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 provide information on concentration and molecular mobility (as revealed by the spin-spin relaxation time, T2) as a function of depth into cross-linking latex coatings during their film formation is reported. These materials are of interest because they provide hard, chemically resistant coatings and because, being waterborne, they do not release organic solvents into the atmosphere. MRI profiles, with a pixel resolution of 9 m, are obtained at regular time intervals from a poly(vinyl acetate-co-ethylene) latex dispersion containing a difunctional cross-linker and a photoinitiator. In this complete 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 of 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 qualitative agreement with those found experimentally.

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