Langmuir 2006, 22, 4923-4927
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Polystyrene-Silica Nanocomposite Particles via Alcoholic Dispersion Polymerization Using a Cationic Azo Initiator Andreas Schmid, Syuji Fujii, and Steven P. Armes* Department of Chemistry, The UniVersity of Sheffield, Dainton Building, Brook Hill, Sheffield, S3 7HF United Kingdom ReceiVed February 1, 2006. In Final Form: March 16, 2006 Submicrometer-sized polystyrene-silica nanocomposite particles have been prepared by alcoholic dispersion polymerization of styrene using commercial alcoholic silica sols of 13 or 22 nm diameter as the sole stabilizing agent. The key to the formation of colloidally stable nanocomposite particles is the selection of a cationic azo initiator (use of nonionic or anionic initiators leads either to the formation of silica-stabilized polystyrene latex particles with very low silica contents or to the precipitation of polystyrene, respectively). Neither surface modification of the silica sol nor the addition of surfactant or polymeric stabilizers is required for successful nanocomposite syntheses. The purified polystyrene-silica nanocomposite particles have relatively narrow particle size distributions, with mean diameters ranging from 331 to 464 nm as judged by disk centrifuge photosedimentometry. Thermogravimetric analyses indicated mean silica contents of 13-26 wt. %, depending on the synthesis conditions. Calcination of the polystyrene-silica nanocomposite particles leads to the formation of hollow silica shells, which indicates a well-defined core-shell morphology for the original nanocomposite particles.
Introduction The field of polymer nanocomposites is of rapidly growing interest.1 It is well-known that the intimate mixing of polymers with inorganic clays or silica on a nanometer scale can lead to composite materials with superior mechanical properties and improved fire retardancy.2-5 Recently, there has been increasing interest in the synthesis of colloidal nanocomposite particles.6 Such syntheses typically involve either emulsion or dispersion polymerization,10-22 although other techniques such as miniemulsion polymerization7 or the entrapment of preformed polymer during sol-gel processes have also been described.8,9 For example, Bourgeat-Lami et al.10-12 encapsulated silica particles * To whom correspondence should be addressed. E-mail: s.p.armes@ sheffield.ac.uk. (1) Percy, M. J.; Amalvy, J. I.; Randall, D. P.; Armes, S. P.; Greaves, S. J.; Watts, J. F. Langmuir 2004, 20, 2184. (2) Gilman, J. W.; Jackson, C. L.; Morgan, A. B.; Harris, R.; Manias, E.; Giannelis, E. P.; Wuthenow, M.; Hilton, D.; Phillips, S. H. Chem. Mater. 2000, 12, 1866. (3) Zhu, J.; Morgan, A. B.; Lamelas, F. J.; Wilkie, C. A. Chem. Mater. 2001, 13, 3774. (4) Manias, E.; Touny, A.; Wu, L.; Strawhecker, K.; Lu, B.; Chung, T. C. Chem. Mater. 2001, 13, 3516. (5) Kashiwagi, T.; Morgan, A. B.; Antonucci, J. M.; VanLandingham, M. R.; Harris, R. H.; Awad, W. H.; Shields, J. R. J. Appl. Polym. Sci. 2003, 89, 2072. (6) Bourgeat-Lami, E. J. Nanosci. Nanotechnol. 2002, 2, 1. (7) Tiarks, F.; Landfester, K.; Antonietti, M. Langmuir 2001, 17, 5775. (8) Sertchook, H.; Avnir, D. Chem. Mater. 2003, 15, 1690. (9) Sertchook, H.; Elimelech, H.; Avnir, D. Chem. Mater. 2005, 17, 4711. (10) Bourgeat-Lami, E.; Lang, J. J. Colloid Interface Sci. 1998, 197, 293. (11) Bourgeat-Lami, E.; Lang, J. J. Colloid Interface Sci. 1999, 210, 281. (12) Bourgeat-Lami, E.; Lang, J. Macromol. Symp. 2000, 151, 377. (13) Yoshinaga, K.; Yokoyama, T.; Sugawa, Y.; Karakawa, H.; Enomoto, N.; Nishida, H., Komatsu, Michio. Polym. Bull. 1992, 28, 663. (14) Luna-Xavier, J.-L.; Bourgeat-Lami, E.; Guyot, A. Colloid Polym. Sci. 2001, 279, 947. (15) Luna-Xavier, J.-L.; Guyot, A.; Bourgeat-Lami, E. J. Colloid Interface Sci. 2002, 250, 82. (16) Luna-Xavier, J.-L.; Guyot, A.; Bourgeat-Lami, E. Polym. Int. 2004, 53, 609. (17) Barthet, C.; Hickey, A. J.; Cairns, D. B.; Armes, S. P. AdV. Mater. 1999, 11, 408. (18) Percy, M. J.; Barthet, C.; Lobb, J. C.; Khan, M. A.; Lascelles, S. F.; Vamvakaki, M.; Armes, S. P. Langmuir 2000, 16, 6913. (19) Amalvy, J. I.; Percy, M. J.; Armes, S. P.; Wiese, H. Langmuir 2001, 17, 4770. (20) Chen, M.; Wu, L.; Zhou, S.; You, B. Macromolecules 2004, 37, 9613. (21) Han, M. G.; Armes, S. P. Langmuir 2003, 19, 4523. (22) Percy, M. J.; Armes, S. P. Langmuir 2002, 18, 4562.
