Synthesis of the combined inter- and intra-crosslinked nanohydrogels by e-beam ionizing radiation

Thermally collapsed poly(vinylpyrrolidone) (PVP) polymer radicals preferably undergo intra-crosslinking reactions producing a nanogel structure rather than a macro-gel that would be caused by inter-crosslinking reactions. In the radiolysis of PVP aqueous solutions, the predominance of the intra-crosslinking reactions was achieved at elevated temperatures up to 77 °C. The dominance of intra-crosslinking under these conditions was confirmed by the molecular weight measurements using asymmetric field flow fractionation (AFFF). A higher yield of C-centered free radicals along the PVP chain at higher electron beam pulse repetition rate (300 Hz) enhances the intra-crosslinking reactions. This represents, mainly, the intra-crosslinking of the radicals of collapsed PVP molecules, along with a minor contribution of inter-crosslinking of the polymer dimers formed by transient occupation by two molecules in the same hydrodynamic volume and the increased diffusivity at the higher temperature.