Self-healing supramolecular block copolymers

Full text of DOI:10.1002/anie.201204840

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Chemie
DOI: 10.1002/anie.201204840
Materials Science
Self-Healing Supramolecular Block Copolymers**
Jens Hentschel, Aaron M. Kushner, Joseph Ziller, and Zhibin Guan*
The ability to spontaneously heal injury is a key feature of
biological materials that increases the survivability and
lifetime of plants and animals. In contrast, synthetic materials
generally fail after damage or fracture. Inspired by nature,
several self-healing polymer systems have been developed
through the incorporation into polymers of mechanically[1] or
photo-activated[2,3] healing agents, reversible covalent
bonds,[4–7] metal-ligand complexes,[8] and dynamic non-cova-
lent bonding.[9–14] Nevertheless, progress towards generally
applicable and mechanically robust self-healing polymers has
been hampered by a fundamental dilemma: the mechanical
stiffness/strength and rapid macromolecular dynamics
(required for spontaneous healing) usually have an inverse
dependent relationship.[15,16] Thus the use of strong reversible
interactions in polymers with high glass transition temper-
Figure 1. The concept of self-healing supramolecular block copolymer
design. a) Conventional PS-b-PBA-b-PS triblock copolymers form
a microphase-separated thermoplastic elastomer. In these systems,
mechanical fracture results in irreversible covalent bond rupture and
permanent loss of properties. b) Supramolecular triblock copolymers
combine the advantageous thermoplastic elastomeric properties of
microphase-separated block copolymer systems with the reversible
H-bonding interactions at the junction of the soft PBA block to afford
dynamic, self-healing properties.
ature (Tg) results in stiff but less dynamic materials,[4,8] while
weak interactions in low Tg polymers afford more dynamic
healing, but yield soft materials.[9,17]
To address this dilemma, our laboratory has been explor-
ing a multiphase design of polymers that combine high
modulus and toughness with spontaneous healing capabil-
ity.[18] Recently, we reported a hydrogen-bonding brush
polymer that self-assembles into a hard/soft two-phase
system, combining the stiff and tough mechanical properties
of the hard phase with the self-healing capacity of dynamic
supramolecular assemblies in the soft matrix.[18] Unlike brush
polymers, block copolymers are important commodity mate-
rials exhibiting well-defined multiphase morphologies and
tunable mechanical properties through the control of block
composition and length. Introducing self-healing capability
into block copolymers would significantly improve the
performance and expand the scope of applications for this
important family of materials. Herein, we report a supra-
molecular block copolymer design for new multiphase self-
healing materials (Figure 1). We reasoned that the supra-
molecular block copolymer should retain the hard/soft two-
phase morphology found in conventional covalent block
copolymer architectures, affording advantageous mechanical
properties (such as thermoplastic elastomeric). Meanwhile,
the supramolecular healing motifs located within the soft
phase should remain dynamic and reversible, providing self-
healing capability (Figure 1).
To demonstrate our concept, we chose a block copolymer
system having poly(n-butyl acrylate) (PBA; Tg = ca. À408C)
as the soft block and polystyrene (PS; Tg = ca. 1008C) as the
hard block. Previous studies have shown that covalent PS-b-
PBA-b-PS triblock copolymers exhibit microphase-separated
morphology and unique thermoplastic elastomer proper-
ties.[19] However, mechanical fracture of this covalent system
would result in irreversible covalent bond rupture and
permanent loss of properties (Figure 1a). We reasoned that
by replacing the covalent linkage in the center of the PBA soft
block with a dynamic quadruple H-bonding junction, the
supramolecular block copolymer should be able to self-heal
after mechanical damage. To demonstrate this, we synthesized
PBA-b-PS diblock copolymers end-functionalized with
a well-defined quadruple H-bonding motif, 2-ureido-4-pyr-
imidinone (UPy). Dimerization between UPy motifs leads to
the formation of supramolecular ABA triblock copolymers
with the flexible PBA blocks connected by a single reversible
UPy dimer (Figure 1b). Importantly, this architecture places
the dynamic H-bonding interaction within the soft phase of
the two-phase system after microphase separation, where
chain motion should facilitate reversible H-bond formation in
the solid state. Whereas several supramolecular block copoly-
mers have been developed,[20,21] to the best of our knowledge,
[*] Dr. J. Hentschel, Dr. A. M. Kushner, Dr. J. Ziller, Prof. Dr. Z. Guan
Department of Chemistry, University of California, Irvine, 1102
Natural Sciences 2 (USA)
E-mail: zguan@uci.edu
Homepage: http://chem.ps.uci.edu/~zguan/
[**] We thank the US Department of Energy, Division of Materials
Sciences (DE-FG02-04ER46162), and the National Science Foun-
dation (DMR-1217651) for financial support. The Alexander von
Humboldt foundation is thanked for financial support through
a Feodor Lynen research fellowship for J.H.
