Variously substituted phenyl hepta cyclopentyl-polyhedral oligomeric silsesquioxane (ph,hcp-POSS)/polystyrene (PS) nanocomposites: The influence of substituents on the thermal stability

Article abstract of DOI:10.1007/s10973-013-2997-3

A comparative study concerning the thermal stability of polystyrene (PS) and six POSS/PS nanocomposites of general formula R₇R′(SiO _1.5)₈/PS (where R = cyclopentyl and R′ = phenyl, 4-methoxyphenyl, 4-tolyl, 3,5-xilyl, 4-fluorophenyl, and 2,4-difluorophenyl) was carried out in both inert (flowing nitrogen) and oxidative (static air) atmospheres. Nanocomposites were prepared by in situ polymerization of styrene in the presence of 5 % w/w of POSS, but the actual filler concentration in the obtained nanocomposites, determined by ¹H NMR spectroscopy, was in all cases slightly higher than that in the reactant mixtures. FTIR spectra of nanocomposites evidenced the presence of filler-polymer interactions. Inherent viscosity (η_inh) determinations indicated that the average molar mass of polymer in methylated and fluorinated derivatives was lower than neat PS, and were in agreement with calorimetric glass transition temperature (T _g) measurements. Degradations were performed into a thermobalance, in the scanning mode, at 10 C min^-1, and the temperatures at 5 % mass loss (T_{5 %}), of various nanocomposites were determined. The effects of various substituents of the POSS phenyl group on the thermal stability of nanocomposites were evaluated. The results were discussed and interpreted.

Full text of DOI:10.1007/s10973-013-2997-3

J Therm Anal Calorim (2013) 112:421–428
DOI 10.1007/s10973-013-2997-3
Variously substituted phenyl hepta cyclopentyl-polyhedral
oligomeric silsesquioxane (ph,hcp-POSS)/polystyrene (PS)
nanocomposites
The influence of substituents on the thermal stability
•
Ignazio Blanco Lorenzo Abate Francesco Agatino Bottino
•
Italian Special Issue
´
´
Ó Akademiai Kiado, Budapest, Hungary 2013
Abstract A comparative study concerning the thermal
stability of polystyrene (PS) and six POSS/PS nanocom-
posites of general formula R7R0(SiO1.5)8/PS (where R =
cyclopentyl and R0 = phenyl, 4-methoxyphenyl, 4-tolyl,
3,5-xilyl, 4-fluorophenyl, and 2,4-difluorophenyl) was
carried out in both inert (flowing nitrogen) and oxidative
(static air) atmospheres. Nanocomposites were prepared by
in situ polymerization of styrene in the presence of 5 %
w/w of POSS, but the actual filler concentration in the
Introduction
Organic–inorganic hybrid materials play currently a key
role in the production of high performance materials
because they possess much better characteristics than
constituents. In particular, the introduction of inorganic
nanostructures into polymers (nanocomposites) leads gen-
erally to the improvement of physical [1], mechanical [2],
barrier [1], and flammability [3] properties in comparison
with virgin polymer. Owing to these reasons, they have
attracted great interest and have driven chemists, materials
scientists, engineers, and physicists to synthesize novel
products and investigate their possible applications in
various fields.
1
obtained nanocomposites, determined by H NMR spec-
troscopy, was in all cases slightly higher than that in the
reactant mixtures. FTIR spectra of nanocomposites evi-
denced the presence of filler-polymer interactions. Inherent
viscosity (ginh) determinations indicated that the average
molar mass of polymer in methylated and fluorinated
derivatives was lower than neat PS, and were in agreement
with calorimetric glass transition temperature (Tg) mea-
surements. Degradations were performed into a thermo-
balance, in the scanning mode, at 10 °C min-1, and the
temperatures at 5 % mass loss (T5 %), of various nano-
composites were determined. The effects of various sub-
stituents of the POSS phenyl group on the thermal stability
of nanocomposites were evaluated. The results were dis-
cussed and interpreted.
