Preparation and characterization of novel naphthyl epoxy resin containing 4-fluorobenzoyl side chains for low-k dielectrics application

Article abstract of DOI:10.1039/c7ra09941j

A new type of epoxy monomer 1,5-bis(4-fluorobenzoyl)-2,6-diglycidyl ether naphthalene (DGENF) was obtained through a three-step procedure involving Friedel-Crafts acylation, demethylation, and followed by nucleophilic reaction. The chemical structures wer

Full text of DOI:10.1039/c7ra09941j

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Preparation and characterization of novel naphthyl
epoxy resin containing 4-fluorobenzoyl side chains
for low-k dielectrics application†
Cite this: RSC Adv., 2017, 7, 53970
*
Tianyi Na, Hao Jiang, Liang Zhao and Chengji Zhao
A new type of epoxy monomer 1,5-bis(4-fluorobenzoyl)-2,6-diglycidyl ether naphthalene (DGENF) was
obtained through a three-step procedure involving Friedel–Crafts acylation, demethylation, and followed
by nucleophilic reaction. The chemical structures were confirmed by 1HNMR, 19FNMR and FT-IR. After
curing with methylhexahydrophthalic anhydride (MeHHPA), the properties of DGENF epoxy resin were
measured and compared with three other kinds of commercial epoxy resins. As a result, DGENF
exhibited excellent thermal stability, hydrophobic and dielectric properties. For example, DGENF had
a higher glass transition temperature of 170 ꢀC than the other three commercial epoxy resins. DGENF
showed a higher contact angle of 116ꢀ, which could satisfy the standard of hydrophobic materials. In
addition, DGENF showed significantly lower dielectric constant (2.97 at 1 MHz) and dielectric loss
(0.0188 at 1 MHz) than those of the other commercial epoxy resins because of the introduction of
fluorine on the side chains, which improved the electronegativity of the epoxy resin and reduced the
polarizability of molecules efficiently. Herein, we believe the novel naphthyl epoxy resin (DGENF) has
demonstrated potential application in electronic industries.
Received 6th September 2017
Accepted 15th November 2017
DOI: 10.1039/c7ra09941j
rsc.li/rsc-advances
According to reports, there are various methods to achieve
low dielectric constant materials, such as the incorporation of
1. Introduction
uorinate groups and stiff non-polar structures, as well as the
introduction of controlled porosity in polymer.21,22 Further-
more, the existence of uorinate and stiff non-polar structure
can also decrease moisture absorption and improve the thermal
stability of materials.23 Sasaki24 reported that the bisphenol A
type epoxy resin cured with uorinated phthalic anhydride had
the lowest dielectric constant (2.77) and dielectric loss factor
(0.01) among all cured epoxy resins. Meanwhile, the water
absorption of this uorinated epoxy resins could be reduced by
75%. Tao and coworkers25 also reported uorinated epoxy
resins with reduced moisture absorption and dielectric
constant. They had lower dielectric constant (3.3) and water
uptake (0.38%) than diglycidyl ether of bisphenol A (DGEBA)
cured with the same curing agent. Xu et al.26 synthesized a series
of novel epoxy resins bearing naphthyl and limonene moieties,
which showed a remarkably higher Tg (171 ꢀC), lower coefficient
(36 ppm ꢀCꢁ1) of thermal expansion, better moisture resistance
(0.13%) and lower dielectric property (3.7). All of the properties
were better than DGEBA cured with the same curing agent.
Therefore, we believe that the introduction of uorinate groups
and stiff non-polar structure into polymer is a promising
method for developing low dielectric materials with improved
thermal stability and excellent moisture resistance.
With the rapid development of micro-electronics industries and
the continuing miniaturization in dimensions of electronic
devices, there is a demand for electronic materials to possess
more functions to satisfy the requirements for optimal perfor-
mance and lifetime of electronic components. These functions
mainly include electric insulation, mechanical support and
overcoming the propagation delay caused by interconnection.
Therefore, advanced electronic materials are required to
possess better mechanical properties, good thermal stability,
excellent chemical and corrosion resistance as well as lower
dielectric properties.1–9
As one of the most important electrical materials, epoxy
resins have been widely used in electrical coatings, electronic
packaging, electrical substrates, adhesives of chips and encap-
sulation.10–19 However, conventional epoxy resins do not have
adequate electrical performance to satisfy the requirements of
advanced microelectronic materials, especially dielectric prop-
erties.20 Therefore, the development of novel epoxy resins pos-
sessing lower dielectric constants and loss (low-k) has attracted
more and more attention of researchers around the world.
