Ambivalent adhesives: Combining biomimetic cross-linking with antiadhesive Oligo(ethylene glycol)

Article abstract of DOI:10.1021/ma300962d

Oligo(ethylene glycol) (OEG) and poly(ethylene glycol) (PEG) exhibit several desirable properties including biocompatibility and resistance to fouling by protein adsorption. Still needed are surgical glues and orthopedic cements, among several other mater

Full text of DOI:10.1021/ma300962d

Article
pubs.acs.org/Macromolecules
Ambivalent Adhesives: Combining Biomimetic Cross-Linking with
Antiadhesive Oligo(ethylene glycol)
Cristina R. Matos-Perez† and Jonathan J. Wilker*,†,‡
́
†Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, Indiana 47907, United States
‡School of Materials Engineering, Purdue University, Neil Armstrong Hall of Engineering, 701 West Stadium Avenue, West Lafayette,
Indiana 47907, United States
S
* Supporting Information
ABSTRACT: Oligo(ethylene glycol) (OEG) and poly-
(ethylene glycol) (PEG) exhibit several desirable properties
including biocompatibility and resistance to fouling by protein
adsorption. Still needed are surgical glues and orthopedic
cements, among several other materials, that display similar
traits. However, the very lack of interactions with other
molecules that prevents toxicity and fouling also makes
adhesion elusive. In work described here the cross-linking
chemistry of marine mussel adhesive is combined with OEG to
make a family of terpolymers. The effect of polymer
composition upon bulk adhesion was examined. High strength
bonding was found with a subset of the polymers containing
appreciable OEG content. These structure−property insights may help the design of new materials for which the properties of
OEG and high strength adhesion are both being sought.
An intriguing approach to materials design is to consider how
marine mussels affix themselves to rocks.46−48 These shellfish
generate a protein-based glue in which 3,4-dihydroxyphenyla-
lanine (DOPA) provides access to cross-linking adhesive
chemistry.46−48 A collection of OEG−DOPA copolymer
systems49 have been developed for surface coatings7,18,20,21
and hydrogels,13,23−26,40 but less bulk adhesion has been
reported (i.e., bonding two substrates together).4,15,29,36,41−44
For copolymers of OEG with DOPA or a pendant catechol
(i.e., 1,2-dihydroxybenzene),49 high strength adhesion has
remained elusive. Common commercial materials such as
white glue and “super” glue can bond with strengths in the
range of 4−11 MPa (see data below).50 So far, experiments
with OEG−DOPA copolymers have shown bulk adhesion only
∼1/10th that of the established products.4,15,29,36,41−44
The exact mechanism of adhesion for DOPA-containing
proteins and catechol-containing polymers has yet to be
described precisely. Both adhesive contacts to the surface and
cohesive forces within the bulk must be considered. Oxidative
cross-linking46,51,52 and metal chelation47,48,52,53 have been
found within the bulk materials. With regard to surface
adhesion, evidence for hydrogen bonding,54 metal chela-
tion,47,48,52,55,56 and radical−surface coupling47,48,52,53 has all
been reported. Indeed, a combination of each process may be at
INTRODUCTION
■
Poly(ethylene glycol) (PEG) and oligo(ethylene glycol)
(OEG) are among the most widely studied synthetic polymers
owing to several intriguing properties including hydrophilicity,1
chain flexibility,1 resistance to protein adsorption,2 and
biocompatibility.3 These characteristics lie at the heart of why
OEG is seen so often in a biomedical context as well as several
other fields.1,3 Cell growth scaffolds,4−6 drug delivery matrices,1
nonfouling surfaces,2,7−10 nonionic surfactants,11 and solid
polymer electrolytes12 are all being developed with OEG. Still
very much in demand are plastics able to deter biomedical (e.g.,
bacteria) or marine (e.g., barnacles) fouling, water compatible
adhesives, surgical glues, and bone cements.2−7,13−17
At first glance, OEG may appear ideal for moving research in
such directions. However, the very traits that allow one
application can prevent another. For example, biocompatibility
and the lack of protein adsorption of OEG are a result of the
polymer not interacting with much more than solvent.1
Developing a bulk adhesive from OEG thus brings about the
seemingly opposed properties of strong chemical bonding to
surfaces and polymer chains versus a lack of interactions with
those same surroundings. Generally speaking, OEG is
considered to prevent adhesion,18 although coatings and
adhesives containing OEG have been reported.2,4,6−8,15−44
The simplest approach to adhering OEG is blending the
polymer with a common glue such as an epoxy.39 Significant
adhesion can result, although potential problems include phase
separation of polymers and domination of the physical
properties by the host adhesive matrix.45
Received: May 13, 2012
Revised: July 17, 2012
Published: July 31, 2012
© 2012 American Chemical Society
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on a Polymer Laboratories PL-GPC 20 using THF for the eluent.
