# Properties Enhancement of PS Nanocomposites through the POSS Surfactants

Authors: Huei-Kuan Fu; Shiao-Wei Kuo; Ding-Ru Yeh; Feng-Chih Chang
Journal: Journal of Nanomaterials
 (2008)

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## Abstract

Polyhedral oligomeric silsesquioxane (POSS)-clay hybrids of polystyrene are prepared by two organically modified clays using POSS-NH2 and  C20-POSS as intercalated agents. X-ray diffraction (XRD)
studies show the formation of these POSS/clay/PS
nanocomposites in all cases with the disappearance of the peaks
corresponding to the basal spacing of MMT. Transmission electronic
microscopy (TEM) was used to investigate the
morphology of these nanocomposites and indicates that these
nanocomposites are composed of a random dispersion
of exfoliated clay platelets throughout the PS matrix. Incorporation of these
exfoliated clay platelets into the PS matrix led to effectively
increase in glass transition temperature (Tg), thermal decomposition temperature (Td), and the maximum reduction in coefficient of thermal expansion (CTE) is ca. 40% for the C20-POSS/clay nanocomposite.


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## Body

## 1. Introduction

Organic-inorganic hybrid materials are
recognized as a new class of advanced material  because they can be   synthesized
or processed using versatile approaches and own tunable  properties [1, 2]. Clays
have been   extensively used as  reinforcement agents  to prepare polymer-layered
silicate nanocomposites with improved thermal and mechanical properties [3–9]. The
incorporation of clay into polymer matrix imparts unique physical and chemical
properties was first reported by the Toyota Research Lab for Nylon 6/organoclay
nanocomposites [10]. These improvements are related to the dispersion of the
layered silicate in the polymer matrix. Typically, the chemical structure of
montmorillonite (MMT) consists of two fused silica tetrahedral sheets
sandwiching an edge-shared octahedral sheet of either magnesium or aluminum
hydroxide. Generally, the naturally occurring clays are hydrophilic characters and require a
modification by intercalating with amino acid, alkylammonium, or phosphonium
salts to become organically compatible [11, 12]. The resulting organophilic
galleries of the organically modified montmorillonite (OMMT) will enhance the
compatibility with polymer [13]. Several methods to make polymer clay
nanocomposites have been demonstrated, including solution mixing, melt
blending, and in situ polymerization [14–16]. The interaction of layered
silicates with polymers leads to two classes of hybrid materials. In the first
class, denoted as intercalated hybrids, one or more polymer chains are inserted
between the host layers, generating ordered lamella with the distance of a few
nanometers. In the second, described as delaminated hybrids, silicate layers of
1 nm thickness are exfoliated and dispersed in the polymer matrix.Polyhedral oligomeric silsesquioxane
(POSS) reagents, monomers, and polymers are emerging as new chemical feedstocks
for the preparation of organic-inorganic nanocomposites [17–21]. Silsesquioxane is the term for all structures with
the formula (RSiO1.5)n,
where R is hydrogen or any alkyl, alkylene, aryl, arylene, or organic functional
derivative groups. POSS compounds with diameters of 1–3 nm can be
possibly considered the smallest particles of silica, but unlike silica,
silicones, or fillers, POSS molecules contain either functionalized or
unfunctionalized substituents at each of the corner silicon atoms. These
substituents can compatibilize POSS molecules with polymers or monomers. POSS macromonomer and POSS-containing polymers inspires
to prepare the functionalized POSS cages, which can be used as the
intercalating agents of layered silicates to prepare the nanocomposites
combining the two types of nanoreinforcement agents and improved mechanical
and thermal properties.In this study, these
nanocomposites were prepared by emulsion polymerization using the POSS-NH2 and C20-POSS-treated clays as shown in Scheme 1. The morphology and
the extent of delamination of the nanocomposites are elucidated using the X-ray
diffraction and transmission electron microscopy. The thermal properties of
these nanocomposites are characterized by thermalgravimetric analysis and
differential scanning calorimetry. The coefficient of thermal expansion of virgin
PS and nanocomposites are measured by thermal mechanical analyzer.Scheme 1
Chemical structures of the
intercalated agents used to prepare the modified clays.


## 2. Experimental

### 2.1. Materials

The sodium montmorillonite (Na+-MMT)
with 1.45 mequiv/g cationic exchange capacity (CEC) was provided by Nanocor Co.
(Ill, USA). The majority of chemicals used in
this study including, acetone, methanol, tetrahydrofuran, acetonitrile, potassium
hydroxide (KOH), triethylamine, and trichloro [4-(chloromethyl)phenyl] silane were
acquired from  Sigma-Aldrich Chemical Co., Inc. The styrene monomer was purchased
from Sigma-Aldrich Chemical Co. and purified by removing the inhibitor with the
aid of an inhibitor-removal column. Sodium dodecyl sulfate (SDS) and
hydrochloride acid (HCl) were both obtained from Curtin Matheson Scientific, Inc. (Tex,
USA).
Potassium persulfate (K2S2O8) and aluminum
sulfate  [Al2(SO4)3] were acquired from
Fisher Scientific, Inc. (Pittsburgh, USA).  N, N-dimethyloctadecylamine (C20)
was obtained from Acros Organics, (NJ, USA). Trisilanolisobutyl polyhedral oligomeric
silsesquioxane (T7-POSS) and aminopropylisobutyl polyhedral oligomeric silsesquioxane
(POSS-NH2) were  obtained from  Hybrid Plastic Corporate Headquarters Inc. (USA).  All reagents, except styrene, were used as
received without further purification.


