Biomimetic design of reversibly unfolding cross-linker to enhance mechanical properties of 3D network polymers

Article abstract of DOI:10.1021/ja0742176

We report here a biomimetic design of a reversibly unfolding modular cross-linker to enhance mechanical properties of 3D network polymers. The inspiration comes from the modular biopolymers observed in nature. A cyclic modular cross-linker based on the quadruple hydrogen bonding 4-ureido-2-pyrimidone (UPy) motif was synthesized via multistep organic synthesis. The modular cross-linker was incorporated into poly(n-butyl acrylate) by free radical polymerization. Stress-strain measurements show that the samples containing our modular cross-linker exhibit significantly enhanced mechanical properties over the control samples. Most strikingly, with increasing cross-linker density, both modulus and tensile strength are significantly improved without sacrificing extensibility. The enhanced tensile properties are attributed to the increased energy dissipating ability of the reversibly unfolding cross-linker. This introduces a novel biomimetic concept to enhance network mechanical properties through design of molecularly engineered cross-linkers. Copyright

Full text of DOI:10.1021/ja0742176

Published on Web 10/31/2007
Biomimetic Design of Reversibly Unfolding Cross-Linker to Enhance
Mechanical Properties of 3D Network Polymers
Aaron M. Kushner, Vahe Gabuchian, Evan G. Johnson, and Zhibin Guan*
Department of Chemistry, UniVersity of California, 1102 Natural Sciences 2, IrVine, California 92697-2025
Received June 9, 2007; Revised Manuscript Received October 18, 2007; E-mail: zguan@uci.edu
Three-dimensional (3D) network polymers are an important
family of materials. For many applications of 3D networks, it is
important to combine high modulus, high tensile strength, and high
extensibility.1 Rigid network materials tend to fail after only a short
extension. While flexible elastomers are more extensible, they
usually have low moduli and exhibit shallow stress response.
Although many engineering approaches and chemical modifications2
have been developed to improve mechanical properties of network
polymers, it remains a challenge to design ideal networks that have
a combination of desired properties. Among chemical methods,
interesting work has been reported on using interchain hydrogen
bonding to improve polymer physical properties.3 Given the
Figure 1. Concept of biomimetic design of biomimetic modular cross-
linker for enhancing network mechanical properties.
importance of cross-linker structure on mechanical properties of
network polymers, it is surprising that there is very limited
investigation on designing molecularly engineered cross-linkers to
enhance network properties. Herein we introduce a novel biomi-
metic design of a reversibly unfolding modular cross-linker that
can increase elastomer stiffness without sacrificing extensibility,
leading to a dramatic tensile strength enhancement.
Scheme 1. Synthesis of the Biomimetic UPy Modular
Cross-Linkera
Our biomimetic concept is based on the modular design observed
in many biopolymers, such as the skeletal muscle protein titin and
connective proteins in both soft and hard tissues, that have a
remarkable combination of strength and elasticity.4 Single-molecule
nanomechanical studies have revealed that their combined mechan-
ical properties originate from their unique modular structure, which
sequentially unfolds upon deformation providing the molecular
mechanism to sustain high force (strength) and to yield high
elongation (elasticity).5 Inspired by nature, our group has been
mimicking this modular domain strategy in the pursuit of synthetic
polymers with advanced mechanical properties. A number of
biomimetic modules have been successfully designed and incor-
porated into linear polymers.6,7 Single-molecule and bulk properties
validated our biomimetic concept of using modular structures to
enhance polymer properties. This communication describes the first
example of introducing a reversibly unfolding modular cross-linker
into 3D networks to enhance their mechanical properties.
Our concept is illustrated in Figure 1. A stress applied from any
direction to a 3D network will be ultimately transferred across the
individual network junctions, where biomimetic modules can be
reversibly unfolded. Since it requires significant forces to unfold
the modules held by strong multiple hydrogen bonds, further
extension can be gained without sacrificing the modulus. In addition,
we propose that the enhanced energy dissipation capability by un-
folding the biomimetic modules should lead to significant increases
in both extensibility and tensile strength. In this study, we use a
simple poly(n-butyl acrylate) network to demonstrate this concept.
The biomimetic cross-linker is based on the quadruple hydrogen
bonding 4-ureido-2-pyrimidone (UPy) motif originally developed
by the Meijer and co-workers.8 The synthesis of the module is
shown in Scheme 1 (also see synthetic details in Supporting
Information). Double alkylation of ethyl acetoacetate followed by
guanidine condensation afforded the alkenyl-pyrimidone interme-
a Conditions: (a)NaH,n-BuLi,THF,Br(CH2)4OBn,52%;(b)Br(CH2)9CHdCH2,
K2CO3, DMF, rt, 12 h, 47%; (c) guanidine carbonate, EtOH, reflux, 12 h,
72%; (d) 4, Py, reflux, 6 h, 83%; (e) DCM; (f) (Cy3P)2RuCl2dCHPh, DCM,
reflux, 1-2 h, 65%; (g) Pd(OH), 1 atm H2, THF, 68%; (h) CHCl3,
2-isocyanatoethyl methacrylate, DBTDL.
diate 3. The isocyanate 4 was coupled to the pyrimidone 3 to yield
5. Upon dimerization in DCM, ring-closing metathesis (RCM)
effectively cyclizes the two UPy units.9 Despite the possibility of
other isomer formation, only the desired product was obtained (see
Supporting Information). A one-pot reduction and deprotection
