Dendritic organic-inorganic hybrid polyphenol and branched benzoxazine monomers with low curing temperature

Article abstract of DOI:10.1039/c4ra07035f

A novel dendritic organic-inorganic hybrid polyphenol (T2) based on cyclotriphosphazene was synthesized by the condensation reaction of T1 and phenol catalysed by phosphotungstic acid. The dendritic polyphenol T2 comprising twelve phenolic hydroxyl groups

Full text of DOI:10.1039/c4ra07035f

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PAPER
Dendritic organic–inorganic hybrid polyphenol and
branched benzoxazine monomers with low curing
temperature†
Cite this: RSC Adv., 2014, 4, 53505
*
Min Zhang, Zhiwei Tan, Hongxia Ma, Jinjun Qiu and Chengmei Liu
A novel dendritic organic–inorganic hybrid polyphenol (T2) based on cyclotriphosphazene was synthesized
by the condensation reaction of T1 and phenol catalysed by phosphotungstic acid. The dendritic
polyphenol T2 comprising twelve phenolic hydroxyl groups exhibited excellent solubility in common
organic solvents. Three branched benzoxazine monomers with different oxazine ring content were
synthesized via Mannich condensation reaction. These branched benzoxazine monomers showed low
initial curing temperature. The properties of corresponding polybenzoxazines, such as thermal stabilities,
mechanical properties, dielectric properties and gel contents significantly depended on the oxazine ring
content of T3. With the increase of the oxazine ring content for T3, these properties could be greatly
improved due to their high crosslinking density.
Received 12th July 2014
Accepted 29th September 2014
DOI: 10.1039/c4ra07035f
www.rsc.org/advances
poly(aryl ether oxazole)s signicantly depended on the nature of
the chain ends, which increases with increasing chain-end
1. Introduction
polarities. Polyphenols are also the amorphous molecular
materials used in advanced photoresists because of their
excellent solubility in alkali.9–12 The branched polyphenol-based
molecular glass resists had high selectivity to radiation and
could form photoresist images as small as 60 nm.9 However, no
reports concerning dendritic polyphenols and their derivatives
was published, and only a few studies on branched polyphenols
have been reported due to the inherent difficulties in the
synthesis and purication.13,14
One of the most interesting applications of polyphenols is as
the key starting raw materials of polybenzoxazine, which is an
ideal alternative of traditional phenol–formaldehyde thermo-
sets.15,16 Polybenzoxazines are also proved to be an excellent
carbon precursor, low-surface-energy and high-thermal-stable
material, novel electrolyte material and microelectronic mate-
rial.17–19 However, unsatised processability and high polymer-
ization temperature limit their application in high-tech
elds.20–22 The feasible structural design of phenols or amines
can overcome these problems to certain extent.15,16 To improve
the curing process, active hydrogen containing groups such as
carboxylic23 and phenolic groups24,25 were incorporated into
benzoxazine monomers, which could effectively decrease curing
temperature due to their catalytic effect on ring-opening
polymerization.
Polyphenols are extensively distributed in renewable natural
resources, such as lignin, Chinese lacquer, and tannin, and
recently they have become rather unique and intriguing prod-
ucts because of their diverse range of physicochemical proper-
ties and unusual structures.1–4 Plant polyphenols display a rich
and complex spectrum of physical and chemical properties,5
giving rise to broad chemical versatility, including the absorp-
tion of UV radiation, radical scavenging, and metal ion
complexation. In addition, human health has been associated
with the consumption of foods and beverages rich in plant
polyphenols.6 Tea, chocolate, and wine-inspired polyphenols
can act to be colorless multifunctional coatings.7 In addition to
the naturally occurring polyphenols, man-made polyphenol
(phenol resins) were also developed 100 years ago and they have
been used to be one type of an engineering material aer
curing. Because of the high reactivity of phenolic hydroxyl
groups, polyphenols act to be reactive intermediates for many
functional polymeric materials. The modication of phenolic
hydroxyls can endow them with particular desirable abilities.
