Biomedical Engineering Reference
In-Depth Information
d
n
4
y
3
n
g
|
1
Figure 13.7
Scattered light intensity graph showing micelle formation. This is for
PEG-b-PCL (5k-b-2.3k) in near-critical trifluoromethane at 100 uC and
5 wt%. (Reproduced from Tyrrell et al.
18
with permission from the
American Chemical Society.)
13.3 Extension to PEG-b-PCL
Therefore, the next logical step in developing NCM was to extend it to a
biomedically relevant polymer, such as poly(ethylene glycol)-block-poly(e-
caprolactone),
17
selected for NCM studies by Tyrrell et al.
18
Poly(ethylene
glycol) (PEG) is a common hydrophilic corona-forming block while poly
(e-caprolactone) (PCL) is a common hydrophobic core-forming block; both
are biodegradable and FDA approved.
Tyrrell et al.
18
chose chlorodifluoromethane and trifluoromethane as
possible solvents for NCM, based on polarity and critical properties. While
chlorodifluoromethane showed no micellization (due to narrow difference in
block CPs), trifluoromethane exhibited evidence of micelle formation, as
shown in Figure 13.7. A compilation of the CP and MP results shown in
Figure 13.8 confirms a robust micellar region for trifluoromethane.
However, such micelles formed at high pressure may in principle undergo a
structural rearrangement or decomposition upon decompression. So, the next
step was to confirm that micelles still exist upon dispersion of the copolymer
precipitate in water. Towards this end, the solvent was removed from the
NCM system depicted in Figure 13.8 by depressurization, and the precipitate
was re-dispersed in water and then filtered through a 200 mm filter. The filtered
solution was then characterized for particle size. As shown in Figure 13.9, the
particle size distribution is narrow and centered around 100 nm, reminiscent of
micelle structure in the NCM solution. This was in sharp contrast to a control
solution (copolymer dissolved in water without NCM treatment), which could
not be dissolved at all.
