Polymers 2015, 7, 1638-1659; doi:10.3390/polym7091474
OPEN ACCESS
polymers
ISSN 2073-4360
www.mdpi.com/journal/polymers
Article
Thermal, Mechanical and UV-Shielding Properties of
Poly(Methyl Methacrylate)/Cerium Dioxide Hybrid Systems
Obtained by Melt Compounding
María A. Reyes-Acosta 1 , Aidé M. Torres-Huerta 1, *, Miguel A. Domínguez-Crespo 1 ,
Abelardo I. Flores-Vela 2 , Héctor J. Dorantes-Rosales 3 and José A. Andraca-Adame 4
1
Instituto Politécnico Nacional, CICATA-Altamira, Km 14.5 Carretera Tampico-Puerto Industrial
Altamira, C.P. 89600 Altamira, Tamps. Mexico;
E-Mails: mreyesa0803@alumno.ipn.mx (M.A.R.-A.); mdominguezc@ipn.mx (M.A.D.-C.)
2
Instituto Politécnico Nacional, CMP+L, Av. Acueducto s/n, Barrio La Laguna,
Col. Ticomán, C.P. 07340, México D.F., Mexico; E-Mail: afloresv@ipn.mx
3
Instituto Politécnico Nacional, ESIQIE, Departamento de Metalurgia, C.P. 07738 México D.F., Mexico;
E-Mail: hdorantes@ipn.mx
4
Instituto Politécnico Nacional, CNMN, Av. Luis Enrique Erro S/N, Unidad Profesional Adolfo López
Mateos, Zacatenco, C.P. 07738, México D.F., Mexico; E-Mail: jandraca@ipn.mx
* Author to whom correspondence should be addressed; E-Mail: atorresh@ipn.mx;
Tel.: +833-260-0125 (ext. 87505); Fax: +833-2649301.
Academic Editor: Lloyd M. Robeson
Received: 30 June 2015 / Accepted: 29 August 2015 / Published: 7 September 2015
Abstract: Thick and homogeneous hybrid film systems based on poly(methyl methacrylate)
(PMMA) and CeO2 nanoparticles were synthesized using the melt compounding method to
improve thermal stability, mechanical and UV-shielding properties, as well as to propose
them for use in the multifunctional materials industry. The effect of the inorganic phase
on these properties was assessed by using two different weight percentages of synthesized
CeO2 nanoparticles (0.5 and 1.0 wt %) with the sol–gel method and thermal treatment at
different temperatures (120, 235, 400, 600 and 800 ˝ C). Thereafter, the nanoceria powders
were added to the polymer matrix by single screw extrusion. The absorption in the UV
region was increased with the crystallite size of the CeO2 nanoparticles and the PMMA/CeO2
weight ratio. Due to the crystallinity of CeO2 nanoparticles, the thermal, mechanical and
UV-shielding properties of the PMMA matrix were improved. The presence of CeO2
Polymers 2015, 7
1639
nanostructures exerts an influence on the mobility of PMMA chain segments, leading to
a different glass transition temperature.
Keywords: PMMA; CeO2 nanoparticles; thermal properties; UV-shielding properties
1. Introduction
In the last years, there has been an increased tendency to use thermoplastic polymers such as
polycarbonate (PC), polymethylmethacrylate (PMMA) and polyethylene-tetrafluoroethylene (PTFE) in
diverse industrial applications due to their low weight and high transparency [1,2]. PMMA is one of the
most commonly used thermoplastic polymers in the construction industry because of its excellent optical
properties (it transmits light in the 360–1000 nm range almost without loss [3]), impact and shatter
properties (compared to inorganic glass), and relatively high weathering resistances. In addition, PMMA
polymer has very low thermal conductivity (~0.0012 cal/s cm K), which makes it a candidate to be used
as a thermal-control interface material [4,5]. Unfortunately, PMMA and other polymers decompose or
degrade under solar UV radiation (200–400 nm) [6]. The most damaging natural UV radiation is between
290 and 350 nm where the highest energy of the solar spectrum occurs [7]. The usage of inorganic UV
absorbers can be a good alternative for the stability of thermoplastic polymers under weather conditions.
Wide band-gap oxides such as titanium dioxide (TiO2 ) and zinc oxide (ZnO) have been added as
inorganic fillers into the PMMA matrix in order to modify its optical properties in the UV region [8–12].
As one of the most versatile semiconductors, cerium dioxide (CeO2 ) can be used as a solar absorber.
CeO2 has shown good thermal and chemical stabilities, a high refractive index in the visible region
(2.1–2.2), good transmittance in the visible spectrum and low cost due to the fact that it is one of the most
abundant rare earth elements [13–15]. It has also potential applications such as in fuel and solar cells,
catalysts, oxygen sensors, polishing agents, gate oxides, optical devices and coating materials [16–18].
Moreover, due to its broad band gap (BG) of 2.7–3.4 eV, depending on the preparation method, effective
absorption in the ultraviolet range can be ensured by this material (λ < 400 nm) [7,19]. The incorporation
of UV-shielding-nano CeO2 into the PMMA polymer matrix can produce functional nanocomposites,
which can be a better way of making effective solar-thermal-control-interface structures for delaying
polymer degradation. Different techniques have been used for the preparation of CeO2 , including
coprecipitation, microemulsion, microwave-assisted thermal decomposition, combustion synthesis, and
mechanochemical, sol–gel, hydrothermal, solvothermal and electrochemical methods [20–22]. Among
them, the sol–gel method has been widely used because it allows the synthesis of ultrafine powders with
well-controlled properties such as homogeneity, purity, microstructure, particle size, and morphology, at
relatively low cost [7,23–26].
Based on the above, it is clear that the development of an interface displaying superior UV-NIR
shielding for degradation control without losing its mechanical properties is in high demand. Recently,
the use of PMMA/CeO2 nanocomposites in the synthesis of a core/shell structure via the electrostatic
interaction and polysiloxane@CeO2 microspheres with multifunctional properties has been reported [13,27],
but until now the effects of the crystallinity and amount of CeO2 nanoparticles on the thermal
stability, mechanical and UV-NIR-shielding properties of these structures have not been reported. In this
Polymers 2015, 7
1640
paper, the preparation and characterization of UV-protective coatings composed of sol–gel-derived CeO2
nanoparticles and dispersed in a PMMA polymer matrix by single screw extrusion are reported. The effect
of different thermal treatments (120, 235, 400, 600 and 800 ˝ C), crystallite sizes and nanoceria contents
(0.5 and 1.0 wt %) on the optical, thermal and mechanical properties of PMMA are studied in detail.
