Sonochemical transformation of epoxy-amine thermoset into soluble and reusable polymers

Article abstract of DOI:10.1021/ma501934p

(Chemical Equation Presented). The degradation and reuse of epoxy thermosets have significant impact on the environments. We report that an epoxy-amine thermoset embedded with Diels-Alder (DA) bonds was transformed into soluble polymers via sonochemistry under mild temperature (ca. 20 °C) for the first time. Sonication could effectively induce the position-oriented cleavage of DA bonds (i.e., retro-DA) of the fully swelled epoxy thermoset in dimethyl sulfoxide (DMSO), leading to the soluble polymers. Of importance, such sonochemical process could be regulated on demand via switching on-and-o ff of the sonication. The obtained soluble polymers could be recured to form epoxy-amine thermosets via DA reaction. This sonochemical method might provide an unprecedented and efficient way to the controlled degradation and recycling of the epoxy thermosets containing the dynamic covalent bonds likes DA groups.

Full text of DOI:10.1021/ma501934p

Article
pubs.acs.org/Macromolecules
Sonochemical Transformation of Epoxy−Amine Thermoset into
Soluble and Reusable Polymers
Yuqin Min, Shuyun Huang, Yuxiang Wang, Zhijun Zhang, Binyang Du, Xinghong Zhang,*
and Zhiqiang Fan
MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science & Engineering, Zhejiang
University, Hangzhou 310027, China
S
* Supporting Information
ABSTRACT: The degradation and reuse of epoxy thermosets have significant impact on the environments. We report that an
epoxy−amine thermoset embedded with Diels−Alder (DA) bonds was transformed into soluble polymers via sonochemistry
under mild temperature (ca. 20 °C) for the first time. Sonication could effectively induce the position-oriented cleavage of DA
bonds (i.e., retro-DA) of the fully swelled epoxy thermoset in dimethyl sulfoxide (DMSO), leading to the soluble polymers. Of
importance, such sonochemical process could be regulated on demand via switching on-and-off of the sonication. The obtained
soluble polymers could be recured to form epoxy−amine thermosets via DA reaction. This sonochemical method might provide
an unprecedented and efficient way to the controlled degradation and recycling of the epoxy thermosets containing the dynamic
covalent bonds likes DA groups.
of 104−106 g/mol) via position-oriented cleavage of the special
INTRODUCTION
■
chemical bonds in the matrix of the thermoset, namely, the
Among of all commercial thermosets, epoxy thermosets present
controlled degradation of the thermoset. As a consequence, the
outstanding chemical, heat-resistant, and mechanical properties
thermoset can be recycled to be a new polymer. Dynamic
and excellent insulation due to their irreversible three-
covalent bonds,5,6 which are unique in the sense that they
dimensional (3-D) cross-linked networks.1,2 Global production
combine the characteristics of the covalent and noncovalent
of the epoxy thermosets such as printed circuit boards,
bonds, meet the above requirement. The dynamic covalent
packaging, coatings, and adhesives are about 2 000 000 t per
bonds, such as imine linkage,7 trithiocarbonate,8 acylhydra-
year in recent years. The majority of them cannot be recycled
due to their insoluble and infusible properties. As a result, the
used epoxy thermosets are generally incinerated or landfilled
zone,9 disulfide,10 and Diels−Alder (DA) bonds,11 can
reversibly form and break under certain conditions.12 Recently,
the so-called covalent adaptable epoxy thermosets containing
like garbage or degraded to small molecules directly using
strong acidic or basic agents.3,4 The separation and reuse of
dynamic covalent bonds have been developed as bulky self-
healing or malleable materials.13−15 However, it is not yet
these small molecules are still big challenges. Indeed, these
reported that the 3-D network of the epoxy thermoset could be
treatments for the used epoxy thermosets cause a huge material
effectively transformed into soluble and reusable polymers.
Aiming to transform the epoxy thermoset into the soluble
polymers under mild temperature, we incorporated DA bonds
into the epoxy thermoset in the present work (Scheme 1). In
chemistry, DA bond is formed by [4 + 2] cycloaddition of furan
and maleimide group at 60−80 °C, while retro-DA (r-DA)
waste, energy consumption, and a great threat to the
environment (e.g., production of dioxins during incineration).
