Phosphorus-containing polymers from tetrakis-(hydroxymethyl)phosphonium sulfate iii. A new hydrolysis-resistant tris(allyloxymethyl)phosphine oxide and its thiol-ene reaction under ultraviolet irradiation

Article abstract of DOI:10.1039/c4ra06080f

Two kinds of new phosphorus-containing crosslinked polymer materials were prepared via thiol-ene photopolymerization and their properties were studied. In order to prepare these crosslinked polymer materials, multifunctional monomer tris(allyloxymethyl)phosphine oxide (TAOPO) was synthesized from an eco-friendly raw material, tetrakis(hydroxymethyl) phosphonium sulfate (THPS). Crosslinked poly(phosphine oxide) networks were then produced by a thiol-ene reaction in which TAOPO reacts with the two kinds of polythiol under ultraviolet (UV) irradiation. The new crosslinked polymers possess long-term hydrolysis-resistance property because the monomer TAOPO contains a phosphorus-carbon bond. The crosslinked polymers have a high gel content, high dielectric constant, low dielectric loss, and excellent transparency with a high refractive index and high Abbe number. DMA and TGA results indicated that all cured poly(phosphine oxide) were uniform networks and exhibited a high thermal stability. This journal is

Full text of DOI:10.1039/c4ra06080f

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PAPER
Phosphorus-containing polymers from tetrakis-
(hydroxymethyl)phosphonium sulfate III. A new
Cite this: RSC Adv., 2014, 4, 41705
hydrolysis-resistant tris(allyloxymethyl)phosphine
oxide and its thiol-ene reaction under ultraviolet
irradiation†
ac
a
a
b
a
Zhiwei Tan, Chengyan Wu, Min Zhang, Wenzhong Lv, Jinjun Qiu
and Chengmei Liu*a
Two kinds of new phosphorus-containing crosslinked polymer materials were prepared via thiol-ene
photopolymerization and their properties were studied. In order to prepare these crosslinked polymer
materials, multifunctional monomer tris(allyloxymethyl)phosphine oxide (TAOPO) was synthesized from
an eco-friendly raw material, tetrakis(hydroxymethyl) phosphonium sulfate (THPS). Crosslinked
poly(phosphine oxide) networks were then produced by a thiol-ene reaction in which TAOPO reacts
with the two kinds of polythiol under ultraviolet (UV) irradiation. The new crosslinked polymers possess
long-term hydrolysis-resistance property because the monomer TAOPO contains a phosphorus–carbon
bond. The crosslinked polymers have a high gel content, high dielectric constant, low dielectric loss, and
excellent transparency with a high refractive index and high Abbe number. DMA and TGA results
indicated that all cured poly(phosphine oxide) were uniform networks and exhibited a high thermal stability.
Received 22nd June 2014
Accepted 13th August 2014
DOI: 10.1039/c4ra06080f
www.rsc.org/advances
carbon double bond, generating a carbon-centered radical that
abstracts a hydrogen atom from a thiol group to regenerate a
Introduction
As one of the most practical and convenient synthetic routes in thiyl radical. Except for the traditional thermally activated
polymer chemistry and material synthesis in recent years, the radical initiators or photoinitiators, transition-metal-based
1
,2
5
6
thiol-ene reaction has attracted signicant attention. The catalysts and ultra-sound have always been involved in the
3
7
reaction was rst reported in early 1900s, and because of its thiol-ene reaction. More recently, Bowman and co-workers
facile, efficient and robust characteristics, it has become a great reported that the thiol-ene click reactions were initiated by
success in the preparation of linear, networked polymers and redox initiation systems. In addition, the thiol-ene reaction
related materials. The process of thiol-ene reaction involves the possess the ability of delaying the gelation of multifunctional
addition of –SH functionality to the C]C bond (hydro- monomer systems, which reduces both the shrinkage and the
1a,8
thiolation), and it has numerous advantages, such as producing shrinkage stress induced by polymerization. Taking all these
high yields, excellent regiospecicity and stereospecicity, merits into account, it is understandable that the thiol-ene click
insensitivity to oxygen or water and reactivity to a wide variety of reaction has been widely utilized as a versatile tool in numerous
1
,2
readily available starting compounds. Therefore, this coupling macromolecular synthetic strategies.
reaction has been included into the category of “click” chem- known sulfur-containing polymer and other heteroatoms, such
Moreover, the well-
1,2,4
9–11
12
13
istry.
germanium and metal atoms have been
alternating combination of propagation and chain transfer successfully introduced into polymers by the thiol-ene reaction.
