Design and development of allyl terminal triaryl pyridine core skeletal modified benzoxazines based polybenzoxazine-silica (PBZ-SiO2) hybrid nanocomposites

Article abstract of DOI:10.1166/jnn.2015.10741

A series of skeletal modified novel allyl terminal benzoxazines (BZ-als) have been successfully designed and developed using triaryl pyridine core diamines via conventional onepot Mannich reaction. The polybenzoxazine-silica (PBZ-SiO₂) nanocomp

Full text of DOI:10.1166/jnn.2015.10741

Article
Journal of
Nanoscience and Nanotechnology
Vol. 15, 9509–9519, 2015
Copyright © 2015 American Scientific Publishers
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Printed in the United States of America
Design and Development of Allyl Terminal Triaryl
Pyridine Core Skeletal Modified Benzoxazines
Based Polybenzoxazine-Silica (PBZ-SiO2)
Hybrid Nanocomposites
R. Padhma Priya, S. G. Gunasekaran, and M. Dharmendirakumar∗
Department of Applied Science and Technology, Anna University, Chennai 600025, India
A series of skeletal modified novel allyl terminal benzoxazines (BZ-als) have been suc-
cessfully designed and developed using triaryl pyridine core diamines via conventional one-
pot Mannich reaction. The polybenzoxazine-silica (PBZ-SiO2ꢀ nanocomposites were prepared
by thermal ring-opening polymerization of benzoxazines with different weight percentages of
3-methacryloxypropyltrimethoxysilane through in-situ sol–gel method. The formation of hybrid
nanocomposites was confirmed by FT-IR. The substantial enhancement in the glass transition tem-
perature was remarked for PBZ-SiO2 hybrids than their respective neat PBZ systems. The dielectric
constant of the nanocomposites was found decreased with an increment in the silica content. The
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shift in the characteristic absorption/emissions corresponding to the neat PBZs toward lower wave-
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Copyright: American Scientific Publishers
length ascertains the successful formation of PBZ-SiO2 nanocomposites. The discernible phase
behaviour observed morphological studies reveals the molecular-level dispersion of silica particles
over the organic networks.
Keywords: Allyl Benzoxazine, Polybenzoxazine-SiO2 Nanocomposites, Glass Transition
Temperature, Thermal Stability, Dielectric Constant, Absorbance, Fluorescence,
Morphology.
1. INTRODUCTION
and also the need of high temperature for the ring open-
ing reaction restrict their utility for high performance
applications.7–15 Polybenzoxazine based organic–inorganic
hybrid nanocomposites can be synthesized via a number
of routes and one that has been particularly successful
is the in-situ polymerization of metal alkoxide precur-
sors in the presence of polymer matrices via the sol–gel
method. The sol–gel technique is a convenient method
for the synthesis of nanocomposite materials consisting of
organic polymers and inorganic compounds.16–21 A variety
of valuable organic–inorganic hybrid polymers have been
synthesized through the sol–gel process.22–25
Over the past decades, organic–inorganic hybrid nanocom-
posites have been attracted considerable research inter-
est for various applications in mechanical, optical and
electronic fields.1–4 Organic–inorganic hybrids comprising
inorganic segments and organic polymers on a molecular
scale possess the advantages of both organic and inor-
ganic components. Organic–inorganic hybrids comprising
inorganic segments and organic polymers on a molecular
scale possess the advantages of both organic and inorganic
components. The properties of organic–inorganic hybrid
materials obtained from the polymeric and inorganic com-
ponents provide an easy processability, better toughness
and good thermal and oxidative stability.5ꢀ6
New types of polybenzoxazine-silica hybrids were
developed by incorporating silica nanoparticles into
allyl terminated benzoxazines through sol–gel process.26
Santhosh Kumar et al. prepared a blend of bisphenol-
Polybenzoxazines (PBZ) are a new class of novel phe-
nolic resins which exhibit more useful properties for dif-
ferent industrial applications. But their brittle behavior
A
based benzoxazine and 2,2ꢀ-bis(8-allyl-3-phenyl-
3,4-dihydro-2H-1,3-benzoxazinyl) propane (Bz-allyl)
with bismaleimide (2,2-bis[4(4-maleimido phenoxy)
∗Author to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2015, Vol. 15, No. 12
1533-4880/2015/15/9509/011
doi:10.1166/jnn.2015.10741
9509

Design and Development of Allyl Terminal Triaryl Pyridine Core Skeletal Modified Benzoxazines
Priya et al.
