Supramolecular functionalized polybenzoxazines from azobenzene carboxylic acid/azobenzene pyridine complexes: Synthesis, surface properties, and specific interactions

Article abstract of DOI:10.1039/c4ra15107k

In this study we synthesized Azo-COOH BZ, a new benzoxazine derivative containing both azobenzene and carboxylic acid units, through the reaction of 4-(4-hydroxphenylazo)benzoic acid (Azo-COOH, itself prepared through a diazonium reaction of 4-aminobenzoi

Full text of DOI:10.1039/c4ra15107k

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Supramolecular Functionalized Polybenzoxazines from Azobenzene Carboxylic
Acid/Azobenzene Pyridine Complexes: Synthesis, Surface Properties, and Specific
Interactions
5
Mohamed Gamal Mohamed,a ChiꢀHui Hsiaoa, KuoꢀChih Hsua, FangꢀHsien Lu,b HisꢀKang Shih,b
and ShiaoꢀWei Kuoa,*
Received (in XXX, XXX) Xth XXXXXXXXX 200X, Accepted Xth XXXXXXXXX 200X
DOI: 10.1039/b000000x
In this study we synthesized AzoꢀCOOH BZ, a new benzoxazine derivative containing both azobenzene and carboxylic acid units,
10 through the reaction of 4ꢀ(4ꢀhydroxphenylazo)benzoic acid (AzoꢀCOOH, itself prepared through a diazonium reaction of 4ꢀaminobenzoic
acid with phenol in the presence sodium nitrite and NaOH) with paraformaldehyde and aniline in 1,4ꢀdioxane. Fourier transform infrared
1
(FTIR) spectroscopy and H and 13C nuclear magnetic resonance spectroscopy confirmed the chemical structure of AzoꢀCOOH BZ. We
used differential scanning calorimetry (DSC), thermogravimetric analysis, and FTIR spectroscopy to investigate the curing behavior of
this new benzoxazine monomer. DSC revealed that the exothermic peak representing the ring opening polymerization of the benzoxazine
15 unit appeared at low temperature relative to those of typical benzoxazines, indicating that the presence of the carboxylic acid and
azobenzene units of this monomer catalyzing the benzoxazine ring opening reaction. In addition, blending with various molar ratios of a
benzoxazine monomer presenting a pyridyl unit (Azopy BZ) led to strong intermolecular hydrogen bonding between the CO2H group of
AzoꢀCOOH BZ and the pyridyl group of Azopy BZ. These supramolecular complex systems also featured significantly lower curing
temperatures (down from ca. 200 °C of AzoꢀCOOH BZ to 150 °C of supramolecular complex), with their products retaining the high
20 water contact angles required for lowꢀsurfaceꢀenergy applications.
reported the attachment of benzoxazine monomers bearing
carboxyl groups onto the surface of magnetite nanopartiecles;33
60 they also developed a PBZ/montmorillonite nanocomposite,
Introduction
Heterocyclic benzoxazine monomers can be synthesized through
Mannich condensations of phenols, aldehydes, and amines in the
25 absence of initiators; subsequent thermal activation can lead to
polymerization of the monomers.1 The diversity of applicable
phenols and amines offers remarkable flexibility in the molecular
design of suitable benzoxazine monomers. Polybenzoxazines
(PBZs) are easyꢀtoꢀprocess phenolic resins that possess excellent
30 thermal and mechanical properties, low water absorption, high
contents of carbon residues, low surface energies, good flameꢀ
retardancy, and nearꢀzero volumetric shrinkage/expansion upon
polymerization.2–14 Furthermore, PBZs have superb molecular
design flexibility and can be synthesized through ring opening
35 polymerizations of aromatic benzoxazines with no need for
strong catalysts and without the production of polymerization
byproducts.15–18
For specific applications, the thermal and mechanical properties
of PBZs can be modified in many ways. For example,
40 benzoxazine monomers presenting allyl,19 acetylene,20
propargyl,21 nitrile,22 and maleimide23 functionalities have been
prepared to facilitate additional crossꢀlinking upon thermal
curing. The thermal and mechanical properties of PBZs can also
be improved through the formation of blends, copolymers, and
45 inorganic nanocomposites.24–32 Another interesting use of
functionalized benzoxazine monomers is their combination with
inorganic materials to fabricate nanocomposites, exploiting the
attractive interactions between the polar functional groups of the
monomers and the inorganic surfaces. For example, Yagci et al.
