Emission and surface properties of main-chain type polybenzoxazine with pyridinyl moieties

Article abstract of DOI:10.1039/c3ra46854b

A main-chain type polybenzoxazine, PBz (1), with pyridinyl moieties was successfully synthesized from the Mannich condensation of 4-phenyl-2,6-bis(4- aminophenyl) pyridine (2), paraformaldehyde, and bisphenol A. For the purpose of property comparison, a s

Full text of DOI:10.1039/c3ra46854b

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Emission and surface properties of main-chain type
polybenzoxazine with pyridinyl moieties†
Cite this: RSC Adv., 2014, 4, 8692
a
a
a
a
Ching Hsuan Lin,* Yu Sin Shih, Meng Wei Wang, Chun Yu Tseng,
b
c
Tzong Yuan Juang and Chih Feng Wang*
A main-chain type polybenzoxazine, PBz (1), with pyridinyl moieties was successfully synthesized from the
Mannich condensation of 4-phenyl-2,6-bis(4-aminophenyl) pyridine (2), paraformaldehyde, and bisphenol
A. For the purpose of property comparison, a structurally similar polybenzoxazine, PBz (2), was prepared.
Unfortunately, PBz (2) is insoluble in organic solvents due to its rigid structure. In contrast, the pyridinyl
group provided solubility for the processing of PBz (1). When the THF dilute solution of PBz (1) was
protonated with HCl, a new absorption signal at 458 nm was observed in the UV-vis spectrum. No
emission at 500–700 nm occurred in the fluorescence spectrum before protonation of PBz (1).
However, upon protonation, the protonated pyridinyl became a stronger acceptor, and then, an
emission at 570 nm occurred between the nitrogen of benzoxazine (donor) and protonated pyridinyl
groups (acceptor) after being excited at 458 nm. After thermal curing, the thin film of PBz (1) thermoset
Received 20th November 2013
Accepted 20th January 2014
ꢀ
ꢀ
ꢁ1
was flexible, with a T
g
value of 261 C, a coefficient of thermal expansion of 38 ppm
C
, and 5%
DOI: 10.1039/c3ra46854b
ꢀ
ꢀ
decomposition temperature at 414 (N
2
) and 419 C (air). The contact angle is as high as 102 , and the
ꢁ2
www.rsc.org/advances
surface energy is as low as 19.6 mJ m
.
prepared pyridine-containing diamines with a triphenylamine
Introduction
2,6,12,13
structure and discussed its emission aer protonation.
Benzoxazines are resins that can be polymerized to ther-
atom, is an aromatic heterocycle that can enhance solubility. mosets by means of thermally activated ring-opening reac-
Pyridine, with a localized lone pair of electrons on the nitrogen
1
4–16
Recently, some reports have been concerned with the intro- tions.
Thermosets with low water absorption, superior
17
18
duction of pyridinyl groups into polymer to increase thermal electrical properties and low surface energy can be obtained
stability due to the molecular symmetry and aromaticity of aer curing. Main-chain-type polybenzoxazines are an active
1–6
pyridine. Pyridine-containing amines are generally prepared concept in the recent development of benzoxazines. They can be
7
–10
by the Chichibabin reaction
and the modied Chichibabin prepared by Mannich-type polycondensation of diamine,
11
19–23
reaction. The modied Chichibabin reaction has been used to bisphenol, and formaldehyde.
However, most of the
prepared pyridine-containing diamines, 4-aryl-2,6-bis(4-amino- discussion on properties focuses on the thermal and mechan-
1
1
3
phenyl) pyridines. Polyamides and polyimides based on the ical properties of benzoxazine thermosets. UV-vis absorption
pyridine-containing diamines show good thermal stability and and emission characteristics of polybenzoxazines have never
solubility in polar aprotic solvents. Pyridine-containing diamine been reported. In this work,
a main-chain type poly-
with
a
diphenoxy structure, 4-phenyl-2,6-bis[4-(4-amino- benzoxazines, PBz (1), with pyridinyl moieties was successfully
phenoxy)phenyl]-pyridine, has been prepared to prepare synthesized from the Mannich condensation of 4-phenyl-2,6-
5
polyimide with transparency, exibility and solubility. Pyri- bis(4-aminophenyl)pyridine (2), paraformaldehyde, and
dine-containing diamines with a naphthalene or pyrene moiety bisphenol A. For the purpose of property comparison, a struc-
6
have been reported by Liaw et al. Polyimides with good thermal turally similar polybenzoxazine, PBz (2), was also prepared.
stability, mechanical properties and high dielectric constant Detailed synthesis, absorption and emission of PBz (1) upon
were prepared based on the diamines. Liaw and Wang et al. protonation, surface energy and thermal properties of PBz (1)
thermoset are provided in this work.
