Recyclable and Malleable Epoxy Thermoset Bearing Aromatic Imine Bonds

Article abstract of DOI:10.1021/acs.macromol.8b01976

A method for preparing reprocessable epoxy thermoset is presented. The process is composed of synthesis of a bisphenol monomer bridged by an imine bond, glycidylation of the bisphenol, and cross-linking with a hardener to form thermoset. The resulting epo

Full text of DOI:10.1021/acs.macromol.8b01976

Article
pubs.acs.org/Macromolecules
Cite This: Macromolecules XXXX, XXX, XXX−XXX
Recyclable and Malleable Epoxy Thermoset Bearing Aromatic Imine
Bonds
Shou Zhao† and Mahdi M. Abu-Omar*,†,‡
‡
†Department of Chemistry & Biochemistry and Department of Chemical Engineering, University of California, Santa Barbara,
Santa Barbara, California 93106, United States
S
* Supporting Information
ABSTRACT: A method for preparing reprocessable epoxy
thermoset is presented. The process is composed of synthesis of a
bisphenol monomer bridged by an imine bond, glycidylation of the
bisphenol, and cross-linking with a hardener to form thermoset. The
resulting epoxy thermoset possesses comparable properties to
conventional high-performance thermosets made from bisphenol
A. However, when treated by a stimulus like acid, temperature, and/
or water, the described thermoset exhibits reprocessability.
Degradation and recycling involve hydrolysis and re-formation of
imine bonds; reshaping and repairing of the thermoset are realized
through imine exchange reactions. All the described processes
require no metal catalyst, press heating, or additional monomer,
which significantly widens thermoset reprocessing.
(BPA) is the most commonly used thermoset precursor, and it
has long been viewed as a problematic and nonsustainable
molecule.31
INTRODUCTION
■
Epoxy thermosetting plastics are among the most versatile and
widely used materials because of their mechanical strength,
chemical and thermal resistance, and excellent insulation.1,2
However, because of irreversible covalent cross-links, epoxy
thermosets cannot be remolded or reprocessed after their
initial formation. This is in sharp contrast to thermoplastics,
which soften when heated. Recently, the literature has seen a
surge in attempts to make thermosets less inert and more
responsive by introducing dynamic covalent bonds into
thermosetting materials. The formed covalent adaptable
networks (CANs), while still covalently cross-linked, can
undergo reversible depolymerization and stress relaxation
through cross-link cleavage and re-formation/exchange.3−6
From the plethora of possible dynamic covalent motifs
available to the chemist, only a couple of variations have
been employed in epoxy cross-linked CANs. These include
ester,7−12 Diels−Alder (DA),13−19 and disulfide bonds.20−30
For example, Leibler et al. developed a malleable thermoset by
epoxy−carboxylic acid reaction.7 Zhang et al. synthesized an
epoxy−amine cross-linked thermoset embedded with DA
bonds.14 The thermoset could be converted to soluble
polymers with the aid of sonication and repolymerized via
DA reaction. Odriozola et al. used diglycidyl ether of bisphenol
A (DGEBA) to react with a disulfide-containing amine
hardener for making fiber-reinforced polymer composites.22
However, the above examples require at least one of the
following treatments: high temperature, press molding, addi-
tional monomer, special processing (e.g., sonication), and/or
metal catalysts to convert the rigid networks into viscoelastic
liquids for recycling/reprocessing. Furthermore, bisphenol A
CANs are broadly classified into two groups, the dynamic
structure of which is obtained either kinetically by bond
exchange (associative) or through equilibrium shifts leading to
reversible depolymerization (dissociative).4,5 Among recog-
nized dynamic bonds, the imine bond is unique because it can
undergo both associative (imine−amine exchange) and
dissociative (imine hydrolysis and re-formation) reactions.
Previous work on imine-containing CANs seldom integrate
good thermomechanical properties with facile reprocessing.
For example, Fukuda et al. connected an imine-incorporated
diol and a poly(butylene succinate) oligomer based diol
through hexamethylene diisocyanate.32 The prepared polyur-
ethane exhibited facile degradation and self-healing properties
without requiring complicated processing like external
pressure. However, because of its linear structure, the prepared
thermoplastic possessed relatively lower mechanical properties.
Similarly, many other easy-to-reprocess imine-incorporated
polymers were also associated with weak thermomechanical
properties.33−36 By comparison, high-performance imine-
containing thermosets often lack facile reprocessing. For
example, Taynton et al. prepared a highly cross-linked
polyimine thermoset by reacting terephthaldehyde with
diethylenetriamine and tris(2-aminoethyl)amine.37 The result-
ing polymer exhibited good mechanical strength. However,
Received: September 14, 2018
Revised: November 7, 2018
© XXXX American Chemical Society
A
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Figure 1. Synthesis and properties of EN-VAN-AP. (A) Synthetic route of EN-VAN-AP. (B) Tensile test, (C) dynamic mechanical analysis, and
(D) thermogravimetric analysis of EN-VAN-AP and EN-BPA. (E) Solvent resistance of EN-VAN-AP in different solvents at 65 °C for 2 days.