The thiol-ene polymerization proceeds through an as silicon,
reactions, in which a thiyl radical propagates through a carbon–
Surprisingly, up to now, there were only a few reports con-
14–16
cerning the phosphorus-involved thiol-ene reaction.
In
a
addition, phosphorus is one of the most abundant elements on
School of Chemistry and Chemical Engineering, Key Laboratory for Large-Format
Battery Materials and System, Ministry of Education, Huazhong University of the earth and small amounts of phosphorus in a polymer can be
1
7–22
Science and Technology, Wuhan, 430074, China. E-mail: liukui@mail.hust.edu.cn
greatly advantageous.
containing polymer have been widely studied, namely poly-
phosphoester) and poly(phosphine oxide). Polyphosphoesters
In general, two kinds of phosphorus-
b
Department of Electronic Science and Technology, Huazhong University of Science and
Technology, Wuhan, 430074, China
(
c
School of Chemistry and Environmental Engineering, Hubei University for
have been attracting signicant attentions in the eld of
phosphorus-containing polymers for its excellent properties,
such as biodegradability, biocompatibility and the ease of
Nationalities, Enshi, 445000, China
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c4ra06080f
1
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vigorous stirring, and the reaction system was maintained
ꢀ
below 5 C. When the pH value of the mixture reached 12, the
reaction was continued for another 2 h, and then successively at
room temperature for half an hour. The reaction temperature
ꢀ
was then raised to 50–60 C. The reaction process was moni-
tored by P NMR and was stopped when a single peak was
obtained at 48.19 ppm. The mixture was neutralized to pH 7 by
3
1
Scheme 1 Synthesis of tris(allyloxymethyl)phosphine oxide (TAOPO).
2 4
the addition of H SO solution (1 M), aer cooling it to room
temperature. Ethanol (1 L) was then added to the mixture and
the precipitated Na SO was removed by ltration. The proce-
2
4
2
ꢁ
dure was repeated for several times, until no SO
by a Ba test. All the volatiles were separated out by rotary
evaporation at 90 C under vacuum, producing 220 g (yield 82%)
was detected
4
2
+
ꢀ
Scheme 2 Thiol-ene reaction of TAOPO with polythiols under UV
irradiation.
1
of a colorless viscous liquid. H NMR (400 Hz, DMSO-d
6
, d): 3.83
, d): 48.19.
IR: n ¼ 3314.28(O–H), 1423.44 (O–H), 2897.86, 2825.13(CH ),
135.73 (P]O), 1042.76 (C–O–C).
3
1
(
2 6
6H, CH ), 5.45(3H, OH). P NMR (400 Hz, DMSO-d
2
1
preparation. However, the main disadvantage of poly-
phosphoester is the lack of long-term hydrolytic stability, which
17
limits their use in many practical applications. Whereas,
poly(phosphine oxide)s show excellent resistant to hydrolysis
because this kind of polymer is formed though P–C bond
Synthesis of tris(allyloxymethyl)phosphine oxide (TAOPO)
To mixture of THPO (7 g, 0.05 mol) and benzyl-
a
23–26
triethylammonium chloride (0.7 g, 10% weight percent of
THPO), an aqueous solution of NaOH (20 mL, 30%) was added
at such a pace that the temperature of the reaction mixture was
linkages.
In recent years, phosphorus–carbon bonds con-
taining monomers and polymers have been the subject of
27–31
extensive research.
ꢀ
maintained below 35 C with mechanical stirring. Aer the
In this article, a convenient method to prepare crosslinked
poly(phosphine oxide) by the aid of thiol-ene reaction is
proposed for the rst time. The corresponding phosphorus–
carbon bonds containing trifunctional monomer, tris(allyloxy-
methyl)phosphine oxide (TAOPO) was prepared from tetra-
kis(hydroxymethyl)phosphonium sulfate (THPS), which is an
environment-friendly chemical (Scheme 1). This monomer then
underwent a thiol-ene reaction with polythiols (Scheme 2)
under UV irradiation to form poly(phosphine oxide)s networks.