2. Experimental Section
2.1. Materials and Synthesis of CeO2 Nanoparticles
CeO2 nanoparticles were synthesized using the sol–gel method starting from cerium nitrate
hexahydrate (Ce(NO3 )3 ¨ 6H2 O, Aldrich, ě98%, St. Louis, MS, USA) as precursor and ethanol
(CH3 CH2 OH, Aldrich 99.5%, St. Louis, MS, USA) as solvent, using the following procedure: cerium
nitrate was mixed with ethanol (0.3 M) at room temperature while it was vigorously stirred by means of a
magnetic stirrer for 2 h to yield a stable and transparent sol without precipitation or turbidity. The pH was
fairly constant during the synthesis process (pH « 6). The CeO2 powders were obtained by gelification
of the solutions and the gels drying at 120 ˝ C for 24 h to remove organic matter; afterwards, the xerogel
samples were ground in an agate mortar and then thermally treated in air at 235, 400, 600 and 800 ˝ C for
2 h to study the structure and particle size effects. Finally, the powders were crushed by a horizontal ball
mill at 350 rpm for 10 h. As is well known, the particle size distribution depends on various factors such
as synthesis method, process parameters and in this case thermal treatment. Thus, the milling process
was only used to uniform the particle size in each experiment before to obtain nanocomposites.
2.2. Preparation of Hybrid Systems
Commercial grade PMMA pellets (Plexiglas V825, Altuglas International, Arkema, Philadelphia, PA,
USA) were used as raw material in this study. As first step, PMMA pellets were ground in a Thomas
Wiley laboratory mill (Arthur H. Thomas Co., Philadelphia, PA, USA) using a 2 mm sieve to obtain
fine particles. Thereafter, the polymeric particles were dried at 80 ˝ C for 1 h before being processed.
PMMA/CeO2 hybrid systems were prepared by using a laboratory single-screw extruder (D = 19 mm,
L/D = 30 mm) with three heating zones. Two different amounts of CeO2 nanostructures (0.5 and 1.0 wt %)
were added to the polymer matrix. The extruder blending temperature profiles were 235 and 240 ˝ C from
hopper to die zone, and the screw rotating speed was 80 rpm; finally, nanocomposite films extrudates were
cooled in a water bath at room temperature. For comparison during the characterizations and performance of
nanocomposites, pure PMMA were ground and extruded under the same process conditions.
2.3. Characterization and Measurements
The IR spectra of the CeO2 nanoparticles and PMMA/CeO2 hybrid systems were obtained within
wavenumbers ranging from 4000 to 450 cm´1 using a Spectrum One Perkin-Elmer spectrometer
(Perkin-Elmer Life and Analytical Sciences, Bridgeport, CT, USA). Transmission IR experiments were
carried out under environmental conditions with 10 scans at a resolution of 4 cm´1 . To determinate
the interaction between the organic and inorganic phases, 1 H NMR and 13 C NMR spectra were carried
out using a Bruker Ascend 750 (750 MHz) NMR spectrometer for liquids (Bruker Instruments, Inc.,
Billerica, MA, USA) at room temperature with deuterated chloroform (CDCl3 , 1 H: δ = 7.26 ppm,
Polymers 2015, 7
1641
13
C: δ = 77.0 ppm, Sigma-Aldrich, 99.96%, St. Louis, MS, USA) as solvent and tetramethylsilane
(TMS, δ = 0.00 ppm, Sigma-Aldrich, NMR grade, ě99.9%, St. Louis, MS, USA) as the internal reference.
Crystalline phase identification of the CeO2 powders and nanocomposites was performed using
a D8 Advance Bruker X-ray diffractometer (Bruker AXS GmbH, Berlin, Germany) with Cu-Kα
monochromatic radiation (Bruker AXS GmbH, Berlin, Germany). The XRD patterns were collected
in the 2θ range from 10 to 90˝ at room temperature. CeO2 nanoparticles were examined by TEM using
a JEOL-2000 FX-II microscope (JEOL Ltd., Akishima, Japan) coupled with an energy dispersive X-ray
spectrometer (EDS, JEOL Ltd., Akishima, Japan) operating at 200 kV. Particle size distributions (PSD)
were determined with a Malvern Zetasizer Nano ZSP (model ZEN5600, Malvern Instruments Ltd.,
Malvern Worcs, UK) by the dynamic light scattering (DLS) technique. A 12-mm round-aperture-glass
cell (PCS8501, Malvern Instruments Ltd., Malvern Worcs, UK) was used for the DLS measurements.
One milliliter of methanol dispersion sonicated for 5 min was added to the cell. The suitable refractive
index was chosen for the CeO2 samples and dispersant: 2.2 and 1.33, respectively. The distribution of
nanoparticles within the polymer was observed by confocal laser scanning microscopy (CLSM) using
a Carl ZEISS microscope (Carl Zeiss, Jena, Germany), LSM 700 model and high resolution scanning
electron microscope (HRSEM) using a JEOL JSM-6701-F microscope (JEOL Ltd., Akishima, Japan).
The fluorescence intensity measurements were performed using the built-in software ZEN of the LSM
710. The intensity peaks, characteristic of a fluorescence emission signal, were 461 nm for PMMA and
539 nm for the CeO2 nanoparticles. The optical properties of the CeO2 nanoparticles were analyzed by
diffuse reflectance UV-vis spectroscopy using a Cary 5000 spectrophotometer (Agilent Technologies
Inc., Santa Clara, CA, USA) with an Internal Diffuse Reflectance accessory (DR) consisting of a
110-mm-diameter-integrating sphere. Data were collected at a 600 nm/min scan rate with a data interval
of 1.0 nm and signal-averaging time of 0.1 s in the 700–200 nm range. The optical transmission of
the PMMA/CeO2 hybrid systems was measured in the 200–1100 nm wavelength range using the same
spectrometer. The thickness of the analyzed samples was about 0.5 mm. TGA studies were carried
out using a Simultaneous Thermal Analyser Labsys Evo 1600 (SETARAM Instrumentation, Caluire,
France). Samples were placed in aluminum crucibles. An empty aluminum crucible was used as
reference. The heating ramp applied to the samples was from 25 to 450 ˝ C at a constant heating
rate of 5 ˝ C/min under argon atmosphere. The nanoindentation data were analyzed with the CSM
Instruments Nanoindentation Tester (TTX-NHT) with a Berkovich triangular diamond pyramid indenter
(CSM Instruments SA, Peseux, Switzerland). Five indents were made in each sample. A loading rate of
15 mN/min was maintained until reaching a maximum load of 5 mN. The load was held at maximum
value for 10 s. The hardness (H IT ), Vickers hardness (HV IT ), elastic modulus (EIT ) and creep (CIT ) were
estimated from the initial gradient of the unloading curves using the Oliver and Pharr method [28].
