Azopyridine-functionalized benzoxazine with Zn(ClO4)2 form high-performance polybenzoxazine stabilized through metal-ligand coordination

Article abstract of DOI:10.1039/c4ra08381d

In this study, we prepared a benzoxazine monomer (Azopy-BZ) that features azobenzene and pyridine units through the reaction of paraformaldehyde, aniline, and 4-(4-hydroxphenylazo)pyridine (Azopy-OH), which is obtained through a diazonium reaction of 4-am

Full text of DOI:10.1039/c4ra08381d

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PAPER
Azopyridine-functionalized benzoxazine with
Zn(ClO4)2 form high-performance
polybenzoxazine stabilized through metal–ligand
coordination
Cite this: RSC Adv., 2014, 4, 50373
Mohamed Gamal Mohamed,a Wei-Chen Su,a Yung-Chih Lin,a Chih-Feng Wang,d
e
f
abc
*
Jem-Kun Chen, Kwang-Un Jeong and Shiao-Wei Kuo
In this study, we prepared a benzoxazine monomer (Azopy-BZ) that features azobenzene and pyridine units
through the reaction of paraformaldehyde, aniline, and 4-(4-hydroxphenylazo)pyridine (Azopy-OH), which
is obtained through a diazonium reaction of 4-aminopyridine with phenol in the presence of sodium nitrite
and NaOH. The azobenzene and pyridine groups in the benzoxazine monomer play the following two roles:
(i) allowing photoisomerization between the planar trans form and the nonplanar cis form of the
azobenzene unit (characterized using UV-vis spectroscopy and contact angle analyses) and (ii) serving as
a catalyst that accelerated the ring opening polymerization of the benzoxazine units, which was
characterized by the exothermic peak shifting to a lower temperature during differential scanning
calorimetry (DSC) analyses. The curing temperature of the model benzoxazine 3-phenyl-3,4-dihydro-
2H-benzoxazine (Pa-type) was 263 ꢀC; it decreased to 208 ꢀC for Azopy-BZ, presumably because of the
basicity of the azobenzene and pyridine groups. Blending with zinc perchlorate [Zn(ClO4)2] not only
improved the thermal properties, as determined through dynamic mechanical analysis (DMA), due to
physical crosslinking of the pyridine units through zinc cation coordination in a metal–ligand bonding
mode, but also further facilitated the ring opening polymerization to occur at a temperature of only
130 ꢀC (DSC). Thus, the presence of Zn(ClO4)2 overcame the problem of high temperature curing (ca.
180–210 ꢀC) required for traditional polybenzoxazines. Introducing the azobenzene and pyridine units
and the zinc salt into this polybenzoxazine system provided a multifunctional material that exhibited
photoisomerization-based tuning of its surface properties, accelerated ring opening polymerization of its
oxazine rings, and increasing physical crosslinking density, through metal–ligand interactions, to
enhance its thermal properties.
Received 8th August 2014
Accepted 22nd September 2014
DOI: 10.1039/c4ra08381d
www.rsc.org/advances
formaldehyde) is also attractive from the view point of devel-
oping novel benzoxazines bearing a variety of functional
groups.7–17 Some of these functional groups serve as reactive
Introduction
1,3-Benzoxazines are receiving considerable attention because
of their promising practical applications as monomers for high
performance materials.1–6 Their facile and versatile syntheses
from relatively abundant materials (e.g., phenols, amines,
sites to increase the degree of crosslinking, thereby varying the
thermal and mechanical properties of the obtained poly-
benzoxazines to fulll specic requirements. Because the
chemical structure of a polybenzoxazine is similar to that of a
traditional phenolic resin, polybenzoxazines possess good heat
resistance, good ame retardancy, good mechanical properties,
low dielectric constants, dimensional stability, low water
absorption, and low surface free energy.1,18–26 As a result, poly-
benzoxazines have potential uses as matrix resins for carbon
ber-reinforced plastics, adhesives, and electronic materials
(e.g., rigid printed circuit boards). Nevertheless, poly-
benzoxazines do have some unattractive properties, including
high curing temperatures for their ring opening polymeriza-
