A novel curing agent for phthalonitrile monomers: Curing behaviors and properties of the polymer network

Article abstract of DOI:10.1016/j.polymer.2016.01.006

A novel methyl tetrahydrophthalic anhydride end-capped imide compound (MODA) was firstly used to promote the curing reaction of phthalonitrile monomer 1,3-bis(3,4-dicyanophenoxy) (3BOCN). The curing behaviors of 3BOCN/MODA blend were studied by differential scanning calorimetry (DSC) and rheological analysis, which suggested that the blend possessed a wide processing window. IR spectra exhibited no triazine has been formed in the curing process and the nitrile group absorption peak almost disappeared after post-cured, the thermogravimetric analysis (TGA) and dynamic mechanical analysis (DMA) revealed that the cured phthalonitrile resins have excellent thermal properties and high glass transition temperature (T_g).

Full text of DOI:10.1016/j.polymer.2016.01.006

Accepted Manuscript
A Novel Curing Agent for Phthalonitrile Monomers: Curing Behaviors and Properties
of the Polymer Network
Suchun Ji, Ping Yuan, Jianghuai Hu, Rui Sun, Ke Zeng, Gang Yang
PII:
S0032-3861(16)30006-4
10.1016/j.polymer.2016.01.006
JPOL 18358
DOI:
Reference:
To appear in: Polymer
Received Date: 14 October 2015
Revised Date: 28 December 2015
Accepted Date: 3 January 2016
Please cite this article as: Ji S, Yuan P, Hu J, Sun R, Zeng K, Yang G, A Novel Curing Agent for
Phthalonitrile Monomers: Curing Behaviors and Properties of the Polymer Network, Polymer (2016), doi:
10.1016/j.polymer.2016.01.006.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
A Novel Curing Agent for Phthalonitrile Monomers: Curing
Behaviors and Properties of the Polymer Network
Suchun Ji, Ping Yuan, Jianghuai Hu, Rui Sun, Ke Zeng*, Gang Yang*
State Key Laboratory of Polymer Materials Engineering, College of Polymer Science and
Engineering, Sichuan University, Chengdu 610065, P.R. China
Correspondence to: G. Yang (E-mail: yanggang65420@163.com) or K. Zeng (E-mail:
zk_ican@sina.com)
ABSTRACT: A novel methyl tetrahydrophthalic anhydride end-capped imide
compound (MODA) was firstly used to promote the curing reaction of phthalonitrile
monomer 1,3-bis(3,4-dicyanophenoxy) (3BOCN). The curing behaviors of
3BOCN/MODA blend were studied by differential scanning calorimetry (DSC) and
rheological analysis, which suggested that the blend possessed a wide processing
window. IR spectra exhibited no triazine has been formed in the curing process and
the nitrile group absorption peak almost disappeared after post-cured, the
thermogravimetric analysis (TGA) and dynamic mechanical analysis (DMA) revealed
that the cured phthalonitrile resins have excellent thermal properties and high glass
transition temperature (Tg).
KEYWORD: curing agent; phthalonitrile resin; thermal properties
INTRODUCTION Phthalonitrile(PN) resins have been studied over the past 30 years
as a class of high temperature/performance polymers, these resins cure via an addition
reaction with no volatile byproducts evolved during the processing stage.[1] The PN
resins have many excellent properties such as high thermal and oxidative stability,
low water absorption, flame retardance, and the absence of a Tg before the thermal
decomposition temperatures.[2,3] These excellent high-temperature properties makes
the phthalonitrile resins attractive for many applications, such as composite
matrices,[4,5] adhesives,[6] electrical conductors,[7] and carbon precursors[8].
The cure of neat PN compounds is slow at temperatures in excess of 300°C which
limited their further applications. [2] The curing process can be promoted by curing
additives. To date, a lot of lewis acids/bases curing agents, such as metallic salts,
strong organic acids, strong organic acids/amine salts and active hydrogen-containing
heterocyclic structures have been developed and used.[9~18] Recently, our laboratory
and C.P. Reghunadhan Nair group reported the cooperative curing effect between
phthalonitrile and alkynyl simultaneously, and they believe that these studies may
offer a new type of resin systems.[19~21] These creative works inspired us to explore
the novel curing agents in addition to the traditional lewis acids/bases curing agents.