3. Results and Discussion
3.1. Characterization of CeO2 Nanopowders
3.1.1. FT-IR Analysis
The FT-IR spectra of CeO2 nanoparticles thermally treated at different temperatures (400, 600 and
800 ˝ C) are shown in Figure 1a. As a reference, FT-IR results for CeO2 particles dried at 120 ˝ C and
Polymers 2015, 7
1642
Polymers 2015, 7
5
thermally-treated at 235 ˝ C were included in the figure. The spectrum of the CeO2 xerogel (120 ˝ C)
shows a broad absorption band located in the region from 3600 to 3000 cm´1 approximately, which
shows a broad absorption band located in the region from 3600 to 3000 cm−1 approximately,
corresponds to the –OH stretching vibration (water or ethanol), while the bands around 1632 and
which corresponds
to the –OH stretching vibration (water or ethanol), while the bands around 1632 and
´1
1503 cm−1
are associated with the H2 O bending vibration [29], confirming the presence of residual
1503 cm are associated with the H2O bending vibration [29], confirming
the presence of residual
´1
physisorbed water. Moreover, the narrow and sharp band near 1384 cm
is attributed to the presence
−1
physisorbed
water. Moreover, the narrow and sharp band near 1384 cm is attributed to the presence of
´
of NO
− 3 groups [30]. As it is evident, the absorption bands mentioned above decrease gradually until
NO3 groups [30]. As it is evident, the absorption bands mentioned above decrease gradually until
disappear with the increasing of treatment temperature. It is also seen that all the CeO2 powders treated at
disappear with the increasing of treatment temperature. It is also seen that all the CeO
2 powders treated
different temperatures show a band in the 750–400 cm´1 region,
−1 which is assigned to the Ce–O stretching
at different temperatures show a band in the 750–400 cm region, which is assigned to the Ce–O
vibration [31,32]. Figure 1b shows the FT-IR spectra of the pure PMMA and PMMA/CeO2 hybrid
stretching vibration [31,32]. Figure 1b shows the FT-IR spectra of the pure PMMA and PMMA/CeO
2
´1
systems.
For
pure
PMMA,
the
bands
around
2996,
2952
(asymmetric)
and
2844
cm
(symmetric)
are
−1
hybrid systems. For pure PMMA, the bands around 2996, 2952 (asymmetric) and 2844 cm (symmetric)
assigned
to C–H
stretching
vibrations.
The bending
vibration
bands of
the methyl
3 ) group
are
assigned
to C–H
stretching
vibrations.
The bending
vibration
bands
of the(–CH
methyl
(–CHappeared
3) group
´1
at
1484
and
1436
cm
in
the
FTIR
spectra,
whereas
the
deformation
mode
of
the
methylene
(–CH2 –)
−1
appeared at 1484 and 1436 cm in the FTIR spectra, whereas the deformation mode of the methylene
´1
´1
bands
at 1728
and 753
cm and
are 753
attributed
group2–)
appeared
at 1387 cm
. In addition,
sharp and
(–CH
group appeared
at 1387
cm−1. In the
addition,
the intense
sharp and
intense
bands
at 1728
cm−1
to the
stretchingtoand
vibrations of the carbonyl
(C=O)
group,
respectively
[33].
The
are
attributed
theout-of-plane-bending
stretching and out-of-plane-bending
vibrations
of the
carbonyl
(C=O)
group,
PMMA/CeO[33].
show2 hybrid
absorption
bands
thatabsorption
matched well
PMMA,
2 hybrid
systems
show
bandswith
thatpure
matched
well indicating
with pure
respectively
The systems
PMMA/CeO
a
weak
interaction
between
PMMA
and
CeO
nanoparticles.
Furthermore,
bands
corresponding
to
2
PMMA, indicating a weak interaction between PMMA and CeO2 nanoparticles. Furthermore, bands
the vibration absorption
of CeO2absorption
nanoparticles
are not
observed owing to the incorporation of a low
corresponding
to the vibration
of CeO
2 nanoparticles are not observed owing to the
percentage ofofnanoparticles.
It isofworth
to noticeItthat
to avoid
altering
full transparency
low
incorporation
a low percentage
nanoparticles.
is worth
to notice
thatthe
to avoid
altering the full
percentages of
CeO
1.0 wt %).
2 were introduced
transparency
low
percentages
of CeO2(0.5
wereand
introduced
(0.5 and 1.0 wt %).
Transmittance (a.u.)
800 °C
600 °C
400 °C
235 °C
1503
120 °C
1632
(a)
3403
1040
1384
4000 3500 3000 2500 2000 1500 1000
500
( cm-1)
Wavenumber
(a)
(b)
Figure 1. FT-IR spectra of (a) CeO thermally treated at different temperatures and (b) pure
Figure 1. FT-IR spectra of (a) CeO22 thermally treated at different temperatures and (b) pure
PMMA and
and PMMA/CeO
PMMA/CeO2 hybrid
hybrid systems.
systems.
PMMA
2
3.1.2. NMR
NMR Studies
Studies
3.1.2.
1
˝
Figure 2a
2ashows
showsthe
the1H
HNMR
NMRspectra
spectraofof
pure
PMMA
PMMA/CeO
and1.0
1.0wtwt%,
%,800
800°C)
C)
Figure
pure
PMMA
andand
PMMA/CeO
and
2 (0.5
2 (0.5
hybrid systems.
systems. The
Thepeak
peakatat3.60
3.60ppm
ppmisisrelated
relatedtotothethe
methoxy
protons,
and
peaksatatthe
the2.2–1.5
2.2–1.5ppm
ppm
hybrid
methoxy
protons,
and
peaks
rangecorrespond
correspondto
tothe
themethylene
methyleneprotons
protonsof
ofPMMA.
PMMA.The
Theα-methyl
α-methylprotons
protonsshow
showpeaks
peaksatat1.22,
1.22,1.01
1.01 and
and
range
0.83
0.83 ppm,
ppm, which
which are
are attributed
attributed to
to the
the presence
presence of
of triads
triads of
of several
several tacticities:
tacticities: isotactic
isotactic (mm),
(mm), heterotactic
heterotactic
(mr)
(mr) and
and syndiotactic
syndiotactic (rr).
(rr). On
On the
the other
other hand,
hand, itit can
can be
be observed
observed that
that the
the addition
addition of
of aa small
small amount
amount of
of
CeO2 nanoparticles (0.5 and 1.0 wt %) to PMMA resulted in a low-field chemical shift of the peak at
1.59 ppm (–CH2– protons). This small chemical shift suggests the existence of electrostatic interactions
Polymers 2015, 7
1643
Polymers
2015,
Polymers
2015, 77 (0.5 and 1.0 wt %) to PMMA resulted in a low-field chemical shift of the peak at66
CeO
2 nanoparticles
1.59 ppm (–CH2 – protons). This small chemical shift suggests the existence of electrostatic interactions
in
the
interfacial
region
between
PMMA
and
CeO
nanoparticles,
which
is
proposed
in
Figure
3.
in the
the interfacial
interfacial region
region between
between PMMA
PMMA and
and CeO
CeO222 nanoparticles,
nanoparticles, which
which is
is proposed
proposed in
in Figure
Figure 3.
3.
in
13
13C NMR spectra of PMMA and the hybrid systems with 0.5 and 1.0 wt % of CeO2 thermally treated at
13
C NMR
NMR spectra
spectra of
of PMMA
PMMA and
and the
the hybrid
hybrid systems
systems with
with 0.5
0.5 and
and 1.0
1.0 wt
wt %
% of
of CeO
CeO22 thermally
thermally treated
treated at
at
C
800
°C
are
reported
in
Figure
2b.
The
resonance
peaks
between
22
and
16
ppm
are
assigned
to
the
methyl
˝
800 °C
are reported
reported in
in Figure
Figure 2b.
