A healable supramolecular polymer blend based on aromatic π-π Stacking and hydrogen-bonding interactions

Article abstract of DOI:10.1021/ja104446r

An elastomeric, healable, supramolecular polymer blend comprising a chain-folding polyimide and a telechelic polyurethane with pyrenyl end groups is compatibilized by aromatic π-π stacking between the π-electron- deficient diimide groups and the π-electron-rich pyrenyl units. This interpolymer interaction is the key to forming a tough, healable, elastomeric material. Variable-temperature FTIR analysis of the bulk material also conclusively demonstrates the presence of hydrogen bonding, which complements the π-π stacking interactions. Variable-temperature SAXS analysis shows that the healable polymeric blend has a nanophase-separated morphology and that the X-ray contrast between the two types of domain increases with increasing temperature, a feature that is repeatable over several heating and cooling cycles. A fractured sample of this material reproducibly regains more than 95% of the tensile modulus, 91% of the elongation to break, and 77% of the modulus of toughness of the pristine material.

Full text of DOI:10.1021/ja104446r

Published on Web 08/10/2010
A Healable Supramolecular Polymer Blend Based on Aromatic
π-π Stacking and Hydrogen-Bonding Interactions
Stefano Burattini,† Barnaby W. Greenland,† Daniel Hermida Merino,†
Wengui Weng,‡ Jonathan Seppala,‡ Howard M. Colquhoun,*,† Wayne Hayes,*,†
Michael E. Mackay,‡ Ian W. Hamley,† and Stuart J. Rowan§
Department of Chemistry, UniVersity of Reading, Whiteknights, Reading RG6 6AD, U.K.,
Department of Materials Science and Engineering, UniVersity of Delaware, Newark,
Delaware 19716, and Department of Macromolecular Science and Engineering, Case Western
ReserVe UniVersity, 2100 Adelbert Road, CleVeland, Ohio 44106
Received May 23, 2010; E-mail: h.m.colquhoun@reading.ac.uk; w.c.hayes@reading.ac.uk
Abstract: An elastomeric, healable, supramolecular polymer blend comprising a chain-folding polyimide
and a telechelic polyurethane with pyrenyl end groups is compatibilized by aromatic π-π stacking between
the π-electron-deficient diimide groups and the π-electron-rich pyrenyl units. This interpolymer interaction
is the key to forming a tough, healable, elastomeric material. Variable-temperature FTIR analysis of the
bulk material also conclusively demonstrates the presence of hydrogen bonding, which complements the
π-π stacking interactions. Variable-temperature SAXS analysis shows that the healable polymeric blend
has a nanophase-separated morphology and that the X-ray contrast between the two types of domain
increases with increasing temperature, a feature that is repeatable over several heating and cooling cycles.
A fractured sample of this material reproducibly regains more than 95% of the tensile modulus, 91% of the
elongation to break, and 77% of the modulus of toughness of the pristine material.
1. Introduction
ent monomers in the system can often be precisely controlled
by application of a suitable external stimulus such as heat5 or
light.6 This may in turn enable a specific physical property of
the polymer, such as its tensile strength, to be modulated rapidly.
Upon removal of the external stimulus, the properties of the
material can return to those it possessed in its original state.
This “switchable” behavior of supramolecular polymers has been
demonstrated in applications as diverse as adhesives, coatings,
and most recently, healable materials.7
The large and still-growing range of areas in which supramo-
lecular materials may find application is driven by the wide
range of molecular components from which they may be
constructed. In recent years, supramolecular polymers that
exploit hydrogen bonding,2,4,8 nucleobase stacking,3b metal-ligand
interations,2,9 and hydrophobic effects,10 among others, have
Self-assembled polymeric materials have been developed and
studied intensively over the past decade.1 Such materials
typically comprise low- to medium-molecular-weight species
capable of strong, directed, supramolecular interactions2 that
impart physical properties, such as high solution viscosity and
tensile strength, that are more traditionally associated with
covalently bonded, high-molecular-weight polymers.3 Self-
assembled polymers are typically stimuli-responsive,4 and the
strength of the supramolecular interactions between the constitu-
† University of Reading.
‡ University of Delaware.
§ Case Western Reserve University.
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J. AM. CHEM. SOC. 2010, 132, 12051–12058 12051

A R T I C L E S
Burattini et al.
