Block Copolymer Assembly via Kinetic Control

Honggang Cui et al.

Science 317, 647 (2007);
DOI: 10.1126/science.1141768

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18. S. C. Greer, Annu. Rev. Phys. Chem. 53, 173 (2002). of Chemistry (I.M.), and the Natural Sciences and Materials and Methods
19. See supporting material on Science Online. Engineering Research Council of Canada. We thank Figs. S1 to S6
20. J. Israelachvili, Intermolecular and Surface Forces I. Herrera for helpful discussions. Tables S1 to S4
(Academic Press, London, ed. 2, 1992), p. 359.
21. Supported by a European Union Marie Curie Chair (I.M.), Supporting Online Material 15 February 2007; accepted 22 June 2007
a Wolfson Research Merit Award from the Royal Society www.sciencemag.org/cgi/content/full/317/5838/644/DC1 10.1126/science.1141382

Block Copolymer Assembly via The system we used consists of a linear poly
(acrylic acid)-block-poly(methyl acrylate)-block-

Kinetic Control
polystyrene (PAA-b-PMA-b-PS) triblock co-
polymer, tetrahydrofuran (THF)/water mixed
solvents, and organic diamines (Fig. 1A). Various
Honggang Cui,1 Zhiyun Chen,2 Sheng Zhong,1 Karen L. Wooley,2* Darrin J. Pochan1* micelles with different packing geometries, such
as disks and toroids, have been constructed using
Block copolymers consist of two or more chemically different polymer segments, or blocks, connected this system (4, 22, 27, 28). This multicomponent
by a covalent linkage. In solution, amphiphilic blocks can self-assemble as a result of energetic assembly system allows control over the thermo-
repulsion effects between blocks. The degree of repulsion, the lengths of the block segments, and the dynamics and kinetics of block copolymer as-
selectivity of the solvent primarily control the resultant assembled morphology. In an ideal situation, sembly in the following ways. The selectivity of
one would like to be able to alter the morphology that forms without having to change the chemistry of water for PAA allows the manipulation of inter-

Downloaded from www.sciencemag.org on June 10, 2012

the block copolymer. Through the kinetic manipulation of charged, amphiphilic block copolymers in facial curvature between the hydrophilic corona
solution, we are able to generate different nanoscale structures with simple block copolymer chemistry. and hydrophobic core within a micelle, thus pro-
The technique relies on divalent organic counter ions and solvent mixtures to drive the organization of viding a means to control local micelle geometry.
the block copolymers down specific pathways into complex one-dimensional structures. Block Organic diamine complexation with chargable
copolymers are increasingly used as templating materials; thus, the ability to control the formation of PAA corona blocks influences both intermicellar
specific patterns and structures is of growing interest and applicability. interactions and the intramicellar PAA corona
block conformation. By the use of different solvent-
he broader development of nanoscale tration, and temperature) (1–7, 21, 23). However, mixing protocols, the pathway through which

