Multifunctional polybenzoxazine nanocomposites containing photoresponsive azobenzene units, catalytic carboxylic acid groups, and pyrene units capable of dispersing carbon nanotubes

Article abstract of DOI:10.1039/c5ra07983g

In this study, we synthesized a new multifunctional benzoxazine monomer Azo-COOH-Py BZ - featuring an azobenzene unit, a carboxylic acid group, and a pyrene moiety - through the reaction of 4-(4-hydroxyphenylazo)benzoic acid (Azo-COOH), paraformaldehyde, and aminopyrene (Py-NH₂) in 1,4-dioxane. Fourier transform infrared (FTIR) spectroscopy and ¹H and ¹³C nuclear magnetic resonance spectroscopy confirmed the structure of this new monomer. Using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and FTIR spectroscopy, we monitored the curing behavior of Azo-COOH-Py BZ leading to the formation of poly(Azo-COOH-Py BZ); we found that the carboxylic acid and azobenzene units acted as catalysts for the ring opening reaction of the benzoxazine unit. The pyrene moiety of Azo-COOH-Py BZ enhanced the dispersibility of carbon nanotubes (CNTs) in THF, leading to the formation of highly dispersible Azo-COOH-Py BZ/CNT nanocomposites stabilized through π-π stacking of the pyrene and CNT units, as detected through fluorescence emission spectroscopy. We also used DSC and TGA to examine the curing behavior of Azo-COOH-Py BZ/CNTs to form poly(Azopy-COOH-Py BZ)/CNTs nanocomposites. Interestingly, DSC profiles revealed that the maximum exothermic peak representing the ring opening polymerization of the benzoxazine unit of Azo-COOH-Py BZ shifted to much lower temperature upon increasing the content of single-walled CNTs (SWCNTs) or multiwalled CNTs (MWCNTs), suggesting that the CNTs acted as catalysts for the ring opening reaction of the benzoxazine. In addition, the curing temperatures for the SWCNT composites were lower than those for the MWCNT composites, suggesting that the SWCNTs were dispersed better than the MWCNTs in their composites and that the thermal stability of the SWCNT nanocomposites was higher than that of the MWCNT nanocomposites. The combination of photoresponsive azobenzene units, carboxylic acid groups, and CNTs enhanced the thermal stability and char yields of the polybenzoxazine matrixes, as determined through TGA analyses.

Full text of DOI:10.1039/c5ra07983g

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Multifunctional polybenzoxazine nanocomposites
containing photoresponsive azobenzene units,
catalytic carboxylic acid groups, and pyrene units
capable of dispersing carbon nanotubes†
Cite this: RSC Adv., 2015, 5, 45201
a
a
b
c
Mohamed Gamal Mohamed, Chi-Hui Hsiao, Faliang Luo, Lizong Dai
and Shiao-Wei Kuo*a
In this study, we synthesized a new multifunctional benzoxazine monomer Azo-COOH-Py BZ—featuring an
azobenzene unit, a carboxylic acid group, and a pyrene moiety—through the reaction of 4-(4-
2
hydroxyphenylazo)benzoic acid (Azo-COOH), paraformaldehyde, and aminopyrene (Py-NH ) in 1,4-
1
13
dioxane. Fourier transform infrared (FTIR) spectroscopy and H and C nuclear magnetic resonance
spectroscopy confirmed the structure of this new monomer. Using differential scanning calorimetry
(DSC), thermogravimetric analysis (TGA), and FTIR spectroscopy, we monitored the curing behavior of
Azo-COOH-Py BZ leading to the formation of poly(Azo-COOH-Py BZ); we found that the carboxylic
acid and azobenzene units acted as catalysts for the ring opening reaction of the benzoxazine unit. The
pyrene moiety of Azo-COOH-Py BZ enhanced the dispersibility of carbon nanotubes (CNTs) in THF,
leading to the formation of highly dispersible Azo-COOH-Py BZ/CNT nanocomposites stabilized through
p–p stacking of the pyrene and CNT units, as detected through fluorescence emission spectroscopy.
We also used DSC and TGA to examine the curing behavior of Azo-COOH-Py BZ/CNTs to form
poly(Azopy-COOH-Py BZ)/CNTs nanocomposites. Interestingly, DSC profiles revealed that the maximum
exothermic peak representing the ring opening polymerization of the benzoxazine unit of Azo-COOH-
Py BZ shifted to much lower temperature upon increasing the content of single-walled CNTs (SWCNTs)
or multiwalled CNTs (MWCNTs), suggesting that the CNTs acted as catalysts for the ring opening
reaction of the benzoxazine. In addition, the curing temperatures for the SWCNT composites were lower
than those for the MWCNT composites, suggesting that the SWCNTs were dispersed better than the
MWCNTs in their composites and that the thermal stability of the SWCNT nanocomposites was higher
than that of the MWCNT nanocomposites. The combination of photoresponsive azobenzene units,
carboxylic acid groups, and CNTs enhanced the thermal stability and char yields of the polybenzoxazine
matrixes, as determined through TGA analyses.
Received 1st April 2015
Accepted 14th May 2015
DOI: 10.1039/c5ra07983g
www.rsc.org/advances
including low degrees of water absorption, near-zero volumetric
shrinkage, low surface free energies, high glass transition
Introduction
Polybenzoxazines (PBZs) are recently developed thermosetting temperatures, excellent resistance toward chemicals and excel-
phenolic resins that are attracting academic and industrial lent physical and mechanical properties relative to those of
1–4
interest because they possess several unique properties, other thermosetting resins.
