Synthesis of tailored nanogels by means of two-stage irradiation

A new method allowing to synthesize polymeric nanogels of independently chosen weight-average molecular weight (Mw) and dimensions in an additive-free system consisting only of linear macromolecules and water is described and tested. This approach, based on generation of free radicals using ionizing radiation in oxygen-free polymer solutions, consists of two steps. In the first step, semi-concentrated polymer solution is irradiated at a moderate dose rate, so that in the stationary state the average number of radicals per chain is <1. This promotes intermolecular cross-linking with increase in Mw and coil dimensions. When the desired Mw is reached, in the second step intra-molecular recombination is induced by pulse irradiation of dilute solution (with the average number of radicals per chain >1 after each pulse), leading to a decrease in nanogel diameter while keeping Mw nearly constant. Experimental data on a model polymer – poly(N-vinylpyrrolidone) – confirm that the proposed method allows synthesizing nanogels of desired properties (independently chosen molecular weight and radius of gyration) in a pure polymer-water system, eliminating the use of monomers, cross-linking agents, or other auxiliary substances.