1
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A.B.V. Kiran Kumar et al. / Reactive & Functional Polymers 73 (2013) 1175–1179
enhanced thermal stability and emission intensity from the inser-
tion of an organic moiety into POSS polymers.
2
. Results and discussion
Nanocomposites of POSS-CEM were synthesized using radical
co-polymerization techniques. The desired POSS-CEM co-polymers
were synthesized using a different CEM wt% with respect to POSS
as shown in Scheme 2 (experimental details may be found in Sec-
tion 3). Earlier reports revealed that POSS cages could be incorpo-
rated into polymers using either single or multiple polymerizable
functional groups. POSS cages with a single polymerizable group
(
pendent or end) on a linear polymer backbone led to composites
and multifunctional groups on polymer chains with star-like 3D
structures, which exhibited improved thermal and mechanical
properties [27]. In the present investigation, an organic moiety
was inserted as a co-monomer into POSS polymers to study
changes in the properties of the copolymer products.
Structures of the resulting POSS-CEM nanocomposites were
characterized using FTIR spectroscopy. FTIR spectra showed a
Fig. 1. FTIR spectra of pure CEM and POSS-CEM nanocomposites with 10, 20, 30,
and 50 wt% CEM.
strong, symmetric SiAOASi stretching absorption band at
À1
1
128 cm , which is the characteristic absorption peak of sils-
esquioxane cages (Fig. 1). The existence of this SiAOASi stretching
band confirmed that the POSS cage was incorporated into the
nanocomposites. Moreover, an absorption band at 1500 cm orig-
À1
of moisture. The second weight loss of the nanocomposite occurred
at approximately 350 °C as expected, and no ceramic yield was ob-
tained. The thermal stability of the nanocomposites was signifi-
cantly enhanced with the inclusion of the inorganic component
in agreement with reports from Chi et al. [31,32]. It has been pro-
posed that the tethered structure is crucial to the improvement of
the thermal stability of POSS-CEM nanocomposites [33,34]. In the
current framework, POSS cages participated in the formation of a
cross-linked network; i.e. the POSS cages were tethered to the
polymer matrix. In addition, the nanoscale dispersion of POSS
cages in the nanocomposites was also an important factor that
contributed to the enhanced thermal stability. It is plausible that
the mass loss from segmental decomposition via gaseous frag-
ments could be suppressed by well-dispersed POSS cubes at the
molecular level. Similar results have been found for fully exfoliated
polymer-clay nanocomposites [35,36]. Therefore, POSS incorpora-
tion led to improved thermal stability of the nanocomposites.
The amorphous nature of the nanocomposites was confirmed
by XRD measurements. Among the synthesized nanocomposites,
the product with the least amount of CEM was in the gel state,
and the remaining products were solids. The existence of solids
is due to increased cross links between CEM and POSS cages. The
diffraction patterns were featureless, showing only broad amor-
phous halos at 2h = 19.23 Å with a d spacing of 4.61 Å (Fig. 4),
which indicates the POSS particles were well distributed within
the polymer matrix, leading to the completely amorphous struc-
ture of these materials [37].
inated from the skeletal vibration of aromatic rings, and stretching
absorption bands of methylene groups were observed at
À1
À1
2
940 cm [28]. The characteristic absorption peak at 1731 cm
was assigned to a carbonyl stretching vibration. Furthermore, the
À1
presence of a
m
OAH vibration peak at 3440 cm , which is not ob-
served in pure CEM, indicated the existence of residual silanol
groups and confirmed the presence of POSS groups in the POSS-
CEM nanocomposites. In addition, characteristic bands for methyl
À1
and methylene groups were observed at ꢀ3000 cm
.
Structural confirmation of the newly synthesized POSS-CEM
29
nanocomposites was determined by
Si NMR spectroscopy
(
Fig. 2). The chemical shift values around À69.47 ppm (for 10
and 20 wt% CEM-POSS nanocomposites; see Fig. 2a and b) and
À69.62 ppm (for 30 and 50 wt% CEM-POSS nanocomposites; see
Fig. 2c and d) clearly confirmed the presence of silicon atoms in
a different chemical environment compared with pure POSS
(
À67.8 ppm) [29]. This down-field chemical shift value indicates
not only an interaction between POSS and CEM, but also that the
product has a uniform structure with high purity. Moreover, the
chemical shift is close to that of silicon atoms in the cage structure
of (RSiO1.5
methacryl-POSS, has a cage structure, and its structural formula
should be (C Si
8
) [30]. This result revealed that the synthetic compound,
7
H
11
O
2
)
8
8 12
O .
TA analysis was carried out for the 20% CEM POSS nanocompos-
ite. As shown in Fig. 3, the first weight loss of 5.6% occurred at
approximately 140 °C and could be attributed to a trace amount
The morphologies of the POSS-CEM nanocomposites were
investigated with FESEM. Fig. 5 shows the fractured surface of a
nanocomposite in which no localized domains were detected. This
indicates that POSS participated in the formation of cross-linked
CEM networks. The surface appears to be free of visible defects
and was quite smooth. No localized areas of POSS aggregates were
observed on a scale of several nanometers, which suggests that the
POSS component was homogeneously dispersed in the continuous
matrix. EDS measurements confirmed the presence of POSS in the
nanocomposites (Fig. 5d). Copper (Cu) peaks resulting from the Cu
grids used to mount the samples were also observed.
Fluorescence spectra for the nanocomposites were recorded in
the solid-state. Fig. 6 shows the excitation and emission curves
for these nanocomposites prepared with various percentages of
CEM. The excitation spectra of nanocomposite materials exhibited
Scheme 2. Synthetic scheme and network structure of the copolymers.
Kiran Kumar
