Biomacromolecules
Article
5, bottom). RH values increased slightly with increasing solution
concentration, the largest shift, from RH = 90 nm (0.25 g L−1)
to RH = 210 nm (5.0 g L−1), being registered in the case of OH-
PMOx10-b-P(BLG)11. No angular dependence was found for
both PEG or PMOx copolymers, the RH being almost identical
at every angle. This suggests that the scattering particles are
isotropic objects, such as sphere.
ACKNOWLEDGMENTS
■
This work was supported by the National Institute of Health
(NIH) and National Eye Institute under the American
Recovery and Reinvestment Act of 2009, grant
2R01EY016674-04A1, and is a collaborative effort within the
Boston Retinal Implant Project. We thank Egen, Inc., in
Huntsville, AL, for their support with light scattering analyses.
We determined the critical association concentrations of the
polymers in water by fluorescence spectroscopy using pyrene
(Py) as probe. In aqueous micellar solutions Py, which is poorly
soluble in water, is preferentially solubilized within hydrophobic
microdomains. The ratio I1/I3 of the intensities of the first to
third bands of the pyrene emission takes a value (∼1.8) for Py
in water and decreases to ≅1.25−1.20 when Py is solubilized in
nonpolar media such as alkanes or within the hydrophobic core
of surfactant micelles.29 Thus, the amphiphile concentration for
which a decrease in I1/I3 value occurs gives an estimate of the
amphiphile critical association concentration. Plots of the
changes in the ratio I1/I3 as a function of polymer concentration
are presented in Figure 6 for solutions of OH-PMOxn-b-
P(BLG)m, CM-PEG5K-b-P(BLG)m (Figure 6, left, open
symbols) and OH-PMOxn-b-P(BLC)m, CM-PEG5K-b-P(BLC)m
(Figure 6, right, closed symbols). The ratio remained constant
(∼1.8) with increasing polymer concentration up to a point
beyond which it decreased gradually toward a plateau value of
∼1.20 in the most concentrated solutions tested (3.0 g L−1).
The decrease took place over a very large concentration range
unlike low molecular weight surfactants; polymers do not
assemble cooperatively.30 We used the polymer concentrations
corresponding to the onset of the drop of I1/I3 as an estimate of
the lowest polymer concentration, cass, for which hydrophobic
domains able to host Py molecules form in solution via
assembly of the end groups (Table 2). The cass values of the
polymer samples with γ-benzyl-L-glutamate block are ∼32%
(average) lower than that samples with S-benzyloxycarbonyl-L-
cysteine block, indicating the enhanced hydrophobicity of the
γ-benzyl-L-glutamate chain compared to S-benzyloxycarbonyl-L-
cysteine. For the samples of similar PMOx and PEG molecular
weights, Mn ∼ 5000 g mol−1, the cass of PMOx block copolymers
was higher than that of the PEG block copolymers, i.e., cass
values ranging from 0.008 to 0.005 g L−1 for γ-benzyl-L-
glutamate end-capped polymers. Furthermore, increasing the
PMOx chain length by doubling the molecular weight
produced, as expected, higher cass values.
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CONCLUSION
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We have described a convenient procedure for the synthesis of
amphiphilic diblock copolymers via ring-opening polymer-
izations. Analyses of aqueous solutions of CM-PEGn-b-
P(BAA)m and OH-PMOxn-b-P(BAA)m block copolymers by
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AUTHOR INFORMATION
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(30) Alami, E.; Almgren, M.; Brown, W.; Franco̧ is, J. Macromolecules
Corresponding Author
1996, 29, 2229−2243.
*Phone: (256) 824 6188. Fax: (256) 824 6349. E-mail:
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dx.doi.org/10.1021/bm201048x|Biomacromolecules 2011, 12, 3797−3804