Solvent-free preparation of uniform styrene/maleimide copolymer microspheres from solid poly(styrene-alt-maleic anhydride) microspheres

A method to prepare uniform styrene/maleimide copolymer (SMI) microspheres from poly(styrene-alt-maleic anhydride) (SMA) and gaseous amine by a solvent-free pathway was developed. In this approach, SMA microspheres were first prepared by a self-stabilized precipitation polymerization, and further reacted with gaseous ammonia, methylamine and ethylamine. Unlike the SMA powder with irregular particle shape and size, the uniform SMA microspheres exhibit an enhanced ammonification degree of 100%. The obtained maleamic acid copolymer was converted to SMI microspheres by a simple heat treatment in a closed vessel. The SMI microspheres can retain the morphology and size of the SMA microspheres without any deformation. The conversion of the maleic anhydride groups during the imidization process was also tracked by FTIR, H-1 NMR, and C-13 NMR spectroscopy and elemental analysis. Compared with the SMA microspheres, the SMI microspheres possess superior thermal properties, and their glass transition and decomposition temperatures increased to 240 degrees C (NH3-SMI microspheres) and 445 degrees C (N-ethyl SMI microspheres), respectively. The solubility test showed that N-methyl SMI and N-ethyl SMI can be easily dissolved in acetone; however, NH3-SMI is not soluble in acetone but can be dissolved in a mixture of acetone and water due to the hydrogen bond formed between the imide groups and water. The static water contact angles of the SMI copolymer films were determined to be in the range from 86 degrees to 100 degrees, which were higher than that of SMA (68 degrees). This simple, low-cost and solvent-free strategy to prepare well-defined SMI microspheres is expected to have potential applications in heat-resistant coating and composite materials.