Cooperativity of Catechols and Amines in High-Performance Dry/Wet Adhesives

Article abstract of DOI:10.1002/anie.202005946

The outstanding adhesive performance of mussel byssal threads has inspired materials scientists over the past few decades. Exploiting the amino-catechol synergy, polymeric pressure-sensitive adhesives (PSAs) have now been synthesized by copolymerizing tra

Full text of DOI:10.1002/anie.202005946

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Accepted Article
Title: Cooperativity of Catechols and Amines in High Performance Dry/
Wet Adhesives
Authors: Brylee David B. Tiu, Peyman Delparastan, Max R Ney,
Matthias Gerst, and Phillip Messersmith
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Angewandte Chemie International Edition
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RESEARCH ARTICLE
Cooperativity of Catechols and Amines in High Performance
Dry/Wet Adhesives
[
a]†
[a]†
[a]
[b]
Brylee David B. Tiu , Peyman Delparastan , Max R. Ney , Matthias Gerst , Phillip B. Messersmith*
[a,c]
†These authors contributed equally
[
a]
B.D.B Tiu, P. Delparastan, M.R. Ney, Prof. P.B. Messersmith
Bioengineering and Materials Science and Engineering
University of California, Berkeley
Berkeley, CA, 94720-1760 (USA)
E-mail: philm@berkeley.edu
[
[
b]
M. Gerst
Polymers for Adhesives
BASF SE
D-67056 Ludwigshafen, Germany
Prof. P.B. Messersmith
Materials Sciences Division
Lawrence Berkeley National Laboratory
Berkeley, CA, 94720 (USA)
c]
Supporting information for this article is given via a link at the end of the document.
Abstract: The outstanding adhesive performance of mussel byssal
threads has been a beacon of inspiration for materials scientists over
the past few decades. Historically, presence of a significant amount
of the unique catecholic amino acid dihydroxyphenylalanine (Dopa)
has been considered to play a key role in strong dry and wet adhesive
properties of the byssal interfacial proteins. In recent years, molecular
and microscopic level studies using short peptides or small molecule
analogs have investigated the roles of other amino acids in mussel
adhesion. In particular, these studies have highlighted the cohesive
role of cation-π interactions as well as the adhesive synergy between
Dopa and flanking lysine residues. Inspired to design advanced
synthetic adhesives that exploit amino-catechol synergy, we
synthesized polymeric pressure sensitive adhesives (PSAs) by
copolymerizing traditional PSA monomers, butyl acrylate and acrylic
acid, with mussel-inspired lysine- and aromatic-rich monomers. Of
particular interest was to compare the consequences of decoupling
amino and catechol moieties from each other (i.e. incorporated as
separate monomers) versus a monomer architecture in which the
catechol and amine were coupled together in a fixed orientation in the
monomer side chain. Comprehensive multi-scale adhesion assays
were used to probe performance at the molecular, microscopic and
macroscopic levels through a combination of AFM-assisted force
spectroscopy, peel and static shear adhesion. We showed that
coupling of catechols and amines together within the same monomer
side chain produced optimal cooperative effects in improving
macroscopic adhesion performance. The findings in this study
improve our understanding of underlying principles of mussel
adhesion and provide a solid framework for rational design of high-
performance bioinspired adhesives.
concepts from mussel adhesives into synthetic materials has been a
[3]
long-standing pursuit for researchers. To achieve this goal, many
fundamental studies have been conducted in order to understand the
underlying principles of mussel adhesion. Early investigations
attributed the unique adhesive properties of mussel plaques to the
presence of unusually high contents (20-30 mol%) of the post-
transitionally modified 3,4-dihydroxyphenylalanine (Dopa) residues in
the mussel foot proteins (Mfps-3,5) at the protein-substrate
[1a, 4]
interface.
Fundamental studies later confirmed the critical role of
catechol moiety in adhesive and cohesive properties of these
interfacial proteins using surface forces apparatus (SFA) and single
[5]
molecule force spectroscopy (SMFS). These initial observations
sparked an era of substantial scientific interest in designing catechol-
containing synthetic peptides and polymers to mimic the mussel
[6]
plaque adhesion. Although incorporating catechols alone shows
[6c, 7]
great promise in improving adhesive performance,
such simplified
approaches are unable to capture the complex interplay between
different amino acids in Mfps.
