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Langmuir 2004, 20, 528-531
Preparation of Nanoparticles of Poly(methyl methacrylate) Latexes at θ Condition Xiao-Jun Xu*,† and Fengxi Chen‡ Department of Materials Science and Engineering, University of Pennsylvania, 3231 Walnut Street, Philadelphia, Pennsylvania 19104, and Institute of Chemical and Engineering Sciences, Block 28, #02-08, Ayer Rajah Crescent, Singapore 139959 Received August 1, 2003. In Final Form: October 7, 2003
Introduction Various polymerization processes, such as emulsion polymerization,1,2 emulsifier-free emulsion polymerization,3,4 dispersion polymerization,5,6 and microemulsion polymerization,7-9 have been well developed to prepare polymer particles of latexes ranging from 20 nm to 600 µm in diameter. Some studies described the formation of particles from bulk polymers via physical methods without using surfactants. Kumaki10 produced monomolecular particles of polystyrene (PS) by spreading a dilute PS solution in benzene over a water surface. A single layer or a multilayer film was obtained from a hydrophobic substrate. Ding et al.11 prepared PS nanoparticles by freeze-drying a very dilute PS solution in cyclohexane or by spraying a dilute PS solution in benzene into methanol with vigorous agitation. The PS particle sizes ranged from 20 to 60 nm in diameter. Chaw et al.12 made microparticles of poly(dl-lactide-co-glycolide) (PLGA) and poly(dl-lactide) (PLA) using the spraying-drying technique. The solution of PLGA or PLA in ethyl acetate was spray-dried through the nozzle (0.7 mm in diameter) of a spray dryer. The diameters of the formed particles were less than 5 µm. In other physical methods, surfactants were used. Langer et al.13 prepared microparticles of novel branched copolymers of lactic acid and amino acid using the single emulsion/solvent evaporation method. Polymer solutions in dimethyl sulfoxide/ methylene chloride were emulsified by poly(vinyl alcohol) (PVA), and solvents were removed by stirring the systems in an open beaker. Precipitated microparticles larger than * To whom correspondence should be addressed. E-mail: xux@ seas.upenn.edu. † University of Pennsylvania. ‡ Institute of Chemical and Engineering Sciences of Singapore. (1) Everett, D. H.; Stageman, J. Faraday Discuss. Chem. Soc. 1978, 65, 230. (2) Bucsi, A.; Forcada, J.; Gibanel, S.; Heroguez, V.; Fontanille, M.; Gnanou Y. Macromolecules 1998, 31, 2087. (3) Goodall, A. R.; Wilkinson, M. C.; Hearn, J. J. Polym. Sci., Polym. Chem. Ed. 1977, 15, 2193. (4) Xu, J.; Li, P.; Wu, C. J. Polym. Sci., Part A: Polym. Chem. 1999, 37, 2069. (5) Paine, A. J. Macromolecules 1990, 23, 3109. (6) Liu, J.; Gan, L. M.; Chew, C. H.; Gong, H.; Gan, L. H. J. Polym. Sci., Part A: Polym. Chem. 1997, 35, 3575. (7) Candau, F. In Polymerization in Organized Media; Paleos, C. M., Ed.; Gordon Breach Science Publishers: New York, 1992; p 215. (8) Candau, F.; Anquetie, J. Y. In Micelles, Microemulsions and Monolayers; Shah, D. O., Ed.; Marcel Dekker: New York, 1998; p 193. (9) Gan, L. M.; Chew, C. H. In Encyclopedia of Polymeric Materials; Salamone, J. P., Ed.; CRC Press: Boca Raton, FL, 2001; Vol. 6, p 35. (10) Kumaki, J. J. Polym. Sci., Part B: Polym. Phys. 1990, 28, 105. (11) Ding, J.; Xue, G.; Dai, Q.; Cheng, R. Polymer 1993, 34, 3325. (12) Chaw, C. S.; Tan, C. W.; Yang, Y. Y.; Wang, L.; Moochhala, S. Biomaterials 2003, 24, 1271. (13) Caponetti, G.; Hrkach, J. S.; Kriwet, B.; Poh, M.; Lotan, N.; Colombo, P.; Langer, R. J. Pharm. Sci. 1999, 88, 136.