Carbon nanotubes, layered silicates, and other [4–10]
are used to prepare organic–inorganic hybrid systems, but,
in the last decade, polyhedral oligomeric silsesquioxanes
(POSSs) have attracted the attention for the use as fillers in
the production of polymer-based nanocomposites [11, 12].
The POSS general formula is (R-SiO1.5)n, where n = 8,
the Si/O ratio is 2/3, while the organic R groups, which can
be the same or different, are linked to a silicon–oxygen
cage.
A typical POSS nanoparticle (Fig. 1) is thus formed by
an inorganic Si8O12 nanostructured cubic cage, surrounded
by eight organic groups such as alkyl, aryl, or any of their
derivatives, linked to the cage silicon atoms by covalent
bonds [13].
Keywords POSS ꢀ Polystyrene nanocomposites ꢀ
Thermal degradation ꢀ Thermogravimetric analysis
The nature of these side groups largely affects both the
solubility in conventional solvents and the compatibility
with polymer matrices, then determining their capability
to undergo nanometric dispersion into a host polymer,
as well as the thermal stability of obtained nanocomposites.
In particular, it has been reported in literature that POSSs
having aliphatic groups bonded with silicon cage show the
I. Blanco (&) ꢀ L. Abate ꢀ F. A. Bottino
Department of Industrial Engineering, University of Catania,
V.le A. Doria, 6, 95125 Catania, Italy
e-mail: iblanco@dii.unict.it
123

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I. Blanco et al.
Fig. 1 Molecular structure of
hcp-POSS a planar view,
b three-dimensional view
(a)
(b)
R
R
Si
O
Si
O
O
Si
O
Si
O
O
O
R
Si
O
Si
O
O
Si
O
Si
R
R
O
R
R
R
improvement of solubility and compatibility, however,
accompanied by the worsening of thermal properties, while
the presence of aromatic groups acts in the opposite way
[14].
thermal stability than neat polymer, which enhanced on
increasing filler concentration up to 5 % POSS about [20],
while it decreased as a consequence of a further filler
percentage increment, probably owing to more POSS–
POSS interactions [21].
Since some years we are interested in the study of
polystyrene (PS)-based nanocomposites [15] to obtain
products having better properties than virgin polymer, in
particular higher thermal stability, which is important
because these products can be subjected to high tempera-
tures during processing or in service. A fine dispersion at
molecular level of a POSS into polymer matrix is requested
to obtain the best exploitation of its properties when used
as filler for nanocomposites. Owing to this reason, we
focused our attention on asymmetric R7R0(SiO1.5)8, owing
to symmetric R8(SiO1.5)8 leads generally to auto-aggrega-
tion phenomena of POSS molecules [16]. So, the opportune
selection of R and R0 groups can allow to obtain products
where the chemical-physical filler-polymer interactions are
prevalent on the POSS–POSS ones.
In order to have a more complete picture of the thermal
behavior of the nanocomposites of our interest, in this work
we carried out a comparative study of the thermal stability
of the PS based nanocomposites having as fillers all the
cyclopentyl POSSs previously studied [18]. The formulae
of various POSSs are reported in Table 1. The samples to
be analyzed were prepared from reactant mixtures at 5 %
w/w POSS/PS, owing to this composition appeared to give
rise, according to some our preceding experiments [20], to
nanocomposites showing most enhanced thermal stability
than neat PS.
The characterization of PS and obtained nanocomposites
was made by 1H NMR spectroscopy, to verify the actual filler
content, and by differential scanning calorimetry (DSC) to
determine glass transition temperature (Tg). Also, inherent
viscosity (ginh) measurements were carried out to check if the
presence of various substituted POSSs affected the average
molar mass of PS. Thermogravimetric (TG) and differential
thermogravimetric (DTG) analysis, in both flowing nitrogen
and static air atmosphere, were finally carried out to study the
thermal behavior of our nanocomposites and to determine
the temperature at 5 % mass loss (T5 %).