phenyl]propane.27 Ishida and Low28 prepared an allyl con-
taining benzoxazine monomer from allyl phenol, aniline
and paraformaldehyde.28 The synthesis of a bi-functional
vinyl terminated benzoxazine based graphene nanocom-
posites was reported and their thermal properties were also
studied.29 Different types of novel benzoxazines containing
allyl groups with improved thermal stability were devel-
oped and characterized.30ꢀ31 Vengatesan et al. prepared a
series of polybenzoxazine (PBZ) hybrid nanocomposites
from new type of allyl terminated benzoxazine monomer
and POSS.32 Agag and Takeichi33 investigated the prepara-
tion and properties of nanocomposites of OMM and allyl
functionalized benzoxazine/OMM. The thermal stability
was significantly improved with increasing clay content
and was evidenced by the higher char yield of these
hybrid materials.33 Dual-cure hybrid polymer networks
were prepared by sequential thiol-ene photo polymeriza-
tion followed by thermal ring-opening polymerization of
benzoxazines with increased glass transition temperature
and improved processibility.34
magnesium sulphate, pyridine and diethyl ether were pro-
cured from SRL India. 4-aminophenol was purchased from
Loba, India. 2-allyl phenol and 3-methacryloxypropyl
trimethoxysilane (MPTMS) were obtained from Sigma
Aldrich and used as received.
2.2. Synthesis of Triaryl Pyridine Core Aromatic
Diamine Derivatives
Triaryl pyridine core aromatic dinitro and diamine deriva-
tives were synthesized by adopting a procedure reported
elsewhere41ꢀ42 as shown in Scheme 1. The structural for-
1
mation of the products were then confirmed by H-NMR,
13C-NMR and FT-IR techniques and matched with earlier
reported data.43
2.2.1. Structure of Phenyl Pendant Pyridine Core
Aromatic Diamine (BBAPP)
IUPAC name: 4-(1-Benzene)-2,6-bis(4-aminophenyl)pyridine
(BBAPP)
FT-IR (KBr, cm−1ꢁ: 3352 (NH2ꢁ, 2952 (symmet-
ric stretching), 2862 (asymmetric stretching), 1597
(Ar stretching), 1227(Ar–O–CH2ꢁ.
The inclusion of heteroaromatic rings into the main
chain of a synthetic polymer would impart certain proper-
ties such as the rigidity and polarizability to the resultant
polymers.35ꢀ36 In addition, the incorporation of pyridine
moiety into the polyimides (PI) might improve their
electron affinity and electron-transporting properties.37
Tamami prepared novel diamines containing pyridine
using Chichibabin reaction and synthesized new PIs.38
1H NMR (400 MHz, CDCl3ꢀꢂꢁ: 8.01 (s, 4 H), 7.56
(s, 2 H), 7.41 (s, 2 H), 7.40 (s, 2 H), 6.78 (s, 1 H), 3.84
(s, 4 H).
13C NMR (400 MHz, CDCl3ꢀꢂꢁ: 158, 150, 145, 138,
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139, 130, 127, 116,115 (aromatic carbon)
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Liaw and his co-workers designed and fabricated a pyrene
pendant core pyridine based diamine for the preparation of
organo soluble polyimides with fluorescence properties.39
The introduction of flexible linkages, heteroaromatic rings
and bulky pendant pyrene group in between the reactive
sites reduced the crosslink density and caused the glass
transition temperature of the cured products lowered.40
In this context, the present work deals the synthesis
of polybenzoxazine-silica (PBZ-SiO2ꢁ hybrids via in-situ
sol–gel method using allyl terminal benzoxazines (BZ-al)
monomers and TEOS in the presence of MPTMS as a
coupling agent. The thermal, dielectric and morphological
properties of PBZ-SiO2 hybrid nanocomposites were stud-
ied by DSC, TGA, UV-vis spectroscopy, PL, impedance
and SEM techniques and the results indicated that the
PBZ-SiO2 hybrids possess better properties for high per-
formance applications.
Copyright: American Scientific Publishers
2.2.2. Structure of Thiophenyl Pendant Pyridine Core
Aromatic Diamine (TBAPP)
IUPAC name: 4-(1-Thiophene)-2,6-bis(4-aminophenyl)pyridine
(TBAPP)
FT-IR (KBr, cm−1ꢁ: 3353 (NH2ꢁ, 2952 (symmet-
ric stretching), 2861 (asymmetric stretching), 1596
(Ar stretching), 1226(Ar–O–CH2ꢁ.
1H NMR (400 MHz, CDCl3ꢀꢂꢁ: 7.91 (s, 4 H), 7.56
(s, 2 H), 7.44 (s, 2 H), 7.34 (s, 2 H), 6.68 (s, 4 H), 4.1
(s, 4 H).
13C NMR (400 MHz, CDCl3ꢀꢂꢁ: 158, 147,144,142,
117,114 (aromatic carbon).
