synthesis and properties of macromer-grafted polymers for noncovalent functionalization of...
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C A R B O N 4 7 ( 2 0 0 9 ) 2 7 1 6 – 2 7 2 6
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Synthesis and properties of macromer-grafted polymersfor noncovalent functionalization of multiwalledcarbon nanotubes
Takuya Morishita*, Mitsumasa Matsushita, Yoshihide Katagiri, Kenzo Fukumori
Toyota Central R&D Labs, Inc., Organic Materials Lab, Nagakute, Aichi 480-1192, Japan
A R T I C L E I N F O
Article history:
Received 17 February 2009
Accepted 22 May 2009
Available online 3 June 2009
0008-6223/$ - see front matter � 2009 Elsevidoi:10.1016/j.carbon.2009.05.032
* Corresponding author: Fax: +81 561 636498.E-mail address: [email protected]
A B S T R A C T
We prepared macromer-grafted polymers (MGPs) containing suitable polymer side chains
for improving solubility and pyrene units for improving adsorption on multiwalled carbon
nanotube (MWCNT) surfaces, and demonstrated that these MGPs act as MWCNT solubiliz-
ers that improve solubility of MWCNTs in typically poor solvents such as alkanes and that
improve flowability of polymer/MWCNT composites. The polydimethylsiloxane (PDMS)–
MGPs, synthesized using PDMS macromers and pyrene-containing monomers, improved
solubility of MWCNTs not only in chloroform but also in hexane, which is a poor solvent
for MWCNTs. Moreover, the addition of PDMS–MGP-adsorbed MWCNTs (MWCNT/PDMS–
MGPs) to epoxy resin monomers or polybutylene terephthalate (PBT) drastically reduced
the viscosity of the obtained epoxy resin monomer/MWCNT/PDMS–MGP mixtures and
PBT/MWCNT/PDMS–MGP composites in comparison to the epoxy resin monomer/MWCNT
mixtures and PBT/MWCNT composites, respectively. The viscosity of PBT/MWCNT/PDMS–
MGP composites including 61 vol% of MWCNTs, in particular, was almost equal to that of
pristine PBT.
� 2009 Elsevier Ltd. All rights reserved.
1. Introduction
Since Iijima’s landmark paper in 1991 [1], carbon nanotubes
(CNTs) have attracted increasing interest because of their
thermal, electrical, mechanical, and structural properties
[2–4]. However, strong interaction between CNTs results in
agglomeration in the presence of solvents and resins. Side-
wall functionalization of CNTs has been carried out exten-
sively through oxidation routes to form carboxylic acid units
that are further derivatized, for example, to yield long-chain
alkylamines on CNT surfaces [5]. Polymer-grafted CNTs with
covalent functionalization were fabricated by in situ radical
polymerization [6] or surface-initiated atom transfer radical
polymerization (ATRP) [7]. However, the graphene structure
of the CNT surface is destroyed by these covalent attach-
ments. Destruction of the CNT surface diminishes various
er Ltd. All rights reserved.co.jp (T. Morishita).
properties, especially the thermoconductive and electrical
properties. To avoid destruction of the CNT surface, noncova-
lent approaches that solubilize CNTs have been reported. For
example, p–p stacking between the aromatic surfaces of CNTs
and cycloaromatic moieties such as pyrene derivatives [8],
pyrene-containing polymers [9–11], conjugated polymers
[12,13], and porphyrins [14] has been reported. However, these
solubilizers are insoluble and do not solubilize CNTs in al-
kanes and cycloalkanes; this is important because good solu-
bility of CNTs in these solvents is required, for example, in the
field of surface coatings. On the other hand, polymer nano-
composites that use CNTs as nanofillers to improve mechan-
ical, electrical, and thermal properties were reported [15–19].
In improving physical properties of polymers in such a man-
ner, it is necessary to use small amounts of multiwalled car-
bon nanotubes (MWCNTs) (0.5–1.5 vol%) in order to cut the
.
C A R B O N 4 7 ( 2 0 0 9 ) 2 7 1 6 – 2 7 2 6 2717
high cost of using CNTs and to dramatically reduce the
amount of fillers such as carbon black, glass fiber, alumina,
and boron nitride that are used together with MWCNTs. Be-
cause physical properties of polymer matrices themselves
are improved by adding small amounts of MWCNTs, physical
properties of polymer/MWCNT/other filler composites can
also be improved by adding small amounts of filler. However,
by adding even small amounts of MWCNTs to polymers, the
viscosity of the obtained polymer/MWCNT composites tends
to increase significantly [20]. Moreover, although improving
MWCNT dispersion improves physical properties, it is also
known to further increase the viscosity. To have good molding
processability of polymer/MWCNT composites, they are re-
quired to have good flowability.
In this paper, we report macromer-grafted polymers
(MGPs) that contain suitable macromer units for improving
solubility in poor solvents and pyrene units for improving
physical adsorption of MGPs on MWCNT surfaces. These
MGPs act as solubilizers that improve solubility of MWCNTs
in typically poor solvents such as alkanes, and impart im-
proved flowability to the polymer/MWCNT composites.