In addition, low dielectric
constants and low ammability make them attractive materials
3–11
a
for use in many applications.
PBZs can be obtained through
Department of Materials and Optoelectronic Science, Center for Functional Polymers
and Supramolecular Materials, National Sun Yat-Sen University, Kaohsiung, Taiwan. polymerization, via heterocyclic ring opening, of corresponding
E-mail: kuosw@faculty.nsysu.edu.tw
benzoxazine (BZ) monomers at elevated temperature, without
the need for strong acids or bases and without releasing
byproducts.
b
Key Laboratory of Energy Resources & Chemical Engineering, Ningxia University,
Ministry-Province Co-cultivated State Key Laboratory Base of Natural Gas
Conversion, Yinchuan 750021, China
12–14
Although they are promising materials, it remains neces-
sary to improve the performance of PBZs. The mechanical
properties of PBZs can be enhanced through the addition of
c
Department of Material Science and Engineering, Fujian Provincial Key Laboratory of
Fire Retardant Materials, College of Materials, Xiamen University, Xiamen, Fujian,
3
61005, China
1
5
16
†
Electronic supplementary information (ESI) available: DSC analyses of the inorganic materials, including glass bers, carbon bers,
thermal curing behavior of Py-BZ, Azo-COOH, Azo-COOH-Py BZ, and the
17–20
polyhedral oligomeric silsesquioxane (POSS),
graphene
Azo-COOH-Py BZ/MWCNT complex. See DOI: 10.1039/c5ra07983g
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2
1
22–30
oxide, and carbon nanotubes (CNTs).
CNTs are hexagonal process and decrease the curing temperature of BZ mono-
2
2
arrays of rolled carbon sheets having lengths of several mers.
Nevertheless, the covalent functionalization of
microns and diameters of a few nanometers. Based on the MWCNTs will destroy their optical, electronic, and mechanical
number of carbon layers in the wall, these structures can be properties, because of the inevitable change in orbital
2
3
26
classied into either single-walled carbon nanotubes hybridization of some of their carbon atoms from sp to sp .
3
1,32
(
SWCNTs) or multiwalled carbon nanotubes (MWCNTs).
The second approach, involving noncovalent interactions
CNTs are materials of interest for academic research and between organic PBZs and CNTs, generally preserves nearly
industrial applications because they possess many unique all of the intrinsic features of the CNTs. Dumas et al.
3
3,34
chemical and physical properties,
including high stiffness, reported that p-stacking interactions between CNTs and a
strength, and thermal conductivity arising from the graphitic phenylenediamine-based BZ led to the formation of a rein-
2
2
nature of the nanotube lattice. Two general approaches have forced network displaying excellent thermal and mechanical
been reported to increase the miscibility of CNTs in PBZ properties, with enhanced dispersion of the unreacted
2
7
matrices: covalent functionalization and noncovalent adsorp- MWCNTs in the PBZ matrix. Wang et al. reported a BZ
tion of organic molecules onto CNT surfaces. The rst derivative that behaved as an attractive noncovalent disper-
approach involves covalently bonding surface-modifying sant for CNTs, leading to functionalized CNTs exhibiting high
2
6
groups or molecules onto the CNTs to enhance their disper- dispersibility in an organic solvent. In addition, we have
2
3,24
sion in solvents and/or polymer matrices.
For example, developed a pyrene-based BZ precursor that forms PBZ/CNT
PBZ-modied MWCNTs have been prepared aer sequential nanocomposites stabilized through strong p–p interactions
2
8,29
treatment of MWCNTs with HNO3 and toluene-2,4- between the pyrene units and CNT surfaces.
We found,
diisocyanate to introduce OH, COOH, and N]C]O groups, however, that the model pyrene-functionalized BZ (Py-BZ)
with the surface COOH groups then catalyzing the opening of prepared from phenol, formaldehyde, and 1-aminopyrene
2
2,25
benzoxazine rings.
In addition, Chen et al. reported that exhibited a relatively high exothermic ring opening tempera-
ꢀ
28
the surface carboxyl groups of CNTs catalyze the ring opening ture of 286.0 C.
Scheme 1 (a) 4-Aminobenzoic acid and synthesis of (b) Azo-COOH, (c) Azo-COOH-Py BZ, (d) poly(Azo-COOH-Py BZ) and (e) further crosslinking
of poly(Azo-COOH-Py BZ).
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Scheme 2 The possible Azo-COOH-Py BZ/CNT hybrid complex and after thermal curing to form PBZ/CNTs nanocomposites.