2.2.3. Structure of Pyrenyl Pendant Pyridine Core
Aromatic Diamine (PBAPP)
IUPAC name: 4-(1-Pyrene)-2,6-bis(4-aminophenyl)pyridine
(PBAPP)
FT-IR (KBr, cm−1ꢁ: 3351 (NH2ꢁ, 2953 (symmet-
ric stretching), 2864 (asymmetric stretching), 1595
(Ar stretching), 1224 (Ar–O–CH2ꢁ.
2. EXPERIMENTAL METHODS
2.1. Materials
Benzaldehyde,
pyrene-1-carboxaldehyde,
4-nitro-
1H NMR (400 MHz, CDCl3ꢀꢂꢁ: 8.77–8.75 (d, 1 H, J =
9ꢃ2 Hz), 8.55 (d, 1 H, J = 9ꢃ6 Hz), 8.36 (s, 2 H), 8.27 (d,
1 H, J = 4ꢃ0 Hz).
acetopheneone were purchased and used as received
from Acros Organics India Ltd. Maleic anhydride and
10 wt% Pd/C were bought from the Lancaster Chem-
icals UK. Thiophene-2-carboxaldehyde, N ,N -dimethyl
formamide (DMF), ammonium acetate, potassium carbon-
ate, paraformaldehyde, calcium hydride, celite, anhydrous
13C NMR (400 MHz, CDCl3ꢀꢂꢁ: 160, 158, 150, 143,
137, 135, 129, 128, 126, 125, 122, 121, 117,114, 91, 54
(aromatic carbon).
9510
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Priya et al.
Design and Development of Allyl Terminal Triaryl Pyridine Core Skeletal Modified Benzoxazines
O2N
NO2
H
O
N
CH3COONH4
CH3COOH
+
O2N
C
R
O
CH3
4-nitroacetophenone
1-arylcarboxaldehyde
Ar
4-(1-aryl)-2,6-bis(4-nitrophenyl)pyridine
Pd/C
EtOH
Reflux
NH2NH2.H2O
H2N
NH2
N
Ar =
S
Ar
4-(1-aryl)-2,6-bis(4-aminophenyl)pyridine
Scheme 1. Synthesis of triaryl pyridine core aromatic diamine derivatives.
2.3. Synthesis of Bisallyl Terminal Triaryl Pyridine
Core Aromatic Diamine Based Benzoxazine
Monomers (BZ-als)
2.5. Characterization
FT-IR spectra were recorded on a Perkin Elmer 6X FT-
IR spectrometer. About 100 mg of optical-grade KBr was
grounded with sufficient quantity of the solid sample to
To 100 ml of 1,4-dioxane solvent, triaryl pyridine diamine
(BBAPP) (0.0148 mol, 5.0 g), 2-allyl phenol (OAP)
(0.03 mol, 3.9 ml) and paraformaldehyde (0.065 mol,
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make 1.0 wt% mixture for making KBr pellets. After the
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sample was loaded, a minimum of 16 scans was col-
lected for each sample at a resolution of 4 cm−1. All
1H NMR and 13C NMR analyses were done in d-CHCl3
and DMSO-d6 recorded on a Bruker 400 spectrometer.
A Netzsch DSC-200 differential scanning calorimeter was
used for the calorimetric analysis. The instrument was
calibrated with Indium supplied by Netzsch. Measure-
ments were performed under a continuous flow of nitrogen
(60 ml/min). All the samples (about 10 mg in weight) were
heated from ambient to 400 ꢁC and the thermograms were
recorded at a heating rate of 10 ꢁC/min. Thermo gravimet-
ric analysis (TGA) was performed in a DSC-2920 from
TA Instruments coupled with a TA-2000 control system.
Copyright: American Scientific Publishers
ꢁ
1.97 g) were added and refluxed at 110 C for 6 h. The
reaction mixture was then filtered and washed once with
1 M NaHCO3 aqueous solution (200 ml) and dried with
anhydrous sodium sulphate for 12 h. Removal of sol-
vent by evaporation and drying under vacuum yielded
benzoxazine as a light brown powder (Scheme 2). The
yield was 83%. The other two monomers such as BZb-
al and BZc-al were also synthesized in a similar way as
reported elsewhere44ꢀ45 and formed the precursors (yel-
low and light yellow) in the form of powder with yields
of 87 and 85% respectively. Here, BZs are denoted as
BZa-al = BBAPP-OAP; BZb-al = TBAPP-OAP; BZc-al =
PBAPP-OAP.
ꢁ
The samples were heated at a scanning rate of 10 C/min
under nitrogen atmosphere in order to diminish oxida-
tion. The dielectric constants of the neat PBZs and PBZ-
SiO2 nanocomposites were determined with the help of an
impedance analyzer (Solartron impedance/gain phase ana-
lyzer 1260) using a platinum (Pt) electrode at 30 C in a
frequency range of 1 MHz.
