Order and Disorder in Monodisperse Latexes
Iridescence due to Bragg diffraction of visible light is exhibited by electrolyte-free monodisperse polymer latexes, and also by redispersions of the polymer spheres in certain polar organic media. The center-to-center particle separation D, the particle diameter Do, and the volume fraction ϕ are related by ϕ(D/Do)3 = 0.74, the value 0.74, being the volume fraction occupied by spheres in a close-packed lattice. The structure responsible for the diffraction is thus identified as a lattice-like array which pervades the entire suspension, and is due to long-range coulombic forces between charged particles. Addition of electrolyte shortens the range of inter-particle forces through a screening action, thereby destroying the lattice and producing a disordered, non-diffracting suspension. At intermediate electrolyte levels, ordered and disordered phases coexist at equilibrium; the volume fractions of the coexisting phases are functions of the ionic strength.
KeywordsPeak Width Benzyl Alcohol Emulsion Polymerization Benzoni Trile Methyl Benzoate
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