Synthesis of Polystyrene-Silica Composite Particles via One-Step Nanoparticle- Stabilized Emulsion Polymerization

Transcription

1 Synthesis of Polystyrene-Silica Composite Particles via One-Step Nanoparticle- Stabilized Emulsion Polymerization Huan Ma and Lenore L. Dai* School of Mechanical, Aerospace, Chemical and Materials Engineering Arizona State University, Tempe, AZ *Corresponding author. address: 1 Introduction Polymer composites are ubiquitous in daily lives such as in automotives, aircrafts, electronics, appliances, and sporting goods. Conventional polymer composites are made by post-formulating and mixing polymer resins and inorganic fillers [1]. In conventional polymer composites, the inorganic fillers are randomly dispersed in the polymer matrix, often in the format of aggregation. The inclusion of inorganic fillers significantly increases the physical, such as mechanical, optical, electrical, and thermal properties of the polymer matrixes [2-5], although the randomness and aggregation often result in uncontrolled or limited property enhancement. Here we report the success of preparing core-shell structured polymer composite particles via one-step nanoparticle-stabilized emulsion polymerization. It is worthwhile to note that the composite structure here is opposite from the often reported coreshell structure in which inorganic particles serve as the core and polymer serves as the shell [6-11]; here the polymer serves as the core and the inorganic particles serve as the shell. Such materials provide a new class of building blocks and can exhibit unusual, possibly unique, properties which cannot be obtained simply by co-mixing polymer and inorganic particles. [12] Surprisingly, there has been remarkably little work in this field. Our approach continues the very recent effort of emulsion or dispersion polymerization to fabricate this class of materials, in addition to the developed post-surface-reaction [13], electrostatic deposition [14], and layer-by-layer self-assembly [7,15-17]. In the past several years, new chemical methods have been utilized to prepare organicinorganic composite particles, including emulsion [18-21], dispersion [22-27], miniemulsion [28-32], and inversed emulsion polymerization [33]. The stabilizing effect is usually provided by surfactants, co-monomers, initiators, or cooperatively with nanoparticles. Very recently, polymerization solely stabilized by nanoparticles has received increasing attention, due to its simplified composition and preparation steps. However, the polymerization mechanism and the nanoparticle incorporation mechanism are still unclear and currently under active investigation [20,34]. During the progress of this work, Schmid and coworkers [22] reported the dispersion polymerization of styrene using silica nanoparticles as the sole stabilizing agent. Polystyrenesilica core-shell composite particles were successfully prepared by one-step alcoholic dispersion polymerization using cationic initiator AIBA (2,2 -azobix(isobutyramidine) dihydrochloride) and negatively-charged silica nanoparticles. The silica incorporation is likely promoted by the electrostatic attraction between the cationic initiator and the anionic nanoparticles [22]. This may be evidenced by the fact that in the absence of the electrostatic interaction, the work using nonionic initiator AIBN (2,2 -azobisisobutyronitrile) only yields latex 1

2 particles with surface Si/C ratio of 0.01 [22]; in other words, the latex particles are poorly covered with silica nanoparticles. Here we report the success of preparing polystyrene-silica core-shell structured composite particles by emulsion polymerization using a nonionic initiator, VA-086 (2,2 -azobis(2-methyl-n-(2-hydroxyethyl) propionamide)), and discuss the role of silica nanoparticles as the sole stabilizing agent. 2 Experimental 2.1 Materials IPA-ST silica solution, obtained from Nissan Chemicals, is nm silica nanoparticles dispersed in 2-isopropanol. The silica concentration is 30-31% by weight. Nonionic azo initiator VA-086 (98%, 2,2 -azobis(2-methyl-n-(2-hydroxyethyl)propionamide)), styrene monomer (99.9%), and HPLC grade water were purchased from Wako Chemicals, Fisher Scientific, and Acro Organics, respectively. All materials were used as received. 2.2 Synthesis of polystyrene-silica composite particles First, water, IPA-ST and styrene were agitated mechanically at 600 rpm for 8 minutes using Arrow 6000 (Arrow Engineering) in an ice bath to emulsify. Second, the emulsion was degassed with nitrogen and kept in nitrogen atmosphere under magnetic stir. When the temperature was raised to 70 C, the initiator solution was added to start the polymerization. The composite particles were sampled at different time intervals ranging from 3 hours to 24 hours. Before characterization, samples were washed twice by centrifuging-redispersing cycles using an Eppendorf 5810R centrifuge. In each cycle, the sample was centrifuged at 7000 rpm for 5 min, the supernatant was replaced with water and the sediment was redispersed by shaking manually. 2.3 Hydrofluoric (HF) etching To remove the silica shell, HF etching procedure described by Han and coworkers [35] was adopted. Approximately 1 ml of original composite particle dispersion was added to 50 ml 10% HF aqueous solution. Then the mixture was stirred at room temperature (approximately 23 C) for 2 hours. After settling for several hours, the supernatant was replaced with water for at least three times and the sediments were obtained. 2.4 Particle size, morphology and composition characterization Particle size and distribution were measured by a Microtrac Nanotrac Particle Size Analyzer using the dynamic light scattering (DLS) technique. The washed composite particles were further dispersed to proper concentrations with water before measurements. Each sample was tested for at least twice and had consistent results. A scanning electron microscope (SEM, Hitachi S-4300) was used to observe the surface morphology of the composite particles. SEM samples were prepared by drying a drop of washed composite particle dispersion on newly cleaved mica substrate, and applying Au/Pd coating of nm thick using a sputter coater. In order to obtain the cross-section image of the composite particles, the particles were firstly embedded in a LX-1122 epoxy resin block and cut into 90-2