2. Experimental section
2.1. Materials
MWNT-7 (diameter 40–90 nm, purity >99.5 wt%, specific grav-
ity 2.1, Nano Carbon Technologies Co., Ltd., Japan) and
Ctube100 (diameter 10–40 nm, purity P93 wt%, specific grav-
ity 2.1, CNT Co., Ltd., South Korea) were used without further
purification. Polydimethylsiloxane (PDMS) macromer (a-bu-
tyl-x-(3-methacryloxypropyl)polydimethyl siloxane, FM-
0711, Mn: 1000, Chisso Corporation) and PEG macromer
(poly(ethylene glycol)methyl ether methacrylate, Mn: 1100,
Aldrich) were used as received. 1-Pyrenebutanol, methacry-
loyl chloride, methyl methacrylate (MMA), 2,20-azobisisobuty-
lonitrile (AIBN), tert-dodecyl mercaptan (TDM), and poly
methyl methacrylate (PMMA, Mn: 46,000) were obtained from
Aldrich and used without further purification. Bisphenol-A
diglycidyl ether, 4-methylcyclohexane-1,2-dicarboxylic anhy-
dride, and 2-ethyl-4-methylimidazole were obtained from To-
kyo Chemical Industry Co., Ltd. and used without further
purification. Polybutylene terephthalate (PBT) was obtained
from WinTech Polymer Ltd. (trade name DURANEX-2002).
2.2. Measurements
1H NMR measurements were carried out with a JEOL JNM-
ECX400P spectrometer (400 MHz, CDCl3). Gel permeation
chromatography (GPC) was performed on a Shodex GPC-101
system using Shodex-K-805L columns in chloroform at
40 �C. Glass transition temperature (Tg) was measured by dif-
ferential scanning calorimetry (DSC, TA Instruments Q1000)
(heating rate = 10 �C min�1). Thermogravimetric analysis
(TGA) was performed on a Rigaku-Thermo plus TG8120
instrument. UV–visible absorption spectra were measured
using a Shimadzu UV–Vis–NIR UV-3600 spectrophotometer.
Fluorescence spectra were measured using a JASCO FP-6500
spectrofluorometer. Raman spectra were measured using a
JASCO NRS-3300 Raman spectrometer equipped with a
532 nm excitation laser source. The viscosity of epoxy resin
monomer/MWCNT mixtures was measured using a shear-
rate controlled rheometer (ARES-100FRT-BATH-STD, Rheo-
metric Scientific) at 80 �C. The viscosity of melt mixing of
polymer/MWCNT/MGP composites was measured using a mi-
cro rheology compounder (HAAKE-MiniLab, Thermo Scien-
tific) at 260 �C and 250 rpm (co-rotational twin screw) for
5 min in a N2 atmosphere. Morphology of frozen fracture sur-
faces of the press-molded test pieces (25 mm · 25 mm; 2 mm
thickness) of polymer/MWCNT/MGP composites were ob-
served using a Hitachi S-3600 N scanning electron microscope
(SEM). Sonication was performed using an ultrasonic cleaner
(Branson B-220). All centrifugation steps were performed at
1220 g for 1 h.
2.3. Synthesis of MGPs
2.3.1. Synthesis of 1-pyrenyl butyl methyl methacrylate1-Pyrenebutanol (5.0 g, 18.2 mmol) and triethylamine (5.3 mL,
38.2 mmol) were added to dry THF (100 mL). Methacryloyl
chloride (2.0 g, 19.1 mmol) was then added dropwise under
stirring at 0 �C. The reaction was conducted at room temper-
ature (rt) for 2 h. The reaction medium was removed by filtra-
tion and washing with ethyl acetate. The filtrate was dried
over magnesium sulfate and filtered. The solvent was evapo-
rated under reduced pressure to yield a solid, which was
purified by column chromatography (silica gel, hexane:ethyl
acetate = 10:1). The product was obtained by evaporation of
the eluent followed by drying (total yield: 62%). 1H NMR
(400 MHz, CDCl3, Me4Si, d): 7.86–8.25 (m, 9H, pyrene-H), 6.10
(s, 1H, CH2@C), 5.53 (s, 1H, CH2@C), 4.23 (t, 2H, CH2O), 3.36 (t,
2H, pyrenyl-CH2), 1.93–2.00 (m, 2H), 1.94 (s, 3H, CH3), 1.83–
1.89 (m, 2H).
2.3.2. Synthesis of PDMS–MGP 1a1-Pyrenyl butyl methyl methacrylate (171 mg, 0.5 mmol),
PDMS macromer (500 mg, 0.5 mmol), MMA (100 mg,
1.0 mmol), AIBN (8.0 mg, 0.048 mmol) and TDM (0.5 mg,
0.0025 mmol) were added to a three-necked glass flask con-
taining dry toluene (2.0 mL), and the reaction system was
purged with nitrogen gas. The solution was then heated to
75 �C. The temperature was maintained at 75 �C for 4 h to
complete the polymerization. After cooling, the solution was
diluted with chloroform (10 mL) and poured into a 10-fold vol-
ume of methanol and purified by reprecipitation. The solvent
was completely removed by drying, producing polymer 1a in a
yield of 80%. From 1H NMR (400 MHz, CDCl3) measurement,
the product contained 30.7 mol% of PDMS macromer units,
37.2 mol% of 1-pyrenyl butyl methyl methacrylate units, and
32.1 mol% of MMA units. From GPC measurement in chloro-
form with PS standards, number-average molecular weight
(Mn) was 38,000 and weight-average molecular weight (Mw)/
Mn was 1.9. 1H NMR (400 MHz, CDCl3, d: 7.4–8.2 (m, pyrene-
H), 3.7–4.1 (m, COOCH2 of PDMS macromer, COOCH2 of 1-
pyrenyl butyl methyl methacrylate), 3.3–3.6 (bs, COOCH3 of
MMA), 2.8–3.3 (m, pyrenyl-CH2), 0.6–2.1 (m, CH2 and CH3 of
the backbone, CH2CH2 of 1-pyrenyl butyl methyl methacry-
late, CH2 and CH2CH2CH3 of PDMS macromer), 0.3–0.5 (m,
SiCH2 of PDMS macromer), �0.2 to 0.2 (m, SiCH3 of PDMS).