tions, brittleness, and moderate glass transition temperatures
(Tg) that are not sufficiently high for applications under harsh
aDepartment of Materials and Optoelectronic Science, National Sun Yat-Sen
University, Kaohsiung, Taiwan
bDepartment of Medicinal and Applied Chemistry, Kaohsiung Medical University,
Kaohsiung, Taiwan
cSchool of Chemical Engineering, East China University of Science and Technology,
Shanghai, China. E-mail: kuosw@faculty.nsysu.edu.tw
dDepartment of Materials Science and Engineering, I-Shou University, Kaohsiung,
Taiwan
eDepartment of Polymer-Nano Science and Technology, Chonbuk National University,
Jeonju, Korea
fDepartment of Materials Science and Engineering, National Taiwan University of
Science and Technology, Taiwan
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conditions.1,23,27 Many approaches have been reported to over- benzoxazine bearing a carboxyl moiety, and they also developed
come these shortcomings. For example, the mechanical and a polybenzoxazine/montmorillonite nanocomposite by using a
thermal properties of the benzoxazine can be improved signif- benzoxazine bearing a 4-pyridyl moiety as a key material.40,41
icantly aer blending with various polymers (e.g., epoxy,28,29
Combining metal ions with benzoxazines is also an inter-
polyurethane,30,31 polyimide32) or through copolymerization. In esting means of preparing new high-performance poly-
addition, the preparation of specically designed novel poly- benzoxazines. Endo et al. reported a novel 1,3-benzoxazine
benzoxazines or high-molecular-weight benzoxazines, featuring bearing a 4-aminopyridyl moiety, synthesized from 4-amino-
another polymerizable group (e.g., ethynyl,33 phenylethynyl,34 pyridine, p-cresol, and paraformaldehyde, with acetic acid
nitrile,35 propargyl,36 allyl37) or hydrogen bonding functional added to neutralize the basicity of the system. The introduction
group,38,39 can be an effective means of enhancing thermal of the 4-aminopyridyl moiety resulted in a polymer exhibiting
properties.
high affinity toward metal ions [e.g., copper(II), cobalt(II)],
Another application of functionalized benzoxazine mono- leading to the production of polymer–metal complexes in the
mers is their combination with inorganic materials (e.g., form of insoluble colored precipitates.41,51 Furthermore, among
clay,40–42 polyhedral oligomeric silsesquioxane (POSS),43–47 known photoresponsive molecules, azobenzene derivatives are
carbon nanotubes,48,49 graphene50) to fabricate nanocomposites, the most popular for photo switching applications52,53 because
stabilized through attractive interactions between the polar they can photoisomerize reversibly between their planar trans
functional groups of the monomer units and the inorganic form and nonplanar cis form upon irradiation with light. Azo-
surfaces. For example, Yagci et al. demonstrated the surface benzene derivatives have two major advantages over other
functionalization of magnetite nanoparticles with
a
molecules when used as photoswitches: their chemical
Scheme 1 (a and b) Syntheses of (a) Azopy-OH and (b) Azopy-BZ. (c) Metal–ligand coordination of Zn(ClO4)2 and Azopy-BZ; (d) possible
structure of Zn(ClO4)2/poly(Azopy-BZ) complexes.
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synthesis is relatively simple (indeed, some are commercially ltered, and concentrated under vacuum to afford an orange
available) and they exhibit chemical stability, allowing repeated residue, which was puried through column chromatography
trans-to-cis photoisomerizations without decomposition—even (SiO2; n-hexane/THF, 1 : 1) to give an orange powder (65%). M.p.
under aqueous conditions. In addition, trans-to-cis isomeriza- (DSC): 163 ꢀC. 1H NMR (d, DMSO-d6): 4.82 (s, 2H, CH2N), 5.61 (s,
tion occurs with irradiation near 350 nm, while cis-to-trans 2H, OCH2N), 6.88 (s, 1H, CH), 6.94 (s, 1H), 6.96 (d, 2H), 7.16 (d,
isomerization is induced by harmless visible light.
2H), 7.23 (d, 2H), 7.84 (d, 2H), 8.79 (d, 2H). 13C NMR (d, DMSO-
In this study, we prepared a benzoxazine derivative, Azopy- d6): 48.59 (CCH2N), 79.79 (OCH2N), 115.76–158.32 (aromatic).
