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.