© Copyright 2006 American Chemical Society
OCTOBER 24, 2006 VOLUME 22, NUMBER 22
Letters On the Stability of Miniemulsions in the Presence of RAFT Agents Genggeng Qi and F. Joseph Schork* School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst DriVe, Atlanta, Georgia 30332 ReceiVed March 21, 2006. In Final Form: June 22, 2006 The colloidal stability of miniemulsions in the presence of RAFT or other control agents for controlled free radical polymerization is examined. A derivation, based on Lifshitz-Slyozov-Wagner (LSW) theory, is proposed here to evaluate the effect of a RAFT agent on the diffusional stability of the miniemulsions before the onset of polymerization. Results indicate that, depending on the hydrophobicity of the control agent, its presence may augment or detract from the effectiveness of the costabilizer in preventing diffusional instability due to Ostwald ripening.
Introduction An aqueous dispersion of relatively stable oil droplets within a size range of 50-500 nm prepared by strong shearing of a system containing oil, water, surfactants, and costabilizers is, for historical reasons, called a miniemulsion.1 Miniemulsion polymerization has developed rapidly in the last two decades; this technique is a very attractive route to new vinyl polymers because it is robust to contamination and operating errors and is generally considered to be a ‘‘green” process using an aqueous emulsion in place of an organic solvent. Moreover, it has the ability to produce polymers of uniform composition and final latexes with excellent shear stability exceeding those of conventional emulsion polymerization. Controlled free radical polymerization such as reversible addition fragmentation transfer (RAFT) polymerization has been demonstrated to be a potentially useful and important new approach to the production of polymers with well-defined or special architectures in a controlled manner.2,3 By applying RAFT polymerization to miniemulsions, RAFT miniemulsion * Corresponding author. E-mail:
[email protected]. Tel: 404 894 3274. Fax: 404 894 2866. (1) Chou, Y. J.; El-Aasser, M. S.; Vanderhoff, J. W. J. Dispersion Sci. Technol. 1980, 1, 129. (2) Chiefari, J.; Chong, Y.; Ercole, F.; Krstina, J.; Jeffery, J.; Le, T.; Mayadunne, R.; Meijs, G.; Moad, C.; Moad, G.; Rizzardo, E.; Thang, S. Macromolecules 1998, 31, 5559. (3) Chong, Y. K.; Le, T. P. T.; Moad, G.; Rizzardo, E.; Thang, S. H. Macromolecules 1999, 32, 2071.
polymerization combines the advantages of both of these techniques and offers a convenient way to synthesize unique or well-defined-structure polymer colloids such as block coreshell nanoparticles.4,5 In addition, RAFT miniemulsion polymerization in continuous reactors provides promising potential industry applications.6-9 The stability of miniemulsions, both before and during polymerization, is a key issue that needs to be addressed because it affects the polymerization kinetics and properties of latex products directly. A number of papers have discussed the stability of emulsions and miniemulsions;10-15 most of these studies focused on the effects of costabilizer in preventing Ostwald (4) de Brouwer, H.; Tsavalas, J. G.; Schork, F. J.; Monteiro, M. J. Macromolecules 2000, 33, 9239. (5) Smulders, W.; Monteiro, M. J. Macromolecules 2004, 37, 4474. (6) Russum, J. P.; Barbre, N. D.; Jones, C. W.; Schork, F. J. J. Polym. Sci., Part A: Polym. Chem. 2005, 43, 2188. (7) Russum, J.; Jones, C.; Schork, F. Ind. Eng. Chem. Res. 2005, 44, 2484. (8) Russum, J. P.; Jones, C. W.; Schork, F. J. Macromol. Rapid Commun. 2004, 25, 1064. (9) Smulders, W. W.; Jones, C. W.; Schork, F. J. AIChE J. 2005, 51, 1009. (10) Lifshitz, I. M.; Slyozov, V. V. Phys. Chem. Solids 1961, 19, 35. (11) Kabal’nov, A. S.; Pertzov, A. V.; Shchukin, E. D. Colloids Surf. 1987, 24, 19. (12) Taylor, P.; Ottewill, R. H. Colloids Surf., A 1994, 88, 303. (13) Kabalnov, A. S.; Shchukin, E. D. AdV. Colloid Interface Sci. 1992, 38, 69. (14) Buscall, R.; Davis, S. S.; Potts, D. C. Colloid Polym. Sci. 1979, 257, 636. (15) Ugelstad, J.; Mork, P. C.; Kaggerud, K. H.; Ellingsen, T.; Berge, A. AdV. Colloid Interface Sci. 1980, 13, 101.