50 ꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀ
aDepartment of Materials and Optoelectronic Science, Center for
Functional Polymers and Supramolecular Materials, National Sun
YatꢀSen University, Kaohsiung, Taiwan
using a benzoxazine bearing a 4ꢀpyridyl moiety as a key
material.33
Azopyridine benzoxazine derivatives have the attractive
features of being lightꢀactive, because of their azobenzene units,
65 and having the ability of selfꢀassemble through hydrogen bonding
of their pyridyl units with organic acids.33 Endo et al. reported a
novel benzoxazine bearing a 4ꢀaminopyridyl moiety, synthesized
from 4ꢀaminopyridine, pꢀcresol, and paraformaldehyde, with
addition of acetic acid being necessary to neutralize the basicity
70 of the system; the presence of 4ꢀaminopyridyl moieties imparted
the resulting polymer with high affinity toward metal ions [e.g.,
copper(II) and cobalt(II)], leading to the formation of the
corresponding polymer–metal complexes, and the formation of
insoluble colored precipitates.34 Although benzoxazine
75 derivatives can be cured at elevated temperatures without the
need for catalysts, the addition of weak or strong acids can lower
the curing temperature.35 Recently, Andreu et al. reported that the
presence of carboxyl groups in the structure of a benzoxazine
monomer impacted the thermal curing process and lowered the
80 processing temperatures.36
Stimuliꢀresponsive materials are receiving a great deal of
attention at present. Various stimuli (e.g., pH; ionic strength;
solvent vapor; light; heat; electrical field) can be used to affect
the properties and conformations of polymers.37–39 Polymers
85 containing azobenzene moieties are of particular interest because
of the reversibility of their photoꢀisomerizations and of their
thermal cisꢀtoꢀtrans isomerizations.40,41 Interconversions between
ground state trans forms and higherꢀenergy cis forms can be
achieved both photochemically (transꢀtoꢀcis) and thermally (cisꢀ
90 toꢀtrans) with high degrees of efficiency and the absence of
competing side reactions. Because one configurational isomer is
furnished photochemically while the other is favored thermally, it
is possible to effectively switch azobenzeneꢀmodified species into
desired geometries and polarities. Thus, the incorporation of
95 azobenzene moieties into PBZs might impart them with photoꢀ
Eꢀmail:kuosw@faculty.nsysu.edu.tw
55 bInstitute of Applied Chemistry, National ChiaoꢀTung University,
Hsinchu, Taiwan
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(c)
*
(b)
(a)
OH
NH2
NH2
OH
N
O
N
*
N
(CH2O)n,
1,4-dioxane
Heating and Curing
1. NaNO2, HCl, 0 oC
2. Phenol, NaOH(aq)
N
N
N
N
COOH
N
COOH
COOH
COOH
Poly(Azo-COOH BZ)
Azo-COOH
Azo-COOH BZ
Scheme 1. Synthesis of AzoꢀCOOH, AzoꢀCOOH BZ, and poly(AzoꢀCOOH BZ).
N
O
N
O
N
N
O
O
H
N
N
N
Heating
Curing
N
N
N
OH
HO
N
O
O
H
N
N
N
5
Scheme 2. Hydrogen bonding between AzoꢀCOOH BZ and Azopy BZ monomers and in their corresponding polymer blends.
and thermoꢀregulated behavior when subjected to changes in
incident light or heat.42–44
PBZs are also interesting materials because of their strong
intramolecular hydrogen bonding after curing and their extremely
low surface energies—in some cases lower than that of pure
Teflon.45 Nevertheless, the need for high curing temperatures
3,4ꢀdihydroꢀ2Hꢀ1,3ꢀbenzoxazinyl)isopropane (Bꢀala) can be
20 cured at a low temperature (120 °C) with the aid of 2,2´ꢀ
azoisobutyronitrile (AIBN), resulting in a PBZ displaying recordꢀ
low surface energy.46 Most polymer coatings possessing
photoresponsive wettability behavior are based on rough surfaces
modified to become more hydrophilic or hydrophobic upon
10
limits the applications of PBZs, especially for use as polymer 25 exposure to light. Jiang et al. reported that covalent attachment of
15 coatings. Recently, a method for lowering the curing temperature
of PBZs attracted great interest because it broadened the
applicability of PBZs in temperatureꢀsensitive substrates (e.g.,
most polymeric materials). Chang et al. reported that bis(3ꢀallylꢀ
azobenzeneꢀfunctionalized polyacrylamides to a rough surface
led to large contact angles after photoisomerization.47
In this study, we synthesized a new benzoxazine derivative,
AzoꢀCOOH BZ, containing both azobenzene and carboxylic acid
2
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Aromatic
OH
N
N
H2O
HOO C
d-DMSO
OH
(a) Azo-COOH
COOH
b
a
O
b
d-DMSO
N
N
Aromatic
N
a
HOOC
(b) Azo-COOH BZ
COOH
14
12
10
8
6
4
2
0
Chemical Shift (ppm)
Figure 1. 1H NMR spectra of (a) AzoꢀCOOH and (b) AzoꢀCOOH BZ.
