A series
of model sterically stabilized diblock copolymer nanoparticles
has been designed to aid the development of analytical protocols in
order to determine two key parameters: the effective particle density
and the steric stabilizer layer thickness. The former parameter is
essential for high resolution particle size analysis based on analytical
(ultra)centrifugation techniques (e.g., disk centrifuge photosedimentometry,
DCP), whereas the latter parameter is of fundamental importance in
determining the effectiveness of steric stabilization as a colloid
stability mechanism. The diblock copolymer nanoparticles were prepared
via polymerization-induced self-assembly (PISA) using RAFT aqueous
emulsion polymerization: this approach affords relatively narrow particle
size distributions and enables the mean particle diameter and the
stabilizer layer thickness to be adjusted independently via systematic
variation of the mean degree of polymerization of the hydrophobic
and hydrophilic blocks, respectively. The hydrophobic core-forming
block was poly(2,2,2-trifluoroethyl methacrylate) [PTFEMA], which
was selected for its relatively high density. The hydrophilic stabilizer
block was poly(glycerol monomethacrylate) [PGMA], which is a well-known
non-ionic polymer that remains water-soluble over a wide range of
temperatures. Four series of PGMAx–PTFEMAy nanoparticles were prepared (x = 28, 43, 63, and 98, y = 100–1400) and
characterized via transmission electron microscopy (TEM), dynamic
light scattering (DLS), and small-angle X-ray scattering (SAXS). It
was found that the degree of polymerization of both the PGMA stabilizer
and core-forming PTFEMA had a strong influence on the mean particle
diameter, which ranged from 20 to 250 nm. Furthermore, SAXS was used
to determine radii of gyration of 1.46 to 2.69 nm for the solvated
PGMA stabilizer blocks. Thus, the mean effective density of these
sterically stabilized particles was calculated and determined to lie
between 1.19 g cm–3 for the smaller particles and
1.41 g cm–3 for the larger particles; these values
are significantly lower than the solid-state density of PTFEMA (1.47
g cm–3). Since analytical centrifugation requires
the density difference between the particles and
the aqueous phase, determining the effective particle density is clearly
vital for obtaining reliable particle size distributions. Furthermore,
selected DCP data were recalculated by taking into account the inherent
density distribution superimposed on the particle
size distribution. Consequently, the true particle size distributions
were found to be somewhat narrower than those calculated using an
erroneous single density value, with smaller particles being particularly
sensitive to this artifact.