Ordered solution structure of a monodispersed polystyrene latex as studied by the transmitted light spectrum method

Abstract

The microstructure of monodispersed polystyrene latices in deionized solution has been systematically investigated by the transmitted light spectrum method. Latices used are between 85 and 246 nm in diameter and the charge density of the latex surface is between 1.0 and 36 µC cm^–2. Clear peaks showing the selective Bragg diffraction are observed in the extinction spectrum. The ordered regions of the face-centred cubic (f.c.c.) and body-centred cubic (b.c.c.) lattices are assigned from the sharp peaks. The nearest-neighbour interparticle distance, 2D_exp, is estimated from the wavelength at the extinction peak. The 2D_exp value decreases with increasing latex concentration and is close to (but smaller than) the calculated average value, 2D₀. The coexistence of ordered and disordered regions, or a two-state structure, is clarified. At low temperatures the f.c.c. structure is stable compared with the b.c.c. lattice, whereas the b.c.c. structure exists predominantly at elevated solution temperatures. The addition of a small amount of a foreign salt (KCl) shifts the lattice structures in the ordered region from f.c.c. to b.c.c. The 2D_exp value increases on the addition of KCl and then begins to decrease upon further addition. The ordered structure is completely broken down by the addition of KCl above 2 × 10^–5 mol dm^–3.