5
moieties. First, we prepared 4ꢀ(4ꢀhydroxphenylazo)benzoic acid
(AzoꢀCOOH) through a diazonium reaction of 4ꢀaminobenzoic
0.100 mol) in water (20 mL). The mixture was stirred at 0 °C and
then diluted with cooled MeOH (400 mL). Phenol (9.41 g, 0.100
acid with phenol in the presence of NaOH [Scheme 1(a)]. We 35 mol), dissolved in a chilled solution of NaOH (10.8 g, 0.190 mol)
then formed AzoꢀCOOH BZ through the reaction of AzoꢀCOOH,
paraformaldehyde, and aniline in 1,4ꢀdioxane [Scheme 1(b)].
10 Subsequently, we examined, before and after curing, the physical
properties of AzoꢀCOOH BZ when blended at various molar
in MeOH (100 mL), was added dropwise to the solution
containing the diazonium salt. A red dye formed; this mixture
was stirred for 2 h and then poured into dilute HCl with stirring.
The red solid was filtered off, washed thoroughly with water,
ratios with a benzoxazine monomer possessing a pyridyl unit 40 dried, and purified through column chromatography (SiO2; nꢀ
(Azopy BZ) that can form intermolecular hydrogen bonds
[Scheme 2]. We used differential scanning calorimetry (DSC),
15 Fourier transform infrared (FTIR) spectroscopy, and contact
angle analyses to examine the thermal properties, specific
hexane/EtOAc, 1:1). Recrystallization (MeOH/H2O) gave the
1
product (9.5 g, 76%). M.p.: 273–275 °C (DSC). H NMR (δ, d6ꢀ
DMSO): 6.88–8.12 (m, 8H), 10.17 (s, OH), 13.13 (s, COOH). 13
C
NMR (δ, d6ꢀDMSO): 167.20 (C=O), 162.28, 154.70, 145.52,
interactions, and surface behavior of these supramolecular PBZ 45 132.35, 130.37, 126.12, 121.83, 116.57. IR (KBr, cm–1): 3204–
materials.
3463 (OH), 3053–2500 (COOH), 1660 (C=O).
(E)ꢀ4ꢀ((3ꢀPhenylꢀ3,4ꢀdihydroꢀ2Hꢀbenzo[e] [1, 3] benzoic Acid
(AzoꢀCOOH BZ)
20 Experimental Section
50 A solution of aniline (0.420 g, 4.54 mmol) in 1,4ꢀdioxane (25
Materials
mL) was added portionwise to
a
stirred solution of
4ꢀAminobenzoic acid, paraformaldehyde (96%), sodium nitrite
(99%), hydrochloric acid (ca. 37%), aniline (99.8%), EtOAc,
EtOH, and hexane were purchased from Acros and used as
25 received. 1,4ꢀDioxane and NaOH were purchased from Aldrich.
paraformaldehyde (0.270 g, 8.67 mmol) in 1,4ꢀdioxane (100 mL)
in a 250ꢀmL flask, cooled in an ice bath. The mixture was then
stirred for 20 min while maintaining the temperature below 5 °C
55 and then a solution of 4ꢀ(4ꢀhydroxphenylazo)benzoic acid (1.00
g, 4.13 mmol) in 1,4ꢀdioxane (30 mL) was added. The mixture
was heated under reflux at 110 °C for 24 h. After cooling, the
solvent was removed in a rotary evaporator; the viscous residue
was dissolved in EtOAc (100 mL) and washed several times with
60 5% NaHCO3 and
AzopyꢀBZ was synthesized using
a previously reported
procedure.48
4ꢀ(4ꢀHydroxphenylazo)benzoic Acid (AzoꢀCOOH)
30 A solution of 4ꢀaminobenzoic acid (13.7 g, 0.100 mol) in dilute
HCl [a mixture of conc. HCl (20 mL) in water (20 mL)] was
diazotized by the addition of a solution of sodium nitrite (6.90 g,
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d6-DMSO
OH
N
N
H OO C
c=o
Aromatic
(a) Azo-COOH
O
b
N
N
N
a
HOO C
Aromatic
a
b
c=o
(b) Azo-COOH BZ
180
160
140
120
100
80
60
40
20
0
Chemical Shift (ppm)
Figure 2. 13C NMR spectra of (a) AzoꢀCOOH and (b) AzoꢀCOOH BZ.
of the solvent had evaporated slowly at room temperature under
25 atmosphere pressure, the mixture was dried in a vacuum oven at
30 °C for approximately 20 h.