a
Department of Chemical Engineering, National Chung Hsing University, Taichung,
Taiwan. E-mail: linch@nchu.edu.tw
b
Experimental
Materials
Department of Applied Chemistry, National Chiayi University, Chiayi, Taiwan
c
Department of Materials Science and Engineering, I-Shou University, Kaohsiung,
Taiwan
Benzaldehyde, bisphenol
purchased from TCI. Phenol, ammonium acetate, and
A and paraformaldehyde were
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c3ra46854b
1
8692 | RSC Adv., 2014, 4, 8692–8698
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
hydrazine hydrate 80%were purchased form Showa. Palladium in water. The sample ID of the lm of PBz (1) thermoset is
on carbon 10%, p-nitroacetophenone were purchased from named PBz (1)-T.
Acros. Glacial acetic acid and acetic anhydride were purchased
from Scharlau. Boron triuoride diethyl etherate was purchased
from ALFA. All solvents were HPLC grade and were
Thin-lm formation for contact angle measurement
A polymer stock solution was prepared by dissolving the poly-
purchased from Tedia. P(B-oda) is a main-chain type poly-
0
benzoxazine precursor in THF at a concentration of 30 mg
benzoxazine prepared from 4,4 -diamino diphenyl methane/
ꢁ1
24
mL . The solution was ltered through a 0.2 mm syringe lter
before spin-coating onto a glass slide (50 ꢂ 50 ꢂ 1 mm). 1 mL of
the appropriate polymer solution was spin-coated onto a glass
slide using a photoresist spinner operating at 1500 rpm for 45 s.
phenol/paraformaldehyde according our previous work.
Preparation of diamine (2)
1
ꢀ
Diamine (2) was prepared by a two-step procedure (Scheme 1),
and the procedure is listed in ESI.†
The sample was le to dry at 60 C for 1 h, and then it was cured
ꢀ
in an oven at a desired time under 240 C.
Preparation of diamine (5)
Characterization
Diamine (5) was prepared by a three-step procedure (Scheme Differential scanning calorimetry (DSC) was performed using a
2
5
S1†), and the procedure is listed in ESI.†
Perkin-Elmer DSC 7 in a nitrogen atmosphere at a heating rate
ꢀ
ꢁ1
of 10 min C . Thermal gravimetric analysis (TGA) was per-
ꢀ
Synthesis of PBz (1) and PBz (2)
formed with a Perkin-Elmer Pyris1 at a heating rate of 20 C
ꢁ1
min in a nitrogen or air atmosphere. Dynamic mechanical
analysis (DMA) was performed with a Perkin-Elmer Pyris Dia-
mond DMA with a sample size of 5.0 ꢂ 1.0 ꢂ 0.006 cm. The
PBz (1) was prepared by a one-pot procedure (Scheme 1). (2)
0.00 g (0.059 mole), bisphenol A 13.53 g (0.059 mole), para-
formaldehyde 7.12 g (0.236 mole), and a mixed solvent of
toluene–ethanol 120 mL (1/1, v/v) were introduced into a round-
bottom 500 mL glass ask equipped with a nitrogen inlet, a
condenser, and a magnetic stirrer. The reaction mixture was
2
0
storage modulus E and tan d were determined as the sample
was subjected to the temperature scan mode at a programmed
ꢀ
ꢁ1
heating rate of 5 C min at a frequency of 1 Hz. The test was
performed by a bending mode with an amplitude of 25 mm.
Thermal mechanical analysis (TMA) was performed with a
ꢀ
stirred at 80 C for 24 h. Aer that, the solution was poured into
methanol. The precipitate was ltered and dried in a vacuum
oven. Yellow powder (85% yield) was obtained. PBz (2) was
similarly prepared except the starting diamine was (5). Yellow
powder (90% yield) was obtained.
ꢀ
Perkin-Elmer Pyris Diamond TMA at a heating rate of 5
C
ꢁ1
min . The coefficient of thermal expansion was recorded in
ꢀ
ꢀ
the temperature range of 50 C to 150 C. NMR measurements
were performed using a Varian Inova 600 NMR in DMSO-d
and the chemical shi was calibrated by setting the chemical
shi of DMSO-d as 2.49 ppm. IR Spectra were obtained from at
least 32 scans in the standard wavenumber range of 400–
6
,
Preparation of PBz (1) thermoset for thermal property
measurement
6
ꢁ1
A 25 wt% PBz (1) in NMP solution was prepared. The solution 4000 cm by a Perkin-Elmer RX1 infrared spectrophotometer.
was then cast onto glass by an automatic lm applicator. The Gel permeation chromatography (GPC) was carried out on a
ꢀ
resulting lms were transparent and colorless. They were dried Perkin-Elmer series 200a using THF as the eluent at 25 C with
ꢀ
ꢀ
ꢀ
ꢀ
ꢁ1
at 60 C overnight and cured at 100 C (1 h), 160 C (1 h), 200 C a ow rate of 1.0 mL min . The data were calibrated with
ꢀ
(
1 h) and 240 C (1 h), respectively. Aer that, brown transparent polystyrene standard. UV-vis spectrum was performed by a

lms (about 60 mm) were obtained by soaking the coated glass Shimadzu UV mini 1240 spectrophotometer. Fluorescence
Scheme 1 Synthesis of PBz (1).
