Ultrahigh Tg Epoxy Thermosets Based on Insertion Polynorbornenes

Article abstract of DOI:10.1021/acs.macromol.5b02648

Thermosetting materials (thermosets) are widely used organic materials derived from 3D-network forming monomers. Achieving high glass transition temperature (T_g) thermosets is often a challenging task due to the complexity of designing efficiently and cheaply monomers which are rigid enough to prevent molecular motions within the thermoset. We report here a very simple route to prepare epoxy thermosets with T_g as high as 350 °C, based on insertion polynorbornenes. The epoxy monomer (PNBE(epoxy)) is prepared by the epoxidation of poly(5-vinylnorbornene) obtained by catalytic insertion polymerization of 5-vinylnorbornene. PNBE(epoxy) can be cross-linked with simple biosourced compounds. Alternatively, polar insertion polynorbornene can also be used as cross-linker in the formulation of an epoxy resin, once again resulting in epoxy resins with T_g higher than 300 °C and devoid of degradation at this temperature. Thus, this study clearly demonstrates the viability of catalytic polymerization to access epoxy thermosets with ultrahigh T_g.

Full text of DOI:10.1021/acs.macromol.5b02648

Article
pubs.acs.org/Macromolecules
Ultrahigh T Epoxy Thermosets Based on Insertion Polynorbornenes
g
†
†
†
†
‡
Basile Commarieu, Jonathan Potier, Moubarak Compaore, Sylvain Dessureault, Brian L. Goodall,
Xu Li, and Jerome P. Claverie*
§
,†
†Quebec Center for Functional Materials, Department of Chemistry, UQAM, Succ. Centre Ville CP8888, Montreal H3C3P8, QC,
Canada
‡
Valicor Renewables, 7400 Newman Blvd., Dexter, Michigan 48130, United States
§
Institute of Materials c. Research and Engineering (IMRE), Agency for Science, Technology and Research (A*STAR), 3 Research
Link, Singapore 117602, Singapore
*
S Supporting Information
ABSTRACT: Thermosetting materials (thermosets) are widely used
organic materials derived from 3D-network forming monomers.
Achieving high glass transition temperature (T ) thermosets is often
g
a challenging task due to the complexity of designing efficiently and
cheaply monomers which are rigid enough to prevent molecular
motions within the thermoset. We report here a very simple route to
prepare epoxy thermosets with T as high as 350 °C, based on insertion
g
polynorbornenes. The epoxy monomer (PNBE(epoxy)) is prepared by
the epoxidation of poly(5-vinylnorbornene) obtained by catalytic
insertion polymerization of 5-vinylnorbornene. PNBE(epoxy) can be
cross-linked with simple biosourced compounds. Alternatively, polar
insertion polynorbornene can also be used as cross-linker in the formulation of an epoxy resin, once again resulting in epoxy
resins with T higher than 300 °C and devoid of degradation at this temperature. Thus, this study clearly demonstrates the
g
viability of catalytic polymerization to access epoxy thermosets with ultrahigh Tg.
INTRODUCTION
and glycerol diglycidyl ether (GDE) are derived from reaction
of epichlorhydrin with butanediol and glycerol, which can all be
■
Thermosetting materials, also referred as thermosets, are 3D-
network molecules obtained by cross-linking multifunctional
monomers. They are widely used for a myriad of applications,
from the everyday convenience products (e.g., cements,
adhesives, protective coatings, automotive applications) to
high-tech applications in the aerospace and microelectronics
industries. Among these materials, epoxy resins occupy a
preponderant position because of the possibility of tuning their
mechanical and thermal properties by changing the precise
composition of the resin. These resins are constituted of a
reactive molecule containing epoxide functionalities (called the
epoxy monomer) and a cross-linker which can react with the
epoxy monomer to lead to the formation of a 3-D network.