Unexpectedly, our lab found that methyl tetrahydrophthalic anhydride end-capped
imide compound (MODA) can also promote the curing reaction of phthalonitrile.
Unlike curable group alkynyl, the neat MODA could not be thermally polymerized,
but the polymerization of phthalonitrile/MODA system is a faster process comparing
with the phthalonitrile/alkynyl system. The respective homopolymerization of
3BOCN or MODA is extremely sluggish even at high temperatures, but the
polymerization rates can be accelerated significantly at elevated temperatures when
they were mixed together at a rational ratio, this phenomenon is defined as “Thermal
Synergistic Polymerization effect” (TSP effect). The TSP effect offer a breakthrough
and promise for seeking new routine to design phthalonitrile resins.
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In this study, the curing behaviors of MODA promoted 3BOCN were investigated
by DSC and rheological analysis, which suggested that the blend possessed a wide
processing window (~100°C). The thermal properties, sectional morphologies and
structures of 3BOCN/MODA system were investigated systematically. The results
showed that the cured resins possessed a void-free structures, high glass transition
temperatures and excellent thermal and thermal oxidation stabilities. Unexpectedly,
the nitrile group absorption of the polymer network was unobserved in IR spectra,
indicating the nitrile group almost consumed in the curing process, and no triazine has
been formed in the polymer network, the curing mechanism is different from the
traditional curing system with lewis acids/bases as agent.
EXPERIMENTAL
Materials
4-nitrophthalonitrile was purchased from Ji’nan Weido Chemica Co.Ltd. Resorcinol
was purchased from Cheng’du Kelong Chemica Co.Ltd. 4, 4’-Diaminodiphenyl ether
was purchased from Shan’dong Wanda Chemic Co.Ltd. The 4-methyl-1,2,3,6-
tetrahydrophthalic Anhydride was purchased from Pu’yang Huicheng electronic
material Co. Ltd. The potassium carbonate, Dimethyl sulfoxide (DMSO), aceticacid
and acetone were purchased from Cheng’du Kelong Chemica Co.Ltd. And all starting
materials were used without further purification.
Synthesis of 1, 3-bis(3,4-dicyanophenoxy)benzene 3BOCN
The phthalonitrile monomer 3BOCN was synthesized according to a reported
literature. [14].
1H NMR (300 MHz, DMSO-d6): δ 8.11-8.14（d, 2H, Ar-H）, 7.91-7.92（d, 2H, Ar-
H）, 7.60-7.63（dd, 1H, Ar-H）, 7.53-7.59（dd, 2H, Ar-H）,7.13-7.14 (s, 2H, Ar-
H), 7.12(s, 1H, Ar-H )。
Synthesis of curing agent MODA
49.84g (0.3mol) 4-Methyl-1,2,3,6-Tetrahydrophthalic Anhydride was dissolved in
320ml acetic acid, then 30.04g (0.15mol) 4,4’-Diaminodiphenyl ether was added in
the solution. The reaction was stirred for 5h at room temperature, and then at
refluxing temperature for 5h under an inert atmosphere. After cooling, the reaction
solution was poured into a large amount of water. The resulting white precipitate was
collected by suction filtration and washed with water until the filtrate was neutral,
dried at 70°C under reduced pressure for 8h. The crude product was chromatographed
on silica gel (CH Cl ) to give the MODA as a white solid. The yield was 67%.
2
2
1H NMR (300 MHz, CDCl3-d6): δ 7.186-7.215(d, 4H, Ar-H), 7.068-7.098(d,4H, Ar-
H), 5.613(s, 2H, =C-H), 3.180-3.203(m, 4H, C H )，2.557-2.686(m, 4H, -CH2-),
2.243-.2343(m, 4H, -CH2-), 1.770(s, 6H, -CH3).
Preparation of the 3BOCN/MODA blend
All 3BOCN/MODA blends (molar ratio 5:5, 6:4; 7:3; 8:2; 9:1) were prepared by
dissolving the compounds in acetone. The mixture was dissolved in acetone at a ratio
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of 25g: 100ml, and the solution was stirred for 24h at room temperature to obtain a
homogeneous mixture. Then the solution mixture vacuum-distilled to remove the
residual solvent, and subsequently grounded into fine powder.