Figure 2. Depolymerization and recycling of EN-VAN-AP through dissociative mechanism. (A) Effects of solvent, HCl concentration, and
temperature on the depolymerization and solubility of EN-VAN-AP. (B) NMR spectra of solid-state (a) original and (c) recycled EN-VAN-AP and
(b) liquid-state depolymerized EN-VAN-AP in acidified DMSO-d6. (C) Dissociative mechanism of imine bond. (D) Tensile and (E) thermal
properties of original and recycled EN-VAN-AP. (F) Recycling steps of EN-VAN-AP. (a) Thermoset was dissolved in DMF with the aid of HCl.
(b) DMF and HCl were removed from the solution, leaving the partially polymerized thermoset as a viscous gel. (c) Polymer gel was heated to
restore the thermoset.
recycling the polymer required external pressure, high
temperature, and additional monomers. Similarly, several
other high-performance imine-containing thermosets also
require press molding or additional monomers when
reprocessing.38−40 By embedding imine bonds into epoxy
cross-linked networks as intermolecular linkages, we herein
report an epoxy thermoset that possesses high thermomechan-
ical properties but can still be easily reprocessed. It can be
degraded, recycled, and repaired/healed on-demand without
requiring complicated processing or additional ingredients
such as a metal catalyst or additional monomer. The described
recyclable thermoset is also based on a bisphenol that employs
vanillin, VAN, a renewable and lignin-derived molecule.41−43
isolated yield). While VAN-AP possesses structure like the
conventional BPA, the attachment of aryl groups to both
nitrogen and carbon atoms of imine bonds is a critical
structural characteristic that leads to the complete reaction of
aromatic aldehyde and amine,33 which improves the efficiency
at the stage of monomer synthesis and polymer recycling. The
structure of VAN-AP was characterized by NMR spectroscopy.
As seen in Figure S1, the proton peak at 8.4 ppm and the
carbon peak at 153 ppm correspond to the imine group,
indicating the coupling of aldehyde and amine. VAN-AP was
then reacted with epichlorohydrin to obtain a glycidyl ether
(GE-VAN-AP), which exhibited new epoxy proton peaks at 2.7
and 2.9 ppm (−CH2− in oxirane), 3.3 ppm (−CH− in
oxirane), and 4.0 and 4.2 ppm (−O−CH2−) in Figure S2. No
aldehyde peak was observed for GE-VAN-AP, suggesting imine
was stable during the glycidylation process. IR spectra were
collected to confirm the structure. The IR spectrum of VAN-
AP exhibited the characteristic absorption band of imine at
1630 cm−1 (Figure S3B). GE-VAN-AP exhibited new oxirane
RESULTS AND DISCUSSION
■
Structural Characterization. The synthetic route of the
epoxy network is shown in Figure 1A. Imine-embedded
bisphenol (VAN-AP) was readily prepared by reacting VAN
with aminophenol (AP) in water at room temperature (95%
B
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band at 912 cm−1, while the OH bond at 3265 cm−1 decreased
significantly, which confirmed the formation of epoxy group
(Figure S3C). When epoxy groups were reacted with amine
hardener to form the epoxy network (EN-VAN-AP), the active
amine protons opened the epoxides while hydroxyls were
created at the same time. This process was reflected by the IR
spectrum of EN-VAN-AP (Figure S4A), which exhibited no
epoxy band at 912 cm−1 (indicating significant conversion of
epoxy group), while the broad hydroxyl band at 3324 cm−1
increased compared to GE-VAN-AP.
Properties of Original Thermoset. EN-VAN-AP was
prepared by reacting GE-VAN-AP with a commercially
available polyfunctional amine (Jeffamine, Mw = 400) at 60
°C for 4 h and then at 120 °C for 20 h. It exhibited tensile
strength and elongation of 46 MPa and 4%, glassy storage
modulus at 30 °C (E30′) of 2004 MPa, and glass transition
temperature (Tg) of 71 °C. These properties were comparable
to BPA-based thermoset (EN-BPA, prepared using the same
cross-linking conditions as EN-VAN-AP) (Figure 1B,C and
Table S1), which was attributed to the structural similarity
between VAN-AP and BPA. By comparison, onset degradation
temperature (temperature at 5 wt % loss) of EN-VAN-AP was
lower than EN-BPA (271 °C vs 350 °C, Figure 1D), which was
attributed to the increased dissociating tendency of imine
bonds at elevated temperature. EN-VAN-AP showed good
resistance to various solvents including benzene, toluene, THF,
ethanol, DMF, DMSO, and water at elevated temperature. EN-
VAN-AP exhibited mainly swelling behavior, with limited
portion dissolved (<15 wt %, Figures 1E and 2A) after being
immersed in the solvents at 65 °C for 2 days. Overall, VAN-
AP-derived epoxy thermoset exhibits properties in line with the
conventional BPA-based counterpart when the same hardener
and curing conditions are employed, which suggests VAN-AP
could be a suitable thermoset precursor for a range of
applications.
Degradation and Recycling via Dissociative Mecha-
nism. By acid-aided hydrolysis of the imine linkages, we then
demonstrated EN-VAN-AP could be transformed into smaller
and soluble oligomers using proper degradation conditions.