PBZs are also interesting materials because strong intra- 1-aminopyrene in 1,4-dioxane [Scheme 1] and then used it to
molecular hydrogen bonding interactions remain aer curing form poly(Azo-COOH-Py)/CNT nanocomposites with SWCNTs
and because of their extremely low surface energies—in some and MWCNTs [Scheme 2]. Differential scanning calorimetry
4
cases lower than that of pure Teon. Nevertheless, the need for (DSC), Fourier transform infrared (FTIR) spectroscopy, and
high curing temperatures can limit the applications of PBZs, thermogravimetric analysis (TGA) revealed the thermal curing
5,9
especially as polymer coatings. In a previous study, we found behavior and thermal stability of the BZ monomer, both before
that Azo-COOH BZ, a monomer containing both azobenzene and aer blending with various weight percentages of SWCNTs
and COOH units, exhibits polymerization temperature much and MWCNTs. In addition, we used photoluminescence spec-
35
lower than that of traditional BZ monomers. Polymers con- troscopy and transmission electron microscopy (TEM) to inves-
taining azobenzene moieties are interesting because of their tigate the interactions and dispersions of the SWCNTs and
ability to undergo reversible photoisomerization or thermal cis- MWCNTs with the PBZ matrices before and aer thermal curing.
to-trans isomerization processes. Thus, the incorporation of
azobenzene moieties into PBZs can impart them with thermally
Experimental section
regulated behavior aer being subjected to changes in the
36–38
amount of incident light or heat.
Indeed, Kiskan et al. Materials
synthesized an azobenzene functionalized BZ that exhibited
high thermal stability arising from the presence of the addi-
tional aromatic group.
As a result, we wished to examine the combination of a pho-
toresponsive azobenzene unit, a catalytic COOH group, and a
pyrene unit (for dispersing CNTs) within a BZ monomer, allowing
the formation of high-performance PBZ/CNT nanocomposites.
Without the need to functionalize CNTs with COOH groups, PBZ/
CNT nanocomposites derived from such a multifunctional BZ
monomer, which can itself catalyze the opening of the BZ ring
Paraformaldehyde (96%), pyrene, NaOH, Na
benzoic acid, CHCl , CH Cl , EtOH, tetrahydrofuran (THF), and
,4-dioxane were purchased from Acros. Ethyl acetoacetate was
purchased from Showa. HNO (65%), AcOH (99.8%), and H SO
65%) were purchased from Scharlau. Hydrazine monohydrate
98%) was purchased from Alfa Aesar. 1-Nitropyrene was
synthesized using a previously reported procedure. SWCNTs
and MWCNTs were purchased from Center Biochemistry
Technology.
2 3
CO , 4-amino-
3
2
2
39
1
3
2
4
(
(
28
(
through its COOH units), would feature high dispersion of the
Pyren-1-amine (Py-NH2)
CNTs in the polymer matrix, stabilized through p–p interactions
28
between the pyrene units and the CNT surfaces, thereby over- In a modied version of a procedure reported our literature, 1-
coming the problems associated with covalently functionalized nitropyrene (10.0 g, 40.5 mmol) and 10% Pd/C (0.2 g) were
CNTs while retaining the photoresponsive azobenzene units to dissolved/suspended in absolute EtOH (250 mL) and THF (150
mediate changes in the surface properties upon the application mL) under N2 in a 500 mL two-neck round-bottom ask
of light and/or heat. Thus, in this study we prepared Azo-COOH- equipped with a stirrer bar. The suspension was heated under
ꢀ
Py BZ through the reaction of Azo-COOH, paraformaldehyde, and reux at 90 C and then hydrazine monohydrate (15 mL) was
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added dropwise. Aer heating under reux for a further 24 h, Photoisomerization of Azo-COOH-Py BZ
the mixture was cooled and ltered; the ltrate was evaporated
to dryness under reduced pressure. The crude product was
recrystallized from cyclohexane and then further puried
through column chromatography (n-hexane/THF, 3 : 1). Drying
À4
A solution of Azo-COOH-Py BZ (1 Â 10 M) in THF (10 mL) was
placed in a quartz tube and irradiated for intervals of time at
room temperature in a merry-go-round-type photoreactor
equipped with ve Philips lamps emitting light nominally at
ꢀ
at 40 C gave greenish yellow crystals (7.50 g, 85%); m.p.: 115–
365 nm.
ꢀ
À1
1
1
17 C (DSC). FTIR (KBr, cm ): 3200–3400 (NH stretch). H
NMR (500 MHz, DMSO-d , d, ppm): 6.34 (s, 2H, NH ), 7.34–8.28
m, 9H, CCH). C NMR (125 MHz, DMSO-d , d, ppm): 113.0
6
2
1
3
Poly(Azopy-COOH BZ) lms
(
(
6
CHCHNH ), 121.1–132.9 (aromatic), 144.4 (CHNH ).
A solution of Azo-COOH-Py BZ monomer (4 wt%) in THF was
passed through a 0.3 mm syringe lter and then a portion (1 mL)
was spin-coated (1500 rpm, 50 s) onto a glass slide (1000 mm Â
2
2
4-(4-Hydroxphenyazo)benzoic acid (Azo-COOH)
1
00 mm  1 mm). Aer thermal curing, the poly(Azo-COOH-Py
In a modied version of a procedure reported in the litera-
BZ) lms were examined for their surface behavior.
3
5
ture, a solution of 4-aminobenzoic acid (13.7 g, 0.100 mol) in
dilute HCl [a mixture of conc. HCl (20 mL) in water (20 mL)]
was diazotized through the addition of a solution of sodium
Azo-COOH-Py BZ/CNT nanocomposites
nitrite (6.90 g, 0.1 mol) in water (20 mL). The mixture was CNTs (SWCNTs or MWCNTs) were rst dispersed at various
ꢀ
stirred at 0 C and then diluted with cold MeOH (300 mL). weight percentages in THF, under ultrasonic vibration for 2 h.