Preparation of phthalonitrile polymer network
Resin1 was fabricated by degassing 3BOCN/MODA blend in a flask(-0.04MPa) at
200°C for 30min with stirring, and then the melted 3BOCN/MODA mixture was
poured into the preheated mold, placed in an oven, and heated under N2 at 200°C for
6h, 230°Cfor 3h. After cooling, the samples were removed from mold, free-standing
samples were cured under an inert atmosphere of argon at elevated temperatures of
270°C for 6h, 310°C for 6h, 330°C for 3h, 350°Cfor 3h, 375°C for 6h. The samples
were sanded to 30×12.5×3.2 for DMA measurements.
Characterization
1H NMR (300 MHz) was obtained with a Bruker Avance-300 NMR spectrometer
with DMSO-d6 or CDCl3 as the solvent and tetramethylsilane (TMS) as the internal
standard. Differential scanning calorimetry (DSC) was performed on a TA instrument
Q200 differential scanning calorimeter under nitrogen atmosphere (50ml·min-1) at a
heating rate of 10°C·min-1. Rheological experiments were conducted on a TA
instruments AR-2000 rheometer in conjunction with an environmental testing
chamber for temperature control. The measurements were made using a 25 mm
diameter parallel plates at low strain values (2.5×10-4) and a frequency of 1 Hz. The
complex viscosity as a function of temperature was measured from 30 to 375°C at a
heating of 5°C·min-1 in air. The complex viscosity of 3BOCN/MODA blend was also
monitored at 230, 250 and 270°C in air as a function of time. Thermo gravimetric
analysis (TGA) was performed on a TA Instruments Q500 at a heating rate of
10°C·min-1 under a nitrogen or air purge of 60 ml·min-1. Dynamic mechanical
measurements (DMA) was performed on a TA Instruments Q800 in 3-Pointing
Bending mode to measure the storage modulus and damping factor (tanδ) of the
material in nitrogen atmosphere from 50°C to 480°C with a heating rate of 5°C·min-1,
a frequency of 1Hz and a strain of 2.5×10-2%. The morphology of the polymer was
determined by a FEI Quanta250 field-emission scanning electron microscope (SEM).
The specimen was coated with Au prior to observation.
RESULT AND DISCUSSION
Monomer Synthesis
The T5% (95% weight retention in TGA measurement) of methyl tetrahydrophthalic
anhydride is 141°C, which is lower than the curing temperature of PN compounds. To
solve this problem, the Methyl tetrahydrophthalic anhydride end-capped imide
compound MODA (Scheme 1), with relatively lower volatility at high temperature
due to its higher molecular weight, was designed and readily synthesized via a typical
reaction between anhydride and amine. The melting temperature of MODA is 162°C
and the T5% is 362°C. The phthalonitrile monomer 3BOCN was synthesized in high
yield by a simple nucleophilic displacement of a labile nitro-substituent from 4-
nitrophthalonitrile by the dialkaline salt of resorcinol in a dipolar aprotic solvent with
the presence of either weak or strong bases. The synthesis of 3BOCN has been
previously reported in the literature and the structures were unambiguously confirmed
1
by H NMR spectra, which agree well with those of the authentic compounds. The
rheological measurements were conducted on 3BOCN/MODA blends (molar ratio 5:5,
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6:4; 7:3; 8:2; 9:1) to determine the optimum molar concentration (Figure S1). The gel
time is shown as a function of MODA concentration in Figure S2 (The gelation is
determined from the crossover point of the storage modulus (G’) and the loss modulus
(G’’)). Since the concentration of MODA increases from 9:1 to 7:3, the gel time
decreases rapidly, while continue to decrease with unobvious changes at the
concentration of MODA from 7:3 to 5:5. A 7:3 molar ratio was used depend on the
gel time to study curing behaviors of the blend and prepare the phthalonitrile polymer
network.
Scheme 1 Synthesis of monomers and curing of 3BOCN/MODA blend
Polymerization Behaviors of 3BOCN/MODA blend
Figure 1 TGA (a) and DSC (b) curves of 3BOCN, MODA and 3BOCN/MODA
blend
Figure 1(a) show the TGA curves of 3BOCN, MODA and the 3BOCN/MODA
blend at the heating rate of 10°C·min-1 in nitrogen. The 3BOCN/MODA blend
showed a 10% weight loss between 275 and 400°C, and the observed weight loss at
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around 400°C slowed down, suggesting that the polymerization was also progressing
which is in agreement with the DSC results(Figure 1(b)). The residual weight
retention at 800°C of 3BOCN/MODA blend is 61%, which is significantly higher
than that of 3BOCN and MODA, indicating the 3BOCN/MODA blend proceeded
curing reaction during the heating run.