2718 C A R B O N 4 7 ( 2 0 0 9 ) 2 7 1 6 – 2 7 2 6
2.3.3. Synthesis of PDMS–MGP 1b1b was prepared by the same method as 1a, except 1-pyrenyl
butyl methyl methacrylate (171 mg, 0.5 mmol), PDMS macro-
mer (300 mg, 0.3 mmol), and MMA (120 mg, 1.2 mmol) were
used instead of the monomer for 1a. The yield of 1b was
78%. From 1H NMR (400 MHz, CDCl3) measurement, the prod-
uct contained 18.5 mol% of PDMS macromer units, 30.1 mol%
of 1-pyrenyl butyl methyl methacrylate units, and 51.4 mol%
of MMA units. From GPC measurement, Mn was 39,000 and
Mw/Mn was 1.8.
2.3.4. Synthesis of PDMS–MGP 1c1c was prepared by the same method as 1a, except 1-pyrenyl
butyl methyl methacrylate (171 mg, 0.5 mmol), PDMS macro-
mer (900 mg, 0.9 mmol), and MMA (60 mg, 0.6 mmol) were
used instead of the monomer for 1a. The yield of 1c was
77%. From 1H NMR (400 MHz, CDCl3) measurement, the prod-
uct contained 50.0 mol% of PDMS macromer units, 23.6 mol%
of 1-pyrenyl butyl methyl methacrylate units, and 26.4 mol%
of MMA units. From GPC measurement, Mn was 26,000 and
Mw/Mn was 1.9.
2.3.5. Synthesis of PEG–MGP 1d1-Pyrenyl butyl methyl methacrylate (171 mg, 0.5 mmol),
PEG macromer (880 mg, 0.8 mmol), MMA (70 mg, 0.7 mmol),
AIBN (8.0 mg, 0.048 mmol), and TDM (0.5 mg, 0.0025 mmol)
were added to a three-necked glass flask containing dry tolu-
ene (2.0 mL), and the reaction system was purged with nitro-
gen gas. The solution was then heated to 75 �C. The
temperature was maintained at 75 �C for 4 h to complete the
polymerization. After cooling, the solution was diluted with
chloroform (10 mL) and poured into a 10-fold volume of hex-
ane and purified by reprecipitation. The solvent was com-
pletely removed by drying, producing 1d in a yield of 78%.
From 1H NMR (400 MHz, CDCl3) measurement, the product
contained 54.8 mol% of PEG macromer units, 16.4 mol% of 1-
pyrenyl butyl methyl methacrylate units, and 28.8 mol% of
MMA units. From GPC measurement in chloroform with PS
standards, Mn was 33,000 and Mw/Mn was 2.3. 1H NMR
(400 MHz, CDCl3, d: 7.5–8.3 (m, pyrene-H), 3.8–4.3 (m, COOCH2
of PEG macromer, COOCH2 of 1-pyrenyl butyl methyl methac-
rylate), 3.4–3.8 (m, COOCH3 of MMA, OCH2CH2O and CH2O of
PEG), 3.4 (s, CH3O of PEG), 3.0–3.3 (m, pyrenyl-CH2), 0.7–2.2
(m, CH2CH2 of 1-pyrenyl butyl methyl methacrylate, CH2
and CH3 of the backbone).
2.4. Synthesis of pyrene-containing polymer
2.4.1. Synthesis of pyrene-containing polymer PP11-Pyrenyl butyl methyl methacrylate (171 mg, 0.5 mmol),
MMA (150 mg, 1.5 mmol) and AIBN (1.0 mg, 0.006 mmol) were
added to a 3-necked glass flask containing of dried toluene
(2 mL), and the reaction system was purged with nitrogen
gas. The solution was then heated to a temperature of 75 �C.
The temperature was further maintained at 75 �C for 4 h to
complete the polymerization. After cooling, the solution was
diluted with chloroform (30 mL) and poured into a 10-fold vol-
ume of methanol and purified by reprecipitation. The solvent
was completely removed by drying, producing PP1 in a yield
of 65%. From 1H NMR (400 MHz, CDCl3) measurement, the
product contained 30.0 mol% of 1-pyrenyl butyl methyl meth-
acrylate units and 70.0 mol% of MMA units. From GPC mea-
surement in chloroform with PS standards, Mn was 98,000
and Mw/Mn was 3.2. 1H NMR (400 MHz, CDCl3, d: 7.5–8.4 (m,
pyrene-H), 3.8–4.2 (m, COOCH2 of 1-pyrenyl butyl methyl
methacrylate), 3.3–3.7 (bs, COOCH3 of MMA), 2.9–3.4 (m, pyre-
nyl-CH2), 0.6–2.1 (m, CH2CH2 of 1-pyrenyl butyl methyl meth-
acrylate, CH2 and CH3 of the backbone).
2.4.2. Synthesis of pyrene-containing polymer PP2PP2 was prepared by the same method as PP1, except AIBN
(8.0 mg, 0.048 mmol) were used instead of the AIBN (1.0 mg,
0.006 mmol) for PP1. The yield of PP2 was 83%. From 1H
NMR (400 MHz, CDCl3) measurement, the product contained
28.9 mol% of 1-pyrenyl butyl methyl methacrylate units and
71.1 mol% of MMA units. From GPC measurement, Mn was
22,000 and Mw/Mn was 2.6.
2.5. Synthesis of PBT/MWNT-7/1a composites
1a (147 mg) was added into MWNT-7 (147 mg) in chloroform
(73.5 mL). After sonication for 1 h, the MWNT-7 dispersed
solution containing 1a was filtered and washed with chloro-
form to eliminate excess 1a; MWNT-7/1a was obtained after
being dried under vacuum. From TGA measurement,
MWNT-7/1a contained 147 mg of MWNT-7 and 4.7 mg of 1a.