To this purpose, in the first part of our research, we syn-
thesized and characterized some R7R0(SiO1.5)8 POSSs, where
R = cyclopentyl or isobutyl group and R0 = unsubstituted
C6H5– group or one of its mono- or di-substituted derivatives,
where the presence of seven cycloaliphatic or aliphatic groups
provides for a good compatibility with polymer [17].
Good thermal stability was found for both series of
studied POSSs with better results for the cyclopentyl
derivatives [18, 19].
Some preliminary investigations were thus carried out in
order to verify if and at which concentration our POSSs
give rise to nanocomposites with better thermal properties
than neat PS [20–22].
Experimental
Materials
Phenyl, hepta cyclopentyl-polyhedral oligomeric silses-
quioxane/polystyrene (ph,hcp-POSS/PS) nanocomposites
at various filler concentration were thus prepared by in situ
polymerization of styrene in the presence of POSS, and
their degradation was studied in both inert and oxida-
tive atmospheres. The results obtained evidenced higher
Styrene (Aldrich Co.) was purified by passing it through
an inhibitor removal column. 2,2-Azobis(isobutyronitrile)
(AIBN) (98 % Aldrich Co.) was re-crystallized twice from
dry ethanol at temperatures less than 40 °C and out of
direct light. Toluene was stirred over calcium hydride for
123

Variously substituted ph,hcp-POSS/polystyrene
423
Table 1 Molecular structure of the various ph,hcp-POSS used for the synthesis of PS nanocomposites
24 h and distilled in a nitrogen atmosphere. Tetrahydro-
furan (THF) was distilled over a Na–benzophenone
mixture.
70 °C for 24 h under stirring. The clear solution was
poured into a large excess of methanol (600 ml), the pre-
cipitated nanocomposite was collected by filtration and
dried under vacuum at 40 °C. The yield was 3.52 g (88 %).
The same polymerization procedure was used to prepare
neat PS and compounds 1 (yield 85 %), 3 (yield 83 %),
4 (yield 80 %), 5 (yield 86 %), and 6 (yield 86 %).
The preparation of variously substituted POSSs was
first carried out. To this aim, phenyltrichlorosilane and
p-tolyltrichlorosilane (Aldrich Co.) were used as received.
4-fluorophenyltrichlorosilane, 2,4-difluorophenyltrichlorosi
lane, 3,5-dimethylphenyltrichlorosilane, and 4-methoxyph-
enyltrichlorosilane were prepared from the appropriate
Grignard reagent and SiCl4 [23, 24]. Cyclopentyltrisilanol
(c C5H9)7–Si7O9 (OH)3 was prepared according to the lit-
erature methods [25, 26]. Substituted POSSs were prepared
by corner capping reaction of cyclopentyltrisilanol with the
suitable aryltrichlorosilane in dry THF. The same proce-
dure was used for all compounds. The details of prepara-
tion are reported in Ref. [18].
1H NMR spectroscopy
1H NMR spectra were recorded using a Varian Unity Inova
instrument (1H 500 MHz), using CDCl3 as solvent and
TMS as internal standard.
IR spectroscopy
Nanocomposites were then prepared by in situ poly-
merization of 5 % w/w POSS/styrene mixtures in toluene.
The details of free-radical polymerization procedure are
described, as an example, for compound 2.
Fourier transform infrared (FTIR) spectra were traced using
a Perkin–Elmer Spectrum 100 spectrometer, using an uni-
versal ATR sampling accessory. Spectra were recorded at
r.t. from 4000 to 600 cm-1 with a resolution of 4.0 cm-1
,
Styrene monomer (3.80 g) and 4-methoxyphenyl hcp-
POSS (0.20 g) were dissolved in 40 ml of toluene, and
AIBN radical initiator (12 mg) was added; the mixture was
frozen in a liquid nitrogen bath, degassed with a vacuum
pump, and thawed. This operation was repeated three times
and then the tube, sealed under vacuum, was heated at
directly on compounds, without any preliminary treatment.