EN-VAN-AP was cut into pieces (ca. 4 × 3 × 2 mm3) and
immersed (no stirring was applied) in 1.5 mL of solvent with
various hydrochloric acid concentrations and temperature for 2
days (Figure S5). At room temperature or 65 °C with no HCl
added, EN-VAN-AP exhibited good resistance to solvents as
mentioned above (Figure S5A,B). Concentrated HCl acid was
selected to depolymerize the thermosets. This selection was
based on its strong acidity, compatibility with hydrophilic
solvents, and low boiling point so it could be readily removed
along with solvent during the drying process. At low HCl
concentration (0.17 mol L−1) and 25 °C, a limited portion (<6
wt %) of EN-VAN-AP was dissolved in toluene and benzene
(Figure 2A). This was related to (1) limited compatibility of
HCl solution with toluene and benzene, as reflected by the
uneven corrosion on thermoset surface, and (2) poor
compatibility of decomposed thermoset in these solvents as
indicated by the aggregation of depolymerized residue in
toluene and benzene (Figure S5C). By comparison, THF,
ethanol, DMF, DMSO, and water exhibited homogeneous
solution of the decomposed residue. Especially, 100% of EN-
VAN-AP was dissolved in water at 25 °C with HCl
concentration as low as 0.17 mol L−1, which was significantly
higher than other solvents (Figure 2A and Figure S5C). This
could be attributed to the specific sensitivity of imine bond to
water. Increasing the temperature to 65 °C and maintaining
the same HCl concentration (0.17 mol L−1) accelerated the
dissociation of thermosets, which resulted in 100 wt %
dissolution of EN-VAN-AP in water and DMSO, 79 wt % in
DMF, 40 wt % in ethanol, 14 wt % in THF, and ∼7 wt % in
toluene and benzene after 2 days (Figure 2A and Figure S5D).
This phenomenon indicated that EN-VAN-AP could be readily
degraded when treated by hydrophilic solvents under mild
conditions. The affinity of thermoset to solvent followed the
order water > DMSO > DMF > ethanol > THF > benzene ≈
toluene (Figure 2A), which was consistent with the polarity of
these solvents. Compared to the degradation of conventional
epoxy thermosets that often involves harsh conditions like high
temperature and strong acid/base,44−48 the facile degradation
and solubility of EN-VAN-AP in slightly acidified hydrophilic
solvents, especially water, highlight its environmental impor-
tance.
While the original EN-VAN-AP was formed through epoxy−
amine reaction, formation of the recycled thermoset was
realized via aldehyde−amine reaction. The recycling process
occurred in three steps: (1) dissolving EN-VAN-AP in
acidified solvents, (2) drying the solution to remove most of
the solvent and HCl, and (3) re-cross-linking at elevated
temperature to recover the thermoset. Solvent used for
thermoset degradation was found to be a major factor that
determined the efficiency and properties of the recycled
thermoset. Optimal solvent should have properties, including
(1) high solubility for thermoset, (2) reasonable boiling point
that can be readily removed, and (3) properties of recycled
thermoset should be comparable to original thermoset.
Because of their relatively lower solubility for EN-VAN-AP,
toluene, benzene, THF, and ethanol are not optimal compared
to DMF, DMSO, and water, which exhibited sufficient
solubility under mild conditions. From the aspect of solvent
removal, DMSO might be problematic due to its high boiling
point (189 °C). In a preliminary experiment, it took more than
3 times longer for DMSO to be mostly removed under reduced
pressure from the polymer solution than when DMF or water
was used as solvent. The use of DMSO increased separation
difficulty, while the presence of DMSO residue in recycled
thermoset might lead to reduced cross-link density, decreased
water resistance, and possibly compromised mechanical and
thermal properties. As for water, the thermoset recovered from
water solution exhibited significantly poor water resistance. It
deformed quickly when immersed in water at room temper-
ature, even though no acid was added (Figure S6). This could
be related to the high affinity of HCl to water, which made
HCl difficult to be completely removed from the polymer
matrix. The trapped trace amount of HCl could still lead to
significant deformation of recycled thermoset when exposed to
water. By comparison, DMF exhibited the highest suitability
among studied solvents for thermoset recycling, since it
demonstrated sufficient solubility and reasonable boiling point,
while the properties of recycled thermoset were retained. The
recycling process and properties of recycled thermoset are
illustrated next.