through hydrogenation gave the diol module 2. Finally, capping 2
with 2-isocyanatoethyl methacrylate at both ends provided the UPy
cross-linker, 1, which was characterized by 1H and 13C NMR, FTIR,
and MALDI-TOF MS (see Supporting Information).
In this model study, we chose poly(n-butyl acrylate) as the
backbone because this polymer exhibits elastomeric properties upon
cross-linking (Tg ) -54 °C)10 and can be easily synthesized by
free radical polymerization. A series of transparent rubbery films
were prepared by copolymerization of n-butyl acrylate with the
cross-linker 1 using AIBN as initiator. A poly(ethylene glycol)
(PEG) dimethacrylate (Mn ) 750) was used as the control cross-
linker. This cross-linker is very soluble and flexible, and when fully
stretched, it has approximately the same length as the fully unfolded
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J. AM. CHEM. SOC. 2007, 129, 14110-14111
10.1021/ja0742176 CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Stress-Strain Data for Both Sample and Control
cross-linker is supported by the much larger loss moduli measured
by DMA for the real samples as compared to the controls (see
Figure S7). Whereas potential nano- and microphases may also
contribute to the bulk mechanical properties, on the basis of the
optical transparency of the samples and our small-angle X-ray
scattering (SAXS) and scanning electron microscopy (SEM) data
(Figures S9 and S10), it is unlikely that nano- or microphases exist
in our system.
In summary, we describe here the biomimetic design of a revers-
ibly unfolding modular cross-linker to enhance mechanical proper-
ties of 3D networks. A UPy-based cyclic modular cross-linker was
synthesized via multistep organic synthesis. Stress-strain measure-
ments show that the poly(n-butyl acrylate) samples containing the
modular cross-linker exhibit significantly enhanced mechanical
properties over the control samples. Most interestingly, with in-
creasing cross-linker density, both modulus and tensile strength are
significantly improved without sacrificing the extensibility. This
introduces a novel biomimetic concept to enhance rubber properties
through design of molecularly engineered cross-linkers. Further
studies are currently ongoing to extend this concept to other network
systems and to probe the mechanisms for the property enhancement.
Systems
elongation
Young’s Modulus (E)
(MPa)
tensile strength
(MPa)
at failure
sample
(mm/mm)
PEG (2%)
PEG (4%)
PEG (6%)
UPy (2%)
UPy (4%)
UPy (6%)
1.24 ((0.07)
1.67 ((0.12)
3.82 ((0.13)
1.67 ((0.09)
3.89 ((0.02)
5.57 ((0.08)
0.52 ((0.04)
0.57 ((0.06)
0.63 ((0.17)
0.95 ((0.12)
2.47 ((0.38)
4.5a
0.59 ((0.06)
0.45 ((0.05)
0.19 ((0.05)
0.75 ((0.07)
0.69 ((0.05)
0.8a
a The data for specimens that did not break within the 10 N load limit
(strain rate: 100 mm/min, at room temperature).
Acknowledgment. We acknowledge the financial support from
the National Institutes of Health (R01EB004936) and Department
of Energy (DE-FG02-04ER46162). We thank Professor Andrew
Putnam for the MTS instrument, and Professor Albert F. Yee for
the DMA instrument. Z.G. acknowledges a Camille Dreyfus
Teacher-Scholar Award and a Humboldt Bessel Research Award.
Figure 2. Stress-strain curves for 6% cross-linked poly(n-butyl acrylate)
rubber for the sample (blue) and control specimen (strain rate: 100 mm/
min, at room temperature).
Supporting Information Available: Synthesis and characterization
of cross-linker and polymers, MALDI-TOF MS, MTS stress-strain
experiments. This material is available free of charge via the Internet
at http://pubs.acs.org.
UPy cross-linker 1. The thermosets were characterized by FTIR,
DSC, and static stress-strain and dynamic mechanical analysis
(DMA). The Tg values measured by DMA for controls and real
samples are all below room temperature (see Figure S8 in
Supporting Information).
References
Table 1 summarizes the mechanical parameters obtained from
the stress-strain studies, and Figure 2 compares the stress-strain
curves for the sample and control specimen having 6 mol % of
cross-linkers. Compared to the control, the key mechanical proper-
ties are significantly improved by introducing our biomimetic cross-
linker. While the improvement is modest for the 2% samples, in
the 4% UPy case, both modulus and tensile strength increase
substantially without sacrificing the maximum elongation. At 6%
incorporation, the UPy sample maintains a continuously improved
modulus and a greater than 700% increase in tensile strength while
maintaining a similar level of elongation. The rubbery plateau
moduli obtained from DMA measurements (see Figure S6 in
Supporting Information) agree well with the static tensile data.
The comparison between the UPy samples and the PEG controls
validates our biomimetic concept. For the PEG controls, the increase
of modulus at higher cross-linker levels trades off with the
maximum elongation. This is typical for regular thermoset elas-
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elastic rubbers.2,11 In contrast, in our UPy samples, we observed a
consistent increase in modulus and tensile strength without sacrific-
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dissipation ability of our modular cross-linker.12 While the modulus
is increased at higher cross-linking density, the reversibly unfolding
modules can act as energy dissipating units to prevent fracture
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by 100%, the free energy change (∆G) is roughly -T∆S, which is
estimated to be ∼0.6 kcal/mol.13 In contrast, it takes ∼11 kcal/mol
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M
in toluene).8 The enhancement of energy dissipation by the modular
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