Self-Assembly of Synthetic and Biological Polymeric Systems of ...

7116 Langmuir, Vol. 24, No. 14, 2008 Juárez et al.
can be expected for 35% < fhydrophilic < 50%, spherical micelles
for fhydrophilic < 45%, and inverted microstructures for fhydrophilic
< 25%. Although sensitivities of these rules to chain chemistry
and molecular weight have not been fully probed, the present
analyzed copolymers seem to be in agreement with these.
Conclusions
We have analyzed the micellization, gelation, and structure
of the aggregates of three poly(ethylene oxide)-polystyrene oxide
block copolymers. Two of them have similar block length but
different structures (E12S10,E10S10E10), and the other has longer
blocks (E137S18E137). In this way, we can compare the effect of
temperature and block length on the self-assembly properties.
The values of the critical micelle concentration show that the
short triblock copolymer has a cmc twice as large as that of the
related diblock. The effect is originated by entropy of the triblock
chains constrained by two block junctions in the core interface
of the aggregate, compared with only one constraint for the diblock
chain. Intensity fraction size distributions showed the existence
of several species in solutions of copolymer E12S10 whereas only
one for copolymers E10S10E10 and E137S18E137. Light scattering,
polarized light microscopy, and transmission and cryo-scanning
electron microscopies provided sufficient evidence to ensure the
coexistence of micelles and polydisperse vesicles for the most
concentrated samples of this copolymer (but even in the dilute
regime). The size of these vesicular structures varies between 60
and 500 nm as shown by TEM, DLS, and cryo-SEM. The classical
birefringent pattern of vesicles was observed by polarized light
microscopy. On the other hand, the single size distribution
observed for copolymers E10S10E10 and E137S18E137 was assigned
to elongated and spherical micelles, respectively. This assignment
was made in light of the high aggregation number derived for
the former copolymer which is incompatible with a spherical
geometry of the aggregated, as also corroborated by TEM. Tube
inversion was used to define the mobile-immobile (soft-hard
gel) phase boundaries. The phase diagram of copolymer
E137S18E137 was further completed to determine the sol-soft
gel-hard gel boundaries. In contrast, copolymers E12S10 and
E10S10E10 did not gel in the concentration range analyzed. Only
certain concentrations of copolymer E10S10E10 were analyzed by
rheometry, for which an upturn in the low-frequency range of
the stress moduli was observed, denoting an evidence of an
emerging slow process, which we assign to the first stages of
formation of an elastic network.
Acknowledgment. We thank the Ministerio de Educación y
Ciencia through projects MAT2007-61604 and Xunta de Galicia
for financial support. J.J. thanks USC for his PhD fellowship,
and E.C. thanks Xunta de Galicia for his “Anxeles Alvariño”
research contract. We are really grateful to Profs. D. Attwood
and C. Booth for providing several of the copolymer samples.
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