By use of the EN-VAN-AP/DMF weight ratio of 1:10, the
HCl concentration of 0.25 mol L−1, and mild stirring, EN-
VAN-AP could be fully dissolved within 30 min at 65 °C
(Figure 2F, step a). Size exclusion chromatography exhibited
Mn = 1529 of the soluble depolymerized thermoset (Figure
S7), indicating the soluble part was mostly prepolymer small
molecules (e.g., one Jeffamine connected to four monophenols
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with free amine or aldehyde; structure illustration of the
soluble thermoset in Figure 2F). The solution was dried by
two steps: (1) the mixture was heated at 80 °C and dried by an
air flow to get rid of around 60% of volume, and (2) the
mixture was slowly dried under vacuum at room temperature
overnight to remove most of volatile components, leaving the
depolymerized EN-VAN-AP as a viscous gel (Figure 2F, step
b). It should be noted that after the acid-aided depolymeriza-
tion, the DMF solution contained a mixture of oligomer chains
with terminal aldehyde and/or amine groups (structural
illustration in Figure 2C,F). During the drying processes, as
HCl was gradually evaporated, the rate of imine formation
increased, and the molecular weight of polymer chains
continuously increased. At the end of the drying processes,
the formed polymer gel was heated at 120 °C for 24 h to
promote solvent residue evaporation, facilitate the aldehyde−
amine reaction, and recreate the polymer network (Figure 2F,
step c). Depolymerization and re-formation of EN-VAN-AP
were confirmed by NMR spectra of original, depolymerized
and recycled samples in Figure 2B and Figures S8−S10. As the
imine linkages within the original EN-VAN-AP were intact, no
aldehyde peak was observed from the solid-state 13C spectrum
(Figure 2B-a and Figure S8). By comparison, depolymerizing
the thermoset in acidified DMSO-d6 revealed the aldehyde
group at 9.8 ppm for proton and 192 ppm for carbon spectrum
(Figure 2B-b and Figure S10), indicating the cleavage of imine
bond. After re-cross-linking, the aldehyde group of recycled
thermoset disappeared again (Figure 2B-c and Figure S9),
suggesting re-formation of network was realized through
aldehyde−amine reaction. The IR spectrum of recycled
thermoset also revealed no aldehyde group, suggesting the
aldehyde−amine reaction was complete (Figure S4B). More-
over, solid-state NMR and IR spectra of original and recycled
EN-VAN-AP exhibited similar pattern (Figures S4, S8, and
S9), which indicated recycled thermoset might retain similar
chemical structure to the original thermoset. Tensile and
thermal properties of the recycled thermoset exhibited no
major decrease as compared to original sample (Figure 2D,E
and Table S1). The glassy modulus E30′ and Tg of original and
recycled thermosets were also comparable (Figure S11 and
Table S1). The dissociative mechanism (bond cleavage and re-
formation) of imine bond conferred epoxy thermoset with
facile degradation and recycling methods, which required no
press heating or metal catalyst. By comparison, high temper-
ature, press molding, and catalyst are often required to convert
the rigid networks into viscoelastic liquids when recycling the
associative CANs.
Figure 3. Malleability and weldability of EN-VAN-AP through
associative mechanism. (A) Associative mechanism of imine bond.
(B) Normalized stress relaxation at different temperature. (C)
Malleability of EN-VAN-AP. (D) Weldability of EN-VAN-AP. (a)
Two thermoset films were overlapped and welded at 120 °C for 4 h. A
dog-bone-shaped sample was punched out, leaving the overlapped
area in the middle. (b) Side view of the welded sample. (c) Broken
pieces after tensile test of the welded sample. (d) Side view of (c).
The sample was broken at another place rather than the overlapped
area. (E) Tensile properties of original and welded EN-VAN-AP.
relaxation. Even though the thermoset was not soluble in
common solvents, the full stress relaxation at elevated
temperature suggested the typical behavior of a viscoelastic
liquid.49 The Maxwell relaxation time (τ, time needed for the
stress to fall to G(t)/G0 = 1/e) was strongly temperature-
dependent and exhibited Arrhenius-like temperature depend-
ence (Figure S12), which was consistent with a previous
study.7 The activation energy was calculated to be 36.9 kJ
mol−1 (Figure S12), which was much lower than the 80 kJ
mol−1 of the polyester cross-linked thermoset.7 Thus, a fast
stress relaxation at lower temperature could be achieved. The
relaxation time was 251 s at 30 °C and reduced significantly to
11 s when temperature increased to 60 °C. By comparison, the
widely studied polyester cross-linked thermosets required
much higher temperature (>200 °C) to get such fast stress
relaxation.7,10,50 The difference in relaxation time among
thermosets could be affected by cross-link density, the
exchange reaction kinetics, density of exchangeable groups,
and the intrinsic rigidity of monomers.4 The fast stress
relaxation at relatively low temperature in this work could
facilitate the thermoset reprocessing like shape transformation
and welding.
Shape transformation is an evidence supporting the stress
relaxation of thermoset at elevated temperature. An EN-VAN-
AP strip with dimensions of 125 × 12.5 × 2.2 mm3 was used
for illustration (Figure 3C). By heating the strip at 80 °C for 2
min, the rigid thermoset was converted into a viscoelastic state.
At this time, the strip was twisted into a helical fusilli-like
shape. When the thermoset was cooled to room temperature,
the helical fusilli-like shape was retained. Applying another
force and heat could recover the thermoset to its original flat
shape. The rigid permanently cross-linked epoxy thermoset
Malleability and Weldability via Associative Mecha-
nism. By utilizing the associative mechanism of imine bonds
(Figure 3A), we then demonstrated EN-VAN-AP was
malleable. The malleability came from the imine exchange
reactions at elevated temperature, which has been proven by
model compound study and imine-containing thermosets.35,37
According to Zhang et al.,35 imine exchange in network was
primarily catalyzed by residual unreacted primary amine
groups via an associative approach. Considering the high
rigidity of EN-VAN-AP, the molar ratio of epoxy:−NH2 was
set as 2:1.05 when curing to facilitate the imine exchange
reactions. A stress relaxation experiment of EN-VAN-AP was
carried out. Figure 3B exhibits the time dependence of the
normalized relaxation modulus G(t)/G0 at different temper-
atures in the range from 30 to 60 °C. The G(t)/G0 values
decreased with time and went to zero, indicating full stress
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Figure 4. Rigidity and toughness of EN-VAN-AP can be adjusted by water. (A) Impacts of imine content and cross-link density on water resistance
of imine-containing thermosets. (B) Wet and soft EN-VAN-AP sample that has been submersed in water for 24 h at room temperature could be
restored to rigid dry sample by heating at 120 °C for 1 day. (C) Tensile properties of wet sample after different periods of heating at 120 °C. The
sample without drying (i) was tested immediately after collecting from water. (D) Water-driven malleability of EN-VAN-AP. (a) Thermoset film
with the thickness of 0.4 mm was immersed in water for 4 h at room temperature. (b) The soft film was stretched on a round glass mold and dried
in a desiccator under reduced pressure. (c) The shape of dried sample was retained. (d) The reshaped sample could support at least 240 g of load
without significant deformation. (e) Enlarged picture of the thermoset that was supporting 240 g of load.
