Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
Christopher W. Bielawski
TOWARD CONDUCTIVE, SELF-HEALING
MATERIALS
Christopher W. Bielawski
The University of Texas at Austin, Department of Chemistry and Biochemistry, 1 University
Station, A5300, Austin, TX 78712 USA
Tel: 512-232-3839
Fax: 512-471-8696
e-mail: bielawski@cm.utexas.edu
URL: http://www.cm.utexas.edu/directory/christopher_bielawski
A novel class of organometallic polymers based on multitopic N-heterocyclic carbenes and transition metals was
shown to have potential as an electrically-conductive, self-healing material. These structurally-dynamic materials
were found to display conductivities on the order of 10-3 S·cm-1. Thin films of these materials were prepared on
silicon wafers, then scored and imaged using scanning electron microscopy (SEM). Heating the scored films
allowed the material to undergo a unique depolymerization process and flow into the vacant regions, as observed
by SEM and surface profilometry. A proposal for how these features may be incorporated into a device that
displays electrically-driven, self-healing functions is presented.
Keywords: self-healing, conductive polymers, dynamic polymers
Stress induced microcrack formation is a main culprit of fatigue in mechanical materials and
failure in electronic componentry [1]. Contemporary research has focused on solving this
deleterious problem by developing “self-healing materials.” Notable examples include epoxybased composites containing encapsulated healing agents [2-4] and thermally-remendable
plastics [5,6]. However, the materials used in these systems are electrically-insulating, which
precludes their use in related applications. By installing features into these materials that
enable electron conductivity, a number of useful features may be realized. For example,
through electronic feedback, one may obtain instantaneous status of a material’s structural
integrity. This feature could lead to new approaches for detecting and quantifying
microcracks and/or materials that are capable of recording their stress/load histories. Other
possibilities include using externally-applied electric fields to drive the self-healing process.
Materials exhibiting self-healing and conductive properties can be expected to offer
advantages in consumer electronics, alternatives to sophisticated redundant electronic
systems, and many other applications.
In order to realize a self-healing material with conductive properties, the development of a
new responsive polymer with the following three features was needed: (a) the polymer must
be electrically-conductive, (b) the polymerization process must be dynamic and respond to
changes in external stimuli, and (c) main-chain unsaturation, a key requirement for
conductivity, must be conserved or increased upon polymerization (see Eq. 1). For practical
reasons, the polymerization process should also not involve the formation of by-products.
Finally, a polymer with modular components was desired to facilitate systematic tuning of
material and electronic properties.
1
© Springer 2007
Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
Christopher W. Bielawski
Although a broad range of polymerizations comprised of dynamic interactions are known, the
resulting polymers are not conductive [7-14]. To create polymers with these characteristics a
chemical interaction that is both dynamic and extends electronic communication is necessary.
The interaction between N-heterocyclic carbenes (NHCs) and transition metals is known to be
reversible and the electronic communication of these systems has been well-studied [15,16].
To utilize this interaction in polymeric materials, compounds possessing more than one NHC
unit (e.g., 1) were needed.
R
N
R
N
R
N
N
R
N
R
N
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R
N
N
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n
2
N
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N
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R
R
E
E
R
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R
N
N
R
N
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N
E
1
E
N
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3
M
R
N
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n
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ar ene li nker
M
transition metal
E
electr ophile
R
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4
N
R
n
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R
Figure 1: Dynamic equilibria between difunctional NHCs, electrophiles, transition metals, and their respective
polymers
Through a series of synthetic developments, a range of multifunctional NHCs were prepared
[17, 18]. The bis(NHC) scaffold was designed to contain many modular components,
including the N-substituent, aryl linker, and heteroatoms. Collectively, these compounds
proved to be versatile building blocks for the synthesis of polymeric materials, as they could
be homopolymerized to form poly(enetetraamine)s (2), reacted with difunctional electrophiles
to produce alternating copolymers (3), and combined with transition metals to obtain welldefined organometallic polymers (4) [19-22]. As described below, each of the polymers
obtained from these reactions display unique characteristics vital for development of
conductive, self-healing materials.
Depending on the size of the N-substituent and temperature, the poly(enetetraamine)s (2)
were found to exist in equilibrium with free monomer in solution [19]. As expected, smaller
N-substituents favored polymer formation, whereas higher temperatures, as well as larger Nsubstituents, resulted in an increase of free monomer. Evidence for this phenomena was
obtained using NMR spectroscopy in conjunction with UV-Vis spectroscopy. The latter
indicated that the effective conjugation length of these polymers increased with molecular
weight. Even though polymers 2 were only stable under oxygen-free conditions, the thermal
control over this reversible polymerization reaction coupled with their electronic
characteristics indicate these materials hold potential in conductive, self-healing materials.
Efforts toward utilizing these compounds in such applications are underway.
2
© Springer 2007
Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
Christopher W. Bielawski
The synthesis of alternating copolymers 3 (E = N3) capitalized on our recently reported results
of reacting NHCs with azides to afford 1,3-disubstituted-triazenes [23]. By combining free
bis(NHC)s with bis(azide)s (e.g., 1,3-diazidobenzene), a new class of conjugated
polytriazenes were prepared. More recently, we have focused efforts on studying the
reversible reaction of NHCs with isothiocyanates. Similarly, free bis(NHC)s are currently
being combined with bis(isothiocyanate)s [24] to form reversible, conjugated polymers.
