Bisphenol-A based benzoxazine monomer (BZ) forms a polybenzoxazine (polyBZ) thermoset after thermal curing. However, when BZ is blended with polystyrene-block-poly(ethylene-co-1-butene)block-polystyrene triblock copolymer (SEBS) matrices and is allowed to polymerize, phase separation at the nanometer level confines BZ in a way that can be controlled by the blending process, resulting in nanospherical polyBZ (polyBZ-NS) in the SEBS matrix. By simply changing the morphology of BZ and SEBS, through blending protocols ranging from sheet to electrospun micro-fibers and to electrospun core-sheath nanofibers, the sizes of polyBZ-NS so obtained vary from similar to 150-500 nm, to similar to 80 nm, and to as low as similar to 12 nm, respectively. This shows a new method to obtain well-defined mono-disperse nanospherical thermoset resins with a controllable size by controlling blend conditions. (C) 2012 Elsevier Ltd. All rights reserved.