T technologies requires methods to fabricate

and manipulate material at the nanometer-
length scale. This includes techniques that ma-
the slow kinetics of block copolymers in solution,
due to the slow exchange of chains between
micelles because of the higher molecular weight of
polymer assembly occurs can be manipulated.
The controlled assembly pathway generally
begins with diamine complexation with the PAA
nipulate individual atoms or clusters, as well as the molecules, hinders assembled structures from block in pure THF solution producing PAA-
materials that will self-assemble into organized reaching global equilibrium states (21, 24–26). diamine aggregates. Subsequent addition of water
patterns, often through solution-based processes. We specifically take advantage of this lack of has the combined effect of aggregating the hydro-
Molecules with varying chemical interactions are global equilibrium in amphiphilic charged block phobic PMA and PS blocks while concurrently
needed to drive the assembly, and this has been copolymers to produce complex, one-dimensional swelling and eventually solubilizing the PAA-
accomplished in block copolymers (1–7), surfac- nanostructures. The block copolymers are con- diamine complexes into micelle coronas. The
tants (8), proteins (9), DNA (10, 11), peptides trollably forced down specific assembly pathways combination of the PAA-diamine complexation
(12, 13), peptide amphiphiles (14), and polypep- through a combination of solvent mixing and the with subsequent solvent mixing produces a unique
tides (15). This broad range of molecules was complexation of a charged, hydrophilic block block copolymer assembly pathway resulting in
needed to tailor nanostructures with potential im- with divalent, organic counterions. The resultant the nanostructures displayed in Figs. 1, B and C.
pact on disparate, emerging fields such as nano- assemblies are kinetically trapped but stable be- The cylindrical nanostructures consist of PAA94-b-
medicine (14), organic photovoltaics (16, 17), cause of the inability of the system to thermody- PMA103-b-PS44 with 2,2'-(ethylenedioxy)di-
and self-assembled spintronics (18, 19) or opto- namically equilibrate. ethylamine (EDDA) as a diammonium counterion
electronic devices (11, 20).
Linear triblock copolymers, produced with
common polymerization techniques, provide an Fig. 1. (A) Molecular A PAA-b-PMA-b-PS Organic diamine, EDDA
opportunity to develop self-assembly strategies structures of triblock 94 103
Br
44
NH2 O
for complex nanostructure formation that do not copolymer, PAA-b-PMA- O NH2
b-PS, and organic di- O O HO O O O
necessarily require the altering of the molecule
chemistry to create a wide range of structures. amine (EDDA). (B and C)
The chemical tunability of amphiphilic block co- TEM images of one-
dimensional assembled B C
polymers has been used previously to produce
micelles and nanostructures in solution (1–7, 21, 22). structures of PAA94-b-
Polymeric micelle size and shape can be designed PMA103-b-PS44 at 67%
THF/water solution in the
through monomer selection, chain architecture
presence of EDDA (molar
design, and variation of solution conditions (e.g.,
ratio of amine groups:
solvent mixtures, pH manipulation, salt concen- acid groups = 1:1). The
1
samples were stained
Department of Materials Science and Engineering and with uranyl acetate aque-
Delaware Biotechnology Institute, University of Delaware,
Newark, Delaware 19716, USA. 2Center for Materials
ous solution. (B, insert)
Innovation, Department of Chemistry and Department of Schematic drawing of
Radiology, Washington University in Saint Louis, Saint Louis, cross section of one-
Missouri 63130, USA. dimensional assembled
*To whom correspondence should be addressed. E-mail: structures. PMA-PS stripes are illustrated as gray and dark blue bands. Light blue bands denote PAA
klwooley@artsci.wustl.edu (K.L.W.); Pochan@udel.edu (D.J.P.) concentrated area. EDDA, which is complexed with the PAA block, is not drawn for clarity.

www.sciencemag.org SCIENCE VOL 317 3 AUGUST 2007 647

REPORTS
by interaction with the acrylic acid residues of the hydrophobic core domains and the hydration of proposed mechanism. Upon quick introduction of
PAA [see table S1 and (29) for block copolymer PAA-diamine into the corona. At high water con- THF, the local packing geometry of isolated
molecular details]. The periodic stripes perpendic- tent (THF:water = 1:4), stable, spherical micelles micelles changes before intermicellar aggregation
ular to the cylinder axes indicate the alternating formed (Fig. 2A). THF was then pipetted into the takes place. Because local chain adjustment is a
layers of hydrophilic PAA complexed with EDDA spherical micelle solution to produce a final much faster process than intermicellar interactions,
and hydrophobic PMA-PS domains. The dark volumetric ratio of THF:water = 2:1. During the oblate spheres or discoidal micelles form with the
stripes are PAA layers that are positively stained original slow addition of water to the block addition of THF (Fig. 2C, step 1). Although the
as a result of uranyl cations interacting with car- copolymer/diamine/THF solution, a 2:1 THF:water local flat interfacial curvature is desired in the
boxylic acid side chains of the PAA. The light ratio produced block copolymer droplets with system, the resultant dispersed structures are not
stripes are composed of PS and PMA hydrophobic local lamellar structure (22). By forcing the sys- stable in the low-water-content solution and
segments with a thickness of ~20 nm (Fig. 1B, tem back to this solvent composition, the spherical undergo aggregation. However, the aggregation
inset).It is notable that this assembly is not a typ- block copolymer micelles were forced to aggre- is one-dimensional because the disklike micelles
ical hydrophobic core/hydrophilic corona micelle gate into a locally ordered lamellar nanostruc- have PAA-diamine faces that experience long-
but rather a cylinder with alternating layers of ture. Transmission electron microscopy (TEM) range, attractive electrostatic interactions with
hydrophilic and hydrophobic components arranged images taken immediately after the THF addition other diamine-rich PAA faces in high THF content
perpendicular to the cylinder axis. demonstrated that all of the micelles polymerized solution. [See figs. S1 to S4 and (29) for data and
The specific assembly pathway to produce the along a preferred growth axis (Fig. 2B). Further accompanying discussion about the electrostatic
striped cylinders is as follows. PAA94-b-PMA103- aging for several hours allowed additional one- nature of the interactions between the PAA blocks
b-PS44 was first dissolved in THF to form a 0.1 dimensional growth of these structures into long and the multivalent amine counterions.] Direct