### 2.2. Preparation of C20-POSS Intercalated Agent

The POSS-Cl compound
was prepared by the method based on Scheme2 [22]. Trisilanolisobutyl
polyhedral oligomeric silsesquioxane (3 g)
and Et3N (1.26 g)
were added into a 100 mL two-neck round bottom flask and stirred continuously for 3 hours  under
nitrogen, then 20 mL of THF was added into
the flask at 0°C for 1 hour. After stirring at 0°C
under nitrogen, triethylamine, and trichloro[4-(chloromethyl) phenyl]silane(1.28 g) in THF (10 mL) were added
dropwisely into the solution and stirred at 0°C. The cooling bath was removed
and stirred continuously for 7.5 hours under nitrogen. The POSS-Cl compound and
HNEt3-Cl byproduct were separated by filtration. The clear THF
solution was dropped into a beaker of acetonitrile and rapidly stirred. The
resulting product was collected and dried in a vacuum   oven for 24 hours. 1H NMR (i.e., proton-NMR spectroscopy)
(CDCl3),δ: 7.59 (d, 2H), 7.33 (d, 2H), 4.52 (s, 2H), 1.92–1.62 (m, 7H), 1.09–0.85 (m, 42H), 0.75–0.48  (m, 14H).Scheme 2
Synthesis of POSS-Cl compound.Scheme 3
Synthesis of theC20-POSS intercalated
agent.The intercalated
agent ofC20-POSS was prepared as shown in Scheme 2. N, N-dimethyloctadecylamine
(C20, 1.49 g) and POSS-Cl
compound (5.68 g) in
acetone (15 mL) were refluxed 80°C under
nitrogen for 24 hours. After cooling, the mixture was evaporated by a rotatory
evaporator and then added Et2O (20 mL). The solution was extracted with deionized water three times (150 mL×3). The organic phase was dried
with MgSO4 and evaporated by arotatory evaporator to
obtain the C20-POSS intercalated agent.
1H NMR (CDCl3), δ:7.68 (d, 2H), 7.58 (d, 2H), 5.03 (s, 2H), 3.42 (t,
2H), 3.30 (s, 6H), 1.81 (m, 7H), 1.41 (m, 2H), 1.26 (m, 30H), 0.90 (m, 42H), 0.82
(t, 3H), 0.59 (m, 14H).


### 2.3. Preparation of POSS-NH2 and C20-POSS
Modified Clays

Na+MMT (0.3 g) in deionized water (50 mL) was stirred
continuously at 80°C for 4 hours. The POSS-NH2(0.38 g) or C20-POSS (0.63 g) in water (5 mL) was placed into another flask and then 10%
hydrochloric acid (1 mL) and ethanol (5 mL) were added and stirred at 80°C
for 1 hour. This intercalated solution was poured slowly into the clay suspension
solution and stirred vigorously at 80°C for 4 hours. The resulting white precipitate
was separated by filtration and then washed thoroughly with warm deionized
water. The final product was dried in a
vacuum oven at
room temperature overnight.


### 2.4. Preparation of Polystyrene/Clay Nanocomposites

Na+-MMT (0.3 g) was dispersed in 40 mL of deionized
water and stirred at 80°C for 4 hours. Based on the CEC value (145mequiv/100g) of the Na+-MMT and the Mw of
the intercalated agent, the calculated weight (the weight of the intercalated agent
needed to fully replace the Na+of the clay) of the intercalated
agent was added and stirred for 4 hours. KOH (0.02 g), SDS (0.4 g),  K2S2O8(0.05 g), and styrene monomer (10 g)
were added into the solution. After emulsification, the dispersion was flushed
with nitrogen for 30 minutes while the temperature was raised to 80°C
under nitrogen protection. Polymerization was carried out at 80°C for 8 hours. After cooling, 10 mL of the
2.5% aqueous aluminum sulfate was added into the polymerized emulsion, followed
by dilute hydrochloric acid (10 mL) with stirring. Finally, acetone was added to
break down the emulsion completely and then the polymer product was washed
several times with methanol and deionized water. The white powder was filtered and
dried in a vacuum oven at 80°C for 24 hours. Similar procedures were
employed to prepare the virgin polystyrene.


### 2.5. Instrumentations

#### 2.5.1. Measurement of the Molecular Weights Characterization

Molecular weight (Mw), number-average(Mn) molecular weight, and polydispersity
index (Mw/Mn) were measured using a
Waters 410 gel permeation chromatography (GPC) system   equipped with RI and UV
detectors and a series of styragel columns (100, 500, and 103Å).
The system was calibrated using polystyrene standards. These polymer chains
were extracted from the clay surface using a reverse ion exchange reaction with
LiCl/DMF to determine the molecular weight and molecular weight distribution.