Characterization of these materials, as well as the evaluation of their potential as self-healing
materials, is currently in progress.
Organometallic polymers (4) with molecular weights up to 1.8 × 106 Da and excellent thermal
stabilities (up to 300 °C) were also prepared using bis(NHC)s [20]. Variations of 4 included:
primary alkyl, benzyl, and aryl N-substituents, benzo, biphenyl, or dioxin aryl linkers, and the
use of Pd or Pt as the transition metal. It was discovered that the molecular weights of
polymers 4 could be controlled through the use of chain-transfer agents (CTAs), and that
these structurally dynamic polymers exhibited conductivities of the order of 10-3 S•cm-1.
However, due to the high affinity of these metals for NHCs, structurally-dynamic properties
were observed only at elevated temperatures. However, in order to utilize metals with weaker
affinities for NHCs, an extra source of chelation was needed. This was achieved by
incorporating chelating N-o-phenol moieties in the basic bis(NHC) structure. This enhanced
stability allowed access to polymers containing Ni in addition to Pd and Pt [21].
Due to the combination of structurally dynamic and conductive character, the potential of
organometallic polymers 4 as conductive, self-healing materials was investigated using
scanning electron microscopy (SEM). Notably, as a result of their high conductivities, the
deposition of gold was found to be unnecessary to obtain visible images of these materials.
Thin films (800 nm) were first cast on silicon wafers and, to emulate microcrack formation,
the films were scored with a sharp razor blade. After imaging (Figure 2A), the films were
then heated (200 °C, 25 min) and then re-imaged (Figure 2B). Comparison of the images
obtained pre- and post-thermal treatment clearly indicated that rough edges introduced by the
razor blade were smoothened
Since the dynamic behavior of the system was previously established in solution, it was
predicted that the presence of solvent might facilitate the reformation of broken NHC-metal
bonds. Hence, a thin film of material was cast on a silicon wafer, and a crack was introduced
and imaged in a manner analogous to the one described above (Figure 2C). The wafer was
then heated (150 °C, 2 h) in a compartmentalized sealed vessel. A pool of solvent
(dimethylsulfoxide) was placed in a contiguous compartment such that solvent vapor would
facilitate healing. Comparison of the SEM image of the treated film (Figure 2D) indicated
that the crack induced by the razor blade was now reloaded with material. This observation
was supported using surface profilometry, which indicated that the depth of the crack had
been significantly reduced (800 nm → ~0 nm) in the healed films. Efforts toward eliminating
the reliance on solvent vapor to facilitate this process are currently underway.
3
© Springer 2007
Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
A
B
C
D
Christopher W. Bielawski
Figure 2: Scanning electron micrographs of a scored organometallic polymer film before (A) and after (B)
exposure to heat (200 ºC, 15 min); a scored organometallic polymer film before (C) and after (D) exposure to
heat (150 ºC, 2 h) in the presence of solvent vapor. Arrows indicate a common point of reference
Shown in Figure 3 is a generalized depiction of how a conductive, self-healing material may
operate once incorporated into a device. Microcrack formation should decrease the total
number of electron percolation pathways within the material resulting in a concomitant rise in
electrical resistance. Integration of the material into a circuit containing an ammeter /
voltmeter, could allow the drop in conductivity (or rise in resistance) to trigger an increase in
the applied electric field. Since the microcrack is the source of the increased resistance, this
voltage bias should result in the generation of heat localized at the microcrack. By harnessing
the produced thermal energy, the system may be electrically-driven back to its original lowresistance/high-current state.
AMPS
VOLTS
AMPS
AMPS
e
e
crack
formation
polymerization
VOLTS
Voltage is biased; The higher
resistance is used to generate heat
which facilitates self-healing.
Microcrack disrupts conductivity;
Resistance increases.
Material is electrically conductive.
e
VOLTS
crack
repair
multifunctional
monomer
polymeric network
network with microcrack
a snapshot of the self-healing process
Figure 3: Operation of an electrically-conductive, self-healing material
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© Springer 2007
Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
Christopher W. Bielawski
Collectively, these results suggest that self-healing capabilities were observed in materials
with electrically-conductive properties. However, many challenges must be solved before
their potential is fully realized. Most importantly, the dependency on solvent vapor to
facilitate healing must be eliminated. Incorporating bulky N-alkyl groups into the
multifunctional NHCs should descrease polymer viscosity, helping material to flow into
microcracks upon depolymerization. In order for these polymeric materials to be broadly
useful, their conductivities must be enhanced to ≥ 1 S·cm-1. We believe this may be achieved
by matching the reduction-oxidation potentials of the transition metal with the N-heterocyclic
carbene through substrate modification [25]. Efforts toward these goals, as well as exploring
the potential of these materials in a variety of electronic applications, are in progress.
ACKNOWLEDGEMENTS
We are grateful to the U.S. Army Research Office (W911NF-05-1-0430), the Welch Foundation (F-1621), the
Petroleum Research Fund as administered by the American Chemical Society (44077-G1), and The University of
Texas at Austin for generous financial support.
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Proceedings of the First International Conference on Self Healing Materials
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Christopher W. Bielawski
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