Recently, a few seminal studies have suggested the critical role of
other amino acids besides Dopa on the strong adhesion of Mfps. For
instance, a possible synergy between Dopa and positively charged
amino acids such as lysine (Lys) and arginine (Arg) has been
[8]
proposed. These amino acids are frequently positioned adjacent to
[1a]
Dopa residues in Mfps.
Although still a subject of active
investigation, presence of these cationic residues and their close
proximity to Dopa are thought to improve the interfacial adhesion by
providing additional surface binding opportunities as well as repelling
the hydration layer and thus preparing a pristine surface for effective
[8-9]
catechol interaction.
In addition to acting as surface primer, amine
groups are also believed to amplify the cation-π interactions and
[10]
enhance cohesive properties. Cation-π interactions are among the
most important non-covalent interactions employed in nature to
The remarkable wet adhesion of marine mussel byssus has been a
major source of bioinspiration for materials scientists during the recent
stabilize and tune the structure and function of many biological
[11]
molecules.
In particular, interactions between hydrophobic
[1]
decades. These sessile organisms are able to tenaciously attach to
aromatic amino acids such as phenylalanine and tyrosine with cationic
residues has been shown to play an essential role in self-assembly,
various surfaces in turbulent, wet, and saline habitats using their
[1a, 2]
byssal threads.
Since many man-made adhesives fail in the
molecular cohesion and adhesion, as well as formation of secondary
presence of moisture, surface contaminants or salts, incorporation of
[12]
structures in proteins and peptides.
Interestingly, a few recent
1
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Angewandte Chemie International Edition
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RESEARCH ARTICLE
investigations have suggested that the aromatic-rich nature of Mfps
provide a clue as to the important function of cation-π interactions in
optimizing the adhesive and cohesive properties of PSAs has proven
to be a challenging task, as improving one property often comes at
[10a, 12b, 12c, 13]
[6b, 14d, 16]
tuning the underwater adhesion of mussel plaques.
the expense of another.
Although these recent findings have improved our understanding
of the underlying mechanisms for the remarkable adhesion of Mfps,
many important questions still remain unanswered. For instance, it is
not clear whether the synergistic effects between catechols and
amines are limited to only enhancing the interfacial adhesion or if
cohesive properties are also affected. More importantly, a controversy
still exists on whether the adjacency of the catecholic and cationic
moieties is crucial in establishing the cooperative effects. In addition,
it remains unclear whether catechol groups are inherently essential to
observe strong synergy with amines or if similar properties can be
achieved by employing phenyl instead of catechol in the bioinspired
molecular designs. Finally, most of the previous investigations have
focused on studying the adhesion of nanometer thin films of small
molecules or short peptides using SFA measurements. Although
these experiments provide invaluable molecular mechanistic insights,
validity of translating these results across many length scales to
design polymer adhesives for practical applications remains an open
question.
The promising results of our earlier study motivated us to take a
step further and apply the recently highlighted mussel adhesion
propositions to our PSA design in order to tackle the above challenges.
To examine the synergy between catechols and cationic amine
groups in PSAs, we designed and synthesized several mussel-
inspired Lys- and aromatic-rich monomers (Figure 1). The monomer
designs were inspired by the high Dopa and Lys content and common
appearance of adjacent Lys-Dopa pairs in Mfps, as well as recent
findings regarding the potential of cation-π interactions between
[8a,
cationic and aromatic residues in tuning the adhesive performance.
8c, 10a]
Acrylic-based PSAs can be considered an ideal platform for
studying the effects of mussel-inspired monomers in improving the
adhesive performance owing to their widespread practical use and
their synthetic modularity in monomer composition. Thus, we
proceeded to polymerize our mussel-inspired monomers with AA and
BA co-monomers using conventional free-radical polymerization to
yield high molecular weight PSAs (Figure 1). Full synthetic schemes
and experimental details for monomer and polymer preparation are
provided in the supporting information (Schemes S1-S6). The
chemical structure and successful synthesis of each compound was
Here, we examined the cooperative effects of amine and aromatic
groups incorporated into a common pressure sensitive adhesive
(
PSA) polymer based on acrylic acid (AA) and butyl acrylate (BA). The
1
13
polymers were subjected to variety of multi-scale adhesion
a
confirmed by H NMR, C NMR, and ESI-MS measurements
(Figures S2-S39). The library of synthesized polymers along with the
selective deprotection of the amine and catechol moieties (Schemes
S6-S7) allowed us to effectively decouple the adhesive and cohesive
contributions of each functional group on the overall adhesive
performance and investigate possible cooperative effects.
measurements including AFM-based force spectroscopy at molecular
and microscopic levels as well as macroscopic peel and static shear
adhesion, providing fundamental insights into the mechanism and
strength of the interfacial interactions between the polymers and the
surface. The rational approach to polymer design combined with
comprehensive multi-scale adhesion characterizations allowed us to
decouple the interfacial adhesive and intermolecular cohesive
contributions of each functional motif on the overall performance and
provided new insights into mussel adhesion principles.