Supporting information, including polymer synthesis and charac-
terizations, material characterizations by DMA, DSC, tensile tests
and polymer morphology study by AFM, for this article is available
on the WWW under http://dx.doi.org/10.1002/anie.201204840.
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no self-healing properties have been reported for such
systems.
We selected the UPy dimer[22] as our reversible interaction
motif because it offers an attractive combination of relatively
high thermodynamic stability (DG = ca. 10 kcalmolÀ1) and
rapid kinetic reversibility (koff = ca. 8 sÀ1),[23] and when
incorporated into polymers, results in adaptive and responsive
supramolecular materials.[24–29] However, the rigid, flat aro-
matic structure of the UPy dimer makes it prone to fibrillar
assembly[30,31] in the condensed phase, which can hinder the
dynamics of reversible H-bond formation. To avoid undesired
long-range aggregation and vitrification of the dynamic
elements, we adapted a design[32] utilizing a 2,6-diisopropyl-
phenyl group to disrupt p–p stacking between UPy dimers,
yielding a new “stack-blocked” UPy motif (SB-UPy) (Sup-
porting Information, Scheme S1). Single crystal X-ray anal-
ysis of the SB-UPy-olefin intermediate confirms that the
bulky isopropyl substituents are oriented perpendicular to the
dimer plane, thus sterically shielding the UPy-dimer from co-
facial long-range assembly (Figure 2).
Scheme 1. Synthesis of SB-UPy end-functionalized PS-b-PBA diblock
copolymers (Poly-1,2,3). Conditions: a) DCC, DMAP, CH2Cl2; b) n-butyl
acrylate, AIBN, DMF, 608C; c) styrene, AIBN, toluene, 708C; d) 20
vol.% TFA in CH2Cl2. AIBN=azobisisobutyronitrile, DCC=dicyclohex-
ylcarbodiimide, DMAP=dimethylaminopyridine, DMF=dimethylfor-
mamide, TFA=trifluoroacetic acid.
of monomer incorporation.[35] Three block copolymers with
high (Protect-1), medium (Protect-2), and low (Protect-3)
relative PS hard-phase fractions were synthesized, varying the
ratio of PBA and PS block size while maintaining a similar PS
block length (Table 1).
Table 1: Molecular weight characterizations of protected SB-UPy func-
tionalized PS-b-PBA diblock copolymers (Protect-1,2,3) as well as
covalent PS-b-PBA-b-PS (SBAS)[a] triblock copolymer control.
Figure 2. a) X-ray structure of SB-UPy showing the perpendicular out-
of-plane orientation of the isopropyl groups. b) Space-filling model
showing steric blockage of co-facial aggregation. Oxygen shown in red,
nitrogen shown in blue.
[b]
[b]
[c]
Polymer
Mn,GPC
Mw/Mn
Mn,NMR
[St]/[nBA][d]
Protect-1
Protect-2
Protect-3
SBAS[a]
23000
29000
36000
73000
1.19
1.27
1.27
1.18
31000
41000
50000
80000
48:52
37:63
32:68
37:63
We employed radical addition fragmentation chain trans-
fer (RAFT)[33] polymerization, a living/controlled radical
polymerization method, to construct our UPy-terminated
block copolymers (Scheme 1). For this purpose, a RAFT
chain transfer agent (CTA) functionalized with a protected
version of our designed SB-UPy motif was first synthesized
following literature procedures. The specific CTA, S-ethyl-S’-
(a,a’-dimethyl-a’’-acetic acid)trithiocarbonate (2), which is
known to be a very efficient and versatile CTA,[34] was
synthesized in a one-pot reaction from carbon disulfide,
ethanethiol, and acetone. This CTA was coupled to the
protected SB-UPy (1) through the hydroxy linker by standard
N,N’-dicyclohexylcarbodiimide (DCC) coupling protocols
and the resulting SB-UPy-CTA (3) was characterized by
NMR spectroscopy and ESI-MS (see the Supporting Infor-
mation).
