PAL-stabilized pickering emulsion
PAL is a kind of natural fibrous phyllosilicate mineral [33]. As shown in Fig. 1, the size of the PAL fibrous crystal used in this research is typically with 0.5-2 µm in length. It is a very effective emulsifier for O/W type pickering emulsion because of its high aspect ratios, suitable hydrophilicity and large surface area.
Because of the plenty of silanol groups on their surfaces, the PAL fibers can be well dispersed in water, forming very stable suspensions. By adding an amount of octane solution that containing MMA, EGDMA and I819 as oil phase in such PAL water suspension, an oil in water (O/W) type Pickering emulsion can be formed after emulsification. Figure 2a provides digital photographs of Pickering emulsions prepared with different percentage of PAL fibers. As can be seen, the emulsion prepared with a low percentage of 1.5 wt% PAL fibers is not stable. Clearly, a phase separation occurred. In comparison to octane and monomer, the density of water is higher. Thus, the formed transparent liquid at the bottom of the bottle is separated water from the emulsion. Along with the increasing of PAL concentration, the fraction of water separated from the emulsion is decreasing (Fig. 2b). When the PAL percentage increased to 3 wt%, there is almost no water separated from the emulsion. Direct evidence was provided by the polarizing microscopy characterization. Figure 2c illustrates the polarizing micrographs of pickering emulsion stabilized by 3 wt% of PAL fibers. As can be seen, there are plenty of spherical rings. Each ring is composed by a light halo surrounding a dark and round spot. As the PAL crystal has birefringent properties, such phenomenon only can be observed during the PAL fibers absorbed on the surface of the emulsion droplets and nearly wrapped up the droplet surface. It suggests that the PAL fibers formed a close packing structure on the emulsion droplet surfaces, which is crucial for the solid particles serving as particulate emulsifier in literature [34].
Preparation and characterization of Polymer/PAL composite microcapsules
The Pickering emulsion photopolymerization can be carried out conveniently. Different experimental conditions were further investigated. Figure 3 displays the morphology of the PMMA/PAL microcapsules prepared with different concentration of PAL fibers. Some spherical microcapsules can be observed only with 1.5 wt% PAL fibers (Fig. 3a). Many fragments of microcapsules also can be seen. Interestingly, the number of formed microcapsules is increased along with the increasing of PAL concentration (Fig. 3b-d). When the PAL concentration reached to 3 wt%, plenty of of PMMA/PAL microcapsules were formed (Fig. 3d). The fragments of microcapsules almost disappeared. However, only broken microcapsules were observed by further increasing the PAL concentration to 3.5 wt% (Fig. 3e) or even higher. It can be expected that the formation of microcapsules became better along with the increasing of PAL concentration as the emulsion became more and more stable. When the PAL concentration reached to 3.5 wt%, the mechanical properties of the formed PAL/PMMA composite shell is not enough to support the microcapsules maintain their structure. The high PAL concentration partially impeded the photopolymerization and probably it leads to such result.
SEM image of PMMA/PAL microcapsules prepared with 3 wt% of PAL fibers (corresponding to Fig. 3d) at higher magnifications is displayed in Fig. 4a. The PAL fibers can be seen clearly that dispersed randomly on the surface of microcapsule. Such result is quite in accordance with the observation of the polarizing micrographs of the emulsion (Fig. 2c). It further proves the PAL fibers was absorbed on the surface of the emulsion droplets and played as the role of particulate emulsifier. Figure 4b shows the SEM image of PMMA/PAL microcapsules after grinding. It demonstrates that the hollow structure of the microcapsule is formed successfully via such pickering emulsion templated photopolymerization process. The PMMA/PAL shell of the microcapsule is very thin, only with few hundred nanometers.
Figure 5 shows the FTIR spectra of PAL fibers and PMMA/PAL microcapsules prepared with 3 wt% of PAL fibers. The absorption bands located at 3,800-3,395 cm− 1 in Fig. 5a can be attributed to the hydroxyl groups of coordinated water in the tunnels of PAL crystals [35]. The absorption bands at 1,030 cm− 1 and 982 cm− 1 can be ascribed to the stretching and the bending vibrations of Si-O-Si bonds [36]. Such absorption bands also can be seen in Fig. 5b, which is the FTIR spectrum of PMMA/PAL microcapsules. Furthermore, the presence of absorption bands at 2970 cm− 1 can be ascribed to C-H stretching vibration of methyl group. The sharp and strong absorption bands at 1732 cm− 1 is the characteristic carbonyl stretching vibration of ester group. These results indicated that the PMMA/PAL composite has been fabricated successfully.