Phenol (9.41 g, 0.100 mol), dissolved in a cooled solution of Each of the resulting solutions was mixed with the Azo-COOH-
NaOH (10.8 g, 0.190 mol) in MeOH (50 mL), was added drop- Py BZ monomer. The solvent was evaporated in a vacuum oven
ꢀ
wise to the solution containing the diazonium salt. A red dye at 50 C for 1 day. The blend mixtures were then poured into a
formed; this mixture was stirred for 2 h and then poured into stainless-steel mold and subjected to thermal curing in a
ꢀ
dilute HCl (0.5 N) with stirring for 1 h. The red solid was stepwise manner (heating at 110, 150, 180, 210, and 240 C,

ltered off, washed many times with deionized water, dried, for 2 h at each temperature). The color of each sample was
and recrystallization (MeOH/H O, 1 : 1) to give the product dark-red.
2
ꢀ
À1
(
3
9.00 g, 66%); m.p.: 273–275 C (DSC). FTIR (KBr, cm ): 3463–
204 (OH stretch), 3053–2500 (COOH), 1660 (C]O stretch). H
1
Characterization
NMR (500 MHz, DMSO-d , d, ppm): 6.88–8.12 (m, 8H, ArH),
6
1
13
H and C nuclear magnetic resonance (NMR) spectra were
1
3
1
0.17 (s, 1H, OH), 13.13 (s, 1H, COOH). C NMR (125 MHz,
recorded using an INOVA 500 instrument with DMSO-d and
6
6
DMSO-d , d, ppm): 167.20 (C]O).
CDCl as solvents and tetramethylsilane (TMS) as the external
3
standard. FTIR spectra of the BZ monomer and PBZ polymer
were recorded using a Bruker Tensor 27 FTIR spectrophotom-
(
E)-4-[3-(Pyren-2-yl)-3,4-dihydro-2H-benzoxazin-6-yl]
diazenylbenzoic acid (Azo-COOH-Py BZ)
eter and the conventional KBr disk method; 32 scans were
À1
A solution of 1-aminopyrene (1.00 g, 4.60 mmol) in 1,4-dioxane collected at a spectral resolution of 4 cm . The lms tested in
(
25 mL) was added portionwise to a stirred solution of para- this study were sufficiently thin to obey the Beer–Lambert law.
formaldehyde (0.290 g, 9.67 mmol) in 1,4-dioxane (50 mL) in a FTIR spectra recorded at elevated temperatures were obtained
50 mL three-neck round-bottom ask cooled in an ice bath. The from cell mounted inside the temperature-controlled
1
a
mixture was then stirred for 30 min while maintaining the compartment of the spectrometer. Dynamic curing kinetics
ꢀ
temperature below 5 C. A solution of 4-(4-hydroxphenyazo)ben- were determined using a TA Q-20 differential scanning calo-
zoic acid (0.810 g, 5.06 mmol) in 1,4-dioxane (30 mL) was added; rimeter operated under a N
2
atmosphere. The sample (ca. 5 mg)
the mixture was degassed three times and then heated under was placed in a sealed aluminum sample pan. Dynamic curing
ꢀ
ꢀ
ꢀ
reux for 24 h at 80–90 C while stirring. Aer cooling the mixture scans were recorded from 30 to 350 C at a heating rate of 20 C
À1
to room temperature, the solvent was evaporated under reduced min . The thermal stabilities of the samples were measured
pressure to give a yellow solid, which was extracted into Et
2
O (3 Â using a TA Q-50 thermogravimetric analyzer operated under a
8
5
0 mL); the combined extracts were washed several times with N atmosphere. A cured sample (ca. 5 mg) was placed in a Pt
2
ꢀ
À1
À1
ꢀ
% Na CO and nally with distilled water. The organic phase cell and heated at a rate of 20 C min from 30 to 800 C under
2
3
4 2
was dried (anhydrous MgSO ) for 1 h and then concentrated a N ow rate of 60 mL min . UV-Vis spectra were recorded
under reduced pressure to afford a yellow solid, which was using a Shimadzu mini 1240 spectrophotometer; the concen-
À4
puried through column chromatography (SiO
1
3
2
; n-hexane/EA, tration of Azo-CCOH-Py BZ in THF was 10
M. Photo-
À1
: 1) to give a yellow solid (0.94 g, 73%). FTIR (KBr, cm ): isomerization of azobenzene moieties was performed directly
053–2500 (COOH), 1232 (asymmetric COC stretching), 1341 under a UV-Vis lamp at 365 nm (90 mW cm ). Photo-
wagging), 931 and 1490 (vibration of trisubstituted benzene luminescence (PL) excitation and emission spectra were
À1
(CH
2
1
ring), 1660 (C]O stretching). H NMR (500 MHz, DMSO-d , d, recorded at room temperature using a monochromatized Xe
6
ppm): 4.84 (s, 2H, ArCH N), 5.59 (s, 2H, OCH N), 6.92–8.43 (m, light source. TEM images were recorded using a JEOL-2100
2
2
1
3
ArH). C NMR (125 MHz, DMSO-d
6
, d, ppm): 48.47 (ArCH
2
N), transmission electron microscope operated at an accelerating
voltage of 200 kV.
8
0.04 (OCH N), 113.7–157.8 (aromatic), 160.5 (C]O).