with diameters ranging between 13 and 629 nm within polystyrene via dispersion polymerization in aqueous ethanol in the presence of a poly(N-vinylpyrrolidone) or poly(styrene-block-ethylene oxide) stabilizer. Encapsulation only occurred when the silica particles were surface-functionalized by grafting 3-(trimethoxysilyl)propyl methacrylate (MPS) prior to polymerization. The resulting composite particles contained differing numbers of silica particles depending on the silica sol diameter; for silica sols of 450 nm, only one silica particle was encapsulated per composite particle. Yoshinaga et al.13 also encapsulated 470 nm silica particles by dispersion polymerization of styrene and methyl methacrylate using a cationic azobisisobutyramidine dihydrochloride (AIBA) initiator. Luna-Xavier et al.14-16 prepared PMMA-silica nanocomposites via emulsion polymerization in the presence of a nonionic surfactant. Electrostatic adsorption of either the cationic AIBA initiator and/or the cationic end groups of the polymer chains onto the anionic silica sol was considered to be essential for successful nanocomposite formation. Raspberrylike nanocomposite particles comprising polymer nodules surrounding a silica core were obtained when using smaller silica sols of 68 nm diameter. In contrast, larger silica sols led to welldefined core-shell morphologies. Armes and co-workers17-19 prepared a wide range of nanocomposite particles by copolymerizing various vinyl monomers with 4-vinylpyridine (4VP) in the presence of an ultrafine aqueous silica sol. The presence of the basic 4VP auxiliary ensured strong interaction of the vinyl copolymers with the acidic silica surface, resulting in an attractive “surfactant-free” route to colloidally stable nanocomposites. Following a similar strategy, Chen et al.20 prepared raspberry-like PMMA-silica nanocomposites using 1-vinylimidazole as an auxiliary comonomer. Recently Armes and co-workers reported that various commercial alcoholic silica sols of 13-22 nm diameter can be used to prepare either poly(3,4-ethylenedioxythiophene)-silica nanocomposite particles21 or various vinyl polymer-silica nanocomposite particles1,22 in an aqueous alcohol milieu in the absence of any comonomer auxiliary. In a recent communication, we reported23 that the dispersion polymerization of styrene in purely alcoholic media using (23) Schmid, A.; Fujii, S.; Armes, S. P. Langmuir 2005, 21, 8103.
10.1021/la060308p CCC: $33.50 © 2006 American Chemical Society Published on Web 04/25/2006
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Figure 1. Schematic representation of the effect of varying the initiator type in the attempted dispersion polymerization of styrene at 60 °C in the presence of a ultrafine silica sol: (a) using the neutral AIBN initiator gives micrometer-sized silica-stabilized polystyrene latex particles; (b) using the anionic ACVA initiator gives a macroscopic precipitate of polystyrene with no stable particles; (c) using the cationic AIBA initiator gives submicrometer-sized polystyrene-silica nanocomposite particles.