Figure 1(b) shows the DSC curves of 3BOCN, MODA and the 3BOCN/MODA
blend at the heating rate of 10°C·min-1 in nitrogen. The 3BOCN and MODA
exhibited an endothermic peak at approximately 185°C and 162°C corresponding to
the melting transition respectively. The 3BOCN/MODA blend exhibited two
endothermic peaks at 168°C and 145°C, which are attributed to the melting transition
of 3BOCN and MODA, respectively. Neither curve of pure 3BOCN or MODA
showed any exothermic peak at elevated temperature indicating that the curing
process of pure 3BOCN and MODA was extremely sluggish and showed very low
polymerization exothermic effect. The 3BOCN/MODA blend shows an exothermic
peak with a total enthalpy of about 309J/g, which could be mainly attributed to the
curing reaction of the blend, though with little thermal weight loss of the 3BOCN or
MODA in TGA curve, indicating the MODA could promote the curing reaction of
phthalonitrile compound.
Figure 2 Melt viscosity vs.
temperature curve for 3BOCN/MODA
blend
Figure 3 Melt viscosity vs. time curve
for 3BOCN/MODA blend at various
temperature
The polymerization behaviors of 3BOCN/MODA blend were studied by rheological
measurements. In Figure 2, the melt viscosity of 3BOCN/MODA blend was
determined as a function of temperature from 30 to 375°C. In this curve, a rapid
decrease was observed in viscosity on melting around 150°C and a minimum melt
viscosity of 0.01-0.1Pa·s was observed for the blend. Above 300°C, a melt viscosity
increase for the 3BOCN/MODA blend was observed, which indicated that the
polymerization was progressing which is in agreement with the DSC results. And the
low melt viscosity of the blend exhibited their good melt stability at high temperatures
which was evidence of their good processability. The Figure 2 also revealed that there
is a large process window (~100°C) between the melting point of the blend and the
polymerization temperature, which is helpful in the fabrication of large size composite
sections.
Rheometric studies on 3BOCN/MODA blend were conducted to monitor the
viscosity changes accompany the curing reaction. Viscosity increase as a function of
reaction time for the polymerization of 3BOCN/MODA blend at 230°C, 250°C and
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270°C are shown in Figure 3. Abrupt viscosity increases were observed which
indicated that the polymerization was progressing. From the plots it is evident that the
low melt viscosities of the blend can be kept for a very long time. The blend did not
show a distinctive increase in viscosity within 10min at 270°C, 25min at 250°C and
70min at 230°C, respectively. And as expected, the higher the temperature, the faster
the rate of viscosity increased.
IR and SEM
In order to observe the polymerization reaction, IR measurement was conducted on
samples cured at elevated temperatures, as shown in Figure 4. Nitrile group
absorption peak was at around 2230cm-1 for 3BOCN compound, which obviously
diminished with increasing extent of polymerization and no nitrile group absorption
peak was observed after cured at 375°C. New absorption peaks at 3290cm-1
correspond to N-H stretching vibration were observed in the early cure stage and
decrease in intensity with increasing extent of polymerization (see Figure S3), new
peak at 1010cm-1 attributed to phthalocyanine ring absorption was observed,
indicating the formation of phthalocyanine during the polymerization process. Typical
isoindoline peaks at 878cm-1 was observed and 1720, 1503cm-1 were overlapped by
strong bands in the polymers. However, no peaks can be assigned to characteristic
absorptions representative of triazine (1520 and 1360cm-1) formations, this may
further explain the nitrile group absorption peak almost disappeared after post-cured
of the 3BOCN/MODA system, as shown in Scheme 2. As we know many
reports[1,13,19,23,24] have revealed that the complete curing of the nitrile groups is
practically difficult even after heating for long duration at elevated temperatures due
to the steric hindrance of the left nitrile groups at the ortho-position of triazine ring.