In order to isolate the effect of composition on adhesion, we chose
to keep the functional monomer feed ratio fixed at 2.85 mol% (Table
1), as this concentration of catechol was demonstrated to be sufficient
[6b]
for boosting the adhesive performance of acrylic PSAs.
Calculations based on the integration of area under the peaks from
functional groups in the NMR spectra showed that the compositional
drifts from the intended feed ratio in the polymerization were not
Results and Discussion
significant (Table S2 and Figures S29-S34). Molecular weight and T
g
PSAs have become an integral component in many consumer
products with widespread applications including adhesive tapes,
values of polymers were measured using gel permeation
chromatography (GPC) and differential scanning calorimetry (DSC)
and are listed in Table 1 (also Table S1, Figures S40-S41).
Compared to the control PSA polymer (BA91.4%:AA8.6%), the functional
[14]
labels, protective coatings, and medical bandages. Among different
classes of PSAs, linear or slightly cross-linked acrylic based polymers
with a high content of BA or other monomers with low glass transition
[14c, 15]
polymers had higher M which can be attributed to the enhanced non-
temperature (T
Although the conventional PSA formulations show satisfactory
performance for non-demanding applications there are few
For instance, the
adhesive properties of PSAs in wet conditions are generally
g
) are extensively used in these contexts.
w
covalent and covalent interactions between the polymer chains owing
to the presence of catechol groups, as described previously in more
a
[6b]
[14b, 14d]
detail. The small variations observed between the molecular weight
and T of the functional polymers can further underline that physical
properties of PSAs are not expected to be a major contributor to the
challenges associated these compositions.
g
[14d]
significantly inferior to their performance in ambient.
In a previous
[17]
differences noticed in their adhesive performance.
effort to improve their wet adhesion, we applied the mussel inspiration
principles to the base PSA polymers and showed that catechol PSAs
outperformed conventional benchmarks in different industry standard
After a thorough physical and chemical characterization of PSAs,
we proceeded to evaluate their interfacial adhesion using an Atomic
Force Microscope (AFM). Owing to its high force resolution and
displacement sensitivity, AFM-assisted force spectroscopy has been
[6b]
tests even at a low ~3mol% catechol content. In addition to the
inferior under water performance, another major shortcoming of
conventional PSAs is their poor cohesive strength and low toughness.
PSAs commonly take the form of viscoelastic polymers with a
sufficiently low modulus to allow for good contact with the substrate
recognized as a versatile technique in studying physical behavior and
[18]
mechanical properties of polymeric films.
Depending on the
cantilever probe used in the measurements, a range of molecular or
micro-scale interfacial phenomena can be investigated using this
technique. For example, colloidal probe spectroscopy (CPS) is a
widely employed method to measure adhesive forces acting between
[15-16]
upon application of light pressure.
However, the low modulus can
lead to poor cohesive strength which can substantially limit the use of
[14c, 16]
PSAs to low stress applications.
Although a few strategies have
been previously proposed to improve the cohesive strength of PSAs,
2
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RESEARCH ARTICLE
Figure 1. Structure of conventional and various mussel-inspired monomers used to synthesize a library of PSAs via free-radical polymerization.
g
Table 1. Monomer feed composition (mol%), molecular weight values, polydispersity index, and T of synthesized PSAs.