(e.g., EN-BPA), however, was prone to fracture when similar
twisting force was applied.
Meanwhile, tensile strength and elongation at break of welded
sample were 46 MPa and 4.4%, respectively, which were
comparable to the original sample (Figure 3E). The reference
material (EN-BPA), however, cannot be welded (two cut EN-
BPA films cannot be attached) under the same welding
condition as EN-VAN-AP.
By use of the imine exchange reaction occurring at the
interface of overlapped thermoset pieces, EN-VAN-AP was
also weldable and repairable. To prove this, a rectangular EN-
VAN-AP film (50 × 12.5 × 0.4 mm3) was cut into two pieces
in the middle. The two pieces were overlapped by ca. 3.2 mm
on a Teflon sheet and preheated at 100 °C for 60 s. During the
preheating period, a ca. 10 N force was manually applied to the
overlapping area to facilitate the welding. It was observed that
the two pieces started to attach to each other. The film was
then transferred to an oven and heated at 120 °C for 4 h for
welding, which required no additional pressure or amine
monomer. After the welding process, a dog-bone-shaped
sample was punched out, while the overlapped area was left in
the middle of the sample (Figure 3D-a,b). The welded sample,
which possessed same dimensions with the original sample
(prepared using the same punch), was subjected to tensile test.
As seen in Figure 3D-c,d, the welded sample always fractured
at a different place rather than the overlapped area, suggesting
the overlapped area was not the weakest part of the sample.
Water Sensitivity. Lastly, we demonstrated that water
resistance of imine-containing thermoset may be impacted by
the content of imine bond and cross-link density of the
thermoset. Previously reported imine-containing thermosets
were mainly prepared through direct condensation of
polyfunctional aldehydes and amines.33,35,37−40,51,52 In these
networks, imine bonds acted as the cross-linking sites.
However, because of the water-sensitive nature of imine
bonds, the strength of thermosets was reported to decrease
when exposed to water.38 In contrast to previous approaches,
we embedded imine bond within the backbone of diglycidyl
ether, while cross-linking was realized through epoxy−amine
reactions. This approach increased the cross-link density and
decreased the content of imine bonds within the network
(epoxy reacts with −NH2 in a 2:1 ratio, while aldehyde reacts
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with −NH2 in a 1:1 ratio), both of which improved the water
resistance of the imine-containing thermoset.
content while still highly cross-linked, EN-VAN-AP exhibited
improved water resistance over the straightforward aldehyde−
amine cross-linked thermosets.
To explore the impacts of content of imine bonds and cross-
link density on the water resistance of thermoset, three
precursorsDGEBA, glycidyl ether of vanillin (GE-VAN),
and terephthaldehyde (TPA), with various epoxy and aldehyde
groupswere compared to GE-VAN-AP (Figure 4A). All
precursors were reacted with polyfunctional amine to obtain
thermosets containing various contents of imine bonds.
Because the aromatic motif could increase the strength of
thermoset, the weight ratio of the aromatic ring in each
thermoset was kept similar by adjusting the molecular weight
of polyamines (MW = 230 or 400). All thermosets were cured
at 120 °C for 1−3 days for complete conversions of epoxy
and/or aldehyde groups as confirmed by IR spectra (Figure
S13). Figure S14 demonstrated the experimental setup for
testing the water sensitivity of thermosets. Even though the
buoyancy of water could retard the thermoset deformation, the
illustrated setup could still provide insight in the relative water
resistance of these thermosets. As seen in Figure 4A, it took
only 3 and 18 min for EN-TPA and EN-VAN to lose most of
their strength when immersed in water at room temperature.
By comparison, EN-VAN-AP exhibited significantly improved
water resistance, as it took more than 220 min to get the same
deformation. As for EN-BPA, it exhibited the highest water
resistance, as only slight deformation was observed even after
24 h.