Downloaded from www.sciencemag.org on June 10, 2012

weight percent homogeneous solution. Next, (up to microns in length) structures with uniform visualization of several separate disklike micelles
EDDA was added to give a molar ratio of amine widths as shown in Fig. 1, B and C. in the intermediate assembly stage can be seen in
group:acid group = 1:1. The EDDA complexed In amphiphilic block copolymer dilute so- Fig. 2D, as marked by black arrows, supporting
with the PAA block affording PAA-diamine lutions, two kinetic processes are possible in the concept that these segmented cylinders are
aggregates. Water was then added slowly (~8 ml response to solvent compositional changes. One formed by one-dimensional collapse of discoidal
water per hour added to 20-ml THF block is a relatively fast intramicellar process of ob- micelles. In addition, the diameter and volume of
copolymer solution with a syringe pump) both to taining local preferred interfacial curvature in each cylindrical segment is comparable to the di-
initiate aggregation of the hydrophobic blocks and isolated micelles through fast local chain adjust- mensions of the separate spherical micelles before
to solubilize the PAA-diamine complexes. This ment. The second is the relatively slow process of aggregation, further supporting the transition
slow addition of water first caused polymer phase reaching a global equilibrium by means of inter- mechanism. Branching appears as growth defects,
separation into polymer-rich domains with a local micellar interactions [through infrequent inter- as observed in Fig. 2, E and F. Theoretical predic-
lamellar nanostructure due to phase segregation of micellar single-chain exchange (21, 24, 25) or tion has shown that branching could occur in the
unlike blocks (22). Once sufficient water was micelle fusion or fission]. When combined, the one-dimensional aggregation of dipolar fluids
added, the phase-separated polymer droplets were mismatch of the two kinetic processes can produce when construction of a branch provides a lower
solublized into discrete micelles through the the well-defined hierarchical structure in Fig. 1, B free energy than the formation of a free chain end
segregation of the hydrophobic blocks into and C, and Fig. 2C schematically demonstrates the (30). In the current work, branching probably oc-

All scale bars: 200nm

Addition of THF Anisotropic growth

Spherical micelles Disklike micelles One-dimensional packing

Fig. 2. (A) Spherical micelles of PAA94-b-PMA103-b-PS44 formed at the 1:4 mechanism of spherical micelles. Sphere-disk transition occurred first as THF was
ratio of THF to water in the presence of EDDA (molar ratio of amine groups:acid introduced. Anisotropic shape of disk-like micelles allows for one-dimensional
groups = 1:1). (B) TEM image of one-dimensional aggregation of spherical preferred growth. Inserted schematic illustrates proposed chain packing of
micelles immediately after introducing THF into original solution to reach a 2:1 spherical micelles, disklike micelles and one-dimensional packing structures. (D)
final ratio of THF to water. Further growth of these short structures led to a giant Separate disklike micelles marked as black arrow. (E and F) Branches appear as
one-dimensional supra-assembly, as shown in Fig. 1, B and C. (C) Growth growth defect. The samples were stained with uranyl acetate aqueous solution.