PBT/MWNT-7/1a composites were produced by melt mixing
(260 �C and 250 rpm (co-rotational twin screw) for 5 min in a
N2 atmosphere) 9.0732 g of PBT with 151.7 mg of MWNT-7/1a
via the micro rheology compounder.
3. Results and discussion
3.1. Synthesis and properties of PDMS–MGPs
Fig. 1 shows a schematic illustration of physical adsorption of
MGP 1 on an MWCNT surface. MGP 1 was designed to have
polymer side chains for improving solubility of MWCNTs in
desired solvents and pyrene units for improving physical
adsorption of MGPs on MWCNT surfaces, resulting from p–p
stacking between the MWCNT surface and pyrene units.
Fig. 2 shows the synthesis of MGP 1 by radical copolymeriza-
tion of macromer 2 with suitable polymer side chains, 1-pyre-
nyl butyl methyl methacrylate 3 as the pyrene unit, and MMA
4 as a spacer unit (macromer method). 1-Pyrenyl butyl methyl
methacrylate 3 was synthesized by reaction of 1-pyrenyl
butanol with methacryloyl chloride in THF (yield = 62%). Like
macromer 2, PDMS macromer was used to impart solubility in
alkanes and cycloalkanes, which are typically poor solvents
for MWCNTs. PDMS–MGPs were synthesized using PDMS
macromer, 3, and 4, as shown in Table 1. Table 1 indicates
the molar composition in the co-monomer feed, polymer unit
of 2 (X in Fig. 2), and the molar composition, molecular weight
(Mn), and glass transition temperatures (Tg) of PDMS–MGPs
(1a–c). In addition, the properties of pyrene-containing poly-
mers (PP1 and PP2) are given for comparison purposes. AIBN
was used as a radical initiator. The yield of each PDMS–MGP
was more than 75%. Tg of PDMS–MGPs tended to decrease
MWCNT1pyrene unit
MWCNT
polymer side chain
Fig. 1 – Schematic illustration of physical adsorption of MGP 1 on an MWCNT surface.
OO OO OO
l m
Xk
O
O
(CH2)4
OOn
O
O
AIBN+ +
2 3 4 1
Xk(CH2)4
Fig. 2 – Synthesis of MGP 1 by radical copolymerization (toluene, 75 �C for 4 h) (X: polymer unit, k = 0 or 3).
Table 1 – Synthesis and properties of PDMS–MGPs.
Code Comp. 2:3:4a Xb Comp. l:m:nc Mnd Tge (�C)
1a 25:25:50 PDMS 31:37:32 38 k 5
1b 15:25:60 PDMS 19:30:51 39 k 58
1c 45:25:30 PDMS 50:24:26 26 k �16
PP1 0:25:75 – 0:30:70 98 k 107
PP2 0:25:75 – 0:29:71 22 k 101
PDMS macromer – – – 1 k –f
a Molar composition (mol%) in the co-monomer feed.
b Polymer unit of 2 in Fig. 2.
c Molar composition (mol%) in the polymers determined by 1H NMR in CDCl3.
d Measured by GPC in chloroform with PS standards.
e Measured by DSC.
f Not determined.
C A R B O N 4 7 ( 2 0 0 9 ) 2 7 1 6 – 2 7 2 6 2719
with increasing PDMS macromer content since Tg of PDMS
was �127 �C [21]. From Tg and 1H NMR measurements
(Fig. 3), units of 2 and PDMS macromer units were found to
be incorporated into the polymer chains of PDMS–MGPs.
PDMS–MGPs (1a–c), PP1, and PP2 were soluble in chloroform,
THF, methyl ethyl ketone, and ethyl acetate. Besides this, 1a–c
were completely soluble in hexane and cyclohexane, whereas
PP1 and PP2 were insoluble. Although the solubility parame-
ter (SP) value of hexane is extremely small (14.9 (MPa)1/2) [22],
the low-polarity PDMS (SP value of dimethyl siloxanes: 10.0–
12.1 (MPa)1/2) side chains in PDMS–GPs are well soluble in
hexane.
Fig. 3 – 1H NMR spectrum of 1a in CDCl3.
2720 C A R B O N 4 7 ( 2 0 0 9 ) 2 7 1 6 – 2 7 2 6
3.2. Evaluation of solubility of MWCNTs with PDMS–MGPs in hexane
To evaluate the effect of PDMS–MGPs on solubility of
MWCNTs in hexane, MWCNTs and PDMS–MGPs (1a, 1b, or
Fig. 4 – Photographs of solutions of (a) MWNT-7 (3 mg) with
PDMS macromer (3 mg) in hexane (30 mL), (b) MWNT-7
(3 mg) with 1a (3 mg) in hexane (30 mL), (c) MWNT-7 (7 mg)
with PDMS macromer (7 mg) in hexane (7 mL), and (d)
MWNT-7 (7 mg) with 1a (7 mg) in hexane (7 mL) after
sonication.
1c) were mixed in hexane under sonication for 1 h at rt.
MWNT-7 was used as MWCNT sample, because its solubility
in hexane was inferior to that of Ctube100. Fig. 4 shows a
solution of MWNT-7 with 1a in hexane (b, d) and MWNT-7
with PDMS macromer in hexane (a, c) at different concentra-
tions. Fig. 4 shows that PDMS macromer could not solubilize
MWNT-7 in hexane and MWNT-7 was completely precipitated
immediately after sonication, but 1a imparted good solubility
to MWNT-7 in hexane. The solutions of 3 mg of MWNT-7 with
3 mg of 1a in 30 mL of hexane were stable for more than a
week, even though the original MWNT-7 are easy to precipi-
tate from hexane, which has a remarkably low SP value and
a low specific gravity (0.66). To our knowledge, this is the first
example that demonstrates an improved solubility of
MWCNTs in alkanes such as hexane. Similar stable solution
was obtained by adding 3 mg of 1c to 3 mg of MWNT-7 in
30 mL of hexane. However, the MWNT-7 dispersed solution
obtained by adding 3 mg of 1b was not stable and some
MWCNTs precipitated after about 24 h. This might be because
the solubility of 1b in hexane is lower than that of 1a and 1c.
