synthesis and characterization of pmma/sio 2 organic-inorganic hybrid...
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Synthesis and Characterization of PMMA/SiO2
Organic–Inorganic Hybrid Materials Via RAFT-MediatedMiniemulsion Polymerization
Jianying Ma,1,2 Mangeng Lu,2 Chunlei Cao,1 Huixuan Zhang1,3
1Engineering Research Center of synthetic resin and special fiber, Ministry of Education, Changchun Universityof Technology, Changchun 130012, People’s Republic of China
2Guangzhou Institute of Chemistry, Chinese Academy of Science, Guangzhou 510650,People’s Republic of China
3Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022,People’s Republic of China
In this article, we first carried out the surface modifica-tion of SiO2 using silane coupling agent KH570, andthen prepared PMMA/SiO2 organic–inorganic hybridmaterials by conventional free radical polymerizationand RAFT polymerization in miniemulsion, respectively.The kinetics comparisons of these two polymerizationswere studied. PMMA/SiO2 hybrid materials werecharacterized by gel permeation chromatography, dif-ferential scanning calorimetry and thermogravimetricanalysis. Experimental results indicated that the poly-merization behavior of MMA in miniemulsion showedcontrolled/living radical polymerization characteristicsunder the control of RAFT agent. Incorporation ofRAFT agent and SiO2 nanoparticles improved the ther-mal properties of polymers, the thermal stability ofpolymers increased with increasing content of SiO2
nanoparticles. The structures and morphologies ofSiO2, modified SiO2, and PMMA/SiO2 hybrid materialswere characterized by FT-IR and TEM. TEM resultsshowed that the addition of modified SiO2 nanopar-ticles to miniemulsion polymerization system obtaineddifferent morphology latex particles. Most of modifiedSiO2 nanoparticles were wrapped by polymer matrixafter polymerization. POLYM. COMPOS., 34:626–633, 2013.ª 2013 Society of Plastics Engineers
INTRODUCTION
Within past few decades, polymer/inorganic nanocom-
posites have attracted immense attention. In general, when
compared with conventional composites, the nanocompo-
sites exhibit significant improvements in physical proper-
ties such as thermal stability [1, 2], mechanical properties
[3], flame retardancy [4], and enhance modulus [5], due to
the much stronger interfacial interactions between nanopar-
ticles and polymer matrix [6–9]. Different types of nano-
particles are used depending on the purpose of the resulting
nanocomposites-common examples are silica, clay, carbon
nanotubes, and montmorillonite (MMT) [10]. The silica is
applied widely among these nanoparticles because of its
high strength and low density.
The mechanical properties of polymer/inorganic nano-
composites depend on the level of adhesion at the inter-
face between the dispersed and continuous phases. If the
surface of nanoparticles is incompatible with polymer
matrix, the polymer matrix and nanoparticles phases sepa-
rate, which will result in the agglomeration of the par-
ticles [11–13]. So the silica surface modification becomes
very important for its application in preparing polymer/
silica nanocomposites. Generally, the silica can be modi-
fied with silane coupling agents to improve the adhesion
between the particles and polymer matrix [14–17]. The
silane coupling agent can react with –OH groups of silica
surface, and an attached functionalized alkyl chain is
more compatible with the polymer matrix than the bare
surface of the silica. According to the functional groups
of coupling agent, either covalent bonds with the polymer
can be created or it may just provide an organic coating
on the silica which solvates the polymer.
In the last decade, the use of controlled/living radical
polymerization (CLRP) has attracted much attention in
the field of nanoscience and nanotechnology [18]. This is
due to many advantages that CLRP offers over other
polymerization techniques, such as precise control over
Correspondence to: Jianying Ma; e-mail: [email protected]
DOI 10.1002/pc.22438
Published online in Wiley Online Library (wileyonlinelibrary.com).