Because of the complexity of the cross-linked network, a
representative polymer segment from each of the four
thermosets is illustrated in Figure 4A for comparison. The
segments were simplified to contain one molecule of precursor
and two molecules of amine hardener chains that reacted at
each side of the precursor. Covalent (formed through epoxy−
amine reaction) and reversible bonds (formed by aldehyde−
amine reaction) within the segments were highlighted. The
aromatic content in each segment was comparable. (One
aromatic ring per segmental molecular weight of 570−668. For
bisphenol precursors, i.e., DGEBA and GE-VAN-AP, poly-
amine with MW = 400 was used, while for monophenol
precursors, i.e., GE-VAN and TPA, polyamine with MW = 230
was used). However, the content of reversible imine bonds
varied significantly among segments. Water resistance of
thermosets decreased as imine content increased. EN-BPA
had no reversible bond, and its permanently cross-linked
network made it the most resistant to water. EN-VAN-AP has
one imine bond per segmental MW of ca. 1115. Although
imine bonds were “protected” by adjacent phenyl groups and
other covalent cross-links, water molecules could still gradually
penetrate the network by hydrolyzing the imine bond and
eventually softened the thermoset. When it came to EN-VAN
and EN-TPA, the imine content was doubled and quadrupled
(one imine bond per segmental MW of 668 and 297)
compared to EN-VAN-AP, respectively. These increments
resulted in the deformation of thermosets in much shorter time
of exposure. While reduced imine content postponed the
deformation of network, increased cross-link density, in a
similar way, helped decrease the exposure of imine bond to
water molecules. It should be noted that epoxy reacts with
−NH2 in a 2:1 molar ratio, while aldehyde reacts with −NH2
in a 1:1 molar ratio. Thus, as illustrated in Figure 4A,
precursors of EN-BPA and EN-VAN-AP could develop in four
directions (potential cross-links were shown in dashed lines),
while EN-TPA was basically linear. Possessing lower imine
As mentioned above, deformation of EN-VAN-AP was
mainly attributed to the hydrolysis of imine bonds, which was
confirmed by the presence of aldehyde group (1675 cm−1) in
the IR spectrum of wet sample after immersion in water for 24
h (Figure S15). Figure 4B exhibits the curvy appearance of the
wet sample. After drying the wet sample at 120 °C for 24 h, the
trapped water was evaporated, imine bonds were re-formed,
and the sample was restored to its original flat shape. Tensile
stress (42 MPa) and strain (4.1%) of fully dried sample (after
heating at 120 °C for 24 h) were comparable to the original
sample (Figure 4C-b), indicating the water-induced deforma-
tion could be recovered. By comparison, the wet sample
exhibited elastic properties, i.e., breaking stress of 4 MPa and
strain of 58% (Figure 4C-i), which might be attributed to (1)
reduced cross-links within the network as caused by the
hydrolysis of imine bonds and (2) increased void volume taken
by water molecules. Tensile properties of thermosets after
drying the wet sample at 120 °C for 5 min to 12 h are also
exhibited in Figure 4C-c−h. As their stresses were much lower
than the original sample, water residue could still exist within
these networks, and thus they were partially dried. The
maximum stress rapidly increased from 4 to 28 MPa within 1 h
of drying and slowly advanced to 31 and 37 MPa after 4 and 12
h of drying. This indicated the water evaporation was very fast
at the beginning of drying, and then it became more difficult to
get the residual water out of the network as drying proceeded.
Unlike the fully dried sample, the partially dried samples were
more ductile with strains ranged between 7.4% and 42.7%.
Because the rigidity and toughness of the thermoset could
be tuned by water, we then demonstrated EN-VAN-AP
exhibited water-driven malleability. A 0.4 mm thickness of
thermoset film was immersed in water for 4 h at room
temperature (Figure 4D-a). The wet sample was stretched over
a round-bottomed flask and placed in a vacuum desiccator to
dry the sample under reduced pressure until fully dried (Figure
4D-b). The shape of the dried sample was retained (Figure
4D-c), and it exhibited robust mechanical properties that could
support at least 240 g of loads without significant deformation
(Figure 4D-d,e). This observation is in agreement with
previous studies on imine-containing thermosets,37 while the
water-driven malleability widens the approaches of thermoset
reprocessing.
CONCLUSIONS
■
In summary, we report a novel epoxy thermoset that exhibits
degradability, recyclability, malleability, and weldability with-
out requiring additional ingredients such as metal catalyst or
additional monomer or complicated processes like press
heating. Through breaking and re-forming the imine bonds
(dissociative mechanism), the epoxy thermoset can be (1)
solubilized in organic or aqueous solutions under mild
conditions and (2) recycled from the solution with the original
thermal and mechanical properties retained. Through imine
exchange reaction (associative mechanism), the epoxy
thermoset at sufficient temperature can be (1) reshaped
through bond exchange within polymer network and (2)
welded through bond exchange at the interface of overlapping
thermoset pieces. The content of imine bond and cross-link
density are related to the water resistance of thermoset.
F
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Article
0.25 mol L−1). The mixture was then heated at 65 °C and mildly
stirred. Thermoset pieces were found to dissolve gradually, and a
homogeneous solution was obtained after 30 min. DMF and HCl in
the homogeneous solution were slowly evaporated when stirred at 80
°C with the aid of an air flow. When thin polymer film started to form
on the surface of solution (at this point, ca. 7 mL of solution was left),
the solution was transferred to a glass mold. The mixture was then
placed in a vacuum desiccator to remove the leftover DMF and HCl.
After 16 h, a viscous polymer gel was formed. The gel was put in an
oven at 120 °C for 24 h to obtain the recycled EN-VAN-AP.