3 150 nm sections using a Reichert-Jung UltraCut E microtome equipped with a glass knife at room temperature. Then, the thin sections were collected with copper grids and observed using the transmission electron microscope (TEM, Hitachi H-8100). The chemical composition of the nanoparticles were analyzed using energy dispersive X-ray spectrometry (EDX), built in the S-4300 SEM, and thermogravimetric analysis (TGA, Mettler-Toledo SDTA851e). In EDX, pure carbon substrate was selected. In TGA, Samples were heated to 800 C at 10 C/min in air. 3 Results and Discussion It is worthwhile to note that we carefully selected VA-086 as the initiator. VA-086 is a water-soluble nonionic initiator and no success has been reported in surfactant-free emulsion polymerization of styrene [36]. In order to verify the sole stabilizing effect of silica nanoparticles, emulsifier-free emulsion polymerization using VA-086 as the initiator in the absence of nanoparticles was performed. No polystyrene particle formation was observed in the product, evidenced by SEM experiments. These experiments show that the initiator VA- 086 has little effect on stabilizing the system in emulsion polymerization and therefore silica nanoparticles are the only source of stabilizer when present. In addition, VA-086 is neutral in charge thus is expected to minimize any electrostatic interactions with the negatively-charged silica nanoparticle surfaces which may complicate identifying silica nanoparticles as the sole stabilizer in emulsion polymerization. (a) (b) (c) (d) Figure 1. (a) An SEM image of composite particles, (b) a TEM image of cross-sectioned composite particles, (c) an SEM image of composite particles after HF etching, and (d) the EDX spectrum of composite particles. All images are representative and the scale bars in (b) and (c) represent 200 nm. 3

4 Unless noted, composite particles were prepared by formulating 8 ml styrene, 52.5 ml water, 20 g IPA-ST, and 0.06 g initiator VA-086. Figure 1(a) is a representative SEM image of composite particles sampled at 5 hour reaction time. The roughness of the composite particle surfaces suggests that the composite particles are covered by silica nanoparticles. The particle sizes, characterized by DLS, are ± 51.6 nm in diameter with mono-distribution (the standard deviation here represents the width of the particle size distribution), consistent with SEM observations. The core-shell structure can be clearly observed in the TEM image presented in Figure 1(b). In many regions, the thickness of the shell is close to the size of one silica nanoparticle (10-15 nm), which may suggest a monolayer coverage. The silica shell can be removed by excess amount of hydrofluoric acid (HF) solution. As shown in Figure 1(c), after HF etching treatment, the particle surfaces become smooth and reveal the polystyrene core. The particle composition was characterized by EDX and TGA. The presence of Si and O elements in the composite particles is indicated by the EDX analysis, as shown by the intensity peaks in Figure 1(d). The silica content is quantitatively determined by TGA. We assume that the major weight loss during heating is associated with the thermo-oxidative degradation of polystyrene and the residue close to 800 C is solely silica. The silica content is approximately 20 wt%, which is significantly higher than the silica content of particles (1.1 wt%) prepared by the dispersion polymerization using nonionic initiator AIBN [22]. The improvement is likely due to the distinct polymerization mechanisms. In contrast to the dispersion polymerization in which the polystyrene monomers are dissolved in alcohols, the emulsion polymerization here contains distinguished liquid-liquid interfaces due to the immiscibility between the monomers and the aqueous continuous phase. Thus the nanoparticles, even in the absence of electrostatic interactions, are thermodynamically favorable to self-assemble and remain at the liquid-liquid interfaces, following the same argument in sold-stabilized emulsions [37-42]. At the initial stage of polymerization, the nanoparticles provide stability to the monomer droplets. During the nucleation stage, silica nanoparticles remain at the interfaces between the monomer and continuous phases. It is worthwhile to note that the role of silica nanoparticles described here is not the same as that in the polymerization involving oppositely charged initiator and nanoparticles [22]. In the latter case, the initiator molecules or residues adsorb onto the silica nanoparticle surfaces after initiation [22] thus the silica nanoparticles function as the surface-active initiator residue in that polymerization. This is our first attempt of applying the concept of solid-stabilized emulsions in polymerization, after studying the fundamentals of solid-stabilized emulsions [40-42], utilizing them as templates to investigate the dynamics of particles [43-45], and developing microrheology at liquid-liquid interfaces [46-47]. Such application may open new avenues of synthesizing advanced materials utilizing the concept of solid-stabilized emulsions. It is also worthwhile to note that the synthesized core-shell composite particles have various potential applications in photonic crystals, optical switches, and display devices. For example, recent work [48] demonstrates that thin films made of polystyrene-silica core-shell composite particles remain transparent, even if the silica loading is as high as 39%. The transparency is due to the well-dispersibility of silica (shell) in the polymer matrix, which cannot be achieved simply by co-mixing the polystyrene and silica particles. 4