Feed Composition (mol%)
M
n
M
w
T
g
Polymer
Đ
Dipeptide
(kDa)
(kDa)
(˚C)
BA
AA
DAc
ABA
Monomer
PSA
91.4%
88.4%
88.4%
88.4%
88.4%
88.4%
85.6%
8.6%
8.74%
8.74%
8.74%
8.74%
8.74%
8.74%
-
-
-
116
280
157
204
233
153
193
346
599
377
413
827
352
544
3.0
-41
-32
-30
-32
-28
-30
-23
PSA-DAc
-
2.85%
-
2.14
2.40
2.03
3.54
2.30
2.82
PSA-Lys(Boc)-DA(ac)
PSA-Lys(Boc)-DA
PSA-Lys-DA
PSA-Lys-PEA
PSA-DAc-ABA
2.85%
2.85%
2.85%
2.85%
-
-
-
-
-
-
-
-
-
2.85%
2.85%
3
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a colloidal particle (i.e., micrometer-sized spherical particle) attached
force of interaction between the probe and the polymer film was
measured. A schematic of the experimental setup as well as a
representative F-D curve for PSA-Lys-DA is shown in Figure 2a. The
sequential rupture of polymer-surface interactions during retraction of
the cantilever lead to the appearance of sawtooth-like peaks in the F-
D curves, reminiscent of what is observed during unfolding of globular
[19]
to a cantilever beam and a planar substrate. Single molecule force
spectroscopy (SMFS) is another powerful AFM-based
characterization method which is well suited to investigate and
quantitively measure the strength of chemical bonds as well as
[20]
adhesive interactions acting between molecules and surfaces. This
technique has been widely used in the past for studying the adhesion
of mussel-inspired peptides and polymers and advances have been
[18d, 21]
polyproteins during stretching.
These rupture events can then
be fitted using polymer elasticity models to confirm single molecule
behavior and extract the strength of polymer-surface interactions
(Figure S43). Histograms and median values of rupture force for
PSAs are presented in Figure 2 and Table 2, respectively (also Table
S3 and Figure S45). We first tested the control PSA polymer where a
median rupture force of 118 pN was measured (Figure 2b). This value
is in the range of weak non-covalent interactions such as hydrogen
bonding and charge-charge interactions and can be attributed to the
presence of AA in the polymer which can accommodate these
[5a]
summarized in a recent review by Li et al. SMFS measurements
are complementary to the CPS experiments and when combined
together can provide a more comprehensive picture of the molecular
basis underlying adhesive and cohesive properties of materials. In
the present work, we utilized both SMFS and CPS techniques to study
adhesive performance of PSA polymers in order to decouple cohesive
contributions from interfacial adhesive interactions acting between
polymer chains and the surface.
[22]
interactions with the substrate. Results of measurements on PSA-
DAc indicated that incorporation of catechol groups into the polymer
can lead to a notable increase in the median rupture force to ~236 pN,
possibly owing to the myriad of interfacial interactions that catechol
can form with the mica surface including strong hydrogen bonding and
bidentate complexation.
SMFS experiments were performed initially to gain a fundamental
insight into the molecular phenomena underlying the interfacial
adhesive properties of PSAs. In our experiments a sharp AFM
cantilever (tip radius ~10-20 nm) was brought into contact with PSA
films on mica substrate and retracted at a specified speed while the
Figure 2. SMFS characterization of PSA polymers. (a) experimental schematic and a representative F-D curve for PSA-Lys-DA are shown. Histograms of the
rupture forces are shown in (b) PSA, (c) PSA-DAc, (d) PSA-Lys-DA, (e) PSA-Lys-PEA, (f) PSA-DAc-ABA.
4
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We observed a stark increase in the rupture forces for PSA-Lys-DA
compared to PSA and PSA-DAc polymers, confirming the cooperative
effects between catechols and positively charged amines at the single
molecule level. The elevated rupture forces for PSA-Lys-DA can be
attributed to the potential role of positive charges of Lys in removal of
solvation layer from the surface for a more effective catechol
interaction as well as possible cooperative surface binding leading to
an increased interaction strength and lifetime as suggested previously
measure of the overall adhesive interactions. More importantly,
owing to the viscoelastic nature of the polymer as well as
roughness and asperities on the colloidal particle, accurate
determination of the contact geometry and radius can be
challenging.[19a-c, 23] Hence, the adhesion energy values calculated
based on the contact mechanics models can be subject to large
errors introduced from the estimated contact radius. An
alternative approach to estimate the magnitude of the overall
adhesive strength between the polymer film and the substrate in
the contact area is to integrate the area under the retraction trace
in the F-D curves.[24] The calculated quantity has the units of
energy and will be referred to as separation work (W) here.[6b, 25]
The separation work calculated here is related to the energy that
is required to detach the probe from the substrate.[19b]
[8]
by Cao and colleagues. Next, we proceeded to study the interfacial
adhesion of PSA-Lys-PEA to evaluate the effects of cation-π
interactions in improving the polymer-surface interactions.