The neat PBZs and PBZ-SiO2 nanocomposites were
characterized by UV-vis-NIR spectrophotometer (UV-vis-
NIR) (U-4100, Hitachi, Japan). The absorbance of the
solution was measured with a wavelength ranging from
200 to 1300 mm. The emission properties of the PBZ-
SiO2 nanocomposites were also studied using Fluores-
cence spectrophotometer (Cary Eclipse, FL1201M002,
Japan) with an excitation wavelength. Scanning electron
2.4. Preparation of Polybenzoxazine-Silica (PBZ-SiO2)
Hybrid Nanocomposites
In a flask, 0.2 g TEOS was dissolved in 4 ml of 1,4-
dioxane and an exact equivalent of dilute acid (0.1 N HCl)
was added.21 Then, 0.2 g BZ-al and varying molar equiv-
alents (0.5, 1.0, 1.5 and 2.0) of MPTMS with respect to
BZ-al were dissolved in 4 ml of 1,4-dioxane separately
in another flask as listed in Table I. Both solutions were
mixed and agitated for 60 min. The curing process was
then performed at 100 ꢁC for 60 min,ꢁ at 150 ꢁC for
120 min and finally post cured at 200 C for 120 min
(Scheme 3).
ꢁ
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Design and Development of Allyl Terminal Triaryl Pyridine Core Skeletal Modified Benzoxazines
Priya et al.
OH
H2N
NH2
N
nCH2O
+
+
Ar
Triaryl pyridine core aromatic diamines
2-allyl phenol
paraformaldehyde
Reflux
7 h
O
O
1,4-dioaxane
N
N
N
Ar
Allyl terminal triaryl pyridine core aromatic diamine based benzoxazines
Ar =
S
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Scheme 2. Synthesis of bis allyl terminal benzoxazines (BZ-als).
microscope (SEM) (Hitachi S-3400N) was used to observe
the surface morphology of the PBZ-SiO2 nanocomposites.
The transmission electron microscope (TEM) observations
were carried on a JEOL JEM-3010, IITM, India, with an
accelerating voltage of 300 KV.
3. RESULTS AND DISCUSSION
3.1. Structure of Bis Allyl Benzoxazine Monomers
Three different types of bis allyl terminal benzoxazine
1
monomers were synthesized by and characterized by H-
NMR, 13C-NMR, FT-IR, UV-Vis and PL techniques and
matched with earlier reported data.12ꢀ21ꢀ27–30
Table I. Curing time cycle of PBZ-SiO2 hybrid nanocomposites.
3.1.1. Structure of BZa-al (BZ-Ph-al)
BZ-al TEOS MPTMS 1,4-dioxane
FT-IR (KBr, cm−1ꢁ: 2956 (symmetric stretching), 2880
(asymmetric stretching), 1505 (substituted benzene), 1387
(C–N), 1245 (C–O), 1122 (allyl), 948 (N–CH2–O).
1H NMR (400 MHz, CDCl3, ꢂꢁ: 8.07 (s, 4 H), 7.63
(s, 2 H), 7.60 (s, 2 H), 7.45 (s, 3 H), 7.10 (s, 2 H), 6.91
(s, 2 H), 6.80 (s, 4 H), 6.09 (s, 4 H), 5.38 (s, 4 H), 5.37
(s, 4 H), 4.68 (s, 4 H).
13C NMR (400 MHz, CDCl3, ꢂꢁ: 159, 158, 150, 143,
132, 129, 128, 126, 124, 122120, 117, 113, 112, 90, 54
(aromatic carbon).
Systems
(g)
(g)
(g)
(ml)
Colour
Neat PBZa
1
1
1
1
1
0
1
1
1
1
0
4
4
4
4
4
Brown
Brown
Light brown
Brown
0.5 wt% PBZa-SiO2
1.0 wt% PBZa-SiO2
1.5 wt% PBZa-SiO2
2.0 wt% PBZa-SiO2
0ꢃ17
0ꢃ34
0ꢃ68
1ꢃ36
Brown
Neat PBZb
1
1
1
1
1
0
1
1
1
1
0
4
4
4
4
4
Brown
Light brown
Brown
Dark brown
Dark brown
0.5 wt% PBZb-SiO2
1.0 wt% PBZb-SiO2
1.5 wt% PBZb-SiO2
2.0 wt% PBZb-SiO2
0ꢃ17
0ꢃ34
0ꢃ68
1ꢃ36
Neat PBZc
1
1
1
1
1
0
1
1
1
1
0
4
4
4
4
4
Brown
Brown
Light brown
Brown
0.5 wt% PBZc-SiO2
1.0 wt% PBZc-SiO2
1.5 wt% PBZc-SiO2
2.0 wt% PBZc-SiO2
0ꢃ17
0ꢃ34
0ꢃ68
1ꢃ36
3.1.2. Structure of BZb-al (BZ-Th-al)
FT-IR (KBr, cm−1ꢁ: 2955 (symmetric stretching), 2881
(asymmetric stretching), 1506 (substituted benzene), 1384
Dark brown
9512
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Priya et al.