The crosslinked material showed excellent optical, thermal and
dielectric properties.
ꢀ
addition, the mixture was cooled to 0 C by immersing it in an
ice-water bath. Allyl chloride (15.23 g, 0.195 mol) was slowly
added dropwise to this solution with stirring for 2 h maintain-
ꢀ
ing the temperature below 5 C. The temperature was then
ꢀ
increased and maintained at 50 C for 10 h. Aer cooling it to
room temperature, required amount of water was added to
dissolve the byproduct, NaCl. The organic layer was separated
ꢁ
and washed several times with water, until no Cl ion was
2 4
detected, and then dried overnight with anhydrous Na SO .
Aer ltration, the remaining allyl chloride was distilled out at
ꢀ
4
0 C under normal pressure. Finally, the remaining product
was placed in high vacuum to remove all the volatiles. 10.4 g
1
Materials and methods
Materials
(
yield 80%) of colorless or light yellow liquid was obtained. H
NMR (400 Hz, DMSO-d , d): 4.07(6H, P–CH –O), 3.87(6H, O–
CH –C), 5.86 (3H, –CH]), 5.22–5.31 (6H, ]CH
Hz, DMSO-d , d): 63.64–64.42 (CH –P), 73.99 (CH
]CH ), 134.58 (]CH–). P NMR (400 Hz, DMSO-d
IR: n ¼ 3080.74 (]C–H), 2982.43, 2855.63(CH ), 1645.05(C]C),
185.22 (P]O), 1091.66 (C–O–C). HRMS(ESI): m/z: calcd for
6
2
1
3
2
2
). C NMR (400
–O), 117.63
, d): 37.43.
Tetrakis(hydroxymethyl)phosphonium sulfate (THPS, 75%) was
purchased from Hubei Xingfa Chemical Company, China; 2,3-
bis[(2-mercaptoethyl)thio]-1-propanethiol(tri-thiol, >98%) was
obtained as a gi from Zhenjiang Junshi Optics Co., Ltd, China;
pentaerythritol tetrakis(2-mercaptoacetate) (tetrathiol, $95%)
was purchased from Qingdao Jiahua Chemical Co., Ltd, China;
allyl chloride (>98%), 2,2-dimethoxy-2-phenylacetophenone
6
2
2
3
1
(
2
6
2
1
+
C H O P [M + Na] 283.2563; found 283.1067.
1
2
21 4
(
DMPA, >98%), benzyltriethylammonium chloride (>98%),
Thiol-ene reaction of TAOPO with polythiols under UV
irradiation
H O (30%), chloroform ($99.0%), Na SO4 ($99.0%), NaOH
2
2
2
(
$99.0%) and anhydrous ethanol ($99.7%) were purchased
Under an argon atmosphere, DMPA (1% weight of ene mono-
mer) was added as a photoinitiator to a certain amount of tri-
thiol(or tetra-thiol), and completely dissolved by stirring. A
certain amount of oxygen-free TAOPO was added to the mixture
and stirred under argon atmosphere until a homogeneous
from Sinopharm Group Co., Ltd and used directly without
further purication.
Synthesis of tris(hydroxymethyl)phosphine oxide (THPO)
THPO was prepared according to a previously reported proce- system was formed. The obtained mixture was transferred to a
32
dure with some modications. NaOH aqueous (30%) was quartz mold under argon and then exposed to UV light to
dropped into THPS solution (500 g, 75%, 0.92 mol) with accomplish the polymerization. In order to avoid excessive
41706 | RSC Adv., 2014, 4, 41705–41713
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31
1
13
Fig. 1 NMR spectrum of TAOPO, (a), P; (b), H; and (c), C.
heating, it was exposed to UV light in a discrete manner with 3 the inuence of the absorbed moisture on the dielectric
min intervals between two successive exposures.
constant. The gel content of photopolymerized polymer was
measured by standard solvent-extraction method, and chloro-
form was used as the extraction solvent.
