Nature Materials 6, 9 - 11 (2007)
doi:10.1038/nmat1812
Nanocomposites: Stiffer by design
Evangelos Manias1
Evangelos Manias is in the Department of Materials Science and Engineering,
Pennsylvania State University, University Park, Pennsylvania 16802, USA. e-
mail:
manias@psu.edu----------------------------------------------------------------------------
----
AbstractThe full potential of nanoparticles in imparting new functionalities
in polymer nanocomposites remains largely untapped. A widely applicable,
two-solvent processing approach provides a hierarchical structure, affording
unparalleled composite performance enhancement.
----------------------------------------------------------------------------
----
IntroductionFor most current applications of plastics, performance
requirements, as well as cost and processing considerations, necessitate the
introduction of additives and fillers into the polymer matrices. The use of
nanoscale fillers push this strategy to the next level by exploiting the
advantages that nanometre-size particulates offer compared with macro- or
microscopic fillers, such as huge surface area per mass, ultra-low filler
levels required for connectivity through the sample (low percolation
threshold), extremely all interparticle separations in a polymer matrix,
and often very high length-to-width, or aspect, ratios. Most of the research
efforts, and almost all of the commercial examples, of polymer-based
nanocomposites to date have revolved around introducing nanoscale fillers
into polymers and simply capitalizing on the filler properties to enhance
performance of the composites — this produces a class of polymer/inorganic
materials that is perhaps better described by the term 'nano-filled polymers
'. On the other hand, the formation of 'genuine' nanocomposites introduces
new physical properties and novel behaviours that are absent in the unfilled
matrices, effectively changing the nature of the original polymer. This
class of materials can be termed as 'polymer/inorganic hybrids' or '
molecular composites'1. On page 76 of this issue, McKinley and colleagues2
present a generally applicable approach to creating polymer nanocomposites
with hierarchical structure, which fall into the latter category. By
selectively reinforcing certain domains of a phase-separated block copolymer
with an inorganic nanofiller using a two-solvent processing method, they
achieve unprecedented performance improvements in stiffness, strength and
heat resistance, without losses in elasticity of the composite. Their
approach should be applicable to other combinations of materials, providing
a new direction for future nanocomposite design.
In polymer systems with an existing phase-separated morphology, such as
polymer blends and copolymers, selective reinforcements can lead to genuine
nanocomposite formation offering exciting opportunities and novel properties
, for example, preferential reinforcement of one phase3 or targeted
reinforcement of the interphase3, 4, filler-induced changes in phase
morphology4 or phase alignment, filler-induced compatibilization of
immiscible polymer phases, and changes of thermo-mechanical transitions5.
Figure 1 shows examples of systems where nanofillers have been selectively
placed in certain regions of phase-separated blends. These have all been
made possible by exploiting thermodynamic mixing of the filler with one
component of the polymer phase.
Figure 1: A variety of multiphase polymer systems with selectively
positioned nanofillers.
a–c, polystyrene-bearing gold nanoparticles in a polystyrene–poly(2-
vinylpyridine) block-copolymer (PS-b-PVP): gold nanoparticles selectively
sequestered in the PS phase of a cylindrical PS-b-PVP morphology3 (a),
copyright (2005) Wiley-VCH; gold nanoparticles preferentially dispersed at
the PS/PVP interface of a lamellar morphology (b) and in the PS lamellae (c)
4, copyright (2006) American Chemical Society. d, Montmorillonite layered
silicates selectively dispersed in the poly(ethylene terephthalate) domains
of a poly(ethylene terephthalate)/polycarbonate blend. All these examples
rely on highly sensitive and system-specific tailoring of thermodynamics to
control the nanofiller partitioning. The approach presented by McKinley and
colleagues2 is a more general solvent-based approach that should enable
similar preferential reinforcements in a wide variety of multiphase polymer
systems.
Full size image (72 KB)
McKinley and co-workers used an elastomeric block copolymer composed of hard
segments and soft segments; the hard segments phase segregate to form hard
crystalline domains through hydrogen bonding. The authors used a two-solvent
processing approach to disperse nanoparticles of Laponite — a layered
silicate clay — preferentially within the crystalline hard domains, while
the soft domains remained largely unaffected. Pristine Laponite is not
thermodynamically miscible with hydrophobic polymers such as the one used by
McKinley and colleagues, but this solvent exchange approach kinetically
traps the inorganic nanofillers within the hard domains. The two solvents
are selected to form a co-suspension of polymer and nanofillers and, during
the evaporation of the second solvent, the inorganic fillers are immobilized
within the hard domains owing to strong hydrogen-bonding interactions.