It is highly desired to develop a useful and controllable method
for degrading the epoxy thermoset, which was predesigned to
be stable in its service state, to soluble polymers.
For such purpose, it would be useful to incorporate special
chemical bonds, which can potentially unzip the 3-D network
under external stimuli, into the epoxy thermoset. It would then
be possible to transform the thermoset with infinite molecular
weight (MW) to soluble high polymer (generally in the range
Received: September 18, 2014
Revised: December 14, 2014
© XXXX American Chemical Society
A
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Scheme 1. Syntheses of 1, 2 and 3 Embedded with DA Bonds and the Proposed r-DA Reaction Induced by Ultrasound
Irradiation
Figure 1. (A) DSC result of 2 under a N2 atmosphere (10 °C/min). (B) The powder of 2 swelled in DMSO for 4 h. (C) The solution obtained from
(B) via sonication in DMSO for 75 min (ice bath). (D) The solid polymers from (C). (E) GPC curves of the soluble polymers obtained at different
sonication times (entries 1−5 in Table 1). (F) The yield and Mn of the resultant soluble polymers as a function of sonication times.
reaction happens at 110−130 °C,11,16−27 releasing the furan
and maleimide groups.16,24 Moreover, r-DA could be activated
by force28 at low temperatures when a DA bond was placed at
the center of a soluble polymer with high MW. In this case,
MW kept unchanged when the polymer reached a MW
threshold (Mlim), below which no further chain scission was
observed.29−32 Since MW of the epoxy thermoset is infinite and
nearly every site in the network could be considered as the
midpoint of a chain, we envisioned that, if the epoxy thermoset
was swelled well by a solvent, DA bonds embedded in the
network could be broken via r-DA reaction under certain
external force, leading to the transformation of the thermoset
into the soluble polymer (Scheme 1). The existence of Mlim for
r-DA reaction might ensure the achievement of high polymers
rather than small molecules. Herein, we shall show the proof-of-
principle of this concept.
RESULTS AND DISCUSSION
■
For testing the aforementioned hypothesis, an epoxy−amine
thermoset was designed and synthesized from the curing
reaction of the bisepoxide (1) (FDB) containing two DA bonds
in one molecule with diethylenetriamine (DETA). 1 was
synthesized by [4 + 2] cycloaddition of furfuryl glycidyl ether
(FGE) [1H (13C) NMR spectra are seen in Figure S1] with
4,4′-methylenebis(N-phenylmaleimide) in refluxed THF (66
°C) for 24 h.18 The obtained 1 had two DA bonds and two
oxirane groups, which were confirmed by H (13C) NMR
1
(Figure S2) and FT-IR spectra (Figure S3).
B
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The choice of the hardener for 1 and the corresponding cure
procedure should strictly obey the rule that just one epoxy−
amine network was formed with DA bonds between two cross-
linked points because the production of the other network
which resulted from the addition reaction of the generated
secondary −OH group with epoxide induced by highly
exothermic epoxy−amine addition reaction at high curing
temperature would interpenetrate with the epoxy−amine
network, and thus the resultant thermoset was difficult to
degrade via sonication. Moreover, the heat-induced r-DA
reaction could occur at above 110 °C in bulk. Therefore, a
common hardener, DETA with five N−H bonds, was employed
for curing with 1 for its moderate reactivity at low
temperatures. 1 was then cured with the equivalent DETA at
25 °C for 4 days and postcured at 60 °C for 12 h under a N2
atmosphere. The long time and low temperature curing
procedure were applied for equivalent reaction of the oxirane
groups with N−H bonds, which was aimed to avoid the
formation of another network. The oxirane groups of FDB were
completely converted after curing reaction, which was proved
by the comparison of FT-IR spectra of FDB/DETA thermoset
(2) (Figure S4) with 1.
