Characterization of oil soluble brush-like latex particles
The schematic representation for the preparation of the oil-soluble PAA-PVK brushes is given in Figure 1. PAA latex particles were synthesized via inverse emulsion polymerization. The aqueous phase contained both acrylic acid and sodium acrylate, which was emulsified by Tween 80 and Span 80 in the oil phase to form W/O emulsion under mechanical stirring. Hexane and toluene was chosen as the continuous phase due to its low viscosity and lipophilic nature. The polymerization of acrylic acid was initiated by thermo-initiator AIBN. At the end of the polymerization, photoinitiator HMEM was added in starving condition to ensure a thin layer of HMEM on the surface of PAA core by copolymerization with the residual AA monomers. Finally, the grafting photo-luminescent monomers, VCz, were polymerized as chains affixed to the surface of PAA nanoparticles via photo emulsion polymerization, forming brush-like structure in oil-phase. The brush-like strucutre could endow the oil-soluble PAA-PVK latex nanoparticles with hydrophilicity in core layer and hydrophobicity in brush layer so it could be utilized to stabilize crude oil in water and reduce its viscosity by forming Pickering emulsions.
In previous reports[18], monodispersed PAA core was synthesized in paraffin. However, utilizing paraffin as the reaction media suffered from many drawbacks such as high viscosity and difficulty to desiccate and characterize. Therefore, solvent applied to disperse and obtain PAA core was changed to hexane to make it readily for further processing. The size and size distribution of PAA core synthesized in hexane is shown in Figure S1.The hydrodynamic diameter (D H) of PAA cores synthesized in hexane was to be around 100 nm and the optimal size distribution of PAA core synthesized in toluene and hexane fitted by Gaussian distribution is of a polydispersity index (PDI) below 0.01, demonstrating the PAA cores are mono-dispersed in hexane.
As polymer chains germinated from the surface of spherical PAA core via “grafting from” method, the hydrodynamic diameter increased and the growth of PVK brush layers could be monitored from the size varization by DLS. The size and size distribution of PAA core and PAA-PVK brushes with different monomer content was shown in Figure 2 (a). After grafting PVK chains through photo emulsion polymerization, theD H of PAA-PVK brushes with 5 %, 10 %, 15 % and 25 % VCz content exhibited marked increase from 100 nm to 132 nm, 164 nm, 192 nm and 238 nm respectively. Moreover, the layer thickness of PAA-PVK brushes, as determined by the size increase of the latex particles could be modulated by the dose of VCz monomer as shown in Figure 2 (b) and the monotonously increasing layer thickness with the VCz concentration could demonstrate that the PVK chains have been attached to the PAA core successfully. The polydispersity index of PAA core and PAA-PVK brushes of different VCz contents was 0.045, 0.034, 0.062, 0.071, 0.037 respectively, which remained within monodisperse range before and after grafting. Moreover, the synthesized PAA core and PAA-PVK brushes could remain stable for over two months. As shown in Figure S2, thanks to the steric protection effect of PVK chains, the average size of PAA-PVK brushes doesn‘t exhibit marked increase even after storage for durable period. The oil-solubility of the obtained PAA-PVK brushes was tested subsequently. After desiccation, the PAA-PVK brushes could be redispersed in various organic solvent like toluene, dimethylformamide (DMF) and N-methyl pyrrolidone (NMP). According to the size distribution of PAA-PVK brushes redispersed in these organic solvents in Figure S3, the overall size of PAA-PVK brushes remained consistent after drying and redispersing in different lipophilic media and the slightly larger size and PDI compared to the original state could be ascribed to partial aggregation during the desiccation process. Furthermore, compared to PAA core, the size variation of PAA-PVK brushes during redispersion process remain less evident due to the protection of PVK layers. Therefore, the stable lipophilic dispersion of the as-synthesized PAA-PVK brushes could ensure stable performance for various applications, such as crude oil emulsification and storage of oil-soluble drugs.
