Langmuir 2002, 18, 941-944
941
Notes Synthesis of Monodisperse Poly(dimethylsiloxane) Micro- and Macroemulsions Matthieu Barre`re, Sergio Capitao da Silva, Robert Balic, and Franc¸ ois Ganachaud* Laboratoire de Chimie des Polyme` res, UMR 7610, CNRSs Universite´ Pierre et Marie Curie, T44 E1, 4 Place Jussieu, 75252 Paris Cedex 05, France Received September 27, 2001. In Final Form: November 12, 2001
batch process. One patent failed in preparing fine microemulsions by dropwise monomer addition in different surfactant and feed conditions,8 but a systematic approach to identify appropriate conditions was not reported. Various parameters, e.g., monomer and surfactant concentrations, surfactant type, and feeding rate were screened so as to determine the best conditions for preparing small particles with high polymer content. The final part deals with the maturation of the resulting translucent latex in the presence of catalyst to give macroemulsions with controlled final particle diameter and polydispersity.
Introduction
Experimental Section
For personal care formulations or textile applications, there is a need for aqueous silicone dispersions exhibiting very small particles (in the range 5-60 nm) defined as poly(dimethylsiloxane) (PDMS) “microemulsions” in the patent literature.1 These are actually “nanolatexes” made of kinetically rather than thermodynamically stable particles. They are very much of interest for their transparency and very large surface area.2 A classical titration technique has been used to prepare D4 thermodynamically stable microemulsions using high loads of surfactant and cosurfactant (typically short-chain alcohols) for comparatively low monomer concentration.1a Polymerizing these microemulsions, however, quickly resulted in the loss of thermodynamic stability and production of particles larger than the original droplets. Such a behavior is frequently observed in free-radical emulsion polymerization.3 Another simple process to produce PDMS microemulsions consisted of polymerizing D4 in a batch emulsion process using relatively high concentrated mixtures of ionic and nonionic surfactants.4 Small particles in the range 25-75 nm with low volatile amounts were produced. The purpose of this work was initially to prepare comparable low particle size latex to that from the batch procedure, with high polymer content and low amount of surfactant(s). Such goals were achieved in free-radical polymerization by adding a subsequent monomer load to a previously polymerized microemulsion, monomer either sitting in a top layer (Winsor I5 systems)6 or added dropwised (starved feed process).7 This latter method avoided the nucleation of new particles especially for partly water-soluble monomers.7b-d The ionic polymerization of D4 in similar starved-feed emulsion process is reported here and compared with the
The various surfactants were used as received from Aldrich (99% purity) unless otherwise stated. The cationic surfactant, cetyltrimethylammonium bromide (CTAB) (Acros, 99%), was used as received. NaOH (1 N) (LABOSI, 99%) in molar equivalent to the surfactant was added as a catalyst for anionic polymerization. Dodecylbenzenesulfonic acid (DBSA) plays the role of both catalyst and surfactant in cationic polymerization. The nonionic surfactants of general formula CxH2x+1[OCH2CH2]yOH, usually noted CxEy, are sold under the BRIJ tradename: BRIJ35 (C12E23); BRIJ78 (C16E28); BRIJ56 (C16E10); BRIJ30 (C12E4). Octamethylcyclotetrasiloxane was purchased from ABCR (99%), and its purity was checked by gas chromatography and 1H NMR. In batch experiments, ionic and nonionic surfactants in various proportions were first dissolved in deionized water. The monomer was then added, and the mixture was poured in a reactor set at 60 or 90 °C equipped with a paddle agitator set at 400 rpm. After half an hour mixing, the catalyst was added; the polymerization was, typically, completed after 4 h. The final dispersion was then neutralized with HCl or NaOH (0.1 N) depending on the catalyst system used to reach a pH of about 8. In starved feed experiments, a primary latex was prepared similarly but by adding only 10 wt % of the monomer. The dispersion was heated at 90 °C before adding the catalyst. After 10 min, the remaining monomer amount was poured dropwised into the primary mixture at various feed rates. The total polymerization time was set at 4 h at the end of which similar neutralization was carried out, except for the ripening experiments, where the emulsion was allowed to evolve for a week. In that latter case, samples were withdrawn to follow the particle diameter and polydispersity variations. Particle size measurements were carried out by photon correlation spectroscopy using a Zetasizer 4 from Malvern Instruments. This apparatus gives z-average diameters (dz). The coefficient of variation (CV) was transformed into a polydispersity index (PDI) according to PDI ) (1 + CV2)1/2.
* To whom correspondence should be sent. E-mail address:
[email protected]. Telephone: (33) 1 44 27 55 01. Fax: (33) 1 44 27 70 89. (1) (a) Halloran, D. J. U.S. Patent 6,071,975, 2000. (b) Hill, R. M.; Kaler, E. W.; Ryan, L. D.; Silas, J. A. U.S. Patent 6,013,683, 2000. (2) Candau, F. In Polymeric Dispersions: Principles and Applications; Asua, J. M., Ed.; Kluwer Academic Publisher: London, 1997; p 127. (3) Antonietti, M.; Basten, R.; Lohmann, S. Macromol. Chem. Phys. 1995, 196, 441. (4) Gee, R. P. European Patent Application EP 459,500, 1991. (5) Winsor, P. A. J. Trans. Faraday Soc. 1948, 44, 376. (6) (a) Gan, L. M.; Lian, N.; Chew, C. H.; Li, G. Z. Langmuir 1994, 10, 2197. (b) Loh, S. E.; Gan, L. M.; Chew, C. H.; Ng, S. C. J. Macromol. Sci. Pure Appl. Chem. 1996, A33, 371.