Scheme 1. Synthesis of Pyrenemethyl End-Capped
Hydrogen-Bonding Polymer 2 and Benzyl End-Capped Polymer 6
Figure 1. Top: Structure of chain-folding polyimide 1. The average value
of n is 6 and those of p and q are ∼7. Bottom: (a) Proposed structure and
(b) energy-minimized model of the electronically complementary π-π
stacking interaction between a bis(diimide) chain fold in polyimide 1
(represented by segment q) and a pyrenyl residue. (c) Schematic of a
supramolecular network formed between polyimide 1 and a bis(pyrenyl)
end-capped polymer.
which this occurs can be predictively tuned by varying the
chemical structure of either the oligomeric core component or
the hydrogen-bonding end group added in the second step of
the reaction.
In the present work, we investigated whether it is possible to
combine the two supramolecular bonding modes of (i) electroni-
cally complementary π-π stacking and (ii) interpolymer
hydrogen bonding into a hybrid polymer system in which the
structural integrity of the bulk supramolecular blend is main-
tained by a designed combination of these interactions. On the
basis of previous studies, we envisaged that such a system might
also afford a high degree of thermal healability, and this has
indeed been demonstrated.
been synthesized. The diverse nature of these supramolecular
motifs offers an immense scope for tailoring the physical,
chemical, and stimuli-responsive nature of the materials produced.
We recently introduced electronically complementary aro-
matic π-π stacking as a noncovalent binding motif through
which supramolecular polymers can be synthesized. Specifically,
we reported a π-electron-deficient bis(diimide) motif that is
capable of encapsulating π-electron-rich species such as pyrene
and its derivatives through induced chain folding of a polyimide
(Figure 1).11 By positioning pyrenyl residues at the chain ends
of a low-molecular-weight polyamide (Mn < 6000 g mol-1) and
blending the resulting telechelic material with a polyimide
component (Mn ≈ 16 000 g mol-1) containing a series of
designed chain-folding residues within its backbone, we were
able to generate novel, highly thermoresponsive supramolecular
blends.5,12 Tailoring the composition of the blend enabled us
to produce a supramolecular material capable of very rapid
healing at temperatures above 50 °C, with a healing efficiency,
in terms of tensile modulus, of 100%.5
In other work, we reported a simple hydrogen-bonding (urea/
urethane-based) motif that can be appended to hydroxy- or
amine-terminated oligomers.13 The robust, one-pot, two-step
synthesis involves addition of diphenylmethane-4,4′-diisocyanate
(MDI) to the telechelic oligomer, with the resulting isocyanate-
terminated prepolymer being subsequently end-capped by an
amine or alcohol to give a polymer with a final Mn ≈ 17 000 g
mol-1 having terminal residues capable of intermolecular
hydrogen bonding.
2. Results and Discussion
Synthesis and Characterization. The synthesis of pyrenyl end-
capped polymer 2 was achieved using a one-pot, two-step
reaction (Scheme 1) analogous to that described previously for
purely hydrogen-bonded supramolecular systems.13 Accordingly,
MDI (3) was added to bis(hydroxy)-terminated polybutadiene
4 (Mw ) 2000 g mol-1; NCO/OH ratio ) 2:1). The resulting
isocyanate-terminated prepolymer was end-capped with 2-ami-
nomethylpyrene (5) to yield a hydrogen-bonding polymer with
π-electron-rich end groups as a pale-yellow elastomer. For
control purposes, the benzyl end-capped polyurethane 6, which
lacks the π-electron-rich pyrenyl residues of 2, was also
synthesized by addition of excess benzylamine to the isocyanate-
terminated prepolymer (Scheme 1). The pyrenyl end-capped
polymer 2 and benzyl end-capped polymer 6 had almost
identical number-average molecular weights (Mn ) 8400 and
8500 g mol-1, respectively).
The resulting supramolecular elastomers show interesting
rheological properties, including a dramatic reduction in storage
modulus above a critical temperature.13 The temperature at
Formation of a supramolecular π-π stacked complex be-
tween 1 and 2 was readily accomplished by mixing an
essentially colorless solution of the pyrenyl end-capped polymer
2 in 6:1 (v/v) chloroform/hexafluoropropan-2-ol with the pale-
yellow solution of chain-folding polyimide 1 in the same solvent.