Viscosity measurements
An Ubbelohde suspended-level viscosimeter and solutions
of polymers in chloroform at the concentration of 1.0 g dL-1
123

424
I. Blanco et al.
and at temperature of 25 0.1 °C were used to measure
inherent viscosities (ginh = lngr/C, where gr = relative
viscosity and C = polymer concentration).
D = (Wo - W)/Wo, and Wo and W were the masses at the
starting point and during scanning.
DSC measurements
Results and discussion
A Mettler DSC 20 differential scanning calorimeter, cou-
pled with a Mettler TC 10 A processor, was used for glass
transition temperature (Tg) determinations. The response of
apparatus in enthalpy and temperature was calibrated
according to the procedure suggested by the manufacturer
[27]. To this purpose, two built-in programs of processor,
which use the fusion of indium for the calibration of
enthalpy and the melting points of three metals, indium,
lead, and zinc, for that of temperature, were employed. The
calibration of enthalpy, checked by the melting of fresh
indium, showed an agreement within 0.25 % with the lit-
erature standard value of 3.273 kJ mol-1 [28, 29], while
the accuracy of temperature, checked by several scans with
fresh indium and tin, was in every case within 0.08 % in
respect to literature data (156.6 and 231.9 °C for indium
and tin, respectively) [28, 29].
Since previous experiments made by us [20, 22] showed
that the filler percentages in nanocomposites obtained by
in situ polymerization of styrene were slightly higher than
those in reactant mixtures, the actual POSS contents in the
nanocomposites prepared starting from mixtures at 5 %
1
w/w POSS/PS were thus determined by H NMR spec-
troscopy. The filler percentages were calculated through
the ratio of hydrogen atoms of filler and those of PS. All
these values were quite in agreement with each other, but
all were slightly higher than 5 % (Table 2).
The FTIR spectra of 1–6 compounds were thus carried
out and compared with those of neat PS and corresponding
POSSs. In all cases, the shift of the sharp band near
1080 cm-1, present in the POSSs spectra and attributable
to Si–O bonds, was observed in the nanocomposites
spectra, thus indicating the presence of filler-polymer
interactions. Since no substantial differences were found
among various investigated POSS, for the sake of shortness
we report here the spectra concerning the compound 6 only
(Fig. 2), in which this band shifts from 1078 cm-1 (for the
POSS) to 1094 cm-1(for the nanocomposite).
The calibrations were repeated every two weeks. Sam-
ples of about 5.0 9 10-3 g, held in sealed aluminum cru-
cibles, a heating rate of 10 °C min-1 and flowing nitrogen
(0.02 L min-1) were used for measurements.
TG analysis
In order to evaluate a possible dependence of nano-
composites thermal stability on polymer average molar
mass, inherent viscosity determinations on studied com-
pounds were thus performed and compared with that of
neat PS. Only the ginh values found for ph,hcp-POSS and
4-methoxy ph,hcp-POSS were the same than that of virgin
polymer, thus indicating that, in these cases, the average
molar mass of PS in nanocomposites was the same than
that of neat polymer. The ginh values found for the
methyl- and dimethyl-derivatives were little lower than
PS. Conversely, the inherent viscosities of F-nanocom-
posite and even more of diF-nanocomposite were much
lower than that of neat polymer, thus suggesting a sharp
decrease of PS average molar mass and then that fluori-
nated POSSs affect the polymerization process. The ginh
values of PS and studied nanocomposites are listed in
Table 2.