Characterization
1
13
31
H, C and P NMR spectra of the monomers were recorded
using a 400 MHz Bruker NMR spectrometer in DMSO-d con-
taining small amounts of TMS as internal standard. Fourier
transform infrared (FTIR) spectra were recorded with a Bruker
Vertex 70 FTIR spectrometer. Liquid samples were prepared by
casting on KBr windows and recorded at room temperature in
6
Results and discussion
Synthesis of THPO and TAOPO
The new phosphorus-containing triallyl ether monomer,
TAOPO was synthesized as shown in Scheme 1. First, the THPS
underwent decomposition reaction under strong basic condi-
tions to form tris(hydroxymethyl)phosphine (THP). The THP
was very sensitive to oxygen, therefore it was immediately
ꢁ
1
ꢁ1
the region of 4000–400 cm with a resolution of 2 cm . Cured
polymer samples attached to an ATR plate with a horizontal ATR
accessory were used for the measurements. The spectra were
obtained by collecting 1024 scans with a spectral range of 4000–
33
oxidized by air to stable THPO, under the reaction conditions.
3
1
ꢁ
1
ꢁ1
The reaction was monitored with P NMR (Fig. S1 in ESI†).
650 cm
with a resolution of 2 cm . Thermogravimetric
According to the results of monitoring, the reaction for
preparing THPO was completed within 5 hours at 50–60 C.
TAOPO was prepared by the standard with the aid of phase
transfer catalyst (PTC). Williamson method involves
analysis (TGA) was performed with a Perkin Elmer Thermal
ꢁ1
ꢀ
ꢀ
Analyser under a N
2
atmosphere at a rate of 20 C min from 40
ꢀ
to 850 C. Mechanical properties were measured using a
dynamic mechanical thermal analysis (DMA) apparatus (Perkin
Elmer, Diamond DMA). Specimens (50 ꢂ 10 ꢂ 1.0 mm) were
tested in three-points bending mode. The thermal transitions
ꢀ
ꢀ
were studied in the scope of 20–200 C at a heating rate of 4 C
ꢁ
1
min and at a xed frequency of 1 Hz. Optical transmission
measurements were performed on polished polymers using an
UV-vis spectrophotometer (Shimadzu UV-2550). Samples were
ground and polished to a path length of 2.5 mm. The refractive
indices were obtained at 546 nm using an Abbe refractometer
ꢀ
(
NAR-4T) at 20 C. The bulk density of the polymers was
obtained by placing dry, preweighed samples in a glass
pycnometer, and lling the rest of the volume with distilled
ꢀ
water at 20 C. The densities were calculated from the weight
ꢀ
difference and the known density of water at 20 C. Five
observations were taken for each sample, and the average was
used for density calculation. Dielectric constant and dielectric
loss were measured at room temperature in an air atmosphere
by two parallel plate modes at 125–18 000 Hz using an Agilent
Fig. 2 FT-IR spectrum of reaction mixture for different UV (0.61 mW
cm ) exposure time. (a): 0 min; (b): 1 min; (c): 2 min; (d): 3 min; (e): 4
ꢁ2
4294A Precision Impedance Analyzer. Prior to each measure-
ment, the sample was thoroughly dried under vacuum to reduce min; and (f): 5 min.
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Table
formulations
1
The gel content of the cured products with different these signals agreed with the monomer structure bearing three
13
allyl moieties. The C NMR (Fig. 1c) spectrum indicated that a
signal at 63.64–64.42 ppm attributed to the carbon connecting
to phosphorus atom, the signal at 73.99 ppm attributed to the
carbon between oxygen and vinyl group, and the signal at 117.63
ppm and 134.58 ppm attributed to the two vinyl carbons,
respectively. ESI-MS results and FTIR spectrum also gave posi-
tive proof for the chemical structure of TAOPO (Please see
n
C]C : n–SH
1 : 0.5
1 : 0.7
1 : 0.9
1 : 1
1 : 1.1
Gel/% (tri-thiol)
Gel/% (tetra-thiol)
79.6
85.6
93.6
95.7
96.1
96.5
96.1
98.0
98.9
98.2
transformation of hydroxyl group into sodium alkoxide with Experimental section).
sodium hydroxide, and successive treatment with allyl chlo-
34
ride. The optimal concentration of NaOH was 30% (w/w) in
this preparation. For the details of the optimal concentration of
NaOH please refer to the literature.