The polymers were combined respectively in a 4:1 weight ratio,
corresponding to an equimolar ratio of end-capping pyrenyl units
and chain-folding bis(diimide) units (q in Figure 1), and instantly
generated a deep-red solution. This dramatic color change
(11) Greenland, B. W.; Burattini, S.; Hayes, W.; Colquhoun, H. M.
Tetrahedron 2008, 64, 8346–8354.
(12) Burattini, S.; Colquhoun, H. M.; Greenland, B. W.; Hayes, W.; Wade,
M. Macromol. Rapid Commun. 2009, 30, 459–463.
(13) (a) Hayes, W.; Woodward, P. J.; Clarke, A.; Slark, A. T. Eur. Pat.
EP792925, 2007. (b) Woodward, P.; Hermida Merino, D.; Hamley,
I. W.; Slark, A. T.; Hayes, W. Aust. J. Chem. 2009, 62, 790–793. (c)
Woodward, P. J.; Hermida Merino, D.; Greenland, B. W.; Hamley,
I. W.; Light, Z.; Slark, A. T.; Hayes, W. Macromolecules 2010, 43,
2512–2517.
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A Healable Supramolecular Polymer Blend
A R T I C L E S
The UV-vis and fluorescence data for both the individual
components 1 and 2 and their blend [1+2] are consistent with
those previously obtained for structurally related polymer blends
that lacked urea and urethane hydrogen-bonding residues in
close proximity to the pyrenyl end group.5a Thus, in solution,
the formation of a π-π stacked complex between the pyrenyl
end groups of 2 and the chain folds in 1 (Figure 1) is unaffected
by the incorporation of the urea-based hydrogen-bonding motif.
Computational modeling of this interaction suggests that such
π-π stacking interactions are reinforced by multiple hydrogen
bonding between the urea and diimide residues, as discussed
below.
While these solution studies were encouraging, it was also
necessary to verify the formation of π-π stacked interpolymer
complexes in the solid state. Films cast from a 1:4 (w/w) blend
of polymers 1 and 2 by evaporation of a solution in 2,2,2-
trichloroethanol indeed produced dark-red elastomeric materials,
clearly demonstrating that complementary π-π stacking is also
retained in the solid state.
Figure 2. UV-vis spectra of the polymeric complex [1+2] (black),
polyimide 1 (blue), and pyrenyl end-capped polyurethane 2 (red). The broad
absorption centered at 525 nm represents a π-π* charge-transfer transition
between the electron-rich pyrenyl and electron-poor diimide residues. The
solution in 6:1 (v/v) chloroform/hexafluoropropan-2-ol was 10-3 M in
pyrenyl units.
The IR spectrum of the solid blend of 1 and 2 showed
absorption bands at 3375, 1660, and 1730 cm-1 assigned to
hydrogen-bonded ν(N-H), hydrogen-bonded ν(CdO), and
non-H-bonded ν(CdO) (urethane) vibrations, respectively (Fig-
ure 4).15 This suggests that not all of the urethane linkages within
the bulk material are able participate in hydrogen bonding.3b
When the sample was heated from 25 to 120 °C, the absorptions
due to the hydrogen-bonded N-H and CdO residues were
appreciably reduced in intensity while those corresponding to
the free carbonyl stretch increased in intensity, as shown in
Figure 4, demonstrating the thermally responsive nature of the
hydrogen bonds in this novel, supramolecular polymer blend.
To investigate the morphology of the supramolecular polymer
blend, variable-temperature small-angle X-ray scattering (SAXS)
analysis of the material was carried out. The sample was initially
held at -20 °C and then cycled sequentially to 80 °C, back to
-20 °C, and finally to 80 °C again at a ramp rate of 5 °C min-1
with a 2 min isothermal hold between successive steps in the
temperature cycle. At all temperatures within the range studied,
the blend showed a single scattering signal at q ) 0.08 Å-1
that is indicative of a phase-separated morphology with a domain
spacing on a 10 nm length scale. While the size of the domain
spacing was essentially invariant within the temperature range
studied, the X-ray contrast between the different domains
increased with increasing temperature. This is demonstrated in
Figure 5 by a plot of the intensity of the scattering signal at q
) 0.08 Å-1 as a function of temperature, where the reversible
nature of this relationship becomes apparent.