VVC 2013 Society of Plastics Engineers
POLYMER COMPOSITES—-2013
molecular architecture, the wide range of monomers that
can be used, and simple reaction conditions required. Up
to now, various controlled polymerization methods such
as nitroxide-mediated polymerization (NMP) [19, 20],
atom transfer radical polymerization (ATRP) [21, 22],
and reversible addition-fragmentation chain transfer
(RAFT) [23–27] are available and have been widely
applied to prepare polymer/inorganic hybrid materials. As
compared with the other CLRP techniques, RAFT poly-
merization has prominent advantages such as good
compatibility with a wide range of monomers and facile
experimental conditions which are similar to conventional
radical polymerization in adding chain transfer agents
(CTA) at the beginning of reaction [28, 29]. In RAFT
polymerization systems, pre-equilibrium and main equilib-
rium reactions lead to controlled and well-defined poly-
mers in a wide range of temperatures. Thus, it is a robust
method to prepare polymer-based nanocomposites having
a matrix with narrow polydispersity index (PDI).
Miniemulsion polymerization is a convenient one-step
technique that can be used for the incorporation of
nanolayered filler materials such as clay [30] and carbon
nanotubes [31] in polymer matrix. This technique offers
several advantages over other dispersion polymerization
techniques: efficient use of surfactant, high conversions,
high rates of polymerization, and yields final latex par-
ticles with high solids content, small particle size, and
high molar mass. The obtained particles are a 1:1 copy of
the miniemulsion droplets [32, 33]. The latter can be
attributed to the fact that the miniemulsion droplets are
directly polymerized, thus, the resulting polymer particles
are often one-to-one copies of the monomer droplets [33].
So in this work, we carried out surface modification of
SiO2 nanoparticles using silane coupling agent KH570,
then the PMMA/SiO2 hybrid materials were prepared via
conventional free radical and RAFT polymerizations in
miniemulsion to study the effects of nanoparticles loading
content on the hybrid materials properties, respectively.
Polymerization kinetics was followed by recording the
variations of monomer conversions, molecular weights
(Mn) and PDI during polymerization. The structural
and morphological characteristics of modified SiO2 and
PMMA/SiO2 hybrid materials were studied by FT-IR and
transmission electron microscopy (TEM) analyses. Mean-
while, the effects of SiO2 nanoparticles and RAFT agent
on the thermal properties of PMMA/SiO2 hybrid materials
were also investigated by differential scanning calorimetry
(DSC) and thermogravimetric analysis (TGA).
EXPERIMENTAL
Materials
Nano-silica (SiO2) was purchased (with a mesh of 20
W, Guangdong Ona New Material, China) and handled at
1008C for 4 h in a vacuum oven to evaporate the water
which adsorbed on the surface. Silane coupling agent
c-methacryloxypropyl trimethoxy silane (KH570) was
purchased (Shanghai Jing Chun Industrial, China) and
used without further purification. Methyl methacrylate
(MMA) was washed with sodium hydroxide aqueous
solution (5 wt%) for three times to remove inhibitors, fol-
lowed by deionized water until neutralization, and then
distilled under reduced pressure prior to miniemulsion
polymerization. The RAFT agent 2-([(tert-butylsulfanyl)-
carbonothioyl] sulfanyl) propanoic acid (BCSPA) was
synthesized similar to the procedure described in detail by
Ferguson et al. [34]. Potassium persulfate (KPS, 99%),
dodecyl sulfate sodium salt (SDS) and cetyl alcohol (CA)
were used as received. All other chemical were used with-
out other purification.
Surface Modification of SiO2
Surface modification of SiO2 was carried out as fol-
lowed. First, 5.0 g SiO2 and 250 mL THF were mixed in
an erlenmeyer flask and subjected to sonication using ul-
trasonic processor at a power output of 600 W for 60 min
at room temperature. Second, 5.0 g silane coupling agent
KH570 was added into the above solution and stirred to
react for 4 h at 708C. The obtained reaction mixture was
centrifuged at the rate of 4,000 r/min, the products were
washed with deionized water for three times and dried
under vacuum for 24 h at 608C, and finally the surface
modified SiO2 was obtained.