2
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2 2
.7 ppm, which we assign to the ArCH N and OCH N units,
Results and discussion
Synthesis of Azo-COOH-Py BZ monomer
respectively; the signals of the NH proton of Py-NH and the OH
proton of Azo-COOH were absent, but the signal for the COOH
2
1
13
H NMR, C NMR and FT-IR spectroscopy conrmed the group remained along with multiple signals at 6.5–8.4 ppm
1
3
chemical structure of the Azo-COOH-Py BZ monomer, which we representing aromatic protons. Fig. 2 presents the C NMR
synthesized in high yield through Mannich condensation of spectra of Azo-COOH, Py-NH , and Azo-COOH-Py BZ. In the
Azo-COOH, 1-aminopyrene, and paraformaldehyde in 1,4- spectrum of Azo-COOH [Fig. 2(a)], the signals of the azobenzene
2
ꢀ
1
dioxane at 90 C (Scheme 1). Fig. 1 displays the H NMR spectra moiety appear in the range 116.6–162.3 ppm, with a signal at
of Azo-COOH, 1-aminopyrene, and Azo-COOH-Py BZ. The 167.2 ppm representing to C]O group. The spectrum of Py-NH
spectrum of Azo-COOH [Fig. 1(a)] features signals at 10.47 and features signals for the pyrenyl aromatic ring in the range
3.13 ppm representing the OH groups of the phenolic and 113.1–144.4 ppm [Fig. 2(b)]. The spectrum of Azo-COOH-Py BZ
COOH units, respectively; the signals of the aromatic protons of [Fig. 2(c)] reveals characteristic signals for the carbon nuclei of
Azo-COOH appeared in the range 6.97–8.10 ppm. The spectrum the oxazine ring at 51.9 and 82.9 ppm (ArCH N and OCH N,
of Py-NH
[Fig. 1(b)] features a characteristic peak at 6.33 ppm respectively) and a signal at 167.5 ppm for the C]O group.
for the NH proton. The H NMR spectrum of Azo-COOH-Py BZ Fig. 3 presents the FTIR spectra recorded at ambient tempera-
monomer [Fig. 1(c)] displays characteristic signals for the oxa- ture of Azo-COOH, Py-NH , Azo-COOH-Py BZ, and, aer thermal
2
1
2
2
2
1
2
zine ring protons of Azo-COOH-Py BZ as two singlets at 4.9 and curing, poly(Azo-COOH-Py BZ). The spectrum of Azo-COOH
À1
[
Fig. 3(a)] features a broad signal at 3053–2523 cm for the
hydrogen-bonded COOH dimer and sharp signals at 3470 and
À1
3
540 cm corresponding to the hydrogen-bonded and free OH
groups, respectively. The spectrum of Py-NH
contains a sharp signal for the amino group at 3200–3460 cm
2
[Fig. 3(b)]
À1
,
with absorption bands of the pyrene unit at 831, 1599, 1619, and
À1
3
033 cm . The FTIR spectrum of Azo-COOH-Py BZ monomer
[Fig. 3(c)] displays characteristic absorption bands at 1228
(
asymmetric COC stretching), 1073 (symmetric COC stretching),
À1
and 920 (stretching vibrations of oxazine ring) cm , as well as a
band at 1376 cm representing the tetrasubstituted benzene
ring and a band at 1670 cm representing the C]O group.
À1
À1
Fig. 3(d) presents FTIR spectra of the pure Azo-COOH-Py BZ
monomer recorded aer each thermal curing cycle. The char-
acteristic absorption bands of the trisubstituted aromatic ring
À1
of Azo-COOH-Py BZ (1490 and 920 cm ) disappeared aer
thermal curing, while broad absorption bands, representing
different types of hydrogen bonding interactions, appeared at
1
À1
1
13
Fig. 1 H NMR spectra of (a) Azo-COOH, (b) Py-NH
2
, and (c) Azo- 2500–3500 cm . All this information from the H, C NMR,
and FTIR spectra is consistent with the successful synthesis of
the Azo-COOH-Py BZ monomer.
COOH-Py BZ.
13
Fig. 2
2 2
C NMR spectra of (a) Azo-COOH, (b) Py-NH , and (c) Azo- Fig. 3 FTIR spectra of (a) Azo-COOH, (b) Py-NH , (c) Azo-COOH-Py
COOH-Py BZ.
BZ, and (d) poly(Azo-COOH-Py BZ), recorded at room temperature.
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Thermal curing behavior of Azo-COOH-Py BZ monomer
but a single peak for the Py-BZ monomer, indicating that the
Azo-COOH unit introduces another mode of curing for the BZ
monomer. We believe the rst exotherm arises from opening of
the oxazine ring, while the second represents decarboxylation.