The cross-linked structure of 2 was evident by the differential
scanning calorimetric (DSC) and solubility test. The glass
transition temperature (Tg) of 2 (Figure 1A) was estimated to
be ca. 97 °C, which was much higher than that of the control
(56 °C) which was the bisphenol A diglycidyl ether (DGEBA)/
DETA thermoset obtained under the same curing condition
(Figure S5-A) because 2 had the rigid bismaleimide moieties.
Moreover, an endothermic peak at 127 °C in the first heating
trace could be attributed to r-DA reaction in 2,16 while the
consecutive cooling and second heating processes exhibited
nearly flat curves because DA reaction of the furan and
maleimide moieties did not occur during DSC measurements.16
The solubility and swelling property of 2 in various solvents
including tetrahydrofuran (THF), acetonitrile (ACN), dimethyl
sulfoxide (DMSO), propylene carbonate (PC), N-methylpyr-
rolidone (NMP), and dimethylformamide (DMF) were
examined at 25 °C (Figure S6). 2 was not dissolved by all
the above solvents, but it was effectively swelled in DMSO,
NMP, and DMF. THF, ACN, and PC presented poor swelling
ability toward 2. NMP and DMF showed the swelling ratios of
157% and 175% to 2, respectively, while DMSO had the
highest degree of swelling of 190% to 2 within 8 h.
Therefore, DMSO was selected as the solvent for the
controlled degradation of 2 under sonication. The ultrasound
irradiation was carried out in the mode of 3 s on followed with
3 s off for the preset time. The crushed 2 (100 mg, the sizes of
the granules were ca. 100 μm−5 mm) was put into DMSO
(10.0 mL) in a 25 mL vial and swelled for at least 4 h under
slow N2 bubbling (Figure 1B). The suspension was frozen to be
solid in the ice bath because the melting point of DMSO is 18.5
°C. Once ultrasound irradiation was started, solid DMSO was
melted immediately; the temperature of the suspension was
raised and maintained at ca. 18−20 °C during sonication. With
increasing the sonication times, the big granules became
invisible gradually. Meanwhile, the color of the solution was
changed from colorless to pale green and finally yellow green
(Figure 1C). After sonication, the suspension was centrifuged.
The supernatant was collected and dialyzed in methanol and
then freeze-dried; a loose and pale yellow solid was obtained
(Figure 1D).
The effects of the sonication intensity and time on r-DA
reaction in the swelled 2 were further examined (Table 1). The
Table 1. Soluble Polymers from the Sonochemical Reaction
a
of the Epoxy Thermosets in the Selected Solvents
b
int
time
yield of soluble
c
d
entry solvent
(%) (min)
polymers (%)
Mn (kg/mol)/PDI
1
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
NMP
30
30
30
30
30
40
50
30
30
30
30
30
25
50
39.8
44.6
78.3
83.5
84.1
71.1
77.9
16.9
3.0
154.5/2.2
110.4/2.3
76.6/2.7
48.8/1.6
48.3/1.7
86.8/2.5
82.7/2.6
577.1/1.5
−/−
2
3
75
4
100
125
50
5
6
7
50
8
75
9
DMF
75
e
e
e
10
11
12
DMSO
NMP
125
125
125
84.5
46.9
3.4
48.6/1.6
106.2/2.0
−/−
DMF
a
Conditions: 100.0 mg of 2 in 10.0 mL of solvent, ice bath; sonication
mode: 3 s on and 3 s off, and 10 min as one cycle (i.e., 5 min
sonication on per cycle), the next cycle was then started after a stop
time of 2 min. The maximum ultrasonic power of the instrument was
650 W; a 6 mm diameter amplitude transformer was immerged into
b
the solvent in a 25 mL vial under a N2 atmosphere. The total time of
c
the on mode of the sonicator. Calculated from the residual solid after
d
sonication, centrifugation, and completely dried process. Determined
by gel permeation chromatography in DMF, 60 °C, PMMA standard.
e
The FHM/DETA thermoset (3).
increase of the sonication intensity from 30% to 50% led to the
decrease of the number-average molecular weight (Mn) of the
resultant soluble polymers from 110.4 to 82.7 kg/mol (entries
2, 6, and 7, the sonication time was 50 min, Table 1) as
determined by gel permeation chromatography (GPC) (Figure
S7). When the sonication intensity was set as 30%, with
increasing the sonication times from 25 to 125 min, Mn
decreased from 154.5 to 48.3 kg/mol (Figure 1E). We noticed
that the decrease of Mn became very slow from 100 to 125 min,
which could be considered as the Mlim (ca. 48.0−49.0 kg/mol)
for the sonication degradation of 2 in DMSO (Figure 1F).
