Fabrication of covalently-bonded polystyrene/SiO₂ composites by Pickering emulsion polymerization

By variety of amount of MPTMS and modification time, SiO₂ particles modified with different wettability were prepared, denoted as Si-10a (MPTMS employed equal to10 wt% of SiO₂ amount and the modification lasted for 8 h), Si-10b (10%, 24 h), Si-15 (15%, 24 h) and Si-30 (30%, 24 h).

The adsorption of colloidal particles at oil/water interface is a crucial factor for preparing stable Pickering emulsions [3, 17]. In order to probe into the wettability of the particles, their behavior when they are suspended in a styrene/water dual-phase mixture was observed. Three drops of SiO₂ suspension were dropped into styrene/water mixture, followed by violent shaking and settling undisturbed. Particles with different modified procedures present different behavior as shown in Fig. 1. Original silica particles disperse only in water due to hydrophilic Si-OH groups on their surface, while Si-30 particles disperse preferentially in styrene due to their hydrophobicity caused by modified MPTMS. Particles with middle degree of modification (Si-10a, Si-10b and Si-15) assemble spontaneously at the water-styrene interface.

The difference of distribution behavior is originated from the difference of MPTMS amount on the particle surface, as evidenced straightly by FTIR spectra in Fig. 2 and TGA in Fig. 3. In FTIR spectra, besides the characteristic bands of SiO₂ at 3,549 cm⁻¹ (-OH stretching), 1,106 cm⁻¹ (Si-O stretching) and 476 cm⁻¹ (Si-O bending vibration), the modified SiO₂ present characteristic bands of MPTMS at 2,985 cm⁻¹ (C-H stretching) and 1,710 cm⁻¹ (C = O stretching), which indicates that reactive MPTMS molecules have bonded to the surface of SiO₂ particles. In Fig. 3, the weight loss observed between 50 °C and 150 °C is attributed to the elimination of adsorbed water. The weight loss of 3.8% (Fig. 3, b) and 7.7% (Fig. 3, c) from 300 °C to 350 °C is assigned to the decomposition of MPTMS. It can be inferred that the grafted ratio of added MPTMS was approximately 51.6% and 34.9% for Si-10b and Si-30, respectively.

It is well established that the effective stabilization of Pickering emulsion is originated from a dense monolayer or multiplayer film formed by particles adsorbed irreversibly on the interface [17‐20]. Many factors, such as character of particles, ratio of phases, pH value and particle concentration, have effect on stabilizing efficiency of particles. In case of spherical particle, the crucial parameter is its contact angle θ which indicates the wettability or hydrophilic-hydrophobic balance of the particle [7, 17, 21]. It has been empirically proved that the phase that preferentially wets the particles becomes the continuous phase of Pickering emulsion.

In this paper, contact angle of SiO₂ particle was determined directly by a contact angle goniometer using water (denoted as θ_aq) and 1-dodecanol (denoted as θ_oil) respectively. 2 uL of water or 1-dodecanol (heated to 40 °C) was putted on a SiO₂ flake obtained by 30 Mpa compressor. The results were shown in Table 1. As the increase of MPTMS amount, θ_aq increases while θ_oil decreases. The results confirmed that the wettability of nano-SiO₂ particles was tunned significantly by modification of MPTMS. It is worthy noting that relative value of θ_aq and θ_oil is unqualified to evaluate the hydrophilic or hydrophobic character, because the method employed to detect θ is a relative method and the surface tension of water (72.8 mN/m) is much larger than that of 1-dodecanol (26.9 mN/m) [22].

Using different SiO₂ particles, O/W type and W/O type emulsions were prepared as shown in Fig. 4a. No emulsion formed when original SiO₂ employed as stabilizing particles due to its high hydrophilicity. W/O type emulsion formed when Si-30 was used because Si-30 particles are wetted preferentially by styrene while O/W type emulsions obtained when Si-10a, Si-10b and Si-15 was used individually. The formation mechanism of Pickering emulsion was illustrated in Fig. 4c. When water-wetted preferentially particles were used, a larger region of particle immerses in water and the curve line deflects alongside styrene droplets. As a result, O/W emulsion forms. When oil-wetted preferentially particles were used, the curve line deflects alongside water droplets and W/O emulsion forms. The emulsion cream quickly after preparation because the droplets are so big as to 20 ~ 50 μm (see Fig. 5 below) and density of styrene is much lower than that of water.

The morphology of droplets was examined by optical microscopy with a high performance digital camera. The diameter of droplets was demarcated by a scale-mark software. The droplets of both O/W and W/O emulsions are spherical with similar size distribution from 20 to 50 μm (Fig. 5). However, the droplets present different clustering character. Droplets stabilized by Si-10a is well dispersed (Fig. 5a), while most of the droplets stabilized by Si-15 (Fig. 5c) cluster together in a disordered fashion. All emulsions are very stable to coalescence and Ostward ripening. Even one month after preparation, the appearance and droplet size have no appreciable change as shown in contrast by Fig. 4a to Fig. 4b and Fig. 5b to Fig. 5e), respectively.