The maximum curing temperature and the enthalpy of the
To better understand the ring opening polymerization and
thermal curing behavior of the Azo-COOH-Py BZ monomer, we
used DSC to investigate the thermal polymerization and curing
temperature of the Azo-COOH-Py BZ monomer and FTIR spec-
troscopy to monitor changes in the absorption signal of the BZ
ring. Fig. 4(A) presents DSC thermograms of Azo-COOH-Py BZ
ꢀ
aer each curing cycle, recorded at a heating rate of 20
C
À1
min . The DSC curves of the uncured Azo-COOH-Py BZ reveal a
maximum with an exothermic peak at 226.0 C, and a reaction
heat of 324.7 J g . This curing temperature is much lower than
ꢀ
À1
that of a model pyrene-functionalized BZ (Py-BZ) lacking the
2
8
ꢀ
Azo-COOH unit; its exothermic ring opening at 286.0 C, as
displayed in Fig. S1(a),† suggests that the Azo-COOH unit in
Azo-COOH-Py BZ acted as a catalyst that contributed to the
lower polymerization temperature. The acidic character of this
reactive monomer increased the concentration of oxonium
species, thereby catalyzing the ring opening reaction of the BZ
unit. Nevertheless, this curing temperature is higher than that
ꢀ
(
sharp exothermic ring opening peak at 199 C) of the uncured
Azo-COOH BZ lacking the pyrene unit [Fig. S1(c)†], indicating
that the bulky pyrene unit hindered the ring opening poly-
merization of the BZ unit. We observed two major curing peaks
Fig. 5 TGA analyses of Azo-COOH-Py BZ, recorded after each curing
each for the Azo-COOH BZ and Azo-COOH-Py BZ monomers, stage.
Fig. 4 (A) DSC thermograms and (B) FTIR spectra of Azo-COOH-Py BZ, recorded after each heating stage.
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ꢀ
exotherms generally decreased upon increasing the curing increased signicantly to 280 C (similar to the exothermic peak
ꢀ
temperature of Azo-COOH-Py BZ. The rst exotherm, corre- at 281 C determined through DSC) when the curing tempera-
ꢀ
ꢀ
sponding to opening of the oxazine ring, disappeared when the ture was 180 C, compared with a value of 230 C when the
ꢀ
ꢀ
curing temperature was 180 C; the second exotherm shied to curing temperature was 150 C. This behavior was due to the
ꢀ
ꢀ
2
81 C when the curing temperature was 180 or 210 C; and all crosslinking structure that formed [Scheme 1(d)] aer thermal
ꢀ
exotherm peaks disappeared completely aer applying a curing curing at 180 C. Further increasing the curing temperature to
ꢀ
ꢀ
ꢀ
temperature of only 240 C, indicating that thermal curing of 240 C caused the value of Td5 to increase signicantly to 396 C,
Azo-COOH-Py BZ was complete at this temperature. indicating a more highly cross-linked structure [Scheme 1(e)],
Fig. 4(B) displays FTIR spectra of the Azo-COOH-Py BZ presumably because of hydrogen bonding between the COOH
monomer and its products aer each curing stage. The inten- groups and/or interactions involving the new reactive positions
40
sities of the characteristic absorption bands at 1496 (stretching produced aer decarboxylation.
of trisubstituted benzene ring), 1234 (asymmetric COC
stretching of oxazine), and 920 (stretching vibrations of oxazine
À1
Photoisomerization of azobenzene chromophore in Azo-
COOH-Py BZ monomer
ring) cm
decreased gradually upon increasing the curing
À1
temperature. The peak at 920 cm disappeared completely and
a new absorption appeared at 3371 cm , corresponding to Azobenzene moieties are well known for their reversible trans-
À1
41,42
[O–H/O] intermolecular hydrogen bonding of OH groups, aer to-cis photoisomerization upon irradiation with light.
opening of the oxazine ring when the curing temperature was Correspondingly, both the molecular morphology and the
ꢀ
1
80 C, consistent with the ndings from our DSC analyses in
which the rst exotherm peak from the ring opening polymer-
ꢀ
ization of the oxazine disappeared completely at 180 C. Upon
further increasing the curing temperature, the main feature in
these spectra was the decreased intensity of the band corre-
À1
sponding to the COOH group at 1677 cm , almost disappear-
ꢀ
ing at 240 C, indicating that thermal polymerization of this
40
monomer proceeds with partial decarboxylation, with one
À1
peak appearing at 1487 cm , attributable to the tetrasub-
stituted benzene ring, that increased in intensity during the
curing cycle to produce PBZ.
Fig. 5 displays the thermal stability of the Azo-COOH-Py BZ
monomer aer each curing stage, as determined through TGA
analysis. We observe that the initial thermal decomposition
temperature (Td5
) and char yield both increased upon
increasing the curing temperature. In addition, the initial
thermal decomposition temperature in Fig. 5 of each curing
stage was strongly correlated with the rst curing exothermic
peak in the DSC traces in Fig. 4(A). For example, the value of Td5
Fig. 7 WCAs of Azo-COOH-Py BZ in its trans and cis isomeric forms,
recorded after each curing stage.
Fig. 6 Changes in UV-Vis absorbance of Azo-COOH-Py BZ in THF Fig. 8 PL spectra of (a) Py-NH
2
and (b) the Azo-COOH-Py BZ/SWCNT
after irradiation at 365 nm.
(1 wt%) hybrid complex.
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dipole moment transition are greatly modied through this hydrogen bonding would increase the WCA and decrease the
photochemical reaction. In the UV-Vis absorption spectra surface free energy relative to that for the uncured BZ mono-
4
À1
(
Fig. 6), the maximum absorption wavelength decreased aer mer. Nevertheless, a new absorption was evident at 3371 cm
photoirradiation. We monitored the photoisomerization [Fig. 4(B)], corresponding to intermolecular [O–H/O] hydrogen
process by measuring the change in absorption of the trans bonding, aer opening of the oxazine ring when the curing
ꢀ
3,4
azobenzene unit at 352 nm, corresponding to its p–p* transi- temperature was 210 C.