With SB-UPy functionalized CTA 3 in hand, we carried
out sequential RAFT polymerization of n-butyl acrylate and
styrene to form the desired diblock copolymer with a single
protected SB-UPy at the PBA terminus (Scheme 1). The CTA
was designed in such a way that the PBA block could be
formed first, avoiding the low cross-over initiation efficiency
observed for block copolymerization with the opposite order
[a] SBAS=covalent PS-b-PBA-b-PS triblock copolymer; Polymer Source
Inc. (Canada). [b] Mn determined by GPC calibrated with PS standards.
[c] Calculated by integration of chain end group and repeat unit from
1H NMR spectra. [d] Calculated by integration of both PS and PBA repeat
units from 1H NMR spectra.
The benzyl protecting group on the SB-UPy attached to
the block copolymers was removed by treating Protect-1,2,3
with 20% trifluoroacetic acid (TFA). Quantitative deprotec-
tion and dimerization of the unprotected SB-UPy was
evidenced by the disappearance of the benzyl peaks and the
appearance of three new peaks in the UPy H-bonding region
(11.9–13.0 ppm) with the expected intensity (Figure S2-A).[28]
Furthermore, gel permeation chromatography (GPC) in
toluene showed the doubling of number-average molecular
weight (Mn) for the copolymers after benzyl deprotection,
confirming the transformation from protected diblock
copolymers into supramolecular triblock copolymers (Fig-
ure S2-B). Thin films were prepared from both protected and
deprotected SB-UPy-copolymers by slow evaporation of
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concentrated chloroform solutions. As a control, a covalent
PS-b-PBA-b-PS (SBAS) triblock copolymer having an iden-
tical ratio of hard/soft blocks was used in comparative studies.
The solid polymer samples were investigated by differential
scanning calorimetry (DSC) and dynamic mechanical analysis
(DMA). As shown by DSC, the Tg values of the PBA blocks
range from À37 to À428C. The Tg values of the PS blocks
were determined by DMA measurements (Table S3) because
they were not obvious by DSC. Our values are consistent with
previously reported Tg values of PS-b-PBA-b-PS triblock
copolymers.[35] The two distinct Tg values suggest that the
block copolymers are microphase separated.
Poly-2 will be used as a representative example for further
discussion, because it has a similar composition ([St]/[nBA] =
37:63) to the commercially available control polymer (basic
mechanical characterization of Poly-1 and Poly-3 can be
found in the Supporting Information). First, the microphase
morphology of the block copolymers was investigated by
tapping mode AFM on dip-coated films (Figure 3). Poly-2 and
its covalent analogue, PS-b-PBA-b-PS (SBAS), show a very
similar cylindrical morphology. AFM micrographs of Poly-3
show a comparable cylindrical morphology to Poly-2 (Fig-
ure S3c), whereas the images of Poly-1 (Figure S3a) show
different phase morphology, owing to the differences in PS/
PBA ratio.
Figure 3. AFM phase-contrast image of a) the SBAS control copolymer
and b) Poly-2. A cylindrical styrenic microphase is observed in both
cases, aligned both parallel and perpendicular to the film surface.
Figure 4. Stress-strain experiments and self-healing tests on both
supramolecular and covalent control triblock copolymers. a) UPy-
protected (Protect-2) vs. deprotected polymer (Poly-2). b) Poly-2 after
different time periods of healing at 458C. c) Covalent triblock copoly-
mer SBAS uncut and after 18 h healing at 458C.
