Depth profiles of polymer mobility during the film formation of a latex dispersion undergoing photoinitiated cross-linking

The first use of one-dimensional magnetic resonance imaging (MRI) to provid e information on concentration and molecular mobility las revealed by the s pin-spin relaxation time, Ttl as a function of depth into cross-linking lat ex coatings during their film formation is reported. These materials are of interest because they provide hard, chemically resistant coatings and beca use, being waterborne, they do not release organic solvents into the atmosp here. MRI profiles, with a pixel resolution of 9 mum, are obtained at regul ar time intervals from a poly(vinyl acetate-co-ethylene) latex dispersion c ontaining a difunctional cross-linker and a photoinitiator. In this complet e formulation, MRI reveals that the rate of cross-linking is fastest in the middle regions of the coating. This result is explained by considering the combined effects of light scattering in the turbid latex, the inhibition o f the free-radical cross-linking reaction by initial molecular oxygen, and the further ingress of oxygen from the atmosphere. A numerical model, using measured and known parameters, predicts MRI profiles that are in good qual itative agreement with those found experimentally.