2b. The
The resonance
resonance peaks
peaks between
between 22
22 and
and 16
16 ppm
ppm are
are assigned
assigned to
to the
the methyl
methyl
800
C are
group;
peaks
ppm
group; peaks
peaks between
between 46
46
and
44
ppm
are
related
to
the
backbone
quaternary
carbon;
the
peak at
at 51.86
51.86 ppm
ppm
backbonequaternary
quaternary carbon;
carbon; the
the peak
peak
at
51.86
group;
between
46 and
and 44
44 ppm
ppmare
arerelated
relatedto
tothe
thebackbone
is
due
to
the
methoxy
group;
the
peak
at
54.47
ppm
is
associated
with
the
backbone
methylene
group;
is due
due to
to the
the methoxy
methoxy group;
group; the
the peak
peak at
at 54.47
54.47 ppm
ppm is
is associated
associated with
with the
the backbone
backbone methylene
methylene group;
group;
is
and
the
peaks
between
179
and
176
ppm
are
correlated
to
the
PMMA
carbonyl
carbon.
No
modification
and the
the peaks
peaks between
between 179
179 and
and 176
176 ppm
ppm are
are correlated
correlated to
to the
the PMMA
PMMA carbonyl
carbonyl carbon.
carbon. No
No modification
modification
and
in
the
chemical
shift
is
observed
with
the
addition
of
CeO
nanoparticles,
indicating
that
no
primary
2
in the
the chemical
chemical shift
shift is
is observed
observed with
with the
the addition
addition of
of CeO
CeO22 nanoparticles,
nanoparticles, indicating
indicating that
that no
no primary
primary
in
chemical
bond
occurred
with
the
polymer.
chemical bond
bond occurred
occurred with
with the
the polymer.
polymer.
chemical
(a)
(a)
-OCH
-OCH3
PMMA/CeO
PMMA/CeO22,, 1wt.%
1wt.%,, 800
800 °C
°C
3
PMMA/CeO
PMMA/CeO22,, 1wt.%
1wt.%,, 800
800 °C
°C
CH3
CH3
C
C
CH2
CH2
α
α-C
-CH
H3
r
r
rr
rr
mm
mm
mm
-CH
-CH22--
(b)
(b)
C
C
3
O
O
n
n
OCH 3
OCH 3
TMS
TMS
PMMA/CeO
PMMA/CeO22,, 0.5wt.%
0.5wt.%,, 800
800 °C
°C
CH2
CH2
-C=O
PMMA
PMMA
CH 3
CH 3
rr -C=O
rr mr
mr
C
C
C
C
OCH 3
OCH 3
PMMA/CeO
PMMA/CeO22,, 0.5wt.%
0.5wt.%,, 800
800 °C
°C
CDCl
CDCl3
-OCH3
-OCH3
-C-C-
mm
mm
O
O
nn
4.5
4.5 4.0
4.0 3.5
3.5 3.0
3.0 2.5
2.5 2.0
2.0 1.5
1.5 1.0
1.0 0.5
0.5 0.0
0.0 -0.5
-0.5
Chemical
Chemical shift
shift (ppm)
(ppm)
180
180
179
179
178
178
177
177
ppm
ppm
176
176
mrrr
mrrr
mm
mm
-CH2-CH2-
175
175
54
54
52
52
50
50
48
48
ppm
ppm
46
46
200
200 180
180 160
160 140
140 120
120 100
100
-CH3
-CH3
3
44
44
80
80
mr
mr
PMMA
PMMA
rr
rr
mm
mm
22
22
21
21
20
20
60
60
19
19
18
18
ppm
ppm
40
40
17
17
16
16
20
20
Chemical
Chemical shift
shift (ppm)
(ppm)
Figure
2.
NMR spectra
spectra of pure
PMMA and
PMMA/CeO22 hybrid
systems by
(a) 111H
NMR
Figure
2.
Figure 13
2. NMR
NMR spectra of
of pure
pure PMMA
PMMA and
and PMMA/CeO
PMMA/CeO2 hybrid
hybrid systems
systems by
by (a)
(a) H
H NMR
NMR
and
(b)
NMR.
13
13C
and
(b)
C
NMR.
and (b) C NMR.
Figure 3. Proposed interaction between the PMMA matrix and CeO2 nanoparticles.
Figure
Figure 3.
3. Proposed
Proposed interaction
interaction between
between the
the PMMA
PMMA matrix
matrix and
and CeO
CeO22 nanoparticles.
nanoparticles.
15
15
Polymers
Polymers2015,
2015,77
16447
3.1.3. Structural
Structural and
and Morphological
Morphological Characterization
Characterization
3.1.3.
Figure
Figure 4a
4a shows
showsXRD
XRDpatterns
patternsof
ofthe
theCeO
CeO22 nanoparticles
nanoparticles thermally
thermally treated
treated at
at different
differenttemperatures.
temperatures.
˝
ItIt can
C exhibited
can be
be observed
observed that
that the
the CeO
CeO22 powders
powders treated
treated at
at 120
120 °C
exhibited low
low intensity
intensity broad
broad peaks
peaks about
about
˝
2θ
2θ == 28.5,
28.5, 33.0,
33.0, 47.5
47.5 and
and 56.3
56.3°,, which
which were
were assigned
assigned to
to the
the (111),
(111), (200),
(200), (220)
(220) and
and (311)
(311) planes,
planes,
respectively,
respectively,and
andcorrespond
correspondtotothe
thecubic
cubicfluorite
fluoritecrystal
crystalstructure
structure(ICDD
(ICDD81-0792).
81-0792).ItItisisclear
clearthat
thatthese
these
˝
reflection
thethe
increasing
temperature
up up
to 800
C,°C,
indicating
an
reflectionpeaks
peaksbecome
becomesharper
sharperand
andnarrower
narrowerwith
with
increasing
temperature
to 800
indicating
˝
increase
in the
average
crystallite
size
andand
crystallinity
ofof
nanoceria
an increase
in the
average
crystallite
size
crystallinity
nanoceriapowders.
powders.The
Thepeaks
peaksatat59.0
59.0°(222),
(222),
˝
˝
˝
˝
69.3
69.3°(400),
(400),76.6
76.6°(331),
(331),79.0
79.0°(420)
(420)and
and88.4
88.4°(422)
(422)also
alsoconfirms
confirmsthe
thepresence
presenceofofthe
thecubic
cubicphase.
phase.
(220)
(311)
(111)
Cubic phase-CeO2, ICDD 81-0792
(422)
PMMA/CeO 2, 1wt.%, 800 °C
PMMA/CeO 2, 0.5wt.%, 800 °C
600 °C
400 °C
235 °C
PMMA/CeO 2, 1wt.%, 600 °C
Intensity (a.u.)
(420)
(331)
(400)
(222)
(311)
(220)
Intensity (a.u.)