C=O
1232 cm-1
921 cm-1
COOH
OH
Characterization
1H and 13C nuclear magnetic resonance (NMR) spectra were
30 recorded using an INOVA 500 instrument, with DMSO as the
solvent and TMS as the external standard. FTIR spectra of the
polymer films were recorded using a Bruker Tensor 27 FTIR
spectrophotometer and the conventional KBr disk method; 32
scans were collected at a spectral resolution of 4 cm–1. The
35 prepared films were sufficiently thin to obey the Beer–Lambert
law. FTIR spectra recorded at elevated temperatures were
obtained using a cell mounted within the temperatureꢀcontrolled
compartment of the spectrometer. The thermal stability of the
samples was measured using a TG Qꢀ50 thermogravimetric
40 analyzer operated under a N2 atmosphere. The cured sample (ca.
5 mg) was placed in a Pt cell and heated at a rate of 20 °C min–1
from 30 to 800 °C under a N2 flow rate of 60 mL min–1. The UV–
Vis spectrum of AzoꢀCOOH BZ (10–4 M in THF) was recorded
(a) Azo-COOH
(b) Azo-COOH BZ
(c) Poly(Azo-COOH BZ)
4000
3500
3000
2500
2000
1500
1000
500
Wavenumber (cm-1)
5
Figure 3. FTIR spectra of (a) AzoꢀCOOH, (b) AzoꢀCOOH BZ,
and (c) poly(AzoꢀCOOH BZ), recorded at room temperature.
using
a
Shimadzu mini 1240 spectrophotometer. The
finally with distilled water. The organic phase was dried
(MgSO4) and concentrated under vacuum to afford an orange
10 residue, which was purified through column chromatography
45 photoisomerizations of AzoꢀCOOH BZ were performed by using
a UV–Vis lamp in conjunction with 365ꢀnm (90 mW cm–2) and
400–500ꢀnm (90 mW cm–2) filters to generate the UV or visible
light for the respective transꢀtoꢀcis and cisꢀtoꢀtrans
isomerizations. The contact angles of 5ꢀµL drops of deionized
50 water on the polymer samples before and after curing were
measured at 25 °C using an FDSA Magic Droplet 100 contact
angle goniometer interfaced with image capture software. We
measured contact angles on the surface of a film by dissolving an
appropriate concentration of AzoꢀCOOH BZ (trans form) in THF,
55 then drop this solution on glass as substrate then curing at various
temperature by spin coating. For measuring contact angle of cis
form, we exposed the trans form solution to UV lamp at 365 nm
to change the isomer then drop this solution on substrate and
curing at various temperature.
(SiO2; nꢀhexane/EtOAc, 1:1) to give an orange powder (1.1 g,
1
70%). H NMR (δ, d6ꢀDMSO): 4.84 (s, 2H, CH2N), 5.59 (s, 2H,
OCH2N), 6.94 (s, 2H, CH), 6.94 (s, 2H), 7.01 (d, 2H), 7.23 (d,
2H), 7.76 (d, 2H), 8.09 (d, 2H), 8.12 (d, 2H). 13C NMR (δ, d6ꢀ
15 DMSO): 48.47 (CCH2N), 80.04 (OCH2N), 117.54–158.02
(aromatic), 167.2 (C=O). IR (KBr, cm–1): 1232 (asymmetric C–
O–C stretching), 1341 (CH2 wagging), 921 and 1490
(trisubstituted benzene ring stretching), 1441 (stretching of trans
N=N bond), 3053–2500 (COOH), 1660 (C=O).