Design and Development of Allyl Terminal Triaryl Pyridine Core Skeletal Modified Benzoxazines
Allyl terminal benzoxazine monomer
O
O
N
N
N
Ar
O
O
TEOS
Michael
addition
HS
Si
O
O
O
O
O
O
O
Si
O
O
Si
O
O
O
O
O
O
Si
S
i
O
O
O
O
O
Si
O
O
O
Si
O
O
S
Si
Si
O
O
Si
O
Si
O
O
O
O
Si
O
O
S
O
O
N
N
N
O
O
O
O
O
O
O
O
Si
O
O
O
Si
Si
O
Si
Si
O
O
S i
O
O
Ar
O
Si
O
Si
O
Si
O
O
O
O
O
O
Si
O
Ring opening polymerization
O
O
O
Si
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O
O
O
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Si
O
OCopyright: American Scientific Publishers
O
O
O
O
S
S
S
H
O
H
O
S
*
*
*
H
*
O
O
H
Si
n
n
O
N
N
O
Si
O
O
O
O
O
O
N
Si
Si
O
Si
Si
O
O
O
Si
O
O
O
O
O
O
O
O
O
S i
Si
O
Si
O
O
O
O
O O
O
O
O
Si
Ar
O
O
Allyl terminal benzoxazine based Polybenzoxazine-Silica (PBZ-SiO2) hybrid nanocomposites
Scheme 3. Schematic representation of synthesis of polybenzoxazine-silica (PBZ-SiO2ꢁ hybrid nanocomposites.
(C–N), 1242 (C–O), 1121 (allyl), 948 (N–CH2–O), 832,
670 (symmetric stretching, C–S).
1H NMR (400 MHz, CDCl3, ꢂꢁ: 8.18 (s, 2 H), 7.68
(s, 2 H), 7.59 (s, 2 H), 7.47 (s, 2 H), 7.15 (s, 2 H), 6.99
(s, 2 H), 6.88 (s, 4 H), 6.03 (s, 3 H), 5.59 (s, 2 H), 5.48
(s, 2 H), 4.80 (s, 4 H).
13C NMR (400 MHz, CDCl3, ꢂꢁ: 159, 158, 150, 143,
133, 132, 129, 128, 126, 124, 122, 120, 117, 113, 112, 90,
56 (aromatic carbon).
3.1.3. Structure of BZc-al (BZ-Py-al)
FT-IR (KBr, cm−1ꢁ: 2955 (symmetric stretching), 2883
(asymmetric stretching), 1502 (substituted benzene), 1384
(C–N), 1244 (C–O), 1125 (allyl), 948 (N–CH2–O).
1H NMR (400 MHz, DMSO-d6, ꢂꢁ: 8.56 (s, 1 H), 8.35
(s, 1 H), 7.88 (s, 1 H), 7.48 (s, 2 H), 7.30 (s, 2 H), 6.67
(s, 2 H), 5.67 (s, 2 H), 5.54 (s, 2 H), 4.74 (s, 4 H), 3.23
(s, 6 H).
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Design and Development of Allyl Terminal Triaryl Pyridine Core Skeletal Modified Benzoxazines
Priya et al.
13C NMR (400 MHz, DMSO-d6, ꢂꢁ: 159, 158, 150, 143,
138, 135, 132, 131, 129, 127, 122, 117, 113, 112, 90, 56
(aromatic carbon).
glass transition temperature (Tg) of neat PBZs and PBZ-
SiO2 hybrid nanocomposites are presented in Table II.
From the DSC thermograms (Fig. 2), it is observed
that the PBZ-SiO2 hybrids showed higher Tg values than
those of the neat PBZs. This may be attributed to the
incorporation of silica components into the PBZ matri-
ces. Furthermore, the enhancement in the Tg values may
be due to the hydrogen bonding interaction between sil-
ica and PBZ matrices, which restricts the mobility of
the polymer chains.15ꢀ21 In addition, the sulfide prod-
uct formed during Michael addition (thiol-ene) led to
increase the glass transition temperature of the resulted
nanocomposites.
3.1.4. UV-Vis Absorption Properties of BZ-al Monomers
Absorption (nm): 321 (BZa-al), 308 (BZa-al), 346 (BZa-
al).
3.1.5. PL Properties of BZ-al Monomers
Emission (nm): 467 (BZa-al), 446 (BZb-al), 450 (BZc-al).