BZ, featuring an azobenzene/pyridine functional group. First, IR (KBr, cmꢁ1): 1230 (asymmetric C–O–C stretching); 1341 (CH2
we prepared 4-(4-hydroxphenyleazo)pyridine (Azopy-OH) wagging); 915, 923, and 1490 (trisubstituted benzene ring); 1441
through a diazonium reaction of 4-aminopyridine with phenol (stretching of trans N]N).
in the presence of NaOH [Scheme 1(a)]. Next, we obtained
Azopy-BZ through the reaction of Azopy-OH, paraformaldehyde,
and aniline in 1,4-dioxane [Scheme 1(b)]. We then investigated
Characterization
the physical properties and specic interactions of Azopy-BZ 1H and 13C nuclear magnetic resonance (NMR) spectra were
when blended with various weight ratios of zinc perchlorate recorded using an INOVA 500 instrument with DMSO as the
[Zn(ClO4)2] before [Scheme 1(c)] and aer [Scheme 1(d)] thermal solvent and TMS as the external standard. Chemical shis are
curing. We employed differential scanning calorimetry (DSC), reported in parts per million (ppm). FTIR spectra of the polymer
Fourier transform infrared (FTIR) spectroscopy, dynamic lms were recorded using a Bruker Tensor 27 FTIR spectro-
mechanical analysis (DMA), and contact angle analyses to photometer and the conventional KBr disk method; moreover,
examine the thermal behavior, specic interactions, and note that 32 scans were collected at a spectral resolution of
surface properties of these materials.
4 cmꢁ1. The lms tested in this study were sufficiently thin to
obey the Beer–Lambert law. FTIR spectra recorded at elevated
temperatures were obtained from a cell mounted inside the
temperature-controlled compartment of the spectrometer.
Dynamic curing kinetics was determined using a TA Q-20
differential scanning calorimeter operated under a N2 atmo-
sphere. The sample (ca. 5 mg) was placed in a sealed aluminum
sample pan. Dynamic curing scans were recorded from 30 to
350 ꢀC at a heating rate of 20 ꢀC minꢁ1. The thermal stability of
the samples was measured using a TG Q-50 thermogravimetric
analyzer operated under a N2 atmosphere. The cured sample
Experimental section
Materials
4-Aminopyridine, paraformaldehyde (96%), sodium nitrite
(99%), hydrochloric acid (ca. 37%), aniline (99.8%), EtOAc,
tetrahydrofuran, and hexane were purchased from Acros. NaOH
and 1,4-dioxane were obtained from Aldrich, as was zinc
perchlorate hexahydrate [Zn(ClO4)2$6H2O], which was dried in a
ꢀ
vacuum oven at 70 C for 24 h prior to use.
ꢀ
(ca. 5 mg) was placed in a Pt cell and heated at a rate of 20ꢁ1C
minꢁ1 from 30 to 800 C under a N2 ow rate of 60 mL min
.
ꢀ
Azopy-OH
DMA was performed using a PerkinElmer instrument DMA 8000
apparatus operated in tension mode and for the preparation of
the samples for DMA; moreover, the sample powder was sand-
wiched in the middle of a single cantilever and the bending was
measured at temperatures from room temperature to 250 ꢀC.
Analyses of the loss tangent (tan d) were recorded automatically
by the system, and the heating rate and frequency were xed at
2 ꢀC minꢁ1 and 1 Hz, respectively. UV-vis spectra were recorded
A solution of phenol (5.0 g, 53 mmol) and sodium nitrite (4.0 g,
58 mmol) in 10% (w/w) aqueous NaOH (20 mL) was added
dropwise to a solution of 4-aminopyridine (6.0 g, 64 mmol) in
ꢀ
7.3 N HCl (45 mL) at 0 C. The mixture was adjusted to pH 6
through the addition 10% aqueous NaOH. The orange precipi-
tate was ltered off, washed with water, dried and recrystallized
(acetone/water) to give the product (3.5 g, 35%). M.p. (DSC):
1
ꢀ
263–265 C. H NMR (d, DMSO-d6): 6.98 (d, 2H), 7.67 (d, 2H),
7.88 (d, 2H), 8.77 (d, 2H), 10.59 (s, 1H). 13C NMR (d, DMSO-d6):
162.29, 156.81, 151.32, 145.24, 125.77, 116.18, 115.77. IR (KBr,
cmꢁ1): 3204–3463 (OH).
using
a Shimadzu mini 1240 spectrophotometer. Photo-
isomerization of the azobenzene unit was performed using a
UV-vis lamp directly at 365 nm (UV, 90 mW cmꢁ2) and through a
lter at 400–500 nm (visible, 90 mW cmꢁ2) to generate the light
for the trans-to-cis and cis-to-trans isomerizations, respectively.
