1
High dielectric multiwalled carbon nanotube-polybenzoxazine nanocomposites for
printed circuit boards applications
Mohan Selvi,1 Muthukumaraswamy Rangaraj Vengatesan,1,2 Pichaimani Prabunathan1, Jang
Kun Song, 2 and Muthukaruppan Alagar 1 a)
1
Polymer Composite Lab, Department of Chemical Engineering, Anna University, Chennai-600 025,
India
2
School of Electronic & Electrical Engineering, Sungkyunkwan University, Suwon, South Korea
Multiwall carbon nanotube (MWCNT) and γ -aminopropyltriethoxysilane (γ-APS) were used
as received from Sigma-Aldrich. Bisphenol-A, aniline, phenol, paraformaldehyde and
calcium hydride were used as received from SRL (India) Ltd.
In order to improve the interfacial adhesion between the CNT and the PBZ matrix, the
surface of the CNT was modified with benzoxazine functional silane, which acts as a
coupling agent and is capable of bridging the CNT, covalently bonded to the PBZ matrix,
through the silylation. The functionalization of MWCNT with BS was confirmed by Raman,
FT-IR, TGA and XPS.
Figure S1 represents Raman spectra (BRUKER: RFS 27) for both CNT-COOH and CNT-BS.
The peak appeared near 1300 cm-1 represents the D band, which indicates disordered
sp2 -hybridized carbon atoms of graphite in the network of CNTs. In contrast, the peak
appeared near 1583cm-1 represents the G band, which is attributed to the rolled graphite
layers of the MWCNTs and is related to the graphitic E2g symmetry of the interlayer mode.
This mode reflects the structural integrity of the sp2 -hybridized carbon atoms of the
nanotubes. It is clear that the acid treatment does not affect the structure of CNTs as both the
characteristic peaks of D and G bands can still be appeared in Raman spectra of CNT-COOH
and CNT-BS. Further, The ID/IG ratio of the CNT-COOH and CNT-BS is 1.22 and 1.35
respectively. Since the CNT used as multiwall, and the inner graphene walls are not modified
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as much as the surface, this difference in ratio should represent significant surface
modification.1
FT-IR spectra (Perkin Elmer 6X) of CNT-COOH and CNT-BS are presented in Figure S2.
From FTIR spectra, the appearance of new characteristic peaks of CNT-COOH observed at
1730 cm-1 (C=O) and 1560 cm-1 (def.–OH) were confirm the presence of carboxyl groups
(–COOH) and hydroxyl groups (–OH) on the surface of CNT-COOH. The appearance of a
band at 947 cm-1 indicates benzene ring attached oxazine which in turn confirms the grafting
of benzoxazine monomer on CNT. This was further confirmed by the appearance of
corresponding new bands at 2932 cm-1 and 2853 cm-1, which are attributed to the asymmetric
and symmetric stretching modes of the –CH2 group present in the CNT-BS.2 Similarly
thermograms of CNT-COOH and CNT-BS are presented in Figure S3. From TGA analysis, it
was observed that the CNT-BS possesses higher weight loss than that of neat CNT-COOH.
There is no complete loss of material up to 800oC, and it can be observed that the degradation
starts above 300oC due to the presence of benzoxazine functional silane in the CNT.
XPS was used to further identify the functionalization of organic molecule on the CNT. The
XPS spectrum of benzoxazine silane modified CNT (CNT-BS) is presented in
Figure S4(a), in which C1s, N 1s, Si 2s, Si 2p and O 1s peaks can be seen, suggesting the
presence of benzoxazine functionality on CNT. The spectra were fitted with Gauss–Lorentz
peaks using XPSPEAK software (Loughborough Surface Analysis Ltd, Leicestershire, UK)
in order to obtain the peak information. A Shirley’s baseline was used in the fitting process.
