Preparation and characterization of the nano-promoters
The nano-promoters were prepared through soap-free emulsion polymerization with St as the hydrophobic monomer, while polymerizable monomers and KPS were applied as the hydrophilic monomers and initiators, respectively. Two polymerizable monomers with different hydrophilic groups (-SO3- and -COO-) were used to give rise to two kinds of nano-promoters, which were defined as -SO3-@PSNS and -COO-@PSNS separately. A 250 mL flask equipped with an agitator was used to synthesize the nano-promoters in nitrogen environment, and a reflux condenser was employed. Firstly, the appropriate amount of deionized water, St and hydrophilic monomers were added into the flask in sequence, while the agitation was turned on at a speed of 300 rpm. Then, when the temperature was reached set value, the initiator solution was dropped into the flask at a rate of 1 drop per second, and subsequently the system was kept for 5 h until the end of polymerization. The recipe of the soap emulsion polymerization is given in Table S1, and the reaction temperature was set as 353.15 K, which was controlled by constant temperature oil-bath pans
The morphologies of the nano-promoters were observed by a transmission electron microscope (TEM, JEM-1200EX, Japan Electronics Co., Ltd., Japan), and it is clear from Figure 1 that both of the nano-promoters are uniform nanospheres. The hydrophilic group on the surface of the nano-promoters was determined with an infrared spectrometer (Nicolet iN 10, Thermo Fisher, USA), the adsorption peaks corresponding to the S=O and C=O confirm the existence of -SO3-and -COO groups on the surface of -SO3-@PSNS and -COO-@PSNS, respectively. The size distribution and zeta potential of the nano-promoters were determined by a Laser Particle Size Analyzer (Malvern Nano-s90, UK), and -SO3-@PSNS and -COO-@PSNS produce the zeta potential of -31.8 mV and -20.36 mV separately. This confirms the existence of corresponding hydrophilic groups on the nano-promoters, and excellent stability is also anticipated through the electrostatic force between the nano-promoters.