Note that Mlim is closely dependent on the experimental
conditions and technique used.29−32
The yield of the soluble polymers increased with increasing
either the sonication intensity or the sonication times, as shown
in Table 1. Note that the plot of the yield of the soluble
polymer versus times presented an “S” type mode (Figure 1F),
and a regime of high degradation rate could be identified for the
sonication times from 50 to 75 min. Of importance, the
sonication was easily to be regulated on demand. The
sonication could be paused and restarted according to the
requirements of the on and off of the external force. In other
words, 2 could be degraded in a controllable manner under
sonication at mild temperature.
The r-DA reaction in the swelled 2 induced by sonication
1
was clearly confirmed by H NMR spectrum (Figure 2) of the
soluble polymer from entry 5 in Table 1. The 1H NMR spectra
of FGE, FDB, and a heat-decomposed 2 (in DMSO, 130 °C, 30
min) are also shown in Figure 2 for comparison. Heat
treatment of 2 in DMSO resulted in nearly complete
disappearance of all DA bonds (6.62−6.51 and 5.20−5.33
ppm). Meanwhile, the furan ring (6.45−6.36 and 7.6−7.7 ppm)
was released (curve 2) after heat treatment. In comparison,
C
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Figure 2. 1H NMR spectra (400 MHz, d6-DMSO) of (1) the soluble polymer of entry 5 in Table 1, (2) heat-degraded product of 2 at 130 °C for 30
min, (3) FDB, and (4) FGE.
ultrasound irradiation caused partial r-DA reaction in the
swelled 2. There were 10.0 and 20.0 mol % DA bonds (curve 1)
in the swelled 2 were broken via r-DA reaction after sonication
for 75 and 125 min, respectively, according to 1H NMR spectra
(the calculations are in Figure S8). The partial cleavage of DA
bonds in the thermoset led to the production of the soluble
polymers, which still contained considerable amounts of DA
bonds. Therefore, the obtained soluble polymers could be
postulated to be the statistical hyperbranched polymers.
of 2 at mild temperature (ca. 18−20 °C). First, DMSO has a
special dipole moment and is also a strong proton accept-
or.36−38 DMSO could swell 2 via the formation of multi-
hydrogen bonding between DMSO and hydroxyl groups
(−OH) that was the strong proton donor in the network,
which likely reduced the energy cost during the mixing of
DMSO with the resultant soluble polymers. Such strong
solvent−polymer interaction aided the extraction of a polymer
chain from the swelled layer of the thermoset (Figure S11).
Second, DMSO has a highly self-association structure when the
temperature was close to its melting point (18.5 °C). Markarian
et al. reported that the physical properties of DMSO had a
sharp transition close to its melting point due to the formation
of the bigger DMSO aggregates (or cluster), and the change of
the temperature near the melting point caused the dramatic
fluctuation concentration of the associates.39 In our system,
because the suitable sonication temperature for 2 in DMSO was
ca. 18−20 °C, the dramatic change of the self-association
structure of DMSO, which was interpenetrated with the swelled
3-D network at the molecular levels, would cause a huge
deformation of such swelled thermoset under sonication and
thus activate the r-DA reaction effectively.