For example, Ching-Fong Shu and coworkers8 have reported
that the hyperbranched poly(aryl ether oxazole)s with terminal
phenolic groups were easily functionalized and the glass tran-
sition temperature and the solubility of the hyperbranched
Cyclotriphosphazene (CP) derivatives are a typical class of
organic–inorganic hybrid compounds,26–28 and the incorpora-
tion of the CP ring into organic compounds or polymers yields
fascinating polymer materials.29–33 We had a benecial attempt
in this area and preliminarily achieved meaningful results.34,35
In this article, a novel CP-based dendritic polyphenol T2 was
School of Chemistry and Chemical Engineering, Key Laboratory for Large-Format
Battery Materials and System, Ministry of Education, Huazhong University of
Science and Technology, Wuhan, 430074, P.R. China. E-mail: liukui@mail.hust.edu.
cn; Fax: +86-27-87559627; Tel: +86-27-87559627
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra07035f
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designed and synthesized through a relatively simple two-step
reaction. Then, highly branched benzoxazine monomer based
on cyclotriphosphazene, T3, was obtained by the Mannich
reaction of T2, aniline and paraformaldehyde. The representa-
tive monomers T3-23.6%, T3-46.6% and T3-75.2%, with oxazine
ring content of 23.6%, 46.6% and 75.2%, respectively, were
chosen to investigate their polymerization behaviors and the
corresponding properties of polybenzoxazine.
2. Experimental
2.1 Chemicals
Hexachlorocyclotriphosphazene [N3P3Cl6] was recrystallized
ꢀ
from dry hexane followed by sublimation (60 C, 0.05 mm Hg)
twice before use. 4-Hydroxybenzaldehyde (98%), phenol (99%),
phosphotungstic acid (99%), thioglycolic acid (99%), para-
formaldehyde (95%) and aniline (99%) were purchased from
Sinopharm Chemical Reagent Co., Ltd., China. Anhydrous
potassium carbonate was dried at 140 ꢀC prior to use. Tetra-
hydrofuran was distilled from sodium under a dry argon
atmosphere.
2.2 Synthetic section
Synthesis of T1. T1 was prepared according to the reported
1
method.36 Yield: 82%. MP ¼ 160–162 ꢀC. H NMR (DMSO-d6,
Scheme 1 Synthesis of dendritic polyphenol T2 and branched ben-
zoxazine monomer T3 (R1 and R2 were a random distribution and the
TMS, ppm): 6.74–7.78 (Ar-H, 8H, m), 9.48 (CH]N, 1H, s), 8.52
(CH]N, 1H, s). 13C NMR (DMSO-d6, TMS, ppm): 156.8 (C–OH), sum was 12).
156.0 (C]N), 151.8 (C–O), 142.7 (C–N), 134.4 (CH), 131.9 (CH),
129.9 (CH), 123.1 (CH), 121.0 (C), 116.1 (CH), 115.5 (CH). 31P
NMR (DMSO-d6, ppm): 8.75.
and a Dean–Stark trap. Aniline (6.69 g, 71.93 mmol) and para-
formaldehyde (4.32 g, 143.64 mmol) were mixed and stirred in
ice bath for 1 h. Then, 150 mL toluene, 50 mL DMAc, T2 (15 g,
7.98 mmol) were added into the mixture with stirring. The
mixture was gently heated to 110 ꢀC and reuxed for 72 h. Water
produced during the reaction was separated out by azeotropic
distillation with toluene. The samples for different reaction
time (3 h, 24 h and 72 h) were removed with a syringe. Aer the
solvents were removed, the residue was collected and dried in
Synthesis of dendritic polyphenol T2. T2 was prepared
according to the reported method37 with a few modications. A
250 mL three-necked ask was tted on a magnetic stirrer with
a thermometer and a Dean–Stark trap. In the ask were placed
T1 (5.0 g, 5.8 mmol), phenol (19.6 g, 208.8 mmol), phospho-
tungstic acid (1.0 g, 0.35 mmol) and 0.1 mL of thioglycolic acid
ꢀ
and 80 mL of cyclohexane. The mixture was heated to 100 C
with the azeotropic removal of the water formed. Water
produced during the reaction was separated out by azeotropic
distillation with cyclohexane. The azeotropic distillation was
continued until no more water came out. The volatile materials
were removed by rotary evaporation and the residue was
ꢀ
vacuum at 50 C for 48 h (Scheme 1).