(200)
(b)
800 °C
(200)
Cubic phase-CeO2, ICDD 81-0792
(111)
(a)
PMMA/CeO 2, 0.5wt.%, 600 °C
PMMA/CeO 2, 1wt.%, 400 °C
PMMA/CeO 2, 0.5wt.%, 400 °C
PMMA/CeO 2, 1wt.%, 235 °C
PMMA/CeO 2, 0.5wt.%, 235 °C
PMMA/CeO 2, 1wt.%, 120 °C
120 °C
PMMA/CeO 2, 0.5wt.%, 120 °C
PMMA
20
30
40
50
60
70
80
90
20
30
40
50
60
70
80
90
2θ (degrees)
2θ (degrees)
Figure 4. XRD patterns of (a) nanocrystalline CeO2 powders thermally treated at different
Figure 4. XRD patterns of (a) nanocrystalline CeO2 powders thermally treated at different
temperatures and (b) pure PMMA and PMMA/CeO2 hybrid systems.
temperatures and (b) pure PMMA and PMMA/CeO2 hybrid systems.
The
was estimated
estimated from
from XRD
XRD patterns
patterns by
by applying
applying the
the full-width
full-width atat
The crystallite
crystallite size
size (t)
(t) of
of CeO
CeO22 was
half-maximum
half-maximum(β)
(β) of
of the
the characteristic
characteristic (111)
(111) peak
peak in
in the
the Scherrer
Scherrer Equation
Equation [34]:
[34]:
0.9λ
0.9
t “=
βcosθβ
(1)
(1)
where
whereλλ isis the
the incident
incidentX-ray
X-raywavelength
wavelength(1.54056
(1.54056Å)
Å)and
andθθββ is
is the
the diffraction
diffraction angle
angle for
forthe
the(111)
(111)plane.
plane.
The
average
CeO
crystallite
diameters
determined
with
the
Scherrer
Equation
were
about
5.5
˘
2
0.1,
The average CeO2 crystallite diameters determined with the Scherrer Equation were about 5.5 ± 0.1,
˝
at
120,
235,
400,
600
and
800
C,
8.6
˘
0.2,
8.8
˘
0.1,
29.3
˘
0.7
and
47.5
˘
1.1
nm
for
CeO
powders
2
8.6 ± 0.2, 8.8 ± 0.1, 29.3 ± 0.7 and 47.5 ± 1.1 nm for CeO2 powders
at 120, 235, 400, 600 and 800 °C,
respectively.
respectively.The
Thecrystallite
crystallitesize
sizeincreased
increasedalong
alongwith
withthe
the increasing-thermal-treatment
increasing-thermal-treatmenttemperature
temperaturedue
due
to
a
typical
diffusion
effect.
Figure
4b
shows
the
XRD
patterns
of
the
PMMA/CeO
hybrid
systems.
2
to a typical diffusion effect. Figure 4b shows the XRD patterns of the PMMA/CeO2 hybrid systems.
˝
The
pattern of
ofpure
purePMMA
PMMAshows
showsa broad
a broad
diffraction
peak
at =2θ
= 14.5
two
The diffraction
diffraction pattern
diffraction
peak
at 2θ
14.5°
with with
other other
two lower
˝
lower
intensity
signals
centered
= 30.7
42.1as, as
a resultofofthe
thediffuse
diffuse scattering
scattering of
intensity
signals
centered
at 2θat=2θ30.7
andand
42.1°,
a result
of amorphous
amorphous
PMMA
PMMA [35].
[35]. The
The shape
shape of
of the
the first
first main
main signal
signal indicates
indicates the
the ordered
ordered packing
packing of
of the
the polymer
polymer chains.
chains.
The
intensity
and
shape
of
the
second
signal
are
attributed
to
the
effect
inside
the
main
chains
The intensity and shape of the second signal are attributed to the effect inside the main chains [36].
[36].
The
diffraction
patterns
of
all
PMMA/CeO
hybrid
systems
show
the
first
two
signals
observed
in
The diffraction patterns of all PMMA/CeO22 hybrid systems show the first two signals observed in the
the
pure
purePMMA,
PMMA,indicating
indicatingthat
thatneither
neitherthe
thepresence
presenceof
ofthe
thenanoparticles
nanoparticlesnor
northe
thepreparation
preparationprocess
processchanges
changes
˝
the
the PMMA
PMMA chains.
chains.The
ThePMMA
PMMA systems
systems with
with CeO
CeO2 thermally
thermally treated
C (1.0
the orientation
orientation of
of the
treated at
at 600
600 °C
(1.0wt
wt%)
%)
˝
and 800 C (0.5 and 1.0 wt %) display a strong tendency toward an amorphous structure with
low-intensity peaks at 2θ = 28.5, 33.0, 47.5 and 56.3˝ , which corresponds to the fcc structure of CeO2 .
Polymers 2015, 7
1645
The lattice parameter was calculated from XRD patterns and the results confirmed that except for
120 ˝ C, where a value of 5.4448 Å was obtained, a quite small change in the lattice parameter was
observed with the thermal treatment: 5.4192 Å (235 ˝ C), 5.4187 Å (400 ˝ C), 5.4107 Å (600 ˝ C) and
5.4127 Å (800 ˝ C). It was determined that the lattice parameter showed a trend of expansion with the
annealing temperature but the changing extent is moderately limited. The lattice distortion rate (∆a/a)
was estimated to be between 0.60% and 0.013%. The highest distortion was obtained at 120 ˝ C and is
undoubtedly due to this temperature that some organic compounds or water or both can be still partially
bonded with cerium powders. The formation of nanoceria powders by sol–gel contains complicated
reactions; in the first instance, they began to nucleate into Ce(OH)3 form, so the supplied energy
during the sol–gel process and subsequent heat treatment must be enough for the complete conversion
to Ce(OH)4 nuclei and thereafter into CeO2 nuclei via dehydration and subsequent growth of highly
crystallized nanoceria.
TEM images of CeO2 in bright field mode with their corresponding selected area electron diffraction
pattern (SAEDP) are shown in Figure 5. The CeO2 nanopowders exhibit a heterogeneous size and
non-uniform shape with a high agglomeration degree. Indeed, a higher treatment temperature seems
to increase the particle size. The images show that the CeO2 powders thermally treated at 120 ˝ C are
constituted of aggregates with sizes between 0.1 and 0.2 µm and composed by crystals of about 1–5 nm
in diameter. CeO2 thermally treated at 235 ˝ C shows aggregates between 0.2 and 0.3 µm with irregular
smaller crystals of about 4–12 nm, while CeO2 thermally treated at 400 ˝ C consists of aggregates formed
of nanocrystals with an average size of 5–13 nm. When the heat treatment of CeO2 is increased to 600 ˝ C,
the resulting samples are 0.45–0.35 µm aggregates with semispherical nanocrystal sizes around 4–22 nm.
Likewise, CeO2 thermally treated at 800 ˝ C shows aggregates between 0.52–0.75 µm constituted by
nanocrystals of about 20–80 nm. The SAEDPs of CeO2 thermally treated at different temperatures
(120, 235, 400, 600 and 800 ˝ C) show concentric rings, which also indicate the polycrystalline nature of
the powder and that the crystallite size is on the nanoscale. The SAEDPs were indexed according to the
ICDD card of face-centered cubic phase of CeO2 . The mean crystal size values and the diffraction rings
of SAED obtained in TEM images for CeO2 matched well with XRD results.