Preparation of crosslinked polymer network
The thiol-ene coupling reaction between TAOPO and trithiol
35
(
vinyl–SH ¼ 1 : 1) was monitored by FTIR spectrum. As shown in
The chemical structure of TAOPO was conrmed by NMR
spectroscopy. Fig. 1a indicates that only one septet peak
appeared, centered at 37.43 ppm, which implies that the
phosphorus atom was surrounded by three carbon atoms. This
Fig. 2, under UV irradiation, the reaction progressed smoothly
and the typical absorption band gradually decreased with
ꢁ
1
extending irradiation time. The absorption peaks at 1644 cm
ꢁ
1
1
for C]C double bond and at 2535 cm
for S–H bond
result was in good agreement with the desired structure. The H
completely disappeared within 5 minutes, which implies that
all monomers were consumed to form a highly crosslinked
NMR spectrum (Fig. 1b) indicated that a signal attributed to P–
CH –O was located at 4.07 ppm, the signal for O–CH –C was at
2
2
36
polymer network.
3.87 ppm, the signal for –CH] was at 5.86 ppm and the signal
According to optimal polymerization conditions, a series of
cured polymer samples were prepared. All samples were trans-
parent with light yellow color. The gel content of the samples
was measured by a solvent-extraction method to estimate the
for ]CH was at 5.22–5.31 ppm. The integration ratio between
2
37
crosslinking degree of cured resins, and the results were
collected in Table 1. At high vinyl/thiol molar ratio (1 : 0.5), the
gel content of cured samples was less than 90%. When the
molar ratio of double bond to thiol group approached to 1 : 1,
the gel content of cured samples increased to about 99%, which
implies that nearly all the monomers joined the crosslinked
network.
Optical properties of crosslinked polymer network
Transparent optical-quality plastics provide large advantages
over metal oxide glasses in terms of weight reduction and
37
fracture resistance. The refractive index is very signicant for
optical resins. A great effort has been made in designing and
preparing high refractive index polymers. The universal method
to improve the refractive index of polymers is to introduce
polarizable atoms and groups into the backbones of polymers
Fig. 3 Digital images of UV-cured resin samples, (a) TAOPO/tri-thiol,
b) TAOPO/tetra-thiol, (the inside characters implies the molar ratio of
vinyl group to thiol group).
(
Table 2 Optical properties of crosslinked polymer networks
TAOPO
nC]C : n–SH
1 : 0.5
1 : 0.7
1 : 0.9
1 : 1
1 : 1.1
a
D
b
Tri-thiol
n
V
1.6270
62.0
304
1.6365
37.7
308
1.6385
36.1
304
1.6456
34.0
306
1.6545
35.1
298
D
c
Cutoff UV/nm
Transmittance /%
Density/g cm
d
70.1
70
70.5
79.4
84.9
ꢁ
3
1.199
1.5835
43.9
308
72.3
1.241
1.5905
36.0
308
76.8
1.292
1.253
1.5945
42.2
310
80.3
1.264
1.5952
38.3
310
84.5
1.267
1.5965
42.4
309
85.4
Tetra-thiol
n
V
D
D
Cutoff UV/nm
Transmittance/%
Density/g cm
ꢁ
3
1.244
1.317
1.327
1.331
a
ꢀ
b
c
d
The refractive indices were obtained at 546 nm at 20 C. Abbe number. UV cut-off wavelength. Optical transmission measurements were
performed on polished polymers at 550 nm.
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Fig. 4
n
D
varying with P and S content% (a) TAOPO/tri-thiol, (b) TAOPO/tetra-thiol.
2
38,39
or as attached pendant groups. Recent interests have been index and Abbe number for optical plastics.
The thiol-ene
focused on the incorporation of sulfur atoms into polymer reaction is obviously an ideal route to reach this target. In order
structure because this approach can provide a high refractive to vividly show the transparency of UV-cured polymers, the
Fig. 5 Transmittance of the cured products, (a) TAOPO/tri-thiol, and (b) TAOPO/tetra-thiol.
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polymer samples of 2 mm thickness were kept on a paper with
The Abbe number is another important character for high
printed characters and their transparency were estimated by quality optical resins. It is a measure of the material's disper-
observing the denition of characters. The digital images of UV- sion (variation of refractive index with wavelength) in relation to
cured resin samples (Fig. 3) indicated that all samples the refractive index, where high value of Abbe number indicates
possessed excellent transparency.