#### 2.5.2. Structure Analysis Characterization

1H NMR spectra were recoded in CDCl3 on a Bruker AM 500 (500 MHz) spectrometer using the solvent signal as an
internal standard. FT-IR spectra were recorded using a Nicolet Avatar 320 FT-IR spectrometer; 32 scans were
collected at a spectral resolution of 1 cm−1. The modified clay was mixed with KBr pellets to press into the small flakes and
dried at 70°C
for 24 hours. The holder was placed in the sample chamber and spectrum was
recorded under N2 purge to maintain the test of the sample dryness.Transmission electron microscopy (TEM)
images were obtained on a Hitachi H-7500 operating at 100 kV. The sample was
thin section to ~70 nm by a Leica ultracut UCT microtome. Wide-angle X-ray diffraction
(WAXD) experiments were carried on a Rigaku D/max-2500 type X-ray diffraction instrument
with Cu Kα radiation (λ = 1.54 Å) using an Ni-filter. Data were recorded in the
range of 2θ = 1 to 20 at the
scanning rate of 0.6°/min.


#### 2.5.3. Thermal and Mechanical Analysis Characterization

Thermal stability of nanocomposite was
investigated by a TA Instruments Q50 apparatus. The sample ~5 mg was placed in
a Pt cell with scan rate of 20°C/min from 30 to 800°C
under a 40 mL/min flow of nitrogen gas. Thermal analysis through differential
scanning calorimetry (DSC) was performed using a Du-Pont (DSC-2010) to measure the
glass transition temperature (Tg) of the nanocomposite. The sample
was preheated at a scan rate of 20°C/min from 30 to 150°C
under a nitrogen atmosphere. A small sample (ca. 5–10 mg) was
weighted and sealed in an aluminum pan. The sample was quickly cooled to 10°C
from the first scan and then scanned between 30 and 150°C
at the scan rate of 20°C/min. The glass transition temperatures
are taken as the
midpoint of the heat capacity transition between the upper and lower points of
deviation from the  extrapolated glass and liquid lines. The coefficient of thermal
expansion (CTE) was measured using a thermomechanical analyzer (TMA TA 2940) by
recording the change in dimension of the specimen with temperature. The
specimen was heated from 25 to 150°C at a heating rate of 5°C/min.


## 2.1. Materials

The sodium montmorillonite (Na+-MMT)
with 1.45 mequiv/g cationic exchange capacity (CEC) was provided by Nanocor Co.
(Ill, USA). The majority of chemicals used in
this study including, acetone, methanol, tetrahydrofuran, acetonitrile, potassium
hydroxide (KOH), triethylamine, and trichloro [4-(chloromethyl)phenyl] silane were
acquired from  Sigma-Aldrich Chemical Co., Inc. The styrene monomer was purchased
from Sigma-Aldrich Chemical Co. and purified by removing the inhibitor with the
aid of an inhibitor-removal column. Sodium dodecyl sulfate (SDS) and
hydrochloride acid (HCl) were both obtained from Curtin Matheson Scientific, Inc. (Tex,
USA).
Potassium persulfate (K2S2O8) and aluminum
sulfate  [Al2(SO4)3] were acquired from
Fisher Scientific, Inc. (Pittsburgh, USA).  N, N-dimethyloctadecylamine (C20)
was obtained from Acros Organics, (NJ, USA). Trisilanolisobutyl polyhedral oligomeric
silsesquioxane (T7-POSS) and aminopropylisobutyl polyhedral oligomeric silsesquioxane
(POSS-NH2) were  obtained from  Hybrid Plastic Corporate Headquarters Inc. (USA).  All reagents, except styrene, were used as
received without further purification.


## 2.2. Preparation of C20-POSS Intercalated Agent

The POSS-Cl compound
was prepared by the method based on Scheme2 [22]. Trisilanolisobutyl
polyhedral oligomeric silsesquioxane (3 g)
and Et3N (1.26 g)
were added into a 100 mL two-neck round bottom flask and stirred continuously for 3 hours  under
nitrogen, then 20 mL of THF was added into
the flask at 0°C for 1 hour. After stirring at 0°C
under nitrogen, triethylamine, and trichloro[4-(chloromethyl) phenyl]silane(1.28 g) in THF (10 mL) were added
dropwisely into the solution and stirred at 0°C. The cooling bath was removed
and stirred continuously for 7.5 hours under nitrogen. The POSS-Cl compound and
HNEt3-Cl byproduct were separated by filtration. The clear THF
solution was dropped into a beaker of acetonitrile and rapidly stirred. The
resulting product was collected and dried in a vacuum   oven for 24 hours. 1H NMR (i.e., proton-NMR spectroscopy)
(CDCl3),δ: 7.59 (d, 2H), 7.33 (d, 2H), 4.52 (s, 2H), 1.92–1.62 (m, 7H), 1.09–0.85 (m, 42H), 0.75–0.48  (m, 14H).Scheme 2
Synthesis of POSS-Cl compound.Scheme 3
Synthesis of theC20-POSS intercalated
agent.The intercalated
agent ofC20-POSS was prepared as shown in Scheme 2. N, N-dimethyloctadecylamine
(C20, 1.49 g) and POSS-Cl
compound (5.68 g) in
acetone (15 mL) were refluxed 80°C under
nitrogen for 24 hours. After cooling, the mixture was evaporated by a rotatory
evaporator and then added Et2O (20 mL). The solution was extracted with deionized water three times (150 mL×3). The organic phase was dried
with MgSO4 and evaporated by arotatory evaporator to
obtain the C20-POSS intercalated agent.
1H NMR (CDCl3), δ:7.68 (d, 2H), 7.58 (d, 2H), 5.03 (s, 2H), 3.42 (t,
2H), 3.30 (s, 6H), 1.81 (m, 7H), 1.41 (m, 2H), 1.26 (m, 30H), 0.90 (m, 42H), 0.82
(t, 3H), 0.59 (m, 14H).