5 µm were obtained. Yang et al.14 produced poly(ortho ester)-poly(ethylene glycol)-poly(ortho ester) (POEPEG-POE) triblock copolymer microspheres using a double-emulsion (water-in-oil-in-water) process with PVA as emulsifier. The resultant particle sizes ranged from 75 to 120 µm in diameter. Poly(methyl methacrylate) (PMMA) latex particles are usually made by various polymerization methods.15-18 These latex particles have relatively large size ranging from about 30 nm to 3 µm in diameter. Most recently, we have successfully explored a physical method to prepare nanoparticles of polystyrene latexes from commercial PS at θ condition.19 In this paper, the preparation of nanoparticles of PMMA latexes having small size (10-30 nm in diameter) stabilized by an anionic surfactant at θ condition is presented. It is well-known that a dilute polymer solution usually deviates from so-called ideal solution. At the ideal condition, interaction energies between solute molecules and solute molecules, solvent molecules and solvent molecules, and solute molecules and solvent molecules are identical. According to the Flory-Huggins theory on polymer solution,20 the deviation of a real dilute polymer solution from ideal solution can be represented by the “excess chemical potential (∆µE)” for solvent (∆µ1E) as follows:
∆µ1E ) RTψ1(θ/T - 1)φ22
(1)
where R, T, ψ1, and φ2 are the gas constant, temperature, entropy parameter, and volume fraction of the polymer in solution, respectively, and θ is called the Flory temperature or θ temperature. When T ) θ, the thermodynamic properties of the polymer solution do not deviate from those of the ideal solution because ∆µ1E ) 0. Under this θ state, the solvent used is a θ solvent and the free energy of interaction of the segments within a volume element is also zero, where the chain attains its unperturbed dimensions and the polymer molecules interpenetrate one another freely without net interactions. At temperatures below θ, they attract each other and precipitation occurs. Experimental Section Chemicals. Poly(methyl methacrylate) standards (Mw ) 3.0 × 104, 1.0 × 105, 2.25 × 105, and 3.5 × 105 g/mol, with Mw/Mn ) 1.10, 1.10, 1.04, and 1.15, respectively) from Polysciences and benzene, n-hexane, toluene, and n-butyl chloride from Aldrich were used as received. Sodium dodecyl sulfate (SDS) from Fluka was recrystallized from methanol. Deionized water was used in all experiments. Experimental Details. The present physical process for preparation of nanoparticles of PMMA latexes is shown in Scheme 1. A surfactant solution consisting of 0.5 g of SDS and 49.1 g of water was thermostated in an oil bath at 25 ( 0.5 °C under magnetic stirring at 500 rpm. A dilute PMMA solution consisting of 0.4 g of PMMA (Mw ) 2.25 × 105 g/mol) and 100 g of n-hexane/ (14) Yang, Y. Y.; Wan, J. P.; Chung, T. S.; Pallathadka, P. K.; Ng, S.; Heller, J. J. Controlled Release 2001, 75, 115. (15) Cutting, G. R.; Tabner, B. J. Macromolecules 1993, 26, 951. (16) Chern, C. S.; Lee, C. K.; Ho, C. C. J. Polym. Sci., Part A: Polym. Chem. 1999, 37, 1489. (17) Hsiao, Y. L.; Maury, E. E.; DeSimone, J. M.; Mawson, S.; Johnston, K. P. Macromolecules 1995, 28, 8159. (18) Gan, L. M.; Chew, C. H.; Ng, S. C.; Loh, S. E. Langmuir 1993, 9, 2799. (19) Xu, X. J.; Chow, P. Y.; Gan, L. M. J. Nanosci. Nanotechnol. 2002, 2, 61. (20) Flory, P. J. Principles of Polymer Chemistry; Cornell University Press: Ithaca, NY, 1953; Chapter 12.