Basic mechanical tests were carried out on both the
protected and deprotected block copolymer samples. All
three deprotected samples show the characteristic stress-
strain behavior of thermoplastic elastomers (Figure S4). As
expected, the volume fraction of the hard PS phase has
a dramatic effect on the bulk mechanical properties. With
increasing volume fraction of the PS hard domain, both the
Youngꢀs modulus (stiffness) and the yield strength of the
sample increase. Figure 4a compares the tensile properties for
Protect-2 and deprotected Poly-2. Protect-2 is relatively weak
and unable to maintain stress after yield. In contrast, the
deprotected Poly-2 is much stronger and displays a distinct
post-yield strain-hardening from 170–450% strain, before the
sample finally breaks at around 600% strain (Figure 4a).
Notably, this dramatic enhancement in mechanical strength
and extensibility results from a single deprotection on the
terminal SB-UPy on each diblock copolymer chain, and
therefore can only be attributed to the spontaneous dimeri-
zation of SB-UPy to form dynamic crosslinks between the
hard domains. The dramatic increase in mechanical properties
following deprotection was observed in all samples, as shown
by their storage modulus (G’), Youngꢀs Modulus (E), and
yield strength (Table S4).
The dynamic nature of the supramolecular triblock
copolymers is revealed by the load-rate dependence of the
mechanical properties in the strain-hardening region. Films of
polymer Poly-2 were stretched at four different load rates
ranging from 10 mmminÀ1 to 1000 mmminÀ1 (Figure S5). The
slope of the post-yield tensile curve rises with increasing load
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rate, indicating that the timescale for the dynamic supra-
molecular polymer formation is on the order of seconds to
minutes, correlating well with the known equilibrium associ-
ation/dissociation dynamics of the UPy motif.[23] On the other
hand, no significant change in the slope of the post-yield curve
is observed when measuring the stress/strain behavior of the
covalent PS-b-PBA-b-PS triblock copolymer (SBAS) at
different load rates (Figure S5), indicating that no dynamic
association is present. It should be noted that the commercial
SBAS control polymer has significantly higher molecular
weight than Poly-2 (Table 1), affording higher tensile strength
and strain at breaking than Poly-2.
Finally, we tested the self-healing ability for the supra-
molecular triblock copolymers in bulk without adding any
solvent, plasticizer, or healing agent. In a typical test
procedure, the specimens were bisected by a razor blade,
then the cut faces were gently pressed together for 1 min and
left for a certain time at 458C before they were stretched to
failure in a standard stress/strain tensile experiment. Previous
studies have indicated that UPy-dimers attached to a polymer
backbone show a significant increase in their association/
dissociation dynamics at 40–508C.[24] Thus, the self-healing
properties of our supramolecular triblock copolymers were
investigated at 458C, a temperature sufficiently high for rapid
SB-UPy on/off dynamics but still well below the Tg of the PS
hard domain. Indeed, a significant recovery of tensile strength
(> 90%) and strain at break (ca. 75%) was observed for Poly-
2 after healing at 458C for 18 h (Figure 4b). In addition, the
time-dependent healing of Poly-2 eventually leads to recovery
of post-yield strain hardening, in sharp contrast to the
covalent control, the SBAS covalent triblock copolymer
(Figure 4c). For the control, despite the significantly higher
molecular weight, owing to the lack of any dynamic inter-
actions, the bisected sample shows only minimal healing
capacity (< 15% recovery of strain at break) and fails
immediately after yield.
concept can be generally applicable to a wide range of block
copolymer systems having different hard/soft blocks and
various types of supramolecular healing motifs for the design
of stiff, strong, and tough self-healing polymers.
Received: June 20, 2012
Published online: && &&, &&&&
Keywords: block copolymers · multiphase · polymer design ·
.
self-healing · supramolecular chemistry
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Communications
Materials Science
J. Hentschel, A. M. Kushner, J. Ziller,
Z. Guan*
&&&& — &&&&
Self-Healing Supramolecular Block
Copolymers
Polymer, heal thyself ! Supramolecular
ABA triblock copolymers formed by
dimerization of 2-ureido-4-pyrimidinone
(UPy) end-functionalized polystyrene-b-
poly(n-butyl acrylate) (PS-b-PBA) AB
diblock copolymers have been synthe-
sized, resulting in a self-healing material
that combines the advantageous
mechanical properties of thermoplastic
elastomers and the dynamic self-healing
features of supramolecular materials.
6
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