The C 1s spectrum of the CNT-BS is shown in Figure S4(b). The peaks that are observed for
the CNT-BS at 283.1, 283.6, 284.5, 285.4 and 287.6 eV which are assigned to C–Si, C–C,
C-H, C–O–C and C–N of the benzoxazine silane, respectively. The N 1s spectrum of the
CNT-BS is depicted in Figure S4(c). Two distinct peaks can be observed at 401.8 and 404.5
eV with the CNT-BS, indicating the binding energy of nitrogen in the two environments such
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as–N-C-O and –N-C–C- respectively. The O 1s spectrum of CNT-BS is presented in Figure
S4(d). After benzoxazine silane functionalization on the CNT, the surface COOH groups of
the CNT are changed to C–O–Si groups. The peak at 532.5 eV for CNT-BS which may
indicate a C–O–Si linkage, and the peaks at 530.6, 532.9 and 534.0 eV indicate a -O=C-O,
Si-O-Si and C-O-C linkage in confirms the presence of silane in the CNT. From the XPS
analysis, we confirmed that the benzoxazine silane makes a perfect covalent bond on the
CNT.
FT-IR spectra of CNT-BS/PBZ composites are presented in Figure S5. The FT-IR spectra
obtained after thermal cure, the bands appeared at 947 cm-1, and 1497 cm-1 were attributed to
the benzene attached oxazine ring and tri-substituted benzene rings are disappeared which
indicates the loss of oxazine ring of the benzoxazine. In addition, new absorption peaks
developed at 870 cm-1 and 1482 cm-1 of the tetra-substituted aromatic ring were observed
indicates the ring-opening polymerization of benzoxazine.3 Furthermore, the bands at 2928
and 2852 cm-1 attributed to the asymmetric and symmetric stretching modes of the –CH2
group in benzoxazine functional silane, which was assigned to the CNT-BS group in
polybenzoxazine.
The polybenzoxazine with varying weight percentages of CNT-BS was subjected to
thermogravimetric analysis (TGA: Netzsch STA 409) to determine their thermal stability in
nitrogen atmosphere and the results are presented in Figure S6. The dynamic thermal
stabilities of CNT-BS nanocomposites in nitrogen atmosphere are much improved as
indicated by the fact that the 5% weight loss temperatures (Td5%) for the composites. Td5%
of 0.5, 1.0 and 1.5 wt% of CNT-BS/PBZ shows 390, 424, and 442°C, respectively, it was
noticed that about 80–130°C increase when compared to that of neat PBZ (312°C). The
presence of CNT-BS provides an additional heat capacity which stabilized the materials
against thermal decomposition. The percentage of char yields are also improved for the
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composite sample having CNT-BS from 36% to 45% (at 800°C) for 1.5wt% of CNT-BS/PBZ
nanocomposites. This also ascertains the incorporation of CNT-BS segments in the PBZ
composites, and in fact the CNT-BS phase contributes positive effects on the stability of
PBZ.
The glass transition temperature (Tg) of the samples were carried out by Netzsch DSC 200.
The values of Tg of CNT-BS/PBZ are increased to 187, 199 and 208°C with an increase in
0.5, 1.0 and 1.5 wt% of CNT-BZS. The interactions resulted between the CNT-BS and the
PBZ possibly influences the formation of an inter phase layer surrounding the CNTs,
restricting the segmental motion of the polymer and consequently enhanced Tg of the
nanocomposites. The increase in Tg may also be associated with the nano-reinforcement
effect of CNTs and the augmentation of cross-linking density, requiring a higher temperature
to provide the necessary thermal energy to make the mobility of polymer segments in the
composite. The improved CNT dispersion and interfacial interactions effected from CNT
functionalization generally resulted in better thermal properties due to the restricted mobility
of polymer chains to a higher degree when they are held by strong interfacial bonds.4 The Tg
appears to be sensitive to interfacial interactions between the polymer matrix and
reinforcement agent as well as the characteristic temperature signifying the motion of
polymer chains. Such an atomistic scale movement of polymer chains is significantly
influenced by the presence of reinforcements. Dimensionally similar to the polymer chain
building units, CNTs influence the alignment of the polymer chains and thereby restrict their
movement. The extent of influence appears to depend on the type of CNTs and their bonding
with the surrounding matrix. In the present study, the difference in Tg may be attributed to
the variation in the extent of cross-linking reaction between PBZ and the silane molecules led
to the formation of covalent bond with CNTs and in turn result higher cross-linking network.