Hygroscopicity of EN-VAN-AP. Placing dried EN-VAN-AP films
(30 × 10 × 1 mm3) at ambient temperature and exposing it to air
moisture for 24 h only led to a limited weight increase of 1.43
0.15%. Immersing EN-VAN-AP in water at ambient temperature,
however, caused more water absorption with weight gain of 30.9
1.8%.
Analysis Methods. The structure of new compounds was
examined using nuclear magnetic resonance (NMR) and Fourier
transform infrared (FTIR) spectroscopy. Liquid-state NMR spectra of
samples were collected on a Bruker Avance ARX-400 spectrometer
using deuterated acetone or chloroform as solvent. Solid-state NMR
spectra of samples were collected on a Bruker Ascend 400 MHz (9.4
T) dynamic nuclear polarization (DNP) NMR spectrometer. FTIR
analyses were conducted using a Thermo-Nicolet Nexus 470 FTIR
spectrometer equipped with an ultrahigh-performance, versatile
attenuated total reflectance (ATR) sampling accessory. The spectra
were scanned over a wavenumber range of 400−4000 cm−1 with a
resolution of 4 cm−1.
Gel permeation chromatography (GPC) was performed at room
temperature using dimethylformamide with 0.01% LiBr as the mobile
phase on a Waters 2695 separation module with a Waters 2414
refractive index (RI) detector and a Waters 2998 photodiode array
detector (PDA). Number-average molecular weights (Mn) and
weight-average molecular weights (Mw) were calculated relative to
polystyrene standards.
Dynamic mechanical properties were characterized using a DMA
850 (TA Instruments). Rectangular specimens with dimensions of 8
mm length, 3.5 mm width, and 0.4 mm thickness were measured in a
tension mode. The measurements were conducted from 30 to 150 °C
at a heating rate of 3 °C min−1 and a frequency of 1 Hz. The
temperature at the maximum in the tan δ curve was taken as Tα
(related to Tg). The cross-link density (v) was calculated from the
equilibrium storage modulus in the rubber region over Tα according
to the rubber elasticity theory using eq 1.53,54
Rigidity and toughness of thermoset can also be tuned by
water, which expands reprocessing methods.
EXPERIMENTAL SECTION
■
General. Vanillin (VAN), 4-aminophenol (AP), epichlorohydrin,
tetrabutylammonium bromide (TBAB), diglycidyl ether of bisphenol
A (DGEBA), terephthalaldehyde (TPA), and Jeffamine (poly-
(propylene glycol) bis(2-aminopropyl ether)) (average molecular
weight of 230 or 400) were purchased from Aldrich Chemical Co.
Concentrated hydrochloric acid (37%) was obtained from Fisher
Scientific. All chemicals were used as received without further
purification.
Synthesis of VAN-AP. A mixture of vanillin (6.08 g, 40 mmol)
and 4-aminophenol (4.36 g, 40 mmol) was stirred in water (125 mL)
at room temperature for 4 h. The afforded powder was collected by
filtration, washed with water, and dried in a desiccator to give VAN-
AP as a yellowish powder (9.23 g, 95% isolated yield, mp = 203−204
1
°C). H NMR (acetone-d6, 400 MHz) δ: 8.44 (s, 1H, −CHN−),
7.59 (d, J = 1.2 Hz, 1H, Ar−H), 7.32 (dd, J = 5.4, 1.2 Hz, 1H, Ar−H),
7.17−7.12 (m, 2H, Ar−H), 6.93 (d, J = 1.2 Hz, 1H, Ar−H), 6.87−
6.82 (m, 2H, Ar−H), 3.90 (s, 3H, − OCH3). 13C NMR (acetone-d6,
400 MHz) δ: 152.9 (−CHN−), 151.6, 145.4, 143.7, 140.1, 125.2,
119.7, 117.9, 111.5, 110.7, 105.4, 51.18 (−OCH3).
Synthesis of GE-VAN-AP. Glycidyl ether of VAN-AP (GE-VAN-
AP) was prepared by reacting VAN-AP (2.43 g, 10 mmol) with
epichlorohydrin (25 g, 266 mmol). Tetrabutylammonium bromide
(0.26 g, 0.85 mmol) was used as a phase transfer catalyst. The mixture
was heated at 85 °C for 3 h and followed by a dropwise addition of 5
g of 20% w/w NaOH solution. The reaction was kept for another 2 h,
and the mixture was introduced with ethyl acetate, filtrated to remove
formed NaCl, washed with water, dried with Na2SO4, and
concentrated with a rotary evaporator to yield GE-VAN-AP as a
1
yellowish solid (3.49 g, 94% isolated yield, mp = 110−112 °C). H
NMR (CDCl3, 400 MHz) δ: 8.34 (s, 1H, −CHN−), 7.57 (d, J =
1.2 Hz, 1H, Ar−H), 7.23 (dd, J = 5.7, 1.4 Hz, 1H, Ar−H), 7.17−7.12
(m, 2H, Ar−H), 6.97−6.87 (m, 3H, Ar−H), 4.29 (dd, J = 7.6, 2.2 Hz,
1H, −O−CH2−), 4.21 (dd, J = 7.3, 2.1 Hz, 1H, −O−CH2−), 4.07−
4.03 (dd, J = 7.3, 2.1 Hz, 1H, −O−CH2−), 3.95−3.90 (dd, J = 7.3,
2.1 Hz, 1H, −O−CH2−), 3.91 (s, 3H, −OCH3), 3.34 (m, 2H, −CH−
in oxirane), 2.93−2.84 (m, 2H, −CH2− in oxirane), 2.79−2.69 (m,
2H, −CH2− in oxirane). 13C NMR (CDCl3, 400 MHz) δ: 158.5
(−CHN−), 157.1, 150.9, 150.1, 145.8, 130.6, 124.1, 122.3, 115.4,
112.9, 109.5, 70.24, 69.30, 56.31 (−OCH3), 50.39, 45.19, 44.99.