Styrene was purified prior to use with an alumina column to remove
the polymerization inhibitor that is included when purchased.
Synthesis of 3,4-Bis[tert-butyldimethylsilyl)oxy]-
benzaldehyde. Modification of literature methods was used to
prepare this compound as well as the following vinyl derivative.58,61,62
tert-Butyldimethylsilyl chloride (TBDMSCl, 30.4 g, 202 mmol) was
dissolved in acetonitrile (200 mL). To this solution, 3,4-dihydrox-
ybenzaldehyde (10.3 g, 74 mmol) was added, and the mixture cooled
to 0 °C for at least 10 min. Addition of 30.0 mL (198 mmol) of 1,8-
diazabicyclo[5.5.0]undec-7-ene was followed by stirring for 48 h at 4
°C. Solvent was removed to obtain the crude product. Purification was
accomplished by silica gel column chromatography (6.3 cm × 45.7
cm) using dichloromethane (DCM) for the mobile phase. The final
product was obtained as a solid 15.2 g (41% yield). 1H NMR (CDCl3,
Figure S1): δ 0.31 (d, J = 6.2 Hz, 6H, SiCH3)2), 1.07 (d, J = 1.0 Hz,
18H, SiC(CH3)3), 7.02 (d, J = 8.7 Hz, 1H, aromatic), 7.39−7.50 (m,
2H, aromatic), 9.88 (s, 1H, CHO).
play, thereby giving rise to the unique bonding properties of
mussel adhesive and synthetic mimics thereof.
The contradictory nature of OEG−DOPA copolymers
with DOPA sticking and OEG preventing most interactions
makes for an interesting target of investigation. We embarked
on a structure−function study in order to understand the
interplay between these two intriguing components when
contained within the same molecule. The ideal system will
allow the amounts of DOPA (or catechol) and OEG to be
varied systematically within a single copolymer chain. If the
DOPA content is held constant but the OEG content is varied,
what changes to bulk adhesive bonding may result? To the best
of our knowledge, there are no systematic studies in which the
direct influence of OEG upon copolymer adhesion has been
examined. These efforts are not necessarily aimed at making the
strongest glue just yet. Rather, we are seeking methodical
insights and fundamental knowledge that will aid the design of
future materials.
Synthesis of 3,4-Bis[tert-butyldimethylsilyl)oxy]-
styrene.58,61,62 Methyltriphenylphosphonium bromide (44.4 g, 12
mmol) was dissolved in 176 mL of THF followed by dropwise
addition of n-butyllithium (45.0 mL, 112 mmol). The solution was
stirred for 4 h at room temperature to allow formation of the ylide.
Slow addition of 3,4-bis[tert-butyldimethylsilyl)oxy]benzaldehyde
(15.2 g, 41.5 mmol) over 10 min was completed and the reaction
stirred overnight at 40 °C. Solvent was removed under vacuum, and
the resulting solid dissolved in DCM. An extraction with water was
carried out. The organic phase was collected and dried over MgSO4.
Silica gel column chromatography (6.3 cm × 45.7 cm) with DCM
To obtain the desired structure−property insights, a
framework was sought to have a flexible composition. Scheme
1 shows a copolymer design in which catechol-containing
Scheme 1. Design of Polymers Containing Both Adhesive
Promoting Groups as Well as OEG Chains That Typically
Deter Bonding
1
eluent provided the product, 9.6 g (64% yield). H NMR (CDCl3,
Figure S2): δ 0.33 (d, J = 2.3 Hz, 6H, SiCH3)2), 1.11 (d, J = 3.8 Hz,
18H, SiC(CH3)3), 5.22 (d, J = 10.8 Hz, 1H, alkene), 5.67 (d, J = 17.5
Hz, 1H, alkene), 6.70 (dd, 1H, J = 17.5 Hz, alkene), 6.85−7.07 (m,
2H, aromatic), 7.44 (s, 1H, aromatic).