10.1021/la030317n CCC: $27.50 © 2004 American Chemical Society Published on Web 12/19/2003
Notes
Langmuir, Vol. 20, No. 2, 2004 529 Scheme 1. Formation of Nanoparticles of PMMA Latexes at θ Condition
Table 1. Effect of θ Solvents on the Characteristics of Latexes aqueous solution
organic solution
latex characteristics
system
SDS (g)
H2O (g)
PMMA (g)
Mw (105 g/mol)
θ solvent (g)
θ temp (°C)
Dh (nm)
polya
npb
1 2 3
0.5 0.5 0.5
49.1 49.1 49.1
0.4 0.4 0.4
2.25 2.25 2.25
benzene/n-hexane (70/30 by volume), 100 n-hexane/toluene (18.8/81.2 by volume), 100 n-butyl chloride, 100
20 25 32.6
20.8 20.3 21.2
1.25 1.27 1.23
13.1 12.2 13.9
a
The particle size distribution. b The number of polymer chains per latex particle.
toluene (18.8/81.2 by volume) conditioned at 25 ( 0.5 °C was then dropwise added to the surfactant aqueous solution during a period of 4 h. It was further stirred for another 4 h, and the resulting milky mixture was transferred to a separating funnel. The mixture was separated into two phases, i.e., excess oil phase (solvent) on the top and the target bluish and transparent latex at the bottom. The particle sizes of all the prepared latexes were determined by light scattering technique. Quasi-elastic light scattering (QLS) measurements were performed with a Brookhaven model BI200SM goniometer, Brookhaven model BI-9000AT correlator, and a Lexel 300 mW Ar laser. All experiments were performed at 90° angle and 25 °C. The intensity-intensity correlation function was obtained, and the data were analyzed using the method of cumulants. The hydrodynamic radius Rh of the particle in the high-dilution limit is as follows:
Rh ) lim φf0
kT 5πη0D
(2)
where k is the Boltzman constant, T is temperature, η0 is the viscosity of system, and D is the diffusion coefficient. The particle size distribution (poly) is also obtained by QLS measurements. The number of polymer chains per latex particle (np) for each latex was estimated from the following equation:21
np )
(1/6)πDh3dp Mw/NA
(3)
where Dh refers to the hydrodynamic diameter of latex particles, dp is the density of PMMA particles (1.04 g/cm3 as reported in our previous work22), and NA is the Avogadro constant. The morphologies of all latexes were observed using a JEOL JEM-100CX electron microscope. One drop of each latex was diluted by 1-2 drops of an aqueous solution containing 5 wt % of SDS to which a drop of 2% phosphotungstic acid (PTA) was (21) Ming, W.; Zhao, J.; Lu, X.; Wang, C.; Fu, S. K. Macromolecules 1996, 29, 7678. (22) Xu, X. J.; Siow, K. S.; Wong, M. K.; Gan, L. M. Langmuir 2001, 17, 4519.
then added. After thorough mixing, a drop of this mixture was put on a copper grid coated with a thin layer of Formvar. The magnification of each transmission electron micrograph (TEM) was fixed at 100 000 times.
Results and Discussion Table 1 shows the effect of θ solvents used on the characteristics of the formed PMMA latexes. The respective θ temperatures for three θ solvents used were 20 °C for benzene/n-hexane (70/30 by volume),23,24 25 °C for n-hexane/toluene (18.8/81.2 by volume),23,25 and 32.6 °C for n-butyl chloride.23,26 It was found that similar particle size (Dh) of about 20 nm and similar particle size distribution (poly) of about 1.25 could be obtained by using the above three different θ solvents. For the systems using PMMA of Mw of 2.25 × 105 g/mol, it was estimated that each latex particle contained about 13 PMMA chains irrespective of three different solvents used at their respective θ temperatures. Such observation may be because that at θ temperature the thermodynamic properties of a dilute polymer solution do not deviate from those of the ideal solution because the excess chemical potential (∆µ1E) is zero. Thus, at θ temperature the thermodynamic properties of a polymer dilute solution do not depend on the solvent used. One TEM micrograph for sample 2 (Table 1) is shown in Figure 1a. Indeed, spherical colloidal particles have been obtained from the present method, although their size distributions were relatively polydisperse. The effect of the molecular weight of PMMA on the latex characteristics is presented in Table 2. With the increase of Mw from 3.0 × 104, 1.0 × 105, 2.25 × 105 to 3.5 × 105 (23) Brandrup, J., Immergut, E. H., Eds. Polymer Handbook; John Wiley & Sons: New York, 1989; Chapter VII, pp 218-219. (24) Gruber, U. Ph.D. Thesis, ETH Zu¨rich, 1964. (25) Elias, H. G.; Etter, O. Makromol. Chem. 1965, 89, 228. (26) Kirste, R.; Schulz, G. V. Z. Phys. Chem. 1960, 27, 310.