The concentration of Azopy-BZ in THF was 10ꢁ4 M for the UV-vis
Azopy-BZ
A solution of aniline (1.03 g, 11 mmol) in 1,4-dioxane (25 mL) spectroscopic study. The contact angles of the polymer samples
ꢀ
was added portion wise to a solution of paraformaldehyde (0.66 before and aer curing were measured at 25 C using a FDSA
g, 22 mmol) in 1,4-dioxane (100 mL) in a 250 mL ask cooled in Magic Droplet 100 contact angle goniometer interfaced with
an ice bath. The mixture was stirred for 20 min at a temperature image capture soware aer injecting a 5 mL liquid drop. To
below 5 ꢀC, and then a solution of Azopy-OH (2.0 g, 10 mmol) in obtain reliable contact data, at least three droplets were
1,4-dioxane (30 mL) was added. The mixture was heated under measured at different regions of the same piece of lm, with at
ꢀ
reux at 90–95 C for 24 h. Aer cooling, rotary evaporation of least two pieces of lm being used. Thus, at least six advancing
the solvent gave a viscous residue that was dissolved in EtOAc contact angles were averaged for each type of lm with each type
(100 mL) and washed several times with 5% NaHCO3 and nally of solvent. Note that deionized water was used as a standard
with distilled water. The organic phase was dried (MgSO4), when measuring the surface properties.
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aer thermal curing. The spectrum of Azopy-OH features a
sharp signal for the OH group at 3442 cmꢁ1 and absorption
bands for the aromatic ring at 3025, 1610, and 1571 cmꢁ1
[Fig. 3(a)]. In the spectrum of Azopy-BZ [Fig. 3(b)], the signal for
the phenolic OH group is absent, but signals are evident for C–C
stretching vibrations of the 1,2,4-substituted benzene ring at
1490 and 923 cmꢁ1 for the oxazine ring, for asymmetric C–O–C
stretching at 1229 cmꢁ1, and for stretching of the trans N]N
bond at 1440 cmꢁ1. These spectroscopic features conrmed the
successful synthesis of Azopy-BZ. Fig. 3(c) presents the FTIR
spectrum recorded aer thermal curing of pure Azopy-BZ. The
characteristic absorption bands at 1490 and 923 cmꢁ1 for the
trisubstituted aromatic ring of Azopy-BZ disappeared aer
thermal curing with the former shiing to 1502 cmꢁ1 for the
tetrasubstituted aromatic ring of poly(Azopy-BZ). The broad
absorption bands in the range of 2600–3500 cmꢁ1 in Fig. 3(c)
represent three different kinds of hydrogen bonding interac-
tions: intramolecular [Oꢁ/H–N+] hydrogen bonding (ca. 2800
cmꢁ1), intramolecular [O–H/N] hydrogen bonding (ca. 3200
cmꢁ1), and intermolecular [O–H/O] hydrogen bonding (ca.
3342 cmꢁ1); the identities of these signals have been reported
previously.18,54
Results and discussion
Synthesis of Azopy-BZ through Mannich reaction of Azopy-OH
with aniline and paraformaldehyde
We prepared Azopy-BZ in high purity and high yield, even
though it was possible that we might generate various side
products, due to the high reactivity of pyridyl and its ability to
form a hydrogen bond intramolecularly with phenol groups.
Fig. 1 presents the 1H NMR spectra of Azopy-OH and Azopy-BZ.
The spectrum of Azopy-OH [Fig. 1(a)] features a signal at 10.58
ppm, corresponding to the proton of the phenolic OH group,
with the signals of the aromatic protons of the azobenzene
pyridine unit appearing as multiplets in the range of 6.97–8.77
ppm. The spectrum of Azopy-BZ [Fig. 1(b)] features the signals
for the CH2N and OCH2N bridges of the oxazine unit at 4.82 and
5.61 ppm, respectively. The signal of the proton of the phenolic
OH group of Azopy-OH is absent, but the signals of the azo-
benzene pyridine moiety are present, conrming the successful
synthesis of Azopy-BZ.
