Small-angle X-ray scattering (SAXS) is a tool which allows the study of the structure and the interaction of polymer latexes with great accuracy. The low electron density of the polymers used for the synthesis of latex particles as e.g. polystyrene allows the matching of the contrast by adding sucrose to the serum. Thus, scattering intensities measured at different contrast, i.e., at different excess electron densities can be evaluated (contrast variation). This yields precise information on the radial electron density of the particles. In this article recent SAXS-investigations on latex particles are reviewed. It is demonstrated that core-shell latexes can be analyzed precisely by contrast variation. The same method can be applied to swollen latex particles to examine the polymer concentration near the boundary to the water phase. Here it is shown that the depletion of the polymer molecules near this boundary is very small which points to a minute wall-repulsion effect. Since the excess electron density of polystyrene latex particles in water is very small, the scattering from adsorbed layers of surfactants dominates the measured intensity in this particular system. Therefore the adsorption equilibrium of a given surfactant as well as the competitive adsorption of two different surfactants on a polystyrene latex can be assessed by SAXS.
A small-angle X-ray scattering (SAXS) study of micelles formed by
a symmetric block
copolymer consisting of poly(styrene) and poly(ethylene
oxide) (molecular weight of each block: 1000
g/mol; concentration up to 15.4 wt %) is given. The contrast of
the micelles toward water was changed
by adding glycerol to the solution. The analysis of the SAXS data
shows that the addition of glycerol
does not change the structure of the micelles. With the aid of
contrast variation, SAXS allows
determination of the following integral parameters: (i) the mean
electron density of the particle ρ together
with the particle volume V
p, (ii) the radius of
gyration at infinite contrast R
g,
∞
together with the
inhomogeneity parameter α, and (ii) the volume
V
c related to the shape of the particles as well
as
parameters characterizing the internal density fluctuation of the
particles. Furthermore the correlation
length l
c has been determined as a function of
concentration and contrast. A comparison of these
parameters and of the scattering intensities at various contrast with
concentric models of the radial
electron density shows the following: The micelles have an overall
spherical shape and a narrow size
distribution (weight-average aggregation number = 400; standard
deviation of the size distribution 10.9%).
The unpolar component poly(styrene) together with a fraction
of the polar poly(ethylene) blocks is located
in the core.
An analysis of the conformation of a ribbon-type polymer in dilute solution is given. By combination of small-angle neutron scattering (SANS) and small-angle X-ray scattering (SAXS) the effect of the finite dimensions of the repeating units can be assessed separately. It is thus possible to evaluate the experimental data to yield the form factor of the corresponding infinitely thin chain. This form factor could be evaluated for three samples differing in molecular weight and compared to the theoretical expression furnished by Kholodenko (Macromolecules 1993, 26, 4179). Good agreement for all molecular weights under consideration here could be achieved. The analysis shows that the ribbon-type polymers under consideration here are wormlike chains with a persistence length of 6.5 nm.
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