The thermal degradations of the studied compounds were
carried out into a Mettler TA 3000 thermobalance, coupled
with the same Mettler TC 10 A processor as control and
evaluation unit used for DSC measurements. The temper-
ature of furnace was calibrated according to the procedure
suggested by Mettler and reported in the user’s manual of
equipment [27]. This procedure, which is considered in
literature the most precise [30], is based on the change of
magnetic properties of three metal samples (Isatherm,
Nickel and Trafoperm) at their Curie points (142.5, 357.0,
and 749.0 °C, respectively). The temperature calibration
was repeated every month.
Degradation experiments were carried out in dynamic
heating conditions, from 35 up to 700 °C, in both flowing
nitrogen (0.02 L min-1), and a static air atmosphere,
at the heating rate (U) of 10 °C min-1. Samples of about
5 9 10-3 g, held in alumina open crucibles, were used and
their masses were measured as a function of temperature and
stored in the list of data of the appropriate built-in program
of processor. The TG and DTG curves were immediately
printed at the end of each experiment and the mass of sample
at various temperatures were then transferred to a PC. These
data were later used to plot the percentage of undegraded
sample (1 - D)% as a function of temperature, where
The glass transition temperatures, which were also
calorimetrically determined, exhibited a trend quite similar
to that of inherent viscosity. Since ginh and Tg are not only
characteristic parameters of polymers but also their values
are connected with the average molecular mass, the similar
trend found for these two parameters indicates that the
observed variations are driven by the same single factor,
it means the average molar mass. Also, the Tg values are
reported in Table 2.
123

Variously substituted ph,hcp-POSS/polystyrene
425
The initial decomposition temperature (Ti) is considered
the key parameter to evaluate the resistance of polymer
materials to thermal degradation [31, 32]. This parameter is
obtained by the degradation TG curves as the intersection
between the starting mass line and the maximum gradient
tangent to the curve [33–35]. Since the Ti values so
obtained largely depend on the slope of the descending
piece of degradation TG curve, we preferred to determine
temperature at 5 % mass loss which is a parameter corre-
lated with initial decomposition temperature [36], but
which is, in our opinion, more reliable to compare the
thermal stability of various compounds because it is not
depending on the kinetics of the most advanced degrada-
tion stages.
thermal degradations and is the same of that of our calo-
rimetric Tg determinations. PS and all studied nanocom-
posites degraded in flowing nitrogen up to complete mass
loss. The corresponding TG degradation curves are repor-
ted in Fig. 3, while the determined T5 % values are listed in
Table 2. Even though PS and its nanocomposites degraded
in all cases up to complete mass loss, the TG curves
exhibited a difference: PS and dimethyl-nanocomposite
degraded completely in a single step, while other nano-
composites showed a first very sharp degradation stage,
followed by a second one at low degradation rate in the last
piece of TG curves, which is more evident for the com-
pounds 1, 5, and 6 (Fig. 3). The behavior of our compounds
in oxidative atmosphere (Fig. 4) was quite similar to that
under nitrogen, and the corresponding T5 % values are also
listed in Table 2.
Degradation experiments were first carried out in inert
atmosphere, in dynamic heating conditions, at the heating
rate of 10 °C min-1, which was selected because it is a
medium scanning rate among those usually employed for
Also, we performed the FTIR spectra of the residues in
air at 430 °C. This temperature, at which the samples mass
is reduced to about the 10 % of the initial one, was selected
because it is roughly the starting temperature of the low
rate degradation stage. The FTIR spectra of the residues so
obtained were compared with that of the residue at 380 °C
(10 % about of initial sample mass) obtained from the
degradation of neat PS and those of corresponding POSSs.
The presence in the FTIR spectra of the residues of
nanocomposites of the band at 697 cm-1 and of some weak
bands in the 2900–3100 cm-1 range, which are present in
PS spectrum, as well as of the band attributable to Si–O
bonds, indicates that, during the second low rate degrada-
tion stage of nanocomposites, both polymer and filler are
present at temperatures largely higher than that at which
the complete decomposition of neat PS occurs (Fig. 4),
thus indicating a further thermal stabilization of polymer.
