Synthesis of thermally degradable epoxy adhesives

Article abstract of DOI:10.1002/pola.26926

We have developed a new strategy for the synthesis of epoxide-containing polymers where the pendant reactive groups are connected to the main backbone via thermally labile oxonorbornene groups. The polymers were synthesized by radical 1,4-polymerization of the appropriate bicyclic diene monomer. The produced polymers can be crosslinked in the presence of a diamine and de-crosslinked by thermal treatment at 160 °C, which induces retro-Diels-Alder reaction and cleaves pendant groups from the polymer backbone, as confirmed by differential scanning calorimetry. The potential for the utilization of this polymer as a thermally removable adhesive was demonstrated by a simple adhesion test. This method provides access to thermally cleavable epoxy networks that can be quickly and irreversibly disintegrated into nonvolatile components upon heating to a specified temperature. Copyright

Full text of DOI:10.1002/pola.26926

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Synthesis of Thermally Degradable Epoxy Adhesives
Kai Luo,1 Tao Xie,2 Javid Rzayev1
1Department of Chemistry, University at Buffalo, The State University of New York, Buffalo, New York 14260
2State Key Laboratory of Chemical Engineering, Department of Chemical and Biochemical Engineering, Zhejiang University,
38 Zhe Da Road, Hangzhou, Zhejiang 310027, People’s Republic of China
Correspondence to: J. Rzayev (E-mail: jrzayev@buffalo.edu)
Received 24 July 2013; accepted 17 August 2013; published online 18 September 2013
DOI: 10.1002/pola.26926
ABSTRACT: We have developed a new strategy for the synthesis
of epoxide-containing polymers where the pendant reactive
groups are connected to the main backbone via thermally
labile oxonorbornene groups. The polymers were synthesized
by radical 1,4-polymerization of the appropriate bicyclic diene
monomer. The produced polymers can be crosslinked in the
presence of a diamine and de-crosslinked by thermal treatment
at 160 ꢀC, which induces retro-Diels–Alder reaction and cleaves
pendant groups from the polymer backbone, as confirmed by
differential scanning calorimetry. The potential for the utiliza-
tion of this polymer as a thermally removable adhesive was
demonstrated by a simple adhesion test. This method provides
access to thermally cleavable epoxy networks that can be
quickly and irreversibly disintegrated into nonvolatile compo-
C
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nents upon heating to a specified temperature.
2013 Wiley
Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51,
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KEYWORDS: adhesives; degradation; Diels–Alder polymers; radi-
cal polymerization; thermosets
a promising approach to circumvent this problem. Such link-
ages allow for the degradation of the crosslinked polymer
network and removal of the adhesive once it has served its
function. Continuous efforts to design such materials have
resulted in epoxy formulations containing carbamate,3 car-
bonate,4 acetal,5 disulfide,6–8 and ester9 linkages. The break-
down of thermally labile linkages usually requires high
INTRODUCTION Epoxy-based materials possess a combina-
tion of many desirable characteristics, such as excellent
chemical resistance, electrical insulating properties, superb
adhesion and heat resistance, as well as good mechanical
properties.1,2 As a result, they have found widespread use in
a range of applications, including paints and coatings, electri-
cal insulators, fiber-reinforced composites, and structural
adhesives. Formulations typically consist of monomers or a
short chain prepolymer “resins” containing epoxide groups
and a polyamine “hardener.” Chemical reaction between
amines and epoxide groups leads to the formation of a cova-
lently crosslinked network. The process of curing can be
controlled by temperature, choice of resin and hardener
compounds, reaction stoichiometry, and the addition of a cat-
alyst. The range of commercially available structural and
compositional variants allow for the preparation of cured
polymers with controlled mechanical properties, thermal
conductivity, and adhesive properties. In general, epoxy
adhesives are able to maintain their desirable properties at
temperatures up to 300 ꢀC depending on the chemical
nature of the epoxy and polyamine components.
3,4,9
ꢀ
temperatures (>200 C),
which limits their applications.