Synthesis of 4-Vinylbenzyl[tetra(ethylene glycol)]. This
compound was prepared according to a literature procedure.63
Tetra(ethylene glycol) (200.0 mL, 1158 mmol) was added to a 1 L
flask that contained (2.1 g, 12.6 mmol) of 4-tert-butylcatechol and
NaOH (16.2 g, 405 mmol). The reaction mixture was heated to 110
°C, followed by dropwise addition of 4-vinylbenzyl chloride (20.0 mL,
138.7 mmol) over 5 min. The solution mixture was stirred at 110 °C
for 2 h and allowed to cool to room temperature. This mixture was
then combined with 250 mL of toluene, and an aqueous−organic
extraction was performed. The organic phase was dried over MgSO4
and solvent removed under vacuum. The yellow oil was purified by
silica gel column chromatography (6.3 cm × 45.7 cm) using 2%
methanol in toluene. The final product was a yellow oil, 7.1 g (21%
yield). 1H NMR (CDCl3, Figure S3): δ 3.57−3.70 (m, 16H,
OCH2CH2O), 4.55 (s, 2H, ArCH2O), 5.23 (d, J = 11.9 Hz, 1H,
alkene), 5.74 (d, J = 16.5 Hz, 1H, alkene), 6.71 (dd, J = 10.9 Hz, J =
17.4 Hz, 1H, alkene), 7.29 (d, J = 6.9 Hz, 2H, aromatic), 7.42 (d, J =
6.9 Hz, 2H, aromatic).
monomers provide cross-linking adhesive chemistry while, at
the same time, OEG-containing monomers are likely to prevent
this adhesion. A catechol-containing 3,4-dihydroxystyrene
monomer represents the simplest possible mimic for the
DOPA chemistry of mussel adhesive proteins (Scheme 2).57−60
Polystyrene proved to be a logical polymer host given the
structural similarities between 3,4-dihydroxystyrene and styr-
ene.57−60 Furthermore, polystyrene provides a clean baseline
for these studies, with no appreciable adhesion to start with.57,59
According to results shown below, a styryl “filler” monomer
helps to ameliorate the conflict between adhesion and
antifouling. A further benefit of this copolymer design is
potential distribution of catechol chemistry throughout the
polymer chains, much like the random distribution of DOPA
residues in mussel adhesive proteins.46 A family of polymers
was prepared in which the catechol content remained
unchanged while the amount of OEG was varied. The bonding
capability of each polymer was quantified. This systematic
approach revealed compositions at which both high strength
adhesion and significant OEG content could coexist.
Synthesis of 4-Vinylbenzyl[methyltetra(ethylene glycol)]. A
literature procedure was followed to obtain this compound.63 4-
Vinylbenzyl[tetra(ethylene glycol)] (7.3 g, 28.9 mmol) was dissolved
in 600 mL of benzene, and sodium hydride in mineral oil was added
(6.62 g, 165.5 mmol). After 2 h, iodomethane (23.0 mL, 369.5 mmol)
was added dropwise over 15 min, and the solution was stirred for 4
days at room temperature. Solvent was removed, and the solid
dissolved in DCM. An extraction was performed with a saturated
solution of sodium bicarbonate, and the organic phase was dried over
CaCl2 followed by removal of solvent under vacuum. A silica gel
column (6.3 cm × 45.7 cm) and 2% MeOH in DCM were used to
1
purify the final product, 4.2 g (59% yield). H NMR (CDCl3, Figure
S4): δ 3.38 (s, 3H, OCH3), 3.54−3.69 (m, 16H, OCH2CH2O), 4.56
(s, 2H, ArCH2O), 5.20 (d, J = 10.9 Hz, 1H, alkene), 5.75 (d, J = 17.6
Hz, 1H, alkene), 6.71 (dd, J = 10.9 Hz, J = 17.4 Hz, 1H, alkene), 7.32
(d, J = 6.9 Hz, 2H, aromatic), 7.43 (d, J = 6.9 Hz, 2H, aromatic).