20
AzoꢀCOOH BZ/Azopy BZ Complexes
AzoꢀCOOH BZ and AzopyꢀBZ, at a chosen molar ratio, were
dissolved in THF to obtain a homogeneous solution. After most
4
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199 oC, 593 J/g
(a) Pure Azo-COOH BZ
o
204 oC, 521 J/g
205 oC, 510 J/g
(b) Curing 60
C
o
(c) Curing 110
C
211oC, 433 J/g
o
(d) Curing 150 C
292 o
C
o
(e) Curing 180
(f) Curing 210
C
188 J/g
o
C
o
(g) Curing 240
C
150
200
250
300
Temperature (oC)
Figure 4. DSC thermograms of the AzoꢀCOOH BZ monomer, recorded after each heating stage (2 h at each temperature).
of the phenolic moiety of AzoꢀCOOH was absent in the spectrum
1680 cm-1
(a) R.T
1232 cm-1
921 cm-1
25 AzoꢀCOOH BZ, but the aromatic signals of AzoꢀCOOH
remained, confirming the successful synthesis of AzoꢀCOOH BZ.
Figure 2 presents the 13C NMR spectra of AzoꢀCOOH and
AzoꢀCOOH BZ. In the spectrum of AzoꢀCOOH, the signals of
the azobenzene moiety appear in the range 116.57–162.28 ppm,
30 with a signal at 167.20 ppm corresponding to the C=O group. In
the spectrum of AzoꢀCOOH BZ, two characteristic peaks appear
at 48.47 and 80.04 ppm, representing the carbon nuclei of the
oxazine moiety; signals for the azobenzene moiety are also
evident, confirming the successful synthesis of AzoꢀCOOH BZ.
35 Figure 3 presents the roomꢀtemperature FTIR spectra of Azoꢀ
COOH and AzoꢀCOOH BZ and, after thermal curing, poly(Azoꢀ
COOH BZ). The spectrum of AzoꢀCOOH features a sharp signal
for the OH group at 3470 cm–1, a broad signal at 3053–2523 cm–1
for the intramolecularly hydrogen bonded CO2H group, and
40 absorption bands of the aromatic ring at 3025, 1610, and 1571
cm–1. In the spectrum of AzoꢀCOOH BZ, the signal for the OH
group of the phenol unit is absent, but signals are evident for the
C–C stretching vibrations of the 1,2,4ꢀtrisubstituted benzene ring
(1490 and 921 cm–1), for asymmetric C–O–C stretching (1232
45 cm–1), and for stretching of the trans N=N unit (1440 cm–1).
Together, these NMR and FTIR spectroscopic data confirmed the
successful synthesis of AzoꢀCOOH BZ. The FTIR spectrum
recorded after thermal curing of pure AzoꢀCOOH BZ at 180 °C
features lessꢀintense characteristic absorption bands for the
50 trisubstituted aromatic ring of AzoꢀCOOH BZ (1490 and 921 cm–
1) was decreased. The broad absorption bands in the range 2600–
3500 cm–1 in Figure 3(c) represent several different kinds of
hydrogen bonding interactions: [O–···H–N+] and [COOH dimer]
intramolecular hydrogen bonding near 2800 cm–1, [O–H···N]
55 intramolecular hydrogen bonding near 3200 cm–1, and [O–H···O]
intermolecular hydrogen bonding near 3342 cm–1; the identities
of these signals have been confirmed previously.49
(b) 60 oC
(c) 100 oC
(d) 110 oC
(e) 150oC
(f) 170 oC
(g) 180 oC
2000
1800
1600
1400
1200
1000
800
600
-1
Wavenumber (cm
)
5
Figure 5. FTIR spectra of the AzoꢀCOOH BZ monomer,
recorded after each heating stage.
10 Results and Discussion
Synthesis of AzoꢀCOOH BZ through Mannich Reaction of
AzoꢀCOOH with Aniline and Paraformaldehyde
We prepared AzoꢀCOOH BZ in high purity and high yield
through a Mannich reaction of AzoꢀCOOH, phenol, and aniline in
15 1,4ꢀdioxane at 110 °C (Scheme 1). Figure 1 presents the 1H NMR
spectra of AzoꢀCOOH and AzoꢀCOOH BZ. The spectrum of
AzoꢀCOOH [Figure 1(a)] features signals at 10.47 and 13.13 ppm
representing the OH groups of the phenolic and COOH units,
respectively. The aromatic protons of AzoꢀCOOH appeared as
20 signals in the range 6.97–8.10 ppm. The spectrum of AzoꢀCOOH
BZ [Figure 1(b)] features two characteristic peaks at 4.82 and
5.64 ppm, corresponding to the CH2N and OCH2N units,
respectively, of the oxazine moiety. The signal for the OH group
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100
80
60
40
20
0
Pure AzoꢀBZ COOH
o
Curing 60 C
o
Curing 110 C
o
Curing 150 C
o
Curing 180 C
o
Curing 210 C
100
200
300
400
500
600
700
Temperature (oC)
Figure 6. TGA analyses of AzopyꢀBZ, recorded after each curing stage
20 temperature. After thermal curing of pure AzoꢀCOOH BZ at 180
°C, the intensity of the exotherm decreased upon increasing the
curing temperature, only few enthalpy of curing after thermal
curing at temperature between 180 and 240 °C. Figure 5 displays
the FTIR spectra of AzoꢀCOOH BZ at various stages during the
25 curing process. The intensities of the characteristic absorption
bands of the benzoxazine moiety—at 921 (C–O–C symmetric),
1232 (C–O–C asymmetric), and 1341 (CH2 wagging) cm–1—
decreased upon increasing the curing temperature. After curing at
180 °C, these characteristic absorption bands of benzoxazine
30 decreased, indicating that ring opening of the benzoxazine units
had occurred. Nevertheless, some of these characteristic
absorption bands—particularly at 921 (C–O–C symmetric) and
1232 (CꢀOꢀC asymmetric) cm–1—remained after curing at 180
°C, implying that some of the benzoxazine rings had not opened.