Fig. 2 presents the 13C NMR spectra of Azopy-OH and Azopy-
BZ. The signals of the azobenzene pyridine moiety appear in the
range from 115.75 to 158.32 ppm in Fig. 2(a). Two peaks appear
at 48.6 and 79.9 ppm in the spectrum of Azopy-BZ in Fig. 2(b),
representing the carbon nuclei of the oxazine ring, and the
signals of the azobenzene pyridine ring conrm the successful
synthesis of Azopy-BZ. Fig. 3 presents the FTIR spectra of Azopy-
OH and Azopy-BZ at room temperature and of poly(Azopy-BZ)
Thermal Polymerization of Azopy-BZ
We used DSC to determine the curing behavior of pure Azopy-
BZ (Fig. 4). The DSC trace of uncured Azopy-BZ featured a sharp
Fig. 1 1H NMR spectra of (a) Azopy-OH and (b) Azopy-BZ.
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Fig. 2 13C NMR spectra of (a) Azopy-OH and (b) Azopy-BZ.
lower polymerization temperature. Aer thermal curing of pure
ꢀ
Azopy-BZ at 110 C, the rst melting peak was absent from the
DSC curve, with the intensity of the exotherm decreasing upon
increasing the curing temperature, almost disappearing aer
thermal curing at temperatures between 150 and 180 ꢀC. Fig. 5
shows FTIR spectra of Azopy-BZ aer various curing stages.
Upon increasing the curing temperature, we observed a
decrease in the intensities of the characteristic absorption
bands of the benzoxazine unit at 923 cmꢁ1 (C–O–C symmetric),
1230 cmꢁ1 (C–O–C asymmetric), and 1490 cmꢁ1.1 Aer curing at
180 ꢀC, these characteristic absorption bands had disappeared
completely, suggesting that ring opening of the benzoxazine
units had reached completion. In addition, aer curingꢁa1t
150 ꢀC, we observed a new absorption band at 2800–3400 cm
,
which is consistent with the formation of a phenolic OH group.
These features are consistent with the progress of the ring
opening polymerization of Azopy-BZ during thermal curing.
Fig. 6 shows TGA thermograms of pure Azopy-BZ before and
Fig. 3 FTIR spectra of (a) Azopy-OH, (b) Azopy-BZ, and (c) poly(-
Azopy-BZ), recorded at room temperature.
ꢀ
aer thermal curing at 60, 110, 150, and 180 C under N2. The
thermal stability, the decomposition temperature (10 wt% loss),
and the char yield all decreased aer curing at 60 and 110 C,
ꢀ
melting peak at 163 C, revealing the high purity of the Azopy-
ꢀ
BZ monomer. A sharp exothermic peak, which we attribute to
the ring opening polymerization of Azopy-BZ, appeared with its
onset at 198 C and its maximum at 208 C; it had a reaction
heat of 579 J gꢁ1. The curing temperature for Azopy-BZ (208 ꢀC)
presumably because of loss of the crystal structure at these
curing temperatures, consistent with the DSC traces in Fig.ꢀ4.
Further increase in the curing temperature to 150 and 180 C
caused the decomposition temperature and char yield to
increase, which is consistent with increased crosslinking
densities enhancing the thermal stabilities of the resulting
poly(Azopy-BZ) species.
ꢀ
ꢀ
ꢀ
shied from 263 C for conventional 3-phenyl-3,4-dihydro-2H-
benzooxazine (Pa-type),1 presumably because of the basicity of
the azo and pyridyl groups, suggesting that the azobenzene and
pyridine moieties acted as basic catalysts that contributed to the
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Fig. 4 DSC thermograms of Azopy-BZ, recorded after each curing stage.
photoisomerization of Azopy-BZ at 60, 110, 150 and 180 ꢀC,
the water contact angles changed completely to 29ꢀ, 88ꢀ, 97ꢀ,
and 100ꢀ, respectively. The water contact angle of the trans
isomer of Azopy-BZ (95ꢀ) was larger than that of the cis isomer
of Azopy-BZ (29ꢀ) aer curing at 60 ꢀC, and this behavior is
consistent with the trans isomer having a smaller dipole
moment and lower surface free energy (and therefore a higher
water contact angle) and the cis form possesses a bigger dipole
moment and higher surface free energy (and therefore a lower
water contact angle).55 The water contact angles increased
upon increasing the thermal curing temperature and time for
both the trans and cis isomers, due to increased degrees of
intramolecular hydrogen bonding and lower surface free
energies aer thermal curing of the benzoxazine monomer.
