Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
Anton W. Bosman
SELF HEALING IN ACTION
Anton W. Bosman
SupraPolix BV
Horsten 1, 5612 AX Eindhoven
The Netherlands
e-mail: bosman@suprapolix.com
www.suprapolix.com
Sophisticated polymeric materials with “responsive” properties like self healing are beginning to reach the
market. The use of reversible, non-covalent interactions is a recurring design principle for these materials. Now,
recently developed hydrogen bonding units allow for taking this design principle to its extreme. Supramolecular
polymers, where hydrogen bonds are responsible for the interactions between the polymer chains, form materials
whose (mechanical) properties strongly respond to a change in temperature or solvent. In this presentation,
hydrogen bonded supramolecular polymers based on SupraB® are presented, and the use of these materials in
self healing applications is discussed.
Keywords: Supramolecular Polymers – self-healing – hydrogen bonding – ureido pyrimidone
– UPy
1
Introduction
In the past 60 years, polymers have revolutionized our lives. Their low prize, high
processability and exceptional mechanical properties have led to the use of polymers in ever
more sophisticated applications. Presently, a strong interest in “smart” polymers to be used in
self healing applications is beginning to expand the potential field of application of polymeric
materials even further. Key to the responsiveness of these smart materials is the reversibility
of the noncovalent interactions that lead to the change in properties. This field of
supramolecular chemistry has been developed over the past 25 years by synthetic chemists,
and was strongly inspired by the ubiquity of reversible yet highly specific intermolecular
processes in nature. i Despite their short history, supramolecular polymers are already
beginning to find commercial use, in applications that take advantage of the reversibility and
responsiveness of non-covalent interactions. ii In this presentation the reversible SupraBpolymers and their (possible) uses as self-healing materials in different applications will be
highlighted.
Autonomous healing, or repairing, of damaged polymeric materials have always been an
attractive means to prolong the service life of polymeric devices, whereby these damages can
range from cracks in polymeric structural engineering materials to scratches in polymeric
coatings. Initially, healing systems used the increasing mobility of polymeric chains with
heat. In this way, chain entanglements in the damaged region could reform upon heating,
thereby repairing the damage. iii
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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
Anton W. Bosman
This approach only works for thermoplastic systems and is hampered by high melt viscosities
of the polymeric chains as this prevent good formation of interfacial entanglements. In
another approach, chemical reactions have been used to repair damages to polymeric resins. iv
Basically, reactive chemical formulations were brought on the damaged interfaces and repair
took place by subsequent hardening of the formulation. As a consequence, the quality of the
repaired crack is strongly depending on the interaction between the fractioned polymer and
the reactive formulation, thus chemical compatibility and surface roughness are very
important. Moreover, the device is not able to autonomously repair itself, as chemicals have to
be added to the device.
Recently, a next step has been made in repairing polymeric resins by embedding the repairing
chemical formulation inside the polymer matrix and using crack formation as initiator for the
healing chemistry. v In this way, a truly self-repairing polymer system is designed. However,
such polymers have to contain reactive chemicals and possible toxic catalysts. Moreover, the
chemical reaction only can take place once, and the presence of the capsules with the healing
ingredients in the polymeric material may deteriorate the material’s mechanical properties.
Therefore, this approach has been improved by designing a polymer that contains thermoreversible linkages, that is a polymeric resin held together by linkages obtained via a DielsAlder reaction. vi As this reaction is thermo-reversible, heating the resin to temperatures above
130°C results in a retro Diels-Alder reaction and consequently reversible opening of the
cross-links followed by flow of the polymer and subsequently repair of the material. Although
no catalyst is needed, this approach still uses chemical reactions in which covalent bonds are
formed that are demanding on relative orientations and prone to side reactions that do not
contribute to repairing.
Here, it is shown that the use of non-covalent, reversible cross-links opens the way to
polymeric materials that behave as they were cross-linked at room temperature but still can be
self-healed at elevated temperatures without the need of forming new covalent bonds, and
hence without the need for reactive chemicals or (toxic) catalysts. This can be done by using
supramolecular polymers, which are formed by the reversible association of low molecular
weight prepolymers.
