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.