Synthesis of polyacrylonitrile nanoparticles at high monomer concentrations by AIBN-initiated semi-continuous emulsion polymerization method

• Semi-continuous emulsion polymerization of AN in the presence of AIBN was studied.
• The AIBN amount has little influences on the size of PAN nanoparticles.
• PAN nanoparticles (〈Dh〉 = 99 nm) was fabricated at [AN] = 202 g L−1 and [SDS] = 10 g L−1.

Due to the large solubility of acrylonitrile (AN) monomer in water and the insolubility of polyacrylonitrile (PAN) in AN monomer, it remains a great challenge to fabricate PAN nanoparticles with sizes of below 100 nm at high monomer concentrations by emulsion polymerization methods. The synthesis of PAN nanoparticles at high monomer concentrations via a hydrophobic initiator 2,2′-azobisisobutyronitrile (AIBN)-initiated semi-continuous emulsion polymerization method was investigated. The influences of various parameters, including the initiator amount, the monomer concentration and feeding mode, the reaction temperature, the surfactant concentration and type, on the polymerization were studied. The results show that narrowly-distributed PAN nanoparticles with smaller sizes can be prepared by decreasing the monomer concentration, modestly slowing down the monomer feeding rate, decreasing the polymerization temperature, or modestly increasing the surfactant amount. The polymerization rate during the AIBN-initiated semi-continuous emulsion polymerization is kept at a very low level due to the hydrophobicity of AIBN. This endows the method with the advantage in the preparation of PAN nanoparticles at high monomer concentrations as compared with the conventional water-soluble initiator potassium persulfate (KPS)-initiated emulsion polymerization method. Narrowly-distributed PAN nanoparticles with a z-average hydrodynamic diameter around 99 nm and a polydispersity index of 0.02 was successfully fabricated at a monomer concentration as high as 202 g L−1 (based on the volume of water) and a sodium deodecylsulfate (SDS) content as low as 4.9 wt% (based on the monomer). As compared with the results of the KPS-initiated emulsion polymerization, the monomer concentration is dramatically higher, and the SDS content is much lower.

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