64 mn m 1 for a d ¼ 40 nm dispersion), which speaks for additional stabilizing energy contributions which do not occur in SDS/styrene, but seem to be typical for the SDG/LMA combination. From the simple measurement of the droplet size before and after polymerization (from ca. 100 nm down to 42 nm), we can also deduce that each primary droplet is producing ca.13 latex particles, that is, each primary droplet undergoes 3 4 budding events on the average. This explanation although the only one left should, however, be taken as provisional, and on-line monitoring of the reaction with respect to surface tension, particle size, and conductivity is currently performed to gain additional information about this very exciting process. Conclusion The aim of this study was the exploration of a new, biodegradable surfactant, N-stearoyl-di(sodium)glutamate (SDG), with high efficiency for miniemulsion polymerization to replace sodium dodecyl sulfate. It was shown that this surfactant produces, in a range of concentrations, latex particles at least as small as those produced with SDS, as exemplified for the model case polystyrene. Whereas the miniemulsion polymerization of pure lauryl methacrylate latexes deviates from the ideal mechanism and leads to significant particle growth, it was shown that a combination of N-stearoyl-di(sodium)glutamate with an appropriate costabilizer allows the making of very small latexes even from this monomer. With KPS as an initiator, the SDG/LMA system deviates from ideal miniemulsion towards smaller particle sizes, as the dispersion reproducibly clears up throughout reaction, and extremely small latexes with very high surface tension and incomplete surfactant coverage are formed. High precision density measurements with the ultracentrifuge also exclude the occurrence of secondary nucleation for this case, and the whole effect was speculatively attributed to a budding phenomenon, which, however, is unusual for droplets. Following vesicle theory, such a transition is driven by domains which are preferentially curved towards the oil phase, which indeed might be realized by flat adsorption of the PLMA backbone on the oil water interface. This peculiarity, however, has to be examined in more detail with additional techniques for an unequivocal description of the mechanism of formation. [1] K. Landfester, Adv. Mater. 2001, 10, 765. [2] M. Antonietti, K. Landfester, Prog. Polym. Sci. 2002, 27, 689. [3] F. Candau, in: Polymerization in Organized Media, E. C. Paleos, Ed., Gordon and Breach Science Publisher, Philadelphia 1992, p [4] M. Antonietti, W. Bremser, D. Müschenborn, C. Rosenauer, B. Schupp, W. Schmidt, Macromolecules 1991, 24, [5] P. J. Blythe, E. D. Sudol, M. S. El-Aasser, Macromol. Symp. 2000, 150, 179. [6] F. J. Schork, G. W. Poehlein, S. Wang, J. Reimers, J. Rodrigues, C. Samer, Colloids Surf. A 1999, 153, 39. [7] K. Landfester, N. Bechthold, F. Tiarks, M. Antonietti, Macromolecules 1999, 32, [8] Y. J. Chou, M. S. El-Aasser, J. W. Vanderhoff, J. Dispersion Sci. Technol. 1980, 1, 129. [9] A. R. M. Azad, J. Ugelstad, R. M. Fitch, F. K. Hansen, in: Emulsion Polymerization, I. Piirma, J. L. Gardon, Ed., ACS, Washington, D.C. 1976, Vol. 24, p. 1. [10] K. Landfester, N. Bechthold, F. Tiarks, M. Antonietti, Macromolecules 1999, 32, [11] K. Landfester, Preparation of Polymer and Hybrid Colloids by Miniemulsion for Biomedical Applications, in: Colloid Polymers: Preparation and Biomedical Applications, A. Elaïssari, Ed., Dekker, New York, Basel 2003, p [12] K. Tauer, Polym. Adv. Technol. 1995, 6, 435. [13] H. G. Müller, F. Hermann, Progr. Colloid Polym. Sci. 1995, 99, 114. [14] M. Antonietti, H. P. Hentze, Adv. Mater. 1996, 8, 840. [15] L. Miao, U. Seifert, M. Wortis, H. G. Dobereiner, Phys. Rev. E 1994, 49, [16] H. G. Dobereiner, J. Kas, D. Noppl, I. Sprenger, E. Sackmann, Biophys. J. 1993, 65, [17] F. Julicher, R. Lipowsky, Phys. Rev. Lett. 1993, 70, 2964.