Langmuir 2004, 20, 3509-3512
3509
Retention of Structure in Microemulsion Polymerization: Formation of Nanolatices David C. Steytler,* Alexandre Gurgel, and Robert Ohly School of Chemical Sciences and Pharmacy, University of East Anglia, Norwich, Norfolk NR4 7TJ, U.K.
Martin Jung E100-1919, BASF Aktiengesellschaft, 67056 Ludwigshafen, Germany
Richard K. Heenan ISIS-CLRC, Rutherford Appleton Laboratory, Chilton, Oxon OX11 0QX, U.K. Received December 1, 2003. In Final Form: February 13, 2004 Polymerization of styrene-in-water microemulsions by photoinitiation using the initiator 2,2′-dimethoxy2-phenyl acetophenone (DMPA) produces small nanolatices of the same size as the parent microemulsion (radius ) 24-31 Å). This behavior is distinct from previously applied methods where significant particle growth accompanies the polymerization reaction. NMR measurements confirmed that polymerization is complete in under an hour and small-angle neutron scattering established the microemulsion structure before and after polymerization. The approach may be of more general application where retention of structure is sought in microemulsion-based polymer templating procedures.
* Corresponding author. Tel.: UK + 1603 592033. Fax: UK + 1603 593139. E-mail:
[email protected].
where As (Å2) is the area occupied by the surfactant at the droplet surface. Provided that the microemulsion structure is not perturbed during the reaction an o/w microemulsion, with a hydrophobic monomer such as styrene as oil (o), would provide a “template” system for control of particle growth on the nanometer scale. However, transfer of the components through interdroplet diffusion and droplet exchange processes may permit fast exchange of both monomer and surfactant between droplets. This can result in uneven growth from which the structure of the droplet template may alter during the reaction or be totally destroyed as a result of phase separation. Although structure appears to be retained in miniemulsion polymerization8 (>500 Å), this is not the case for smaller microemulsion droplets where the dimensions of the resulting particles obtained may significantly exceed those of the “parent” microemulsion.3 Modification of the surfactant structure/properties has led to significant advances in the control of latices formed in microemulsions. Using cuprated tetradecyldiethanolamine as surfactant, Antonietti9 has shown that growth of particles during the polymerization could be restricted to 140 Å, about half the dimension obtained in the absence of copper. Using a polymerizable surfactant10 (surfmer), Pileni et al.11,12 were, for the first time, able to maintain droplet structure in microemulsion polymerization. Particle growth accompanying the polymerization reaction is, of course, advantageous when particles larger than the microemulsion are sought, but to date, there does not appear to be any reports of size retention using simple dispersed monomer systems.
(1) Candau F. Macromol. Symp. 1995, 92, 169-178. (2) Antonietti, M.; Basten, R.; Lohmann, S. Macromol. Chem. Phys. 1995, 196, 441-466. (3) Desai, S. D.; Gordon, R. D.; Gronda, A. M.; Cussler, E. L. Curr. Opin. Colloid Interface Sci. 1996, 1 (4), 519-529. (4) Hentze, H.-P.; Kaler, E. W. Curr. Opin. Colloid Interface Sci. 2003, 8, 164-178. (5) Materials Aspects. Curr. Opin. Colloid Interface Sci. 1997, 2 (2). (6) Pileni, M. P. Nat. Mater. 2003, 2, 145-150. (7) Antonietti, M.; Lohmann, S.; Van Niel, C. Macromolecules 1992, 25, 1139-1143.
(8) Landfester, K.; Bechthold, N.; Tiarks, F.; Antonietti, M. Macromolecules 1999, 32, 5222-5228. (9) Antionetti, M.; Nestl, T. Macromol. Rapid Commun. 1994, 15, 111-116. (10) Summers, M.; Eastoe, J. Adv. Colloid Interface Sci. 2003, 100, 137-152. (11) Pileni, M. P.; Hammouda, A.; Gulik, T. Langmuir 1995, 11, 36563659. (12) Mackay, R. A.; Pileni, M. P.; Moumen, N. Colloids Surf., A 1999, 151, 409-417.