648 3 AUGUST 2007 VOL 317 SCIENCE www.sciencemag.org

 REPORTS
curs because of polydispersity in size and shape of could be polydispersity in the sizes of disks, with immersing assembled one-dimensional structures
assembling micelle units. As intermicellar interac- slightly larger disks having more PAA-diamine into primary amine-coated gold nanoparticle
tion proceeds, there is a chance that some spherical surface area, so that two additional disks could aqueous suspension for several minutes. Dark
micelles do not have enough time to adjust their assemble, thus forming a branch. stripes in Fig. 3, A and B, are due to the high
packing geometry before assembling into a The periodic spacing of PAA was then used as electron density of gold nanoparticle–rich PAA
growing cylinder. This curved surface could then a template to interact with oppositely charged in- regions. Lattice fringes of gold nanoparticle single
allow two disklike micelles to attach, forming a organic nanoparticles to construct periodic hybrid crystals can be clearly seen in high-resolution
branch. An alternative branching mechanism materials. Hybrid superstructures were created by TEM imaging (Fig. 3C). In high-angle annular
dark-field (HAADF) imaging, the gold stripes are
Fig. 3. TEM images of visualized as parallel bright lines (Fig. 3, D and E).
directed gold nanoparticle Another advantage of using charged corona
assembly in the charged blocks is that multivalent counterions, such as
PAA region. (A and B) functionalized inorganic nanoparticles, can be used
Bright-field images. Dark to influence local micelle structure and act as
stripes are concentrated the stimulus for the formation of one-dimensional
gold nanoparticle areas. segmented nanostructures. This approach may be
Insert shows proposed used in concert with, or as an alternative to, the
structures. Yellow dots de- addition of solvent. If the block copolymer is
note gold nanoparticles. designed correctly, when spherical micelles come

Downloaded from www.sciencemag.org on June 10, 2012

(C) High-resolution TEM
into contact with functionalized inorganic nano-
(HRTEM) imaging of lat-
particles, they should transform into disks that
tice structure of gold single
crystals. (D and E) high- attractively collapse into segmented cylinders.
angle annular dark field This concept has been implemented with the
(HAADF) imaging of peri- addition of positively charged gold nanoparticles
odic gold stripes. Gold into a 50% THF/water suspension of PAA94-b-
particles appear as bright PMA103-b-PS130 spherical micelles, assembled
stripes. (F) TEM image of without added diamine. Each gold nanoparticle
periodic gold stripes had, on average, six primary amine groups on
when polyamine func- the surface and functioned as a multivalent
tionalized gold particles counterion to complex with PAA. When they
are used as counterions. were added to the block copolymer spheres, the
spheres collapsed into one-dimensional struc-
tures with alternating stripes of gold nano-
particle–rich layers and hydrophobic layers,
both perpendicular to the primary axis of the
assembly. The distance between PAA gold-laden
stripes was then 40 nm, as compared to approx-
imately 20 nm between PAA layers for the
sample in Fig. 1, B and C. This increased inter-
layer spacing was due to the increase of the PS
block length to 130 from 44 monomer repeat
units. Apparently, the structures shown in Fig. 3F
were not a consequence of pure one-dimensional
growth of spherical micelles, because the diameter
of the final cylindrical structures is about twice as
large as the original spherical micelles. However,
the addition of charged gold nanoparticles was able
to influence the assembly of spheres in a preferred
direction. The multivalent functionalized nanopar-
ticle can be chosen independently, and the spac-
ing between inorganic-rich layers of the final
one-dimensional assembly can be tuned by choos-
G 94 103
Br
117
ing different relative polymer block lengths.
O O HO O O O By taking advantage of slow kinetics of block
copolymer chains in solution and the complexation
of charged blocks with multivalent counterions,
Br
93 99
F
100
F
one can also produce complex micelles containing
O O HO O O O multiple hydrophobic blocks within the same
F F micelle core that can undergo local, intramicellar
F
phase separation. To obtain a single polymeric
Fig. 4. Nanostructured multicompartment cylinders. (A and B) Bright-field TEM images. Dark regions micelle geometry, such as cylinders, with each
present polypentafluorostyrene-chain rich area, (C and D) HAADF images of cylindrical micelles with micelle core constituted by multiple hydrophobic
internal phase-separated cores. (E) Cryogenic TEM (cryo-TEM) image of uniform cylindrical micelle at blocks, at least two different, linear triblock
40% water/THF solution. (F) Cryo-TEM image of cylindrical micelles with internal phase-separated cores copolymers are required, with similar overall
at 67% water/THF solution. (G) Schematic illustration of formation of multicompartment cylinders. molecular weight and relative block ratios but