10.1021/la060762t CCC: $33.50 © 2006 American Chemical Society Published on Web 09/21/2006
9076 Langmuir, Vol. 22, No. 22, 2006
ripening. Ostwald ripening describes the diffusional instability that occurs when monomer diffuses from the small droplets, across the aqueous phase, and into the larger droplets, thus reducing the overall interfacial chemical potential of the colloid. This can be countered by the addition of a highly water-insoluble costabilizer that diffuses sparingly through the aqueous phase and so remains in the smaller droplets. The concentration of costabilizer as the monomer diffuses out of the small droplets causes an increase in the overall chemical potential of the colloid and limits Ostwald ripening. There have been few papers dealing with the stability of miniemulsions in the presence of RAFT or other control agents. It has been widely reported that there is a slight loss of stability for miniemulsions containing RAFT agents as compared with the stability of conventional miniemulsions.4,9 A common phenomenon is a tiny oil phase separated at the vortex early in the polymerization. A superswelling theory was recently proposed to explain the observed loss of stability during controlled miniemulsion polymerization.16 This theory is based on the idea that in controlled radical polymerization a great number of oligomeric species are present early in the polymerization. These species can act as very effective swelling agents and can, under certain conditions, partially destabilize the miniemulsion. However, colloidal instability has been observed in miniemulsions before the onset of polymerization (and the formation of oligomers). This is especially observed in the feed tanks of continuous polymerization systems.6-8,17 Until now, there has been no theory dealing with this situation. Therefore, another derivation based on Lifshitz-Slyozov-Wagner (LSW) theory is proposed here to evaluate the effect of a RAFT agent on the stability of the miniemulsions before the onset of polymerization.10,13 It should be pointed out that the derivation here describes a different situation from superswelling theory.16 The situation discussed here is from a purely physical perspective. The RAFT agent was assumed to have no reaction with the monomers. The process occurs before the onset of oligomerization or polymerization. The previous superswelling theory describes a situation after the oligomerization or polymerization when there is an uneven oligomerization in the RAFT miniemulsion. The RAFT oligomers are assumed to have interactions with the monomers in superswelling theory.
Stability of Miniemulsions in the Presence of RAFT (or Other) Control Agents A miniemulsion is a metastable emulsion of fine oil droplets dispersed in water. The droplets must be stabilized against coalescence by collisions and against Ostwald ripening by diffusion processes. Stabilization against coalescence can be achieved by adding an appropriate surfactant such as sodium dodecyl sulfate (SDS); diffusional stabilization is achieved by adding a small quantity of a highly monomer-soluble and extremely hydrophobic costabilizer such as hexadecane. Because only a small amount of the RAFT agent, typically hydrophobic dithioester compounds, is utilized in controlled miniemulsion polymerization, which is partitioned in the monomer droplets and is structurally different from surfactants, the addition of a RAFT agent has very limited effects, if any, on changing the collision environment around the droplets. Therefore, we will focus on the Ostwald ripening process after the introduction of RAFT agents. In the following text, subscripts 1, 2, and 3 refer to the monomer, RAFT agent, and costabilizer, respectively. The following derivation is based on LSW theory. The main results of LSW theory are the following: (16) Luo, Y. W.; Tsavalas, J.; Schork, F. J. Macromolecules 2001, 34, 5501. (17) Smulders, W.; Jones, C.; Schork, F. Macromolecules 2004, 37, 9345.
Letters
(1) During the ripening process, there exists a critical radius ac in polydisperse droplets. Particles with a larger radius grow while smaller ones shrink. For those with a radius of ac, the radius of the particle does not change. ac increases during the ripening process. (2) The Ostwald ripening rate w is constant
w)
daj3 8Vmiσ ) DC′ dt 9RT i i
(3) The particle size distribution P∞(u) is independent of time, and the droplet sizes are scaled to ac.
P∞(u) ) P∞
()
81eu2 exp
[
1 (2u/3) - 1
]
a ) 3 ac x32(u + 3)7/3(1.5 - u)11/3
0 eµ e1.5
and
P∞(u) ) 0
µ > 1.5
The derivation proposed here is based on the following assumptions: (1) The surfactants will not interfere with the mass transfer of different species. The present of the RAFT agent does not change the surface tension of the miniemulsions. (2) The molar fractions of the RAFT agent and costablizer in a RAFT miniemulsion system are small. There is no reaction between the RAFT agent and the monomer. (3) The concentration of the dispersed phase is the same throughout the medium except in the neighborhood of the droplet surfaces. (4) The dispersed-phase transport is by molecular diffusion from one droplet to the others, following Fick’s first law. (5) The solubility of the RAFT agent in water is comparable to or less than that of the monomer. The chemical potential of component i in one droplet of a miniemulsion is
) µi′ + µdrop i
2σVmi + RT ln xi a
(1)
The chemical potential of component i in the bulk is
µi′ ) µ0i + RT ln Ci′
(2)
When the droplets and the medium reach equilibrium, the chemical potential of composition i in the droplets should equal that close to the surface of the droplets.