Preparation of PMMA/SiO2 Hybrid Materials
PMMA/SiO2 hybrid materials were prepared in minie-
mulsion mediated by RAFT mechanism as followed. The
miniemulsion polymerizations were performed in a 500-
mL four-necked flask which was placed in a water bath
thermostated at desired temperature. A number of minie-
mulsions were run with different contents of modified
SiO2 and RAFT agent. About 1.2 g surfactant SDS, 0.6 g
cosurfactant CA and different content of SiO2 were added
into 300 mL deionized water and stirred. RAFT agent
BCSPA and monomer MMA were added into the above
emulsion, and then the emulsion was subjected to sonica-
tion using ultrasonic processor at a power output of 600
W for 30 min at room temperature to obtain miniemul-
sion. The miniemulsion was moved into the 500-mL four-
necked flask with a mechanical stirrer, a thermometer, a
reflux cooler, and a N2 bubbler. About 5 mL deionized
water contained 0.12 g initiator KPS was injected to the
miniemulsion. The miniemulsion polymerization was ini-
tiated at 758C and lasted for 4 h under N2 protection. Dif-
ferent samples were taken to measure monomer conver-
sions at different reaction time during polymerization.
The polymer products were precipitated into methanol
and washed with distilled water for several times, then
dried under vacuum for 24 h at 508C, finally the PMMA/
SiO2 hybrid materials were obtained.
DOI 10.1002/pc POLYMER COMPOSITES—-2013 627
Solubility of PMMA/SiO2 Hybrid Materials
About 0.5 g pure PMMA and PMMA/SiO2 hybrid
materials with different content of modified SiO2 (0.5,
1.0, 2.0, and 5.0%) were added into 20 mL chloroform,
respectively. These mixtures were stirred 30 min to
observe the solubility.
Instrumentation
FT-IR. FT-IR spectra were carried out using RFX-65A
(Analects, America) Fourier transform-infrared spectrome-
ter at room temperature in the range from 4,000 to 500
cm–1, with a resolution of 2 cm–1 and 20 scans. Samples
were prepared by dispersing well the complexes in KBr
and compressing the mixtures to form disks.
Gel Permeation Chromatography. Gel permeation
chromatography (GPC) analysis were performed at a flow
rate of 0.80 mL min–1 and 258C in THF by using a
Waters 515 GPC (Waters, America) after calibrated with
standard polystyrene (PSt).
Differential Scanning Calorimetry. DSC measure-
ments were performed on Perkin-Elmer Pyris diamond
TA lab system (Perkin-Elmer, America) with a heating
rate of 108C min–1 under a nitrogen atmosphere.
Thermogravimetric Analysis. Thermal decomposition
behaviors were examined by means of TGA with a heat-
ing rate of 108C min–1 under a nitrogen atmosphere on a
Perkin-Elmer Pyris 1 thermogravimetric analysis (Perkin-
Elmer, America).
Transmission Electron Microscopy. The microstruc-
tures of samples were imaged using a JEM-100CX TEM
(JEOL, Japan). TEM samples were prepared by casting
one drop of a dilute miniemulsion onto a carbon-coated
copper grid.
RESULTS AND DISCUSSION
Structural Characterization
FT-IR spectra can characterize the molecular structures
of organic compounds, and different functional groups of
compounds can be distinguished based on different char-
acteristic peaks. In this article, we carried out the surface
modification of SiO2 using silane coupling agent KH570,
then the structures of SiO2 and modified SiO2 were con-
formed by FT-IR spectra.
According to the results shown in Fig. 1(a), some char-
acteristic peaks of SiO2 were observed. The characteristic
peaks at 1,000�1,150 cm–1 and 808 cm–1 corresponded to
the stretching vibration peaks of Si–O bond. The charac-
teristic peak at 1,640 cm–1 corresponded to the bending
vibration peaks of O-H bond. The characteristic peak at
3,412 cm–1 corresponded to the stretching vibration peaks
of Si–O–H and O–H, ascribed to the hydroxyl and
adsorbed water.
The general formula of silane coupling agent is repre-
sented by Y-R-SiX3. Here, X represents the alkoxy hydro-
lysis, and Y represents the reactive groups which can
react with polymers. During the surface modification of
SiO2, the X group firstly hydrolysis to silanol, then
reacted with the hydroxyl on the surface of SiO2 to form
chemical bonds. Meanwhile, the silanol of silane molecu-
lar also associated each other and formed network mem-
brane to cover on the surface of nanoparticles.
Figure 1(b) was the FT-IR spectra of modified SiO2.