Since no structural transition occurred when DMSO was
heated from 20 to 60 °C,38−40 and the hydrogen bonding
interaction between DMSO and water decreased with
increasing the temperatures,41 we wondered if the degradation
efficiency of 2 would decrease at a higher temperature or in
both DMF and NMP which had no self-association structures
in their liquid states because of their low melting points (DMF:
−61 °C; NMP: −24.4 °C). To verify this idea, 2 was irradiated
by ultrasound in DMSO without ice bath (ca. 40 °C) for 75
min; although the swelling ratio of 2 was 280% at 40 °C, the
resultant solution was turbid (Figure S9−C) and only 9 wt %
soluble polymer with a Mn of 219 kg/mol was obtained. When
2 was irradiated by ultrasound in DMF and NMP at ca. 10 °C
(entries 8 and 9, Table 1), it was difficult to be degraded
The partial degradation of the swelled 2 was solely caused by
r-DA reaction rather than the degradation of epoxy−amine
adducts in the thermoset (Scheme 1). It was confirmed by the
sonication test for the swelled DGEBA/DETA thermoset (Tg =
56 °C, the degree of swelling was 156% in DMSO, 25 °C, 8 h)
in DMSO under the same condition as that of entry 5 in Table
1. No soluble products were collected (Figure S9A,B). The
weight loss of the DGEBA/DETA thermoset after sonication
was ca. 3.0 wt %. Moreover, r-DA reaction in the swelled 2 was
not caused by the local strongly exothermic heat, although the
cavitation was formed and hot spots were produced under
sonication.33,34 In a control experiment, we observed that r-DA
reaction of FDB (1), which could be efficiently activated by
heat,35 did not occur in d6-DMSO under the same sonication
1
condition as entry 5 in Table 1 that was proved by H NMR
spectra (Figure S10).
The force-induced partial degradation of 2 to soluble
polymers could be ascribed to the cavitation effect induced
by ultrasound irradiation in DMSO.29,30 It is worthy to note
that the sonochemical r-DA reaction of 2 was heterogeneous
because 2 was cross-linked. The thermoset could only swell but
not evenly disperse in DMSO. The sonication irradiation would
be also helpful for the swelling of 2 in solution besides
generating the mechanical force. Furthermore, the r-DA
reaction may occur preferentially on the polymer chains that
were pulled out by the solvation. DMSO was the effective
solvent among of the above solvents for the partial degradation
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although both solvents had good swelling abilities toward 2 and
could easily dissolve the obtained polymers from sonication test
in DMSO. Only 16.9 wt % of 2 was degraded to the soluble
product (entry 8, Table 1) in NMP, while 2 was not degraded
in DMF at all (entry 9, Table 1; Figure S9D,E) under
sonication. Of interest, Mn of the soluble polymer obtained
from the sonication-induced degradation of the thermoset in
NMP was 577.1 kg/mol, which was higher than that (76.6 kg/
mol) obtained with DMSO as solvent under the same
sonication conditions and the soluble polymers obtained after
a 25 min sonication (154.5 kg/mol, entry 1, Table 1). This
result suggested that the degradation rate of the soluble
polymer in NMP was slower than that in DMSO. Such low
efficiency of the mechanical activation in the control experi-
ments might conversely prove the effective mechanical
activation of r-DA reaction of 2 in DMSO at the temperatures
close to the melting point of DMSO.
As a result, the partial degradation of 2 to soluble polymers is
a combination of the swelling and pulling-out effect caused by
ultrasound irradiation in DMSO. The effective swelling of 2 is
the prerequisite for converting the cross-linked network into
soluble polymers, while the strong solvent−polymer interaction
and highly self-association structure could facilitate the
extraction of a polymer chain from the swelled layer of the
thermoset (Figure S11) via the cavitation effect induced by
ultrasound irradiation. Such sonochemical method represents a
new effective route to transform epoxy thermosets into soluble
polymers at mild temperature.
MPa). G′ at 50 °C of 2 was 2600 MPa and higher than that of
the recured 2 at the same temperature (2260 MPa), suggesting
reduced storage modulus of the recured 2. Similar to the Tg
values from DSC determination, both samples also showed
nearly the same Tgs (85.9 and 85.5 °C for 2 and recured 2,
respectively) from the peak position of tan δ.