Figure 3. Fluorescence spectra of polyurethane 2 (red), polyimide 1 (blue),
and supramolecular polymeric network [1+2] (black) in 6:1 (v/v) chloroform/
hexafluoropropan-2-ol solution at 20 °C, showing essentially complete
quenching of the pyrene emission bands upon complexation. Excitation was
at 345 nm; the concentration was 10-7 M in pyrenyl units.
resulted from the presence of a new absorption band at 525 nm
in the visible spectrum of the blend (Figure 2). Absorption at
this wavelength is indicative of charge-transfer between the
π-electron-rich (pyrene) and π-electron-deficient (diimide) spe-
cies.11 The absorption is entirely analogous to the one at 530
nm reported previously for the complex formed between discrete
compounds designed to model this interaction.11
Irradiation of a solution of the pyrenyl end-capped polymer
2 under a standard laboratory UV source (310 nm) revealed
the familiar blue fluorescence response originating from the
pyrenyl residues on polymer 2, as shown in Figure 3.12,14
However, this fluorescence is almost completely quenched in
the solution blend [1+2], confirming the occurrence of comple-
mentary π-π stacking [see p S12 in the Supporting Information
(SI)].12 Moreover, the emission spectrum of the pyrenyl end-
capped polyurethane 2 shows characteristic emission bands for
the monomeric pyrenyl group at 395, 410, and 437 nm as well
as the excimer emission at 467 nm originating from the
association of two pyrenyl moieties.12,14 These emission bands
are completely absent in the fluorescence spectrum of the blend
of the complementary polymers 1 and 2.
We previously demonstrated that a driving force for producing
a stable, homogeneous blend is the formation of complementary
π-π stacking complexes between two polymers.5 In the present
system, the SAXS data suggest that supramolecular complex-
ation is reduced at elevated temperatures, with the result that
the polymers become less miscible, thereby driving phase
separation, most probably into polyimide-rich and polybutadi-
ene-rich domains.16 This conclusion was supported by com-
(15) (a) Ning, L.; De-Ning, W.; Sheng-Kang, Y. Polymer 1996, 37, 3045–
˙
3047. (b) Yilgo¨r, E.; Burgaz, E.; Yurtsever, E.; Yilgo¨r, I. Polymer
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J. Polym. Sci., Part B: Polym. Phys. 2009, 47, 471–483.
(16) Analogous phase behavior has recently been reported for a supramo-
lecular diblock copolymer. See: Feldman, K. E.; Kade, M. J.; Meijer,
E. W.; Hawker, C. J.; Kramer, E. J. Macromolecules 2010, 43, 5121–
5127.
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A R T I C L E S
Burattini et al.
Figure 4. Variable-temperature FTIR spectroscopic analysis of the supramolecular network formed by 1 and 2.
packing of the self-assembled blend. Several peaks were
observed, corresponding to d spacings of 15.4, 9.2, 5.1, and
3.4 Å. The 5.1 Å peak is in close agreement with that previously
measured for the urea-urethane separation in hydrogen-bonded
elastomers.17 The smallest spacing, d ) 3.4 Å, has been
previously attributed to π-π stacking within supramolecular
polymers.3b It may also be compared to the distance predicted11
from energy-minimized models (e.g., Figure 1b) in which
pyrenyl groups are bound within the designed chain fold in
polymer 1 (3.4-3.5 Å) as well as the interplanar distances
measured by single-crystal X-ray analysis of related π-π-
stacked model complexes.18
We previously showed that π-π stacking and hydrogen
bonding between polymer chains can be successfully modeled
using molecular mechanics simulations with charge equilibra-
tion.11 In the present work, this approach enabled a detailed,
atomistic analysis of the interaction between the pyrenemethy-
lurea end groups of polymer 1 and the designed bis(diimide)
chain folds of polyimide 2. Thus, Cerius2 (v.3.5, Accelrys Inc.,
San Diego, CA) was used to construct a model based on segment
q of polyimide 1, which contains two naphthalenediimide
residues linked by a triethylenedioxy chain (see Figure 1). This
unit was designed specifically to generate a chain fold that would
be complementary to an electron-rich pyrenyl unit.11 A model
compound representing a pyrenemethylurea end group was
inserted into the chain fold and, after calculation of the atomic
charge distribution, the system was energy-minimized using the
Dreiding2 force field.19 After initial minimization, the position
of each atom was disordered by 0.1 Å and the system
reminimized. The minimize-disorder-reminimize cycle was
repeated until a consistent structure was obtained. This approach
provided some reassurance that a deep (though not necessarily
global) energy minimum had been identified.