When compared with Fig. 1(a), we found the appearance
of new characteristic peaks at 1,505 and 2,928 cm21. The
characteristic peak at 1505 cm21 was the vibration peak of
C–C, and at 2928 cm21 were the vibration peaks of –CH3
and –CH2. These results indicated the appearance of or-
ganic groups on the surface of modified SiO2, which would
improve the adhesion between the particles and polymer.
We prepared PMMA/SiO2 hybrid materials using
modified SiO2 in miniemulsion mediated by RAFT mech-
anism. Figure 1(c) was the FT-IR spectra of PMMA/SiO2
FIG. 1. FT-IR spectra of SiO2, modified SiO2 and PMMA/SiO2 hybrid
materials.
628 POLYMER COMPOSITES—-2013 DOI 10.1002/pc
hybrid materials. We can see from this spectra that the
deformation vibration characteristic peak of –CH2–
appeared at 1,450 cm–1, but the characteristic peak of
hydroxyl at 3,400 cm–1 obviously weakened compared
with the SiO2, this result indicated the hydroxyl on the
surface of modified SiO2 reacted with silane coupling
agent. In addition, the appearance of stretching vibration
of Si–O at 1,450 cm–1 indicated that the double bond of
silane coupling agent successfully linked onto the surface
of SiO2. The characteristic absorption peak at 1,630 cm–1
of –CH¼CH– further proved this conclusion, and weaken
of this characteristic absorption peak indicated that the
double bond of silane coupling agent had reacted with
monomer MMA. Moreover, the characteristic peak at
1,760 cm–1 corresponded to the absorption peak of
carbonyl, which illustrated the existence of PMMA, and
the stretching vibration of Si–C at 790 cm–1 indicated the
existence of silane coupling agent.
Polymerization Kinetics
In our study, PMMA/SiO2 hybrid materials were pre-
pared via conventional free radical polymerization and
RAFT polymerization in miniemulsion, respectively. We
investigated the effects of modified SiO2 nanoparticles
addition on polymerization kinetics. The recipes of pre-
paring PMMA/SiO2 hybrid materials were listed in Table
1. Here, the content of modified SiO2 nanoparticles was
based on the content of monomer MMA. For these RAFT
polymerizations in miniemulsion, the initial molar ratio of
[initiator]0/[RAFT agent]0/[monomer]0 was set at 1/4/900,
and the content of surfactant SDS and cosurfactant CA
were also shown in Table 1.
The monomer conversions, Mn and PDI of PMMA/
SiO2 hybrid materials with different content of modified
SiO2 nanoparticles were shown in Table 2. For the case
of sample 1 and sample 2, that is, the samples of pure
PMMA and PMMA/SiO2 hybrid materials which were
prepared via conventional miniemulsion polymerization,
the obtained polymers had high monomer conversions and
high molecular weights, but wide PDI. However, once the
RAFT agent was added into the polymerization system,
that is, the polymerization was performed via RAFT
mechanism, the PDI became narrow. Meanwhile, we can
see from these four hybrid materials (PMMA/SiO2-
3�PMMA/SiO2-6) prepared by RAFT polymerization that
the monomer conversions and molecular weights
decreased with increasing content of modified SiO2 nano-
particles in hybrid materials.
According to these results shown in Table 2, we
depicted the curves of molecular weights and PDI of
PMMA/SiO2 hybrid materials. From these curves in Fig.
2, we can clearly observe the effects of modified SiO2
nanoparticles content on molecular weights and PDI of
PMMA/SiO2 hybrid materials. For the hybrid materials
prepared by RAFT polymerization, though the PDI slightly
increased with increasing content of modified SiO2 nano-
particles in hybrid materials, the PDI were controlled in a
relatively narrow range. For the molecular weights of
hybrid materials, we can see that the experimental values
of molecular weights (Mn,exp) were close to the theoretical
value (Mn, th). The theoretical values of molecular weights
were calculated according the following equation [35].
Mn;th ¼½monomer�03Conversion
½RAFT�03MWmonomer þMWRAFT
Here, MWmonomer and MWRAFT were the molecular
weights of monomer and RAFT agent, [monomer]0 and
[RAFT]0 were their initial molar concentrations, respec-
tively. Therefore, based on the discussion on the effects of
modified SiO2 nanoparticles content on molecular weights
TABLE 1. Recipes of preparing PMMA/SiO2 hybrid materials via
RAFT polymerization in miniemulsion.