To further verify the structures of the latex particles, the TEM images of as-synthesized PAA core and PAA-PVK brushes were obtained as shown in Figure 3. In Figure 3 (a) and (b), the PAA core was clearly identified with well-defined spherical shape with average diameter of 100 nm and the result was in good agreement with DLS analysis. After grafting PVK chains onto the surface, the overall distribution was relatively good without apparent aggregation as shown in Figure 3 (c) and from the enlarged image of PAA-PVK brushes with higher resolution in Figure 3 (d), spherical shaped corona around the PAA core marked with red dotted circle could be clearly observed, indicating the existence of PVK brush layer around the PAA core. Moreover, measurements of the corona thickness in the dried state according to the TEM images reveal values of about 60 nm, coinciding with the DLS results.
Since PVK, a photo-luminescent polymer was grafted onto polymer beads, the oil-soluble PAA-PVK brushes could exhibit well-tuned fluorescence property. The UV-vis and fluorescent spectra of PAA core and PAA-PVK brushes dispersed in hexane is shown in Figure 4, in which the characteristic absorption peak at 328 nm and 340 nm and emission peak at 360 nm of PVK could be clearly observed for PAA-PVK brushes in both of the spectra. Moreover, with the increasing dose of polymerized VCz, PL intensity of PAA-PVK brushes enhanced correspondingly, which could serve as another convincing evidence of successful grafting of PVK chains. Interestingly, maximum emission peak exhibited evident red shift with increasing VCz monomer dose, which might be attributed to π-π stacking effect of the carbazole groups caused by the hydrophobic interaction between the polymer backbone of the grafted PVK chains[19-21].
Emerging as a powerful characterization method to present structural information of macromolecules, small angle X-ray scattering (SAXS) has been widely used to provide detailed information about the nanostructure of nanoparticles[22-24]. Therefore, SAXS was also utilized to characterize the core-shell structure and size distribution of PAA-PVK brushes. The scattering intensity curves of PAA core and PAA-PVK brushes and the optimized fits of the corresponding distribution of excess electron density Δρe(r) are displayed in Figure 5. As shown in Figure 5 (a), compared to PAA core, the scattering intensity of PAA-PVK brushes increased dramatically and the first maximum peak shifted to smaller q values, demonstrating the increased electron density due to the successful grafting of PVK chains onto the PAA core. Five-layer model was then chosen to analyze the structure of PAA-PVK brushes. From the radial distribution of excess electron density shown in Figure 5 (b), the average radius of PAA core and PAA-PVK brushes was about 34 nm and 108 nm, which was slightly lower than the analyzing results of DLS. The difference could be attributed to the different mechanism of these two characterization methods. According to previous reports[25-27], DLS is used for the detection of hydrodynamic diameter which is sensitive to the extension of exterior chains and determined by the longest chains of polymer brush while SAXS focuses on the average size calculated from statistical modeling analysis. Moreover, it is noteworthy that the dramatic decrease of excess electron density of the second layer could be clearly observed while the excess electron density declined slightly for outer layers, which differs from the gradual decreasing tendency observed in our previous studies of water-soluble SPBs by SAXS[23, 28]. Such phenomenon could be ascribed to the distinctive structure of the PAA-PVK brushes. Due to the hydrophobic nature of the PVK chains, the oil-soluble PAA-PVK brushes dispersed in hexane lacked the electrostatic repulsion prevalent in traditional spherical polymer brushes dispersed in water. Hence, the PVK brush layer may exhibit a locally collapsed state around the PAA core with chains coiling and entangled with each other while the end of the chains could stretch in the outer space, leading to the uneven distribution of surface electrons. Therefore, judged from results by SAXS, we could conclude that PVK chains have been successfully grafted onto PAA core, forming distinctive brush structure. Moreover, to deal with further applications such as oil-soluble drug delivery and storage, the loading position and stability could be precisely monitored in situ by SAXS.