Synthesis of Poly{(3,4-di(OTBDMS)styrene)-co-{4-
vinylbenzyl[methyltetra(ethylene glycol)]}-co-styrene}. In
EXPERIMENTAL SECTION
■
General Procedures. Nuclear magnetic resonance (NMR)
spectroscopy was performed on a Varian INOVA 300 MHz
spectrometer in CDCl3. Full spectra are provided in the Supporting
Information. Gel permeation chromatography (GPC) was carried out
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Scheme 2. (A) Synthesis of the Protected Adhesive Monomer; (B) Preparation of the OEG-Containing Monomer; (C)
Polymerization and Deprotection To Yield the Poly{(3,4-dihydroxysdtyrene)-co-{4-vinylbenzyl[methyltetra(ethylene glycol)]}-
co-styrene} Terpolymers
order to obtain the desired polymers, radical polymerizations (AIBN)
were performed. Polymerizations were typically carried out in benzene
but for polymers containing higher percentages of OEG (∼30+%),
anhydrous toluene, or freshly distilled THF were used. Polymer-
izations were performed in an air-free flask and sealed for 7 or 10 days
at 80 °C.
A typical polymerization consisted of styrene (0.8560 g, 8.2 mmol),
3,4-di(OTBDMS)styrene (0.3688 g, 1.0 mmol), and 4-vinylbenzyl-
[methyltetra(ethylene glycol)] (0.4687 g, 1.4 mmol) dissolved in 10.0
mL of benzene. The radical initiator AIBN (0.0044 g, 0.03 mmol) was
added, and the solution was stirred for 7 days at 80 °C. Once cooled to
room temperature, polymers were precipitated with MeOH (or
hexanes) at least three times to remove starting materials. Solvent was
removed under vacuum to obtain the polymer, 1.5 g (88.6% yield). 1H
NMR (CDCl3, Figure S5): δ 0−1.2 ppm (broad, TBDMS groups),
1.2−2.3 ppm (broad, polymer backbone), 3.1−4.6 ppm (broad, OEG
groups), and 6.0−7.4 ppm (broad, aromatic). These 1H NMR spectra
were used to determine the monomer ratios in the final polymers
(Table 1). GPC provided molecular weights of the polymers, shown in
Table 2.
Table 1. Monomers in the Starting Feeds Compared to
Composition of the Final Polymers (All Values in %)
feed
final polymer composition
OEG
catechol
styrene
OEG
catechol
styrene
0
2
9
10
8
91
88
84
77
76
50
0
0
2
10
11
9
90
87
82
78
73
52
0
8
9
14
15
38
91
9
13
18
35
88
9
9
9
12
9
13
12
deprotected to expose the pendant catechol groups. In a general
procedure, a protected polymer (1.5 g) was dissolved in 60.0 mL of
dioxane, and 13.0 mL of concentrated HCl were added. The reaction
was stirred overnight at 80 °C and allowed to cool to room
temperature. Solvent was removed under vacuum, and the solid
dissolved in 1:1 acetone/DCM. An extraction was performed with
water and the organic phase dried over MgSO4. Solvent was removed
Synthesis of Poly{(3,4-dihydroxystyrene)-co-{4-vinyl-benzyl-
[methyltetra(ethylene glycol)]}-co-styrene}. The polymers were
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(ethylene glycol)]}-co-styrene}, melting at ∼71−73 °C was
observed. Increasing the OEG content to 9% brought the
melting temperature down to ∼43 °C. At 42% OEG the
copolymer was already an oil at room temperature.
Table 2. Molecular Weights and Polydispersity Data for
Each Poly{[3,4-di(OTBDMS)styrene]-co-{4-
vinylbenzyl[methyltetra(ethylene glycol)]}-co-styrene}
Terpolymer
Adhesion Measurements. With a synthetic route to cross-
linking OEG polymers in hand, an exploration of bonding
properties was then carried out. To see if both high strength
adhesion and high OEG content could coexist, a family of seven
different polymers was prepared. For each case, the catechol
monomer content was maintained at close to 10 mol %,
approximating the DOPA found in mussel adhesive proteins.66
The OEG monomer was placed into the polymer at 0%, 88%,
and several intermediate loadings. Styrene comprised the
reminder.
OEG (%)
catechol (%)
styrene (%)
Mn
Mw
PDI
0
2
10
11
9
90
87
82
78
73
52
0
39 792
16 612
16 927
20 673
19 626
12 627
13 711
49 712
19 255
19 722
25 295
22 119
21 240
19 972
1.2
1.2
1.2
1.2
1.1
1.7
1.5
9
13
18
35
88
9
9
13
12
Adhesion of each polymer was examined by bonding
together two pieces of polished aluminum in a lap shear
configuration (Figure 1).64 Several methods exist for testing
1
under vacuum to provide 1.2 g of product (80% yield). H NMR
(CDCl3, Figure S6): δ 1.2−2.3 ppm (broad, polymer backbone), 3.1−
4.6 ppm (broad, OEG groups), and 6.0−7.4 ppm (broad, aromatic).