commercial alcoholic silica sols of either 13 or 22 nm diameter as the sole stabilizer leads to the formation of micrometer-sized, silica-stabilized polystyrene latex particles, provided that the polymerization is initiated using a neutral azo initiator. These latexes had narrow size distributions and relatively low silica contents (around 1.0% by mass). Herein we report that the nature of the initiator has a dramatic effect on the size and morphology of the resulting particles, see Figure 1. Using an anionic initiator leads to gross precipitation of polystyrene, with little or no particle formation. In contrast, using a cationic initiator (AIBA) leads to the formation of submicrometer-sized polystyrene-silica nanocomposite particles. These new examples of colloidal nanocomposites are characterized in terms of their particle morphology, size distribution, mean silica content and surface composition using electron microscopy, disk centrifuge photosedimentometry, thermogravimetric analysis and aqueous electrophoresis measurements, respectively. Experimental Details Materials. Styrene (Aldrich) was passed through a column of basic alumina to remove the inhibitor and then stored at -20 °C prior to use. 2,2′-Azobis(isobutyramidine) dihydrochloride (AIBA) (Aldrich), 4,4′-azobis(4-cyanovaleric acid) (ACVA) (Acros), 2,2′azobisisobutyronitrile (AIBN) (BDH), methanol, and 2-propanol (both ex. Fisher Scientific) were all used as received. Three commercial alcoholic silica sols were obtained from Nissan Chemicals, Texas (U.S.A.): MT-ST (13 nm silica sol in methanol at 30 wt %), MA-ST (22 nm silica sol in methanol at 30 wt %), and IPA-ST (13 nm silica sol in 2-propanol at 30 wt %); each sol was used as received. Particle Synthesis. A typical nanocomposite synthesis was carried out as follows. Styrene monomer (2.50 mL), the appropriate volume of an alcoholic silica sol (equivalent to 2.0 g solids of silica), and, depending on which type of silica sol was used, either methanol or 2-propanol (17.0 mL) were added in turn to a round-bottomed flask equipped with a condenser and magnetic stir bar. The mixture was degassed with nitrogen and then heated to 60 °C in an oil bath. The
initiator (23.0 mg; 1.0 wt. % based on monomer) was added as a 2.0 mL solution in either methanol (for syntheses carried out in methanol) or a 3:1 2-propanol/water mixture (for syntheses carried out in 2-propanol). Each polymerization was allowed to continue for 24 h under nitrogen. The resulting milky-white colloidal dispersions were purified by eight centrifugation-redispersion cycles (3000 rpm for 30 min), with each successive supernatant being carefully decanted and replaced. Initially, the supernatant was replaced three times with pure methanol (or 2-propanol) and then with mixtures of the alcohol and deionized water (2:1, 1:1 and 1:2 by volume, respectively), and finally it was replaced twice with deionized water. Transmission electron microscopy confirmed that all excess silica sol had been removed by this purification protocol. Excessive centrifugation rates (>8000 rpm) and times (>1 h) were avoided, since these would otherwise result in sedimentation of the excess silica sol and also make redispersion of the sedimented particles more difficult. Disk Centrifuge Photosedimentometry (DCP). DCP (Brookhaven Instruments) was used to obtain the weight-average particle size distributions of the purified aqueous latexes, as described previously.23 Solid-state particle densities were measured by helium pycnometry (Micromeritics AccuPyc 1330 instrument). Thermogravimetric Analyses (TGA). TGA were carried out using a Perkin-Elmer Pyris 1 TGA instrument. Dried samples were heated in air to 800 °C at a heating rate of 10 °C min-1, and the observed mass loss was attributed to the quantitative degradation of the polystyrene, with the remaining incombustible residues being assumed to be pure silica (SiO2). Transmission Electron Microscopy (TEM). TEM samples were prepared by drying a drop of a dilute dispersion onto a carboncoated copper grid, and analyzed using a Phillips CM100 electron microscope operating at 100 kV. SEM studies were performed on a JEOL JSM 6400 scanning electron microscope. Samples were dried on adhesive carbon disks and sputter-coated with a thin layer of gold prior to examination. Particle Growth Curves. Particle growth curves were determined by periodic sampling of the polymerizing solution. Quenching was achieved by a 10-fold dilution (0.20 mL aliquot into 2.0 mL methanol containing 20 mM hydroquinone), followed by DLS studies of the mean particle diameter at 25 °C.