Despite more than 50 years of epoxy chemistry, the number of
epoxy monomers commercially available is actually quite
limited. The majority of the monomers derive from diglycidyl
ether of bisphenol molecules such as bisphenol A (BPA), F, or
S or of oligomers of phenol−formaldehyde (Novolac). The
resulting resins have glass transition temperatures (T ) which
are usually lower than 250 °C. The T is a fundamental
characteristic of any thermoset, higher T ’s being actively
sought for a variety of high-performance applications. Out of
concern for an improved sustainability, and also sparked by the
fact that BPA is a known endocrine disruptor, the quest for
produced from the biomass. However, the T of the resulting
g
3
epoxy resins prepared with such monomers are low. Recently,
it has been shown that phenyl-based epoxy monomers can be
replaced by furan-based ones, leading to thermosets with T up
g
4
to 193 °C.
In order to achieve high T epoxy resins, it is necessary to use
g
rigid building blocks for which molecular motions are as
restricted as possible and to achieve high cross-link density
during the curing process. For example, rigid-rod cross-linkers
(
such as sulfanilamides) have been shown to yield epoxy resins
with T s which are higher than those cured with a flexible cross-
g
5
linker. However, the T s remain lower than 250 °C. Highly
g
rigid fluorinated polyimides end-capped by maleic anhydride
can even be used as cross-linker, leading to epoxy resins with
6
T ’s comprised between 250 and 300 °C. With the exception
g
of this polyimide example, to our knowledge, no epoxy resin
with a Tg as high as 300 °C has ever been reported.
Considering the difficulty and cost associated with the
1
g
7
preparation of fluorinated polyimides, an alternative route
g
based on simple and readily accessible synthons would be
highly preferable.
g
2
Received: December 10, 2015
Revised: January 14, 2016
novel epoxys has aroused the interest of numerous research
3
groups. For example, 1,4-butanediol diglycidyl ether (BDE)
©
XXXX American Chemical Society
A
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Our work is based on the realization that insertion
polynorbornene (PNBE) obtained by the catalytic polymer-
ization of readily available norbornene (NBE) leads to a rigid
amonium hydroxide (BTH), tetrahydrofuran (THF), dimethyl-
formamide (DMF), N-methyl-2-pyrrolidone (NMP), toluene, ethyl
acetate, methanol, and deuterated solvents for NMR were all
purchased from Sigma-Aldrich. Solvents and liquid monomers used
for polymerization were dried over molecular sieves and deoxygenated
by bubbling nitrogen.
8
,9
polymer with T as high as 350 °C. Substituted NBEs bearing
g
polar groups can be obtained in a single step by a simple Diels−
Alder reaction using fundamental organic molecules such as
dicyclopentadiene (DCPD), acrylic acid, or butadiene as
feedstocks (Figure 1). Once polymerized, they lead to the
Synthesis of Functionalized NBEs (Figure 1). Synthesis of
NBE(CO H). In a 50 mL round flask equipped with a condenser, acrylic
2
acid (10.9 g, 150 mmol, 2 equiv) and hydroquinone (165 mg, 1.5
mmol, 1 mol %) were heated under stirring until it began to boil (150
°
C). DCPD (10 g, 75 mmol, 1 equiv) was then added in a single
portion, and the mixture was stirred at 170 °C until the reflux stopped
1
and the reaction mixture turned brown (around 16 h). The H NMR
spectrum of this mixture showed 95% yield of 5-norbornene-2-
carboxylic acid (40% exo) which was purified by simple distillation to
yield a colorless liquid (bp = 153 °C at 5 mmHg).
Synthesis of Functionalized PolyNBEs (Figure 1). Synthesis of
PNBE(CO H) (Polymer 1). In a 250 mL round-bottom flask equipped
2
with a condenser and under a nitrogen flow, [PdCl(C H )] (26 mg,
3
5
2
7
1
1.06 μmol, 142.12 μmol of Pd, 1 equiv) and NBE(CO H) (40% exo,
00 g, 724.63 mmol, 5100 equiv) were mixed and heated at 70 °C
2
until the full solubilization of the [PdCl(C H )] yellow crystals. Then,
3
5
2
AgSbF (60 mg, 174.61 μmol, 1.23 equiv) were added under vigorous
6
Figure 1. Overview of synthesis of the functionalized polyNBEs.
stirring at 70 °C. The reaction was kept for 36 h at 70 °C. At the end
of the reaction, the flask contained a cracked solid. This solid was
collected and broken with a spatula to form a small powder. The solid
was purified using a Soxhlet apparatus with water as solvent until the
formation of highly functionalized PNBEs which can be used
for the preparation of ultrahigh T epoxy resins. Two routes
g
1
were explored in this report. In the first one (route A, Figure 2),
amount of residual monomer was below quantification by H NMR.