FTIR spectra revealed (Fig. 5) that four different types of
hydrogen bonds were present upon increasing the curing
temperature: intramolecular [Oꢁ/H+N] hydrogen bonds (2830
cmꢁ1), intramolecular [OH/N] (from the Mannich bridge)
hydrogen bonds (3200 cmꢁ1), intermolecular [OH/O] and
[OH/N] (from pyridyl) hydrogen bonds (3400–3370 cmꢁ1),
and intramolecular [OH/p] hydrogen bonds (3459 cmꢁ1), as
has been noted in previous reports.18,54 As a result, the strong
intramolecular [OH/N] hydrogen bonds (from the Mannich
bridge) dominated for low surface free energy than the pho-
toisomer of Azopy-BZ monomer, and we also found that the
water contact angle of the trans isomer remained larger than
that of the cis isomer aer thermal curing.
Photoisomerization of azopyridine chromophore in Azopy-BZ
Fig. 7 presents UV-vis absorption spectra, revealing the photo
-isomerization of the azopyridine chromophore in Azopy-BZ.
Fig. 7(a) shows the change in the UV-vis absorption spectra of
the Azopy-BZ solution upon irradiation with UV light at 365
nm. The maximum absorption at 360 nm was due to the p–p*
transition of the trans-azopyridine group. Upon irradiation
with UV light, the intensity of the absorbance at 360 nm
decreased gradually, eventually reaching a relatively stable
value. At the same time, the intensity of the signal near
333 nm, which we assign to the p–p* transition of the cis
isomer increased. Eventually, the wavelength of maximum
absorption shied from 360 to 333 nm. These variations are
consistent with the trans-to-cis isomerization of the azopyr-
idine group in Azopy-BZ. Fig. 7(b) reveals the change in the
UV-vis absorption spectra of the Azopy-BZ solution upon
irradiation with UV light at 443 nm; moreover, the intensity of
the absorbance at 360 nm increased gradually until reaching a
stable state aer 3 min, consistent with cis-to-trans isomeri-
zation of the azopyridine group. Fig. 8 displays the contact
angles of water for pure Azopy-BZ before and aer its photo-
isomerization under a UV lamp at 365 nm, as well as aer
thermal curing at 60, 110, and 150 ꢀC for 3 h and at 180 ꢀC for
2 h. The water contact angles of Azopy-BZ at room tempera-
ꢀ
ture and aer thermal curing at 60, 110, and 150 C were 89ꢀ,
95ꢀ, 98ꢀ, and 101ꢀ, respectively, and aer thermal curing at
180 ꢀC, it reached 108ꢀ. Notably, aer trans-to-cis
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Fig. 5 FTIR spectra of Azopy-BZ, recorded after each curing stage.
containing 1 or 2 wt% of Zn(ClO4)2, we observed three major
ꢀ
curing peaks: near 130, 190, and 233 C, respectively. The rst
two curing peaks presumably arose from ring opening of the
benzoxazine units coordinated with Zn2+ ions, while the latter
corresponds to the curing of the non-coordinated benzoxazine
units. The curing peak shied to lower temperature upon the
addition of Zn(ClO4)2, indicating that the Zn2+ ions could
initiate the ring opening process.56,57 Ishida et al. reported that
transition metal salts initiate ring opening, but not the poly-
merization of Ba-type benzoxazine.56 The temperature for ring
opening of the benzoxazine units shied from 208 ꢀC in the
absence of Zn(ClO4)2 to 233 ꢀC in its presence because the initial
ring opening in the presence of Zn(ClO4)2 resulted in a certain
degree of crosslinking that inhibited the purely thermal curing
of the remaining benzoxazine groups. Furthermore, increasing
the Zn(ClO4)2 content to 3–5 wt% caused the second peak at
190 ꢀC to shi completely to 130 ꢀC, thereby increasing the
enthalpy of curing at the rst peakꢀtemperature. Notably, the
intensity of the curing peak at 233 C did not change accord-
ingly, and the total enthalpy of curing of the overall reaction
remained relatively unchanged.