Figure 5. Intensity of the SAXS scattering signal at q ) 0.08 Å-1 as a
function of temperature.
Figure 6. False-color ESEM images of (left) the π-π stacked network
[1+2] and (right) the non-π-stacking blend [1+6] at 20 °C. The fine, parallel
lines visible in the left-hand image reflect the texture of the PTFE substrate
on which the films were cast.
parison of environmental scanning electron microscopy (ESEM)
images of the 1:4 (w/w) supramolecular polymer blend [1+2]
and an analogous control material produced from benzyl end-
capped polyurethane 6 and chain-folding polyimide 1 (Figure
6). It can be seen that the film cast from the electronically
complementary components [1+2] is homogeneous at the
micrometer scale, whereas the material comprising the chain-
folding polyimide and the benzyl end-capped polyurethane is
very clearly phase-separated at this length scale. This demon-
strates the necessity of π-π complexation to drive miscibility
of the polymers and thus enable the formation of stable
elastomeric films, in agreement with our studies on related
polymer blends.5
The final model is shown in Figure 7, which reveals not only
close, electronically complementary π-π stacking between the
pyrenyl end group and the two naphthalenediimide residues of
the chain fold but also, less predictably, a pair of strong,
convergent hydrogen bonds from the urea NH groups to a single
diimide carbonyl group. The hydrogen-bond parameters are
(17) Kaushiva, B. D.; McCartney, S. R.; Rossmy, G. R.; Wilkes, G. L.
Polymer 2000, 41, 285–310.
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D. J. Org. Lett. 2003, 5, 4353–4356.
Wide-angle X-ray scattering (WAXS) analysis of the polymer
film [1+2] was carried out at 25 °C to investigate the local
(19) Mayo, S. L.; Olafson, B. D.; Goddard, W. A., III. J. Phys. Chem.
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A Healable Supramolecular Polymer Blend
A R T I C L E S
Figure 8. Master curve for the supramolecular network [1+2], with a
reference temperature of 30 °C. The storage modulus profile is shown in
blue, the loss modulus profile in red, and tan δ in black.
Figure 7. Minimized computational model of the interaction between a
pyrenemethylurea end group of polyurethane 2 and a designed chain fold
of polyimide 1. Key binding features are the electronically complementary
triple π-stack (with mean interplanar spacings of 3.45 and 3.41 Å) and a
pair of strong, convergent hydrogen bonds (shown in magenta) from the
urea unit to a diimide carbonyl group (N · · ·O ) 2.95, 2.84 Å; H · · ·O )
2.03, 1.91 Å; N-H· · ·O ) 160, 161°).
a good atomistic model for such interactions that is supported
by spectroscopic studies in solution. With these data in hand,
our attention turned to the physical properties of the new
material.
Rheological Studies. Initial studies of the rheological behavior
of the supramolecular polymer blend [1+2] were carried on a
parallel-plate rheometer. The Boltzmann time-temperature
superposition (TTS) technique was used to create a master curve
with a reference temperature of 30 °C; the curve exhibited a
plateau modulus of 2 × 107 Pa as determined by the MIN
method,20 which considers the plateau modulus to be the value
of the storage modulus when tan δ reaches a minimum (Figure
8).20c The storage modulus (G′) and the loss modulus (G′′)
appear to be essentially parallel in the low-frequency region
and slow to achieve their limiting power laws of 2 and 1,
respectively, with frequency.21 This behavior is in contrast to
that observed for isotropic covalent and noncovalent polymers,
which are known to exhibit slopes of 2 and 1, respectively, for
G′ and G′′ in this region.20a,21 This result is consistent with a
structural model for [1+2] in which multiple chain-fold recep-
tors within the backbone of each polyimide lead to formation
of a noncovalently cross-linked network rather than extended
linear chains in the solid state.