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 8692–8698 | 8693

View Article Online
RSC Advances
Paper
spectrum was performed by a Hitachi F-2500 spectrophotom- 4.6 ppm conrm the structure of benzoxazines. No signal was
eter. The concentration of polybenzoxazine in THF is 0.01 M for observed corresponding to N–CH -ph at around 4.1 ppm,
2
both UV and Fluorescence spectrum. The contact angle of the which resulted from the ring opening of benzoxazine. A further
ꢀ
polymer sample was measured at 25 C using a FDSA Magi- increase in the ethanol ratio led (67%) to a low yield (Run 7).
cDroplet-100 contact angle goniometer interfaced with image- This might be due to the product being somewhat soluble in
1
capture soware by injecting a 5 mL liquid drop. To obtain the precipitation process. Fig. S4† shows the H-NMR spectrum
reliable contact data, at least three droplets were in at different of PBz (1) prepared by Run 6, Table S1.† The characteristic
regions of the same piece of lm, and at least two pieces of lm oxazine peaks at 4.6 and 5.4 ppm support the structure of PBz
ꢁ
1
were used to obtain reliable contact angle data. Thus, at least (1). In the IR spectrum (not shown), absorptions at 945 cm
ꢁ
1
six advancing contact angles were averaged for each type of lm for oxazine, and absorptions at 1032 cm (symmetric stretch)
and each kind of liquid. Deionized water, ethylene glycol, and and 1232 cm (asymmetric stretch) for Ar–O–C were observed,
diiodomethane were used as standards to measure the surface supporting the structure of PBz (1). The integral area of
ꢁ1
free energies.
2
methylene in N–CH -ph/methyl in isopropylidene is 0.48,
which is lower than the theoretical value of 0.67. The lower
ratio can be explain by the existence some ring-opened struc-
ture in the PBz (1). The other possible reason is that PBz (1) is
phenol-terminated telechelic oligomer with number average
Results and discussion
Synthesis of PBz (1)
ꢁ1
Diamine (2), a precursor for PBz (1), was prepared by a modied molecular weight 3500 g mol (Table 1), and the phenol-
Chichibabin reaction, followed by reduction. Fig. S1† shows terminated structure leads to higher internal area of iso-
the H-NMR of (2). The signals are consistent with the structure. propylidene signals.
1
1
PBz (1) was prepared from the Mannich condensation of (2),
paraformaldehyde, and bisphenol A (Scheme 1).
Synthesis of PBz (2)
Table S1† lists the solvent effect on the synthesis of PBz (1).
Using homo-solvent, such as 1,4-dioxane (Run 1), chloroform Diamine (5), a precursor for PBz (2), was prepared by a three
2
5
(
Run 2), xylene (Run 3), and toluene (Run 4) led to insoluble step procedure according to Scheme S1.† Fig. S5† shows the
1
gelation. Although gelation occurs for PBz (1) synthesis in
H-NMR spectrum of (5). The assignment is consistent with the
homo-solvent (Run 1–4), the gelation phenomenon was structure. PBz (2) was prepared from the Mannich condensation
signicant improved using toluene–ethanol as reaction solvent of (5), paraformaldehyde, and bisphenol A (Scheme S1 and
1
(
Runs 5–7). A slurry form was obtained for toluene–ethanol (2/ Runs 8–10 of Table S1†). Fig. S6† shows the H-NMR spectrum
1, Run 5). Increasing the ratio of ethanol led to a homogeneous of PBz (2). The characteristic oxazine peaks at 4.6 and 5.4 ppm
solution (Runs 6–7). Fig. S2† shows the NMR trace for PBz (1) support the structure of PBz (2). While PBz (1) shows good
synthesized in Run 5. A triazine signal at 4.88 ppm was solubility during preparation, there was powder precipitation
observed aer 2 h, and reached a maximum aer 4 h. Aer that, (not gelation) aer reacting for 1 h due to the poor solubility of
the signal decreased gradually with reaction time. However, PBz (2). Therefore, the early precipitation lowers the purity of
residual phenol and triazine signals suggest the reaction was PBz (2), as shown by the residual phenolic OH in Fig. S6.† Table
not complete. Fig. S3† shows the NMR trace for PBz (1) S2† lists the solubility data of PBz (1) and PBz (2). PBz (1) was
synthesized Run 6. Unlike those shown in Fig. S2,† no triazine soluble in most organic solvents. In contrast, PBz (2) was
signal was observed. According to literature, preparation of insoluble in most organic solvents. It was only soluble in NMP
main-chain type polybenzoxazine is proceeded by two steps: at high temperature, but precipitation occurred aer cooling.