Glycidyl ether of vanillin (GE-VAN) was prepared from vanillin
and epichlorohydrin using the same method as GE-VAN-AP (95%
isolated yield, mp = 80−82 °C).1H NMR (CDCl3, 400 MHz) δ: 9.83
(s, 1H, −CHO), 7.47−7.33 (m, 2H, Ar−H), 7.00 (d, J = 8.4 Hz, 1H,
Ar−H), 4.36 (dd, J = 11.4, 3.1 Hz, 1H, −O−CH2−), 4.08 (dd, J =
11.4, 5.7 Hz, 1H, −O−CH2−), 3.91 (s, 3H, −OCH3), 3.41 (dd, J =
14.8, 13.1 Hz, 1H, −CH− in oxirane), 2.92 (dd, J = 6.1, 2.8 Hz, 1H,
−CH2− in oxirane), 2.80 (dd, J = 6.1, 2.8 Hz, 1H, −CH2− in
oxirane). 13C NMR (CDCl3, 400 MHz) δ: 191.7 (−CHO), 154.3,
132.7, 107.3, 74.87, 56.99 (−OCH3), 51.18, 45.21.
v = E′/(ΦRT)
(1)
where E′ is the storage modulus at Tα + 30 °C. ϕ is the front factor
(approximated to 1 in the Flory theory),55,56 while R and T are the
gas constant and absolute temperature at Tα + 30 °C, respectively.
Stress relaxation experiments were also conducted on a DMA. The
measurements were conducted at different temperatures in the range
from 30 to 60 °C. The thermoset was first heated to the test
temperature. After the temperature was equilibrated for 5 min, a 1%
strain was applied, and the stress was recorded over time. A constant
normal force of 5 N was applied to maintain a good contact of the
sample with the parallel plate. The relaxation modulus G(t) were
normalized by the “plateau” value G0, which corresponded to the
elastic response of the material to the applied strain. This treatment
provided an easier comparison between the different temperatures.
Tensile testing was performed on dog-bone-shaped specimens
according to the ASTM D638 standard on a custom-built setup on a
vertical TwinRail positioning table (Lintech, Monrovia, CA) with a
200 N load cell. The crosshead speed was set to 0.5 mm min−1.
Thermal stability studies were performed on a Discovery
thermogravimetric analyzer (TGA, TA Instruments) under a nitrogen
flow of 40 mL min−1. Samples (5−10 mg) were placed in a platinum
pan and scanned from 40 to 600 °C at a ramp rate of 20 °C min−1.
Formation of Epoxy Networks. GE-VAN-AP was first melt at
115 °C. Then, Jeffamine (MW = 400) with a 2:1.05 molar ratio of
epoxy vs −NH2 was dropwise added. The mixture was vigorously
stirred at 100 °C for 1 min, degassed under vacuum to remove
trapped air, and quickly poured into mold and cured at 60 °C for 4 h
and then 120 °C for 20 h. Thermosets for tensile and welding tests
had the thickness of ca. 0.4 mm, while for solvent resistance,
depolymerization and twisting tests, the thickness was prepared to be
ca. 2 mm. The obtained epoxy network was denoted as EN-VAN-AP.
BPA-based epoxy network (EN-BPA) was prepared from DGEBA and
Jeffamine (MW = 400) using the same curing method with EN-VAN-
AP.
Recycling Methods. 1.5 g of cured EN-VAN-AP was cut into
pieces (ca. 12.5 × 5 × 2 mm3) and placed in a 20 mL glass vial. To
this vial was added successively 15 mL of DMF and 12 drops of
concentrated HCl (HCl concentration in DMF solution was about
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ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.macro-
mol.8b01976.
Liquid and solid-state NMR, IR, and GPC spectra of
new compounds, solvent resistance, dynamic mechanical
analysis and stress relaxation behavior of thermosets
(PDF)
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thermoset into soluble and reusable polymers. Macromolecules 2015,
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail abuomar@chem.ucsb.edu.
ORCID
Shou Zhao: 0000-0002-1953-5852
Mahdi M. Abu-Omar: 0000-0002-4412-1985
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported by the Center for direct
Conversion of Biomass to Biofuels (C3Bio), an Energy
Frontiers Research Center (EFRC) funded by the U.S.
Department of Energy, Office of Science, Office of Basic
Energy Sciences under Award DE-SC000097, the University of
California, Santa Barbara, and the Mellichamp Academic
Initiative in Sustainability. We acknowledge the use of the
MRL Shared Experimental Facilities, supported by the
MRSEC Program of the National Science Foundation under
Award NSF DMR 1121053, a member of the NSF-funded
Materials Research Facilities Network.
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