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

Self-Assembly Process of Different Poly(oxystyrene)-Poly(oxyethylene)
Block Copolymers: Spontaneous Formation of Vesicular Structures
and Elongated Micelles
Josué Juárez, † Pablo Taboada,* ,† Miguel A. Valdez, ‡ and Víctor Mosquera †
Departamento de InVestigación en Polímeros y Materiales y Departamento de Física, UniVersidad de
Sonora, Sorales Resales y TransVersal, 83000 Hermosillo Sonora, Mexico, and Laboratorio de Física de
Coloides y Polímeros, Grupo de Sistemas Complejos, Departamento de Física de la Materia Condensada,
Facultad de Física, UniVersidad de Santiago de Compostela, Spain
ReceiVed February 12, 2008. ReVised Manuscript ReceiVed March 26, 2008
In the present work, we investigated the micellization, gelation, and structure of the aggregates of three poly(ethylene
oxide)-polystyrene oxide block copolymers (E12S10, E10S10E10, and E137S18E137, where E denotes ethylene oxide and
S styrene oxide and the subscripts the block length) in solution. Two of them have similar block lengths but different
structures (E12S10 and E10S10E10) and the other has longer blocks (E137S18E137). For the first time, the spontaneous
formation of vesicles by a poly(oxystyrene)-poly(oxyethylene) block copolymer is reported. These vesicular structures
are present when copolymer E12S10 self-assembles in aqueous solution in coexistence with spherical micelles, as
confirmed by the size distribution obtained by dynamic light scattering and pictures obtained by polarized optical
microscopy, and transmission and cryo-scanning electron microscopies. Vesicle sizes vary between 60 and 500 nm.
On the other hand, for copolymers E10S10E10 and E137S18E137, only one species is found in solution, which is assigned
to elongated and spherical micelles, respectively. If we compare the high aggregation number derived by static light
scattering for the triblock block copolymer micelles, with the maximum theoretical micellar dimensions compatible
with a spherical geometry, we can see that the micellar geometry cannot be spherical but must be elongated. This
is corroborated by transmission electron microscopy images. On the other hand, tube inversion was used to define
the mobile-immobile (soft-hard gel) phase boundaries. To refine the phase diagram and observe the existence of
additional phases, rheological measurements of copolymer E137S18E137 were done. The results are in good agreement
with previous values published for other polystyrene oxide-poly(ethylene oxide) block copolymers. In contrast, copolymers
E12S10 and E10S10E10 did not gel in the concentration range analyzed. Thus, 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.
Introduction
Block copolymers of hydrophilic poly(oxyethylene)
(OCH2CH2, E) oxyethylene unit) and hydrophobic poly(oxystyrene)
(OCH2CH(C6H5), S ) oxyphenylethylene unit) are
denoted as either EmSn,SnEmor EmSnEm,SnEmSn, depending on
the sequence of polymerization or architecture, respectively. They
associate to usually form spherical micelles in dilute aqueous
solution. 1,2 Increases in concentration lead to the formation of
lyotropic crystal mesophases (gels) where the structural objects
are the micelles usually efficiently packed in a body-centered or
face-centered cubic structure as a result of being effectively
interacting as hard spheres. 3
Aggregate morphology is determined primarily by a force
balance among three contributions: the core-chain stretching,
corona-chain repulsion, and interfacial tension between the core
and the outside solution. Consequently, factors that influence the
above balance can be used to control the aggregate architecture:
* Author to whom correspondence should be addressed. Telephone:
0034981563100, ext.: 14042. Fax: 0034981520676. E-mail: fmpablo@usc.es.
† Universidad de Santiago de Compostela.
‡ Universidad de Sonora.
(1) Crothers, M.; Attwood, D.; Collett, J. H.; Yang, Z.; Booth, C.; Taboada,
P.; Mosquera, V.; Ricardo, N. P. S.; Martini, L. G. A. Langmuir 2002, 18, 8685–
8691.
(2) Yang, Z.; Crothers, M.; Ricardo, N. M. P. S.; Chaibundit, C.; Taboada,
P.; Mosquera, V.; Kelarakis, A.; Havredaki, V.; Martini, L.; Valder, C.; Collett,
J. H.; Attwood, D.; Heatley, F.; Booth, C. Langmuir 2003, 19, 943–950.
(3) Castelletto, V.; Hamley, I. W.; Crothers, M.; Attwood, D.; Yang, Z.; Booth,
C. J. Macromol. Sci., Phys. 2004, 43, 13–27.
Langmuir 2008, 24, 7107-7116
10.1021/la8004568 CCC: $40.75 © 2008 American Chemical Society
Published on Web 06/12/2008
7107
polymer molecular weight, relative block lengths, hydrophobic
block structure, solution conditions, and so forth. For a given
value of the association number (Nw) for a poly(oxyalkylene)
block copolymer, the average volume of a micelle core is readily
estimated, and the corresponding spherical radius (rc) can be
compared with the length l (or half-length, l/2, in the case of
triblock copolymers) of the extended hydrophobic blocks. With
allowance made for a Poisson distribution of hydrophobic blocks,
experience shows that spherical (or near-spherical) micelles will
be formed if l (l/2 for triblock architecture) is approximately
equal or greater than the value of rc. 4,5 If this does not occur,
other types of assemblies are observed under certain solution
conditions. For example, a cylindrical geometry is expected as
the solution temperature is increased because of an increase in
the aggregation number derived from block copolymer dehydration:
6 the core blocks are stretched as the radius of a spherical
micelle core is increased eventually to reach a point at which the
enthalpy penalty forces the transition to a cylindrical geometry.
Thus, formation of cylindrical micelles has been widely studied
in block poly(oxyalkylene) copolymers with hydrophobic blocks
(4) Taboada, P.; Velazquez, G.; Barbosa, S.; Castelletto, V.; Nixon, S. K.;
Yang, Z.; Heatley, F.; Hamley, I. W.; Ashford, M.; Mosquera, V.; Attwood, D.;
Booth, C. Langmuir 2005, 21, 5263–5271.
(5) Ricardo, N. M. P. S.; Pinho, M. E. N.; Yang, Z.; Attwood, D.; Booth, C.
Int. J. Pharm. 2005, 300, 22–31.
(6) Booth, C.; Attwood, D. Macromol. Rapid Commun. 2000, 21, 501–527.
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