2.3 Measurements
precipitated in a large amount of water, the resulting red solid The structure of the compounds were veried by solution-state
was ltered off and washed with chloroform and dried in proton (1H), carbon (13C) and phosphorus (31P) nuclear
vacuum at 40 ꢀC for 48 h. The crude product was recrystallized magnetic resonance spectroscopy (NMR) using Bruker
from ethyl alcohol/water (1/2). Light yellow powder was AV400NMR spectrometer at a proton frequency of 400 MHz, as
ꢀ
collected and dried in vacuum at 50 C for 481 h (Scheme 1).
well as the corresponding phosphorus frequency at room
Yield: 78%. 1H NMR (DMSO-d6, TMS, ppm): 6.67–6.84 (Ar-H, temperature. Chemical shis were reported in ppm downeld
12H, m), 5.23 (Ar-CH, 1H, s), 9.29 (Ar-OH, 2H, s). 13C-NMR from SiMe4. Thermal transitions were monitored with a differ-
(DMSO-d6, TMS, ppm): 56.0 (C–OH), 155.06 (C–O), 148.42 (C), ential scanning calorimeter (DSC), Model 204F1 from NETZSCH
134.85 (CH), 130.26 (CH), 120.72 (CH), 15.52 (CH), 54.21 (C– Instruments, and a scan rate of 10 ꢀC minꢁ1 over a temperature
CH). 31P NMR (DMSO-d6, ppm): 8.99.
range of 30–300 ꢀC and nitrogen ow rate of 20 mL minꢁ1 were
Synthesis of branched benzoxazine monomers. T3 was used in DSC experiments. Thermogravimetric analysis (TGA)
prepared according to the reported method38 with a few modi- was performed with a NETZSCH Instruments' High Resolution
cations. The typical procedure was as follows: A 250 mL three- STA 409 PC thermogravimetric analyzer that was purged with
necked ask was tted with a magnetic stirring, a thermometer nitrogen at a ow rate of 70 mL minꢁ1. A heating rate of 20 C
ꢀ
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minꢁ1 was used and scanning range was from RT to 1000 C. where the Wt and W0 represent the sample weights aer and
Infrared spectra were recorded using a Bruker VERTEX 70 before (dry sample) humidity absorption, respectively.
Fourier transform infrared spectrometer (FT-IR) under ambient Humidity absorption content was measured for 5 samples of
condition. The potassium bromide disks were prepared by each material group and the average values were recorded.
compressing the powder. Mechanical properties were measured
ꢀ
using a dynamic mechanical thermal analysis (DMA) apparatus
3. Results and discussion
3.1 Synthesis and characterization of polyphenol T2
(PerkinElmer, Diamond DMA). Specimens (50 ꢂ 10 ꢂ 1.0 mm)
were tested in 3 point bending mode. The thermal transitions
were studied in the scope of 20–300 ꢀC at a heating rate of 4 ꢀC
minꢁ1 and at a xed frequency of 1 Hz.
Branched polyphenol T2 was facilely prepared via the conden-
sation of T1 with phenol catalysed by phosphotungstic acid and
thioglycolic acid (Scheme 1). Initially, we performed the reac-
tion in the solvent of toluene, but 1H NMR analysis showed that
the purity of the product was poor due to high azeotropic
temperature in toluene–water system. To reduce side reactions,
which might result from the oxidation of phenolic hydroxyls
and the deactivation of catalysts, cyclohexane was used in the
reaction. Main impurities (phenol and catalysts) in the crude
product were easy to remove through repeated dissolution in
ethanol and precipitation in water. T2 with high purity were
obtained by recrystallization from ethyl alcohol/water. The
dendritic polyphenol exhibited excellent solubility in common
organic solvents such as methanol, ethanol, acetone, 1,4-
dioxane, THF, ethyl acetate, DMSO and DMF.