35 The result is consistent with the DSC analysis data, which
revealed some enthalpy of curing (ca. 188 J gꢀ1) after thermal
curing at 180 °C. Figure 6 displays TGA thermograms of Azoꢀ
COOH BZ before and after thermal curing under N2 at 60, 110,
150, 180, and 210 °C. The thermal stability, the decomposition
40 temperature (10 wt% loss), and the char yield all increased upon
increasing the curing temperature, implying that increased
crosslinking density (through hydrogen bonding) enhances the
thermal stability of poly(AzoꢀCOOH BZ). We observed that the
degradation steps at 180 oC because of the decarboxalytion
45 process of carboxyl group.
0 min
1 min
2 min
3 min
4 min
5 min
250
300
350
400
450
Wavelength (nm)
5
Figure 7. UV–Vis absorption spectra of AzoꢀCOOH (10–4 M in
THF) after treatment under UV light at 365 nm for various
periods of time.
Thermal Polymerization of AzoꢀCOOH BZ
10
We used DSC to determine the curing behavior of pure Azoꢀ
COOH BZ (Figure 4). The DSC trace of uncured AzoꢀCOOH BZ
features a sharp exothermic peak, attributed to its ringꢀopening
polymerization, with the onset at 190 °C, the maximum at 199
°C, and a reaction heat of 593 J g–1. The curing temperature for
Photoisomerization of Azobenzene Chromophore in Azoꢀ
COOH BZ
15 AzoꢀCOOH BZ (199 °C) shifted from 263 °C for conventional 3ꢀ
phenylꢀ3,4ꢀdihydroꢀ2Hꢀbenzooxazine (Paꢀtype, without azo and
carboxylic groups),50 presumably because the azobenzene and
carboxylic acid moieties acted as basic and acidic catalysts,
respectively, that contributed to the lower polymerization
Figure 7 reveals evidence for the photoisomerization of the
50 azobenzene chromophore in AzoꢀCOOH BZ, through the change
in the UV–Vis absorption spectrum of a solution of AzoꢀCOOH
BZ upon irradiation with UV light at 365 nm. The maximum
6
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O
H
O
1232 cm-1
Azo-COOH BZ/Azopy-BZ Complex
180
170
160
150
140
130
120
110
100
90
R
R
O
H
O
921 cm-1
trans Azo-COOH BZ
cis Azo-COOH BZ
O
H
N
R
O
(a) 1/0
(b) 3/1
(c) 1/1
(d) 1/3
(e) 0/1
1800
1600
1400
1200
1000
800
600
80
-1
)
Wavenumber (cm
15
70
Figure 10. FTIR spectra of the AzoꢀCOOH BZ/Azopy BZ
supramolecular complex prior to thermal curing
60
BZ. The isomerization degree of monoꢀcyclic monomers at the
50
20 photostationary state is calculated based on this equation:51
Isomerization degree = (A0ꢀAi)/A0 ×100%
40
where A0 is the absorbance at λmax before the light irridation and
Ai is the absorbance at the same wavelength measured at the
photostationary state. The isomerization degree of monoꢀcyclic
25 monomers was 64 %. Figure 8 displays the contact angles of
water on AzoꢀCOOH BZ before and after its photoisomerization
under a UV lamp at 365 nm, as well as after thermal curing at
110 and 150 °C for 3 h, at 180 °C for 2 h, and at 210 °C for 2 h.
