First stage of film formation by latexes investigated by small-angle X-ray scattering

The packing of latex spheres achieved in the first step of film formation is considered. The latexes consisting of PMMA or poly(styrene) particles were dried at temperatures far below the glass temperature of both materials. The weight-average diameters were between 69 and 106 nm whereas the standard deviation of the size distribution was located between 4.8 and 12%. The spatial order in the bulk samples thus obtained was investigated using small-angle X-ray scattering (SAXS). SAXS allows to study the latex in dilute suspension and the dried samples. These data yielded the structure factor S(q) (q = (43 pi/lambda) sin(theta/2); theta, scattering angle; lambda, wavelength) of the system of dried spheres. The SAXS analysis showed that no deformation of the particles occurred when going from the dilute suspension to a system of closely packed spheres. In all cases studied herein it could be demonstrated that no true long-range order was achieved in the process of drying. Model calculations showed that ordering of the spheres into small but perfect crystallites can be ruled out experimentally. It is shown that the order achieved upon drying these latexes is intimately connected to the polydispersity of the system: With increasing polydispersity of the latex particles the maximum of the first peak of S(q) decreases whereas its breadth increases. This feature could be described quantitatively by the Percus-Yevick theory augmented by Vrij for systems of polydisperse hard spheres.