5 600 (a) 3 h (b) 11 h (c) 24 h 500 Particle size [nm] (d) 3 h (e) 11 h (f) 24 h Reaction time [h] Figure 2. Plot of particle size at 50% of the particle size distribution versus reaction time and representative SEM images with different initiator concentrations: 0.83 wt% (, inset images a, b and c), 2.5 wt% ( ) and 4.2 wt% (, inset images d, e and f). The error bars indicate the width of the particle size distribution and the scale bars represent 100 nm. In addition, the influence of initiator concentration was studied by increasing the initiator amount from 0.06 g to 0.18 g and 0.30 g (0.83 wt%, 2.5 wt% and 4.2 wt%, respectively, based on monomer amount), maintaining other experimental parameters. Samples were taken at different time intervals between 3 and 24 hours and analyzed by SEM and DLS. Figure 2 is the plot of particle size measured by the dynamic light scattering (DLS) technique versus the reaction time. During the first 3 to 7 hours of reaction, the particles in all three systems experienced comparable particle size growth. The two systems with higher initiator concentrations ceased to grow soon after 7 hours of reaction time, whereas the system with the lowest initiator amount continued to grow significantly and produced the largest composite particles at 24 hour reaction time. The trend is confirmed by SEM experiments (images not shown). Comparing the particle sizes at 24 hour reaction time, a decrease in particle size occurred when the initiator amount was increased from 0.06 g to 0.18 g. The dependence of particle size on initiator concentration herein is in reverse trend with that in solid-stabilized alcoholic dispersion polymerization reported by Schmid and coworkers [22]. The difference probably originates from the distinct particle growth mechanisms [49]. In our emulsion polymerization, a larger number of nuclei likely formed at the initial stage of polymerization, so the particle size decreased with increasing initiator concentration, given a fixed amount of monomer and assuming the same monomer conversion. Further increasing the initiator amount from 0.18 g to 0.30 g did not have significant influence on particle size. One possible explanation would be that the available amount of silica nanoparticles did not provide sufficient 5

6 stability to an even larger number of nuclei. With the particle size growth, the changes in surface coverage occurred and were observed using SEM. Selected representative images are shown in the insets of Figure 2. In most cases, silica coverage decreased with reaction time after 5 hours. This is likely due to the increase of surface area during particle growth. One exception is the system with the highest initiator concentration (4.2 wt%, based on monomer amount). An increase of silica coverage at 24 hour reaction time was repeatedly observed. The underlying mechanism for the unexpected coverage increase at 24 hour reaction is still unclear. To the best of our knowledge, this is the first study of tracking the change of surface coverage along the reaction time. 4 Conclusions In summary, polystyrene-silica core-shell structured composite particles were successfully synthesized by nanoparticle-stabilized emulsion polymerization. Silica nanoparticles act as the sole stabilizer during the emulsion polymerization, following the same mechanism in solid-stabilized emulsions. In the polymerization formulated with 8 ml styrene, 52.5 ml water, 20 g IPA-ST, and 0.06 g initiator VA-086, composite particles sampled at 5 hour reaction time exhibit core-shell structure in which the polystyrene core is covered by silica. The silica content, determined by TGA, is approximately 20 wt%. In addition, we further explore the polymerization mechanism by studying the particle growth as a function of initiator concentration and reaction time: when the initiator/monomer ratio is increased from 0.83 wt% to 2.5 wt%, the particle size at 24 hour reaction time decreases for a fixed monomer amount, probably due to a larger number of nuclei at the initial stage of polymerization. The dependence of particle size on initiator concentration herein is in reverse trend with that in solid-stabilized alcoholic dispersion polymerization [22]. Further increasing the initiator/monomer ratio to 4.2 wt% does continually decrease the particle size, which may be limited by the stabilization provided by a fixed concentration of silica nanoparticles. The surface coverage also changes with initiator concentration and reaction time although the underlying mechanism is not fully understood. Acknowledgements We would like to acknowledge the Department of Chemical Engineering, the Department of Chemistry, and the Imaging Center at Texas Tech University for instrumental usage and the National Science Foundation for financial support (CBET and CBET ). References [1] P.C.P. Watts, W.K. Hsu, G.Z. Chen, D.J. Fray, H.W. Kroto, D.R.M. Walton, J. Mater. Chem. 11 (2001) [2] C. Sanchez, B. Julian, P. Belleville, M. Popall, J. Mater. Chem. 15 (2005)

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