1
the formation of triazine network (R ) and the dissociation of PBz (2) is structurally similar to PBz (1) except for the pyridinyl
26
the resulting triazine network (R
and R plays a key role for the preparation. Gelation can group in solubility. The poor solubility of PBz (2) makes it
occur if reaction rate of R is much faster than that of R . In difficult to process for property comparison between the ther-
2
). The competition between group, so the result demonstrates the advantage of pyridinyl
R
1
2
1
2
preparing bisphenol/monoamine-based benzoxazine, the gel mosets of PBz (1) and PBz (2).
will react with bisphenol, giving the desired product. However,
in preparing diamine/monophenol-based benzoxazine, we
found that the decomposition of gel is not as easy as that in
Molecular weight of PBz (1)
preparing bisphenol/monoamine-based benzoxazine. There- Table 1 lists the GPC and viscosity data of the PBz (1). The
3
fore, reducing R
1
is a strategy to prepare diamine/monophenol- number-average molecular weight (M
n
) ranges from 2.4 ꢂ 10 to
3
ꢁ1
based benzoxazine with high yield and purity. The fact that no 3.5 ꢂ 10 g mol , and the weight-average molecular weight
3
4
ꢁ1
triazine signal was observed in Fig. S3† suggests that reaction (M
rate of R was reduced at a higher ethanol concentration. This prepared in toluene–ethanol. In contrast, M
result is consistent with our proposal in a previous work that 10 to 1.7 ꢂ 10 g mol , and M
w
) ranges from 9.2 ꢂ 10 to 1.4 ꢂ 10 g mol for PBz (1)
1
n
ranges from 1.2 ꢂ
3
3
ꢁ1
3
ranges from 3.5 ꢂ 10 to 4.3 ꢂ
ethanol, which is structurally similar to methylol (–NCH OH), 10 g mol for PBz (1) prepared in xylene and toluene. The
w
3
ꢁ1
2
24
solvates the methylol. The solvation hindered the condensa- inherent viscosities of PBz (1) prepared in toluene–ethanol were
tion of methylol and hydrogen, and thus reduced the reaction in the range of 0.14–0.17 dL g , which are higher than those
rate of R . The characteristic oxazine peaks at 5.3 ppm and (0.10–0.12 dL g ) prepared in xylene and toluene. The GPC and
1
ꢁ1
ꢁ
1
8694 | RSC Adv., 2014, 4, 8692–8698
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
Table 1 Molecular weight and viscosity of PBz (1)
a
ꢁ1
Sample ID
Preparation solvent
Yield (%)
Inherent viscosity (dL g
)
M
w
M
n
PDI
b
3
3
3
4
3
PBz (1)
PBz (1)
PBz (1)
PBz (1)
Xylene
30
35
70
80
0.10
0.12
0.14
0.17
3.5 ꢂ 10
4.3 ꢂ 10
9.2 ꢂ 10
1.4 ꢂ 10
1.2 ꢂ 10
1.7 ꢂ 10
2.4 ꢂ 10
3.5 ꢂ 10
3.01
2.51
3.75
3.93
c
3
3
3
Toluene
d
Toluene–ethanol (2/1)e
Toluene–ethanol (1/1)
a
ꢁ1
ꢀ
.5 g dL in NMP at 30 C. Prepared by Run 3, Table S1. Note that PBz (1) was obtained from the precipitation of its xylene solution into n-
c
b
0
hexane. The gelation part was not included in the precipitation. Prepared by Run 4, Table S1. Note that PBz (1) was obtained from the
d
precipitation of its toluene solution into n-hexane. The gelation part was not included in the precipitation. Prepared by Run 5, Table S1.
Prepared by Run 6, Table S1.
e
viscosity data demonstrate the advantage of toluene–ethanol in
preparing PBz (1) with higher molecular weight.