2.4 Preparation of polybenzoxazines
Samples for dynamic mechanical analysis (50 ꢂ 10 ꢂ 1.0 mm),
dielectric measurement (20 ꢂ 10 ꢂ 1.0 mm) and humidity
absorption test (50 ꢂ 10 ꢂ 1.0 mm) were prepared as follows:
three benzoxazine monomers were added to the tinfoil mold
and gently melted at 150 ꢀC under vacuum for 1 h. Subse-
quently, the samples were heated stepwise and cured in a
temperature-controlled oven at 160 ꢀC and 180 ꢀC each for 8 h,
ꢀ
ꢀ
then post-cured at 200 C and 220 C each for 2 h. Thereaer,
the samples were allowed to slowly cool to room temperature to
prevent cracking. Finally, the samples were polished to be
regularly shaped.
1
Fig. 1 shows the H NMR spectra of T2. Peaks at 9.29 ppm
(Ar-OH) and 5.23 ppm (Ar-CH) veried the successful intro-
duction of phenol to the T1 structure. Signals at 6.67–6.84 ppm
for Ar-H were also observed. In the corresponding 13C NMR
spectrum in Fig. 2, resonance appearing at 54.21 ppm was
assigned to the methyne carbon of Ar-CH. Other chemical shis
(ppm) were assigned to the aromatic carbon resonances: 156.0
(C–OH), 155.06 (C–O), 148.42 (C), 134.85 (CH), 130.26 (CH),
120.72 (CH), 115.52 (CH). The 31P NMR is shown in Fig. 3, and a
single peak appeared at 8.98 ppm, which indicated that the
substituted reaction of phenol onto T1 was complete.
2.5 Gel content of polybenzoxazines
The gel fraction of the samples (50 ꢂ 10 ꢂ 1.0 mm) was deter-
mined by standard extraction method39 using chloroform. The
process involved continuous extraction with chloroform in a 500
mL round bottom ask for 72 h until constant weight was
obtained. Aer the extraction, the samples were dried and the gel
content was calculated according to the formula (1) as follows:
À
Á
mt
Gel content %
¼
ꢂ 100%
(1)
m0
where mt and m0 represent sample weights aer and before
extraction, respectively.
3.2 Synthesis of branched benzoxazine monomers
T3 was synthesized using phenol (T2), aniline, and formalde-
hyde according to Scheme 1. As shown in Scheme 1, the ben-
zoxazine and phenol groups were randomly distributed as
2.6 Dielectric measurements of polybenzoxazines
Dielectric constant and dielectric loss were measured at room
temperature under atmospheric air by the two parallel plate
modes at 125 Hz–18 MHz using Agilent 4294A Precision
Impedance Analyzer.40,41 A sample (about 20 mm ꢂ 10 mm ꢂ 2
mm) was placed between the two copper electrodes to form a
parallel plate capacitor. Prior to each measurement, the sample
ꢀ
was dried under vacuum at 100 C for 3 h.
2.7 Humidity absorption
In the humidity absorption measurements, the cured samples
ꢀ
were conditioned under vacuum at 90 C for 20 h before they
were placed in air (75% and 33% RH).42 All these experiments
were conducted at room temperature. Then, the weight
percentages of humidity absorption of the cured samples were
calculated according to the formula (2) as follows:
À
Á
Wt ꢁ W0
Humidity absorption content %
¼
ꢂ 100% (2)
W0
Fig. 1 1H NMR spectrum of T2 (solvent: DMSO-d6).
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Fig. 2 13C NMR spectrum of T2 (solvent: DMSO-d6).
Fig. 4 1H NMR spectra of branched benzoxazine monomers (solvent:
DMSO-d6). The different oxazine ring contents at given time interval
are marked in the spectra.
À
Á
3R
C % ¼
ꢂ 100%
(3)
1 ꢁ 2R
where C is the oxazine ring content at given time interval, R the
ratio of the integral area of methylene protons for O–CH2–N and
the integral area of all aromatic protons. The equation was
established in the condition where the opening of oxazine ring
was not considered. Different oxazine ring contents for all
monomers at a given time interval is listed in Table S1.† The
estimated oxazine ring content aer 3 h, 24 h and 72 h were
23.6%, 46.6% and 75.2%, respectively. The peak assigned to the
phenolic hydroxyl still exists aer 72 h; thus, it is not necessary
to prolong the reaction time.