Interestingly, we observed a median rupture force of ~125 pN for PSA-
Lys-PEA. The sharp contrast in the rupture forces detected for PSA-
Lys-DA and PSA-Lys-PEA clearly highlights the advantages of
catechols over unsubstituted phenyl rings for improving interfacial
interactions with substrates.
Histograms and median values of W for the PSA polymers are
presented in Figure 3 and Table 2, respectively (also Table S4 and
Figure S46). CPS results show that incorporating catechol as DAc
into the polymer backbone can lead to a 20-fold increase in the
separation work. A similar trend was observed in our earlier study
where incorporating 5 wt% dopamine methacrylamide into PSAs
After observing the performance of PSA-Lys-DA we were intrigued
to study the effects of the molecular architecture and adjacency of
amine and catechol groups on the interfacial adhesion. Since Lys and
Dopa residues are most commonly positioned adjacent to each other
along the backbone in Mfp-3 and 5, it was initially hypothesized that
the close proximity of the cationic Lys residues and catecholic motifs
resulted in a significant increase in the separation work for SiO
2
[6b]
probes. The large increase in adhesion to silica upon incorporation
of DAc may be attributed to strong binding mechanisms such as
hydrogen bonding and bidentate complexation as described in detail
[8a, 8c]
are necessary to establish the synergistic effects.
However,
recently it has been suggested that the adjacency of the two functional
motifs might not be required for the synergistic effects and only co-
presence of cationic and catecholic groups might lead to major
[1c, 5b, 6b]
previously.
As expected, incorporating a protected form of the
bifunctional monomer Lys-DA into the polymer backbone did not
result in major changes in adhesion compared to the control PSA
polymer (Figure S44). However, upon removal of the protecting
groups the resulting PSA-Lys-DA outperformed all other polymers by
showing a remarkable ~36-fold increase in the separation work
compared to the control PSA. This striking increase further highlights
that co-presence of amine and catechol groups can lead to significant
improvements in adhesive performance of PSA polymers. The
cooperative effects between amines and catechols can be attributed
to the role of positive charges of amines in removing solvation layer
and providing an immaculate surface for catechol interaction or
resulted from a better load distribution through synergistic surface
binding of the two groups as suggested in previous molecular
[9a]
improvements in adhesive performance. To further test this notion
we studied interfacial adhesion of PSA-DAc-ABA. The median rupture
force for PSA-DAc-ABA was ~351 pN, much higher than that of the of
amine-free PSA-DAc and only modestly less than the values
observed for PSA-Lys-DA. Compared to PSA-Lys-DA, catechol- and
amine-containing monomers are located randomly along the
backbone in PSA-DAc-ABA, resulting in a stochastic distribution of
spacing between DAc and ABA groups in the polymer chains. The
SMFS results indicate that even when randomly distributed, the amine
groups can possibly be partially effective in desolvating the surface
and facilitating the interaction of catechols with the substrate. More
importantly, in SMFS measurements the polymer chains can be
picked up at any random point along their contour length. Owing to
this random attachment to the probe as well as flexibility of the
backbone and the presence of a large number of amine and catechol
groups in polymer molecules, the synergistic effect due to the
simultaneous rupture of catechol- and amine-surface interactions
might still be observed, leading to elevated rupture forces as
compared to PSA-DAc.
[8-9]
studies.
To determine if similar adhesive performance can be achieved by
substituting phenyl for catechol groups in the monomer design, we
tested the adhesion of PSA-Lys-Phe. Although the results showed a
notable ~10x improvement in the separation work compared to the
control PSA, PSA-Lys-Phe underperformed both PSA-Lys-DA and
PSA-DAc. Considering the results obtained in CPS and SMFS
experiments together, the findings indicate the potential benefits of
cation-π interactions in improving the cohesive interactions between
the macromolecules leading to an enhanced adhesion as measured
Next, we proceeded to perform CPS experiments on PSAs in
order to complement the SMFS studies and to gain a microscopic
view of adhesion performance.[19c, 19d] In our measurements an AFM
[10a,
cantilever with a colloidal probe of silica (SiO
2
, diameter = 3.5 µm)
here in the CPS experiments as well as in previous SFA studies.
12b, 12c]
was used and brought into contact with a PSA film adsorbed on
mica substrate. Schematic of the experimental setup as well as a
representative F-D curve for PSA-Lys-DA are shown in Figure 3a.