Retro-Diels–Alder (retro-DA) reaction is a cycloreversion pro-
cess, which has also been used for the disintegration of
epoxy resins10–12 and other crosslinked networks.13–17
A
ubiquitous example of such linkages is based on a reversible
maleimide furan coupling18–20 (Scheme 1). For example, Liu
et al. reported self-healing polyamide networks using the
reversibility of Diels–Alder reaction between polymers bear-
ing maleimide side groups and a furan-containing crosslink-
ing agent.15 The observed weight loss at 150 ꢀC was
attributed to the degradation of crosslinked polymers and
volatility of the crosslinking agent. Gandini et al. developed a
silicone polymer bearing furan side groups, which can also
be crosslinked utilizing Diels–Alder reaction with maleimide
at room temperature.21 The de-crosslinking occurred at 80
ꢀC and the silicone soft segments were preserved during the
crosslinking and de-crosslinking processes.
Upon curing, epoxy resins form insoluble and infusible cross-
linked networks, which are very difficult or impossible to
thoroughly remove postapplication without damaging the
underlying substrate. Incorporating thermally or chemically
degradable linkages into the epoxy resin structure has been
Thermal reversibility of the linkages is certainly appealing
for the preparation of recyclable and self-healing
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their applications in some challenging environments, such as
aerospace nanocomposites and optical component mounting.
EXPERIMENTAL
Materials
(3-Glycidoxypropyl)tetramethyldisiloxane was purchased
from Gelest, Jeffamine D-400 from Huntsman, while all other
chemicals were obtained from Sigma-Aldrich or Acros
Organic and used without further purification unless stated
otherwise. Dichloromethane was purified by a commercial
solvent purification system (Innovative). Azobis(isobutyroni-
trile) (AIBN) was purified by recrystallization from methanol.
cis-2,3-Bis(4-methylbenzensulfonate)bicyclo-[2.2.1]hept-5-ene-
2,3-dimethanol was synthesized according to literature
procedures.23
SCHEME 1 Reversible and irreversible retro-Diels–Alder
chemistry.
networks,14,16 but it may not be suitable for applications
requiring temporary adhesion and removal of the crosslinked
materials. The reversible character often results from a
Diels–Alder reaction between maleimide and furan groups at
low temperatures, and the activation of cycloreversion reac-
tion at high temperatures. The former process is driven by a
high reactivity of maleimide as a dienophile, while the latter
process is due to a weak aromatic character of furan. By
appropriately designing the chemical structure of the Diels–
Alder adduct to contain fragments of furan and a poor dieno-
phile (nonactivated olefin), one can produce irreversible
cleavage and degradation of polymer networks (Scheme 1).
Monomer Synthesis
Karstedt’s catalyst (0.381 g, 1 mmol) was added to cis-2,3-
bis(4-methylbenzensulfonate)bicyclo-[2.2.1]hept-5-ene-2,3-
dimethanol (5.15 g, 10 mmol) and (3-glycidoxypropyl)tetra-
methyldisiloxane (1.94 g, 10 mmol) in 20 mL dichloro-
methane. The mixture was stirred for
3 days and
dichloromethane was evaporated. The obtained oil was used
without purification and dissolved in 20 mL DMSO. Potas-
sium tert-butoxide (4.48 g, 40 mmol) was added and stirred
for overnight. Water was added to the mixture and the prod-
uct was extracted with pentane. After drying with Na2SO4,
pentane was evaporated and the residue was purified by col-
umn chromatography using hexanes/ethyl acetate (20:3) as
a mobile phase. The product (1) was obtained as a colorless
liquid (3.2 g, 86% yield).
We have previously reported that polymerization of 2,3-
dimethylene-7-oxanorbornane produced polymers containing
oxanorbornene fragments in the backbone.22 Upon heating
to 150 ꢀC, the polymer undergoes cycloreversion to furan
containing polymer and ethylene gas. In this contribution,
we take advantage of this structural motif for the prepara-
tion of thermally removable epoxy adhesives (Scheme 2).