Adhesion Studies. Bonding performance was quantified with lap-
shear adhesion.64 Each terpolymer (13.5 mg in 45 μL of 1:1 acetone/
DCM) was added to two polished aluminum adherends. The
adherends were overlapped with an area of 1.2 × 1.2 cm and allowed
to set for 1 h at room temperature. Weights (∼54 g) were applied to
hold the assemblies together. Samples were cured in an incubator for
22 h at 70 °C and then cooled at room temperature for 1 h. The shear
tests were performed using an Instron 5544 materials testing system.
Each measurement was repeated 10 times, averaged, and error bars of
90% confidence intervals are shown.
RESULTS AND DISCUSSION
■
Polymer Synthesis and Characterization. Scheme 2
shows the six-step synthetic route developed to obtain the
target terpolymers. A catechol monomer was realized in two
steps by modifying published methods, starting from an
aldehyde, progressing through silyl protection, and a Wittig
Figure 1. (A) Two aluminum adherends bonded together with 225
mg of a 15% OEG-containing polymer, able to support a 25 pound
(∼11 kg) weight. (B) Representative data from a lap shear adhesion
test in which a sample is pulled apart until failure. The plot rises with
increasing force applied to the bond until a sudden drop, indicating
that the joint has broken, thereby providing the maximum adhesive
force. (C) Photograph of two bonded substrates in the materials
testing system, ready to be pulled apart for measurement of adhesive
strength.
reaction to reveal the vinyl group (Scheme 2A).58,61,62
A
literature approach was used to couple vinylbenzyl chloride and
OEG, followed by CH3I capping, for the styryl OEG monomer
(Scheme 2B).63 This OEG monomer was combined with the
protected catechol monomer, styrene, and a radical initiator to
obtain the protected terpolymer (Scheme 2C). Acid removed
the silyl protecting groups, thereby yielding the final poly{(3,4-
dihydroxystyrene)-co-{4-vinylbenzyl[methyltetra(ethylene gly-
col)]}-co-styrene} polymers.
adhesive bonding including lap shear, tensile, peel, and
burst.50,67 Lap shear is used the more often than other bonding
schemes.50,67 Making direct comparisons between different
adhesion studies can be difficult (see below), but lap shear
bonding of metal substrates will provide what may be the
closest thing to a general condition.
Polymer characterization was accomplished by 1H NMR
spectroscopy and GPC. The ratio of monomers in the final
1
polymer was revealed with H NMR spectroscopy (Table 1).
Generally speaking, the polymeric monomer mole percent was
a reflection of the ratios in the starting feed (Table 1). The
GPC data for all of the terpolymer derivatives showed number-
average molecular weights (Mn’s) within a range of 13 000−40
000 and polydispersity indices (PDI’s) between 1.2 and 1.7,
indicating consistency among the polymers (Table 2). These
low PDI values are likely a result of the long OEG substituents
shielding propagating radicals from chain termination.65
Thermal characterization by methods such as differential
scanning calorimetry (DSC) was complicated by some
polymers appearing to exhibit Tg values below room temper-
ature. With low OEG content the copolymers were solid, but
when the OEG monomer percentage increased, the copolymers
were oils. Consequently, a melting temperature apparatus was
used in an effort to gain some insight on the thermal behavior.
For a 100% polystyrene control, the polymer deformed at ∼140
°C and was still solid up to 230 °C. With 3% OEG in poly-
{(3,4-dihydroxystyrene)-co-{4-vinylbenzyl[methyltetra-
The polymers were dissolved in solvent for application onto
the adherends. The second adherend was overlapped onto the
first, and the assembly was cured. Bonding strengths were
quantified by pulling apart the glued substrates until failure.
This failure force (in newtons) was divided by the measured
adherend overlap area (in m2) and is reported in pascals (Pa =
N/m2). Adhesion data for each polymer are provided in Figure
2.
A 100% polystyrene control adheres only weakly, at ∼0.2−
0.6 MPa.57,59 For another control without OEG, the 10%
catechol, 90% styrene copolymer adhered at 1.3
0.2 MPa.
Comparing the performance of these two polymers indicates
that introduction of the catechol group enhances adhesion.
For the poly{(3,4-dihydroxystyrene)-co-{4-vinylbenzyl-
[methyltetra(ethylene glycol)]}-co-styrene} copolymers, addi-
tion of 2% OEG monomer (and removal of 2% styrene)
displayed comparable attachment at 1.8 0.4 MPa (Figure 2).