Addition of 3 mg of PP1 or PP2 to 3 mg of MWNT-7 in 30 mL of
hexane did not improve the solubility of MWNT-7, because
PP1 and PP2 were insoluble in hexane. On the other hand,
when the MWCNT sample used was Ctube100 instead of
C A R B O N 4 7 ( 2 0 0 9 ) 2 7 1 6 – 2 7 2 6 2721
MWNT-7, a stable solution of Ctube100 dispersed in hexane
was obtained by adding of each of the PDMS–MGPs (1a, 1b,
or 1c).
To evaluate the solubility of MWCNTs with PDMS–MGPs
(1a–c) in hexane more precisely, a comparison of the absor-
bance of supernatant solutions after centrifugation of
MWCNTs dispersed solutions (3 mg of MWCNTs and 3 mg of
PDMS–MGPs were mixed in 30 mL of hexane under sonication
for 1 h at rt) was carried out. Fig. 5 and Table 2 show the absor-
bance at 600 nm of these supernatant solutions. The absor-
bance of the supernatant solutions of MWNT-7 dispersed in
hexane with 1a, 1b, or 1c was greater than that with PDMS
macromer. By comparison of the supernatant solution absor-
bances, the solubility of the MWCNTs with PDMS–MGPs was
found to be better than that with PDMS macromer. This im-
proved solubility is considered to be due to the presence of
many PDMS side chains in the PDMS–MGPs on MWCNT sur-
faces. Fig. 6 shows changes in absorbance at 600 nm of the
supernatant solutions of MWCNTs dispersed in hexane by
adding 1a (0–6 mg) to 3 mg of MWCNTs in 30 mL of hexane.
The absorbance of the supernatant solution of MWNT-7 dis-
persed in hexane was saturated by addition of P0.9 mg of
1a, as shown in Fig. 6a. On the other hand, that of Ctube100
dispersed in hexane was saturated by addition of P3 mg of
1a, as indicated in Fig. 6b. This difference in the amounts
needed for a stable solution of MWCNTs dispersed in hexane
was considered to be mainly due to the difference in the spe-
cific surface area of the MWCNTs used (MWNT-7: 25–
30 m2 g�1 and Ctube100: 150–250 m2 g�1).
3.3. Confirmation of physical adsorption of PDMS–MGPson MWCNT surfaces
To confirm the physical adsorption of PDMS–MGPs on
MWCNTs in hexane, MWCNTs dispersed in hexane solution
with PDMS–MGPs were filtered, washed with hexane to elimi-
0
0.5
1
1.5
2
2.5
Abs
orba
nce
(a.u
.)
Wavelength (nm)e
d
acb
300 400 500 600 700 800 900
Fig. 5 – UV–visible spectra of supernatant solutions after
centrifugation of hexane solutions of (a) MWNT-7 with 1a,
(b) MWNT-7 with 1b, (c) MWNT-7 with 1c, (d) MWNT-7 with
PDMS macromer, and (e) 1a.
nate the excess PDMS–MGPs, and then dried under vacuum to
yield MWCNT/PDMS–MGP. Fig. 7 shows results of TGA on
MWCNT/PDMS–MGP and MWNT-7 treated with PDMS macro-
mer, which was prepared by the same method as MWCNT/
PDMS–MGP, except PDMS–MGP was replaced with PDMS macr-
omer. 1a (a) and PDMS macromer (b) were almost completely
decomposed before reaching 500 �C. MWNT-7 and Ctube100
did not show weight loss (wt%) in going from rt to 500 �C;
MWNT-7 treated with PDMS macromer also did not show
weight loss (g). Therefore, the weight loss observed from
approximately 270 to 500 �C in MWNT-7/1a (e) and Ctube100/
1a (f) can be attributed to MGPs immobilized on MWNT-7.
The grafting ratio (GR) of MGPs was defined as the mass ratio
of the MGPs immobilized on MWCNTs, which was estimated
by the weight loss determined by TGA. The GR of MWNT-7/
1a (e) was 0.036 and that of Ctube100/1a (f) was 0.293. This dif-
ference in GR was mainly due to the difference in the specific
surface area of the MWCNTs used. Table 2 shows the GRs of
MWCNT/PDMS–MGP (1a–c) in hexane. As shown in Table 2,
the solubility of MWCNTs with PDMS–MGPs was found to tend
to increase with increasing the GRs of MWCNT/PDMS–MGP.
3.4. Confirmation of p–p stacking between PDMS–MGPsand MWCNT surfaces
To confirm p–p stacking between pyrene units in PDMS–MGPs
and the MWCNT surfaces, we measured fluorescence spectra
of 1a and MWNT-7/1a in hexane after sonication for 1 h. As
shown in Fig. 8, the fluorescence of the MWNT-7/1a solution
was quenched relative to that of the 1a solution, although
the absorbance values of 1a at the excitation wavelength
(344 nm) in both UV–visible spectra were equal. The fluores-
cence quenching is considered to be due to the energy trans-
fer from the pyrene units of PDMS–MGPs to the MWCNTs
through p–p interactions [14].