3.2. Characterization of PBZ-SiO2 Nanocomposites
Figure 1 shows the FT-IR spectra of the neat PBZs
and PBZ-SiO2 hybrid nanocomposites. The ring opening
polymerization was evidenced by the disappearance of
the absorption peaks at 948 cm−1 (oxazine ring) and at
1505 cm−1 (trisubstituted benzene), which confirms the
formation of neat polybenzoxazines matrices (PBZs).
The appearance of the new absorption peaks at
1480 and 1120 cm−1 (Si–O–Si linkages) also speci-
fied the ring opening polymerization of the PBZ-SiO2
nanocomposites.47 The strong bands at 838 and 712 reveal
the occurrence of C–S stretching of thiophene ring present
in the PBZ and PBZ-SiO2 systems. The absence of the
bands at 3421, 2500 (S–H stretching) and 1652 cm−1
(C C stretching) and indicates the formation of coꢁva-
3.3.2. Thermo Gravimetric Analysis
The thermo grams of all the PBZ-SiO2 systems are shown
in Figure 3. The PBZ-SiO2 hybrids showed enhanced ther-
mal stability (as evidence from the thermal degradation
temperature) when compared to that of the cured neat
PBZs (Table II), due to the presence of highly thermally
stable silica networks.12 Especially, the hybrids containing
2.0 wt% silica particles exhibit higher initial degradation
temperature of 330 ꢁC and char yield of 60.2% for 2.0 wt%
PBZa-SiO2, 63.1% for 2.0 wt% PBZb-SiO2 and 62.7% for
2.0 wt% PBZc-SiO2 respectively. This may be due to the
presence of sulfur, partial ionic nature, high bond energy
of Si–O–Si network and higher silica content.
In addition, this enhanced thermal stability could be
attributed to better adhesion between the polymeric matrix
and inorganic Si–O–Si network domains via the construc-
tion of hydrogen bonding between the polar groups from
the thiol functional silane and the free –OH groups of
the polybenzoxazine matrix. Moreover, the formation of
Michael adducts between the benzoxazine monomers and
thiol end silane coupling agents afford the higher initial
degradation temperature for the hybrids. The increasing
char residue can reduce the exothermic nature of the pyrol-
ysis reaction, which in turn, lowers the thermal conduc-
tivity and inhibits the combustion of the burning moieties,
led to the higher thermal stability than those of neat PBZ
systems.21ꢀ34ꢀ46ꢀ47
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lent bonding between MPTMS and BZ-al at 180 C,
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resulted from the ring-opening polymerization of benzox-
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azine along silica precursors.
3.3. Thermal Properties
3.3.1. Differential Scanning Calorimetry
Differential scanning calorimetric analysis was carried out
to determine the glass transition temperature of the neat
PBZs and PBZ-SiO2 hybrid nanocomposites and the DSC
thermo grams were shown in Figure 2. The values of the
3.3.3. Limiting Oxygen Index
The flame retardant behavior of the neat PBZs and PBZ-
SiO2 nanocomposites was studied for their char yield at
700 ꢁC in terms of LOI values and is presented in Table II.
The LOI values were found to be in the range of 49.0–
63.0 (Fig. 4). The burning behavior was correlated to
the higher char yield. Furthermore, the flame retardancy
was increased due to the presence of sulfur content as
well as inorganic Si–O–Si network formed during the
thermal ring opening polymerization. Thus, the formation
of inorganic network hinders the hybrid materials from
combustion.
Figure 1. FT-IR spectra of neat PBZ and PBZ-SiO2 hybrid
nanocomposites.
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J. Nanosci. Nanotechnol. 15, 9509–9519, 2015

Priya et al.
Design and Development of Allyl Terminal Triaryl Pyridine Core Skeletal Modified Benzoxazines
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DSC thermograms of PBZ-SiIOP:h9yb1r.id21na5n.o1c3om6p.1os7it5es.On: Sat, 18 Jun 2016 19:43:15
Figure 2.
2
Copyright: American Scientific Publishers
3.4. Dielectric Constant
In the present work, the lowest values of dielectric con-
stant were obtained for a maximum (2.0 wt%) loading
of silica in the PBZ matrices (Fig. 5). It is inferred that
the incorporation of silica into the PBZ matrices low-
ered the values of the dielectric constant, compared to
that of neat PBZs. This is attributed to the presence of
the low polar nature of Si–O–Si networks present in the
nanocomposites.
The development of new low k materials is warranted
for advanced microelectronic applications. In this direc-
tion, new types of PBZ-SiO2 hybrid material were devel-
oped and its insulating behavior has been assessed. The
values of dielectric constant of the neat PBZs and PBZ-
SiO2 hybrid nanocomposites at the frequency range of
1 MHz in room temperature are presented in Table II.
Table II. Thermal and dielectric data of neat PBZ and PBZ-SiO2 hybrid nanocomposites.
