Origin of nanostructural inhomogeneity in polymer-network gels

Polymer-network gels often display nano-to microstructural spatial inhomogeneity of their polymer-segmental and crosslinking densities, especially if they are formed by uncontrolled free-radical crosslinking copolymerization of mono-and multifunctional monomers and crosslinkers. This structural complexity markedly affects their optical clarity, mechanical strength, and permeability, which is of central relevance for their performance in everyday-life and high-tech products. This review summarizes and inter-relates a rich amount of existing studies on the origin of this inhomogeneity. It arcs from early fundamental work in the 1990s, development and assessment of highly ideal and super tough model-network gels in the 2000s, up to more recent work on hierarchically structured gels.