Experimental
Materials Acrylic acid (AA, Lingfeng Chemical Reagent
Co. Ltd.) was purified through reduced pressure distillation to remove
polymerization inhibitor and stored in refrigerator in 4oC before use. N-Vinylcarbazole (VCz, 98%, Adamas)
was purified by recrystallization from methanol. Photoinitiator
2-[p-(2-hydroxy-2-methylpropiophenone)]-ethylene glycol-methacrylate
(HMEM) and water-soluble PS-PSS spherical polymer brushes were
synthesized and used as reported previously[18-20]. Span 80 and Tween 80 were purchased from
Sinopharm Chemical Regent Co., Ltd. The heavy crude oil sample was
kindly donated by Sinopec Shengli Oilfield, and its properties are
summarized in Table S1. All other materials of analytical quality were
from Lingfeng Chemical Reagent Co. Ltd. and used as received. All water
used in this work was purified by reverse osmosis and subsequent ion
exchange (Millipore Milli-Q).
Synthesis of PAA latex particles PAA latex particles
were prepared through inverse emulsion polymerization based on previous
reports [21]. In a typical reaction, 0.75 g Span 80 and 0.25 g Tween
80 were mixed in 34 g hexane as the continuous lipophilic phase and the
aqueous phase was prepared by dissolving 5 g AA in 7.5 g DI water
neutralized by 2.5 g NaOH. The aqueous phase was added into the oil
phase dropwise under mechanical stirring (rate: 300 rpm) to yield a
water-in-oil emulsion. The emulsion was degassed and charged with
nitrogen repeatedly to ensure oxygen-free environment. After adjusting
the temperature to 50 oC, 0.0625 g oil-soluble
initiator azodiisobutyronitrile (AIBN) dissolved in 5 mL toluene was
injected into the emulsion. The polymerization reaction lasted for 2.5 h
under nitrogen atmosphere, after which a mixed solution containing 0.2 g
photoinitiator HMEM, 2 g hexane and 0.5 g toluene was added at a rate of
6 seconds per drop. An extra 2.5 h was needed for HMEM to attach to the
surface of PAA particles. Finally, the obtained emulsion was filtered to
remove possible aggregation and then purified through dialysis against
hexane for three days.
Synthesis of polymer brush by photoemulsion
polymerization The purified PAA latex particles coated with a thin
shell of HMEM were diluted to 1 wt% with hexane and transferred into a
home-made photo reactor. Then grafting monomer VCz, with different
amount was added into the reactor and mixed with the latex particles by
vigorous magnetic stirring. After repeating evacuation and subsequent
charging of nitrogen for three times, the whole reactor was exposed
under UV light (wavelength: 200-600 nm, power: 150 W) to conduct photo
emulsion polymerization for 2.5 h. Finally, the prepared PAA-PVK polymer
brush was purified by dialysis against hexane to remove unreacted
monomers. The relative monomer content w was used to designate PAA-PVK
brushes prepared at different VCz dose and it could be calculated by the
following equation:
\begin{equation}
w=\frac{m_{\text{VCz}}}{V_{\text{PAA}}x_{\text{PAA}}}\nonumber \\
\end{equation}Where mVCz, VPAA and
xPAA refers to the mass of VCz monomer, the volume and
solid content of PAA latex particles. In our experiment, PAA-PVK brushes
with relative monomer content up to 5 %, 10 %, 15 % and 25 % was
prepared for further application devlopment.
Preparation of Oil-in-Water (O/W) emulsions The PAA core
and PAA-PVK brushes of 25 % monomer content (solid content: 250 mg/mL)
was chosen as the oil-in-water emulsifier. The oil-in-water emulsions
were generated by adding 1 mL obtained latex particles into 25 mL of the
mixture of oil and water and then homogenizing for certain times. For
model oil, the emulsion was prepared by sonication for 5 minutes with
the volume ratio of water/decane set as 3:7. 53 KHz and 350 W were
chosen as the sonication parameters by employing a sonicator (SK7210LHC
from KUDOS Co.) in the entire experiments. For crude oil, the emulsion
was prepared by homogenizing with a mechanical stirrer Ultra-Turrax T25
for 5 minutes (15000 rpm) with the mass ratio of water/crude oil set as
5:1. The dispersion state and morphology of oil-in-water emulsions was
observed by polarizing optical microscopy (Leica DM2500 P microscope)
and the apparent viscosity of the crude oil emulsions was recorded by
rheometer (Anton Paar, MCR 501).
Characterizations Dynamic light scattering (DLS) was
carried out using a particle sizing system, NICOMP 380 ZLS equipped with
a scattering angle of 90 °. Transmission electron microscopy (TEM) was
performed by a JEOL JME-1400 transmission electron microscope at
acceleration voltage of 100 kV. TEM samples were prepared by adding one
drop of particle solution on a copper grid (300 mesh) and then dried at
ambient environment overnight. The optical properties of PAA-PVK brushes
were characterized by UV-3250 UV-Vis spectrophotometer, (SHIMADZU) and
RF-5301PC fluorometer (SHIMADZU). SAXS measurements were performed at
BL16B1 beamline in Shanghai Synchrotron Radiation Facility (SSRF,
China). The detector used was MAR165 CCD detector and the
sample-to-detector distances were set as 5 m. During each SAXS
measurement, 0.1-0.2 mL solution was added into a sample cell by syringe
and the sample cell was a 1-mm-thick polystyrene plastic template
wrapped by polyimide film on both sides. The morphology of emulsion
droplets was observed with a Leica DM2500 P microscope. A small drop of
emulsion was placed on a microscope slide for observation and images
were captured with a charge-coupled device camera connected to a
computer via WT-1000GM imaging board. Software ImageJ was then utilized
to analyze the average size and size distribution of the formed
emulsions. The apparent viscosity of crude oil emulsions in different
temperatures was tested by Anton Paar MCR 501 rheometer with 30 points
recorded at a interval of 10 s. The shear rate was set as 100
s-1 and apparent viscosity is recorded at temperature
range of 25-70 °C.