) µsurf µdrop i i
(3)
The chemical potential of component i around the droplet surface can be written as
) µ0i + RT ln Csurf µsurf i i From the above equations,
()
Csurf ) Ci′ exp i
Ri x a i
in which
Ri )
2σVmi RT
(4)
Letters
Langmuir, Vol. 22, No. 22, 2006 9077
From Fick’s first law, the mass-transfer rate is
∂Ci Ji ) -4πa2Di |r)a ) 4πaDi(Ci - Csurf i ) ∂r
w1 ) (5)
At every instant, there exists a critical radius ac in the polydisperse distribution of droplet sizes at which the flux of the droplets is close to zero. For an arbitrary droplet, the mass flux of costablizer is
[ ()
J3 ) 4πaD3C3′ x3,c exp
( )]
R3 R3 - x3 exp ac a
(6)
Because miniemulsions can be stable for a relatively long time, there should be a quasi-stationary state in which J1 ≈ 0 (J2 and J3 are therefore also close to zero because the monomer flux J1 accounts for the main part of the total mass transfer); that is,
C1′ exp
()
()
R1 R1 (1 - x2c - x3c) ≈ C1′ exp (1 - x2 - x3) (7) ac a
From eq 7, considering that the mole fractions of the RAFT agent and costablizer in a RAFT miniemulsion system are typically quite small, by expanding the exponents in a Taylor series truncated at the second term we have
x2c - x2 + x3c - x3 )
R1 R1 ac a
(8)
Two observations have been made in previous reports of RAFT miniemulsions:4,6,9,18 (1) The stability of RAFT miniemulsions was found to be reduced to some extent compared with those without RAFT; however, the stability was still quite good in these situations. There is no obvious phase separation or cream except a thin layer of oil at the vortex over long storage times. (2) The molecular weight distribution (PDI) of the polymer product is very narrow. The particle size distribution (PSD) of the final polymer latex is also quite narrow, though slightly broader than in the free-radical miniemulsion protocol. The living nongrowing chains in RAFT miniemulsion polymerization account for only a very small part of the total amount of polymer produced.9 The above phenomena indicate that the relative concentration change of the components in RAFT miniemulsion droplets is relatively small. Therefore, the ratio of the component fluxes is assumed to be approximately equal to the volume fractions φi of the components in the droplets though it is a multivariable function of volume fractions, the diffusion coefficient, and the solubility of the components:
J1/J2/J3 ) φ1/φ2/φ3
(9)
This leads to
J 1 + J2 + J3 ) -
dVdroplet ) dt
(
4πR1a
φ2 φ3 + D2C2′ D3C3′
)( ) -1
(
1 1 (10) ac a
Note that eq 10 has a form similar to that of the one-component particle evolution equation described decades before by Lifshitz et al.10 From LSW theory, the rate of droplet growth should be (18) Butte, A.; Storti, G.; Morbidelli, M. Macromolecules 2001, 34, 5885.