3.5. Evaluation of MWCNTs with PDMS–MGPs inchloroform
The PDMS–MGPs (1a, 1b, or 1c) improved solubility of
MWCNTs in chloroform, which is a better solvent than hex-
ane for MWCNTs. The solutions of Ctube100 (3 mg) with 1a
(3 mg) in chloroform (30 mL) were stable for more than
2 months. As shown in Table 3, the absorbance of the super-
natant solution after centrifugation of MWCNTs (3 mg) in
chloroform (30 mL) with PDMS–MGPs (1a, 1b, or 1c) (3 mg)
was improved remarkably compared to that with PP1, PP2,
PDMS macromer, and PMMA. By comparison of the superna-
tant solution absorbances, the solubility of the MWCNTs with
PDMS–MGPs was found to be better than that of the MWCNTs
with pyrene-containing polymers, PDMS macromer, and
PMMA. Besides this, as shown in Table 3, the GRs of
MWCNT/PDMS–MGP (1a–c) in chloroform, which were esti-
mated from TGA-determined weight loss by the same method
as GRs in hexane, were larger than those of the MWCNT/pyr-
ene-containing polymer (PP1 and PP2). The comparison of
PP1 and PP2 shows that the GR and solubility of MWCNTs
barely change with an increase in Mn. However, after using
PDMS–MGPs, the GR and solubility did increase, even though
the PDMS–MGPs had lower Mn. This improved solubility is
Table 2 – Properties of MWCNT/PDMS–MGPs in hexane.
Code Abs.a (MWNT-7) Abs.a (Ctube100) GRb (MWNT-7) GRb (Ctube100)
1a 0.64 0.75 0.036 0.293
1b 0.23 0.45 0.032 0.235
1c 0.48 0.65 0.034 0.258
PDMS macromer 0.005 0.007 0.000 0.004
a Absorbance of supernatant solution at 600 nm by UV–visible spectrometry.
b The mass ratio of the polymers immobilized on MWCNTs.
a b
0
0.2
0.4
0.6
0.8
1
Abs
orba
nce
at 6
00 n
m
1a (mg)
0
0.2
0.4
0.6
0.8
1
Abs
orba
nce
at 6
00 n
m
1a (mg)0 1 2 3 4 5 6 0 1 2 3 4 5 6
Fig. 6 – Changes in absorbance at 600 nm by adding 0–6 mg of 1a to of (a) a solution of 3 mg of MWNT-7 in 30 mL of hexane
and (b) a solution of 3 mg of Ctube100 in 30 mL of hexane.
-100
-80
-60
-40
-20
0
Wei
ght (
%)
Temperature /
g
a
f
e d
b
c
0 100 200 300 400 500
Fig. 7 – TGA (heating rate = 20 �C min�1, under N2) of (a) 1a,
(b) PDMS macromer, (c) MWNT-7 (dashed line), (d) Ctube100,
(e) MWNT-7/1a, (f) Ctube100/1a, and (g) MWNT-7 treated
with PDMS macromer.
0
50
100
150
360 460 560 660
Fluo
resc
ence
inte
nsity
(a. u
.)
Wavelength (nm)
b
a
Fig. 8 – Fluorescence spectra (excitation wavelength:
344 nm) of (a) 1a and (b) MWNT-7 with 1a in hexane. The
absorbances due to the 1a moiety were adjusted to be 0.23
at 344 nm in both spectra.
2722 C A R B O N 4 7 ( 2 0 0 9 ) 2 7 1 6 – 2 7 2 6
considered to be due to the increase in surface repulsion be-
tween MWCNTs because of the presence of many side chains
in the PDMS–MGPs on MWCNT surfaces.
3.6. Synthesis and properties of PEG–MGPs
Other MGPs can be easily synthesized by a similar macromer
method using various macromers such as poly(ethylene
glycol) (PEG) macromer, polystyrene macromer and PMMA
macromer. As shown in Table 4, PEG–MGPs were synthesized
Table 3 – Properties of MWCNT/PDMS–MGPs in chloroform.
Code Abs.a (MWNT-7) Abs.a (Ctube100) GRb (MWNT-7) GRb (Ctube100)
1a 1.14 1.09 0.032 0.258
1b 1.03 0.90 0.030 0.236
1c 1.08 0.99 0.030 0.243
PP1 0.61 0.46 0.025 0.172
PP2 0.56 0.45 0.024 0.144
PDMS macromer 0.09 0.08 0.000 0.000
PMMA 0.09 0.08 0.000 0.000
a Absorbance of supernatant solution at 600 nm by UV–visible spectrometry.
b The mass ratio of the polymers immobilized on MWCNTs.
C A R B O N 4 7 ( 2 0 0 9 ) 2 7 1 6 – 2 7 2 6 2723
using PEG macromer, which are soluble in both organic sol-
vents and water. PEG–MGP 1d imparted better chloroform sol-
ubility to MWCNTs than PP1, PP2, and PEG macromer, as
shown in Table 5. Moreover, 1d imparted improved water sol-
ubility to MWCNTs. Fig. 9 shows the absorbances of the
supernatant solution after centrifugation of Ctube100 with
1d, sodium dodecylbenzenesulfonate (SDBS) [23], and PEG
macromer in water. The absorbance of the supernatant solu-
tion of Ctube100 dispersed in water with 1d was larger than
that with SDBS and PEG macromer.