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Langmuir, Vol. 20, No. 2, 2004
Notes
Figure 2. Linear relationships between (a) log Dh and log(Mw/104) and (b) log np and log(Mw/104) for PMMA latex particles obtained from dilute PMMA solutions with different Mw at θ condition. Figure 1. TEM micrographs of PMMA latexes obtained from dilute PMMA (Mw ) 2.25 × 105 g/mol) solutions in n-hexane/ toluene (18.8/81.2 by volume) at θ temperature (25 °C) using (a) 0.5 g of SDS, Dh ) 20.3 nm, and (b) 0.25 g of SDS, Dh ) 31.1 nm.
g/mol, Dh of latexes obtained increased significantly from 13.2, 17.6, and 20.3 to 22.5 nm. Their poly values decreased slightly from about 1.31 to 1.23. At θ temperature, polymer solution is in a substable state. Because of the fluctuation of temperature, it may be difficult to avoid the formation of some concentrated microphases (Scheme 1). Upon the addition of the dilute polymer solution to an aqueous solution of surfactant, rather small nanoparticles corresponding to those small microphases may be formed by adsorbing enough surfactant molecules. The sizes of the concentrated microphases for polymer with higher Mw will generally be larger than those with lower Mw because of their different solubilities at a given θ temperature. As a result, the formed nanoparticle sizes increased with the increase of Mw as shown in Table 2. However, their np (the number of polymer chains per latex particle) decreased by more than 2-fold from about 25 to 11. It is interesting to note that there exists a linear relationship between log Dh and log Mw with a slope c of 0.214 24 and a linear relationship between log np and log Mw with a slope c′ of -0.352 08 as shown in parts a and b of Figure 2, respectively. Actually, the linear relationship between log np and log Mw just follows from
a simple calculation using eq 3:
np )
(1/6)πDh3dp w Mw/NA log np ) log(πNAdpDh3/6) - log Mw (4)
Since there exists a linear relationship between log Dh and log Mw with a slope c, one can write
log Dh ) c log Mw w Dh ) Mwc
(5)
Substituting eq 5 into eq 4 results in
log np ) log(RMw3c) - log Mw w log np ) log(R) + log(Mw3c-1) (6) with R ) πNAdp/6, and
log np ) constant + (3c - 1) log Mw
(7)
Thus, it is trivial that log np versus log Mw is linear, since eq 5 is valid. The calculated slope 3c - 1 for log np vs log Mw is about -0.357 28, which is consistent with the slope value (c′ ) -0.352 08) for the plot log np vs log Mw. This further confirms that the relationship between log np and log Mw is linear. The above observation is very similar to that reported in our previous work19 that for PS latex particles obtained at θ condition, there existed a linear relationship between log np and log Mw, and the micro-
Notes
Langmuir, Vol. 20, No. 2, 2004 531 Table 2. Effect of Molar Mass of PMMA on the Characteristics of Latexes aqueous solution
organic solution
latex characteristics
system
SDS (g)
H2O (g)
PMMA (g)
Mw (105 g/mol)
n-hexane/toluene (18.8/81.2 by volume) (g)
θ temp (°C)
Dh (nm)
polya
npb
1 2 3 4
0.5 0.5 0.5 0.5
49.1 49.1 49.1 49.1
0.4 0.4 0.4 0.4
0.3 1.0 2.25 3.5
100 100 100 100
25 25 25 25
13.2 17.6 20.3 22.5
1.31 1.28 1.27 1.23
25.1 17.9 12.2 10.7
a
The particle size distribution. b The number of polymer chains per latex particle. Table 3. Effect of SDS Concentration on the Characteristics of Latexes aqueous solution
organic solution
latex characteristics
system
SDS (g)
H2O (g)
PMMA (g)
Mw (105 g/mol)
n-hexane/toluene (18.8/81.2 by volume) (g)
θ temp (°C)
Dh (nm)
polya
npb
1 2 3 4
0.25 0.5 0.75 1.0
49.35 49.1 48.85 48.6
0.4 0.4 0.4 0.4
2.25 2.25 2.25 2.25
100 100 100 100
25 25 25 25
31.1 20.3 15.6 9.5
1.20 1.27 1.31 1.34
43.8 12.2 5.5 1.2
a
The particle size distribution. b The number of polymer chains per latex particle. Table 4. Effect of PMMA Concentration on the Characteristics of Latexes aqueous solution
organic solution
latex characteristics
system
SDS (g)
H2O (g)
PMMA (g)
Mw (105 g/mol)
n-hexane/toluene (18.8/81.2 by volume) (g)
θ temp (°C)
Dh (nm)
polya
npb
1 2 3 4
0.5 0.5 0.5 0.5
49.3 49.1 48.9 48.7
0.2 0.4 0.6 0.8
2.25 2.25 2.25 2.25
100 100 100 100
25 25 25 25
16.5 20.3 23.4 25.9
1.32 1.27 1.25 1.22
6.5 12.2 18.9 25.3
a
The particle size distribution. b The number of polymer chains per latex particle.