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Table 1. Effect of Varying the Initiator Type, Silica Sol Type, and Silica Sol Particle Diameter on the Particle Morphology, Mean Particle Diameter, and Silica Sol Content of the Polystyrene-Silica Nanocomposite Particles Produced via Alcoholic Dispersion Polymerization at 60 °Ca entry no.
silica sol typeb
silica sol diameter (nm)
solvent
initiator type
1 2 3 4 5 6 7
MT-ST MT-ST MT-ST MA-ST IPA-ST None None
13 13 13 22 13 n/a n/a
methanol methanol methanol methanol 2-propanol methanol 2-propanol
AIBN AIBA ACVA AIBA AIBA AIBA AIBA
silica contentc (wt %)
silica sol incorporation efficiency (%)
particle densityd (g cm-3)
Dw e
0.7 0.8 1.07 1.36 ( 0.12 µm 26 40 1.24 331 ( 43 nm macroscopic precipitation of polystyrene with very few particles 18 25 1.16 362 ( 22 nm 13 17 1.14 464 ( 45 nm 0.0 n/a 1.05 760 ( 50 nm 0.0 n/a no particle formation
a Polymerization conditions: 2.50 mL styrene, 2.0 g silica sol (i.e., 6.67 g of a 30 wt. % alcoholic solution), 1.0 wt. % initiator based on styrene and a total reaction volume of 25 mL. All polymerizations were carried out at 60 °C for 24 h under a nitrogen blanket. b IPA-ST, 13 nm silica sol in 2-propanol; MA-ST, 22 nm silica sol in methanol; MT-ST, 13 nm silica sol in methanol. Each of these alcoholic silica sols was obtained as a 30 wt. % dispersion from Nissan Chemicals, U.S.A. The silica sol concentration used in these experiments was 8.0 w/v% based on the total solvent volume. c Determined from thermogravimetric analyses assuming that the incombustible residues were SiO2. d As measured by helium pycnometry. e Weight-average diameter ( standard deviation for the redispersed, purified particles as determined by disk centrifuge photosedimentometry.
Aqueous Electrophoresis Measurements. Aqueous electrophoresis measurements were carried out using a Malvern Zetasizer NanoZS instrument. Measurements were carried out in the presence of 1 mM Na2SO4 as background salt, and the solution pH was adjusted by the addition of HCl or NaOH.
Results and Discussion The type of initiator proved to be crucial in determining the physical nature of the polystyrene produced in these syntheses. Using a neutral azobisisobutyronitrile initiator (AIBN) led to micrometer-sized, silica-stabilized polystyrene latexes, as previously reported. In contrast, an anionic initiator simply led to a macroscopic precipitate, with little or no colloidal particles being obtained (see the Supporting Information). Only the cationic AIBA initiator led to submicrometer-sized polystyrene-silica nanocomposite particles. The effect of varying the silica sol diameter and sol type on the nanocomposite particle diameter, particle density, and silica content is summarized in Table 1. It is also noteworthy that the presence of the silica sol is essential for the formation of colloidal particles for polymerizations conducted in 2-propanol; in the absence of silica, only a macroscopic precipitate was obtained (see the Supporting Information). In contrast, a silica-free polymerization conducted in methanol led to the formation of colloidally stable chargestabilized polystyrene particles with a mean particle diameter of 760 nm (see the Supporting Information). Colloidally stable polystyrene-silica nanocomposite particles were obtained using each of the three commercial alcoholic silica sols. Nanocomposite densities ranged from 1.14 to 1.24 g cm-3, which are significantly higher than that of pure polystyrene (1.05 g cm-3). Assuming additivity and taking the mean density of the silica sols to be approximately 2.15 g cm-3, these nanocomposite densities allowed the estimation of mean silica contents ranging from 15 to 29 wt %. Thermogravimetric analysis confirmed silica contents of 26 wt % for nanocomposites prepared with the 13 nm silica sol in methanol, 18 wt % for nanocomposites prepared with the 22 nm silica sol in methanol and 13 wt % for nanocomposites prepared with the 13 nm silica sol in 2-propanol (see entries 2, 4, and 5 in Table 1). These silica contents correspond to silica incorporation efficiencies of up to 40% for nanocomposites prepared with the 13 nm methanolic silica sol and 17% for the particles prepared with the 13 nm silica sol in 2-propanol. These efficiencies should be compared to those previously calculated23 for the silica-stabilized polystyrene latex particles prepared with the neutral AIBN initiator, where less than 1% of the original silica sol is actually incorporated into the particles.