As an alternative to the Soxhlet extraction procedure, once the
polymerization reaction was performed, the cracked solid was washed
with ethyl acetate via vigorous stirring. When ethyl acetate is added,
the large pieces are broken into a thin powder which was filtered and
washed on ethyl acetate on a Buchner filter. The polymer was dried at
5
0 °C under vacuum overnight. Yield (46 g, 46%). M = 105 000 g/
n
mol, PDI = 1.7.
Monomer Recovery. The liquid solution recovered after the
filtration was concentrated on the rotary evaporator and was purified
by simple distillation to yield a colorless liquid (bp =153 °C at 5
mmHg). NBE(CO H) was collected to be used again. Yield: 44 g,
2
8
1%.
Synthesis of PNBE(Vinyl) (Polymer 2). In a 250 mL round-bottom
flask, tris(dibenzylideneacetone)dipalladium(0) (76 mg, 0.083 mmol,
.166 mmol of Pd, 1 equiv), AgSbF (68.2 mg, 0.198 mmol, 1.19
0
6
Figure 2. Overview of the synthesis of the thermosets.
equiv), and triphenylphosphine (43.5 mg, 0.166 mmol, 1 equiv) were
solubilized in 100 g of toluene at 70 °C. Then, 5-vinyl-2-norbornene
(
100 g, 832 mmol, 5000 equiv) was added under vigorous stirring and
poly(5-norbornene-2-carboxylic acid) (PNBE(CO H)) was
used as cross-linker of bio-based epoxy formulations. In the
second (route B, Figure 2), a substituted PNBE (PNBE-
2
heated at 70 °C for 72 h. A black viscous solution was obtained. The
polymer was precipitated with 600 mL of methanol and washed three
times with methanol. Then the gray powder was filtered and dried
(
epoxy)) was used as epoxy monomer, using simple and
under vacuum at room temperature overnight. Yield: 85 g, 85%. M =
n
inexpensive hardeners. In all cases, epoxy thermosets with Tg
comprised between 250 and 350 °C were obtained. Thus, we
have demonstrated that by using the catalytic polymerization of
17 400 g/mol, PDI = 2.8.
Synthesis of PNBE(Epoxy) (Polymer 3). Procedure A. In a 500 mL
round-bottom flask, polymer 2 (15 g, 0.10 mol, 1 equiv) was dissolved
in 120 mL of dichloromethane. Then successively acetic acid (4.7 g,
substituted NBEs, very high T epoxy thermosets can be
g
0
.08 mol, 0.8 equiv), formic acid (28 g, 0.61 mol, 6.1 equiv), and H O
prepared from widely available synthons, using a remarkably
simple synthetic route. We envision these novel ultrahigh Tg
epoxy will find application in high-power electronics, composite
fabrication, structural adhesive applications, and other
applications where thermosets have to endure elevated
temperatures.
2 2
(30% w:w in water) (74 g, 0.65 mol, 6.5 equiv) were added under
vigorous stirring and stirred for 24 h. During addition, the temperature
was kept at 0 °C by cooling with an external ice/water bath. During
reaction, the solution turned white, and a white supernatant foam was
formed. At the end of the reaction, the solution was precipitated into
acetone under vigorous stirring and then rapidly filtered over a filter
paper. The solid was collected and washed twice with acetone. Then
the white thin powder was dried under vacuum at room temperature
EXPERIMENTAL METHODS
■
Materials. Allylpalladium chloride dimer, tris(dibenzylidene-
acetone) dipalladium(0), silver hexafluoroantimonate, triphenyl-
phosphine, acrylic acid, DCPD, hydrogen peroxide, formic acid, acetic
acid, hydroquinone, butanediol diglycidyl ether (BDE), glycerol
diglycidyl ether (GDE), isophoronediamine (IPDA), glycerol, sebacic
acid, 5-vinyl-2-norbornene, 2,4,6 tris(dimethylaminomethyl)phenol
for 18 h. Yield: 85%. M = 19 600 g/mol, PDI = 2.1.