## 2.3. Preparation of POSS-NH2 and C20-POSS
Modified Clays

Na+MMT (0.3 g) in deionized water (50 mL) was stirred
continuously at 80°C for 4 hours. The POSS-NH2(0.38 g) or C20-POSS (0.63 g) in water (5 mL) was placed into another flask and then 10%
hydrochloric acid (1 mL) and ethanol (5 mL) were added and stirred at 80°C
for 1 hour. This intercalated solution was poured slowly into the clay suspension
solution and stirred vigorously at 80°C for 4 hours. The resulting white precipitate
was separated by filtration and then washed thoroughly with warm deionized
water. The final product was dried in a
vacuum oven at
room temperature overnight.


## 2.4. Preparation of Polystyrene/Clay Nanocomposites

Na+-MMT (0.3 g) was dispersed in 40 mL of deionized
water and stirred at 80°C for 4 hours. Based on the CEC value (145mequiv/100g) of the Na+-MMT and the Mw of
the intercalated agent, the calculated weight (the weight of the intercalated agent
needed to fully replace the Na+of the clay) of the intercalated
agent was added and stirred for 4 hours. KOH (0.02 g), SDS (0.4 g),  K2S2O8(0.05 g), and styrene monomer (10 g)
were added into the solution. After emulsification, the dispersion was flushed
with nitrogen for 30 minutes while the temperature was raised to 80°C
under nitrogen protection. Polymerization was carried out at 80°C for 8 hours. After cooling, 10 mL of the
2.5% aqueous aluminum sulfate was added into the polymerized emulsion, followed
by dilute hydrochloric acid (10 mL) with stirring. Finally, acetone was added to
break down the emulsion completely and then the polymer product was washed
several times with methanol and deionized water. The white powder was filtered and
dried in a vacuum oven at 80°C for 24 hours. Similar procedures were
employed to prepare the virgin polystyrene.


## 2.5. Instrumentations

### 2.5.1. Measurement of the Molecular Weights Characterization

Molecular weight (Mw), number-average(Mn) molecular weight, and polydispersity
index (Mw/Mn) were measured using a
Waters 410 gel permeation chromatography (GPC) system   equipped with RI and UV
detectors and a series of styragel columns (100, 500, and 103Å).
The system was calibrated using polystyrene standards. These polymer chains
were extracted from the clay surface using a reverse ion exchange reaction with
LiCl/DMF to determine the molecular weight and molecular weight distribution.


### 2.5.2. Structure Analysis Characterization

1H NMR spectra were recoded in CDCl3 on a Bruker AM 500 (500 MHz) spectrometer using the solvent signal as an
internal standard. FT-IR spectra were recorded using a Nicolet Avatar 320 FT-IR spectrometer; 32 scans were
collected at a spectral resolution of 1 cm−1. The modified clay was mixed with KBr pellets to press into the small flakes and
dried at 70°C
for 24 hours. The holder was placed in the sample chamber and spectrum was
recorded under N2 purge to maintain the test of the sample dryness.Transmission electron microscopy (TEM)
images were obtained on a Hitachi H-7500 operating at 100 kV. The sample was
thin section to ~70 nm by a Leica ultracut UCT microtome. Wide-angle X-ray diffraction
(WAXD) experiments were carried on a Rigaku D/max-2500 type X-ray diffraction instrument
with Cu Kα radiation (λ = 1.54 Å) using an Ni-filter. Data were recorded in the
range of 2θ = 1 to 20 at the
scanning rate of 0.6°/min.


### 2.5.3. Thermal and Mechanical Analysis Characterization

Thermal stability of nanocomposite was
investigated by a TA Instruments Q50 apparatus. The sample ~5 mg was placed in
a Pt cell with scan rate of 20°C/min from 30 to 800°C
under a 40 mL/min flow of nitrogen gas. Thermal analysis through differential
scanning calorimetry (DSC) was performed using a Du-Pont (DSC-2010) to measure the
glass transition temperature (Tg) of the nanocomposite. The sample
was preheated at a scan rate of 20°C/min from 30 to 150°C
under a nitrogen atmosphere. A small sample (ca. 5–10 mg) was
weighted and sealed in an aluminum pan. The sample was quickly cooled to 10°C
from the first scan and then scanned between 30 and 150°C
at the scan rate of 20°C/min. The glass transition temperatures
are taken as the
midpoint of the heat capacity transition between the upper and lower points of
deviation from the  extrapolated glass and liquid lines. The coefficient of thermal
expansion (CTE) was measured using a thermomechanical analyzer (TMA TA 2940) by
recording the change in dimension of the specimen with temperature. The
specimen was heated from 25 to 150°C at a heating rate of 5°C/min.