3.3 DSC studying for the curing behavior of branched
benzoxazine monomers
It is known that the high polymerization temperature is a major
drawback that limits the application of polybenzoxazine. The
polymerization temperature of the three hydroxyl-containing
benzoxazine monomers was examined by DSC and the results
are shown in Fig. 5 and summarized in Table 1. A wide
exothermic peak corresponding to the ring-opening polymeri-
zation was observed for T3-23.6%, in which the onset and
maximum temperatures of the exotherm were 97.2 ꢀC and 201.5
ꢀC, respectively. With the increase of oxazine ring content, the
exothermic peaks obviously changed from the DSC results. For
T3-46.6%, the onset and maximum temperatures of the exo-
therm were increased to 169.7 ꢀC and 222.4 ꢀC, respectively; for
T3-75.2%, the onset and maximum temperatures of the exo-
therm were separately increased to 170.8 ꢀC and 206.4 ꢀC.
Comparing to T3-46.6% and T3-75.2%, the polymerization
temperature for T3-23.6% was obviously reduced. The results
indicated that the hydroxyl groups could reduce the onset
polymerization temperature of benzoxazine monomers, which
was in accordance with other report.24 Nevertheless, the
maximum temperature of the exotherm gave the ascending
order of T3-23.6%, T3-75.2% and T3-46.6%. The reason for the
Fig. 3 31P NMR spectrum of T2 (solvent: DMSO-d6).
branches of benzoxazine monomer T3. The exact position and
numbers of the benzoxazine and phenol groups in T3 molecule
were time-dependent and could not be determined. However,
the sum of the benzoxazine and phenol groups in T3 molecule
was 12. As the reaction proceeds, the phenol groups gradually
turned into benzoxazine groups through Mannich reaction. The
1
formation of oxazine rings was monitored by H NMR and the
results are presented in Fig. 4. Resonances appearing at 4.50
ppm and 5.27 ppm are assigned to the methylene protons of O–
CH2–N of the benzoxazine ring, respectively. The multiplets at
6.66–6.80 ppm are assigned to the aromatic protons. The singlet
peak at 9.28 ppm is assigned to the unreacted phenolic hydroxy
proton. The single peak at 5.73 ppm is assigned to the methine
proton connected with three phenyl rings.
The oxazine ring content for benzoxazine monomers at a
1
given time interval was determined from the H NMR spectra
using the eqn (3) as follows:
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Fig. 5 DSC curves of polybenzoxazines by curing monomers T3-
Fig. 6 Gel contents of the polybenzoxazines PT3-23.6%, PT3-46.6%
23.6%, T3-46.6% and T3-75.2%.
and PT3-75.2%.
higher maximum temperature of T3-46.6% than T3-75.2% may
be due to the different oxazine ring distribution in T3-46.6%
and T3-75.2%. It was reported that the side groups of the CP
derivatives were arranged approximately perpendicular to the
CP ring forming parallel triplets that were pointing upward and
downward to give a calamitic superstructure capable of being
organized in a nematic phase.43,44 This meant that the oxazine
rings and hydroxyl groups were separately distributed in the top
and bottom faces of the CP plane. In addition, the position of
hydroxyl group relative to benzoxazine structure played a
signicant role in accelerating the polymerization.45 For T3-
46.6%, the oxazine ring content was close to 50%, and the
oxazine rings and hydroxyl groups were almost distributed at far
intervals as the branches of T3-46.6%, while for T3-75.2% with
the monomers. The increase of the oxazine ring content in the
monomer resulted into high gel content and high crosslinking
density. For the polybenzoxazine PT3-23.6% cured by monomer
T3-23.6%, in which the oxazine ring content was 23.6%, the gel
content reached 90.8%, which was the lowest among the three
monomers. This meant that even small amount of benzoxazine
units attached on the middle molecular weight polyphenol T2
(mol. Wt ¼ 1882 g molꢁ1) could act to be effective crosslinkers to
form thermosets with high crosslinking density. When the
oxazine ring content exceeded 50%, the gel content of the cor-
responding polybenzoxazine was above 95%.