n
Procedure B. In a 50 mL round-bottom flask equipped with a
magnetic stirrer and a condenser, 1 g (6.7 mmol) of polymer 2 and 33
mg (0.13 mmol, 2 mol %) of MTO (methyltrioxorhenium(VII)) were
dissolved in 10 mL of CH Cl . To this solution were added dropwise
2
2
64 μL (0.8 mmol, 12 mol %) of pyridine followed by 4.1 mL (75
(
DMP 30), zinc nitrate hexahydrate (ZN), benzyl tetramethyl-
mmol, 6 equiv) of 30% w/w aqueous H O . During the addition the
2
2
B
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temperature was kept at 15 °C by cooling with an external cold water
bath. The mixture was stirred at room temperature for 42 h. Then, the
polymer was precipitated in acetone (100 mL) and washed three times
with acetone. The polymer was finally dried under vacuum overnight
at room temperature, leading to the formation of polymer 4 as a fine
white powder (75% yield).
Preparation of PolyNBEs Thermosets (Figure 2). Sample
Preparation. The thermosets were prepared in either a clean silicon
mold which was 2 in. in length, 1 in. in width, and 0.4 in. in height or
an aluminum mold depicted in Figure S1. The polymer and 90% of the
volume of the solvent were mixed together until a homogeneous
solution was obtained. Then, the solution was centrifuged (3500 rpm
for 10 min). In some cases, the dissolution of the polymer required
moderate heating, but once dissolved, the solution could be used at
room temperature without polymer precipitation. Then, the other
component (nonpolymeric cross-linker or epoxy monomer) and the
catalyst, dissolved in the remaining 10% of solvent, were added to the
polymer solution. The solution was then mixed for 4 min with a vortex
stirrer at room temperature. The solution was then immediately
poured into the mold, which was then placed into the oven. The
curing procedure was as indicated in the Table S1.
tensile test was performed on an Instron 5569 machine with the
crosshead speed of 1 mm/min.
Thermogravimetric Analysis. The samples were analyzed by TGA-
MS using a TA Q500 instrument coupled to a Discovery MS, with a
temperature range from 30 to 1000 °C at a 20 °C/min rate.
Swelling Experiments. A piece of the cross-linked polymer was
precisely weighted (Wdry, 150−350 mg) and was immersed in NMP at
room temperature for several days. The plastic piece was blotted dry
on filter paper and weighed to determine the wet mass Wwet. This step
was repeated every day to obtain the swelling evolution over time.
Wwet − Wdry
Wup (wt %) =
× 100
Wdry
RESULTS AND DISCUSSION
■
Our novel thermoset platform stems from the chemistry of
functional norbornenes (NBEs), which are derived from Diels−
Alder reactions between an essential C5 feedstock, DCPD, and
fundamental synthons such as acrylic acid or butadiene (Figure
10
11
Solvent-Free Preparation. Polymer 3 and hardener (IPDA, 6) were
manually mixed until a homogeneous viscous solution was obtained.
The resulting solution was poured into the mold (see Figure S1 or S2),
heated at 70 °C for 12 h and then at 150 °C for 4 h, and postcured at
1
). Both acrylic acid and butadiene can be prepared from
bio-based feedstocks if necessary. These Diels−Alder reactions
12
can be performed in the absence of organic solvent and lead
to the formation of the primarily endo substituted NBE (kinetic
isomer), which can be collected by simple distillation. In our
hands, the solvent-free reaction of acrylic acid and DCPD leads
to the formation of 5-norbornene-2-carboxylic acid, NBE-
190 °C for 4 h. Note on the curing rate: a very slow curing process was
deliberately chosen in order to prepare coatings which are free of
defects (such as cracks or bubbles), as shown in Figures S1 and S2.
Such procedure is necessary to ensure that mechanical measurements
be reproducible. Visually acceptable coupons are nonetheless obtained
at significantly faster curing rates such as 3 °C/min with 1 h plateau at
(
CO H), in ≥95% yield prior to distillation. As starting
2
materials for the novel thermoset platform, we used NBE-
8
(
0 °C and 1 h plateau at 110 °C before reaching the final temperature
180 °C).