Furthermore, due to the silane functionalization, a strong interfacial bond and good
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dispersibility reduce the mobility of PBZ molecules around the CNTs and, thus, result in
increased thermal stability.
1
Y. H. Wang, C. M. Chang, and Y. L. Liu, Polymer 53, 106 (2012).
M. R. Vengatesan, S. Devaraju, K. Dinakaran, and M. Alagar, J. Mater. Chem. 22, 7559
(2012).
3
C. Jubsilpa, T. Takeichi, and S. Rimdusit, Polym. Degrad. Stabil. 96, 1047 (2011).
4
P. C. Ma, J. K. Kim, and B. Z. Tang, Carbon 44, 3232(2006).
2
D
(a) CNT-COOH
(b) CNT-BS
G
Intensity (a.u.)
(b)
1300
1583
(a)
600
800
1000
1200
1400
Raman shift
1600
-1
(cm )
1800
2000
FIG. S1. Raman spectra of (a) CNT-COOH and (b) CNT-BS
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Transmittance (a.u.)
(a) CNT-COOH
(b) CNT-BS
1730
1560
947
2853
2932
3500
3000
2500
2000
Wave number
1500
1000
(cm-1)
FIG. S2. FT-IR spectra of (a) CNT-COOH and (b) CNT-BS
110
(a) CNT-COOH
(b) CNT-BS
100
Weight (%)
90
(a)
80
(b)
70
60
50
40
30
100
200
300
400
500
600
Temperature ( C)
700
FIG. S3 TGA of (a) CNT-COOH and (b) CNT-BS.
800
7
30000
25000
O 1s
(a)
12000
C 1s
10000
Intensity (a.u.)
Intensity (a.u.)
20000
15000
10000
N 1s
5000
Si 2pSi 2s
800
600
400
200
0
600
(c)
4000
2000
280
12000
Intensity (a.u)
400
300
200
0
-100
404
288
290
292
406
O 1s
4000
2000
402
286
Curve fitting
O=C-O 530.6
C-O-Si 532.5
Si-O-Si 532.9
C-O-C 534.0
6000
0
400
284
(d)
8000
100
398
282
14000
10000
500
Intensity (a.u)
6000
Binding Energy (eV)
Curve fitting
N-C-C 401.8
N-C-O 404.5
N 1s
8000
-2000
278
Binding Energy (eV)
700
Curve fitting
C-Si- 283.1
C-C 283.6
C-H 284.5
C-O 285.3
C-N 287.6
0
0
1000
C 1s
(b)
528
530
532
534
536
538
Binding Energy (eV)
Binding Energy (eV)
FIG. S4. XPS spectrum for CNT-BS (a) Survay, (b) C1s binding energy, (c) N1s binding
energy and (d) O1s binding energy
Transmittance (a.u.)
1.5% CNT-BS/PBZ
1.0% CNT-BS/PBZ
0.5% CNT-BS/PBZ
2928
3500
3000
870
2852
1482
2500
2000
Wave number
1500
(cm-1)
1000
FIG. S5. FT-IR spectra of CNT-BS/PBZ nanocomposites
8
100
90
Weight (%)
80
70
0.5% CNT-BS/PBZ
1.0% CNT-BS/PBZ
1.5% CNT-BS/PBZ
60
50
40
30
100
200
300
400
500
600
700
Temperature ( C)
FIG.S6. TGA for CNT-BS/PBZ nanocomposites
800