3.5 Thermal properties of cured polymers
far more oxazine ring content than that of T3-46.6%, the high Thermal stability of the three polybenzoxazines was investi-
concentration of oxazine rings were closer to the hydroxyl gated by TGA. The TGA proles are shown in Fig. 7, and the
groups, which beneted the catalytic efficiency of the hydroxyl details are shown in Table 1. The 5% and 10% weight loss
groups.
temperatures (T5% and T10%) for the polybenzoxazine PT3-
23.6% were 375 and 407 C, respectively, while for PT3-46.6%
ꢀ
and PT3-75.2%, the T5% and T10% were all higher than that of
23.6%: for PT3-46.6%, they were 408 and 437 C, respectively;
3.4 Gel contents of the polybenzoxazines
ꢀ
The gel contents of the polybenzoxazines cured by monomers for PT3-75.2%, they were 427 and 474 ꢀC, respectively. Similarly,
T3-23.6%, T3-46.6% and T3-75.2% were determined using the the maximum weight loss temperature (Tmax) of the poly-
methodology described in the experimental part, and the benzoxazine PT3-75.2% was also lower than those of PT3-23.6%
results are shown in Fig. 6. The gel contents of PT3-23.6%, PT3- and PT3-46.6%. The Yc of PT3-75.2% was as high as 56.2%, and
46.6% and PT3-75.2% were 90.8%, 95.5% and 99%, respectively. that of PT3-23.6% and PT3-46.6% were considerably lower than
The results indicated that the gel contents of all poly- that of PT3-75.2%; the PT3-23.6% sample held the lowest yield,
benzoxazines depended on the different oxazine ring content of only 46.8%.
Table 1 Thermal stability of polybenzoxazines by curing T3-23.6%, T3-46.6% and T3-75.2%
a
b
c
d
e
f
Samples
T0 (ꢀC)
Tmax (ꢀC)
T5% (ꢀC)
T10% (ꢀC)
Tmax (ꢀC)
Yc (%)
T3-23.6%
T3-46.6%
T3-75.2%
97.2
169.7
170.8
201.5
222.4
206.4
375
408
427
407
437
474
403
435
498
46.8
51.9
56.2
a
Onset temperature of exothermic peak. b Maximum of the polymerization exotherm. c The temperature for 5% weight loss. d The temperature for
10% weight loss. e Maximum weight loss temperature. f Char yields at 850 ꢀC.
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Fig. 7 TG curves of polybenzoxazines. (a) PT3-75.2%, (b) PT3-46.6%,
(c) PT3-23.6%.
From the TGA results, it can be deduced that the organic–
inorganic hybrid polybenzoxazines, by curing the above
monomers T3-23.6%, T3-46.6% and T3-75.2%, showed
outstanding thermal stability. It was considered that the rigid
inorganic CP ring enhanced the thermal stability of the poly-
benzoxazines, which provide thermally stable sites compared to
the traditional main-chain type polybenzoxazine such as
bisphenol-based polybenzoxazines.23 Furthermore, the number
of benzoxazine moieties on the CP ring also played an impor-
tant role in the thermal stability of polybenzoxazines by
improving cross-linked density and minimizing dangling side
groups. Therefore, the higher the oxazine ring content, the
better was the thermal stability of polybenzoxazine.
Fig. 8 Storage moduli (A) and tan d (B) of polybenzoxazines PT3-
23.6%, PT3-46.6% and PT3-75.2%.
3.6 Mechanical properties of polybenzoxazines
The dynamic mechanical properties of the CP-based poly-
benzoxazines PT3-23.6%, PT3-46.6% and PT3-75.2% were
measured from room temperature to the rubbery plateau of
each material. The storage moduli (E0) of these poly-
benzoxazines are presented in Fig. 8(A). The E0 value of PT3-
23.6% at room temperature was 0.319 GPa, while they were 1.03
PT3-23.6% and PT3-46.6%, which meant that the composition
distribution in PT3-75.2% was not considerably narrow as that
in PT3-23.6% and PT3-46.6%.