3.7. Evaluation of epoxy resin monomer/MWCNT/MGPmixtures
Polymer nanocomposites prepared using MWCNT/MGPs were
evaluated. As MWCNTs, MWNT-7, which has a relatively
higher G/D value (G/D = �8.0, which is the Raman intensity ra-
tio of G-band (1590 cm�1) to D-band (1350 cm�1)) than
Ctube100 (G/D = �0.9) was used, as MWCNTs with higher G/
D values have higher potential to improve physical properties
such as thermal conductivity because of the reduced amount
of defects in the graphene structures. Addition of the MWNT-
7 (420 mg) to epoxy resin monomers (47.17 g, specific gravity:
�1.19) and sonication for 1 h generated an epoxy resin mono-
mer/MWNT-7 (99.5 vol%/0.5 vol%) mixture. As epoxy resin
monomers, mixtures of bisphenol-A diglycidyl ether
(25.22 g), 4-methylcyclohexane-1,2-dicarboxylic anhydride
(21.74 g), and 2-ethyl-4-methylimidazole (0.20 g) as hardening
agent were used. However, the appearance of the obtained
epoxy resin monomer/MWNT-7 mixtures was uneven, and
their viscosity was increased significantly compared to that
of epoxy resin monomers alone. This caused the productivity
in hardening processes to deteriorate. On the other hand, by
Table 4 – Synthesis and properties of PEG–MGPs in chloroform
Code Comp. 2:3:4a Xb
1d 40:25:35 PEG
PEG macromer – –
a Molar composition (mol%) in the co-monomer feed.
b Polymer unit of 2 in Fig. 2.
c Molar composition (mol%) in the polymers determined by 1H NMR in C
d Measured by GPC in chloroform with PS standards.
e Absorbance of supernatant solution at 600 nm by UV–visible spectrom
adding MWNT-7/1a (433.4 mg, consisting of MWNT-7
(420 mg) and 1a (13.4 mg)) to epoxy resin monomers
(47.16 g) and sonication for 1 h, well-dispersed epoxy resin/
MWNT-7/1a (99.47 vol%/0.5 vol%/0.03 vol%) mixtures were ob-
tained, and the viscosity of epoxy resin monomer/MWNT-7/
1a mixtures barely changed in comparison to epoxy resin
monomers alone. Fig. 11a shows well-dispersed epoxy resin/
MWNT-7/1a composites after hardening at 150 �C for
15 min. The viscosity of the epoxy resin monomer/MWCNT/
1a mixtures was evaluated more precisely using a shear-rate
controlled rheometer. Fig. 10 shows that by adding MWNT-7
(420 mg) to the epoxy resin monomers (47.17 g), the viscosity
(at 80 �C) of the obtained epoxy resin monomer/MWNT-7 mix-
tures after sonication for 1 h was significantly increased.
However, addition of MWNT-7/1a or MWNT-7/1d (each sam-
ple contains 420 mg of MWNT-7) to the epoxy resin mono-
mers (47.16 g) significantly decreased the viscosity. On the
other hand, the addition of MWNT-7/PP2 to the epoxy resin
monomers caused the viscosity to decrease slightly at a shear
rate of 0.1–20 s�1. However, at a shear rate greater than 20 s�1,
the viscosity of the epoxy resin monomer/MWNT-7/PP2 mix-
tures was almost equal to that of epoxy resin monomer/
MWNT-7 mixtures. In particular, the viscosities of epoxy resin
monomer/MWNT-7/1a mixtures and epoxy resin monomer/
MWNT-7/1d mixtures were found to be almost equal to that
of pristine epoxy resin monomer at a shear rate of
�1000 s�1. Improved flowability (a decrease in the viscosity)
of epoxy resin monomer/MWNT-7 mixtures is effective for
improving productivity in molding processes. It is not clear
why there is an improvement in the flowability when MGPs
are used. The MGPs with polymer side chains certainly im-
prove flowability in comparison with PP2, which does not
have polymer side chains. Therefore, the improvement of
.
Comp. l:m:nc Mnd Tge (�C)
55:16:29 33 k 5
– 1.1 k �35
DCl3.
etry.
Table 5 – Properties of MWCNT/PEG–MGPs.
Code Abs.a in chloroform(MWNT-7)
Abs.a in chloroform(Ctube100)
Abs.a in water(MWNT-7)
Abs.a in water(Ctube100)
GRb in water(MWNT-7)
GRb in water(Ctube100)
1d 1.05 0.92 0.55 0.44 0.036 0.348
PEG macromer 0.09 0.09 0.10 0.09 0.004 0.085
a Absorbance of supernatant solution at 600 nm by UV–visible spectrometry.
b The mass ratio of the polymers immobilized on MWCNTs.
0
0.4
0.8
1.2
1.6
2
Abso
rban
ce (a
.u.)
Wavelength (nm)
c
b
a
300 400 500 600 700 800
Fig. 9 – UV–visible spectra of supernatant solutions after
centrifugation of water solutions of (a) Ctube100 with 1d, (b)
Ctube100 with SDBS, and (c) Ctube100 with PEG macromer.
0.1
1
10
100
Visc
osity
(Pa
)
Shear rate (s−−1)0.1 1 10 100 1000
Fig. 10 – Viscosities of epoxy resin monomer (j), epoxy
resin monomer/MWNT-7 (m), epoxy resin monomer/MWNT-
7/PP2 (d), epoxy resin monomer/MWNT-7/1a (�), and epoxy
resin monomer/MWNT-7/1d (·) after sonication for 1 h,
measured with a shear-rate controlled rheometer.
2724 C A R B O N 4 7 ( 2 0 0 9 ) 2 7 1 6 – 2 7 2 6
flowability is considered to be mainly due to a decrease in
interaction between MWCNTs because of the presence of
bulky polymer side chains in the MGPs on MWCNT surfaces.