From measurements of dynamic light scattering, different particle diameter moments (number-average
diameter Dn ; weight-average diameter Dw and z-average diameter Dz ) were calculated using
Equations (2–4), and the polydispersity index (PDI) was determined using Equation (5):
ř
ni Di
Dn “ ř
(2)
ni
ř
ni Di4
ř
(3)
Dw “
ni Di3
ř
ni Di6
Dz “ ř
(4)
ni Di5
Dw
Dn
where ni is the number of CeO2 nanoparticles with diameter Di .
P DI “
(5)
Polymers 2015, 7
Polymers 2015, 7
1646
9
Figure
Figure 5.5. TEM
TEM images
images and
and SAEDPs
SAEDPs of
of nanocrystalline
nanocrystalline CeO
CeO22 powders
powders thermally
thermally treated
treated atat
different
differenttemperatures.
temperatures.
The particle
particle size
size distributions
distributions (PSDs)
(PSDs) for
for the
the samples
samples are
are shown
shown in
in Figure
Figure 6,6, whereas
whereas the
the obtained
obtained
The
averageparticle
particlesize
sizeand
andcalculated
calculatedpolydispersity
polydispersity
index
(PDI)
shown
in Table
1. From
average
index
(PDI)
areare
shown
in Table
1. From
thesethese
data,data,
the
˝
sizeincreased
was increased
as the temperature
to 400
°C. After this
the 2CeO
2 particle
CeO
particle
size was
as the temperature
varied fromvaried
120 tofrom
400 120
C. After
this temperature,
a
temperature,
decreasesize
in the
size(600
wasand
observed
(600
800 °C). to
It is
noteworthy
to mention
decrease
in thea particle
wasparticle
observed
800 ˝ C).
It isand
noteworthy
mention
that the
particle
that
the
particle
size
distribution
can
be
influenced
by
sonification
and
dispersant
agent
[37].
size distribution can be influenced by sonification and dispersant agent [37]. Furthermore, the CeO
2
Furthermore,
CeO2 particle
powderssizes
haveand
different
particle
andconsidered
therefore they
can be considered
as
powders
have the
different
therefore
they sizes
can be
as polydispersed
systems
polydispersed
systemsthese
(PDIresults
> 1).confirm
In general,
these results
confirm the
presence ofdue
agglomerated
(PDI
> 1). In general,
the presence
of agglomerated
nanoparticles
to Van der
nanoparticles
due
to
Van
der
Waals
forces
and
their
high
surface
energy.
Waals forces and their high surface energy.
Polymers 2015, 7
Polymers 2015, 7
1647
10
Figure 6. Size distribution of CeO2 thermally treated at different temperatures.
Figure 6. Size distribution of CeO2 thermally treated at different temperatures.
Table 1. Particle size distribution of the as-prepared CeO powders.
Table 1. Particle size distribution of the as-prepared CeO22 powders.
˝
CeO
C
2 120
CeO
2 120 °C
Size
Mean
Size
Mean
d, nm number %
d, nm number %
68.06
0.40.4
68.06
78.82
3.73.7
78.82
91.28
11.1
91.28
11.1
105.7
19.0
105.7
19.0
122.4
22.4
122.4
22.4
141.8
21.3
141.8
21.3
164.2
15.1
164.2
15.1
190.1
5.6
190.1
5.6
220.2
0.7
220.2
0.7
255.0
0.3
255.0
295.3
0.20.3
295.3
342.0
0.10.2
342.0
396.1
0.10.1
396.1
Dz = 212 nm0.1
˝
CeO
C
2 235
CeO
2 235 °C
Size
Mean
Size
Mean
d, nm number %
d, nm number %
190.1
3.9
190.1
3.9
220.2
10.2
220.2
10.2
255.0
17.1
255.0
17.1
295.3
25.9
295.3
25.9
342.0
25.3
342.0
25.3
396.1
13.9
396.1
13.9
458.7
3.7
458.7
3.7
-----D-z = 368.6 nm
Dz = 212 nm
Polydispersity
index
= 1.2483
Polydispersity
Dz = 368.6 nm
Polydispersity
index
= 1.1234
Polydispersity
index = 1.2483
index = 1.1234
˝
CeO
400 °C
C
CeO22 400
Size
Mean
Size
Mean
d, nm number %
d, nm number %
220.2
4.7
220.2
4.7
255.0
14.0
255.0
14.0
295.3
18.9
295.3
18.9
342.0
19.2
342.0
19.2
396.1
13.8
396.1
13.8
458.7
6.6
458.7
6.6
531.2
7.4
531.2
7.4
615.1
9.2
615.1
9.2
712.4
5.1
712.4
5.1
825.0
1.1
825.0
1.1
-Dz = 635.1 -nm
D
Polydispersity
z = 635.1 nm
index
= 1.4051
Polydispersity
index = 1.4051
˝
CeO2600
600°C
C
CeO
2
Size
Mean
Size
Mean
d, nm number %
d, nm number %
190.1
5.5
190.1
5.5
220.2
17.8
220.2
17.8
255.0
24.8
255.0
24.8
295.3
22.0
295.3
22.0
342.0
15.6
342.0
15.6
396.1
9.1
396.1
9.1
458.7
4
458.7
4
531.2
1.1
531.2
1.1
615.1
0.1
615.1
0.1
------- Dz = 392.7- nm
DPolydispersity
z = 392.7 nm
index = 1.1964
Polydispersity
index = 1.1964
˝
CeO800
2 800
CeO
°C C
2
Size
Mean
Size
Mean
d, nm number %
d, nm number %
141.8
14.9
141.8
14.9
164.2
29.9
164.2
29.9
190.1
20.0
190.1
20.0
220.2
15.0
220.2
15.0
255.0
15.1
255.0
15.1
295.3
5.1
295.3
5.1
- - - - - - - - - Dz = 242.7nm
-
DzPolydispersity
= 242.7nm
index = 1.1523
Polydispersity
index = 1.1523
Polymers 2015, 7
Polymers 2015, 7
1648
11
3.2. Characterization
Characterization of
of PMMA/CeO
PMMA/CeO22 Hybrid
Hybrid Materials
Materials
3.2.
3.2.1. Dispersion
Dispersion Analysis
Analysis
3.2.1.
Dispersion is
is the
the key
key factor
factor that
that determines
determines quality
quality and
and properties
properties of
of hybrid
hybrid compounds
compounds [38].
[38]. During
During
Dispersion
the dispersion
dispersion process
process two
two kinds
kinds of
of interaction
interaction can
can occur:
occur: (i)
(i) polymer
polymer filler
filler interaction,
interaction, adhesion
adhesion of
of the
the
the
polymer to
to the
the surface
surface of
of the
the particles;
particles; and
and (ii)
(ii) filler-filler
filler-filler interaction,
interaction, where
where the
the particles
particles interact
interact to
to form
form
polymer
aggregates. Figure
Figure 77 shows
shows the
the 3D
3D images
images revealing
revealing the
the CeO
CeO22 nanoparticles’
nanoparticles’ location
location within
within the
the PMMA.
PMMA.
aggregates.