2
Supramolecular polymers
To obtain polymers with sufficient cross-linking density, a high association constant between
the repeating units is a prerequisite. The reversible interaction that is used here is based on
hydrogen bonding. Although hydrogen bonds between neutral organic molecules are not
among the strongest non-covalent interactions, they hold a prominent place in supramolecular
chemistry due to their directionality and versatility. vii,viii,ix Meijer and Sijbesma have shown
that combining four hydrogen bonds in a functional unit, employing a particular arrangement,
results in a strong increase of association strength between those units. x,xi,xii These selfcomplementary quadruple H-bonding units based on 2-ureido-4[1H]-pyrimidinones
(SupraB®, see Figure 1), are reported to dimerize in chloroform with an association constant
of Kdim=6×107 M-1.xi,xiii
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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
Anton W. Bosman
Figure 1: chemical structure of a hydrogen bonded dimer of SupraB® or 2-ureido-4[1H]-pyrimidone (left) and its
X-ray structure with R = Me and R’ = n-Bu
The reversibility of the linkages between the building blocks is instrumental in the
development of materials that change their properties in response to environmental changes,
so called ‘smart materials’. Although the supramolecular polymers based on bifunctional
ureidopyrimidinone derivatives in many ways behave like conventional polymers, the strong
temperature dependence of their mechanical properties really sets them apart from
macromolecular polymers. At room temperature, the supramolecular polymers show polymerlike viscoelastic behavior in bulk and solution, whereas at elevated temperatures liquid-like
properties are observed (Figure 2).
Figure 2: Illustration of the phase-changes in a rubberlike material with SupraB ® upon heating or dissolving
As the SupraB unit is readily accessible from bulk chemicals on a large scale, the application
of the SupraB- technology in polymers of industrial relevance is straightforward. This has
resulted in the development of SupraB-materials based on, amongst others, polysiloxanes, xiv,xv
poly(ethylene/butylenes), xvi polyethers, xvii polyesters, xviii,xix, and poly(meth)acrylates. xx,xxi In
all these compounds, the material properties have been shown to improve dramatically upon
functionalization, and materials were obtained that combine many of the mechanical
properties of conventional macromolecules with the low melt viscosity of organic
compounds. Consequently, in this strategy the gap is closed between polymers and oligomers
using the best of both worlds.
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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
Anton W. Bosman
Moreover, the use of monomers with a functionality of three or more, will give rise to
network formation. However, in contrast to condensation networks, the ‘self-healing’
supramolecular network can reassemble to form the thermodynamically most favorable state,
thus forming denser networks.xvii
3
Self-healing properties
Because of the thermo-reversible nature of supramolecular polymers with SupraB, applying
heat results in a decrease in the hydrogen bonding interactions between the polymer chains
and consequently a strong decrease in dynamics of the individual polymers. The reduced melt
viscosities allow easy flow of the polymeric materials and can be used to repair damages.
Upon cooling, the hydrogen bonding interactions between the chains are rebuilt and
consequently the non-covalent cross-linking in the material, giving thematerial its properties
back.
The applicability of this approach has been demonstrated in coatings made of elastomers
consisting of hydrogenated polybutadiene modified with SupraB (polymer 1). Polymer 1
shows visco-elastic behavior and flows at temperatures above 100°C. Consequently, a coating
consisting of polymer 1 was damaged with a sharp object, followed by heating to 140°C. At
this temperature the hydrogen bonding between the polymers is strongly reduced and because
of the relative low molecular weight of the polymer, a good flow of the material is observed.
This results in leveling out of the coating in the damaged area. After cooling down the
original coating properties are restored again and the damaged area is not visible anymore
(Figure 3a).
In another approach, a hard siloxane coating was made using polymer 2, a telechelic PDMS
functionalized with SupraB. This material has a thermal transition above 100°C. Indeed, a
spin-coated film of polymer 2 could be repaired after damage by heating the film to 140°C.
Again, the thermal reversibility of the SupraB-units allowed good flow at this temperature and
subsequent healing of the film (see figure 3b).
Figure 3: Self-healing at 140°C for polymer 1 (top) and polymer 2 (bottom)
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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
Anton W. Bosman
The self-healing approach can also be used for functional coatings. Here, an adhesive is
combined with the SupraB technology resulting in polymer 3: a soft PDMS prepolymer
functionalized with SupraB. This supramolecular PDMS adhesive shows good adhesion to
glass, probably because of the high flow of the material at high temperatures allowing good
wetting of glass-surface. Not only the adhesion is good, but also the cohesive strength because
of the presence of the hydrogen-bonding interactions in the polymeric material. As a result,
gluing two glass slides together with this SupraB-adhesive, resulted in a very strong adhesion.
However, heating the bonded glass-slides to 140°C weakened the hydrogen bonding
interactions and, therefore, resulted in cohesive failure and consequently in debonding of the
two glass slides.
In addition, these debonded slides could be bonded again by repositioning them followed by
cooling down to room temperature. In conclusion, the thermo-reversible nature of this Supraadhesive allows strong temporarily bonding of substrates and the possibility to repair the
adhesive linkage by applying heat.