## 2.5.1. Measurement of the Molecular Weights Characterization

Molecular weight (Mw), number-average(Mn) molecular weight, and polydispersity
index (Mw/Mn) were measured using a
Waters 410 gel permeation chromatography (GPC) system   equipped with RI and UV
detectors and a series of styragel columns (100, 500, and 103Å).
The system was calibrated using polystyrene standards. These polymer chains
were extracted from the clay surface using a reverse ion exchange reaction with
LiCl/DMF to determine the molecular weight and molecular weight distribution.


## 2.5.2. Structure Analysis Characterization

1H NMR spectra were recoded in CDCl3 on a Bruker AM 500 (500 MHz) spectrometer using the solvent signal as an
internal standard. FT-IR spectra were recorded using a Nicolet Avatar 320 FT-IR spectrometer; 32 scans were
collected at a spectral resolution of 1 cm−1. The modified clay was mixed with KBr pellets to press into the small flakes and
dried at 70°C
for 24 hours. The holder was placed in the sample chamber and spectrum was
recorded under N2 purge to maintain the test of the sample dryness.Transmission electron microscopy (TEM)
images were obtained on a Hitachi H-7500 operating at 100 kV. The sample was
thin section to ~70 nm by a Leica ultracut UCT microtome. Wide-angle X-ray diffraction
(WAXD) experiments were carried on a Rigaku D/max-2500 type X-ray diffraction instrument
with Cu Kα radiation (λ = 1.54 Å) using an Ni-filter. Data were recorded in the
range of 2θ = 1 to 20 at the
scanning rate of 0.6°/min.


## 2.5.3. Thermal and Mechanical Analysis Characterization

Thermal stability of nanocomposite was
investigated by a TA Instruments Q50 apparatus. The sample ~5 mg was placed in
a Pt cell with scan rate of 20°C/min from 30 to 800°C
under a 40 mL/min flow of nitrogen gas. Thermal analysis through differential
scanning calorimetry (DSC) was performed using a Du-Pont (DSC-2010) to measure the
glass transition temperature (Tg) of the nanocomposite. The sample
was preheated at a scan rate of 20°C/min from 30 to 150°C
under a nitrogen atmosphere. A small sample (ca. 5–10 mg) was
weighted and sealed in an aluminum pan. The sample was quickly cooled to 10°C
from the first scan and then scanned between 30 and 150°C
at the scan rate of 20°C/min. The glass transition temperatures
are taken as the
midpoint of the heat capacity transition between the upper and lower points of
deviation from the  extrapolated glass and liquid lines. The coefficient of thermal
expansion (CTE) was measured using a thermomechanical analyzer (TMA TA 2940) by
recording the change in dimension of the specimen with temperature. The
specimen was heated from 25 to 150°C at a heating rate of 5°C/min.


## 3. Results and Discussion

### 3.1. Morphologies of Modified Clays and Nanocomposites

Microstructures
of polymer-layered silicate nanocomposites were characterized by XRD and TEM. Figure4 shows the X-ray diffraction curves of the pristine clay and modified clays in
the 2θ region of 2–10°. For the
pristine clay, the Bragg diffraction peak at 2θ = 6.92° corresponds to d-spacing of
1.28 nm. For the POSS-NH2/clay and C20-POSS/clay, the 2θ
value shifts from 6.92° (1.28 nm) to 5.51° (1.61 nm) and 2.33° (3.80 nm) after
ion exchange, indicating that the basal spacing is expanded as the sodium
cations in the interlayer galleries are replaced by intercalated agents of POSS-NH2 and C20-POSS. The increase of the basal spacing indicates that the
clay can be efficiently intercalated by POSS-NH2 and C20-POSS.
The  d-spacing of the C20-POSS-modified clay is substantially
greater than the POSS-NH2-modified clay. Larger interlayer spacing favors
the penetration of styrene monomer and the formation of exfoliated nanocomposite
by providing more hydrophobic environment. The pure POSS-NH2 has
characteristic diffraction peaks arising from the aggregation of the POSS [23].
Figure 5 shows XRD patterns of the unmodified clay and the C20-POSS
modified clay. The pure C20-POSS has a broad peak in the region of 5–12° arising from
the C20 long aliphatic chain. When the C20-POSS is
inserted between  the galleries of the clay, the d spacing is increased from 1.28 nm for original
clay to 3.80 nm, implying that the organic modifier is incorporated between and
pushing the clay layers. Both nanocomposites do not show XRD diffraction peak
as shown in Figure 6, indicating the silicate layers are exfoliated in the
polymer matrix. TEM images for POSS-NH2/clay and C20-POSS/clay
nanocomposites at 3% inorganic clay loading are shown in Figures 7(a) and 7(b),
indicating that the exfoliated clay platelets are distributed in the matrix
homogeneously and randomly.