Characterization. Nuclear Magnetic Resonance. The H NMR
(CO H) (endo:exo = 60:40) and 5-vinyl-2-norbornene (NBE-
2
(vinyl) endo:exo = 75:25). The 1,2-insertion polymerization of
NBE leads to a saturated polymer with extremely high Tg
1
spectrum was recorded on a Bruker Ultrashield 300 MHz
(
≥350 °C, too high to be measured accurately) and unique
1
spectrometer at ambient temperature. H chemical shifts were
8,13,14
optical and mechanical properties.
The polymerization of
referenced to the solvent signal.
substituted NBE’s was historically plagued by the lack of
reactivity of the endo isomer, but a novel rectification−insertion
polymerization mechanism was recently discovered whereby
the low reactivity endo isomer is converted in situ into the more
Gel Permeation Chromatography. The molecular weight dis-
tributions of the polymers were determined by gel permeation
chromatography (GPC) using a Viscotek instrument equipped with
one PL-Gel mixed A LS 20 μm column, one PL-Gel mixed B LS 10
μm column, and one polypore 5 μm column, a Wyatt DSP
refractometer, and a Wyatt Dawn light scattering detector. For the
PNBE(vinyl) and PNBE(epoxy) polymers, elution was performed in
THF at 40 °C, and all samples were analyzed using a dn/dc of 0.16 and
15
reactive exo isomer. Using this mechanism, the solvent-free
3
polymerization of NBE(CO H) catalyzed by (η -allyl)PdSbF
2
6
(0.01 mol % relative to monomer) leads to the formation of
PNBE(CO H), 1, in 45% yield. The polymerization stops
2
0
.20, respectively. For PNBE(CO H), elution was performed at 40 °C
because the polymer/monomer mixture reaches a gel point.
The polymer can be purified by Soxhlet extraction with water.
2
in DMF containing 1 g/L of LiCl (dn/dc = 0.04).
Differential Scanning Calorimetry. Differential scanning calorim-
etry measurements (DSC) of solid polymers were performed on a
DSC823e (TOPEM modulation) equipped with an FRS5 sample cell,
a sample robot, a Julabo FT400 intracooler, and an HRS7 sensor from
Mettler Toledo. Samples were heated from 50 to 300 °C at a rate of 10
The recycled NBE(CO H) is pure enough to be used for
2
another polymerization without any change in yield. Thus, the
preparation of polymer 1 from widely available starting
materials (DCPD and acrylic acid) did not require any organic
solvent.
°C/min, and data were analyzed with STAR software. The data
Known catalytic systems reported for the polymerization of
NBE(vinyl) have either very low reactivity or react with both of
the double bonds present on the monomer leading to an ill-
associated with the second heated ramp are shown.
Fourier Transform Infrared Spectroscopy. The solid polymers
FTIR spectra were recorded on a Nicolet 6700 Spectrometer equipped
with Smart ATR accessory (ThermoSci).
Dynamic Mechanical Thermal Analysis (DMTA). The DMTA
analysis were performed on a TA Q800 dynamic mechanical thermal
analysis using a single cantilever on a clamped sample over a
temperature range of 30−340 °C with a 5 °C/min heating ramp.
Samples of 17.6 × 14.5 × 2.0 mm were analyzed, using an aluminum
or silicon mold as described above (see Figure S1).
1
6,17
defined polymer structure.
The rectification−insertion
3
polymerization catalyst, (η -allyl)PdSbF , leads to the formation
6
of cross-linked polymers. We tested several other nickel- or
palladium-based catalysts which were found to react with the
pendant double bonds (see Supporting Information) either via
insertion or via chain transfer. Recently, Wakatsuki et al.
discovered that the catalyst Pd(dba) /PCy /CPh [B(C F ) ]
2
3
3
6 5 4
Thermomechanical Analysis (TMA). The TMA analysis was
performed on a TA Q400 instrument over a temperature range of
efficiently polymerizes NBE(vinyl) in the absence of side
18
reaction for the pendant double bond, leading to a very high
3
0−340 °C with a 5 °C/min ramp. Sample sizes were identical to
molecular weight polymer (M ≥ 200 000 g/mol). In solution,
those of DMTA.