The selective reinforcement, combined with filler loadings above the
percolation threshold and a choice of particulates that are comparable in
size to the hard domains, leads to the formation of an extended
interconnected network of hard microdomains. This morphology gives rise to
unprecedented increases in stiffness of up to 23-fold and concurrent
improvements in strength and heat deflection temperature (by up to 100 oC),
while the elastic properties of the polymer are retained. This is a
remarkable and unique set of property improvements that is impossible to
obtain through traditional composite approaches and originates directly from
the hierarchical morphology of these nanocomposites. The improvement in
stiffness without sacrificing elasticity arises from the fact that the
nanofillers are preferentially located in the hard domains. When the
nanofillers are also dispersed in the soft domains, the elastic properties
are lost2; in the extreme case, where the nanofillers are preferentially
dispersed in the soft domains, the strength and ultimate extensibility of
the nanocomposite are dramatically reduced6. In addition to this
preferential reinforcement of the hard domains, the ultra-high magnitude of
stiffness improvement is only possible because the fillers are of similar
size to the hard domains (about 10 nm) and their volume fraction is high
enough to allow for the development of interconnected network structures.
When the volume fraction is below the level needed to create these extended
structures, the magnitude of stiffness improvement is substantially aller
(about threefold2) and comparable to that of polyurethane elastomers 'nano-
filled' by montmorillonite layered silicate particles7 that are ten times
larger. Larger nanofillers usually provide greater stiffness enhancements,
so the preferential location of the filler in McKinley and co-workers'
composites is critical to the magnitude of improvement in properties.
The impacts of the above study reach far beyond improving the
thermomechanical properties of polyurethane elastomers by layered-silicate
fillers, particularly because this approach is not limited to the system
studied. For example, a natural extension of the above design principles
aiming to selectively disperse carbon-nanotube fillers in shape-memory
polyurethane matrices5 would almost certainly lead to large improvements of
the shape-memory properties of these systems — an idea that is pointed out
by the authors. In another example, selective reinforcement of the
hydrophobic phase of a fuel-cell proton-exchange membrane — aiming to
increase its stiffness by formation of an extended network — would also
prevent collapse of the hydrophilic domains at low hydration levels, thus
extending the fuel-cell operation to much higher temperatures than currently
possible. The two-solvent approach put forward by McKinley and co-workers
would allow for such a preferential reinforcement of the selected/
appropriate phase in both the above examples, as it relies on selective
solvation of filler and polymer rather than polymer/filler thermodynamics of
mixing.
These materials are a paradigm of exploiting the hierarchical nanocomposite
structure, rather than the inherent physical properties or the reinforcement
ability of the nanofillers, to give rise to novel properties. This work
thereby exemplifies a shift in focus from 'nano-filled' composites to
genuine 'molecular-composite hybrids', where the interconnection of
selectively reinforced domains leads to new functionalities.
Top of pageReferences
Vaia, R. A. & Wagner, H. D. Mater. Today 7, 32–37 (November, 2004). |
ChemPort |
Liff, S. M. et al. Nature Mater. 6, 76–83 (2007). | Article |
Bockstaller, M. R., Mickiewicz, R. A. & Thomas, E. L. Adv. Mater. 17, 1331–
1349 (2005). | Article | ChemPort |
Kim, B. J., Bang, J., Hawker, C. J. & Kramer, E. J. Macromolecules 39, 4108
–4114 (2006). | Article | ChemPort |
Koerner, H., Price, G., Pearce, N. A., Alexander, M. & Vaia, R. A. Nature
Mater. 3, 115–120 (2004). | Article | ChemPort |
James-Korley, L. T., Liff, S. M., Kumar, N., McKinley, G. H. & Hammond, P. T
. Macromolecules 39, 7030–7036 (2006). | Article | ChemPort |
Xu, R., Manias, E., Snyder, A. J. & Runt, J. Macromolecules 34, 337–339 (
2001). | Article | ChemPort |