### 3.2. Fourier Transfer Infrared Analyses

The representative FT-IR spectra of the organophilic
clay, POSS-NH2-modified and C20-POSS modified clays are
given in Figure 1. After ion exchange, FT-IR spectroscopy can provide important
information regarding the difference between intercalated agents and modified
clays. In Figure 1(a), characteristic vibration bands of the pure clay are 1030 cm−1(Si–O), 520 cm−1 (Al–O), and 470 cm−1 (Mg–O) [24–26]. In Figure
1(b), the absorption peaks in the region of 2950–2800 is assigned
to the stretching vibration of aliphatic C–H. The symmetrical Si–O–Si band in
the silsequioxane cage is characterized by the stretching band at 1109 cm−1. In Figure 1(d), C20-POSS
contains both alkyl chain and POSS moiety where the POSS moiety exhibits
characteristic absorption peaks at 2950–2800 cm−1 (C–H bonds), 1230 cm−1 (Si–C bonds), 1109 cm−1 (Si–O–Si bonds of the
cage structure). The characteristics
of  the
vibration band of alkyl chain appear at 2920, 2850, and 1475 cm−1 (–CH2–vibration
bands). Figures 1(c) and 1(e) show the features of combination of
characteristic bands of pure clay, POSS-NH2, and C20-POSS.
IR analysis further confirms the existence of these intercalated agents in these
intercalated clay samples, implying that these intercalations of the intercalated agents are indeed
present within the gallery gap. These observations support the explanation in
the earlier observation from XRD.Figure 1
IR spectra of the two intercalated agent,
intercalated clay, and pure clay.


### 3.3. Thermal Properties

Figure2
presents DSC traces of these nanocomposites with different intercalated agents.
All DSC thermograms display single glass transition
temperatures in the   experimental temperature range. The glass transition
temperature of PS occurs at 100°C. With the
addition of the POSS and C20-POSS modified MMT to the polymer
matrix, the glass transition temperatures (Tgs) of the POSS-NH2/clay/PS
and C20-POSS/clay/PS are 108, and 105°C,
respectively. From the DSC results that the incorporation of the organoclay resulted in
an   increase in the Tg relative to
virgin PS, as summarized
in Table 1. The addition of clay results in Tg increase which can be
attributed to the retardation of PS chain movement.Table 1
Results of thermal and mechanical properties of
polystyrene and polystyrene nanocomposites.
SampleTg,(°C)(a)T0.05,(°C)(b)T0.5,(°C)(c)Char at 600(X°C)
%CTE(μm/m°C)PS100±0.5390±1.7424±0.80164±2POSS/Clay/PS108±0.6411±1.3446±1.22.998±1C20-POSS/Clay/PS105±0.3415±1.1457±0.92.4100±3(a)Glass transition temperature (Tg).(b)5% Degradation
temperature (T0.05).(c)50% Degradation
temperature (T0.5).Figure 2
DSC curves glass transition temperature of (a) PS, (b) the nanocomposites
formed used POSS-NH2, and (c) the nanocomposites formed used C20-POSS.Figure3 presents
the thermal stabilities of POSS-NH2- and C20-POSS-modified
clays and nanocomposites investigated by TGA. Both nanocomposites show improved
thermal stabilities than the virgin PS. The improvement in the degradation
temperature is mainly due to the homogeneous dispersion of silicate
nanoplatelets in the PS matrix [27–30]. In Figure 3, the C20-POSS/clay
decomposes at 262°C while the POSS-NH2/clay
decomposes at higher temperature of 38°C. The POSS-NH2-modified clay is
relatively more stable than the C20-POSS-modified clay. Essentially, all
nanocomposites give higher decomposition temperatures than the pristine PS and
the improved thermal stability can be attributed to the diffusion hindrance of
the decomposited volatiles. The values of 5% and 50% weight loss temperatures
and the char yields are summarized in Table 1.Figure 3
TGA curves of (a) pure Clay, (b) POSS-NH2/Clay, (c) C20-POSS/Clay,
(d) pure PS, (e) the nanocomposite formed with POSS-NH2, and (f) the
nanocomposite formed with C20-POSS.Figure 4
X-Ray
diffraction patterns of pure clay, and intercalated clay.Figure 5
XRD spectra
ofC20-POSS,  pure clay, and C20-POSS/Clay.Figure 6
XRD
spectra of the two surfactant-containing nanocomposites indicating the extent
of delamination.TEM images of (a) POSS-NH2 and (b) C20-POSS-treated
nanocomposites.
(a)(b)


### 3.4. Molecular Weights of the Nanocomposites

Molecular weight
and molecular weight distribution (PDI) by GPC analyses of polymer samples recovered
after excluding all clay content are listed in Table2. From Table 2, molecular
weight (Mw or Mn) of the PS in the PS/clay nanocomposites is higher than the
pure PS, suggesting that clay may act as a catalytic agent responsible for the
observed higher molecular weight of the PS with the proceeding emulsion
polymerization.Table 2
Molecular weights of polystyrene and polystyrene nanocomposites.
SampleMn(×104)(a)Mw(×104)(b)PDI(Mw/Mn)(c)PS34.553.11.54POSS-NH2/Clay/PS40.954.01.32C20-POSS/Clay/PS47.758.71.23(a)Number-average molecular weights  (Mn).(b)Weight-average
molecular weights (Mw)
were determined by GPC.(c)Polydispersity index, Mw/Mn.