However, as one considers the very low frequency region,
the complex viscosity approaches a constant value (see p S14
in the SI). The zero-shear (i.e., terminal) viscosity η0 was
obtained from a Carreau-Yasuda fit, which converted the
dynamic data to a steady-shear model using the Cox-Merz
rule,20b resulting in an extrapolated value of η0 greater than 3.5
× 1013 Pa s. The material appears to be an extremely viscous
liquid, with a relaxation time in excess of 1010 s (∼300 years)
at 30 °C. We note that the magnitude of the viscosity is larger
than that frequently associated with the glassy state (1012 Pa
s),22 although this value is disputed23 and may not be applicable
to our case.
Scheme 2. Model Chain Fold 811 and Synthesis of the End-Group
Model Compound 9
N· · ·O ) 2.95, 2.84 Å; H · · ·O ) 2.03, 1.91 Å; and N-H· · ·O
) 160, 161°.
Experimental support for this binding mode was provided
by Job analysis using the π-π* charge-transfer band at 530
nm resulting from complexation of model chain fold 8 with
pyrenylurea compound 9 (Scheme 2). The Job plot peaks at
0.50 (see p S7 in the SI), confirming the 1:1 complexation
1
predicted by the simulation shown in Figure 7. Moreover, H
spectra of an equimolar mixture of 8 and 9 over a range of
concentrations (2-14 mg mL-1) show progressive downfield
shifts of both NH protons (∆δ ) 0.42 and 0.21 ppm) with
increasing concentration, consistent with the hydrogen-bonding
pattern shown in Figure 7. The diimide aromatic proton
resonance shows a corresponding upfield shift (∆δ ) 0.30 ppm)
under these conditions, as a consequence of increasing ring-
current shielding by the associating pyrenyl residue, again in
good agreement with the proposed bonding model (see p S8 in
the SI).
Analysis of the supramolecular blend [1+2] using both
spectroscopic (UV-vis/IR) and X-ray scattering (SAXS/WAXS)
techniques has demonstrated that the material is phase-separated
on the nanoscale and that there is clear evidence for hydrogen
bonding and π-π stacking interactions, as proposed. Compu-
tational simulation based on molecular mechanics has provided
Figure 9a shows a plot of the rheometric shift factor aT, a
parameter related to melt viscosity, as a function of temperature
for the noncovalent network [1+2]. It can be seen that aT
(20) (a) Ferry, J. D. Viscoelastic Properties of Materials; Wiley: New York,
1980. (b) Mezger, T. G. The Rheology Handbook; Vincentz Network:
Hannover, Germany, 2002. (c) Liu, C.; He, J.; van Ruymbeke, E.;
Keunings, R.; Bailly, C. Polymer 2006, 47, 4461–4479.
(21) Kautz, H.; van Beek, D. J. M.; Sijbesma, R. P.; Meijer, E. W.
Macromolecules 2006, 39, 4265–4267.
(22) Barlow, A. J.; Lamb, J.; Matheson, A. J. Proc. R. Soc. London, Ser.
A 1966, 292, 322–342.
(23) Soesanto, T.; Williams, M. C. J. Phys. Chem. 1981, 85, 3338–3341.
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Burattini et al.
a healing time of ∼240 min, as shown in Figure 10a. This
healing efficiency relative to the pristine material was maintained
over five break-heal cycles of a single sample, as shown in
Figure 10b.
In addition to recovery of the tensile modulus (stiffness) of
the material, it is important from an application-driven point
of view to recover the modulus of toughness.26 The modulus
of toughness is a measure of the energy required to break the
material and is readily calculated from the area under the
stress-strain curve. The elastomeric nature of the pristine
supramolecular blend [1+2] allows it to be stretched to 2.7 times
its original length before breaking, i.e., it shows 170% elongation
(Figure 11a), and the blend has a modulus of toughness of 510
MPa. This can be compared with an elongation to break of 22%
and modulus of toughness of 59 MPa for the pyrenyl end-capped
polymer 2 alone and with a value of 18 MPa measured here
(see p S15 in the SI) for a previously reported healable
π-stacking blend based on a polyamide rather than polybutadiene
backbone.5a
After one break-heal cycle, the present supramolecular blend
[1+2] regained 91% of the original elongation to break and
70% of its original modulus of toughness (360 MPa). As shown
in Figure 11b, the modulus of toughness remained in the range
108-109 Pa over five break-heal cycles. In these repeat
experiments, fracture was generally observed at or near the site
of the original break. Several attempts were made to heal the
pyrenyl end-capped polymer 2 in the absence of the polyimide
component 1 (100 °C for up to 24 h), but these were completely
unsuccessful. The polyimide itself is not film-forming, so its
mechanical properties could not be evaluated.5
Analysis of rheometry scans for the supramolecular polymer
blend [1+2] and for the pyrenyl end-capped polyurethane 2
showed very disparate responses to the healing conditions
(Figure 12). For blend [1+2], the storage modulus G′ and loss
modulus G′′ converge as temperature increases and cross at 93
°C (i.e., just below the observed healing temperature). Thus,
above 93 °C, the viscous properties of the material dominate
over the elastic properties, allowing reorganization of the
polymer chains and rapid re-engagement of the supramolecular
interactions, facilitating healing. This is in contrast to G′ and
G′′ for the pyrenyl end-capped polymer 2 alone, which diVerge
over the temperature range studied, showing that the elastic
properties dominate at all temperatures accessed.