www.sciencemag.org SCIENCE VOL 317 3 AUGUST 2007 649

REPORTS
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23. T. P. Lodge, Macromol. Chem. Phys. 204, 265 (2003).
complexation with the PAA blocks, thereby unfavorable energetic interactions between PPFS
24. R. Lund, L. Willner, D. Richter, E. E. Dormidontova,
forming aggregates with PAA-diamine cores. and PS, are only possible kinetically because of Macromolecules 39, 4566 (2006).
Notably, these aggregates contained each of the the forced mixing of unlike hydrophobic core 25. Y. Y. Won, H. T. Davis, F. S. Bates, Macromolecules 36,
triblock copolymers with both PS and PPFS blocks as a result of PAA complexing with 953 (2003).
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27. Z. Y. Chen et al., J. Am. Chem. Soc. 127, 8592 (2005).
of unlike hydrophobic blocks in the same Both the multicompartment cylinders with 28. Z. B. Li et al., Langmuir 21, 7533 (2005).
aggregate by PAA-diamine complexation. Next, phase-separated cores and the cylindrical nano- 29. Materials and methods are available as supporting
introduction of water into the THF solution to a structures with alternating layers of chemistry material on Science Online.
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aggregates, as a result of PAA and diamine copolymer architectures and chemistries. The key 32. We thank NSF for funding, specifically the Nanoscale
complexation, forced the local co-assembly of parameters are the combination of charged block Interdisciplinary Research Teams program under grant
DMR-0210247. Any opinions, findings, conclusions, or
unlike third hydrophobic blocks into the same interactions with multivalent counterions to influ- recommendations expressed in this material are those of
micelle core. In addition, the lack of chain ence both intra- and intermicellar interactions and the authors and do not necessarily reflect the views of
exchange in solution that disallows global chain solvent mixing to control the assembly pathways. NSF. We also thank the W. M. Keck College of
migration and maintains nonequilibrated micelle Engineering electron microscopy laboratory at the
References and Notes University of Delaware and the nuclear magnetic
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solution of mixed hydrophobic core cylinders.
Internal phase separation is clearly indicated by
the strong undulations along the cylinder surfaces
and the TEM contrast variation along the Capillary Wrinkling of Floating
Thin Polymer Films
cylinders. The larger, darker, and more spherical
regions within the cylinders are hypothesized to
be regions that are concentrated in PAA94-b-
PMA103-b-PPFS100 triblock copolymer. First, Jiangshui Huang,1,2 Megan Juszkiewicz,1 Wim H. de Jeu,2,3 Enrique Cerda,4 Todd Emrick,2
there is a higher interfacial energy between PPFS Narayanan Menon,1* Thomas P. Russell2*
and PMA, relative to PS and PMA, causing more
chain stretching within PPFS-rich core domains A freely floating polymer film, tens of nanometers in thickness, wrinkles under the capillary force
so as to limit PPFS interactions with surrounding exerted by a drop of water placed on its surface. The wrinkling pattern is characterized by the
PMA blocks. Second, the greater electron density number and length of the wrinkles. The dependence of the number of wrinkles on the elastic
of the PPFS block provides a greater ability to properties of the film and on the capillary force exerted by the drop confirms recent theoretical
scatter electrons and produce darker images in predictions on the selection of a pattern with a well-defined length scale in the wrinkling
the TEM. The thinner region of the undulating instability. We combined scaling relations that were developed for the length of the wrinkles with
cylinder would then be occupied primarily by those for the number of wrinkles to construct a metrology for measuring the elasticity and
PAA93-b-PMA99-b-PS117 (Fig. 4G). This internal thickness of ultrathin films that relies on no more than a dish of fluid and a low-magnification
cylinder phase separation only occurred at microscope. We validated this method on polymer films modified by plasticizer. The relaxation of
relatively higher amounts of water in the mixed the wrinkles affords a simple method to study the viscoelastic response of ultrathin films.
solvent solutions. Cryo-TEM showed uniform
cylinders without undulation on the surface at hin sheets are much more easily bent than deform out of plane to form wrinkles. This is an
only 40% water/THF solution after 4 days (Fig.
4E). However, multicompartment cylinders T stretched by external forces. Even under
purely planar tension, a sheet will often
everyday phenomenon that can be seen on our
skin as it is stretched by smiling, scars, or age;

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