10% weight
loss (ꢁC)
20% weight
loss (ꢁC)
Char yield
Dielectric
constant (ꢄ)
LOI at 700 ꢁC
0.4ꢅꢆꢁ+17ꢃ5
Systems
Tg (ꢁC)
(at 700 ꢁC) (%)
Neat PBZa
210
217
223
227
234
360
350
375
390
401
408
412
414
422
430
49.1
51.2
52.3
57.6
60.2
3.70
3.42
3.01
2.57
2.39
37.1
38.0
38.4
40.5
41.6
0.5 wt% PBZa-SiO2
1.0 wt% PBZa-SiO2
1.5 wt% PBZa-SiO2
2.0 wt% PBZa-SiO2
Neat PBZb
213
219
225
230
236
370
350
390
400
412
406
410
422
451
468
49.5
52.0
58.0
61.6
63.1
3.65
3.22
2.71
2.42
2.13
37.3
38.3
40.7
42.1
42.7
0.5 wt% PBZb-SiO2
1.0 wt% PBZb-SiO2
1.5 wt% PBZb-SiO2
2.0 wt% PBZb-SiO2
Neat PBZc
211
220
223
229
232
356
380
360
395
410
410
413
420
450
464
48.8
52.0
56.0
60.1
62.7
3.75
3.46
2.82
2.50
2.21
37.0
38.3
39.9
41.5
42.6
0.5 wt% PBZc-SiO2
1.0 wt% PBZc-SiO2
1.5 wt% PBZc-SiO2
2.0 wt% PBZc-SiO2
J. Nanosci. Nanotechnol. 15, 9509–9519, 2015
9515

Design and Development of Allyl Terminal Triaryl Pyridine Core Skeletal Modified Benzoxazines
(a) (b)
Priya et al.
100
90
80
70
60
50
100
90
80
70
60
50
Neat PBZb
Neat PBZa
0.5 wt% PBZb-SiO2
1.0 wt% PBZb-SiO2
1.5 wt% PBZb-SiO2
2.0 wt% PBZb-SiOa
0.5 wt% PBZa-SiO2
1.0 wt% PBZa-SiO2
1.5 wt% PBZa-SiO2
2.0 wt% PBZa-SiOa
100
200
300
400
500
600
700
100
200
300
400
500
600
700
Temperature (°C)
Temperature (°C)
(c)
100
90
80
70
60
Neat PBZc
0.5 wt% PBZc-SiO2
1.0 wt% PBZc-SiO2
1.5 wt% PBZc-SiO2
50 Delivered by Ingenta to: McMaster University
2.0 wt% PBZc-SiOa
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200
300
400
600
700
Temperature (°C)
Figure 3. TGA thermograms of PBZ-SiO2 hybrid nanocomposites.
3.5. Optical Properties
3.5.1. UV-Vis Absorption Spectra
The UV-vis absorption studies of the neat PBZ matri-
ces were illustrated in Figure 6. It was found that
the neat PBZ matrices display a characteristic absorp-
tion band corresponding to the benzoxazine moieties.
Hence, this study confirms the successful formation of
polybenzoxazines.
PBZa-SiO2
42
4.0
PBZa-SiO2
PBZb-SiO2
PBZb-SiO2
PBZc-SiO2
PBZc-SiO2
3.5
40
3.0
2.5
2.0
38
36
0.0
0.5
1.0
1.5
2.0
0.0
0.5
1.0
1.5
2.0
Weight percentages of silica
Weight percentages of silica
Figure 4. LOI values of neat PBZ and PBZ-SiO2 hybrid
nanocomposites.
Figure 5. Dielectric behaviour of PBZ-SiO2 hybrid nanocomposites.
J. Nanosci. Nanotechnol. 15, 9509–9519, 2015
9516

Priya et al.
Design and Development of Allyl Terminal Triaryl Pyridine Core Skeletal Modified Benzoxazines
respectively. The blue shift was occurred due to the pres-
ence of silica particles in the form of Si–O–Si networks
within the matrix. Hence, such these emission peaks reveal
the photoluminescence properties and in turn, it ascertains
the successful formation of PBZ-SiO2 nanocomposites.
Neat PBZa
Neat PBZb
Neat PBZc
3.6. Morphology of the PBZ-SiO2 Nanocomposites
3.6.1. Scanning Electron Microscopy
The surface morphology of the neat PBZs and PBZ-SiO2
hybrid nanocomposites was investigated by SEM tech-
niques. Figures 8(a)–(f) shows a uniform and smooth
surface morphology of neat PBZ matrices. The surface
of the hybrid thin film is quite smooth, indicating that
the synthesized hybrids have a molecular-level dispersion
of silica particles, distributed uniformly over the organic
phase.21 In addition, the excellent structural uniformity is
resulted from both the covalent bonding and Michael addi-
tion of thiol end silane coupling agent and benzoxazine
matrix.