Sample
code SiO2 (g)
Monomer
(MMA, g)
RAFT
agent
(BCSPA, g)
Surfactant
Initiator
(KPS, g)SDS (g) CA (g)
PMMA/SiO2-1 0 40 0 1.2 0.6 0.12
PMMA/SiO2-2 0.2 40 0 1.2 0.6 0.12
PMMA/SiO2-3 0.2 40 0.423 1.2 0.6 0.12
PMMA/SiO2-4 0.4 40 0.423 1.2 0.6 0.12
PMMA/SiO2-5 0.8 40 0.423 1.2 0.6 0.12
PMMA/SiO2-6 2.0 40 0.423 1.2 0.6 0.12
TABLE 2. Results of preparing PMMA/SiO2 hybrid materials via
RAFT polymerization in miniemulsion.
Sample code Conversion (%) Mn (g/mol) PDI
PMMA/SiO2-1 98.4 38000 2.7
PMMA/SiO2-2 95.4 35000 2.2
PMMA/SiO2-3 92.6 21000 1.2
PMMA/SiO2-4 87.4 20000 1.2
PMMA/SiO2-5 83.2 19000 1.3
PMMA/SiO2-6 80.4 18000 1.4
FIG. 2. Effects of SiO2 content on the molecular weights and polydis-
persity index of PMMA/SiO2 hybrid materials.
DOI 10.1002/pc POLYMER COMPOSITES—-2013 629
and PDI of hybrid materials, we can conclude that the
RAFT polymerization in miniemulsion showed the charac-
teristics of controlled/living radical polymerization.
Solubility of PMMA/SiO2 Hybrid Materials
Generally, the solubility is an important influence
factor for applying polymer materials in some special
occasions, such as polymer coatings. The introduction of
inorganic nanoparticles to polymers can significantly
improve some properties, such as thermal stability, me-
chanical strength, flame retardance, and optical properties
[36–40]. Meanwhile, the inorganic nanoparticles also
affect the solubility of polymers. In our experiments, we
introduced modified SiO2 nanoparticles to PMMA and
investigated the solubility of hybrid materials in chloro-
form. The dissolving time of hybrid materials with differ-
ent content of modified SiO2 nanoparticles in chloroform
were different. Experimental results were shown in Table
3. From these results, we can see the pure PMMA could
quickly and completely dissolve in chloroform. However,
the solubility of polymers decreased with introduction of
modified SiO2 nanoparticles. PMMA/SiO2 hybrid materi-
als partly dissolved when the content of modified SiO2
nanoparticles was more than 2.0%. So we can speculate
that the PMMA/SiO2 hybrid materials can only partly dis-
solve or swelling with increasing content of modified
SiO2 nanoparticles. The pure PMMA and PMMA/SiO2
hybrid materials with lower content of modified SiO2
nanoparticles (0.5%, 1.0%) can completely dissolve in
chloroform. The SiO2 nanoparticles had higher surface
energy, and the PMMA molecular chains can be adsorbed
on the surface of SiO2 nanoparticles, then these molecular
chains entangled each other and formed physical cross-
linking. Meanwhile, the double bonds group of silane
coupling agent which were connected to the surface of
SiO2 nanoparticles reacted with the PMMA matrix and
formed chemical bonds, thus finally formed chemical
crosslinking. Therefore, the structures of hybrid materials
gradually became to insoluble crosslinked networks from
soluble linear molecular chains, which resulted in the sol-
ubility of PMMA/SiO2 hybrid materials decreased from
dissolving to swelling.
Thermal Properties of PMMA/SiO2 Hybrid Materials
Figure 3 illustrated the differential scanning calorime-
try traces for pure PMMA (1) and PMMA/SiO2 hybrid
materials (2�6). From the curves (2�6), we can see that
the Tg of these PMMA/SiO2 hybrid materials firstly
decreased after adding SiO2 nanoparticles to polymeriza-
tion system, then the Tg increased with increasing content
of modified SiO2 nanoparticles. The reason for decreasing
of Tg can be explained as followed. First, the silane cou-
pling agent with low Tg formed ductile interface layer
with polymers during polymerization, good compatibility
between polymer matrix and silane coupling agent made
the Tg of matrix decrease. Second, the small amount of
silane coupling agent can proliferate into matrix from the
surface of polymer particles and generate internal plastici-
zation with polymer matrix, resulting in decreasing of Tg.Third, the addition of SiO2 nanoparticles made the free
volume of its surrounding matrix slightly increase which
also resulted in decreasing of Tg. The SiO2 nanoparticles
which were modified by silane coupling agent played the
role of crosslinking points. On the one hand, these cross-
TABLE 3. Solubility of PMMA/SiO2 hybrid materials.