UV-vis characteristic of PBz (1)
Fig. 1 shows the UV-vis spectra of PBz (1) and protonated PBz (1)
ꢁ
2
in a THF dilute solution (10 M). PBz (1) displays an absorption
at 338 due to the p–p* transition of unpaired electrons of the
pyridinyl group. When the THF dilute solution of PBz (1) was
protonated with HCl, the intensity at 338 nm decreased due to
the lone pair of electrons of the pyridinyl group was quaternized
by HCl. A new absorption at 458 nm for a lower p–p* transition
of the quaternated PBz (1) appeared when the concentration of
HCl was higher than 0.1 M. Fig. 2 shows the photos of PBz (1)
and protonated PBz (1) in a THF dilute solution. The THF
solution of protonated PBz (1) shows yellow color, comple-
mentary colors of the absorption at 458 nm. The spectroscopic
data suggests PBz (1) can be used as a proton sensor via a
protonation process.
Fig. 2 Photos of THF dilute solution of (a) PBz (1), and protonated PBz
1) with HCl (b) 0.001 M (c) 0.01 M (d) 0.1 M (e) 0.5 M and (f) 1.0 M.
(
Emission characteristic of PBz (1)
Fig. 3 shows the emission spectra of PBz (1)and protonated
PBz (1) aer being excited at 458 nm. No obvious uorescence
at 500–700 nm was observed in the uorescence spectrum for
PBz (1). However, a new strong uorescence at 570 nm that
Fig.
3 Emission spectra of THF dilute solution of PBz (1) and
protonated PBz (1) after being excited at 458 nm.
increased with the concentration of HCl was observed. The
spectroscopic data suggest that the emission is a lower tran-
sition related to the protonated pyridinyl group. Interestingly,
the emission intensity increased with the concentration of HCl
when the concentration was below 0.5 M, and decreased when
the concentration reached 1.0 M. Before protonation, both
pyridinyl and benzoxazine groups were donors, so no emission
occurred aer excitation. The speculation for the phenom-
enon was proposed as following. On protonation at low HCl
concentration, the protonation occurred at the pyridinyl, as
shown by PBz (1)-H in Scheme 2, since the nitrogen of the
Fig. 1 UV-vis Absorption spectra of PBz (1) and protonated PBz (1) in a
THF dilute solution (10 M).
ꢁ2
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 8692–8698 | 8695

View Article Online
RSC Advances
Paper
as small as 60 mm ꢂ 2.5 cm ꢂ 0.8 cm. The thermal properties of
PBz (1)-T were evaluated by DMA and TMA. Fig. S9† shows the
DMA thermograms of the PBz (1)-T. The T obtained from the
g
ꢀ
peak temperature of tan d was 261 C. Takeichi et al. reported
that the T
g
of pyridinyl-containing benzoxaine thermoset is
ꢀ
40
C higher than bisphenol A/formaldehyde/aniline-based
benzoxazine thermoset due to the strong hydrogen bonding
27
between the pyridinyl moiety and phenolic group. However,
no PBz (2)-T data are available for property comparison because
of the poor solubility of PBz (2) in organic solvents. The TMA
measurement (Fig. S10†) shows that the T value of PBz (1)-T is
g
ꢀ
2
3
46 C, and the coefficient of thermal expansions (CTE) is
ꢀ
ꢁ1
8 ppm C , which is a relative lower one compared with other
benzoxazine thermosets. Fig. S11† shows the TGA thermograms
ꢀ
of the PBz (1)-T. The 5% degradation temperature is 414 C in
ꢀ
ꢀ
nitrogen, and 418 C in air. The char yield at 800 C is 56 in
nitrogen, and 47 in air. These values are relatively high when
compared with other benzoxazine thermosets. Unfortunately,
PBz (2) which is structurally similar to PBz (1) except for the
pyridinyl, was insoluble in solvent for lm formation. There-
fore, the effect of pyridinyl on thermal properties was not
available.
Dielectric properties of PBz (1)-T
The dielectric constant of the PBz (1)-T was measured by an
Agilent E4991A RF Impedance Material Analyzer. The dielectric
constant at 1 GHz and 100 MHz was 3.77 and 3.91, respectively.
The value is much higher than uorinated polybenzoxazines,
Scheme 2 The proposed structure for the protonated PBz (1).
pyridinyl group is more basic than that of benzoxazine. The
protonated pyridinyl became a stronger acceptor, and then an
emission at 570 nm occurred between the nitrogen of ben-
zoxazine (donor) and the protonated pyridinyl groups
28
and is relatively higher than that of P-oda, and F-a thermosets.