Fig. 6 TGA analyses of Azopy-BZ, recorded after each curing stage.
Thermal Polymerization of Azopy-BZ blended with Zn(ClO4)2
We used DSC to investigate the thermal curing behavior of pure
Azopy-BZ in the presence of various amounts of Zn(ClO4)2
(Fig. 9). The melting peak disappeared aer blending with this
zinc salt, indicating that the crystal structure was destroyed
upon the interactions of the Zn2+ ions with the pyridine or azo
groups. Interestingly, prior to thermal curing of the samples
Fig. 10(a) presents FTIR spectra of Azopy-BZ blended with
various weight ratios of Zn(ClO4)2 prior to thermal curing.
Signals for the stretching of the pyridine ring appeared at 1598,
1490, and 993 cmꢁ1, with these peaks sometimes overlapping
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Fig. 7 UV-vis absorption spectra of Azopy-BZ (10ꢁ4 M in THF) (a) before and (b) after irradiation with UV light at 365 nm for different periods of
time.
with those representing the aromatic ring; indeed, we observed
three major peaks at 1612, 1598, and 1576 cmꢁ1 for aromatic
stretching of the C]C bonds in the phenyl–O–C, pyridine ring,
and phenyl–N–C groups of Azopy-BZ. Upon addition of
Zn(ClO4)2, a new band appeared at 1644 cmꢁ1, the fraction of
which increased upon increasing the content of the salt. In a
previous study of Zn(ClO4)2/P4VP blend systems, we found that,
upon addition of Zn(ClO4)2, a similar new band appeared at a
relatively high wavenumber, and we assigned this signal as
resulting from the Zn2+ coordinating to pyridine rings.58 Pires
et al. reported that the pyridine units in P4VP behave as p-
bonding ligands when coordinated to this cation.59 Therefore,
the higher energy of this new absorption band is the result of
the formation of such a coordination complex. Ishida et al.
found, however, that treatment of benzoxazines with metal salts
resulted in an increase in the content of OH groups, indicating
the presence of ring-opened structures. They also detected a
band at 1654 cmꢁ1 aer blending with metal salts, tentatively
assigned to conjugated C]O groups of possibly quinone-like
structures. The OH groups of phenols can be oxidized by metal
ions to form quinone structures.56 The structure of Azopy-BZ,
however, differs from that of a Ba-type benzoxazine because it
possesses another pyridyl group, which could also interact with
Zn(ClO4)2. To conrm whether such an interaction existed,
Fig. 10(b) displays the region of the FTIR spectra featuring the
ꢁ
stretching bands of ClO4 (from 660 to 600 cmꢁ1) for pure
Fig. 8 Water contact angles of Azopy-BZ in its trans and cis isomeric
Zn(ClO4)2 and for various Zn(ClO4)2/Azopy-BZ blends. In this
forms, recorded after each curing stage.
region, absorptions at 627 and 635 cmꢁ1 represent vibrations of
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Fig. 9 DSC thermograms of Azopy-BZ in the presence of various amounts of Zn(ClO4)2.
ꢁ
the free and contacted ClO4 ions, respectively.60 When the suggesting that the ring opening polymerization of Azopy-BZ
concentration of Zn(ClO4)2 decreased, the band for the con- was complete in the presence of 5 wt% Zn(ClO4)2.
tacted ion shied to lower frequency and became symmetric.
Fig. 12 presents the FTIR spectra of Azopy-BZ blended with
We attribute the asymmetric shape of the band for pure 5 wt% Zn(ClO4)2 aer thermal curing at various temperatures.
Zn(ClO4)2 to the existence of both free ions and ion pairs. Aer thermal curing at 60 ꢀC, the absorption peaks at 1640 and
Blending with Azopy-BZ caused the fraction of contacted ions to 1092 cmꢁ1 disappeared, presumably because of dissociation
decrease, consistent with the Zn2+ ion interacting with thꢁe and the Zn2+ ions coordinated with the pyridyl units. Aer
pyridyl group of Azopy-BZ, causing the fraction of free ClO4
thermal curing at 150 and 180 ꢀC, the characteristic absorption
anions to increase. We found that the degree of coordination of bands of the trisubstituted aromatic rings of the oxazine units
the pyridine rings increased upon increasing the content of at 1490 and 925 cmꢁ1 disappeared, with signals for the phenolic
Zn(ClO4)2.