Introduction Nanofabrication techniques that employ self-assembly systems as templates are of considerable commercial and academic interest and are undergoing rapid development. Such methodology has been used to limit the particle size of dispersed systems formed by both polymerization1-4 (latices) and precipitation reactions5,6 (inorganic particles). Applications of the resulting materials are widespread in paints and surface coatings, in catalysis and separation media and, more recently, as drug delivery systems. Techniques for the control of small-scale structures of nanometer dimension are becoming increasingly important in the development of well-defined nanoparticles and high surface area porous materials. Moreover, control of the surface chemistry of these materials offers potential for selective adsorption of specific targeted components.7 Microemulsions, composed of surfactant, water, and oil, may be formed as spherical nanodroplets of oil-in-water (o/w) or water-in-oil (w/o) stabilized by a monolayer of surfactant. At constant surfactant concentration [S] (mol dm-3), the radius Rd (Å) of the droplets then increases linearly with the volume fraction of the dispersed phase (φd) and is therefore readily controlled through this parameter
Rd ) (4.98 × 103)φd/As[S]
(1)
10.1021/la0304262 CCC: $27.50 © 2004 American Chemical Society Published on Web 03/30/2004
3510
Langmuir, Vol. 20, No. 9, 2004
Using high initiator and cross-linker concentrations, Antonietti13 and Wu14 have demonstrated a linear dependence (eq 1) of particle radius on φ for CTAB-stabilized cross-linked polystyrene microemulsions. However, the smallest latices that could be formed were of radius ∼100 Å, which is much larger than the microemulsion precursors. In related studies, using cationic surfactants, polymerized microemulsions are found to comprise particles up to an order of magnitude larger than the parent microemulsion droplets.15,16 Kaler et al. have pioneered polymerization in aqueous cationic surfactant microemulsions. Systems using single (dodecyltrimethylammonium bromide, DTAB) and mixed surfactants (DTAB and didodecyldimethylammonium bromide (DDAB)) have been examined. Mixed surfactant microemulsions have enabled the monomer hydrophobicity (styrene and alkyl methacrylates) to be systematically varied. In parallel, theoretical models17 have been developed that describe the growth of polymer particles during the course of the reaction. Significantly the results demonstrated an increase in particle size with increasing monomer solubility in the continuous aqueous phase. This lends support for a mechanism whereby monomer is transferred between droplets by diffusion through water and not as a result of droplet collisions.18 Small-angle neutron scattering (SANS) has been employed19 to directly monitor the structural changes occurring during the reaction. These measurements have established a mechanism of growth whereby initiated droplets “recruit” monomer from uninitiated droplets to ultimately form a small population of polymer particles coexisting with a much larger number of empty micelles, i.e., a bimodal size distribution. More recently, SANS has been employed to examine monomer partitioning by mixing uninitiated monomer droplets and polymerized particles.20 As a result of this and related studies the kinetic model has been refined21,22 to include effects of nonlinear monomer partitioning, bimolecular termination, and diffusion limitation to termination. In this paper we report an alternative photoinitiation procedure for styrene-in-water microemulsions stabilized by DTAB that maintains the microemulsion structure throughout the polymerization reaction. This appears to result from simple reaction kinetics (compared with thermal initiation) in which initiation and growth are synchronous and rapid within the droplet population. The method provides a route to very small nanolatices that exhibit optical transparency. Experimental Section For polymerization and characterization experiments a constant surfactant concentration of 3% w/v was employed. The styrene content of the microemulsions was varied within the range 0.6-1.4% w/v (below the maximum uptake of ∼1.7%23). (13) Antonietti, M.; Bremser, W.; Muschenborn, C.; Rosenauer, B.; Schupp, B.; Schmidt, M. Macromolecules 1991, 24, 6636-6643. (14) Wu, C. Macromolecules 1994, 27, 298-299. (15) Gan, L. M.; Lee, K. C.; Chew, C. H.; Ng, S. C. Langmuir 1995, 11, 449-454. (16) Full, A. P.; Kaler, E. W.; Arellano, J.; Puig, J. E. Macromolecules 1996, 29, 2764-2755. (17) Jordan, J. D.; Lusvardi, K. M.; Kaler, E. W. Macromolecules 1997, 30, 1897-1905. (18) Lusvardi, K. M.; Shubert, K.-V.; Kaler, E. W. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 373-379. (19) Carlos, C. C.; Kaler, E. W. Macromolecules 1998, 31, 32033210. (20) Co, C. C.; de Vries, R.; Kaler, E. W. Macromolecules 2001, 34, 3224-3232. (21) de Vries, R.; Co, C. C.; Kaler, E. W. Macromolecules 2001, 34, 3233-3244. (22) Co, C. C.; Cotts, P.; Burauer, S.; de Vries, R.; Kaler, E. W. Macromolecules 2001, 34, 3245-3254.