### 3.5. Coefficient of Thermal Coefficient

Thermal mechanical analyzer (TMA) was used to
determine the coefficient of thermal expansion of the POSS/clay nanocomposites.
The thermal expansion coefficient is an   important issue for polymers in
engineering applications. The CTE was measured from the initial linear slope of the
thermal strain-temperature plot. A low thermal expansion coefficient is
often desirable to achieve dimensional stability and can be achieved by
incorporation of a rigid and low CTE filler material. From the data in Table1, the CTE
of the virgin PS is 164 μm/m°C and the addition 3 wt% organically modified clays reduces the CTE values to 98 and 100 μm/m°C
for POSS/clay/PS
and C20-POSS/clay/PS approximately 40% reduction relative to
the virgin PS.In general, the extent of CTE reduction depends on the
particle rigidity and fine dispersion of the clay platelets in the PS matrix
and also due to
efficient stress transfer to clay layers. The retardation of PS chain segmental movement through
incorporation of organically modified clays also leads to decrease in the
coefficient of thermal expansion (CTE). The incorporation of the organically modified clays
results in significant improvement in dimensional stability of the PS matrix.


## 3.1. Morphologies of Modified Clays and Nanocomposites

Microstructures
of polymer-layered silicate nanocomposites were characterized by XRD and TEM. Figure4 shows the X-ray diffraction curves of the pristine clay and modified clays in
the 2θ region of 2–10°. For the
pristine clay, the Bragg diffraction peak at 2θ = 6.92° corresponds to d-spacing of
1.28 nm. For the POSS-NH2/clay and C20-POSS/clay, the 2θ
value shifts from 6.92° (1.28 nm) to 5.51° (1.61 nm) and 2.33° (3.80 nm) after
ion exchange, indicating that the basal spacing is expanded as the sodium
cations in the interlayer galleries are replaced by intercalated agents of POSS-NH2 and C20-POSS. The increase of the basal spacing indicates that the
clay can be efficiently intercalated by POSS-NH2 and C20-POSS.
The  d-spacing of the C20-POSS-modified clay is substantially
greater than the POSS-NH2-modified clay. Larger interlayer spacing favors
the penetration of styrene monomer and the formation of exfoliated nanocomposite
by providing more hydrophobic environment. The pure POSS-NH2 has
characteristic diffraction peaks arising from the aggregation of the POSS [23].
Figure 5 shows XRD patterns of the unmodified clay and the C20-POSS
modified clay. The pure C20-POSS has a broad peak in the region of 5–12° arising from
the C20 long aliphatic chain. When the C20-POSS is
inserted between  the galleries of the clay, the d spacing is increased from 1.28 nm for original
clay to 3.80 nm, implying that the organic modifier is incorporated between and
pushing the clay layers. Both nanocomposites do not show XRD diffraction peak
as shown in Figure 6, indicating the silicate layers are exfoliated in the
polymer matrix. TEM images for POSS-NH2/clay and C20-POSS/clay
nanocomposites at 3% inorganic clay loading are shown in Figures 7(a) and 7(b),
indicating that the exfoliated clay platelets are distributed in the matrix
homogeneously and randomly.


## 3.2. Fourier Transfer Infrared Analyses

The representative FT-IR spectra of the organophilic
clay, POSS-NH2-modified and C20-POSS modified clays are
given in Figure 1. After ion exchange, FT-IR spectroscopy can provide important
information regarding the difference between intercalated agents and modified
clays. In Figure 1(a), characteristic vibration bands of the pure clay are 1030 cm−1(Si–O), 520 cm−1 (Al–O), and 470 cm−1 (Mg–O) [24–26]. In Figure
1(b), the absorption peaks in the region of 2950–2800 is assigned
to the stretching vibration of aliphatic C–H. The symmetrical Si–O–Si band in
the silsequioxane cage is characterized by the stretching band at 1109 cm−1. In Figure 1(d), C20-POSS
contains both alkyl chain and POSS moiety where the POSS moiety exhibits
characteristic absorption peaks at 2950–2800 cm−1 (C–H bonds), 1230 cm−1 (Si–C bonds), 1109 cm−1 (Si–O–Si bonds of the
cage structure). The characteristics
of  the
vibration band of alkyl chain appear at 2920, 2850, and 1475 cm−1 (–CH2–vibration
bands). Figures 1(c) and 1(e) show the features of combination of
characteristic bands of pure clay, POSS-NH2, and C20-POSS.
IR analysis further confirms the existence of these intercalated agents in these
intercalated clay samples, implying that these intercalations of the intercalated agents are indeed
present within the gallery gap. These observations support the explanation in
the earlier observation from XRD.Figure 1
IR spectra of the two intercalated agent,
intercalated clay, and pure clay.


