INFLUENCE OF CROSS-LINKING ON STRUCTURE, MECHANICAL-PROPERTIES, AND STRENGTH OF LATEX FILMS

Two series of emulsion polymers with n-butyl (meth)acrylate and the bifunctional monomer methallyl methacrylate were prepared, covering the whole range from un-cross-linked to highly cross-linked particles. Dynamic mechanical measurements showed that films from slightly or moderately cross-linked particles behave like homogeneous networks in the linear viscoelastic range. Measurements of the fracture energy per unit volume, W(B), of poly(butyl methacrylate) films gave insight into the development of mechanical strength by interdiffusion across particle boundaries. When annealed above T(g), un-cross-linked films show a transition from brittle to tough fracture at short annealing times, followed by a further gradual increase of W(B) linear with the square root of the annealing time. On the basis of these measurements and small-angle neutron scattering experiments on the same samples,7 a three-stage process is proposed for the development of film strength. Films from particles so highly cross-linked that the mean molecular mass between cross-links becomes smaller than the entanglement length M(e) remain brittle upon annealing. Interdiffusion and the formation of interparticular entanglements are impossible in these latex films.