This sonochemical method could be applied to another
epoxy thermoset, the FHM/DETA thermoset (3) containing
DA bonds. The synthesis and characterization of FHM and 3
are given in the Supporting Information (Figures S12−S15, Tg
= 15 °C). 3 was degraded to soluble polymers in DMSO under
the same sonication condition (entry 10, Table 1) based on the
same DA chemistry. The yields of the obtained soluble
polymers in DMSO and NMP were 84.5% and 46.9%,
respectively (entries 10 and 11, Table 1). Mn of the obtained
soluble polymer in DMSO were 48.6 kg/mol and close to Mlim
that was clearly lower than that (106.2 kg/mol) in NMP
(entries 10 and 11 in Table 1), while 3 was hardly degraded in
DMF (entry 12 in Table 1) under sonication. The partial
degradation of 3 induced by sonication showed the similar
results as that of 2. These obtained soluble polymers contain
polar ether linkages, amino, carbonyl, and hydroxyl groups,
which could be potentially used for scavenging the heavy metal
ions or dyes in water.
CONCLUSIONS
■
In summary, we reported the first example of force-induced
transformation of the epoxy thermoset into a soluble and
reusable polymer via partial position-oriented cleavage of DA
bonds in the thermoset in DMSO. This force-induced
degradation protocol provides an unprecedented, useful, and
efficient way to recycle the epoxy thermosets with dynamic
covalent bonds like DA groups under mild temperature.
The obtained soluble polymers (Figure 1D) could be easily
reused. It was dissolved into DMF and poured into a mold.
After evaporating DMF, the sample was cured at 70 °C for 2
days. A semitransparent flexible film was obtained (Figure 3,
EXPERIMENTAL SECTION
■
Materials. Furfuryl alcohol was purchased from Aladdin Reagent Ltd.
4,4′-Methylenebis(N-phenylmaleimide) (DPMBMI) and diethylene-
triamine (DETA) were purchased from J&K Scientific Ltd.
Epichlorohydrin (ECH) and solvents were bought from Sinopharm
Chemical Reagent Co., Ltd. (Shanghai, China). Diglycidyl ether of
bisphenyl A (DGEBA, E-51, Brand KB-28) was granted kindly by
Jiangsu Chemical Industry (Kingboard) Co., Ltd. (Wuxi, China). All
solvents and the above reagents were used as received without further
purification.
Equipment and Measurements. Fourier transform infrared (FTIR)
spectra were recorded by a Vector 22 FTIR spectrophotometer (KBr
pellet). 1H nuclear magnetic resonance (1H NMR) spectra were
obtained on a Bruker Advanced DMX 400-MHz spectrometer with
tetramethylsilane (TMS) as internal standard. Ultrasound-induced
degradation was carried on a sonifier cell disrupter (XO-650, Nanjing
Xianou Instruments Manufacture Co. Ltd., Nanjing, China.). To
obtain gel permeation chromatography (GPC) data, a PL-GPC 220
chromatograph (Polymer Laboratories Ltd.) equipped with an HP
1100 pump from Agilent Technologies. The GPC columns were
eluted with DMF (0.05 M LiBr) at a rate of 1 mL/min at 60 °C. The
injection volume was 100 μL. The molecular weight standard was
PMMA. Differential scanning calorimetric (DSC) measurements were
conducted on a TAQ200 instrument (New Castle, DE) with a
ramping rate of 10 °C/min under a N2 atmosphere. The dynamic
mechanical analysis (DMA) measurements were carried out on a
Q800 analyzer (TA Instruments Corporation). The single cantilever
mode was used, and the measurement was carried out from 0 to 140
°C with a heating rate of 3 °C/min and an oscillatory frequency of 1
Hz.
Figure 3. DSC result of the recured sample (inserted chart) from the
recycled epoxy polymer in DMF (cure condition: 70 °C/2 days).
inserted chart) and was not dissolved by DMF and DMSO.