Although such intermolecular
tion. Upon irradiation with UV light, the intensity of the
absorbance at 352 nm decreased gradually, eventually reaching
a relatively stable value; the signal for the cis isomer would
increase accordingly, as expected. Fig. 7 displays the water
contact angles (WCAs) for pure Azo-COOH-Py BZ before and
aer its photoisomerization under a UV lamp at 365 nm, as well
ꢀ
as aer thermal curing at 110, 150, and 210 C for 2 h. The WCA
of the trans isomer of Azo-COOH-Py BZ was larger than that of
the cis isomer of Azo-COOH-Py BZ for all curing temperatures;
this behavior is consistent with the trans isomer having a
smaller dipole moment and lower surface free energy (and,
therefore, a higher WCA) and the cis form possessing a bigger
dipole moment and higher surface free energy (and, therefore, a
43
lower WCA). The WCAs increased upon increasing the thermal
ꢀ
curing temperature up to 150 C, probably because of a drying
effect. Aer thermal curing of the Azo-COOH-Py BZ monomer at
ꢀ
2
10 C, the WCA remained higher than that of the uncured Azo-
COOH-Py BZ monomer, consistent with the network structure
that formed (cf. DSC and FTIR analyses in Fig. 4) and the
presence of different kinds of hydrogen bonding interactions.
Fig. 10 DSC thermograms of the curing behavior of Azo-COOH-Py
Most importantly, the degree of intramolecular [O–H/N] BZ/SWCNT hybrid complexes at various SWCNT contents.
Fig. 9 (a–c) Photographs of (a) Azo-COOH-Py BZ, (b) pristine SWCNTs, and (c) Azo-COOH-Py BZ/SWCNTs (5 wt%) in THF. (d–h) TEM images of
(
(
d) pure SWCNTs, (e and f) Azo-COOH-Py BZ/SWCNTs (5 wt%) (e) before and (f) after thermal curing, and (g and h) Azo-COOH-Py BZ/MWCNTs
5 wt%) (g) before and (h) after thermal curing.
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Azo-COOH-Py BZ/SWCNT (1 wt%) complex features completely
quenched uorescence of the pyrene unit, indicating that
strong p–p stacking existed between the pyrene moieties of the
44
Azo-COOH-Py BZ monomers and the SWCNTs. Fig. 9(a–c)
display photographs of THF solutions of pure Azo-COOH-Py BZ,
pure SWCNTs, and the Azo-COOH-Py BZ/SWCNT complex,
respectively. Pure Azo-COOH-Py formed a clear solution,
whereas the pure SWCNTs precipitated completely with strong
aggregation, as conrmed in the TEM image in Fig. 9(d). In
contrast, the addition of Azopy-COOH-Py BZ into the suspen-
sion of the SWCNTs resulted in a clear black solution [Fig. 9(c)],
suggesting that stable complexes formed as a result of non-
covalent p–p stacking. Fig. 9(e) and (f) reveal the uniform
dispersions of the SWCNTs within the Az-COOH-Py BZ matrix
before and aer thermal curing, respectively. Similar behavior
appeared for the Az-COOH-Py BZ/MWCNT nanocomposites,
with Fig. 9(g) and (h) revealing the uniform dispersions of the
MWCNTs within the Az-COOH-Py BZ matrix before and aer
thermal curing, respectively. Scheme 2 presents a possible
morphology for the Az-COOH-Py BZ/CNT hybrid complexes.
We used DSC to monitor the curing behavior of the Azo-
COOH-Py BZ monomer in the presence of various weight
percentages of SWCNTs (Fig. 10). The addition of SWCNTs
caused the exotherm peak of pure Azo-COOH-Py BZ to shi to
lower temperature. For example, the temperature of the exo-
Fig. 11 DSC thermograms of the curing behavior of the Azo-COOH-
Py BZ/SWCNT (5 wt%) hybrid complex after each curing stage.
4
[
O–H/O] hydrogen bonding would decrease the WCA, we also
found that the WCA of the trans isomer remained larger than
that of the cis isomer aer each thermal curing.
Poly(Azo-COOH-Py BZ)/CNTs nanocomposites
Fig. 8 displays the uorescence spectra of Py-BZ and the Azo-
COOH-Py BZ/SWCNT complex in solution aer excitation at
ꢀ
therm peak for Azo-COOH-Py BZ shied from 226 C to 215,
ꢀ
1
94, 193, 192, and 190 C aer blending with 0.5, 1, 2, 3, and 4
343 nm. The spectrum of Py-NH
2
features a strong uorescence
wt% of SWCNTs, respectively, and then increased slightly to
signal for the pyrene unit at 426 nm, while the spectrum of the
Fig. 12 DSC thermograms of the curing behavior of (a) Azo-COOH-Py BZ and (b–e) its nanocomposites with (b) 1 wt% MWCNTs, (c) 5 wt%
MWCNTs, (d) 1 wt% SWCNTs, and (e) 5 wt% SWCNTs.