## 3.3. Thermal Properties

Figure2
presents DSC traces of these nanocomposites with different intercalated agents.
All DSC thermograms display single glass transition
temperatures in the   experimental temperature range. The glass transition
temperature of PS occurs at 100°C. With the
addition of the POSS and C20-POSS modified MMT to the polymer
matrix, the glass transition temperatures (Tgs) of the POSS-NH2/clay/PS
and C20-POSS/clay/PS are 108, and 105°C,
respectively. From the DSC results that the incorporation of the organoclay resulted in
an   increase in the Tg relative to
virgin PS, as summarized
in Table 1. The addition of clay results in Tg increase which can be
attributed to the retardation of PS chain movement.Table 1
Results of thermal and mechanical properties of
polystyrene and polystyrene nanocomposites.
SampleTg,(°C)(a)T0.05,(°C)(b)T0.5,(°C)(c)Char at 600(X°C)
%CTE(μm/m°C)PS100±0.5390±1.7424±0.80164±2POSS/Clay/PS108±0.6411±1.3446±1.22.998±1C20-POSS/Clay/PS105±0.3415±1.1457±0.92.4100±3(a)Glass transition temperature (Tg).(b)5% Degradation
temperature (T0.05).(c)50% Degradation
temperature (T0.5).Figure 2
DSC curves glass transition temperature of (a) PS, (b) the nanocomposites
formed used POSS-NH2, and (c) the nanocomposites formed used C20-POSS.Figure3 presents
the thermal stabilities of POSS-NH2- and C20-POSS-modified
clays and nanocomposites investigated by TGA. Both nanocomposites show improved
thermal stabilities than the virgin PS. The improvement in the degradation
temperature is mainly due to the homogeneous dispersion of silicate
nanoplatelets in the PS matrix [27–30]. In Figure 3, the C20-POSS/clay
decomposes at 262°C while the POSS-NH2/clay
decomposes at higher temperature of 38°C. The POSS-NH2-modified clay is
relatively more stable than the C20-POSS-modified clay. Essentially, all
nanocomposites give higher decomposition temperatures than the pristine PS and
the improved thermal stability can be attributed to the diffusion hindrance of
the decomposited volatiles. The values of 5% and 50% weight loss temperatures
and the char yields are summarized in Table 1.Figure 3
TGA curves of (a) pure Clay, (b) POSS-NH2/Clay, (c) C20-POSS/Clay,
(d) pure PS, (e) the nanocomposite formed with POSS-NH2, and (f) the
nanocomposite formed with C20-POSS.Figure 4
X-Ray
diffraction patterns of pure clay, and intercalated clay.Figure 5
XRD spectra
ofC20-POSS,  pure clay, and C20-POSS/Clay.Figure 6
XRD
spectra of the two surfactant-containing nanocomposites indicating the extent
of delamination.TEM images of (a) POSS-NH2 and (b) C20-POSS-treated
nanocomposites.
(a)(b)


## 3.4. Molecular Weights of the Nanocomposites

Molecular weight
and molecular weight distribution (PDI) by GPC analyses of polymer samples recovered
after excluding all clay content are listed in Table2. From Table 2, molecular
weight (Mw or Mn) of the PS in the PS/clay nanocomposites is higher than the
pure PS, suggesting that clay may act as a catalytic agent responsible for the
observed higher molecular weight of the PS with the proceeding emulsion
polymerization.Table 2
Molecular weights of polystyrene and polystyrene nanocomposites.
SampleMn(×104)(a)Mw(×104)(b)PDI(Mw/Mn)(c)PS34.553.11.54POSS-NH2/Clay/PS40.954.01.32C20-POSS/Clay/PS47.758.71.23(a)Number-average molecular weights  (Mn).(b)Weight-average
molecular weights (Mw)
were determined by GPC.(c)Polydispersity index, Mw/Mn.


## 3.5. Coefficient of Thermal Coefficient

Thermal mechanical analyzer (TMA) was used to
determine the coefficient of thermal expansion of the POSS/clay nanocomposites.
The thermal expansion coefficient is an   important issue for polymers in
engineering applications. The CTE was measured from the initial linear slope of the
thermal strain-temperature plot. A low thermal expansion coefficient is
often desirable to achieve dimensional stability and can be achieved by
incorporation of a rigid and low CTE filler material. From the data in Table1, the CTE
of the virgin PS is 164 μm/m°C and the addition 3 wt% organically modified clays reduces the CTE values to 98 and 100 μm/m°C
for POSS/clay/PS
and C20-POSS/clay/PS approximately 40% reduction relative to
the virgin PS.In general, the extent of CTE reduction depends on the
particle rigidity and fine dispersion of the clay platelets in the PS matrix
and also due to
efficient stress transfer to clay layers. The retardation of PS chain segmental movement through
incorporation of organically modified clays also leads to decrease in the
coefficient of thermal expansion (CTE). The incorporation of the organically modified clays
results in significant improvement in dimensional stability of the PS matrix.


## 4. Conclusions

The POSS-clay
hybrids of polystyrene are prepared via emulsion polymerization using two
organically modified clays, POSS-NH2 and C20-POSS, as
intercalated agents. X-ray
diffraction (XRD) results indicate that the clay is successfully intercalated by
POSS-NH2 and C20-POSS. The random dispersion of these
exfoliated silicate layers in these nanocomposites are identified by XRD and
TEM. These well dispersed clay platelets in PS matrix result in improved
thermal properties in terms of thermal decomposition temperature (Td) and glass
transition temperature (Tg). In addition, the incorporation
of these organoclay results in significant reduction in coefficient of thermal
expansion of virgin PS.



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*Source: 739613-2008-03-30.xml*