0
Note that P-oda is a benzoxazine based on 4,4 -diamino
diphenyl methane/phenol/paraformaldehyde, and F-a is a ben-
zoxazine based on bisphenol F/aniline/paraformaldehyde. Liaw
et al. reported the dielectric constant of poly(pyridine imide) at
(
acceptor). On protonation at higher HCl concentration, the
nitrogen of benzoxazine was protonated, as shown by PBz (1)-
H in Scheme 2, changing the benzoxazine from a donor to a
1
kHz (4.20) was higher than that of the commercially available
2
4,6
polyimide lm Kapton HN (3.50 at 1 kHz). Hougham et al.
reported that the dielectric constant is affected by a molecule's
hydrophobicity and free volume, and polarizability. The pyr-
weak acceptor. In this case, both the protonated pyridinyl and
protonated benzoxazine groups were donors, so no emission
occurred aer excitation. This explains the decreased emis-
sion of protonated PBZ (1) at HCl concentration higher than
29
idinyl group that increases the polarizability of a polymer under
an electrical eld is thought to be responsible for the increased
dielectric constant.
0
.5 M.
Fig. S7† shows the emission spectra of P(B-oda) and
protonated P(B-oda) in THF dilute solution. Note that P(B-oda)
is a main-chain type polybenzoxazine prepared from 4,4 -dia-
Contact angle and surface free energy of PBZ (1)-T
0
Fig. 4 shows the contact angle of water for PBz (1) aer curing
mino diphenyl methane/phenol/paraformaldehyde. No obvious
emission was observed (the longitudinal scale is the same as
that in Fig. 3) for either P(B-oda) or protonated P(B-oda), in
which no pyridinyl group was present. The spectroscopic data
further supports that the emission in Fig. 3 is relative to the
protonated pyridinyl group. The result further suggests that the
pyridinyl-containing PBz (1) can be used as a proton sensor via a
protonation process.
ꢀ
at 240 C for (a) 0, (b) 1, (c) 4, and (d) 8 h. Table 2 lists the data
of contact angles and the surface free energies of all tested
ꢀ
specimens aer various curing times at 240 C. Although the
surface free energy of a solid can be used as a guide or indi-
cator of its relative adhesive properties, it is difficult to
measure the surface free energy of a solid directly. For prac-
tical reasons, procedures based on contact angle measure-
3
0,31
ments are commonly employed.
free energies using Van Oss and Good's three-liquid method.
Table 2 lists the relationship between the surface free energy
We evaluated the surface
3
2
Thermal properties of PBz (1) thermoset
PBz (1) was thermally cured to PBz (1) thermoset, the PBz (1)-T. and the curing time. The surface free energy initially
Fig. S8† shows a photo of PBz (1)-T at a thickness of 60 mm and decreased and then increased steadily with the increase in
550 mm. The lm of PBz (1)-T can be bent even with dimensions curing time. The lowest value of surface free energies of PBz
8696 | RSC Adv., 2014, 4, 8692–8698
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
ꢀ
Fig. 4 The contact angle of water for PBz (1) after curing at 240 C for
(
a) 0, (b) 1, (c) 4, and (d) 8 h.
ꢁ
2
(
(
1)-T was 19.6 mJ m , which is less than that of pure Teon
ꢁ
2
21.0 mJ m ) when using the same testing liquids and
3
3
calculation method.
1
8,34–36
Previous studies
have suggested that an increase in
ꢀ
the degree of intramolecular hydrogen bonding will decrease Fig. 5 Curve fitting of the FTIR spectra of PBz (1) curing at 240 C for
ꢁ
+
(
a) 1 h, (b) 4 h and (c) 8 h. 1: –O /H N intramolecular hydrogen
the surface energy of a polymeric material. To determine the
degrees of hydrogen bonding with these polybenzoxazines, we
performed curve-resolving of their FTIR spectra with respect
bonding. 2: –OH/N (from Mannich bridge) intramolecular
hydrogen bonding. 3: –OH/O and –OH/N (from pyridinyl) inter-
molecular hydrogen bonding. 4: –OH/p intramolecular hydrogen
to the corresponding frequencies of each hydrogen bonding bonding.
ꢁ
1
species. Fig. 5 displays expand FTIR spectra (4000–2000 cm
)
ꢀ
for the PBz (1) curing at 240 C for (a) 1 h, (b) 4 h, and (c) 8 h.
ꢁ
Four different kinds of hydrogen bonds were evident: –O / fraction of these hydrogen bonds increased upon increasing
+
ꢁ1
H N intramolecular hydrogen bonds (2830 cm ), –OH/N the curing time. It is clear that increasing the degree of
(
(
from Mannich bridge) intramolecular hydrogen bonds intramolecular hydrogen bonding will lead to a decrease in
ꢁ
1
3171 cm ), –OH/O and –OH/N (from pyridinyl) intermo- the surface free energy, whereas increasing the fraction of
ꢁ1
lecular hydrogen bonds (3400–3370 cm ), and –OH/p intermolecular hydrogen bonding has the opposite effect. Our
ꢁ1
intramolecular hydrogen bonds (3550 cm ), which have been observations are in a good agreement with the results of
2
7,37
ꢁ1
18,24,34–36
discussed in previous reports.