Figure 2. TEM images and disk centrifuge particle size distribution curves of polystyrene-silica nanocomposite particles prepared by the alcoholic dispersion polymerization of styrene in the presence of commercial alcoholic silica sols using the cationic AIBA initiator. The silica sols used were (a) MT-ST 13 nm silica sol in methanol, (b) MA-ST 22 nm silica sol in methanol, and (c) IPA-ST 13 nm silica sol in 2-propanol.
Investigation of the particle morphology by TEM confirmed the nanocomposite character of these particles, since the adsorbed silica sol is visible at the surface of each nanocomposite particle (see Figures 2 and 5). Mean particle diameters were determined by disk centrifuge photosedimentometry and ranged from 331 nm for nanocomposite particles prepared using the 13 nm methanolic silica sol up to 464 nm for those prepared with the 13 nm silica sol in 2-propanol. These sizes agreed reasonably well with the mean particle diameters estimated from transmission electron microscopy. As the choice of initiator is the only difference between the syntheses, it is likely that electrostatic attraction between the cationic initiator and the anionic silica sol promotes successful nanocomposite formation. To test this hypothesis, the extent of adsorption of the cationic AIBA initiator from methanol onto the 22 nm methanolic silica sol at room temperature was determined by the depletion method, using UV-visible absorption spec-
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Figure 3. Particle growth curves determined by dynamic light scattering of (a) silica-stabilized polystyrene latex particles prepared using the nonionic AIBN initiator; (b) polystyrene-silica nanocomposite particles prepared using the cationic AIBA initiator in the presence of a 22 nm MA-ST methanolic silica sol; (c) cationic polystyrene latex particles prepared using the cationic AIBA initiator in the absence of any silica sol.
trometry to monitor the characteristic AIBA absorption at λmax ) 368 nm. A Langmuir-type isotherm was obtained, as expected (see the Supporting Information). The maximum adsorbed amount of AIBA was determined to be 3.48 mg g-1, which equates to 0.028 mg m-2. This should be compared to the value of 2 mg g-1 (or 0.050 mg m-2) reported by Luna-Xavier and co-workers14 for AIBA adsorbed onto an aqueous 68 nm silica sol (Klebosol 30N50). Presumably these differing values simply reflect differences in surface charge density and/or hydroxyl content for the two types of silica sol. Such differences in surface charge density between aqueous and alcoholic silica sols are clearly evident in zeta potential measurements, as reported by Percy et al.1,22 On the other hand, in the case of the nonionic AIBN initiator, a π-electron interaction between the aromatic ring of the polymerized styrene residues and the silica surface has been suggested by Agarwal et al.24 on the basis of 13C MAS solidstate NMR studies. We propose that such an interaction is also likely to be important for the formation of the silica-stabilized polystyrene latex particles. Under the chosen polymerization conditions, we calculate that the AIBA concentration is approximately three times greater than the minimum amount required for monolayer coverage. Thus, the silica particles are completely coated with initiator and excess initiator is also present in solution. In this context, it is important to note that, on addition of the cationic AIBA initiator, there is a reduction in zeta potential from -47 mV (measured for the MA-ST sol at pH 7 prior to initiator addition) to -15 mV (for the same sol at pH 7 after initiator addition). However, there is no visible flocculation of the silica sol in the presence of this AIBA initiator and there is no significant increase in the mean sol diameter, as judged by DLS studies. Given the significant proportion of adsorbed AIBA initiator, some degree of surface polymerization probably occurs, which should favor more efficient silica incorporation. Nevertheless, it is difficult to be certain that surface polymerization is essential for the formation of colloidally stable nanocomposite particles. Particle growth curves were obtained by DLS studies of the polymerization of styrene in the presence of the 22 nm methanolic MA-ST silica sol initiated by both AIBN and AIBA, as well as for a cationic polystyrene latex prepared with AIBA in the absence of any silica sol, see Figure 3. For the styrene polymerization conducted using the AIBN initiator, the final particle diameter is attained (24) Agarwal, G. K.; Titman, J. T.; Percy, M. J.; Armes, S. P. J. Phys. Chem. B 2003, 107, 12497.