A characteristic feature in most of the collected F-D curves was
the appearance of multiple spikes in the retraction trace. The
presence of these spikes can be attributed to detachment of
multiple polymer chains from either the colloidal probe or the
substrate upon retraction of the AFM cantilever, or alternatively to
rupture of specific polymer-surface adhesive interactions between
individual chains and the surface. As a result, extracting only the
maximum detachment force from the F-D curves is not a proper
Nevertheless, the SMFS results imply that Lys-PEA pairs have
minimal effects in strengthening the interfacial adhesive interactions
with surfaces. In other words, due to the nature of the CPS and SFA
experiments where detachment of an ensemble of molecules is
studied, cohesive interactions between the molecules can affect the
measured values for separation work or work of adhesion. Thus,
possible contributions of intermolecular cohesive and interfacial
adhesive interactions on the improved performance cannot be
equivocally deconvoluted solely based on the results these
measurements.
5
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Figure 3. CPS characterization of PSA polymers. (a) experimental schematic and a representative F-D curve for PSA-Lys-DA are shown. Histograms of the
separation work are shown in (b) PSA, (c) PSA-DAc, (d) PSA-Lys-DA, (e) PSA-Lys-PEA, (f) PSA-DAc-ABA.
To gauge the possible role of molecular architecture and proximity
of catechol and amine groups on the adhesive performance of PSAs,
we performed CPS experiments on PSA-DAc-ABA. Interestingly, the
polymer displayed a median separation work close to that of PSA-Lys-
DA polymer. The trend observed in CPS measurements is similar to
what was noted earlier in SMFS experiments and can indicate that
architecture of the polymer and the adjacency of catechol- and amine-
containing groups might not be necessary in establishing the
cooperative effects to improve the adhesion, at least when measured
at the microscopic or molecular levels. Similar observations have
Combined with the results obtained in the interfacial adhesion
measurements, this holistic multi-scale approach could allow us to
decouple the adhesive and cohesive contributions as well as effects
of polymer composition and architecture on the PSA performance.
For the macroscopic adhesion measurements, the PET films were
coated with PSAs, dried and then cut into 1 inch-wide strips to prepare
test samples (Figure S1). The shear holding power of the adhesives
was evaluated in both dry and wet conditions by adhering the PSA
tapes onto a stainless steel test plate and measuring the time to
rupture under the influence of a constant shear force (Figure S47).
This test simulates a constant load creep condition and can be used
for determining the shear failure time, a measure of the cohesive
strength or shear holding power of the adhesive. Similar to the results
obtained in our previous study, incorporation of catechol groups into
the polymer backbone led to a substantial increase in the failure time
for PSA-DAc as compared to the control PSA, from 14 and 8 to 123
and 77 minutes under dry and wet conditions, respectively (Figure
4a). While the improved shear holding power of the PSA-DAc can be
mostly attributed to the enhanced cohesive and intermolecular
interactions due to the presence of catechol groups, we should note
that in addition to the chemical composition, the shear failure times
[9a]
been made recently by Degen et al.,
where results of SFA
measurements implied that incorporating glycine residues as spacer
between the cationic and catecholic groups did not lead to significant
changes in the force required to separate peptide-coated mica
surfaces.
Despite the wealth of mechanistic information that interfacial
adhesion measurements on thin-films can provide, the bulk behavior
still cannot be unambiguously predicted from CPS and SMFS
experiments that probe the microscopic and molecular levels,
respectively. Thus, we proceeded to perform static shear and 180°
peel measurements as industry-standard macroscale adhesion tests.
w g
are also dependent on additional factors including M and T .
6
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Figure 4. Macroscopic characterization of PSA tapes. (a) Shear failure time after 10 min conditioning in ambient (D) and wet (W) conditions. (b) 180° peel
adhesion after 24 h conditioning in ambient (D) and wet (W) conditions. The failure modes are denoted as: C=cohesive, A=adhesive, and AT=adhesive transfer.
The most interesting finding was the remarkably large failure times
observed for PSA-Lys-DA, which was on the order of 20,000 minutes
or more compared to <600 minutes for all other samples. This
outstanding performance can indicate the significant enhancements
in the intermolecular interactions in PSA-Lys-DA, possibly owing to
the augmented covalent and non-covalent interactions such as
additional hydrogen bonding due to the presence of amine groups as
well as strong cation-π interactions between the charged amine and
catechol motifs, in a similar manner to their recently highlighted role
amine moieties in mussel inspired adhesives across many length
scales.