The main component in this adhesive is a polyepoxy com-
pound where epoxide groups are linked to the polymer back-
bone via thermally labile groups. Thermal treatment of the
crosslinked network induces retro-DA reaction and cleaves
the linkages, which do not reform when the sample is cooled
down to room temperature. One of the advantages of using
this method is that there are no volatile side products pro-
duced during cleavage, which makes the process more eco-
friendly and practically useful. Additionally, by incorporating
silicon-based linkages into these materials, we have taken
advantage of excellent low temperature and optical proper-
ties of silicon materials, which extends the range of physical
properties of conventional epoxy resin systems enabling
Rf 5 0.43. 1H NMR (CDCl3): d 5.23(s, 1H), 5.09 (s, 1H), 4.97
(s, 1H), 4.84 (s, 2H), 4.75 (s, 1H), 3.74 (m, 1H), 3.40–3.50
(m, 3H), 3.18 (m, 1H), 2.82 (m, 1H), 2.64 (m, 1H), 1.76 (m,
2H), 1.62 (m, 2H), 1.06 (m, 1H), 0.56 (m, 2H), 0.02–0.21 (m,
12H). 13C NMR (CDCl3): d 150.54, 148.57, 100.93, 98.05,
81.51, 81.38, 74.29, 71.43, 50.87, 44.34, 31.34, 31.38, 30.86,
23.43, 14.15, 1.23, 1.01, 0.25.
Synthesis of Prepolymer 3
A mixture of AIBN (10 mg, 0.061 mmol), n-butanthiol (40
mg, 0.444 mmol), and 1 (300 mg, 0.815 mmol) was heated
to 65 ꢀC for 72 h. GPC (light scattering): Mn 5 1.6 kg/mol,
Mw/Mn 5 1.60.
SCHEME 2 Synthesis and thermal decomposition of the new epoxy adhesive. [Color figure can be viewed in the online issue,
which is available at wileyonlinelibrary.com.]
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Monomer Synthesis
Based on the concept shown in Scheme 3, we targeted the
synthesis of a monomer containing a polymerizable diene
group and epoxide pendant units linked together via the oxa-
norbornane skeleton. The monomer was synthesized in a
sequence of five steps, as shown in Scheme 4. Diels–Alder
reaction between furan and maleic anhydride produced the
bicylic skeleton in high yield. The anhydride group was then
converted to a diol by LiAlH4 mediated reduction, and subse-
quently esterified by reacting with p-toluenesulfonyl chloride
SCHEME 3 Direct incorporation of a thermally labile group
into the polymer backbone. [Color figure can be viewed in the
online issue, which is available at wileyonlinelibrary.com.]
Specimen Preparation and Adhesion Test
to
produce
cis-2,3-bis(4-methylbenzensulfonate)bicyclo-
Before application of adhesive materials, the adhered surfa-
ces were prepared by cleaning the aluminum plate surfaces
with methyl ethyl ketone followed by abrading with silicon
carbide paper (76-mm grain size). After surface treatment,
aluminum pieces (100 3 25 3 1.4 mm) were assembled
into single lap shear joints with 12.5 mm of overlap length.
The applied contact pressure was maintained by a 2 lb plate,
which allowed obtaining probes with the same uniform
adhesive thickness.
[2.2.1]hept-5-ene-2,3-dimethanol. Hydrosilation of the double
bond was then carried out to introduce epoxy groups using
a commercially available silane and Karstedt catalyst in high
yield. The diene monomer 1 was obtained by elimination of
p-toluenesulfonic acid in the presence of potassium t-butox-
ide. It should be noted that the obtained monomer was a
mixture of endo- and exo- isomers.
Prepolymer Synthesis
Measurements
Monomer 1 can be polymerized under free radical polymer-
ization conditions, and high molecular weight (ꢁ30 kg/mol)
polymers can be obtained by using AIBN as the initiator. But
for the application as an adhesive, short oligomers are pre-
ferred due to their moderate viscosities and relatively high
reactivities. Thus, we conducted prepolymer synthesis by
radical polymerization of monomer 1 in the presence of n-
butanethiol as a radical chain transfer agent (Scheme 5). The
obtained oligomer 2 had Mn 5 1600 g/mol (GPC with poly-
styrene standards), corresponding to an average of 4.5 epox-
ide groups per chain. From 1H NMR analysis, comparing
peak areas under signals “n” and “t” in Figure 1 the average
number of epoxide groups per chain can be estimated to be
3.5. The dispersity index of 1.6 obtained from GPC suggests
that there is a relatively broad distribution of chain lengths,
as expected. However, for the purposes of this work, any
polymer chain with at least two epoxide groups will
GPC analysis was carried out on Viscotek’s GPCMax with two
PolyPore columns (Polymer Laboratories, Varian) and
TDA302 Tetradetector Array system, which contained refrac-
tive index, UV, viscosity, low (7ꢀ), and right angle light scat-
tering detector modules. Tetrahydrofuran was used as a
mobile phase at 30 ꢀC. The system was calibrated with 10
polystyrene standards from 1.2 3 106 to 500 g/mol. NMR
measurements were performed on a Varian Inova-500 (500
MHz) spectrometer by using CDCl3 as a solvent. Thermal
gravimetric analysis was performed on a Perkin Elmer TGA7
under nitrogen flow at 10 ꢀC/min heating rate. Differential
scanning calorimetry (DSC) was conducted on a TA Instru-
ments Q200 system with an RCS-90 cooling device.