Results and Discussion Mechanisms of D4 Polymerization in Batch and Starved Feed Processes. The direct generation of silicone latexes by ionic polymerization in emulsion (IPE) (7) (a) Rabelero, M.; Zacarias, M.; Mendizabal, E.; Puig, J. E.; Dominguez, J. M.; Katime, I. Polym. Bull. 1997, 38, 695. (b) Roy, S.; Devi, S. Polymer 1997, 38, 3325. (c) Xu, X.; Ge, X.; Yin, Y.; Zhang, Z.; Zou, J.; Niu, A. J. Polym. Sci., Part A 1998, 36, 2631. (d) Ming, W.; Jones, F. N.; Fu, S. Macromol. Chem. Phys. 1998, 199, 1075. (e) Ming, W.; Jones, F. N.; Fu, S. Polym. Bull. 1998, 40, 749. (f) Ming, W.; Zhao, Y.; Cui, J.; Fu, S.; Jones, F. N. Macromolecules 1999, 32, 528. (g) Xu, X. J.; Chew, C. H.; Siow, K. S.; Wong, M. K.; Gan, L. M. Langmuir 1999, 15, 8067. (h) Sosa, N.; Peralta, R. D.; Lopez, R. G.; Ramos, L. F.; Katime, I.; Cesteros, C.; Mendizabal, E.; Puig, J. E. Polymer 2001, 42, 6923. (8) Graiver, D.; Tanaka, O. U.S. Patent 4,999,398, 1991.
10.1021/la0155956 CCC: $22.00 © 2002 American Chemical Society Published on Web 12/20/2001
942
Langmuir, Vol. 18, No. 3, 2002
Notes
Table 1. Recipe and Final Features for Batch (Labeled B*) and Starved Feed (Labeled SF*) Microemulsionsa run B1b,d B2b B3 SF1 SF2a SF2b SF2c SF2d SF3a SF3b SF4a SF4b SF4c SF5a SF5b SF5c SF5d B4c SF7c
monomer amt initial (g) feed (g) 10 10 10 1 1 1 1 1 0.4 1 1 1 1 1 1 1 1 20.1 2.1
9 9 9 9 9 3.8 13.6 9 9 9 9 9 9 9 18
ionic (g) 2.7 3.3 3.3 3.6 3.6 3.6 3.6 3.6 3.6 3.6 3.6 4.3 4.3 4.8 4.3 2.4 1.2 4.7 4.7
surfactant amt nonionic (g)
type
1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.3 0.8 0.4
C12E23 C12E23 C12E23 C12E23 C12E23 C12E23 C12E23 C12E23 C12E23 C12E23 C16E28 C16E10 C12E4
1.8 1.8
C12E23 C12E23
water amt (g) 10.2 20.3 20.3 22.6 22.6 22.6 22.6 22.6 30 19 23.3 24 24 24 24 24 24 40 40
feed rate (g/h)
7 20 20 7 13 7 7 7 7 7 7 7 7 7 7
z-average diameter (nm) 41 56 37 25 50 52 27 12 39 33 72 56 30 36 60 74 79 66 43
PDI 1.12 1.16 1.008 1.007 1.003
1.03 1.006
a Other parameters (unless otherwise stated): ionic surfactant, CTAB; temperature, 90 °C; catalyst, 4 mL of NaOH (1 N); stirring rate, 400 rpm. b At 60 °C. c Using DBSA as the catalyst in cationic polymerization. d Formed a physical gel at room temperature.