The strongest bonding of 2.3 0.5 MPa was found with 13%
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difficult to make. Differences in bonding modes, substrates,
cure conditions, and sample environments prevent putting
various materials on equal footing. The lap shear bonding of a
high energy substrate used in this current study was chosen to
provide the most broadly applicable insights on polymer
adhesive performance. Nonetheless, the results presented do
show that significant bonding and appreciable OEG copolymer
content need not to be mutually exclusive. OEG-containing
copolymers can be turned into high-strength adhesives.
CONCLUSIONS
■
The compelling properties of OEG have driven development of
applications in several fields of study. High strength adhesion,
however, has remained elusive given that OEG prevents
interactions with other molecules. Here a systematic
structure−property study was carried out to provide insights
on how OEG and a DOPA mimic influence each other when
contained within a polymer chain. At the maximum possible
OEG content adhesion was not detectable. Lower loadings of
OEG did display adhesion with no discernible penalty relative
to the starting polymer without any OEG. We hope that these
results will aid the design of new materials such as coatings,
hydrogels, and bulk glues for which both OEG and DOPA can
be so well suited.
Figure 2. Influence of OEG monomer content on adhesive bonding of
poly{(3,4-dihydroxystyrene)-co-{4-vinylbenzyl[methyltetra(ethylene
glycol)]}-co-styrene} terpolymers. The adhesive catechol-containing
monomer was held constant at ∼10% while the amount of OEG-
containing monomer was varied. Styrene provided the remainder.
OEG. Total OEG monomer content could be pushed to 18%
without any major penalty on adhesion (1.6
0.2 MPa).
Higher OEG loadings, however, did inhibit adhesion
significantly (Figure 2). The 35% OEG derivative only stuck
at 0.26 0.04 MPa. At 88% OEG there was not even sufficient
bonding of the substrates to allow handling for measurements.
When examining adhesion as a function of OEG content, we
must also be mindful of effects from varying the styrene
monomer content. Recently, we showed that incorporation of
3,4-dihydroxystyrene into polystyrene to form poly[(3,4-
dihydroxystyrene)-co-styrene] can generate high strength
adhesives, with bonding up to ∼7 MPa.59 Polystyrene exhibits
minimal adhesion of ∼0.2−0.6 MPa when measured under
conditions similar to those used here.57,59 For data presented in
the current study, decreased adhesion with increased OEG
content coincided with less styrene in the polymer. These
results lead us to believe that the styrene monomers are
providing a “filler” for the polymer composition and are not
necessarily bringing about any detriment to adhesion.
ASSOCIATED CONTENT
■
S
* Supporting Information
1H NMR spectra of monomers and polymers. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: wilker@purdue.edu.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We appreciate support for this work provided by the Office of
Naval Research, the National Science Foundation, and a Ruth
L. Kirschstein National Research Service Award from the
National Institutes of Health to C.R.M-P. We thank Mildred M.
Rodriguez and Jeffrey P. Youngblood for helpful discussions,
Kaumba Sakavuyi for processing NMR data, and Allison O.
Mattes for assistance with GPC.
Overall, these data are consistent with the general notion that
OEG typically prevents adhesion. To provide a perspective on
this adhesive performance, three established commercial glues
were measured under similar conditions. White glue (poly(vinyl
acetate), Elmer’s) adhered at 4
1 MPa, Krazy Glue
(cyanoacrylate) at 7 1 MPa, and a two part epoxy (Loctite)
at 11 2 MPa.
REFERENCES
■
Comparisons to Other OEG-Containing Adhesives.
Bulk adhesion data for OEG-containing copolymers have been
reported in some cases. These OEG adhesives include a
hydrogel composed of cross-linked OEG-aldehyde able to bond
at 0.7 MPa,33 a poly(acrylic acid)−OEG sticking at ∼0.15
MPa,16 scaffolds of OEG−DOPA−polycaprolactone with
adhesion of 0.11 MPa,4 four-arm OEG−(DOPA)4-type systems
bonding with strengths of 0.03515 or 0.03036 MPa, and an
eight-arm OEG aldehyde matrix at 0.019 MPa.37 A recent
report with both OEG and dopamine yielded impressive
bonding at 1.2 MPa, although juxtaposition may not be
appropriate here given that each component was in a separate
polymer chain.29
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