The resultant thermoset showed similar thermal behaviors
(curve in Figure 3) with that of the pristine 2 (Figure 1A) and a
Tg of 96 °C, which was higher than that (50 °C, Figure S5B) of
the soluble polymers after sonication. Moreover, this film could
be degraded to soluble polymers (yield: 51%) with a Mn of 103
kg/mol under ultrasound irradiation. Dynamic mechanical
analysis (DMA) test was performed for obtaining the
mechanical and thermal properties of the pristine 2 and
recured 2 from the soluble polymers after sonication, as shown
in Figure 4. The initial storage moduli (G′) at 0 °C of both
pristine 2 and the recured 2 were nearly the same (ca. 3250
Synthesis of Furfuryl Glycidyl Ether (FGE).18 51 g (0.55 mol) of
epichlorohydrin (ECH) and 3.8 g of tetrabutylammonium bromide
E
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Figure 4. Storage moduli (G′) and tan δ of the FDB/DETA thermoset (A) and the recured FDB/DETA thermoset (B) from soluble polymers as a
function of temperature from dynamic mechanical analysis (DMA) scans. The glass fiber paper was used for strengthening 2 and recured 2, and the
molding method was used for preparing the samples with a pressure of ca. 1−2 kg/cm2. The single cantilever mode was used, and the measurement
was carried out from 0 to 140 °C at a heating rate of 3 °C/min and an oscillatory frequency of 1 Hz.
(TBAB) were charged into a 250 mL three-necked round-bottom flask
equipped with a magnetically stirred, a thermometer, and an inlet of
dry nitrogen. When the solution was maintained at 25 °C with a water
bath, 49 g (0.5 mol) of 2-furanemethanol was added dropwise in 30
min. The reaction mixture was stirred for 2 h at room temperature.
After that, 80 mL of sodium hydroxide aqueous solution (50% (w/v))
was added dropwise within 1 h. The reaction proceeded for an
additional 2 h at room temperature. Afterward, 100 mL of ethyl ether
was added, and the organic layer was collected, washed with 50 mL
waters for 3 times, and dried with anhydrous sodium sulfate, filtrated,
and evaporated to get a light yellow liquid. The colorless liquid
product was obtained after distillation of the crude product at 103−
Sonication Test for 2. 100.0 mg of 2 was added into 10.0 mL of
solvent in a 25 mL vial for at least 4 h, which was then cooled with an
ice bath (0 °C) and subjected to pulsed ultrasound irradiation. The
sonication test was carried on a sonicator with an 6 mm diameter
amplitude transformer that was immerged into the solvent in a 25 mL
vial under a N2 atmosphere. The maximum ultrasonic power was 650
W, and the intensity rate was set as 30% (195 W). The sonication
mode: 3 s on and 3 s off, and 10 min as one cycle (i.e., 5 min
sonication on per cycle); the next cycle was started after a stop time of
2 min. After the ultrasound degradation, the mixtures were centrifuged
(3000 rpm, 20 min). The precipitates were dried and weighted to
calculate the residual solid percentage. The supernatant were collected
and loaded into the dialysis bags with molecular cutoff of MW3500
and placed in CH3OH for dialyzing 3 days at ca. 0 °C to replace the
solvent with CH3OH. After that, the solutions in the dialysis bags were
dried in vacuum oven and solid powders were obtained. For GPC
measurement, 10.0 mg of obtained solid powders was first dissolved in
3.0 mL of DMF and filtered with 0.22 μm organic membranes; the
obtained clear solutions were then subjected to GPC test. As a control
experiment, 15.0 mg of cured sample and 1.0 mL of DMSO-d6 were
loaded in a NMR tube. The NMR tube was placed in an oil bath with
temperature of 130 °C for 30 min and then quenched to room
temperature by ice water. The obtained heat-induced degradation
1
105 °C in a vacuum (11 mmHg). H NMR (400 MHz, CDCl3): δ
7.44−7.39 (m, 1H), 6.38−6.31 (m, 2H), 4.53 (q, J = 12.8 Hz, 2H),
3.75 J = 11.5, 3.1 Hz, 1H), 3.44 (J = 11.5, 5.8 Hz, 1H), 3.16 (J = 5.8,
4.0, 2.9 Hz, 1H), 2.80 (J = 9.2, 4.4 Hz, 1H), 2.61 (J = 5.0, 2.7 Hz, 1H).
13C NMR (100 MHz, CDCl3): δ 151.41, 142.89, 110.30, 109.58,
77.54, 77.28, 76.96, 70.57, 64.97, 50.68, 44.18. FIIR (KBr, cm−1):
3125, 3062, 3000, 1504, 1257, 1217, 1150, 1084, 1008, 916, 850, 751.