College of Chemistry, Jilin University, Changchun 130012, P. R. China. E-mail:
zhaochengji@jlu.edu.cn; Fax: +86-431-85168870; Tel: +86-431-85168870
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c7ra09941j
In this work, we synthesized a novel naphthyl epoxy mono-
mer, 1,5-bis(4-uorobenzoyl)-2,6-diglycidyl ether naphthalene
(DGENF) containing 4-uorobenzoyl side chains and stiff
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non-polar naphthalene moieties simultaneously. Aer the
addition of methylhexahydrophthalic anhydride (MeHHPA) as
the curing agent and 2-methylimidazolate (2-MI) as the accel-
erator with a stoichiometric amount, the cured epoxy resins
with low dielectric properties and improved thermal stabilities
were obtained. Additionally, we prepared other three kinds of
cured commercial epoxy resins in the same method and char-
acterized their properties for comparison, including bisphenol
A epoxy resin (BPA-EP), bisphenol F epoxy resin (BPF-EP)
and 4,40-diglycidyl(3,30,5,50-tetramethylbiphenyl) epoxy resin
(TMBP).
2. Experimental
2.1 Materials
2,6-Dimethoxynaphthalene (99.0%), 4-uorobenzoyl chloride
(98.0%), boron tribromide (BBr3, 99.99%) and ferric chloride
(FeCl3, 97.5%) were purchased from Sigma-Aldrich Chemical
Co., Ltd. Epichlorohydrin (ECH, 98.0%), MeHHPA (98.0%), 2-MI
(99.0%), NaOH, tetrabutylammonium bromide (purity 98.0%)
were obtained from Sinopharm Chemical Reagent Co., Ltd.
BPA-EP (KDS-8128 EEW ¼ 172 g per eq.) was purchased by
Kukdo Chemical (Kunshan) Co., Ltd and BPF-EP (NPEF-170
EEW ¼ 170 g per eq.) were obtained by Nanya plastics
(Taiwan) Co., Ltd. TMBP was prepared according to our method
reported previously.27 All other solvents were obtained
commercially and used without further purication.
Scheme
monomers.
1
The synthesis routes of DMNF, DHNF and DGENF
2.2 Synthesis of 1,5-bis(4-uorobenzoyl)-2,6-dimethoxy-
naphthalene (DMNF)
The synthetic procedure of DMNF is described in Scheme 1. 2,6-
Dimethoxynaphthalene (9.4 g, 0.05 mol) and 4-uorobenzoyl
chloride (17.4 g, 0.11 mol) were dissolved in chloroform
(CHCl3). Then anhydrous ferric chloride (1.65 g, 0.01 mol) was
added to the mixture under 5 ꢀC and the reaction was then kept
at ambient temperature for 24 h. The resulting mixture was
poured into hydrochloric acid. The liquid phase was removed by
decantation and the brown solid was precipitated in methanol.
The crude product was then recrystallized from dime-
thylformamide (yield: 78%).
0.20 mol) and tetrabutylammonium bromide (0.096 g,
0.3 mmol) were stirred in a three-neck ask at 90 C for 6 h.
ꢀ
Aer cooling to room temperature, the excess ECH was removed
at a reduced pressure. Subsequently, 30 mL toluene and
2.88 g NaOH (50 wt% aqueous solution) were added and the
mixture was kept at 90 ꢀC for another 3 h. Then the mixture was
washed and distilled under reduced pressure. Aer drying in
ꢀ
a vacuum oven at 60 C for 12 h, the white crystalline product
DGENF was nally obtained(yield: 88%).
2.3 Synthesis of 1,5-bis(4-uorobenzoyl)-2,6-
dihydroxynaphthalene (DHNF)
2.5 Preparation of cured epoxy resin samples
As shown in Scheme 2, a stoichiometric amount of MeHHPA
was added into DGENF, BPA-EP, BPF-EP and TMBP, respec-
tively. The mixtures were stirred continuously for 1 h, and 2-MI
was then added into the mixtures as the curing accelerator.