Letters Polymerization. Inhibitor (4-tert-butylcatechol) was first removed from the styrene monomer using a prepacked column (available from Aldrich). The oil-soluble initiator 2,2′-dimethoxy2-phenylacetophenone (DMPA, Aldrich, 98%) was then dissolved in the styrene at 6% w/v. This level was chosen such that, when dispersed, each microemulsion droplet would contain at least one initiator molecule. A 3% aqueous solution of DTAB was then prepared from which dissolved oxygen was removed by sparging with nitrogen for 5 min. The required amount of the initiatorcontaining monomer was then added to the aqueous surfactant solution at 25 °C and the mixture vigorously shaken for half an hour to form the optically transparent microemulsion. The microemulsion was then placed in a standard 1 cm path length quartz UV cell (Hellma). Photoinitiation was carried out for 1 h inside a thermostated incubator (Termaks) set at 25 °C. A UV light source (Philips HPR 125 W lamp), with a power spectrum matching the initiator absorption, was located approximately 40 cm from the cell. NMR Measurements. 1H NMR measurements were performed at room temperature (20-25 °C) using a Gemini 300MHz Fourier transform NMR spectrometer (Varian). The microemulsions were prepared in D2O and spectra recorded both before and after polymerization. The peaks corresponding to the double bonds and aromatic protons of the styrene monomers were used to assess the extent of the reaction. Small-Angle Neutron Scattering. Samples were prepared as described above using D2O to provide contrast. Time-of-flight SANS measurements were performed on the LOQ instrument24 on the ISIS pulsed neutron source at the Rutherford-Appleton Laboratory, U.K. The instrument uses a 64 cm square twodimensional gas detector positioned at a distance of 4.1 m from the sample. The samples were contained in 2 mm thick circular quartz cells (Hellma) and were thermostated at 25 °C by means of a Julabo FP 52 circulating bath. After normalization by standard procedures the measurements yield the absolute scattering cross section I(Q) (cm-1) as a function of momentum transfer Q ) 4π/λ sin(θ/2), with λ the incident neutron wavelength (2.2 Å < λ < 10 Å) and θ the scattering angle. SANS from D2O was used as background and was subtracted from all sample data.
Results Proton NMR spectra for DTAB micelles and a styrenein-water microemulsion before and after polymerization are shown in Figure 1. The results show that polymerization is complete within an hour of irradiation as evidenced by the disappearance of the styrene dCH2 signal at 5.1 ppm. Due to restricted motion, a dramatic broadening of the proton NMR signals from the polymer is observed which is characteristic of polymerization in microemulsion environments.25 SANS measurements were made on the systems before and after polymerization. Comparison of the two sets of data provide clear “fingerprint” evidence that the droplet structure has not altered as a result of the polymerization. However, a more detailed analysis can be employed. The scattering cross section for a polydisperse system of particles can be written
I(Q) ) Np[S(Q)|〈F(Q)〉Q|2 + 〈|F(Q)|2〉Q - |〈F(Q)〉Q|2] (2) where Np is the particle number density, F(Q) is the particle form factor, S(Q) is the interparticle structure factor, and the average 〈 〉 is over particle size. For coreshell particles with a contrast step (F1 - F2) at the core(23) Perez-Luna, V. H.; Puig, J. E.; Castano, V. M.; Rodriguez, B. E.; Murphy, A. K.; Kaler, E. W. Langmuir 1990, 6, 1040-1044. (24) Heenan, R. K.; Penfold, J.; King, S. M. J. Appl. Crystallogr. 1997, 30, 1140-1147. (25) Summers, M.; Eastoe, J.; Davis, S.; Du, Z. P.; Richardson, R. M.; Heenan, R. K.; Steytler, D.; Grillo, I. Langmuir 2001, 17, 5388-5397.
Letters
Figure 1. NMR spectra for DTAB micelles (A) and for a DTABstabilized styrene-in-water microemulsion before (B) and after (C) polymerization. [DTAB] ) 3% w/v; [styrene] ) 1.2% w/v. All measurements were made in D2O.