emulsion polymerization of a low content of styrene (4-8 wt %) at higher surfactant concentrations (6-10 wt %) would produce Mw and np relationships similar to those obtained directly from dilute PS solutions at θ temperature. It seems that such a linear relationship between log np and log Mw is applicable for either polymers from water-insoluble monomers such as styrene or polymers from partly water-soluble monomers such as MMA. Table 3 shows the effect of SDS concentration on the characteristics of latexes. The particle sizes decreased sharply from about 31 to 10 nm as the SDS concentration increased from about 0.5, 1, 1.5 to 2 wt %. At higher concentration of SDS of about 2 wt %, np was as small as 1. This means that one polymer particle contained only one polymer chain at a higher concentration of surfactant. At a very dilute polymer solution, it is possible that each concentrated microphase contains only an isolated polymer chain. Upon addition to surfactant solution, these isolated polymer chains would adsorb enough surfactant molecules to form latex particles with sufficient colloidal stability at or above an optimum surfactant concentration. On the other hand, when the corresponding PMMA dilute solution (about 0.4 wt %) was added to a lower concentration of SDS solution of 0.5 wt %, a relatively large particle size of about 31 nm was obtained, and the np value reached about 44. This indicates that unstable small particles containing a single chain may agglomerate each other to form larger particles containing more than one polymer chain below the optimum concentration of surfactant. Figure 1b shows the TEM micrograph for sample 1 (Table 3) with a relatively larger particle size obtained at lower SDS concentration. Similarly, spherical colloidal particles were observed again. The concentration effect of PMMA on the latex characteristics is summarized in Table 4. The latex particle sizes increased from 16.5, 20.3, 23.4 to 25.9 nm with
decreasing poly from 1.32, 1.27, 1.25 to 1.22 as the PMMA concentration in the organic phase was increased from about 0.2, 0.4, 0.6 to 0.8 wt %, respectively. Meanwhile, the np value increased by about 4-fold from about 6 to 25. The above results imply that with the increase of polymer concentration, the ideal solution cannot be achieved even at θ temperature. Hence, more polymer chains can coexist in a given microphase of the polymer solution, and the higher is the polymer concentration, the more polymer chains are present in a given microphase. This results in forming larger latex particles containing more than one polymer chain from the polymer solution upon addition to the surfactant solution. In conclusion, small spherical PMMA nanoparticles of latexes stabilized by the anionic surfactant SDS can easily be produced from the dilute solutions of commercial PMMA in different solvents at their respective θ temperatures. Stable PMMA latexes with particle sizes ranging from about 10 to 30 nm in diameter can be obtained under different experimental conditions. This is closely related to the effects of Mw of PMMA and concentrations of SDS as well as PMMA rather than the effect of solvents used. There exists a linear relationship between log Dh and log Mw as well as between log np and log Mw for the present PMMA latexes formed at θ condition. It seems that such linear relationship between log np and log Mw is applicable for either polymers from water-insoluble monomers such as styrene or polymers from partly water-soluble monomers such as MMA. This simple physical method for preparing nanosized polymer latexes from their corresponding bulk polymers may have some potential practical applications for those polymers whose latexes are difficult to produce, and it deserves to be further studied. LA030317N
