Nature Materials 6, 94 - 95 (2007)
doi:10.1038/nmat1820
Nanocomposites: Economy at the nanoscale
Anna C. Balazs1
Anna C. Balazs is in the Chemical Engineering Department, University of
Pitturgh, Pitturgh, Pennsylvania 15261, USA. e-mail:
balazs1@engr.pitt.
edu
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AbstractExploiting the interplay between entropic and enthalpic
contributions in block copolymer–nanoparticle blends permits construction
of composites with specified structures. Disassembly can then provide well-
defined structural units as building blocks for future applications.
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IntroductionNature is parsimonious in its use of ingredients and design
principles. Even using them sparingly, however, it achieves a tremendously
diverse assortment of materials. For example, twenty amino acids are
sufficient to create a large array of complex macromolecules, which interact
to perform a stunning variety of functions. Applying this approach to the
fabrication of synthetic materials could have significant benefits; for
example, being able to fashion a host of structures from a all set of
building blocks would be both economical and efficient. On page 156 of this
issue, Ulrich Wiesner and colleagues1 take a step in this direction, by
creating a range of hybrid materials using just diblock copolymers and a
high volume fraction of nanoparticles. The self-assembling diblocks permit
access to various architectures, including lamellar, cylindrical,
bicontinuous and spherical forms. The diblocks, however, do not simply
template the ordering of the particles. Rather, the final morphology is
determined by a complex interplay between entropy and enthalpy within the
system. The 'art' in designing these materials is knowing how to exploit
this interplay to create the desired structure and expand the repertoire of
available morphologies.
The experiments reported by Wiesner et al. demonstrate that particle size is
a key variable in tailoring the morphology of the composite, and thus
confirm previous theoretical predictions2, 3. The synthesized particles are
chemically compatible with one of the blocks (for example, the B segment) of
the AB diblocks; hence this creates an enthalpic driving force for
incorporating the particles into the B phase of the material. If the
particle diameter is all compared with the contour length — the distance
between the two ends — of the B block, the particles are dispersed within
the B phase and the overall structure of the material is dictated by the
diblock copolymers. If, however, the particle diameter is large compared
with the contour length, the polymers must stretch around the particles to
accommodate these solids. Significant stretching lowers the entropy (degrees
of freedom) of the polymers because the extended chains can access fewer
conformations than the relaxed coils. When the particle diameter reaches a
critical size, this cost in free energy becomes too great and the particles
can no longer be lodged in the morphology that would be assumed by the
chains in the absence of the particles; instead, the particles now dictate
the structure. For example, the addition of relatively large particles
promotes the formation of onion-like or cylindrical3 mesophases, even though
the diblocks alone would form a lamellar structure. Through these
experiments, the authors show that by judiciously choosing the particle
sizes, the designer can tune the macroscopic morphology of the material.
The results of Weisner et al. also point to the possibility of synthesizing
more heterogeneous composites by using a mixture of larger and aller
nanoparticles with a B-like coating. In particular, they hypothesize that
one could assemble composites where the larger particles are segregated to
one region and the aller ones are localized in a different area. For
example, the larger particles with a B-like coating would nucleate a
cylindrical phase and localize in the core of the cylinders; the aller
particles would form a ring around the larger, central species. An example
of this structure is shown in Fig. 1, which is the result of prior
theoretical calculations4. Because the aller particles are excluded from
the central core, a all volume fraction is driven into the chemically
incompatible A region; thus, entropic effects overwhelm the unfavourable
enthalpic interactions. Such gradient materials could exhibit remarkable
behaviour; in particular, if the particles are semiconductors, then the
graded layers could display novel opto-electronic properties5.
Figure 1: Density (a,b) and surface plots (c,d) for a composite containing a
bidisperse mixture of nanoparticles and diblock copolymers (see ref. 4 for
details).
The density and surface plots for the larger particles are in (a) and (c),
respectively, and for the aller particles in (b) and (d), respectively.
The light regions in the density plots mark the presence of the species, and
the black regions mark the absence of the species. Thus, the density plots
clearly show that the larger particles are localized in the centre of the
cylindrical phase, and the aller particles form a corona around the larger
species.
Full size image (33 KB)
This study could have even greater implications; that is, by controlling the
entropic contributions through variations in particle size and enthalpic
contributions through variations in particle coatings, one can create
inorganic composites that could not be formed through any other means. This
is particularly important at high particle loadings. For example, by mixing
both the larger B-particles (as in Fig. 1) and aller A-particles with the
matrix AB diblocks, particle-filled cylinders of B could be formed within a
matrix filled with A particles. Heating the sample would burn away the
organic component, sinter the particles and potentially yield an ordered
inorganic blend6. This approach could open new pathways for creating
inorganic alloys, which combine materials that would not form a periodic
material in nature.
In addition to describing an elegant approach to assembling well-ordered
composites, Wiesner and colleagues go on to disassemble the composites into
aller, but well-defined, structural components. Through appropriate choice
of solvent and some form of agitation (stirring or sonication), larger
systems fall apart along chemically weak or physically stressed planes to
yield nanoscale objects with specific architectures. Filtration of the
system yields monodisperse nanotubes, hexapods and other complex, well-
defined units.
The fabrication of nanoscale devices would be greatly facilitated by the
availability of a tool box of nanoscale components that could be used as
basic building blocks. The retro-synthesis demonstrated in this study
produces a range of such structural building elements, which might be highly
challenging to fabricate through other means. What is intriguing is that,
like nature, the retro-synthetic approach is also parsimonious in that one
sample provides a number of different structural units. The studies carried
out by Wiesner et al. therefore point to an effective approach that mimics
design principles adopted by nature.
Top of pageReferences
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