In this study, O/W emulsion was employed to prepare PS/SiO₂ composites by radical polymerization initiated by oil-soluble AIBN. The suspension obtained after polymerization was diluted and dropped onto a glass slide, followed by volatilizing to dry and being characterized by optical microscope. The optical micrograph of composites using 2.0 wt% of Si-10b as stabilizer was shown in Fig. 6a. The composites are spherical with the diameter of 20 ~ 50 μm, which is consistent with the droplet template. SEM micrograph in Fig. 6b and c indicates that the composites have rugged surface. This morphology may be attributed to two mechanisms. Firstly, SiO₂ aggregations form during polymerization and locate on the surface. Secondly, PS particles formed by secondary nuclei assemble on the surface of composites. Although most of hydrophobic AIBN decomposes and initiates polymerization inside the droplets, a small amount of decomposed initiator diffuses into the aqueous phase because of its slight solubility in water, and initiates the monomer dissolved in aqueous phase [23]. After the oligomer reaches a critical length, it no longer dissolves in water, but precipitates to form secondary nuclei. SiO₂ particles are too large in quantity to move as free as low molecular surfactant employed in traditional emulsion, so the secondary nuclei cannot enter the droplet as freely as those in traditional emulsion polymerization. The secondary nuclei absorb monomer and polymerize on the droplet surface, resulting in the formation of micron-sized PS particle on the surface of prepared composites.

In order to verify that chemical bonds exist between PS core and SiO₂ shell, FTIR spectra were employed. The composites were dissolved in toluene, followed by centrifugation to separate SiO₂ precipitation and PS solution. The PS solution was diluted into ethanol to precipitate PS and the precipitation SiO₂ was redispensed in toluene and separated by centrifugation twice to remove residual PS. FTIR spectra of Si-10b, PS/SiO₂ composites, PS and SiO₂ separated from composites were shown in Fig. 7. No particle larger than 1 μm presents in the optical micrograph of precipitated SiO₂ (shown in Fig. 7, e), from which we can infer that all composites were destroyed by toluene and PS ungrafted on SiO₂ was removed entirely from the precipitation SiO₂. Both the characteristic bands of SiO₂ (3,549, 1,106 and 476 cm⁻¹) and polystyrene (1,650 ~ 1,400, 760 and 693 cm⁻¹) present in spectrum of composite (Fig. 7, b), verifying that PS/SiO₂ composites were successfully synthesized. Furthermore, the characteristic bands of PS also present in the spectrum of precipitated SiO₂ (Fig. 7, c), from which it can be inferred that the C = C bands on SiO₂ particle surface have polymerized with styrene, thus PS core and SiO₂ shell were covalently bonded in the prepared composites.

It has been verified that particle concentration has an important effect on the properties of Pickering emulsion. In this work, the effect of SiO₂ concentration on polymerization and morphologies of PS/SiO₂ composites were studied. After polymerization, the appearance of emulsion changed from cream to suspension of individual composites in case of SiO₂ concentration as 4% and 2% or conglutinating blocks in case of SiO₂ concentration as 1% and 0.5%. Figure 8 shows the morphologies of composites prepared with different concentration of Si-10b. The difference of result originates from the difference of particle coverage, defined as the percent of droplet surface covered by particles. Large amount of stabilizing particles results in a denser layer to protect droplets against coalescence and high stability during polymerization. Consequently, individual composites form when SiO₂ concentration is larger than 2.0%. When SiO₂ concentration is 1.0%, the stability of emulsion decline during polymerization and some SiO₂ particles detach away from the droplet surface to the continuous phase, which results in the formation of SiO₂ aggregation as shown in Fig. 8c. When SiO₂ concentration is 0.5%, the emulsion changed to conglutinating blocks containing aggregated SiO₂ and composites.

The average diameter of emulsion droplets increases as the particle concentration decreases [24, 25]. In this study, size distribution of prepared composites at different SiO₂ concentration was illustrated in Fig. 9. The average diameter of composites obtained at SiO₂ concentration of 4%, 2% and 1.0% was 13.6 μm, 26.4 μm and 48.1 μm, respectively. Especially, size distribution of composites obtained at SiO₂ concentration of 1.0% is shown exponentially in the inset graph in Fig. 9. Three kinds of different particle with average diameter of 70 nm, 420 nm and 48.1 μm may be corresponded to original Si-10b particles, aggregations of Si-10b particles and composites respectively. This is consistent with the optical micrographs as shown in Fig. 8c.