)
φ3 daj3 8Vm1σ φ2 ) + dt 9RT D2C2′ D3C3′
-1
(11)
The one-component droplet system without a costablizer and RAFT agent should be
w)
daj3 8Vm1σ ) DC′ dt 9RT 1 1
(12)
A retardation factor F, defined as the droplet growth rate ratio of a miniemulsion and an emulsion, could be used to evaluate the relative stability of a miniemulsion system. Therefore, the retardation factor F for miniemulsions with both a RAFT agent and costablizer is
F2,3 )
(
φ2 φ3 w ) D1C1′ + w1 D2C2′ D3C3′
)
(13)
and the retardation factor F for a miniemulsion system having only a costablizer is
F3 )
φ3′ w ) D1C1′ w1′ D3C3′
(14)
The effect of the RAFT agent can be evaluated by a factor R
φ2 φ3 + F2,3 D2C2′ D3C3′ ) R) F3 φ 3′ D3C3′
(15)
Unfortunately, there is no experimental report of the diffusion coefficient of RAFT agents. Because the diffusion coefficient is around 0.5 × 10-9 to 2 ×10-9 m2/s for a wide range of systems, the above equation can be simplified to
C3′ φ 2 + φ3 C2′ R) φ3′
(16)
by assuming that D2 ≈ D3. Figure 1 shows the effect of the RAFT agent on the stability of miniemulsions. With the higher hydrophilicity of the RAFT agent, the miniemulsions tend to be less stable. It is noteworthy that there is a low boundary when C3′/C2′ ) 0, that is, when the costabilizer has zero solubility. However, when the hydrophobicity of the RAFT agent is comparable to that of the costablizer, the miniemulsions are even more stable than conventional miniemulsions. To understand the above result, the RAFT agent can be considered to be a special costabilizer, but not of the same stabilizing effectiveness as costablizers such as hexadecane. On one hand, the RAFT agent improves the stability of miniemulsions as when its solubility is comparable to that of hexadecane. On the other hand, when the RAFT agent is more hydrophilic than the costabilizer, it dilutes the costablizer, making the miniemulsion less stable and contributing to oil-phase separation. Considering that the solubility of RAFT is typically much higher than that of hexadecane, the stability of miniemulsions with RAFT should be close to the lower boundary C3′/C2′ ) 0. There are some supplementary explanations that need to be made for the derivation: (1) It is worth pointing out that even after introducing RAFT agents the critical radius ac ) aj, which is similar to that of
9078 Langmuir, Vol. 22, No. 22, 2006
Letters
(3) Although the assumption that the ratio of the component fluxes is approximately equal to the volume fractions of the components in the droplets works well for most of the RAFT miniemulsion systems previously reported, there maybe few cases in which the assumption does not apply under certain conditions (e.g., when the RAFT agent is extremely hydrophilic or there is significantly different diffusion behavior between the RAFT agent and the other components in the droplets). In these cases, the RAFT agent will redistribute in the droplets because of Oswald ripening and cause an uneven distribution of RAFT concentration in the droplets. The high concentration of RAFT in certain droplets could favor the retardation of the following polymerization process in these droplets, which leads to a much broader PDI in the final latex.21-24 Acknowledgment. We thank the National Science Foundation for funding this work (CTS-0553516). Nomenclature Figure 1. Relative stability comparison of the miniemulsion containing a RAFT agent with the miniemulsion without a RAFT agent, assuming that the volume of the monomer equals 100 and the volume of the costablier equals 1.
conventional emulsions.19 This relationship can be derived easily from the mass conservation. For N droplets, N
∑i (J1 + J2 + J3)droplet i ) N
∑i
[ ( 4πR1
φ2
D2C2′
+
φ3
) ( )]
D3C3′
-1
1
ac
-
1
a
) 0 (17)
droplet i
thus
ac )
1
N
∑i a ) aj
N
(18)
(2) The derivation is not limited to miniemulsions with RAFT agents. Actually, it can be applied to evaluate the stability of miniemulsions with any small amount of controlled agents similar to RAFT agents, such as many controlled agents utilized in ATRP.20 (19) Finsy, R. Langmuir 2004, 20, 2975. (20) Qiu, J.; Charleux, B.; Matyjaszewski, K. Prog. Polym. Sci. (Oxford) 2001, 26, 2083.
µdrop i µi′ µsurf i µ0i Csurf i Ci′ xi φi σ Vmi a aj Ji Ci Di ac xic
chemical potential of component i in the droplets chemical potential of component i in the bulk chemical potential of component i at the droplet surface chemical potential of component i in the standard state concentration of component i at the droplet surface concentration of component i in bulk molar fraction of component i in the droplets volume fraction of component i in the droplets surface tension of miniemulsions partial molar volume of component i radius of droplets number average radius of droplets flux of composition i between the droplets and the medium concentration of component i in the medium diffusion constant of component i in the medium critical radius molar fraction of component i in the droplets at ac
LA060762T (21) Barner-Kowollik, C.; Quinn, J. F.; Morsley, D. R.; Davis, T. P. J. Polym. Sci., Part A: Polym. Chem. 2001, 39, 1353. (22) McLeary, J. B.; Calitz, F. M.; McKenzie, J. M.; Tonge, M. P.; Sanderson, R. D.; Klumperman, B. Macromolecules 2004, 37, 2383. (23) Calitz, F. M.; McLeary, J. B.; McKenzie, J. M.; Tonge, M. P.; Klumperman, B.; Sanderson, R. D. Macromolecules 2003, 36, 9687. (24) Vana, P.; Davis, T. P.; Barner-Kowollik, C. Macromol. Theory Simul. 2002, 11, 823.