OH groups, arising from ring opening, appearing at 1630 and
Fig. 11 presents the thermal curing behavior of Azopy-BZ 1512 cmꢁ1. Liu et al. reported that the metal salts perform three
containing 5 wt% Zn(ClO4)2. As mentioned above, two major major steps during the ring opening of benzoxazine. First, the
curing peaks appeared at 130 and 233 ꢀC, corresponding to ring catalyst coordinates with the oxygen and nitrogen atoms of the
opening of the Zn2+-coordinated benzoxazine units and to the oxazine ring, and then subsequent electrophilic reactions
original curing of benzoxazine units, respectively. Aer thermal involve O attack, N attack, and aryl attack, and nally, rear-
curing at 60 ꢀC, the intensity of the rst exotherm peak rangement occurs from a phenoxy structure to a phenolic
decreased, eventually disappearing aer thermal curing at structure.61 We conclude that the Zn2+ ions were effectively
110 ꢀC. The rst curing peak shied to 190 ꢀC, the same curing coordinated with the oxygen and/or nitrogen atoms during ring
temperature as that for the second peak of the non-cured Azopy- opening of the benzoxazine units. In addition, the azopyridine
BZ aer blending with 1 wt% Zn(ClO4)2 (see Fig. 9). Aer units in Azopy-BZ provided another coordination site to
thermal curing at 150 ꢀC, this curing peak also disappeared and increase the physical crosslinking density, thereby improving
shied to 228 ꢀC, corresponding to the temperature of thermal the thermal properties aer thermal curing.
curing of the original benzoxazine. Aer thermal curing at
Finally, we used DMA to investigate the glass transition
180 ꢀC, the exothermic peaks disappeared completely, temperatures of these species. Fig. 13 indicates that the glass
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Fig. 10 FTIR spectra of Azopy-BZ in the presence of various amounts of Zn(ClO4)2, recorded at room temperature.
Fig. 11 DSC thermograms of Azopy-BZ in the presence of 5 wt%
Zn(ClO4)2, recorded after each curing stage.
Fig. 12 FTIR spectra of Azopy-BZ in the presence of 5 wt% Zn(ClO4)2,
recorded after each curing stage.
transition temperature of poly(Azopy-BZ) increased from 159 to
the ion–polymer and ion–ion interactions of the poly-
benzoxazine. Scheme 2 displays some possible modes of metal–
ligand coordination in the complex formed between Zn(ClO4)2
and Azopy-BZ.
198 ꢀC aer blending with 5 wt% Zn(ClO4)2 and thermally
ꢀ
curing at 180 C. The minor peaks at higher temperature may
come from another thermal transition for poly(Azopy-BZ)
matrix. Clearly, the ionic interactions or ionic cluster formation
of ionomers usually resemble physical cross-linking.58,60,62 The
mobility of polymer chains was restricted by such physical
cross-links, thereby resulting in higher glass transition
temperatures than that of the mother polymer, due to increased
Conclusions
In this study, we synthesized an azo/pyridine-functionalized
polybenzoxazine. DSC revealed that the exothermic peak for
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Scheme 2 (Left) Possible morphology of a Zn(ClO4)2/poly(Azopy-BZ) complex. (Right) Possible modes of metal–ligand coordination between
Zn(ClO4)2 and poly(Azopy-BZ).
the ring opening polymerization of benzoxazine itself shied
to lower temperature because the azo and pyridyl groups acted
as basic catalysts for the ring opening process. In addition, the
azobenzene group also allowed photoisomerization between its
planar trans form and its nonplanar cis form upon irradiation
with light, thereby allowing tuning of the surface properties.
Furthermore, the azopyridyl moiety in the benzoxazine
exhibited high affinity toward Zn2+ ions, not only promoting
the ring opening polymerization to occur at a relatively low
curing temperature (only 130 ꢀC) but also leading to the
formation of corresponding polymer–metal complexes and,
thereby, improving the thermal properties, based on DMA
analyses. Because of such metal–ligand bonding, we predict
that such polymers would be useful as metal scavenging
materials and as components within polymer/inorganic hybrid
materials.
Acknowledgements
This study was supported nancially by the National Science
Council, Taiwan, Republic of China, under contracts MOST103-
2221-E-110-079-MY3 and MOST102-2221-E-110-008-MY3.
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