shell interface and (F2 - Fs) at the shell-solvent interface the form factor is given by
F(Q) ) V1(F1 - F2)F0(QR1) + V2(F2 - Fs)F0(QR2) (3) where F1, F2, and Fs are the scattering length densities of the core, shell, and solvent, V ) (4/3)πR3 and F0 ) 3j1(QR)/(QR). SANS from the microemulsion droplets and nanolatices was fitted26 using a polydisperse core-shell form factor model with a mean particle radius R2. The surfactant shell thickness (t) was fixed at 15 Å such that the core radius R1 ) R2 -15 Å. Scattering length densities for the styrene core, DTAB shell, and D2O solvent were taken as F1 ) 1.2, F2 ) -0.3, and Fs ) 6.4 × 1010 cm-2, respectively. Size distribution was introduced into the model using the Schultz polydispersity function27 with polydispersity index σ/R2 of 0.1. To account for charge repulsion between the droplets, the Hayter-Penfold spherical macroion structure factor28 was employed. This is determined by the effective radius (RΦ) of the charged surface, the effective charge (Φ) per sphere, the volume fraction (φ) of spheres, and the inverse Debye screening length (κ). The latter parameters were fixed; φ by the sample composition and κ was calculated from standard procedures.28 The data and model fitting (Figure 2) provide an accurate deter(26) Heenan, R. K. FISH Data Analysis Program, Rutherford Appleton Laboratory Report, RAL-89-129; 1989. (27) Kotlarchyk, M.; Chen, S.-H. J. Chem. Phys. 1983, 79, 2461. (28) Hayter, J. B.; Penfold, J. Mol. Phys. 1981, 42, 109-118.
Langmuir, Vol. 20, No. 9, 2004 3511
Figure 2. SANS data (symbols) and model fit (lines) for DTABstabilized styrene-in-water microemulsions before (A) and after (B) photoinduced polymerization. [DTAB] ) 3% w/v. Table 1. Parameters Obtained from SANS Data of Figure 2 Using a Polydisperse Charged Core-Shell Sphere Model with a Hayter-Penfold S(Q) Term (see text for details)a styrene content (% w/v)
R2/Å
RΦ/Å
Φ/e
0.6 0.8 1.2 1.4
24.0 (24.0) 25.5 (25.0) 28.5 (28.0) 31.0 (31.0)
25.5 (25.5) 27.1 (27.0) 30.5 (28.0) 32.0 (32.0)
12.5 (12.5) 10.8 (10.9) 17.9 (17.9) 19.1 (20.0)
a Quantities in parentheses are for the styrene-containing microemulsions before polymerization. [DTAB] ) 3% w/v.
mination of overall particle size (through R2 and RΦ) but, owing to the small core-shell contrast step and significant contribution of the S(Q) term, the method is relatively insensitive to the magnitude of t. The parameters obtained (Table 1) from the fitting procedure show the microemulsions before and after polymerization to be identical within error. This invariance in size during polymerization was also obtained for systems for which a proportion (up to 6%) of the styrene was replaced by the cross-linking monomer, divinylbenzene. The particle radii RΦ, given from the S(Q) term, were marginally higher than Rav, obtained from P(Q), by approximately 1-2 Å. The data for the UV initiation approach are compared with published data from thermal initiation procedures using the representation employed by Antonietti and Wu (Figure 3).
3512
Langmuir, Vol. 20, No. 9, 2004
Letters
lie in the choice of light source and initiator used for photoinitiation. The lamp employed was a high-pressure mercury lamp and was chosen specifically for its spectral power distribution that exhibits a high output of the 366 nm emission (5 W). This provides an intense luminosity on the high wavelength side of the adsorption peak of the initiator (centered at λmax ) 345 nm). Studies of photoinitiation of poly(methyl methacrylate) sheets under similar conditions have suggested a mechanism showing efficient dissociation of the initiator DPMA.29 Conclusion
Figure 3. “Antonietti-Wu” plot comparing particle size as a function of monomer content for the thermal initiation method (refs 13 and 14) and the photoinitiation method (reported in this paper). (Wm and Ws represent weight of monomer and surfactant, respectively.)
Many factors may contribute to this behavior. Simultaneous initiation of polymerization within the population of microemulsion droplets is aided through choice of initiator concentration. Also monomer diffusion between droplets is lower (relative to thermal initiation) by virtue of a lower monomer solubility in water at ambient temperature. However, previous methods using UV initiation have not preserved the microemulsion structure to give such small latices. The key to the behavior observed may
A photoinitiation procedure has been shown to maintain the original droplet structure in the polymerization of styrene-containing microemulsions stabilized by the surfactant DTAB. The behavior is distinct from alternative thermal (and photo-) initiation methods employed in which particle growth occurs during polymerization. It is anticipated that the approach could be more widely applied to polymerization reactions in microemulsions where retention of structure is sought. Acknowledgment. We thank CLRC for allocating beam time at ISIS and contributions toward consumables and travel. Studentship support from CNPq-Brazil for Alexandre Gurgel is also gratefully acknowledged. LA0304262 (29) Havermeyer, F.; Pruner, C.; Rupp, R. A.; Schubert, D. W.; Kratzig, E. Appl. Phys. B: Lasers Opt. 2001, 72, 201-205.