of cyclosiloxanes was first patented 40 years ago by Dow Corning,9 but only a handful of scientific investigations were conducted so as to better understand the polymerization mechanism.10 Systematic studies on the anionic polymerization of D4 in miniemulsion (i.e., polymerization in metastable droplets prepared by pre-emulsification of the monomer in the aqueous phase)10,11 showed that it proceeds somehow differently than its radical polymerization counterpart. Indeed, polymerization occurs at the particle interface through the catalyst (typically OH-) borne by the surfactant (typically quaternary ammonium). Every single droplet is thus nucleated in this system. A recent report11c showed that this interfacial process favored the chain polymerization over condensation or redistribution reactions, resulting in better control of the polymer synthesis compared to D4 bulk polymerization (a low small cycle amount, no macrocycle formation, perfectly difunctionalized polymer chains). The batch (B) and starved feed (SF) experiments carried out with similar recipes are reported in Table 1 (examples B3 and SF1 and B4 and SF7 for the anionic and cationic polymerization processes, respectively). The batch experiments consisted of mixing all the ingredients together in the reactor set at 60 or 90 °C and then adding the catalyst. The starved feed process consisted in first polymerizing 10 wt % of the monomer before slowly adding the remaining amount (see Experimental Section). It is difficult to identify whether a thermodynamically stable microemulsion of D4 forms prior to polymerization. Indeed, quaternary ammonium surfactants bearing hydroxide counterions are formed in situ and, in the meantime, catalyze the polymerization. A recent paper showed that mixtures of cationic and nonionic surfactants were good candidates for preparing one-phase D4 microemulsions, even at high monomer content.12 Sampling and droplet/particle diameter measurements are currently under study to investigate this issue. (9) Hyde, J. F.; Wehrly, J. R. U.S. Patent 2,891,920, 1959. (10) De Gunzbourg, A.; Favier, J.-C.; He´mery, P. Polym. Int. 1994, 35, 179 and references therein. (11) (a) de Gunzbourg, A.; Maisonnier, S.; Favier, J.-C.; Maitre, C.; Masure, M.; He´mery, P. Macromol. Symp. 1998, 132, 359. (b) Barre`re, M.; Maitre, C.; Ganachaud, F.; He´mery, P. Macromol. Symp. 2000, 151, 359. (c) Barre`re, M.; Ganachaud, F.; Bendejacq, D.; Dourges, M.-A.; Maitre, C.; He´mery, P. Polymer 2001, 42, 723. (12) Silas, J. A.; Kaler, E. W.; Hill, R. M. Langmuir 2001, 17, 4534.
We observed that the initial emulsion prepared at high monomer content (batch process) quickly led to phase separation at high temperature. The upper layer was essentially composed of monomer, whereas analysis of the lower aqueous layer showed the presence of small particles. This is typical of a Winsor I type microemulsion.5 On the contrary, the low monomer content dispersion (starved feed process) stayed clear with time. Further polymerization by continuous monomer addition led to particles with smaller diameters and much lower polydispersities than for the batch process, independently of the surfactant-catalyst systems used (see Table 1). Proposed mechanisms for batch and starved feed processes are given in Figure 1. In the batch process, the phase separation induces a sharp decrease of the particle number and the release of excess surfactant and monomer. Large droplets of monomer thus form and nucleate to create new particles. In the meantime, monomer diffuses toward the small polymer-swollen particles. Due to the formation of new particles during the course of polymerization, the final microemulsion still exhibits a low average particle diameter, although with a large polydispersity. In the starved feed process, the seed dispersion is first polymerized without demixing, which prevents the release of surfactant in the aqueous phase. In addition, the very slow monomer addition ensures its polymerization in the existing droplets so that the number of particles stays constant. This latter point is relevant when explaining the low polydispersity of diameters. Hence, the starved feed process, by reducing the number of monomer droplets, gives smaller particles than those obtained in the batch process. Optimization of the Starved Feed Process. The different recipes and final latex characteristics are summarized in Table 1. Several parameters were first optimized in the batch process. Preliminary results showed that a strong shearing of the dispersion (400 rpm) ensures a quick diffusion of the monomer from droplets to particles. In addition, raising the temperature (typically at 90 °C) ensures high polymerization rates to yield smaller particles (see examples B2 and B3 in Table 1). The type and content of surfactants dramatically influence the final particle diameter. A cationic surfactant alone (Table 1, examples SF5a-d) gave small particles,
Notes
Langmuir, Vol. 18, No. 3, 2002 943
Figure 1. Proposed mechanism for batch (A) and starved feed (B) D4 emulsion polymerization. (A) The demixing of the emulsion releases excess monomer and surfactants to form new large D4 droplets. These slowly nucleate, whereas monomer diffuses toward the smaller particles. A small and broad microemulsion is finally obtained. (B) The initial dispersion containing only part of the monomer polymerizes without droplet degradation. The particles grow by slowly feeding the monomer that swells existing particles rather than forming new droplets. This process ultimately gives a monodisperse microemulsion with lower particle diameter than in batch.
which is an improvement compared to the batch procedure where only a nonionic + ionic surfactant mixture was shown to give particles of comparable diameters.4 In addition, relatively high surfactant concentration was required so as to drastically decrease the particle diameter. Mixtures of ionic/nonionic surfactants were also used to tune the final particle diameter (examples 1, SF4a-4c). The length of the ethoxylated chain seems to be of minor importance for controlling the particle diameter (examples SF1 and SF4c). On the other hand, nonionic surfactants with small hydrophobic tails gave the smallest particle diameters. At comparable surfactant content, the cationic surfactant alone led to slightly larger particles (examples SF1 and SF5a). Microemulsions with high polymer content prepared by the batch process formed a gel at room temperature (example B1) due to insufficient coverage of the particles with surfactant. This physical gel was already observed for similar systems and constitutes a network made of percolated “unstable” particles.7f Its formation occurs above a given monomer/surfactant ratio; hence, as proposed by Gee,4 excess surfactant addition is enough to reverse gelation. Using comparable monomer/surfactant ratio (example SF3b), the starved feed process led to a stable and low viscosity microemulsion. As a matter of fact, decreasing the water fraction in the starved feed formulation still produced particles with low diameter (