n
Tensile Test. An aluminum mold was used as shown in Figure S2 to
this polymer is extremely viscous, rendering any postfunction-
prepare dogbones following testing standard of ASTM D638, and
alization step difficult unless a huge excess of organic solvent is
C
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Table 1. Main Characteristics of the Epoxy Materials Derived from Functional Polynorbornenes
c
d
E′ (MPa)
CTE (ppm/°C)
a
b
expt
mon
cross-linker
wt %
Tg (°C)
40 °C
250 °C
40 °C
250 °C
route A (PNBE(CO H) as cross-linker)
2
1
2
4
5
1
1
26−74
26−74
≥350
≥320
4100
2000
690
685
107
135
142
133
route B (PNBE(epoxy) as epoxy monomer)
3
4
5
6
7
8
3
3
3
3
3
3
7
7
8
6
6
9
92−8
240
235
1100
2400
1550
3130
2700
<100
<100
110
89
154
121
52
151
591
185
59
75−25
69−31
76−24
62−38
81−19
245
≥340
≥340
≥340
1240
900
75
18
3150
b
912
72
66
a
Weight compositions correspond to the relative weight of epoxy monomer to cross-linker. Measured as the peak of the tan δ peak in DMTA.
Storage modulus as measured in DMTA. Coefficient of thermal expansion as measured in TMA.
c
d
used. We discovered that the catalyst Pd (dba) :AgSbF :PPh
3
2
3
6
(1:1.2:1) leads to the formation of a low molecular weight
polymer, 2 (M = 12 000 g/mol, PDI = 2.2), which contains
n
one pendant double bond per inserted monomer, as measured
1
by H NMR. Using 0.01 or 0.02 mol % of the catalyst, the
polymer is obtained in 45% or 85% yield, respectively (Figure
1). Once again, the unreacted monomer can be collected by
distillation and reused for further polymerization. Epoxidation
of 2 can be accomplished using H O /formic acid/acetic acid in
2
2
yields as high as 85%. Alternatively, the epoxidation can also be
performed using H O as oxidizing agent with 2 mol % of
2
2
19,20
methyltrioxorhenium(VII) as catalyst in 75% isolated yield.
1
Characterization by H NMR and GPC indicates that more
than 95% of the pendant double bonds are epoxidized (Figure
S3) without any alteration of the molecular weight distribution
of the polymer (Figure S4).
An epoxy thermoset is obtained upon cross-linking of an
epoxy monomer containing at least two epoxide functionalities
with a cross-linker molecule. In this work, we found that
polymers 3 and 1 can serve as epoxy monomer and cross-linker,
respectively (Figure 2). Below, we refer to them as epoxy
monomers even though they are in fact multifunctional reacting
species. Upon thermally reacting polymer 1 with either GDE
(4) or BDE (5), two bio-based epoxy monomer, thermosets
with T ≥ 350 °C (Table 1) are obtained. Using DMTA, the
g
precise value of the T (corresponding to the maximum of tan δ
g
peak) cannot be determined (Figure 3) as the thermosets start
to degrade in air before T is attained (see below). The Tg
g
onsets (measured at the base of the tan δ peak) of the
thermosets prepared with 4 and 5 (Figure 2) are respectively
2
85 and 304 °C. These values are extremely high when
Figure 3. Storage modulus (A) and tan δ (B) of several thermosets
(Table 1) as measured by DMTA (50−340 °C).
compared to the T ’s of thermosets prepared by reaction of 4 or
g
5
with a bisphenol containing cross-linker, bisphenol A diamine
with the same ratio epoxy/hardener (68 and 12 °C,
respectively, Figure S26). The mechanical properties of the
thermosets measured by DMTA (Table 1 and Figure 3)
indicate a high storage modulus, even at 250 °C.
PNBE(epoxy) (3) as epoxy monomer can be cross-linked
with IPDA (6), glycerol (7), sebacic acid (8), and maleic
anhydride (9), which are all bio-based cross-linkers (Figure 2).