In these
these images,
images,ititcan
canbe
beobserved
observedthat
thatininallallthethePMMA/CeO
PMMA/CeO
systems
nanoparticles
tend
2 hybrid
In
systems
thethe
nanoparticles
tend
to
2 hybrid
According
to
Yang
et
al.,
the
to
form
agglomerates
of
different
sizes
and
semispherical
morphologies.
form agglomerates of different sizes and semispherical morphologies. According to Yang et al., the
nanoparticles tend
tend to
to form
form aggregates
aggregates to
to reduce
reduce the
the surface
surface energy
energy due
due to
to their
their high
high surface
surface energy
energy and
and
nanoparticles
small size
size [39].
[39]. ItIt can
can also
also be
be observed
observed that
that the
the CeO
CeO22 nanoparticles
nanoparticles thermally
thermally treated
treated at
at 120,
120, 235
235 and
and
small
˝
400 °CCshow
showaauniform
uniformdistribution
distributionwithin
within
PMMA
matrix.
In PMMA/CeO
the PMMA/CeO
systems
2 hybrid
systems
with
400
thethe
PMMA
matrix.
In the
2 hybrid
˝
with
nanoparticles
thermally
treated
at
600
and
800
C,
a
small
quantity
of
nanoparticles
can
be
seen
nanoparticles thermally treated at 600 and 800 °C, a small quantity of nanoparticles can be seen because
because
the fluorescence
decreases
the increasing
size. However,
last systems
the
fluorescence
intensityintensity
decreases
with thewith
increasing
crystalcrystal
size. However,
these these
last systems
also
also
show
a
uniform
dispersion,
which
is
a
relevant
aspect
for
increasing
the
thermal
stability
of
the
show a uniform dispersion, which is a relevant aspect for increasing the thermal stability of the
PMMA/CeO22 hybrid
hybrid systems.
systems.
PMMA/CeO
Figure 7.
7. 3D
3D CLSM
CLSM representations
representations of
of PMMA/CeO
PMMA/CeO22 hybrid
hybrid systems.
systems.
Figure
To
the homogeneous
homogeneous distribution
distribution of
of CeO
CeO22 nanoparticles
To confirm
confirm the
nanoparticles into
into the
the polymer
polymer matrix,
matrix, SEM
SEM
observations
observations of
of PMMA/CeO
PMMA/CeO22 cross
cross sections
sections were
were analyzed
analyzed for
for CeO
CeO22 powders
powders sintered
sintered at
at 400,
400, 600
600 and
and
Polymers 2015,
2015, 77
Polymers
1649
12
˝C (Figure 8a–c). The samples were analyzed at different magnifications using both secondary
800
800 °
C (Figure
The samples were analyzed at different magnifications using both secondary
electrons
electrons (SE)
(SE) and
and backscattered
backscattered electrons
electrons (BSE).
(BSE). The
The present
present morphology
morphology confirms
confirms that
that even
even with
with the
the
tendency
tendency to
to form
form some
some aggregates
aggregates with
with aa very
very variable
variable size, the
the shear
shear stress
stress during
during melt
melt mixing
mixing separate
separate
the
the CeO
CeO22 nanoparticles
nanoparticles to
to obtain
obtain aa good
good dispersion
dispersion with
with an
an interaction
interaction type
type (i)
(i) throughout
throughout hydrodynamic
hydrodynamic
forces;
forces; in
in such
such aa case,
case, the
the effectiveness
effectiveness is also
also distributed
distributed in the entire
entire sample.
sample. SEM
SEM micrographs
micrographs reveal
reveal
that
that PMMA
PMMA incorporates
incorporates semispherical
semispherical CeO
CeO22 domains,
domains, which
which are
are an
an ideal
ideal physical
physical form
form to
to improve
improve
polymer
polymer properties.
properties.
Figure 8.
8. Cross-section
Cross-section SEM
SEM micrographs
micrographs of
of PMMA
PMMA with
with 1.0
1.0 wt
wt %
% of
of CeO
CeO22 nanoparticles
nanoparticles
Figure
˝
˝
˝
sintered at
at (a)
(a) 400
400 °C;
C; (b)
(b) 600
600 °C
C and
and (c)
(c) 800
800 °C.
C.
sintered
3.2.2.
3.2.2. Optical
Optical Properties
Properties
The
The Kulbeka–Munk
Kulbeka–Munk Equation
Equation was used to obtain the accurate
accurate band gap energy of CeO2
nanoparticles
nanoparticles (E
(Egg).). The
Thereflectance
reflectance data
data were
were converted
converted to the absorption coefficient F(R8
values using
using
∞)) values
the
the following
following equation:
p1 ´ R8 q22
(6)
F pR8 q) “ (1 − 𝑅∞ )
𝐹(𝑅∞ =
(6)
2R
8
2𝑅∞
where R8 is the diffused reflectance at a given wavelength of an infinitely thick sample and
where R∞ is the diffused reflectance at a given wavelength of an infinitely thick sample and
hC
E peV q “ ℎ𝐶
(7)
𝐸(𝑒𝑉) = λ
(7)
𝜆
´34
8
´1
where hh isisPlanck’s
Planck’sconstant
constant(6.626
(6.626× 10
ˆ −34
10 J s),JCs),is the
C is light
the light
speed
8
−1 ) and λ is the
where
speed
(3.0(3.0
׈
1010
msms
) and λ is the
2
wavelength
(nm)
[40].
The
relationship
between
(F(R
)hv)
and
the
photon
energy
of
CeO
nanoparticles
wavelength (nm) [40]. The relationship between (F(R∞8
)hv)2 and the photon energy of CeO22nanoparticles
thermally treated
treated at
at different
different temperatures
temperatures are
are shown
shown in
in Figure
Figure 9.
9. The
The indirect
indirect band
band gap
gap energy
energy (E
(Egg)) was
was
thermally
obtained from
from the
the intersection
intersection of
of the
the extrapolated
extrapolated linear
The EEgg values
values of
of the
the CeO
CeO22 powders
powders
obtained
linear portion.
portion. The
˝
thermallytreated
treatedatat120,
120,235,
235,400,
400,600
600and
and800
800°CC were
were 3.40
3.40 ˘
0.01, 3.34
3.34 ˘
0.02, 3.33
3.33 ±
˘ 0.03,
0.03, 3.40
3.40 ±
˘ 0.02
0.02
thermally
±0.01,
±0.02,
and 3.44
3.44 ±
˘0.01,
0.01,respectively.
respectively. ItItreveals
reveals that
that the
the EEgg decreases
decreases with
with increasing
increasing the
the treatment
treatment temperature
temperature
and
˝
˝
up to
to 400
400 °C
andand
800800
C.°C.
In
up
C and
and then
then aa slight
slight increase
increaseisisobserved
observedwith
withthe
thepowders
powderstreated
treatedat at600
600
22
addition,
in in
thethe
inset,
powders show
show aa strong
strong
8 )hv)
In
addition,
inset,(F(R
(F(R
vsλλcurves,
curves,itit was
was found
found that
that all
all the
the CeO
CeO22 powders
∞)hv) vs
charge-transferbetween
betweenthe
the O
O 2p
2p and
and
absorptionband
bandatat210–350
210–350nm
nmininthe
theUV
UVrange
rangeoriginated
originatedfrom
fromthe
thecharge-transfer
absorption
2´
4+
Ce 4f
4f states
states in
in O
O2− and
andCe
Ce4+ [41].