In addition to shear, conventional 180˚ peel adhesion tests were
performed both in dry conditions and after immersion in water for 24
hours prior to the measurement (Figure S48). The results of the peel
adhesion for PSA-DAc show that both dry and wet adhesion increase
~4-fold compared to the control PSA polymer (Figure 4b and Table
2). More importantly, the failure mode also changes from cohesive to
adhesive transfer in ambient condition. This behavior can be
attributed to the strong interactions of catechol with the metal
substrate as compared to the PET backing material which leads to
detachment of the polymer from the backing layer. In the case of PSA-
Lys-DA polymer, although both dry and wet performance was
improved compared to the control PSA, the increase in peel adhesion
was not as significant as that observed for PSA-DAc. However, the
most interesting observation was the change in the failure mode from
cohesive to adhesive for both dry and wet measurements. This
behavior can be ascribed to the magnified intermolecular and
cohesive forces between the polymer chains in PSA-Lys-DA owing to
the presence of cation-π interactions. The adhesive and cohesive
performance of PSA-Lys-DA, in terms of both force values and failure
mode, may be attractive for many practical applications of PSAs.
Peel adhesion results of PSA-Lys-PEA showed significant
improvements in comparison to the control PSA which can partly be
[10a, 26]
in improving cohesive strength of Mfps.
To understand the effects of the aromatic molecular structure on
the strength of the cohesive cation-π interactions, we tested the shear
holding power of PSA-Lys-PEA. Interestingly, the failure time in wet
condition was not statistically different from that shown by PSA-DAc,
although PSA-Lys-PEA outperformed other polymers except for PSA-
Lys-DA when considering dry shear holding power. The results
underline the key role of cation-π interactions in boosting the cohesive
strength of the adhesive films. However, when compared against
PSA-Lys-DA, the catechol-containing polymer displayed significantly
larger failure times, both in ambient and wet conditions. Such a
notable difference between failure times of PSA-Lys-DA and PSA-
Lys-PEA polymers can possibly be attributed to the role of hydroxyl
groups of the catechol in providing additional opportunities for
intermolecular hydrogen bonding as well as acting as an anionic site
for auxiliary charge interactions with cationic Lys groups.
g
attributed to the increase in the T and consequently changes in the
To evaluate the possible impact of molecular design on cation-π
interactions and cohesive strength of catechol-amine containing
polymers, we studied shear holding power of PSA-DAc-ABA.
Surprisingly, we did not observe a substantial increase in the dry and
wet failure times as compared to PSA-DAc. Unlike the behavior noted
at the molecular level in this study and recent SFA measurements by
mechanical properties at room temperature. In dry condition,
adhesive failure was observed, which can further underline
enhancements in the intermolecular interactions resulting from cation-
aromatic pairs in the polymer backbone. The PSA-DAc-ABA polymer
showed average peel adhesion values between those measured for
g
PSA-DAc and PSA-Lys-DA. However, since T of this polymer is
[
9a]
others, the static shear test results indicate that close proximity of
cation-aromatic binding pairs can significantly affect the macroscopic
cohesive strength. The results might also explain why marine mussels
have evolved to secrete adhesive proteins with Dopa and Lys pairs
located mostly adjacent to each other rather than separated by spacer
amino acids such as glycine. The findings here also motivates further
studies to investigate the effects of proximity of catechol and cationic
somewhat different from those of the other two polymers,
interpretation and direct comparison between the peel adhesion
values should be done with caution.
Overall, the results of the static shear and peel adhesion
measurements highlight the importance of balancing adhesive and
cohesive properties to achieve optimal performance of mussel-
inspired PSAs. In this sense, the performance shown by PSA-Lys-DA
clearly stands out compared to all other polymers for its remarkable
7
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Angewandte Chemie International Edition
10.1002/anie.202005946
RESEARCH ARTICLE
shear strength conferred by strong cohesive interactions between
catechol and amine functional groups. In addition, PSA-Lys-DA also
showed improvements in both dry and wet peel adhesion compared
to the control PSA, and more importantly we observed a change in
the failure mode from cohesive to adhesive which is preferred for
many practical applications.
Table 2. Summary of the adhesion characterization results of PSAs measured using SMFS, CPS, static shear test, and 180˚ peel adhesion experiments. The values
in parenthesis for SMFS and CPS results correspond to the total number of events used to calculate the averages.