Aer stirred for 30 min, the mixtures were put into the vacuum
oven in order to remove the entrapped air. Finally, the mixtures
DMNF (12.975 g, 0.03 mol) obtained from above procedure was
added into 300 mL CHCl3 and mixed under 5 ꢀC (ice bath). Then
1 mol Lꢁ1 CHCl3 solution of BBr3 (300 mL) was added into the
mixture dropwise during 2 h. The mixture was stirred at room
temperature for 48 h before pouring into ice-water to absorb
excess BBr3, and then washed with deionized water. The
resulting yellow powder (DHNF) was dried under vacuum at
ꢀ
were poured into a polish mould and cured at 120 C for 2 h,
ꢀ
ꢀ
160 C for 2 h and 200 C for 2 h to obtain the epoxy resins.
ꢀ
60 C for 24 h (Scheme 1) (yield: 80%).
2.4 Synthesis of epoxy monomer 1,5-bis(4-uorobenzoyl)-
2.6 Characterization
1H-NMR spectra were measured on a 500 MHz Bruker Avance
2,6-diglycidyl ether naphthalene (DGENF)
The synthesis route of the epoxy monomer DGENF is illustrated 510 spectrometer at 298 K with DMSO-d6 as the solvent and
in Scheme 1. A mixture of DHNF (4.05 g, 0.01 mol), ECH (16 mL, tetramethylsilane as the standard.
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Fig. 1 1H-NMR spectra of DMNF and DHNF monomers.
Scheme 2 Procedure of preparing epoxy resins.
signal of –OCH3 at 3.7 ppm disappeared completely in the
spectrum of DHNF (Fig. 1b). Instead, the characteristic peak for
the proton of –OH can be observed at 10.0 ppm. This variation
indicated that methoxyl groups in DMNF were completely
converted into hydroxyl groups in DHNF.
FT-IR spectra were obtained with a Nicolet Impact 410
Fourier transform spectrometer using KBr pellets.
Dynamic mechanical analysis (DMA) was performed on
a DMA Q800 analyzer (TA Instruments, New Castle, DE, USA)
using a single cantilever clamp over a temperature range of
40–200 ꢀC at a heating rate of 3 ꢀC minꢁ1 with a constant
frequency of 1 Hz.
1
Fig. 2 shows the H-NMR spectrum of the epoxy monomer
DGENF. Compared to DHNF, the proton peak of –OH at
10.0 ppm disappeared, while the proton peaks (He and Hg) for
the methylene group (–CH2–) appeared at 4.30 ppm, 3.98 ppm,
2.62 ppm and 2.48 ppm due to the incorporation of epoxy
groups. The results indicated that the hydroxyl groups were
converted to epoxy groups thoroughly by reacting with ECH.
Fig. 3 shows the 19F NMR of DGENF. The only one peak at
ꢁ105 ppm corresponds to the terminal uorine atom on 4-u-
orobenzoyl side chains. Therefore, we could conrm that
4-uorobenzoyl side chains exist in the structure of our target
DGENF product.
Thermogravimetric analysis (TGA) was conducted on a Pyris
1 TGA analyzer (Perkin-Elmer, USA). The samples were heated
ꢁ1
ꢀ
ꢀ
ꢀ
from 100 C to 700 C at a rate of 10 C min under nitrogen
atmosphere. Before the heating scan, all the samples were pre-
dried under N2 at 100 ꢀC for 10 min to remove the residual water
and solvent thoroughly.
Dielectric constants of the samples were measured using
a Hewlett-Packard 4294A apparatus at room temperature over
a frequency range of 0–106 Hz.
Contact angle measurements were carried out on a JC2000C2
contact angle goniometer (Shanghai Zhongchen Powereach
Company, China) by the sessile drop method with a micro
syringe at room temperature.