300
350
400
450
500
Wavelength (nm)
Figure 6. UV-vis absorption spectra of PBZ matrices.
3.5.2. Photoluminescence Spectra
Figure 7 shows photoluminescence spectra of the neat
PBZs and PBZ-SiO2 hybrid nanocomposites at an exci-
tation wavelength of 365 nm. The strong fluorescent
peaks were observed in the wavelength region of 459 nm
(PBZaꢁ, 449 nm (PBZa-SiO2ꢁ, 460 nm (PBZbꢁ, 441 nm
(PBZb-SiO2ꢁ, 449 nm (PBZcꢁ and 441 nm (PBZc-SiO2ꢁ
3.6.2. Transmission Electron Microscopy
The TEM analyses of the PBZ-SiO2 hybrid nanocompos-
ites were performed and their micrographs were presented
(a)
(b)
Neat PBZa
2.0 wt % PBZa-SiO2
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2.0 wt % PBZb-SiO2
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Copyright: American Scientific Publishers
300
350
400
450
500
550
600
300
350
400
450
500
550
600
Wavelength (nm)
Wavelength (nm)
(c)
Neat PBZc
2.0 wt % PBZc-SiO2
300
350
400
450
500
550
600
Wavelength (nm)
Figure 7. Photoluminescence spectra of neat PBZ and PBZ-SiO2 hybrid nanocomposites.
J. Nanosci. Nanotechnol. 15, 9509–9519, 2015
9517

Design and Development of Allyl Terminal Triaryl Pyridine Core Skeletal Modified Benzoxazines
Priya et al.
Figure 8. SEM images of (a) neat PBZa (b) neat PBZb, (c) neat PBZc, (d) PBZa-SiO2, (e) PBZb-SiO2 and (f) PBZc-SiO2 nanocomposites.
Delivered by Ingenta to: McMaster University
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Copyright: American Scientific Publishers
Figure 9. TEM images of (a) PBZa-SiO2 (b) PBZb-SiO2 and (c) PBZc-SiO2 hybrid nanocomposites.
in Figure 9. The incorporation of silica particles led to
distinguishable phase morphology, as can be seen from
Figure 9. The dark area (the portion of the hybrid) in the
polymer matrix represents the dispersions of SiO2 parti-
cles whereas the area surrounding the dark patches indi-
cates the PBZ matrices. Consequently, this study reveals
the successful inclusion of the silica network into the poly-
benzoxazine matrices.
wt% of silica (2.0 wt%) possess higher Tg than that of
hybrids prepared using low wt% silica. Theses hybrid
polymer networks exhibited good thermal stability and
higher char yield (about 60%) than those of neat PBZs,
due to the higher content of silica and sulfur present in the
hybrids. The blue fluorescent emissions were observed in
the wavelength region 439–464 nm for polybenzoxazine-
silica hybrid nanocomposites, as evidenced by PL stud-
ies. The molecular level dispersion of silica particles
in the PBZ matrix was ascertained from morphologi-
cal studies. Hence, the development of polybenzoxazine-
silica nanocomposites with tunable thermal and dielectrical
properties is expected to find application in the fields of
aerospace and microelectronics.
4. CONCLUSION
A new series of novel polybenzoxazine-silica (PBZ-SiO2ꢁ
hybrid nanocomposites were successfully prepared from
three different types of novel allyl terminal benzoxazine
monomers (organic phase) and TEOS (inorganic phase)
using MPTMS as a coupling agent through in-situ sol–gel
method. The developed nanocomposites were then charac-
terized and confirmed by various physico-chemical meth-
ods. The values of the dielectric constant were found lower
for PBZ-SiO2 hybrid systems (k = 2.39 for 2.0 wt% PBZa-
SiO2, 2.13 for 2.0 wt% PBZb-SiO2 and 2.22 for 2.0 wt%
PBZc-SiO2 than those of neat PBZ systems (k = 3.70
for PBZa, 3.65 for PBZb and 3.75 for PBZcꢁ. Among
the PBZ-SiO2 hybrids, the hybrids obtained using high
Acknowledgments: The authors acknowledge the
Indian Institute of Technology Madras, Chennai, India
for providing the Transmission electron microscope anal-
ysis facility. The authors thank R. Selvakumar, Centre for
Nanoscience and Nanotechnology for studying dielectri-
cal behaviour and K. Srinivasan, Department of Chemistry,
Anna University, Chennai, India for providing photolumi-
nescence analysis facility.
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Priya et al.
Design and Development of Allyl Terminal Triaryl Pyridine Core Skeletal Modified Benzoxazines
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Received: 17 November 2014. Accepted: 11 December 2014.
J. Nanosci. Nanotechnol. 15, 9509–9519, 2015
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