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Figure 4 IR spectra of 3BOCN/MODA blend and polymers cured at elevated
temperatures
(o) 3BOCN/MODA blend
(a)200°C for 6h, 230°C for 3h;
(b) 200°C for 6h, 230°C for 3h, 270°C for 3h, 310°C for 6h;
(c) 200°C for 6h, 230°C for 3h, 270°C for 3h, 310°C for 6h, 330°C for 3h, 350°C for 3h, 375°C for 6h
Scheme 2 Curing mechanism of the system
The surface morphology of the Resin1 was investigated by SEM under
magnifications of 500 times (a), and 10000 times (b) (Figure 5). As a result, no void
can be observed from the pictures which proves the void-free structure of Resin1 and
it also guarantees the excellent thermal and mechanical properties. Moreover, it is
noted that the resin exhibits a homogeneous continuous phase, indicating the initial
3BOCN/MODA blend is homogeneous and no phase separation in the currently
tested size range occurred during the polymerization process.
Figure 5 SEM image of Resin1
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Thermal and Dynamic Mechanical Properties
Figure 6 TGA and DTG curves of Resin1 in N2(a) and air(b)
The thermal and thermo-oxidative stability of Resin1 was evaluated by TGA in air
and nitrogen atmosphere (Figure 6). The Resin1 exhibited excellent thermal property
with 72% weight retention approximately at 800°C in nitrogen atmosphere, and it
showed the weight residual of 99%(T1%), 95%(T5%), 90%(T10%) at 475.1°C, 517.7°C
and 542.9°C respectively. It also exhibited outstanding stability in air atmosphere as
show in this figure, and it showed the T1%, T5%, T10% at 460.7°C, 513.1°C and
536.5°C respectively. The corresponding DTG curves of the Resin1 in nitrogen and
air atmosphere are also displayed in Figure 4. The maximum decomposition
temperature (Tdmax) appears at 534°C in nitrogen atmosphere and 619°C in air. The
phthalonitrile polymer network that formed from 3BOCN cured in the presence of
amine as curing agent, showed oxidative stability to about 450°C with catastrophic
decomposition occurring between 500 and 700°C.[14] The Resin1 has a comparable
thermal and thermo-oxidative stability with phthalonitrile polymer that cured with
amine curing agent. The data indicate a large ratio of MODA did not compromise the
thermal and thermo-oxidative stability of the phthalonitrile polymer network.
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Figure 7 Dynamic mechanical properties of Resin1
Dynamic mechanical analysis (DMA) was performed on solid samples of Resin1
obtained by curing 3BOCN/MODA blend under inert atmosphere. We observed
(Figure 7) that the storage modulus for Resin1 gradually decreased from 50°C to
480°C indicating some flexibility and mobility within the polymer network. However,
this mobility dose not result in a complete loss of mechanical stability. Upon heating
from 50 to 400°C, the storage modulus for Resin1 changed from 4303MPa to
2180MPa, this is evidence that the polymers retained very good mechanical properties
at elevated temperatures. Continued heating to 480°C resulted in an abrupt decrease in
storage modulus, meanwhile the tanδ started to rise but not peaking as the temperature
approached 480°C indicating a slight softening of the sample.
CONCLUSION
A novel curing agent (MODA) which is different from traditional lewis acids/bases
agents was designed and used to promote the curing reaction of 3BOCN. The mixture
of 3BOCN and MODA can be thermally polymerized to yield a void-free resin. The
nitrile group absorption peak in IR spectra almost disappeared and no triazine has
been formed in the curing process. These results will contribute to develop a new type
of curing agents for phthalonitrile compounds. The prepared phthalonitrile polymer
network Resin1 exhibits excellent thermal stability together with high Tg and storage
modulus. And the 3BOCN/MODA system shows a lower melting point (168°C) and a
large processing window (~100°C), which makes the system is favorable to
processing into complex composite structures by RTM or RIM.
ACKNOWLEDGMENTS
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The authors sincerely thank to the National Natural Science Foundation of China (No.
51203098) for the financial support.
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·A novel curing agent was designed and synthesized for phthalonitrile monomer.
·The new phthalonitrile/curing agent system has great process ability and the cured
polymer has excellent thermal properties.
·No triazine absorption peak was observed in IR spectra of the polymer network.
·Phthalocyanine and isoindoline were formed during the curing process.