RESULTS AND DISCUSSION
Synthetic Strategy
The synthetic strategy is based on a unique prepolymer
where reactive epoxide groups are linked to the backbone
via thermally labile oxanorbornene linkages. Owing to the
requirement of having a nonactivated olefin fragment as a
part of the oxanorbornene unit, such structures cannot be
prepared by a direct Diels–Alder reaction. Therefore, we uti-
lized radical 1,4-addition polymerization of
a bicyclic
diene22,24 to install oxanorbornene units in the polymer
backbone in situ (Scheme 3). The polymerization was con-
ducted in the presence of a chain transfer agent to produce
low molecular weight oligomers to be used as an epoxy pre-
polymer with a low glass transition temperature (Tg 5 240
ꢀC). Crosslinking can be achieved by mixing the prepolymer
ꢀ
with a polyamine at 80 C. The formed network can be disin-
tegrated by heating to 160 ꢀC to initiate the retro Diels–
Alder process. The network degradation process does not
release volatile compounds, and the products of degradation
are not reactive enough at room temperature to reform the
network, thus allowing convenient dismounting and removal
of the adhesive.
SCHEME 4 Synthesis of the epoxy-containing bicyclic diene
monomer.
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SCHEME 5 Covalent attachment and thermally activated detachment of substituents.
contribute to crosslinking. As we reported previously, the
polymerization of similar bicyclic dienes proceeds predomi-
nantly through 1,4-addition, which is necessary in this case
for the formation of oxanorbornene linkages.22,24 The com-
parison of 1H NMR spectra of the monomer and the pro-
duced oligomer [Fig. 1(a,b)] confirmed the disappearance of
olefinic proton signals at 4.8–5.3 ppm. The remaining
resonances in the 4.6–5.0 ppm region of the oligomer spec-
trum can be attributed to the bridgehead protons (compli-
cated by contributions from 1,4- and 4,1-additions) and to
the remaining olefinic protons as a result of 1,2-addition
(ꢁ20%).
Polymer Network
We next explored the formation of thermally cleavable net-
works by using prepolymer 2 as an epoxy “resin” and a pol-
ydiamine (Jeffamine D-400, Tg < 275 C) as a “hardener.” As
there are multiple reactive epoxide groups on each prepoly-
mer chain and potentially four reactive sites on Jeffamine D-
ꢀ
Prepolymer Reactivity
The reactivity of the prepared oligomers was first evaluated
by utilizing n-hexylamine as a model compound (Scheme 5).
Even though each primary amine can potentially react with
two epoxide groups, the second reaction is much slower
than the first. Therefore, by using equimolar amounts of the
amine and the epoxide groups in the polymer, we targeted
the preparation of soluble oligomers suitable for NMR analy-
sis (Fig. 1). First, neat n-hexylamine was allowed to react
ꢀ
with oligomer 2 at 80 C, which resulted in the quantitative
ring opening of the epoxide groups. After the reaction, char-
acteristic epoxide proton signals at 3.15, 2.85, and 2.65 ppm
completely disappeared from the 1H NMR spectrum [Fig.
1(c)], while new resonances were observed at 3.50 and 2.60
ppm, corresponding to the methylene and methine protons
of the ring-opened epoxide groups, and at 1.40 and 1.00
ppm, corresponding to the n-hexyl protons of the amine. The
regiochemistry of the amine addition could not be distin-
guished from the NMR spectrum. Subsequently, the amine-
functionalized polymer was thermally treated at 160 ꢀC to
induce retro-Diels–Alder reaction of the oxanorbornene
1
groups in the polymer backbone. In the H NMR spectrum of
the thermally treated polymer [Fig. 1(d)], we observed new
vinyl group signals at 5.70–6.20 ppm and furan proton
resonances at 7.15 ppm, formed as a result of cycloreversion.