In a final demonstration of the importance of π-π stacking
interactions in maintaining the stiffness, elongation, and tough-
ness of material [1+2] and in enabling it to heal, the “control”
blend of benzyl end-capped polyurethane 6 with chain-folding
polyimide 1 was subjected to the same tensile tests (Figure 13).
The pristine blend [1+6] exhibited an elongation of 40% before
breaking (cf. 170% for [1+2]) and a modulus of toughness of
Figure 9. (a) Shift factor aT of the supramolecular network [1+2] as a
function of temperature. (b) Plot of aT for [1+2] as a function of temperature
normalized to Tg.
changes by some 14 orders of magnitude between -10 and
+110 °C. This is approximately the same rate of change of aT
with temperature as observed in previous healable supramo-
lecular blends, though it is seen here over a much wider
temperature range.5a The very large change in aT over a small,
readily accessible temperature range is consistent with dissocia-
tion of the supramolecular network, resulting in a large drop in
viscosity that thereby facilitates healing.
It has recently been postulated24 that for all amorphous,
covalently bonded polymers there is a linear relationship
between aT and temperature (normalized to the glass transition
temperature Tg). However, this relationship has also been
shown25 to break down for supramolecular polymers such as
telechelic ionomers, where the plot of aT as a function of
temperature deviates significantly from linearity at high
temperature.
Figure 9b shows a shift factor plot for the supramolecular
polymer network [1+2]. At low temperatures the plot is
essentially linear, whereas at higher temperatures the plot
gradient increases dramatically, in line with results for other
supramolecular polymer systems.25 This deviation is attributed25
to disengagement of the supramolecular (π-π stacking and
hydrogen-bonding) interactions, leading to a change in the
apparent molecular weight of the noncovalent polymer blend
and thus resulting in a rapid change in viscosity with temperature.
Healing Studies. In an initial investigation of the healing
potential of the polymer blend [1+2], a strip of film (0.4 mm
in thickness and 2.6 mm in width) was analyzed to break by
tensometry. The pristine elastomer exhibited a tensile modulus
of ∼3 × 105 Pa. The two ruptured portions of the film were
then positioned with their broken edges in contact, but not
overlapped, on a preheated PTFE plate at 100 °C and then placed
in an oven at 100 °C for varying periods of time (20-1000
min), after which the tensile modulus was measured again. The
resulting data are displayed in Figure 10, which shows the tensile
modulus and associated healing efficiency for the healed material
as function of healing time. From these data it is evident that
the material achieves its maximum healing efficiency (95%) after
Figure 10. (a) Recovery of tensile modulus as a function of healing time for a sample of [1+2]. (b) Recovery of the tensile modulus for the network [1+2]
sample over five break-heal cycles.
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A Healable Supramolecular Polymer Blend
A R T I C L E S
Figure 11. (a) Stress-strain curves for elastomer 2 (2), the pristine network [1+2] (9), and the healed network [1+2] after annealing at 100 °C for 4 h
(O). (b) Recovery of the modulus of toughness for the network [1+2] over five break-heal cycles.
Figure 12. Rheology of (left) supramolecular network [1+2] and (right) elastomer 2, showing the temperature dependence of the dynamic shear moduli.