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Figure 4. Aqueous electrophoresis curves obtained for the three pristine commercial alcoholic silica sols (MT-ST, 9; MA-ST, [; IPA-ST, 2) [open symbols] and the corresponding three polystyrenesilica nanocomposites [filled symbols] prepared with the cationic AIBA initiator using each of these commercial silica sols.
Figure 5. Electron microscopy images of (a) the original polystyrenesilica nanocomposite particles; (b) after treatment with 20% HF to remove the surface-adsorbed silica sol (note the smoother particle surface); (c) and (d) after heating to 800 °C to pyrolyze the polystyrene and produce free-standing hollow silica shells.
after around 12 h, whereas no further change in particle diameter was observed after 3-4 h for the two styrene polymerizations conducted with the AIBA initiator. This simply reflects the differing half-lives for these two initiators at 60 °C (approximately 440 min for AIBA and around 1300 min for AIBN). More importantly, no significant differences were observed for the particle growth curves obtained from the two AIBA-initiated polymerizations conducted in the presence and absence of the silica sol. This suggests that if surface polymerization does occur under these conditions it does not appear to affect the overall kinetics of polymerization. Aqueous electrophoresis measurements of the three types of nanocomposite particles indicated negative zeta potentials over the whole pH range, as observed for the three pristine silica sols (see Figure 4). This suggests that the surface of the nanocomposite particles is silica-rich. This finding is corroborated by the following observations. Treatment of one of the nanocomposites prepared with the 22 nm silica sol (entry 4 in Table 1) with excess 20% aqueous HF solution ensured complete digestion of the silica sol component but left the polystyrene component untouched. Subsequent examination by transmission electron
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microscopy revealed that the original surface roughness due to the presence of the nanosized silica sol was replaced with a much smoother particle surface, see Figure 5. Moreover, calcination of the same nanocomposite particles at 800 °C led to selective pyrolysis of the polystyrene component, leaving selfsupporting hollow silica shells. Thus, these three experiments are all consistent with the original polystyrene-silica nanocomposite particles having a “core-shell” morphology. This core-shell structure is best explained by consideration of the surface thermodynamics. The hydrophilic silica nanoparticles form the shells of the nanocomposite particles, remain solvated by the alcoholic continuous phase, and are responsible for the long-term colloid stability of these dispersions. On the other hand, the relatively hydrophobic polystyrene component forms the nonsolvated cores of the nanocomposite particles. Elucidation of the mechanism(s) of formation of such nanocomposite particles and strategies for increasing the silica sol incorporation efficiency in such syntheses will form part of our further work in this field.
Conclusions Submicrometer-sized polystyrene-silica nanocomposite particles can be readily prepared by alcoholic dispersion polymerization of styrene in the presence of ultrafine commercial alcoholic silica sols, provided that a cationic azo initiator is utilized. Particle diameters range from 331 to 464 nm, with silica
contents of up to 26% by mass. The incorporation efficiency of the silica sol within the nanocomposite particles is up to 40%, which is much higher than that found for the micrometer-sized silica-stabilized polystyrene particles obtained using a neutral azo initiator. The silica and polystyrene components of these new nanocomposite particles can be selectively removed by HF treatment and calcination, respectively. Such degradation experiments indicate that the nanocomposite particles have a welldefined core-shell morphology. Acknowledgment. The University of Sheffield is acknowledged for a Ph.D. studentship for A.S. Dr. J. Du and Mr. H. Bagshaw are thanked for assistance with TEM and SEM studies. Mr. R. Ducker is thanked for his assistance in handling HF. S.P.A. is the recipient of a five-year Royal Society-Wolfson Trust Research Merit Award. Nissan Chemicals (U.S.A.) is thanked for donating the alcoholic silica sols. Supporting Information Available: Control experiments (including three scanning electron micrographs) demonstrating that the cationic AIBA initiator is essential for successful nanocomposite formation. Langmuir-type isotherm (obtained by UV-visible absorption spectroscopy using the depletion method) for the adsorption of the AIBA initiator onto one of the alcoholic silica sols at 20 °C. This material is available free of charge via the Internet at http://pubs.acs.org. LA060308P