The optical properties of samples are shown in Table 2. The high Abbe number due to the aliphatic characteristics of all the
refractive index (n ) of cured resins increased with the amount monomers and the large atomic refraction of sulfur and phos-
of polythiol. The refractive index values of TAOPO, tri-thiol and phorus atoms.
tetra-thiol, were 1.4874, 1.5530 and 1.6324, respectively. Aer The UV cut-off wavelength of all cured resin samples were
low dispersion. From Table 2, it is clear that all samples have
D
40
photopolymerization, the refractive index of TAOPO/tri-thiol about 300 nm, which indicated that currently prepared poly-
based resin changed from 1.6270 to 1.6545 at different mers had excellent blocking effect on short wavelength UV light,
monomer ratios, while, the refractive index of TAOPO/tetra- although this formulation did not contain any aryl unit. All
thiol based cured resins changed from 1.5835 to 1.5965. All resins showed high transmittance at 550 nm in the range of
samples showed high refractive index and possessed potential 70% to 85% (Fig. 5). The methods for further improving the
to be used as optical resins. The relationship between refrac- transmittance include decreasing the amount of photoinitiator,
tive index and content of P and S atom is shown in Fig. 4. The or adopting new initiating system, as suggested by Stiegman
12
refractive index improved with the increase of S content. In and coworkers. In fact, our recent results showed that the
general, S atom is a highly polar element and introducing it transmittance of cured resins could be improved over 90% at
into polymer chains would result in a high refractive index 550 nm if the self-initiation method was adopted to prepare the
resin. The idea has been widely accepted for the prescription optical polymer. The density data of all the resins are also listed
of eyewear design.
in Table 2, showing that the density increased with the amount
of polythiol.
0
Fig. 6 E and tan d curves of the cured products (a) TAOPO/tri-thiol and (b) TAOPO/tetra-thiol.
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0
ꢀ
Table 3
products
T
g
, E (in T
g
+50 C) and Young's modulus (E) of the cured Another interesting fact was that all loss tangent peaks of the
two series had very sharp appearance and the widths were very
narrow. From these results, we could deduce that the triallyl
n
C]C : n–SH
1 : 0.7
1 : 0.9
1 : 1
1 : 1.1
ether and polythiol formed a uniform cross-linked network
ꢀ
41
Tri-thiol T
Tetra-thiol T
Tri-thiol E /MPa
Tetra-thiol E /MPa
Tri-thiol E/MPa
Tetra-thiol E/MPa
g
/ C
ꢁ18.7
ꢁ5.6
14.26
14.02
7.52
ꢁ11.2
11.7
18.18
19.29
10.34
6.09
ꢁ8.9
15
19.07
19.7
10.54
7.88
ꢁ8
aer photopolymerization. The results were signicantly
ꢀ
g
/ C
15.7
19.16
20.9
10.82
8.27
consistent with the aforementioned conclusion and proposed
polymerization mechanism. Young's moduli of all samples are
also given in Table 3. The values of Young's modulus of all
resins were not high enough to meet practical need. One of the
next efforts is to improve the mechanical properties of the
cured resin.
0
0
3.4
The thermal stabilities of all polymers were studied by TGA
and the results are shown in Fig. 7 and Table 4. All the
samples, except for the formulation in which the ratio of
double bond vs. thiol is 1 : 0.5, showed high thermal stability
Mechanical properties and thermal stabilities of crosslinked
polymer networks
The mechanical properties of cured resins were estimated by
DMA analysis (Fig. 6) and the results are collected in Table 3.
ꢀ
below 350 C, however, the crosslinked polymers decomposed
0
The storage modulus (E ) and the glass transition temperature
ꢀ
ꢀ
rapidly around 400 C. The char yields of all samples at 850 C
and the limited oxygen index (LOI) based on the char yield are
listed in Table 4. Though the calculated LOI value cannot
essentially reect the re-retardant property of polymer, it is
indeed a useful reference for choosing suitable polymers for
g
(T ), determined from the peak in the loss tangent, increased
with the molar ratio of vinyl group to thiol group. Moreover,
the storage modulus and Tg of TAOPO/tetra-thiol series are
higher than those of TAOPO/tri-thiol series at the same
monomer ratio due to the different crosslinking density.