We observed several interesting phenomena. First, the water
30 contact angle, at room temperature, of the trans isomer of Azoꢀ
COOH BZ (105°) was larger than that of the cis isomer of Azoꢀ
COOH BZ (88°); this behavior is consistent with the trans isomer
having a smaller dipole moment and lower surface free energy.
Second, the surprisingly high water contact angle of the trans
35 isomer (105°) was presumably due to strong intramolecular
hydrogen bonding of the CO2H group of AzoꢀCOOH BZ. A
similar result was found for a superhydrophobic surface (contact
angle = 171°) consisting of fibers fabricated from amphiphilic
poly(vinyl alcohol).52 Third, after thermal curing at 110, 150,
40 180, and 210 °C, the contact angles of the trans isomer were 112,
118, 96, and 90° respectively. Notably, after transꢀtoꢀcis
photoisomerization of the AzoꢀCOOH BZ samples that had been
cured at 110, 150, 180, and 210 °C, the water contact angles
became 101, 115, 118, and 90°, respectively. The water contact
45 angles for the samples of the trans isomer increased after curing
at 110 and 150 °C, presumably the result of a drying effect
because our DSC analyses confirmed that the benzoxazine ring of
AzoꢀCOOH BZ would not open at a temperature at or below 150
°C. In contrast, the water contact angles decreased for the
50 samples after curing at 180 and 210 °C because of ring opening
of AzoꢀCOOH BZ. This ring opening polymerization would
result in the OH groups of the phenolic units forming
intermolecular hydrogen bonds with the carboxylic acid groups,
thereby decreasing the water contact angle. In addition, the trans
55 and cis isomers of the AzoꢀCOOH BZ sample displayed similar
water contact angles after thermal curing at 210 °C, presumably
because the cis isomer of AzoꢀCOOH BZ transformed back to the
trans isomer after thermal treatment at this elevated temperature.
We emphasize that the water contact angles of these two isomers
60 were complicated because they depended on the ratio of the
photoꢀisomers of the azobenzene groups, the degree of thermal
30
20
30
60
90
120
150
180
210
Curing Temperature (oC)
Figure 8. Water contact angles of AzoꢀCOOH BZ in its trans and
cis isomeric forms, recorded after each curing stage.
5
Azo-COOH BZ/Azopy BZ Complex
(a) 1/0
(b) 3/1
(c) 1/1
(d) 1/3
(e) 0/1
50
100
150
200
250
Temperature (oC)
Figure 9. DSC thermograms of the AzoꢀCOOH BZ/Azopy BZ
supramolecular complex prior to thermal curing.
10 absorption at 360 nm is due to the π–π* transition of transꢀ
azobenzene group. Upon irradiation with UV light, the intensity
of the absorbance at 360 nm decreased gradually, ultimately
reaching a stable value. These variations are consistent with
transꢀtoꢀcis isomerization of the azobenzene unit in AzoꢀCOOH
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1232 cm-1
Azo-COOH BZ/Azopy BZ = 50/50
180
921 cm-1
Azo-COOH BZ
Azo-COOH BZ/Azopy BZ = 50/50
170
160
150
140
130
120
110
100
90
O
(a) R.T
o
N
N
N
(b) 60
C
o
HOOC
(c) 110 C
o
(d) 150
C
o
(e) 180
C
o
(f) 210
(g) 240
C
o
C
80
2600 2400 2200 2000 1800 1600 1400 1200 1000
800
600
-1
70
Wavenumber (cm
)
Figure 11. FTIR spectra of the AzoꢀCOOH BZ/Azopy BZ =
50/50 complex, recorded after each heating stage.
N
60
N
50
N
OH
5
curing, and the degree of intermolecular hydrogen bonding after
thermal curing.
HO
N
O
O
H
N
N
40
N
30
Supramolecular
Complexes, Stabilized Through Carboxylic Acid–Pyridine
10 Hydrogen Bonds
Figure 9 presents the thermal curing behavior of Azoꢀ
AzobenzeneꢀContaining
Benzoxazine
20
30
60
90
120
150
180
210
Curing Temperature (oC)
COOH BZ/Azopy BZ supramolecular complexes. Pure Azoꢀ
COOH BZ and AzopyꢀBZ exhibited maximum curing
temperatures of 199 and 208 °C, respectively. Interestingly,
15 blending AzoꢀCOOH BZ with AzopyꢀBZ at different molar ratios
(1:3, 1:1, 3:1) resulted in two maximum curing temperatures for
the complexes, each shifted to lower temperature. The first
exothermic peak shifted to 150 °C from 199 °C, consistent with
the high basicity of the pyridine ring facilitating nucleophilic
20 attack on the benzoxazine ring; the second exothermic peak
appeared at 190–255 °C for the completely ringꢀopened
benzoxazine. Figure 10 displays FTIR spectra of AzoꢀCOOH
BZ/Azopy BZ supramolecular complexes at various molar ratios
(1:3, 1:1, 3:1), prior to thermal curing. In all complexes, the FTIR
25 spectra confirmed the formation of hydrogen bonds between the
Figure 12. Water contact angles of the AzoꢀCOOH BZ/Azopy
BZ = 1:1 complex in its trans and cis isomeric forms, recorded
50 after each curing stage.