The peak at 3380 cm
previous studies on surface free energy effects.
As
corresponds to the intermolecular hydrogen bonding of the shown in Table 2, the fraction of intermolecular hydrogen
polybenzoxazine, and it is clear that the fraction of these bonding is 34–48%, which is much higher than that in our
1
6,19,27–29
intermolecular hydrogen bonds increased upon increasing previous work.
The hydrogen bonding between the
the curing time (Table 2). Wang and Chang et al. reported that phenolic OH and pyridinyl moiety is responsible for the
ꢀ
the curing time inuences the surface free energy of poly- increased fraction. However, high contact angle (102 ) and low
ꢁ
2
benzoxazines; they found that the lowest surface free energy surface energy (19.6 mJ m ) can still be obtained even with
occurred when the system possesses the minimal extent of the high Fraction of intermolecular hydrogen bonding. This
1
8
–
OH/O intermolecular hydrogen bonding.
The area phenomenon has never been reported.
Table 2 The contact angles for water, ethylene glycol (EG) and diiodomethane (DIM) on PBz (1) film (standard deviations in the range 0.3–2.0).
The corresponding surface free energies and fraction of intermolecular hydrogen bonding are also given
ꢀ
Contact angle ( )
Surface free
energy (mJ m )
Fraction of intermolecular
hydrogen bonding (%)
ꢀ
ꢁ2
Temp. ( C)
Time (h)
H
2
O
EG
DIM
6
0
1
1
4
8
80.1
102.2
91.5
60.0
83.4
76.2
65.2
30.3
77.0
75.6
72.5
44.2
19.6
21.7
26.0
—
34.7
41.3
47.9
2
40
80.3
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 8692–8698 | 8697

View Article Online
RSC Advances
Paper
1
0 A. Katritzky and C. Rees, Comprehensive Heterocyclic
Chemistry, Pergamon, Oxford, 1984, vol. 2, p. 395.
Conclusions
We have successfully prepared a pyridinyl-containing poly- 11 M. Weiss, J. Am. Chem. Soc., 1952, 74, 200–202.
benzoxazines, PBz (1), from the Mannich condensation of (2), 12 Y.-C. Huang, K.-L. Wang, C.-H. Chang, Y.-A. Liao, D.-J. Liaw,
phenol, and paraformaldehyde using toluene–ethanol (1/1) as
the reaction solvent. When the THF dilute solution of PBz (1)
K.-R. Lee and J.-Y. Lai, Macromolecules, 2013, 46, 7443–
7450.
was protonated with HCl, the intensity at 338 nm decreased, 13 D.-J. Liaw, K.-L. Wang, E.-T. Kang, S. P. Pujari, M.-H. Chen,
and a new absorption at 458 nm appeared in the UV-vis
spectra. No obvious uorescence at 500–700 nm was observed
Y.-C. Huang, B.-C. Tao, K.-R. Lee and J.-Y. Lai, J. Polym.
Sci., Part A: Polym. Chem., 2009, 47, 991–1002.
in the emission spectrum for PBz (1). However, a new strong 14 N. N. Ghosh, B. Kiskan and Y. Yagci, Prog. Polym. Sci., 2007,
uorescence at 570 nm was observed in the emission spec-
32, 1344–1391.
trum for protonated PBz (1). In contrast, no obvious emission 15 K. Zhang, Q. Zhuang, X. Liu, R. Cai, G. Yang and Z. Han, RSC
was observed for either P(B-oda) or protonated P(B-oda), in
Adv., 2013, 3, 5261–5270.
which no pyridinyl group was present. These results suggest 16 P. Zhao, Q. Zhou, Y. Deng, R. Zhu and Y. Gu, RSC Adv, 2013,
that the pyridinyl-containing PBz (1) can be used as a proton
4, 238–242.
sensor via a protonation process. To the best of our knowl- 17 X. Ning and H. Ishida, J. Polym. Sci., Part A: Polym. Chem.,
edge, the absorption and emission of protonated poly-
1994, 32, 1121–1129.

benzoxazines has not been reported. Aer curing, the 18 C.-F. Wang, Y.-C. Su, S.-W. Kuo, C.-F. Huang, Y.-C. Sheen and
ꢀ
thermoset of PBz (1), PBz (1)-T, shows a T
g
value at 261 C, and
F.-C. Chang, Angew. Chem., Int. Ed., 2006, 45, 2248–2251.