Synthesis of 2,2′-(Methylenebis(4,1-phenylene))bis(4-((oxiran-
2-ylmethoxy)methyl)-3a,4,7,7a -tetrahydro-1H-4,7-epoxyisoindole-
1,3(2H)-dione) (FDB, 1).18 7.14 g (0.02 mol) of DPMBMI was
dissolved in 50 mL of anhydrous THF. The solution was charged into
a 100 mL three-necked round-bottom flask equipped with magnetic
stirring and condenser and a thermometer. 6.16 g (0.04 mol) of FGE
was then slowly dropped into the solution by a constant dropping
funnel. Afterward, the solution was refluxed at about 66 °C for 24 h
under a N2 atmosphere and then cooled down to room temperature.
The reaction solution was poured into a large excess amount of diethyl
ether. The precipitate was filtered and then dried under vacuum at
room temperature. The obtained powder was dissolved into 5 mL of
acetone and poured into 500 mL of diethyl ether again. Finally, the
precipitate was filtered and dried under vacuum at room temperature.
The precipitate was further purified by chromatography on a silica gel
column with mixed solvent of CHCl3/CH3OH = 100/1 (Rf = 0.38) to
obtain the yellow powder product. The exo and endo ratio of FDB was
determined by 1H NMR as exo/endo = 7:1. Exo isomer 1H NMR (400
MHz, DMSO-d6): δ 7.39−7.32 (2H), 7.15 (2H), 6.57 (2H), 5.21
(1H), 4.23 (2H), 4.08−3.98 (1H), 3.85−3.75 (2H), 3.36−3.32 (1H),
3.19 (1H), 3.06 (1H), 2.74−2.68 (1H), 2.56−2.51 (1H). 13C NMR
(100 MHz, CDCl3): 13C NMR (100 MHz, DMSO-d6): δ 176.29−
173.03, 140.80, 138.57−136.44, 128.93, 90.45−89.10, 80.63, 72.00,
68.34, 50.99−47.90, 43.85−37.67.
1
product was directly characterized by H NMR spectroscopy.
Preparation of Sample Bars for DMA Test. The glass fiber paper
was used for strengthening 2 and the recured 2 because both were
brittle and hard to remove bubbles by pouring method. The glass fiber
paper was immersed into CH2Cl2 solution of FDB and DETA, and
then the solvent was evaporated under vacuum for 3 h and cured at 25
°C for 4 days and 60 °C for 12 h under pressure of ca. 1−2 kg/cm2;
the sample bar with 50 × 10 × 1.0 mm3 was obtained. For testing the
mechanical properties of recured 2, the powder obtained by sonication
degradation of 2 in DMSO were dissolved into DMF; the glass fiber
paper was then immersed into the solution. Afterward, DMF was
evaporated under vacuum for 3 days and then cured at 75 °C for 2
days under a pressure of ca. 1−2 kg/cm2, leading to the sample bar
with 50 × 10 × 1.0 mm3.
ASSOCIATED CONTENT
■
S
* Supporting Information
Text, figures, and tables giving general experimental procedures
and characterization data. This material is available free of
charge via the Internet at http://pubs.acs.org.
Preparation of the FDB/DETA Thermoset (2). First, 0.06 g of
curing agent DETA was dissolved into 1.0 mL of 1,2-dichloroethane.
Then 1.0 g of FDB was dissolved in 2.0 mL of CH2Cl2. The two
solutions were then mixed until a clear transparent mixture was
formed. The mixture was degassed under vacuum for 3 h. Afterward,
the mixture was cured at room temperature for 4 days and postcured
at 60 °C for 12 h to get the cross-linked epoxy thermoset.
AUTHOR INFORMATION
■
Corresponding Author
*Fax +86-571 87953732; e-mail xhzhang@zju.edu.cn (X.Z.).
F
DOI: 10.1021/ma501934p
Macromolecules XXXX, XXX, XXX−XXX

Macromolecules
Article
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors are grateful for financial support by the National
Science Foundation of the People’s Republic of China (No.
21274123 and 21474083).
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DOI: 10.1021/ma501934p
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