3.8. Evaluation of PBT/MWCNT/MGP composites
The effect of the addition of MWCNT/PDMS–MGPs on the
viscosity of thermoplastic polymer composites was evalu-
ated. As the thermoplastic polymer, PBT, which is generally
used as a material for automobile parts, was used. By adding
MWNT-7/1a (151.7 mg, consisting of MWNT-7 (147 mg. 1 vol%)
and 1a (4.7 mg, 0.06 vol%)) to PBT (9.0732 g, 98.94 vol%), PBT/
MWNT-7/1a (98.94 vol%/1 vol%/0.06 vol%) composites were
produced using a micro rheology compounder. An SEM image
of these composites (Fig. 11b) shows that MWCNTs are well
dispersed in these composites. The micro rheology com-
pounder was also used to measure the viscosity (260 �C and
250 rpm for 5 min) of melt mixing PBT/MWNT-7/1a compos-
ites. By adding MWNT-7/1a (151.7 mg) to PBT (9.0732 g, with
the melt viscosity 18.8 Pa s at 260 �C at a shear rate of
�900 s�1 measured by the micro rheology compounder), the
viscosity of melt mixing for PBT/MWNT-7/1a composites
was reduced from 23.8 Pa s for PBT/MWNT-7 composites to
18.9 Pa s, as shown in Fig. 12. Moreover, to our surprise, the
viscosity of PBT/MWNT-7/1a (98.94 vol%/1 vol%/0.06 vol%)
composites was found to be almost equal to that of pristine
PBT (18.8 Pa s). On the other hand, the viscosity of PBT/
MWNT-7/PP2 (98.95 vol%/1 vol%/0.05 vol%) composites was
23.4 Pa s, which was almost equal to that of PBT/MWNT-7
composites. The comparison of PBT/MWNT-7/1a composites,
PBT/MWNT-7/PP2 composites, and pristine PBT shows that
PDMS side chains in the PDMS–MGPs decrease the viscosity
significantly. As shown in Fig. 13, the reduction in viscosity
of PBT/MWNT-7/1a (98.94 vol%/1 vol%/0.06 vol%) composites
in comparison with that of PBT/MWNT-7 (99 vol%/1 vol%)
composites was found to be 20.6%. On the other hand, when
the content of MWNT-7 in PBT/MWNT-7/1a composites in-
creased from 1 vol% to 3 vol%, the reduction in viscosity in
comparison with the PBT/MWNT-7 composites was only
5.4%. Improvement of flowability of the PBT/MWNT-7/1a
composites including small amounts of MWNT-7 (63 vol%)
in comparison with PBT/MWNT-7 composites was considered
to be mainly due to a decrease in interaction between
MWCNTs because of the presence of bulky side chains in
the MGPs on MWCNT surfaces. Therefore, by increasing the
MWNT-7 content in the PBT/MWNT-7/1a composites to
P3 vol%, the reduction in viscosity in comparison with the
PBT/MWNT-7 composites approaches 0%, which is due to an
increase in interaction between individual MWNT-7 nano-
tubes through an increase in MWNT-7 concentration. In
15
20
25
30
35
Visc
osity
(Pa
s)
Time (min)
b
ca
Fig. 12 – Changes in the viscosities of (a) PBT/MWNT-7/1a
(98.94 vol%/1 vol%/0.06 vol%) composites, (b) PBT/MWNT-7
(99 vol%/1 vol%) composites, and (c) pristine PBT.
15
20
25
30
35
40
0 1 2 3 4
Visc
osity
(Pa
s)
MWNT-7 content (vol%)
Fig. 13 – Changes in the viscosities of PBT/MWNT-7/1a
composites (d), PBT/MWNT-7 composites (m), and pristine
PBT (j).
Fig. 11 – SEM images of frozen fracture surfaces of (a) epoxy resin/MWNT-7/1a (99.47 vol%/0.5 vol%/0.03 vol%) composites and
(b) PBT/MWNT-7/1a (98.94 vol%/1.0 vol%/0.06 vol%) composites.
C A R B O N 4 7 ( 2 0 0 9 ) 2 7 1 6 – 2 7 2 6 2725
addition, it may also be due to a decrease in the friction
between MWCNT/MGPs and the inner wall of the cylinder of
the micro rheology compounder because of the presence of
side chains. Improved flowability of the PBT/MWCNT com-
posites using MWCNT/PDMS–MGPs at a high shear rate of
900 s�1 is especially important for injection molding.
4. Conclusion
We report noncovalent functionalization of MWCNTs using
MGPs containing suitable polymer side chains for improving
solubility and pyrene units for improving adsorption on
MWCNT surfaces. PDMS–MGPs were easily synthesized by
radical copolymerization of PDMS macromer and a pyrene-
containing monomer. The PDMS–MGPs showed strong physi-
cal adsorption on the MWCNT surfaces and improved the sol-
ubility of MWCNTs in both chloroform and hexane, which is
typically a poor solvent for MWCNTs. In addition, various
MGPs such as PEG–MGPs were easily synthesized by a similar
macromer method using various macromers. The PEG–MGPs
imparted improved water solubility to MWCNTs. Through
the addition of the MWCNT/PDMS–MGPs to epoxy resin
monomers, the viscosity of the obtained epoxy resin mono-
mer/MWCNT/PDMS–MGP mixtures was reduced drastically
in comparison with that of epoxy resin monomer/MWCNT
mixtures. Moreover, by addition of the MWCNT/PDMS–MGPs
to PBT, the viscosity of the PBT/MWCNT/PDMS–MGP compos-
ites was reduced drastically in comparison with that of the
PBT/MWCNT composites. The viscosity of PBT/MWCNT/
PDMS–MGP composites including 61 vol% of MWCNTs, in
particular, was almost equal to that of pristine PBT. Further
studies including evaluation of the thermal, electrical, and
mechanical properties of various polymer/MWCNT/MGP
composites are in progress.
2726 C A R B O N 4 7 ( 2 0 0 9 ) 2 7 1 6 – 2 7 2 6
Acknowledgements
We are grateful to Minoru Takahara for performing the shear-
rate controlled rheometer measurements and to Takashi Ohta
and Dr. Hisato Takeuchi for helpful discussions.
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