Polyethylene Glycol Polyester Block Copolymers: Biocompatible Carriers for Nanoparticulate Drug Delivery - Harnessing Biomaterials in Nanomedicine: Preparation, Toxicity, and Applications - page 48

PEO block length of 5 kDa, and relatively short PCL blocks (0.35,
0.95, 1.5, 1.85, 2.2, and 3 kDa). The conformation of the PEO chains
in the corona were determined to exist in a moderately extended
geometry based on steric hindrance at the corona-core interface
depicted by its low surface area. The surface area of this interface
was calculated based on N agg . Aggregation numbers ranged from 12
to 126 for PCL block lengths of 0.35-2.2 kDa. SANS and SAXS data
were fit to various models to determine the conformation of core,
which was depicted as a dense core of rods, based on the size and
model. Micelle size was determined to be 20-25 nm in diameter
for PCL block lengths of 0.95-2.2 kDa. Jérôme and coworkers have
also shown that when micelles are formed from organic solvent and
water, lower concentrations of copolymers lead to larger micelles,
and depending on the solvent, the size can range from 30-100 nm
[37]. The CMCs determined from surface tension data were on the
order of 10 -5 -10 -6 M for PEO-PCL micelles with PCL block lengths
ranging from 0.95-2.2 kDa. Using self-diff usion coefficients obtained
from NMR, a higher content of unimers was present with smaller PCL
blocks because their diff usion coefficients were very close to that of
free PEO in solution. Transmission electron microscopy confirmed
the spherical shape and monodispersity of the micelles. Stĕpánek
and coworkers characterized PEO-PCL micelles with larger PCL
blocks (5, 13, and 32 kDa) at a constant PEO block length of 5 kDa
with static (SLS) and dynamic light scattering (DLS) [23]. Scattering
intensities and relaxation time distributions from DLS and SLS data
do not indicate any dissociation dependence on concentration at
low concentrations (i.e., kinetically frozen). The longer PCL blocks
stabilize the micelles as is often evident through lower CMC values
(Table 3.1). The geometry, block arrangement and functional groups
substituted onto PCL also influence the CMC of micelles containing
PEO and PCL blocks.
As expected, hydrodynamic radii and radii of gyration were
comparable and thus indicative of a spherical shape. Average-density
aggregation numbers (Z) were calculated from hydrodynamic radii
to range from 2,300-10,900 for PCL blocks 5-32 kDa in length. These
aggregation numbers were consistent with other micelles known to
form vesicles at similar sizes. This implies that the inner core was
liposome-like, or hollow. The diameter of these micelles increased
from 80-200 nm across PCL blocks of 5-32 kDa as expected. Size
distributions are normally characterized on many micelles.