Content of
SiO2 (wt%) Solubility
0 Quickly and completely dissolved (3 min)
0.5 Completely dissolved after stirring a moment (5 min)
1.0 Completely dissolved after stirring a period of time (10 min)
2.0 Partly dissolved after stirring a period of time (10 min)
5.0 Partly dissolved after stirring a long time (30 min)
FIG. 3. DSC curves of PMMA/SiO2 hybrid materials with different
content of modified SiO2.
FIG. 4. TGA curves of PMMA/SiO2 hybrid materials with different
content of modified SiO2.
630 POLYMER COMPOSITES—-2013 DOI 10.1002/pc
linking points were beneficial to chain entanglement of
PMMA molecular segments, forming physical crosslink-
ing. On the other hand, the silane coupling agent reacted
with polymer matrix and formed the interface layer
between the nanoparticles and polymer matrix, forming
chemical crosslinking. The crosslinking density increased
with increasing content of modified SiO2, finally resulted
in increasing of Tg.Figure 4 showed the typical TGA thermograms of
weight loss as a function of temperature for pure PMMA
and PMMA/SiO2 hybrid materials. The figure showed no-
ticeable improvements in the thermal stability of PMMA/
SiO2 hybrid materials compared to pure PMMA,
expressed by a shift in the decomposition temperature to
higher temperatures. So the addition of modified SiO2
nanoparticles to polymers increased the thermal stability
of PMMA, and the thermal stability increased with
increasing content of modified SiO2 nanoparticles.
Generally, the addition of inorganic nanoparticles to
polymers can affect the thermal decomposition tempera-
tures Td of polymers. The Td was affected by these fac-
tors. First, the modified SiO2 nanoparticles played the role
of crosslinking points and resulted in increasing of ther-
mal decomposition temperatures. Second, the thermal
decomposition temperatures may decrease if the silane
coupling agent can not completely react with polymer
matrix during polymerization. In addition, the unreacted -
OH on the surface of SiO2 nanoparticles may continue to
TABLE 4. Initial decomposition temperature of PMMA/SiO2 hybrid materials with different content of modified SiO2.
PMMA/SiO2-1 PMMA/SiO2-2 PMMA/SiO2-3 PMMA/SiO2-4 PMMA/SiO2-5 PMMA/SiO2-6
T5%/8C 209 250 263 277 291 306
FIG. 5. TEM images of PMMA/SiO2 hybrid materials with different content of modified SiO2. (a) PMMA/SiO2-1, (b) PMMA/SiO2-2, (c) PMMA/
SiO2-3, (d) PMMA/SiO2-4, (e) PMMA/SiO2-5, and (f) PMMA/SiO2-6.
DOI 10.1002/pc POLYMER COMPOSITES—-2013 631
react and generate water. The existence of these small
molecules can decrease the thermal decomposition tem-
peratures.
From Fig. 4, we can see that the addition of modified
SiO2 nanoparticles increased the thermal decomposition
temperatures, and the thermal stability increased with
increasing content of modified SiO2 nanoparticles. The
phenomenon of decreasing of the thermal decomposition
temperatures did not appear. This result indicated that the
crosslinking points played the main role for the thermal
stability of polymers. The existence of modified SiO2
nanoparticles caused the appearance of a large number of
crosslinking networks, and the crosslinking networks den-
sity increased with increasing content of modified SiO2
nanoparticles. The length of motive molecular chains seg-
ments decreased when the crosslinking networks density
increased to a certain extant, and prevented the thermal
decomposition of materials, finally improved thermal sta-
bility of polymers.