42
specic purposes.
Fig. 7 TGA curves of the cured products (a) TAOPO/tri-thiol and (b) TAOPO/tetra-thiol.
Table 4 TGA data of the cured products
nC]C : n–SH
1 : 0.5
1 : 0.7
1 : 0.9
1 : 1
1 : 1.1
a
ꢀ
TAOPO/tri-thiol
T
T
Y
5% ( C)
204.8
262.5
14.29
23.22
214.7
292.7
10.79
21.82
285.8
309.8
14.16
23.16
310.3
354.7
14.92
23.47
291.6
307.2
14.24
23.2
315.8
355.3
12.36
22.4
293.7
313.4
14.26
23.2
331.7
356.6
14.72
23.39
303.3
322.6
6.03
19.91
343.7
365.7
14.58
23.33
a
ꢀ
10% ( C)
b
c
ꢀ
(850 C)
c
ꢀ
ꢀ
LOI
TAOPO/tetra-thiol
T
T
Y
5% ( C)
10% ( C)
ꢀ
c
(850 C)
LOI
a
ꢀ
ꢁ1
b
Temperature at which 5 or 10% weight loss was recorded by TGA at a heating rate of 20 C min under N
2
.
Weight percentage of material le
ꢀ
c
ꢀ
aer TGA analysis at a maximum temperature of 850 C under N
formula: LOI ¼ 17.5 + 0.4Y
2
.
Limited oxygen index based on char yield Y
c
(850 C) from semi-empirical
c
.
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Fig. 8 The dielectric constant and dielectric loss of cured products. (a) TAOPO/tri-thiol and (b) TAOPO/tetra-thiol.
Dielectric properties of crosslinked polymer network
friendly tetrakis(hydroxymethyl)phosphonium sulfate. The
phosphorus–carbon bonds containing monomer showed a high
reactivity with polythiol under UV irradiation through a thiol-
ene radical mechanism. As the molar ratio of the double bond
to the thiol was near to 1, the gel contents of all crosslinked
samples were above 96%. Because of its uniform nature, all
samples were highly transparent. The highest refractive index
was 1.6545, and the corresponding Abbe number was 35.1. The
resulting network polymers showed high thermal stability
below 350 C, and the char yield was above 10% at 850 C, and
the highest LOI based on the char yield from semi-empirical
formula was 23. In addition, the TAOPO based thiol-ene cross-
linking network polymers showed high dielectric constant and
low dielectric loss compared with the reported results.
The dielectric properties of the thiol-ene systems have attracted a
signicant attention in recent years because such systems provide
a convenient method to prepare crosslinking polymer networks
with desired dielectric properties and facilitates the preparation
41,43–45
procedure of some microelectronic devices.
Therefore, we
studied the dependence of the dielectric constant and dielectric
loss of TAOPO-polythiol systems on the frequency of the applied
eld at room temperature. As indicated in Fig. 8, both dielectric
ꢀ
ꢀ
constant and dielectric loss of all the samples showed similar
variation trends with an increase of frequency because the
dielectric constant and dielectric loss of materials depend on the
electronic, atomic, and dipolar polarization rate. As the frequency
of the applied eld increases, the orientation of the polarization
of dipolar is suppressed, resulting in a decrease in the dielectric
constant and dielectric loss of materials. Comparing with repor- Acknowledgements
43
ted results, the TAOPO based thiol-ene crosslinking networks
showed high dielectric constant and low dielectric loss.
The authors thank the NSF of China (no. 21274049), Opening
Project of Key Laboratory of Optoelectronic Chemical Materials
and Devices of the Ministry of Education, Jianghan University
(
no. JDGD-2013-06) and the Fundamental Research Fund for
Conclusions
Central Universities (2013QN159) for nancial support. All
Tris(allyloxymethyl)phosphine oxide, a novel multifunctional authors give sincerely thanks to Analysis and Testing Center of
monomer, was successfully prepared from an environment- HUST for NMR, TGA and DMA test.
41712 | RSC Adv., 2014, 4, 41705–41713
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