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ꢀ
1
94 C at 5 wt% of SWCNTs. The maximum decrease in the atoms in the CNTs (as does covalent functionalization with
ꢀ
curing temperature was approximately 36 C (4 wt% SWCNTs) COOH groups) because the COOH groups of the Azo-COOH-Py
when compared with that of the pure Azo-COOH-Py BZ mono- BZ monomers acted as catalysts for thermal curing. In addi-
ꢀ
mer and approximately 96 C when compared with that of the tion, the total exotherm for the Azo-COOH-Py BZ/SWCNT
À1
Py-BZ monomer, implying increasingly rapid thermal curing of nanocomposites decreased from 324.7 J g
(for pure Azo-
À1
the BZ monomers. Previous studies have conrmed that CNTs COOH-Py BZ) to 201.5 J g at 5 wt% SWCNT content, indi-
can act as catalysts for polymer resins and initiate early state cating that blending with SWCNTs hindered the mobility of the
3,45,46
curing.
temperatures to the catalytic effect of the SWCNTs on the the PBZ.
opening of the BZ ring, thereby accelerating the curing process We also studied the curing behavior of the Azo-COOH-Py BZ/
Thus, we attribute the decrease in the curing ring-opened BZ, thereby decreasing the crosslinking density of
46
at lower temperatures. Our results suggest that both the COOH SWCNT (5 wt%) hybrid complex aer each curing stage as
unit on the aromatic ring and the SWCNTs led to acceleration of shown in Fig. 11. The enthalpy of the exotherm of the mixture
the curing process at lower temperature. As mentioned above, gradually decreased upon increasing the temperature of the
Chen et al. reported that the surface COOH groups of CNTs curing process, nally reaching zero when the curing temper-
ꢀ
catalyzed and decreased the curing temperature of the BZ ature was 240 C. The result is similar to the curing behavior of
22
monomer; such covalent functionalization of CNTs would, pure Azo-COOH-Py BZ [Fig. 4(A)], where rst exotherm dis-
ꢀ
however, worsen their optical, electronic, and mechanical appeared at 180 C, but the rst exotherm peak temperature
properties. Our approach described herein has two advantages: aer adding 5 wt% SWCNTs increased upon increasing the
(i) the Azo-COOH-Py BZ monomers facilitated dispersion of the curing temperature. Therefore, SWCNTs have a catalytic effect
CNTs through p–p stacking between the pyrene units and the on the ring opening reactions of BZ monomers, but they also
2
CNTs; (ii) it does not lower the number of sp -hybridized carbon hinder the mobility of the PBZ polymers aer thermal curing.
Fig. 13 TGA analyses of (a) Azo-COOH-Py BZ and (b–e) its nanocomposites with (b) 1 wt% MWCNTs, (c) 5 wt% MWCNTs, (d) 1 wt% SWCNTs, and
(
e) 5 wt% SWCNTs after thermal curing.
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We observed similar curing behavior during DSC analysis of the aer blending Azo-COOH-Py BZ with CNTs, the exothermic
ꢀ
Azo-COOH-Py BZ/MWCNT (5 wt%) nanocomposite (Fig. S2†).
curing peak decreased from 226 to 190 C (SWCNTs), conrm-
We also examined the curing behavior of the Azo-COOH-Py ing that incorporation of the CNTs into the PBZ matrix catalyzed
BZ monomer aer blending with SWCNTs or MWCNTs at the the thermal curing process. The thermal decomposition
same weight percentages (Fig. 12). The maximum exothermic temperature and char yield increased signicantly, however,
temperatures and the enthalpies of the curing exotherm aer addition of the CNTs into poly(Azo-COOH-Py BZ) because
decreased signicantly upon increasing the content of of both the crosslinked structure and the presence of the inor-
MWCNTs or SWCNTs. More interestingly, the SWCNTs lowered ganic CNTs. In addition, the thermal decomposition tempera-
the curing temperature, but increased the enthalpy of the exo- ture and char yield in the presence of the SWCNTs were better
therm, indicating that they acted as stronger catalysts and than those in the presence of the MWCNTs, presumably
provided a higher crosslinking density than did the MWCNTs because the greater thermal conductivity of the SWCNTs
for the ring opening of the BZ monomer in this study. This enhanced the crosslinking density of the PBZ matrix. This
result was probably due to the better thermal conductivity of the combination of multifunctional units on the BZ structure
SWCNTs enhancing the thermal curing behavior of the BZ signicantly decreased the curing temperature, but provided a
47
ring.
Fig. 13 presents the thermal stabilities under N
polymeric matrix exhibiting signicantly greater thermal
of pure Azo- stability.
2
COOH-Py BZ and its blends with SWCNTs and MWCNTs aer
ꢀ
thermal curing at 210 C, to eliminate the effect of decarboxyl-
Acknowledgements
ation, as investigated using TGA. The addition of MWCNTs or
SWCNTs increased both the thermal decomposition tempera- This study was nancially supported by the Ministry of Science
ture and the char yield of Azo-COOH-Py BZ, because the incor- and Technology, Taiwan under contracts MOST103-2221-E-110-
porated CNTs blocked the premature evaporation of 079-MY3 and MOST102-2221-E-110-008-MY3. We also thank Mr
decomposed molecular fragments and led to the formation of Hsien-Tsan Lin of the Regional Instruments Center at National
45,46
network structures for the nanohybrids.
Nevertheless, the Sun Yat-Sen University for help with the TEM measurement. In
thermal decomposition temperature and char yield in the addition, Prof. Dai would like to thank the National Natural
presence of 1 wt% MWCNTs or SWCNTs were better than those Science Foundation of China (U1205113).
in the presence of 5 wt% MWCNTs or SWCNTs. We attribute the
lower thermal decomposition temperatures to the lower cross-
linking densities aer blending with 5 wt% MWCNTs or
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