Figure 6 further illustrates the monomer conversion value of emulsions with different percentage of PAL fibers after the photopolymerization process. The monomer conversion is decreased along with the increasing of PAL concentration. When the PAL concentration is 1.5 wt%, the monomer conversion is 83.6%. By increasing the PAL concentration to 3.5 wt%, the monomer conversion decreased to 70.2%. Such result is probably because of the light scattering induced by the PAL fibers located at the surface of emulsion droplets (Fig. 2c and Fig. 4a). It hinders the photolysis of photoinitiator to initiate the radical polymerization of monomers. The higher amount of PAL concentration will scatter more light, which lead to the lower monomer conversion.
After the investigation of the PAL concentration effect, further studies on the influence of photoinitiator concentration were carried out. Figure 7 displays the SEM images of PMMA/PAL microcapsules prepared with different concentration of photoinitiator. No microcapsule or microsphere is formed only with 2 wt% of I 819 photoinitiator (Fig. 7a). The number of formed microcapsules is increasing along with the growth of the photoinitiator concentration (Fig. 7b-c). As can be seen from Fig. 7c, there are plenty of microcapsules when the concentration of I 819 reached to 6 wt%. As it is well-known, 2 wt% of photoinitiator is sufficient enough in most cases for the conventional radical photopolymerization [37]. However, here it required a higher photoiniatiator concentration due to the light scattering of the solid emulsifier in Pickering emulsion photopolymerization.
Figure 8 shows the SEM images of PMMA/PAL microcapsules prepared under different light irradiation intensity. As can be seen from Fig. 8a, plenty of PMMA/PAL microcapsules were formed with 20 mW/cm2 irradiation intensity. The morphology of the most microcapsules was not spherical. Along with the decreasing of light irradiation intensity, the spherical microcapsules became more and more. Besides, the number of broken microcapsules was increasing. As it is well-known, the photoinitiator decomposed into free radical fragments will be faster with higher light irradiation intensity for photopolymeriztion process, leading to higher polymerization rate and lower molecular polymer chain [38]. In this case, the light reached to the photoinitiator that inside of the emulsion droplets is decreased dramatically due to the light shielding of PAL fibers. It leads to the insufficient polymerization and the large amount of broken microcapsules (Fig. 8c). Combining the removal of octane, the shell composed by low molecular weight polymer (high light irradiation intensity) and PAL fibers couldn’t afford the shrinkage, inducing the non-spherical morphology of the formed microcapsules.
After optimal the light intensity, further studies were carried out on the crosslinker and monomer radios. Figure 9 displays the SEM images of PMMA/PAL-3 microcapsules prepared at different volume ratios of crosslinker (EGDMA) and monomer (MMA). When the EGDMA/MMA ratio was 6/10, many cracked microcapsules were formed (Fig. 9a). Obviously, spherical shape microcapsules were formed and their number was increased along with the increasing of the EGDMA/MMA ratio (Fig. 9b and Fig. 9c). However, only fragments of microcapsules could be observed by further increasing the EGDMA/MMA ratio to 12/10 (Fig. 9d).
However, it is still unclear whether a wide range of radically polymerizable monomers can be used for the preparation of microcapsules via Pickering emulsion templated photopolymerization process. Figure 10 shows the SEM images of Polymer/PAL-3 microcapsules prepared with different monomers (MA, MMA, BA, BMA, St and DEAEMA). Interestingly, the microcapsules can be formed for all of the monomers. However, the best monomer for the formation of spherical microcapsules is DEAEMA (Fig. 10f). In comparison to MA monomer (Fig. 10a), the formation of microcapsules is better by using BA as monomer (Fig. 10c). It is probably because the monomer of BA has a butyl tail, which endowed the formed poly(n-butyl acrylate) with better toughness [39]. Such phenomenon is really interesting, but it still requires further investigation in order to find out the proof.
The formation mechanism of PMMA/PAL microcapsules was schematically displayed in Fig. 11. Firstly, the homogeneous oil phase of octane solution that containing monomers and photoinitiators was dispersed in PAL water gel. The oil droplets were surrounded by PAL fibers and forming stable oil in water type Pickering emulsion, as it has been proved by polarizing microscopy observation in Fig. 2c. In the second step, the formed Pickering emulsion was further polymerized via light irradiation in a homemade quartz reactor. The formation of PMMA polymers that induced them separated from the octane in each PAL wrapped droplet, as the octane is a poor solvent for the polymer. Along with the light irradiation, the droplets became PMMA/PAL microcapsules which encapsulated octane inside. Finally, PMMA/PAL microcapsules were achieved by removing the octane.