ꢀ
ꢁ1
a CTE at 38 ppm C , and 5% decomposition temperature at 19 T. Takeichi, T. Kano and T. Agag, Polymer, 2005, 46, 12172–
ꢀ
4
14 (N
2
) and 419 C (air), demonstrating moderate-to-high
12180.
thermal stability. FTIR spectra show that the surface free 20 T. Agag and T. Takeichi, J. Polym. Sci., Part A: Polym. Chem.,
energy is strongly related with the fraction of intermolecular
2007, 45, 1878–1888.
hydrogen bonding. In addition, the fraction of intermolecular 21 J. Liu, T. Agag and H. Ishida, Polymer, 2010, 51, 5688–5694.
hydrogen bonding in this work is much higher than that in 22 T. Agag, S. Geiger, S. M. Alhassan, S. Qutubuddin and
our previous work due to the hydrogen bonding between the
phenolic OH and pyridinyl moiety. However, high contact 23 C. Altinkok, B. Kiskan and Y. Yagci, J. Polym. Sci., Part A:
H. Ishida, Macromolecules, 2010, 43, 7122–7127.
ꢀ
ꢁ2
angle (102 ) and low surface energy (19.6 mJ m ) can still be
obtained even with the high Fraction of intermolecular 24 C. H. Lin, S. L. Chang, T. Y. Shen, Y. S. Shih, H. T. Lin and
Polym. Chem., 2011, 49, 2445–2450.
hydrogen bonding.
C. F. Wang, Polym. Chem., 2012, 3, 935–945.
25 I. K. Spiliopoulos and J. A. Mikroyannidis, Macromolecules,
1
998, 31, 1236–1245.
Acknowledgements
2
6 Z. Brunovska, J. P. Liu and H. Ishida, Macromol. Chem. Phys.,
1999, 200, 1745–1752.
The authors thank the National Science Council of the Republic
of China for nancial support.
27 Y. Shibayama, T. Kawauchi and T. Takeichi, High Perform.
Polym., 2013, DOI: 10.1177/0954008313495245.
2
8 C. H. Lin, S. L. Chang, H. H. Lee, H. C. Chang, K. Y. Hwang,
A. P. Tu and W. C. Su, J. Polym. Sci., Part A: Polym. Chem.,
2008, 46, 4970–4983.
Notes and references
1
2
B. Tamami and H. Yeganeh, Polymer, 2001, 42, 415–420.
K.-L. Wang, W.-T. Liou, D.-J. Liaw and S.-T. Huang, Polymer, 29 G. Hougham, G. Tesoro and J. Shaw, Macromolecules, 1994,
008, 49, 1538–1546.
27, 3642–3649.
B. Tamami and H. Yeganeh, J. Polym. Sci., Part A: Polym. 30 F. M. Fowkes, J. Phys. Chem., 1962, 66, 382–382.
Chem., 2001, 39, 3826–3831.
31 W. A. Zisman, Ind. Eng. Chem., 1963, 55, 18–38.
D.-J. Liaw, K.-L. Wang, F.-C. Chang, K.-R. Lee and J.-Y. Lai, 32 C. J. Van Oss, M. K. Chaudhury and R. J. Good, Chem. Rev.,
2
3
4
J. Polym. Sci., Part A: Polym. Chem., 2007, 45, 2367–
374.
X. Wang, Y. Li, C. Gong, S. Zhang and T. Ma, J. Appl. Polym.
Sci., 2007, 104, 212–219.
1988, 88, 927–941.
2
33 J. Tsibouklis, M. Stone, A. A. Thorpe, P. Graham, T. G. Nevell
and R. J. Ewen, Langmuir, 1999, 15, 7076–7079.
34 S.-W. Kuo, Y.-C. Wu, C.-F. Wang and K.-U. Jeong, J. Phys.
Chem. C, 2009, 113, 20666–20673.
5
6
7
8
9
D.-J. Liaw, K.-L. Wang and F.-C. Chang, Macromolecules,
2
007, 40, 3568–3574.
A. Chichibabin and O. Zeide, Zh. Russ. Fiz.-Khim. O-va., 1914,
6, 1212.
35 H.-C. Lin, C.-F. Wang, S.-W. Kuo, P.-H. Tung, C.-F. Huang,
C.-H. Lin and F.-C. Chang, J. Phys. Chem. B, 2007, 111,
3404–3410.
4
R. L. Frank and R. P. Seven, J. Am. Chem. Soc., 1949, 71, 2629– 36 L. Qu and Z. Xin, Langmuir, 2011, 27, 8365–8370.
2635.
37 H.-D. Kim and H. Ishida, J. Phys. Chem. A, 2001, 106, 3271–
J. J. Li, in Name Reactions, Springer, 2009, pp. 107–109.
3280.
8698 | RSC Adv., 2014, 4, 8692–8698
This journal is © The Royal Society of Chemistry 2014