These studies have confirmed that the prepolymer 2 can be
used for attachment of substituents via the epoxide group
and thermal detachment of the substituents via retro-Diels–
Alder reaction.
FIGURE 1 1H NMR spectra of (A) monomer 1, (B) prepolymer
2, (C) amine-functionalized prepolymer 3, and (D) thermally
treated prepolymer 4.
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400, the reaction between the two should produce a cross-
linked network. Both components have low glass transition
temperatures and therefore flow easily at ambienꢀt conditions.
The reaction between the two components at 80 C overnight
produces an insoluble material. Upon heating to 160 ꢀC, the
network disintegrates to produce mostly soluble by-products.
Thermogravimetric analysis of the crosslinked network con-
firmed that there were no volatile by-products forming at 160
ꢀC, as evidenced by no weight loss observed at temperatures
as high as 200 ꢀC. The degradation of the polymer network
could also be observed by DSC (Fig. 2). In the first heating
scan, the polymer network exhibited a glass transition tem-
perature of 240 ꢀC followed up by an endothermic peak at 160
ꢀC. The latter was attributed to the retro-DA reaction, which
proceeds with the release of ring strain and the formation of
aromatic furan moieties. The sample was then cooled down
and reheated to evaluate the reversibility of the DA reaction. In
the second heating scan, glass transition temperature observed
FIGURE 3 Adhesion demonstration: pieces held together by a
cured mixture of oligomer 2 and diamine (left, middle), and
adhesion failure after thermal treatment at 160 ꢀC for 5 s
(right). [Color figure can be viewed in the online issue, which
is available at wileyonlinelibrary.com.]
ꢀ
for the thermally cleaved polymer decreased by about 10 C,
consistent with increased segmental mobility in the cleaved
sample. Concurrently, there was no evidence of other thermal
transitions, indicating that the cycloreversion process was com-
plete and irreversible. Importantly, since there was no weight
loss observed by TGA upon heating to 200 ꢀC, we conclude that
the network disintegrates into smaller nonvolatile oligomeric
fragments. As a control experiment, we analyzed a commercial
epoxy glue without any degradable linkages. In this case, DSC
almost no difference in terms of adhesion before and after ther-
mal treatment. The pieces cannot be easily separated and the
surface cannot be cleaned without scratching. Further experi-
ments evaluating adhesion strength and rheological properties
of these degradable polymeric networks are underway.
ꢀ
only showed a glass transition temperature of 90 C and the
CONCLUSIONS
sample was thermally stable up to 200 ꢀC. Theseꢀresults further
indicate that the endothermic transition at 160 C observed in
our sample is a result of a retro-DA reaction that leads to net-
work degradation.
A new bicyclic diene with an epoxide group substituent has
been synthesized and polymerized by free radical polymer-
izations with n-butylthiol as a radical transfer agent. Pendant
epoxide groups can be used for the attachment of functional
groups to the polymer, while bicyclic groups in the backbone
allow thermal detachment of substituents. The obtained
oligomers were crosslinked with a diamine to produce poly-
meric networks where chains are connected through ther-
mally labile oxonorbornene linkages. Heating the sample to a
specified temperature promotes retro-DA reaction with con-
current network degradation. The thermally labile linkages
were specifically designed to irreversibly disintegrate at tem-
The formation and degradation of the polymer network was
further evaluated in a form of adhesive holding two aluminum
plates together. The plates were treated with a mixture of
oligomer 2 and Jeffamine D-400, and held together at 80 ꢀC to
promote adhesion. Upon curing, the adhesion between the two
plates was strong enough to support up to 2 kg weight (Fig. 3).
ꢀ
After thermal treatment at 160 C for 5 s, the two pieces were
easily separated without any force and the surface can be easily
cleaned with acetone. For comparison, we tried the same
experiment using commercial epoxy glue. In this case, there is
ꢀ
peratures below 200 C, where no formation of other volatile
products is observed. Preliminary testing showed that such
materials can be used as thermally removable adhesives. We
are exploring applications of such adhesives in temporary
component mounting and protective coatings.
ACKNOWLEDGMENTS
The authors thank the University at Buffalo and the Donors of
the American Chemical Society Petroleum Research Fund for
support of this research.
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