Single Molecule Force
Spectroscopy (SMFS)
Colloidal Probe
Shear Failure Time (min)
Peel Adhesion (N/25mm)
Spectroscopy (CPS)
Median Rupture Force
Median Separation Work
Polymer
Dry
Wet
Dry
Wet
(
pN)
(J×10-16
)
PSA
118 (2862)
235.8 (6088)
430 (4822)
0.27 (10996)
5.41 (8061)
9.72 (6292)
3.51 (8850)
8.76 (8020)
10.5 ± 3.3
122.9 ± 23.5
>20000
3.9 ± 1.3
76.7 ± 50
14.5 ± 1.3
62.5 ± 9.3
23.7 ± 3
8 ± 0.7
PSA-DAc
33.9 ± 2.4
12.6 ± 1.8
27.3 ± 1.5
26.9 ± 2.3
PSA-Lys-DA
PSA-Lys-PEA
PSA-DAc-ABA
19858.6 ± 327.4
82.9 ± 15.4
55.2 ± 7
125 (29728)
350.7 (7568)
531.6 ± 55.6
182.1 ± 47.8
60.8 ± 22
42.4 ± 13.4
Conclusion
Acknowledgements
Altogether, in this work we applied the recently highlighted
compositional and structural aspects of Mfps into our modular PSA
designs and investigated the effects of molecular architecture and
aromatic structure on the cooperative adhesive effects of catechols
and amines. An important finding of this work is that comprehensive
adhesion measurements across different length scales are crucial in
decoupling contributions from interfacial adhesive and intermolecular
cohesive interactions on overall PSA performance, highlighting the
benefits of a multi-scale approach.
This work was supported by the BASF California Research Alliance
CARA). The authors wish to acknowledge the UC Berkeley College
(
of Chemistry NMR Facility, UC Berkeley QB3 Mass Spectrometry
Facility, UC Santa Barbara Materials Research Laboratory (MRL), Dr.
Rachel Behrens for GPC analysis, Dr. Rita Nichiporuk for ESI-MS
measurements, and Dr. Joshua C. Speros for intellectual discussion.
The MRL Shared Experimental Facilities are supported by the
MRSEC program of the NSF under award no. DMR 1720256, a
member of the NSF-funded Materials Research Facilities Network
Our results from AFM-based molecular and microscopic force
spectroscopy experiments showed significant improvements in
interfacial adhesion of catechol-amine PSAs compared to the amine-
free polymers, possibly owing to the synergistic effects between
catechols and amines. Macroscopic adhesion results further indicated
that coexistence of catechol and amine moieties in a hybrid monomer
architecture can lead to optimization of overall performance of the
catechol-amine adhesives and developing PSAs with remarkable
resistance to flow under shear. Moreover, by substituting phenyl for
catechol in the hybrid monomer structure we showed that
incorporating phenyl-amine pairs into polymer backbone cannot
provide a clear benefit in improving interfacial adhesion with surfaces.
However, PSA with phenyl and amine groups coupled together
demonstrated a satisfactory performance in macroscopic shear and
peel adhesion tests, underlining the key role of cation-aromatic pairs
as compelling molecular modules in enabling robust cohesion of
mussel-inspired adhesives and hydrogels. Finally, since the precise
positioning of catechol and amine motifs adjacent to each other in a
fixed architecture is synthetically demanding, the intriguing possibility
of achieving similar synergistic effects through a random distribution
of these groups motivated us to study adhesive performance of PSA
polymer with catechol and amine moieties incorporated as separate
monomers. Interestingly, both SMFS and CPS results indicated that
molecular architecture and adjacency of these groups is not
necessary for establishing the synergistic effects. However, the
cooperative effects between catechols and amines were not observed
at macroscopic level in static shear or peel adhesion tests when
amine and catechol groups were decoupled from each other in
monomer architecture.
(
www.mrfn.org).
Keywords: Mussels • Catechol-amine Synergy • Bioinspired •
Adhesives • AFM Force Spectroscopy
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Entry for the Table of Contents
We report design and synthesis of a new generation of amino-catechol adhesives by copolymerization of traditional monomers with
novel mussel-inspired monomers. Our multi-scale adhesion characterization results indicated that coexistence of catechol and amine
moieties in a hybrid monomer architecture can lead to optimization of overall performance of the catechol-amine adhesives and
developing PSAs with remarkable resistance to flow under shear.
1
0
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