Importantly, polymer 3 is soluble in 6, thus precluding the need
of organic solvent to prepare the thermoset. At room
temperature, the mixture of 3 and 6 does not cross-link for
up to a week, as shown by the absence of viscosity increase.
Thus, it is possible to prepare solvent-free formulations based
on 3 and 6 with relatively long pot-lives. Again, once cured, the
thermosets exhibit very high T and excellent mechanical
properties (Table 1 and Figure 3). Comparative thermosets in
which polymer 3 is replaced by diglycidyl ether of bisphenol A,
g
DGEBA, exhibit T ’s which are all less than 100 °C (Figure
g
S26). Using IPDA and maleic anhydride as cross-linkers, the
peak of the tan δ curve is not attained at 350 °C (Figure 3B), a
temperature at which the sample starts to degrade (see below).
For experiments 1, 2, 6, and 7, a small tan δ peak (≤0.1) is
observed at ca. 120 °C. This subglass peak is due to the
presence of a beta transition, as confirmed by dielectric
spectroscopy (results not shown) and also by the fact that it
occurs while the storage modulus remains high (at T , the
g
storage modulus falls below 100 MPa). Therefore, the PNBE-
D
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based thermosets exhibit very high T ’s and storage moduli.
g
These remarkable thermomechanical properties can be ascribed
to the high rigidity of the bicyclic backbone with hindered
internal rotations as well as to the very high cross-link density
21,22
achieved with these multifunctional PNBEs (see Figure 4).
Figure 5. TGA spectra of several obtained thermosets (Table 1).
Analysis performed under nitrogen flow. All samples were heated to
2
00 °C prior analysis in order to dehydrate them.
determine the coefficient of thermal expansion (CTE). For
adhesive and coating applications, low values of CTE which
remain constant over a wide temperature range are desirable in
order to minimize the risk of stress cracking due to a CTE
mismatch between a low-CTE substrate and the thermoset.
Remarkably, the PNBE thermosets exhibit a constant and low
CTE values over a wide range of temperatures, as shown by a
linear TMA curve (Figure 6). The small inflections in the TMA
Figure 4. (A) FTIR spectra of top: BDE (5), middle: polymer 1,
bottom: thermoset sample 2 (Table 1). (B) FTIR spectra of top:
polymer 3, middle: IPDA (6), bottom: thermoset sample 6 (Table 1).
−1
Note the disappearance of the epoxide band at 874 cm (starred
peak).
The disappearance of the characteristic epoxide band at 874
−
1
cm , as observed by FTIR spectroscopy (Figure 4 for
characteristic examples and Supporting Information for each
formulation) is indicative of the complete conversion of the
epoxide ring upon reaction during curing. The thermosets are
also found to be impervious to swelling, as shown by less than
1
.5% swelling after 1 week of immersion in NMP whereas all
the starting materials are soluble in NMP. The disappearance of
the 874 cm− band and the absence of swelling are indicative of
the high cross-linking degree achieved in these samples.
1
The PNBE thermosets possess very high thermal stability.
With the exception of samples 1, 3, and 4, the onset of
degradation in air, defined at the point where the weight change
derivative dWtchange/dT is greater than 0.05%/°C, is comprised
between 340 and 360 °C (Figure 5). Thermosets 1, 3, and 4
have onsets of degradation above 280 °C. These samples are
prepared with monomers (GDE) or cross-linkers (glycerol)
which contain free hydroxyl (OH) groups. The ring-opening of
an epoxide by an hydroxyl group is known to be slow compared
to the reaction of an amine with an epoxide. Once it is ring-
opened, the epoxide is itself transformed into an alcohol, which
can again react with another epoxide. Thus, only a fraction of
the hydroxyl (OH) groups of glycerol initially introduced in the
formulation are consumed. Therefore, compared to the other
samples, thermosets prepared with glycerol (samples 3 and 4)
have a lower cross-link density and lower thermal stability, as
shown by lower mechanical properties and thermal stability
Figure 6. TMA analysis of five different thermosets (Table 1), with the
Tg onset indicated by a star.
curve (indicated by stars in Figure 6) correspond to the onset
of T , in agreement with the values measured by DMTA
g
(Figure 3). For glycerol-containing thermosets (experiment 3),
Tg is observed ca. 50 °C higher, resulting in high CTE values at
250 °C (Table 1). All other themosets have constant and low
CTE values in the range 50−275 °C, indicative of a remarkable
dimensional stability.