This finding confirms our previous results [20]. The FTIR
spectra of the residues at 430 °C of compound 6 and the
Table 2 POSS percentages, inherent viscosity (ginh), glass transition
temperature (Tg), temperature at 5 % mass loss (T5 %) of PS and
various nanocomposites in static air atmosphere, and in flowing
nitrogen
Compounds Air static atmosphere
Nitrogen
flow
T5 %/K
POSS/%
g
inh/dL g-1 Tg/K
T5 %/K
PS
1
–
0.17
0.17
0.17
0.16
0.16
0.15
0.12
372
372
373
369
369
370
368
511
558
569
539
509
556
550
573
606
588
581
565
581
570
5.9
6.8
6.4
6.6
6.8
6.9
2
3
4
5
6
Fig. 2 FTIR spectra of 2,4-
difluorophenyl hcp-POSS (a),
sample 6 (b) and PS (c)
a
1078
b
1094
c
4000.0 3600
3200
2800
2400
2000
1800 1600
1400
1200
1000
800 650.0
Wavenumber cm–1
123

426
I. Blanco et al.
100
corresponding POSS are reported as an example in Fig. 5,
100
80
together with that at 380 °C of PS.
80
60
40
20
0
The data in Table 2 suggest some considerations:
–
–
the T5 % values obtained for the unsubstituted 5 %
ph,hcp-POSS/PS nanocomposite were largely higher
(33 and 47 °C in flowing nitrogen and static air
atmosphere, respectively) than those of neat PS, thus
indicating a strongly enhanced resistance to the thermal
degradation of nanocomposite in respect to virgin polymer;
the introduction of a methoxy group on the POSS phenyl
ring does not lead to any change of the inherent viscosity
with respect to that of unsubstituted ph,hcp-POSS/PS.
Conversely, the introduction of a methyl group and,
more, of a fluorine atom, leads to the decrease of ginh
value, which is larger if two fluorine atoms are
introduced. These results indicate that the presence of
fluorinated POSSs reduces the PS polymerization degree
and appear in agreement with Tg determinations owing to
it is well known that glass transition temperature of
polymers increases on increasing average molar mass
[37];
60
200
300
400
Temperature/°C
PS
1
2
3
4
5
6
0
100
200
300
400
500
600
700
Temperature/°C
Fig. 3 TG degradation curves at 10 °C min-1, under nitrogen flow,
of PS and various ph,hcp-POSS/PS nanocomposites
100
100
80
80
–
the unsubstituted ph,hcp-POSS/PS nanocomposite
appears the most thermally stable in inert environment.
The introduction of one substituent on the POSS phenyl
group lowers the thermal stability of obtained nanocom-
posite independently on its electron-donor (as methyl
or methoxy group) or electron-withdrawing (as fluorine
atom) character, such as shown by the decrease in
T5 % values, which however is still higher than that of
neat PS;
60
60
PS
200
300
400
Temperature/°C
1
2
3
4
5
6
40
20
0
0
100
200
300
400
500
600
700
–
the introduction on the POSS phenyl group of a second
substituent, identical to the first one, leads to a further
Temperature/°C
Fig. 4 TG degradation curves at 10 °C min-1, in static air atmo-
sphere, of PS and various ph,hcp-POSS/PS nanocomposites
T5 % decrease, but, in this case, up to lower values than
that of virgin polymer;
Fig. 5 FTIR spectra of the
residues of 2,4-difluorophenyl
hcp-POSS (a), sample 6 (b), and
PS (c) after thermal degradation
in static air atmosphere
a
1039
3027
2868
2948
b
c
697
1029
3026
2925
2850
2800
697
800 650.0
4000.0 3600
3200
2400 2000
1800 1600
1400
1200
1000
Wavenumber cm–1
123

Variously substituted ph,hcp-POSS/polystyrene
427
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