3.2 FT-IR spectra of cured epoxy resins
DGENF epoxy resin was prepared using MeHHPA as the curing
agent and 2-MI as the accelerator. Another three kinds of cured
commercial epoxy resins including BPA-EP, BPF-EP and TMBP
were also prepared at the same condition for comparison. Fig. 4
3. Results and discussion
3.1 Synthesis and characterization of monomers
As depicted in Scheme 1, the epoxy monomer DGENF was
synthesized via a three-step synthetic procedure. Firstly, the
monomer DMNF was synthesized by a Friedel–Cras acylation
reaction between 2,6-dimethoxynaphthalene and 4-uo-
robenzoyl chloride using anhydrous ferric chloride as the
catalyst. Then, DMNF containing two methoxyl groups was
demethylated to DHNF bearing hydroxyl groups by using BBr3
solution. Finally, DGENF containing epoxy groups was obtained
by a nucleophilic reaction between DHNF and ECH under the
catalyst of NaOH. The structure of DMNF and DHNF interme-
diates as well as the epoxy monomer DGENF was conrmed by
1
their H NMR spectra in DMSO-d6. As displayed in Fig. 1, the
proton peak at 3.7 ppm was assigned to the protons on –OCH3
groups of DMNF (Fig. 1a). Aer the demethylation reaction, the
Fig. 2 1H-NMR spectra of epoxy monomer DGENF.
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Fig. 3 19F NMR of the epoxy monomer DGENF.
Fig. 5 Storage modulus curves of cured epoxy resin samples.
TMBP had a highest storage modulus (2375 MPa) among all
epoxy resin samples within the experimental temperature range
due to the biphenyl structure in TMBP, which improved the
stiffness of the molecule chains and enhanced the mechanical
strength. Compared to BPA-EP and BPF-EP, DGENF possessed
a higher storage modulus (1976 MPa) at 40 ꢀC and showed
a slower decreasing rate with the temperature increasing. This
result was ascribed to the naphthyl group with sufficient stiff-
ness in DGENF, which also provided good thermal stability for
the cured epoxy resin. Although DGENF possessed rigid naph-
thyl group, the existence of 4-uorobenzoyl side chains
decreased the cross-linking density of the cured epoxy resin,
thus resulting in a relatively lower storage modulus than TMBP.
As shown in Fig. 6, the Tg of DGENF resin samples derived
from tan dmax is 170 ꢀC, which is much higher than those of
other three kinds of epoxy resin materials (Table 1). The result
can be explained by the existence of stiff naphthy group in
DGENF, thus improving the rigid nature of polymer chains.
Meanwhile, DGENF resin shows a relatively broad tan delta
peak, which is indicative of the chain branching within the
epoxy resin due to the introduction of 4-uorobenzoyl side
chains. DMA results indicated that this novel naphthyl epoxy
Fig. 4 FT-IR spectra of epoxy monomer DGENF and cured epoxy
resins.
shows the FT-IR spectra of the epoxy monomer DGENF and four
kinds of cured epoxy resins. For the DGENF monomer, the
absorption bands observed from 1500 cmꢁ1 to 1650 cmꢁ1 can
be attributed to the skeleton vibration of naphthalene rings. A
sharp and strong characteristic band at 1740 cmꢁ1 was assigned
to the C]O absorption on the side chains. The characteristic
absorption peak of oxirane ring vibration was observed at
912 cmꢁ1, which demonstrated that the epoxy group had been
successfully introduced into DGENF monomer. In order to
conrm that the curing reaction was completed, we also used
FT-IR to characterize the cured epoxy samples. As shown in
Fig. 4, the characteristic peak of epoxy group (912 cmꢁ1) dis-
appeared aer curing with MeHHPA for all the samples. It is
believed that the anhydride group of MeHHPA reacted with
epoxy ring thoroughly to form ester groups in the presence of
cured regent and accelerator at high temperature.
3.3 Mechanical and thermal properties
The mechanical property is essential for epoxy materials for
applications in electronic eld. In this study, we used DMA to
investigate the mechanical properties and glass transition
temperature (Tg) of the cured epoxy resins. As shown in Fig. 5, Fig. 6 Tan delta curves of cured epoxy resin samples.
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Table 1 TGA, DMA and dielectric properties of cured epoxy resins
Char yield
[%]
Dielectric properties
at 1 MHz
TGA (ꢀC)
DMA
Storage modulus
(MPa)
Samples
T5%
T10%
T50%
At 700 ꢀ
C
Tg (ꢀC)
Dk
Df
BPA-EP
BPF-EP
TMBP
334
334
313
288
384
381
340
313
453
446
450
532
14.5
26.8
7.7
1852
1800
2375
1976
129
139
150
170
3.81
3.95
3.77
2.97
0.0219
0.0205
0.0244
0.0188
DGENF
34.5
material could satisfy the demand of thermal and mechanical cured epoxy resins at high temperatures efficiently and delays
stabilities to be used as the electronic materials.
the degradation process of the material.