4
Conclusions
Polymeric materials modified with ureido-pyrimidones (SupraB®) show strong temperature
dependant properties because of the presence of strong hydrogen-bonding interactions:
material properties belonging to cross-linked materials at room temperature, and low viscous
melts at elevated temperatures. This property allows these materials to be used in self-healing
applications in which the temperature can be used as a trigger for the healing process. In this
way, no reactive chemicals are needed and no covalent bond formation has to take place,
increasing not only the fidelity of the healing process but also the number of times that
healing can take place. These features can be applied to generate self-healing coatings and
adhesives that fully benefit of the reversibility of hydrogen bonding interactions.
REFERENCES
i
ii
iii
iv
v
vi
vii
viii
ix
x
xi
xii
xiii
Brunsveld, L.; Folmer, B. J. B.; Sijbesma, R. P.; Meijer, E.W. Chem. Rev. 2001, 101, 4071.
Bosman, A. W.; Sijbesma, R. P.; Meijer, E. W. Mater. Today 2004, 7, 34.
Bucknall, C. B.; Drinkwater, I. C.; Smith, G. R. Pol. Eng. Sci. 1980, 20, 432.
Raghavan, J. Wool, R. P. J. Appl. Pol. Sci. 1999, 71, 775.
White, S. R. Sottos, N. R.; Geubelle, P. H.; Moore, J. S.; Kessler, M. R.; Sriram, S. R.; Brown, E. N.;
Viswanathan, S. Nature 2001, 409, 794.
Chen, X.; Dam, M. A.; Ono, K.; Mal, A.; Shen, H.; Nutt, S. R.; Sheran, K.; Wudl, F. Science, 2002,
295, 1698.
Krische, M. J.; Lehn, J.-M. Struct. Bond. 2000, 96, 3.
Sijbesma, R. P.; Meijer, E. W. Curr. Opin. Colloid Interface. Sci. 1999, 4, 24.
Fredericks, J. R.; Hamilton, A. D. Comprehensive Supramolecular Chemistry, chapter 16, Lehn, J.-M
(ed.), Pergamon, New York, 1996.
Beijer, F. H.; Sijbesma, R. P.; Kooijman, H.; Spek, A. L.; Meijer, E. W. J. Am. Chem. Soc. 1998, 120,
6761.
Beijer, F. H; Kooijman, H.; Spek, A. L.; Sijbesma, R. P.; Meijer, E. W. Angew. Chem., Int. Ed. Engl.
1998, 37, 75.
Sijbesma, R. P.; Beijer, F. H.; Brunsveld, L.; Folmer, B. J. B.; Hirschberg, J. H. K. K.; Lange, R. F. M.;
Lowe, J. K. L.; Meijer, E. W. Science 1997, 278, 1601
Söntjens, S. H. M.; Sijbesma, R. P.; van Genderen, M. H. P.; Meijer, E. W. J. Am. Chem. Soc. 2000,
122, 7487.
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Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
xiv
xv
xvi
xvii
xviii
xix
xx
xxi
Anton W. Bosman
Hirschberg, J. H. K. K.; Beijer, F. H.; van Aert, H. A.; Magusin, P. C. M. M.; Sijbesma, R. P.; Meijer,
E. W. Macromolecules 1999, 32, 2696.
Bosman, A. W.; H. M. Janssen, van Gemert, G. M. L.; Versteegen, R. M.; Meijer, E. W.; Sijbesma, R.
P. WO2004052963, 2004, patent to SupraPolix BV.
Folmer, B. J. B.; Sijbesma, R. P.; Versteegen, R. M.; van der Rijt, J. A. J.; Meijer, E. W. Adv. Mater.
2000, 12, 874.
Lange, R. F. M.; van Gurp, M; Meijer, E. W. J. Polym. Sci. A 1999, 37, 3657.
Dankers, P. Y. W.; van Leeuwen, E. N. M.; van Gemert, G. M. L.; Spiering, A. J. H.; Harmsen, M. C.;
Brouwer, L. A.; Janssen, H. M.; Bosman, A. W.; van Luyn, M. J. A.; Meijer, E. W. Biomat. 2006, 27,
5490.
Janssen, H. M.; van Gemert, G. M. L.; Meijer, E. W.; Bosman, A. W. EP1687378, 2005, to SupraPolix
BV.
Janssen, H. M.; van Gemert, G. M. L.; ten Cate, A. T.; van Beek, D. J. M.; Sijbesma, R. P.; Meijer, E.
W.; Bosman, A. W. US20040034190, 2004, to SupraPolix BV
Yamauchi, K.; Lizotte, J. R.; Long, T. E. Macromolecules, 2002, 36, 1083.
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