Some thermal properties of PMMA/SiO2 hybrid mate-
rials were summarized in Table 4. According to these
results, we can see that the addition of modified SiO2
nanoparticles had a considerable effect on the thermal sta-
bility of polymers. The pure PMMA (1) began to decom-
pose at a low initial decomposition temperature 2098C,but the PMMA/SiO2 hybrid materials prepared by con-
ventional miniemulsion (2) had a higher initial decompo-
sition temperature 2508C compared with pure PMMA.
Meanwhile, the effect of RAFT agent on the thermal sta-
bility of polymers was similar to the effect of RAFT
agent on the Tg of polymers. The existence of RAFT
agent made the molecules chains regularly arrange in
polymers and finally increased the decomposition temper-
ature of polymers. We can clearly see from Table 4 that
the decomposition temperature of PMMA/SiO2 hybrid
materials prepared by RAFT polymerization in miniemul-
sion (3�6) were higher than 2608C. In addition, for these
several hybrid materials (3�6), the decomposition temper-
ature increased with increasing content of modified SiO2
nanoparticles in hybrid materials. These results indicated
that the addition of modified SiO2 nanoparticles can effec-
tively improve the thermal stability of polymers.
Morphological Characteristics of Hybrid Materials
The TEM images of pure PMMA and PMMA/SiO2
hybrid materials with different content of modified SiO2
nanoparticles were shown in Fig. 5. From these TEM
micrographs, we can see that the pure PMMA latex par-
ticles prepared by conventional miniemulsion polymeriza-
tion had smooth surface, and uniform sphericity, but
uneven particle sizes distribution. The addition of modi-
fied SiO2 nanoparticles to the polymerization system
obtained different morphology latex particles. Most of
modified SiO2 nanoparticles were wrapped by polymer
matrix after polymerization. So from these TEM micro-
graphs of PMMA/SiO2 hybrid materials, we can see the
brighter spherical particles of PMMA latex particles, and
there were no the existence of SiO2 nanoparticles.
CONCLUSIONS
A series of experiments were carried out to synthesize
PMMA and PMMA/SiO2 hybrid materials via conven-
tional free radical and RAFT polymerization in miniemul-
sion. The effects of modified SiO2 nanoparticles content
and incorporation of RAFT agent on the kinetics of poly-
merization and properties of corresponding hybrid materi-
als were investigated, respectively. Polymerization
kinetics studies showed that the monomer conversions
and molecular weights decreased with increasing content
of modified SiO2 nanoparticles in hybrid materials, but
the PDI increased with increasing content of modified
SiO2 nanoparticles. The PMMA/SiO2 hybrid materials
prepared by RAFT mechanism had narrower PDI. The
structures of modified SiO2 and PMMA/SiO2 hybrid
materials were analyzed by FT-IR. Experiments results
showed that organic groups were successfully introduced
to the SiO2 nanoparticles and reacted with polymer ma-
trix. The thermal properties of hybrid materials were char-
acterized by DSC and TGA. DSC characterization indi-
cated that the Tg of polymers first decreased with increas-
ing content of modified SiO2 nanoparticles then increased
again. TGA results showed that the introduction of modi-
fied SiO2 nanoparticles effectively improved the thermal
stability of polymers mainly because of the role of cross-
linking points of modified SiO2 nanoparticles, and the
thermal stability of polymers increased with increasing
content of modified SiO2 nanoparticles.
ABBREVIATIONS
SiO2 Nano-silica
KH570 c-methacryloxypropyl trimethoxy silane
MMT Montmorillonite
MMA Methyl methacrylate
BCSPA 2-([(tert-butylsulfanyl)-carbonothioyl] sulfanyl) pro-
panoic acid
KPS Potassium persulfate
SDS Dodecyl sulfate sodium salt
CA Cetyl alcohol
CLRP Controlled/living radical polymerization
NMP Nitroxide mediated polymerization
ATRP Atom transfer radical polymerization
RAFT Reversible addition fragmentation chain transfer
CTA Chain transfer agents
Mn Molecular weights
PDI Polydispersity index
FTIR Fourier transform-infrared
GPC Gel permeation chromatography
DSC Differential scanning calorimetry
TGA Thermogravimetric analysis
TEM Transmission electron microscopy
632 POLYMER COMPOSITES—-2013 DOI 10.1002/pc
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DOI 10.1002/pc POLYMER COMPOSITES—-2013 633