Biomedical Engineering Reference
In-Depth Information
polymer collapses the retention time in the GPC increases compared
to the one for the linear precursor. Thus, the nanoparticles show an
apparent lower molecular weight, but what this really means is that
intramolecular cross-linking took place (Figure 2.1a).
In case of intermolecular cross-linking the outcome is a polymer
network with higher weight than the original polymer. The intrinsic
viscosity also decreases in value when the coil polymer collapses,
and according to Beck and co-workers, analogous nanoparticles
display equivalent viscosities even if the molecular weights diff er by
50% (Figure 2.1b).
This is due to the similarity between the nanoparticles and
constant density spheres [26].
Figure 2.1
(A) Overlay of GPC traces for the starting copolymer 1, (Mn =
45,300), the corresponding linear control polymer 2 and nanoparticle 3
incorporating ca. 10 mol % NCO cross-linking functional groups. (B) Plot
of reduced viscosity versus concentration for control copolymers 2 (
, 150
kDa;
D
, 100 kDa) and their analogous cross-linked nanoparticles 3 (
, 150
kDa;
, 100 kDa) in THF. Mol % NCO is 10 in this case. Reproduced from ref.
[42] with permission.
Nuclear magnetic resonance (NMR) is also a useful technique to
confirm the formation of SCNPs. There are signals that disappear
once the linear polymer collapses to form the nanoparticle, as is
clearly shown by Kim
et al.
[51]. When the particles are in solution,
their radius can be measured by dynamic light scattering (DLS),
even though the average sizes of this kind of nanoparticles are in the
lower size limit of most of the commercial instruments. Also, if the
SCNPs are dried on a surface, they can be analyzed by atomic force
microscopy (AFM) and the dimensions of the collapsed coils can
be visually studied. One of the most detailed studies of this type of
nanoparticle has been carried out by Berda and co-workers, where
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