Scan rate ) 1 °C min-1
.
samples of the blend [1+2] exhibit a tensile modulus of 3 ×
105 Pa, are elastomeric in nature (170% elongation), and have
a high modulus of toughness (510 MPa) when compared with
previously reported supramolecular healable materials.5 When
it was broken and rehealed, the present material repeatedly
regained 95% of its tensile modulus, 91% of its elongation to
break, and 77% of its modulus of toughness after annealing for
240 min at 100 °C. In contrast, control experiments on a pure
sample of the pyrene end-capped polymer (2) failed to show
significant healing even after extended periods at 100 °C, a result
that is consistent with the very disparate temperature dependence
of G′ and G′′ for polymer 2 and the blend [1+2]. Analysis of
a second control material, a blend of a benzyl end-capped
polymer 6 and chain-folding polyimide 1 (essentially identical
to the healing blend [1+2] but lacking the π-electron-rich
pyrenyl groups), demonstrated the importance of interpolymer
π-π stacking in producing a tough, coherent, and healable
material. Pristine samples of the control blend [1+6] exhibited
a low modulus of toughness (91 MPa) and a healing efficiency
of only 8% even under forcing conditions. Computational
modeling of the interaction between a polyimide chain fold and
a pyrenemethylurea end group confirmed the stability of the
proposed triple π-stack and showed that this is reinforced by a
pair of strong, convergent hydrogen bonds between the adjacent
urea unit and a diimide carbonyl group.
Figure 13. Stress-strain curves for the pristine blend of benzyl end-capped
polyurethane and polyimide [1+6] (blue) and the healed blend, annealed
at 100 °C for 24 h (black). The recovery of the modulus of toughness (area
under the curve) for this blend was no more than 8%.
91 MPa (cf. 510 MPa for [1+2]). Even after an extended
annealing time (24 h), the healed material exhibited a greatly
reduced elongation to break (20%) and a modulus of toughness
of only 7 MPa (cf. 360 MPa for [1+2]), resulting in a healing
efficiency of 8% (cf. 77% for [1+2]).
3. Conclusions
A novel pyrenemethylurea end-capped polymer (2) that forms
a thermally healable material when blended with a chain-folding
polyimide (1) has been designed and synthesized. Pristine
The supramolecular polymer [1+2] shows a significant
enhancement of tensile modulus over that of the purely
hydrogen-bonded system described by Leibler and co-workers
(3 × 105 vs 1 × 104 Pa),4 though the elongation to break of the
(24) (a) Liu, C.-Y.; He, J.; Keunings, R.; Bailly, C. Macromolecules 2006,
39, 8867–8869. (b) Liu, C.-Y.; Halasa, A. F.; Keunings, R.; Bailly,
C. Macromolecules 2006, 39, 7415–7424.
(25) Stadler, F. J.; Pyckhout-Hintzen, W.; Schumers, J. M.; Fustin, C. A.;
Gohy, J. F.; Bailly, C. Macromolecules 2009, 42, 6181–6192.
(26) Jastrezebski, Z. D. The Nature and Properties of Engineering
Materials, 2nd ed.; Wiley: New York, 1976.
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latter material (400%) is considerably greater than the 170%
shown by [1+2]. Conversely, the thermally healable networks
described by Wudl and co-workers27 based on reversible
covalent (Diels-Alder) chemistry show very much higher
tensile moduli (typically in the region of 109 Pa) but relatively
low degrees of strain to failure.
Supporting Information Available: Full experimental details
of materials synthesis and characterization and rheology, X-ray
scattering, and healing studies; raw scattering data for the
variable-temperature SAXS experiments; Job plot and 1H NMR
spectra for complexation of 8 with 9; WAXS data; unprocessed
ESEM images and photographs of the healing polymer blend
in solution under visible light and UV irradiation; photograph
of a sample of the healable material [1+2]; plot of the complex
viscosity η* of [1+2] as a function of frequency; and a
stress-strain plot for the supramolecular π-stacking polymer
blend reported in ref 5a. This material is available free of charge
via the Internet at http://pubs.acs.org.
Acknowledgment. We thank the EPSRC and NSF for grants
in support of this work (EP/D07434711, EP/G026203/1, and NSF-
DMR-0602869) and Henkel Adhesives Ltd. for Ph.D. studentship
funding to D.H.M.
(27) Chen, X.; Wudl, F.; Mal, A. K.; Shen, H.; Nutt, S. R. Macromolecules
2003, 36, 1802–1807.
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