Thermosets 6 and 7 (Table 1) have been selected to perform
a tensile test analysis because of their high thermomechanical
properties. As measured by tensile testing, the Young’s moduli
of thermosets prepared in experiments 6 and 7 are 2.83 ± 0.18
and 3.21 ± 0.12 GPa, respectively (average of four dogbones).
The fracture in sample 6 is brittle, as shown by SEM analysis of
the broken sample (Figure S24), whereas the fracture of sample
7 is ductile (Figure S25). This behavior can be explained by the
higher amount of rigid PNBE in sample 6 compared to sample
7.
23
(
Figure 5). Nonetheless, these properties remain very high in
comparison to conventional epoxy thermosets.
The mechanical properties of the PNBE thermosets were
further examined by TMA analysis (Table 1), thus allowing to
E
DOI: 10.1021/acs.macromol.5b02648
Macromolecules XXXX, XXX, XXX−XXX

Macromolecules
Article
The thermosets based on either PNBE(CO H) or PNBE-
(10) Bozell, J. J.; Petersen, G. R. Green Chem. 2010, 12, 539−554.
2
(
11) Angelici, C.; Weckhuysen, B. M.; Bruijnincx, P. C. A.
(
epoxy) have to our knowledge the highest T ’s ever reported
g
ChemSusChem 2013, 6, 1595−1614.
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for epoxy thermosets. They also offer unique mechanical
properties, as illustrated by the low CTE at 250 °C and very
high storage moduli (as high as 4 GPa at room temperature for
experiment 1 and 1.2 GPa at 250 °C for sample 6). They also
contain as much as 38 wt % of IPDA (6) or 26 wt % of GDE
(
1
(
ization. In Polymer Science: A Comprehensive Reference, 10 Vol. Set;
Matyjaszewski, K., Moller, M., Eds.; Elsevier: Amsterdam, 2012; Vol. 3,
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A.; Yankelevich, D.; Knoesen, A. Macromolecules 2004, 37, 5163−
5178.
(
4) or BDE (5) which are all bio-based and renewable.
Furthermore, the synthesis of PNBE(CO H) from starting
2
materials (acrylic acid and DCPD) to isolated polymer did not
require any solvent and did not generate any waste. Thus, these
polymers offer a convincing proof of principle that green
chemistry principles can be used to design a bisphenol-free
epoxy platform with a set of properties which exceeds those of
conventional containing epoxys.
(15) Commarieu, B.; Claverie, J. P. Chem. Sci. 2015, 6, 2172−2181.
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(
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30, 1−9.
CONCLUSIONS
■
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(19) Herrmann, W. A.; Fischer, R. W.; Marz, D. W. Angew. Chem.,
To conclude, one of the most important challenges currently
facing synthetic polymer chemists is to design novel materials
with properties surpassing those of existing materials while
using synthetic routes which are acceptable in terms of
environmental, cost, and safety constraints as well as societal
expectations. In order to address this challenge, we
demonstrated here that catalytic PNBEs form a versatile
platform for the facile construction of epoxy resins with
unsurpassed thermomechanical properties.
Int. Ed. Engl. 1991, 30, 1638−1641.
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Chem. Soc. 1997, 119, 6189−6190.
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000, 10, 221−233.
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Chem. B 1998, 102, 9783−9790.
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Thermochim. Acta 2006, 447, 167−177.
(
(
2
(
(
ASSOCIATED CONTENT
Supporting Information
■
*
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.macro-
mol.5b02648.
Synthesis of polymers 1−3, of all thermosets, pictures,
and analytical details (NMR, FTIR, DMTA, TMA, AFM,
SEM) (PDF)
AUTHOR INFORMATION
Corresponding Author
Tel 1 (514) 987-3000, ext 6143; Fax 1 (514) 987-4054; e-mail
claverie.jerome@uqam.ca (J.P.C.).
■
*
Notes
The authors declare no competing financial interest.
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DOI: 10.1021/acs.macromol.5b02648
Macromolecules XXXX, XXX, XXX−XXX