Assessment of spatial order in dried latexes by small-angle X-ray scattering

The spatial order present in a dried latex consisting of poly(styrene) (PS) particles (diameter: 106 nm) is investigated by small-angle X-ray scattering (SAXS). The process of drying is conducted below the glass transition temperature of PS to avoid deformation of the particles when going from the concentrated latex to a system of close-contact particles. The assessment of spatial order rests on the analysis of the structure factor S(q) (q = (4 pi/lambda) sin(theta/2); theta = scattering angle; lambda = wavelength) of the dry latex. S(q) has been obtained by SAXS measurements of the particles in the dilute latex and in the dried system. Special attention is paid to the different contrast between the particles and the surrounding medium when going from the waterborne latex particles to the dry film. It is demonstrated that the system under consideration here exhibits no true long-range order but a more liquidlike short range order. The data show furthermore that the deformation of the particles during the process of drying must be small if not negligible.