from AzoꢀCOOH BZ/Azopy BZ exhibited enhanced thermal
curing because the high basicity of the pyridine ring facilitated
nucleophilic attack on the benzoxazine ring.
Figure 12 displays the water contact angles of pure Azoꢀ
COOH BZ and the AzoꢀCOOH BZ/Azopy BZ supramolecular
complex. After interacting with Azopy BZ, the water contact
angle of the trans isomer of the supramolecular complex was
smaller than that of the trans isomer of pure AzoꢀCOOH BZ after
55
pyridyl moiety and the CO2H group. Some of the characteristic 60 thermal curing at all temperatures, consistent with intermolecular
peaks of pyridyl moieties are sensitive to the presence of
hydrogen bonds; for example, the signals at 1590, 1050, 993, and
625 cm–1 shifted to 1600, 1067, 1011, and 634 cm–1, respectively,
30 after pyridyl groups hydrogen bonded with the CO2H groups of
hydrogen bonding between the CO2H group of AzoꢀCOOH BZ
and the pyridyl group of Azopy BZ decreasing the
hydrophobicity of the surfaces.
poly(ethyleneꢀcoꢀmethacrylic acid).53 In this present study, we 65 Conclusions
observed that the signal of the pyridyl group shifted to 1646 cm–1
from 1590 and 1684 cmꢀ1 after blending Azopy BZ with Azoꢀ
COOH BZ, consistent with strong intermolecular hydrogen
35 bonding in their AzoꢀCOOH BZ/Azopy BZ complexes.
We have prepared a benzoxazine, AzoꢀCOOH BZ,
containing both azobenzene and CO2H units. The polymerization
temperature of AzoꢀCOOH BZ is much lower than that of
traditional benzoxazine monomers, because the CO2H and
Figure 11 presents FTIR spectra of the thermal 70 azobenzene groups on the benzoxazine monomer serves as a
polymerization products of the AzoꢀCOOH BZ/Azopy BZ =
50/50 complex after thermal curing at various temperatures. The
spectrum of pure AzoꢀCOOH BZ features signals for the C–C
40 stretching vibrations of its 1,2,4ꢀtrisubstitued benzene ring at
catalyst for the polymerization. Furthermore, TGA revealed that
the presence of the CO2H group significantly improved the
thermal stability and char yield of AzoꢀCOOH BZ. When we
performed polymerizations of AzoꢀCOOH BZ after blending with
1490 and 921 cm–1, as mentioned above. After thermal curing at 75 various ratios of AzopyꢀBZ, DSC revealed that the curing
150, 180, 210, and 240 °C, these characteristic bands had
disappeared. Interestingly, a curing temperature of 150 °C was
sufficient to complete benzoxazine ring opening of this
45 supramolecular complex, in contrast to the behavior (Figure 5) of
pure AzoꢀCOOH BZ. Thus, the supramolecular complex formed
temperature of AzoꢀCOOH BZ decreased from 199 to 150 °C.
Thus, blending Azopy BZ into AzoꢀCOOH BZ facilitated thermal
curing and enhanced the hydrophilicity of their surfaces as a
result of intermolecular hydrogen bonding between their CO2H
80 and pyridyl groups.
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32. S. W. Kuo, and F. C. Chang, Prog. Polym. Sci. 2011, 36,
1649ꢀ1696.
Acknowledgment
65
33. Y. Yagci, B. Kiskan, L. A. Demirel, and O. Kamer, J. Polym.
Sci. Part A: Polym. Chem. 2008, 46, 6780ꢀ6788.
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Sudo, T. Endo, and Y. Yagci, J. Polym. Sci. Part A: Polym.
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Chem 1999, 37, 1913ꢀ1921.
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This study was supported financially by the Ministry of Science
and Technology, Republic of China, under contracts MOST103ꢀ
2221ꢀEꢀ110ꢀ079ꢀMY3 and MOST102ꢀ2221ꢀEꢀ110ꢀ008ꢀMY3.
5
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