Probing the structure of inhomogeneous colloidal particles by small-angle x-ray scattering

Small angle X-ray scattering (SAXS) is well suited for a detailed study of both the shape and the inner structure of suspended colloidal particles. A "trial-and-error" modeling was used in a SAXS study of two-stage latices (TSL), composed of polystyrene (PS) and polytribromostyrene (PTBrS, 15 wt%). The TSL particles were found to have a concentric core-shell structure. When a PTBrS latex was used as a seed, its particles were overcoated with a PS shell during the second-stage polymerization. However, only a small portion of the seed particles were overcoated with a PTBrS shell when using a PS seed. The size distributions of the TSL and the PTBrS latex particles were determined from the scattering curves, using the method of indirect Fourier transformation. The resulting average radii were in good agreement with the values obtained from TEM observations. The contrast variation method, designed to separate the information on the particle as a whole from that of its inhomogeneities, was employed in a SAXS study of a model copolymer latex, composed of styrene and pentabromobenzyl acrylate (PBBA, 40 wt%). The separation of the homogeneous function allows direct calculation of the size distribution of the spherical particles (volume average diameter, (26.7 ± 1.3) nm). The SAXS analysis reveals a particle's inner structure described as a continuous copolymer phase, of composition being slightly richer in PBBA, within which domains of PS are randomly distributed. The volume fraction of the PS domains was estimated as 8 vol%, and their characteristic length as 5.1 nm.