3.7 Dielectric analysis of polybenzoxazines
GPa and 2.16 GPa for PT3-46.6% and PT3-75.2%, respectively. Dielectric property of polymers is one of the key characters for its
The E0 value of the polybenzoxazine were inuenced by the application in microelectronic devices.40 The dependence of the
number of the benzoxazine moieties, which led to more spatial dielectric constant and dielectric loss of the three poly-
crosslinking sites. Therefore, the high oxazine ring content was benzoxazine samples PT3-23.6%, PT3-46.6% and PT3-75.2%
greatly in favor of the increase of the E0 value for the corre- were studied in the frequency range of 125 Hz to 106 Hz at room
sponding polybenzoxazine.
temperature. Fig. 9 shows that both the dielectric constant (3)
The glass transition temperature (Tg) is also an important and the dielectric loss (tan d) largely depended on the oxazine
property of the polybenzoxazines, which could be deduced from ring content. Because the dielectric constant and dielectric loss
the corresponding peak temperature of tan d value, as shown in were directly related to the polarizability of material, which
Fig. 8(B). The Tg of PT3-23.6% was 173.3 ꢀC, while they were could be increased by increasing the polarizability, the dielectric
175.1 ꢀC and 179.2 ꢀC for PT3-46.6% and PT3-75.2%, respec- constant and dielectric loss were strongly dependent on its
tively. The highest cross-linked degree for PT3-75.2% could chemical structure.41 The dielectric constants of the poly-
constrain the move of the cross-linked chains under heat and benzoxazine PT3-23.6% at 125 Hz was about 5.74, for PT3-46.6%
move them at higher temperature. Thus, the Tg of PT3-75.2% and PT3-75.2%, the value changed to 4.57 and 4.45, respectively.
was higher than that of PT3-23.6% and PT3-46.6%. In addition, In the frequency range of 125 Hz to 106 Hz, the dielectric
the PT3-75.2% sample produced a broader glass transition than constant for each sample showed a small decrease. When the
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Fig. 10 Humidity adsorption of the polybenzoxazines PT3-23.6%,
PT3-46.6% and PT3-75.2% at different relative humidity (RH) at room
temperature. A. RH ¼ 75%; B. RH ¼ 33%.
Fig. 9 Frequency dependence of the dielectric constant (A) and
dielectric loss (B) of the polybenzoxazines PT3-23.6%, PT3-46.6% and
PT3-75.2%.
temperature. As shown in Fig. 10, all the polybenzoxazine
samples exhibited a low water uptake value (<1.0 wt%) at room
temperature aer 10 days regardless of the RH value of the air
was high or low due to the complete hydrogen-bond network
formed between phenolic OH and nitrogen atoms, which
hinder the water absorption and transportation.42 Moreover, the
water uptake ability of the samples regularly changed with the
increase of the oxazine ring content. The nal water absorption
values of the three polybenzoxazines decreased in the order of
PT3-23.6%, PT3-46.6% and PT3-75.2%. The value of PT3-23.6%
was about 0.95 wt%, and the value decreased to about 0.79 wt%
and 0.66 wt% for PT3-46.6% and PT3-75.2%, respectively. The
lower water uptake ability of PT3-75.2% was attributed to the
higher crosslinking density in the network.
frequency of the applied eld increased to 106 Hz, the dielectric
constants of PT3-23.6%, PT3-46.6% and PT3-75.2% declined to
4.85, 4.09 and 3.75, respectively. This was because of the fact that
PT3-75.2% was a highly cross-linked polymer and most of its
components linked together by chemical bonds, thus it showed
lowest polarizability. However, for PT3-23.6% and PT3-46.6%,
which were cured by the monomers with the oxazine ring
content of 23.6% and 46.6%, respectively, the uncrosslinked
groups acted to be polar pendant groups linked to the network
by chemical bond and increased the polarizability. The dielectric
loss of PT3-23.6%, PT3-46.6% and PT3-75.2% at 125 Hz were
0.061, 0.042 and 0.040, respectively. These values showed small
changes in the frequency range of 125 Hz to 106 Hz. The reason
for these changes was the same as that of the dielectric constant.
4. Conclusions
We have synthesized a novel dendritic organic–inorganic hybrid
polyphenol (T2) based on cyclotriphosphazene. The dendritic
polyphenol possessed good solubility in common solvents.
3.8 Humidity absorption of the polybenzoxazines
The humidity absorption of the three polybenzoxazine samples Based on the dendritic phenol, three branched benzoxazine
PT3-23.6%, PT3-46.6% and PT3-75.2% were measured at monomers T3-23.6%, T3-46.6% and T3-75.2% with different
different relative humidity (RH ¼ 75% & 33%) at room content of phenolic hydroxyl groups were synthesized via
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