Fig. 7 shows the TGA curves of the cured epoxy resin samples
measured under N2 atmospheres. The relevant thermal
decomposition data, including the decomposition temperature
of 5% weight loss (T5%), 10% weight loss (T10%) and 50% weight
loss (T50%), and the char residues at 700 ꢀC are listed in Table 1.
It was obviously found that DGENF exhibited a lower 5% weight
3.4 Contact angle and dielectric properties
Water absorption as one of the important property of materials
not only greatly impair the mechanical and thermal properties
of epoxy resins, but also increase relative dielectric constant
even at small quantities, which widely limit the usage of epoxy
resins as low-k materials. Therefore we measured contact angles
of all epoxy samples to determine the relative wettability of the
surface. Fig. 8 exhibits the photographs and the result of
contact angles. Photos represented the contact angles of BPA-EP
(w ¼ 78.2ꢀ), BPF-EP (w ¼ 80.2ꢀ), TMBP (w ¼ 83.2ꢀ) and DGENF
(w ¼ 116ꢀ), respectively. All the contact angles were less than 90ꢀ
except DGENF, which was consistent with the standard of
hydrophobic materials. This result was attributed to the stiff
naphthalene moieties as the skeleton of the molecular chains
and the hydrophobic uorine in the side chains, thus
decreasing the surface wettability of the material.
To the best of our knowledge, the signal propagation loss is
related to the dielectric constant (Dk) and the dielectric loss
factor (Df), which are the most important parameters for epoxy
resins used for the electronic materials. Therefore, selecting
lower Dk and Df material is benecial for improving propagating
speed and reducing the propagation loss. Fig. 9 and 10 showed
the dielectric constant and dielectric loss of all these epoxy
samples. It was clearly observed that DGENF had a remarkably
lower dielectric constant (Dk ¼ 2.97 at 1 MHz) than that of other
ꢀ
loss temperatures (T5%) of 288 C than those of other kinds of
epoxy resin materials (BPA-EP 334 ꢀC, BPF-EP 334 ꢀC and TMBP
ꢀ
313 C). This result was mainly caused by two reasons. Firstly,
the existence of 4-uorobenzoyl side chains decreased the cross-
linking density of the cured resins. Secondly, the side chains
group could be degraded prior to the rigid main chain. This is
also demonstrated by its tan delta curve with a relatively broad
peak due to the introduction of 4-uorobenzoyl side chains.
However, as the temperature increased, the degradation rate of
DGENF became much slower than other three kinds of epoxy
resin samples. For example, as the temperature reached up to
400 ꢀC, DGENF displayed a slow decreasing tendency in the
thermal degradation, while the other epoxy resins maintained
the continuous process of thermal degradation. Consequently,
T50% ꢀof DGENF (532 ꢀꢀC) became much higher than BPA-EP
ꢀ
(453 C), BPF-EP (446 C) and TMBP (450 C). Meanwhile, the
char yield of the epoxy resin samples at 700 ꢀC listed in Table 1
indicated DGENF showed more residual (34.5%) than other
three kinds of epoxy resins. It can therefore be deduced that the
introduced naphthyl group improves the thermal stability of
Fig. 7 TGA curves of cured epoxy samples.
53974 | RSC Adv., 2017, 7, 53970–53976
Fig. 8 Contact angles of cured epoxy samples.
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followed by nucleophilic reaction with ECH. Then a novel
naphthyl epoxy resin containing 4-uorobenzoyl side chains
were prepared by curing with MeHHPA at high temperature. By
comparing the properties of DGENF with other three kinds of
conventional epoxy resins (BPA-EP, BPF-EP and TMBP), this
novel naphthyl epoxy resin showed more hydrophobic and
much lower dielectric properties. Therefore, DGENF is consid-
ered to be one of the most promising candidates to substitute
future low-k dielectrics in microelectronic. These results could
motivate the research on the design of lower dielectric constant
materials and the investigation of the relationship between the
structure and properties of epoxy resins to be used as thermoset
materials.
Fig. 9 Dielectric constants of cured epoxy samples.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
We acknowledge the nancial support from the Natural Science
Foundation of China (No. 21374034 and 21474036).
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