[41].UV
UVshielding
shieldingmaterials
materialsare
aregenerally
generallyexpected
expectedtotoabsorb
absorbUV
UVlight
light at
at less
less
Ce
than400
400nm
nmwavelengths,
wavelengths, as
as obtained
obtained for
for the
the CeO
CeO22 nanoparticles
nanoparticles thermally
thermally treated
treated at
at different
different temperatures.
temperatures.
than
Transmissionspectra
spectraininthe
theUV-Vis
UV-Visregion
regionfor
forpure
purePMMA
PMMAand
andPMMA
PMMAsamples
sampleswith
with0.5
0.5and
and 1.0
1.0 wt
wt %
%
Transmission
CeO22 at
at different
different thermal-treatment
thermal-treatment temperatures
temperatures are
are shown
shown in
in Figure
Figure 10.
10. ItItcan
canbe
beseen
seenthat
thatthe
theUV-Vis
UV-Vis
CeO
Polymers 2015, 7
1650
Polymers 2015, 7
13
absorption of the PMMA/CeO2 nanocomposites increased with the CeO2 nanoparticle content in the
nanoparticlesthermally
thermallytreated
treatedatat120
120 and
and 235
235 ˝°C
C
nanocomposite.
ThePMMA/CeO
PMMA/CeO2 2hybrid
hybridsystem
systemwith
withnanoparticles
nanocomposite. The
shows
shows similar
similar behavior
behavior with
with aa transmittance
transmittance decrease
decrease of
of about
about 22
22 and
and 40
40 percent
percent (0.5
(0.5 and
and 1.0
1.0 wt
wt %)
%) in
in
comparison
with
pure
PMMA.
This
strong
decrease
is
primarily
attributed
to
the
organic
matter
present
comparison with pure PMMA. This strong decrease is primarily attributed to the organic matter present
in
as it
it was
was observed
in CeO
CeO22 as
observed by
by FTIR,
FTIR, as
as aa result
result of
of the
the low
low thermal-treatment
thermal-treatment temperatures.
temperatures. On
On the
the other
other
˝
hand,
the
PMMA/CeO
hybrid
systems
with
0.5
and
1.0
wt
%
CeO
thermally
treated
at
400
C
display
2
2
hand, the PMMA/CeO2 hybrid systems with 0.5 and 1.0 wt % CeO2 thermally treated at 400 °C display
aa transmittance
transmittance reduction
reduction of
of approximately
approximately 7–15
7–15 percent
percent in
in comparison
comparison with
with pure
pure PMMA,
PMMA, which
which are
are the
the
most
transparent
systems.
It
can
be
noticed
that
of
all
the
hybrid
systems,
nanocomposites
with
0.5
wt
most transparent systems. It can be noticed that of all the hybrid systems, nanocomposites with 0.5 wt %
%
˝
of
at 400
400 °C
C showed
showed the
of CeO
CeO22 at
the highest
highest percentage
percentage of
of transmittance,
transmittance, whereas
whereas the
the PMMA
PMMA with
with 1.0
1.0 wt
wt %
%
˝
of
CeO
treated
at
high
temperatures
(600
and
800
C)
showed
lower
values
in
comparison
with
systems
2
of CeO2 treated at high temperatures (600 and 800 °C) showed lower values in comparison with systems
˝
formed
C. The
formed by
by CeO
CeO22 at
at 400
400 °C.
The aforesaid
aforesaid is
is related
related to
to the
the formations
formations of
of small
small crystalline
crystalline domains
domains
embedded
embedded in
in the
the amorphous
amorphous PMMA
PMMA as
as well
well as
as crystal
crystal size
size growth
growth with
with sintered
sintered temperature.
temperature. Figure
Figure 11
11
shows
shows photographs
photographs of
of these
these films,
films, where
where itit is
is evident
evident that
that even
even the
the presence
presence of
of aa small
small percentage
percentage of
of
CeO
leads
opacity
or
turbidity
due
to
the
fact
that
the
agglomerated
CeO
nanoparticles
act
as
a
strong
CeO2 leads opacity or turbidity due to the fact that the agglomerated CeO22 nanoparticles act as a strong
scattering
scattering center.
center. This
This point
point also
also suggests
suggests that
that the
the UV
UV shielding
shielding properties
properties of
of the
the hybrid
hybrid films
films are
are mainly
mainly
due
due to
to the
the scattering
scattering of
of these
these agglomerates
agglomerates in
in the
the film.
film. As
As it
it was
was mentioned,
mentioned, the
the particles
particles agglomeration
agglomeration
enhanced with the thermal treatment and crystallite size; however, the aggregates are homogeneously
distributed within the polymeric
polymeric matrix.
matrix.
Figure
Figure 9.
9. Cont.
Cont.
Polymers 2015, 7
Polymers
Polymers2015,
2015,77
1651
14
14
Figure
9. Kubelka–Munk modified
spectra and
their equivalence
in hν
vs λλ (insert)
of CeO
Figure
Figure9.9.Kubelka–Munk
Kubelka–Munkmodified
modifiedspectra
spectraand
andtheir
theirequivalence
equivalencein
inhν
hνvs
vs λ(insert)
(insert)of
ofCeO
CeO222
powders
thermally treated
at different temperatures.
powders
powdersthermally
thermallytreated
treatedatatdifferent
differenttemperatures.
temperatures.
Likewise,
nanocomposites
Likewise,
spectra
of
pure
PMMA
and
PMMA/CeO
show
an
absorption
band
atat
Likewise, spectra
spectra of
of pure
pure PMMA
PMMA and
and PMMA/CeO
PMMA/CeO222 nanocomposites
nanocomposites show
show an
an absorption
absorption band
band at
about
isisattributed
to
transition
of
C=O
the
This
band
about
272nm,
nm,which
which
isattributed
attributed
to n-π*
the
transition
thegroup
C=Oin
in the[42].
PMMA
about272
nm,
which
tothe
the
n-π*n-π*
transition
ofthe
theof
C=O
group
ingroup
thePMMA
PMMA
[42].
This[42].
band
isis attenuated
of
atat2 different
causing
This
band is with
attenuated
with the incorporation
of CeO
treated attemperatures
different temperatures
causing
a
attenuated
with the
the incorporation
incorporation
of CeO
CeO22 treated
treated
different
temperatures
causing aa significant
significant
decrease
transmittance
in
nm
which
can
be
associated
band-gap
significant
decrease
in transmittance
in the 250–360
nm range,
which
associatedwith
withthe
the band-gap
decrease in
in
transmittance
in the
the 250–360
250–360
nm range,
range,
which
cancan
bebe
associated
with
band-gap
absorption
of
CeO
by
UV
and
visible
absorption
In
general,
the
synthetic
polymers
are
susceptible
to
degradation
absorptionof
ofCeO
CeO222...In
Ingeneral,
general,the
thesynthetic
syntheticpolymers
polymersare
aresusceptible
susceptibleto
todegradation
degradationby
byUV
UVand
andvisible
visible
light;
irradiation between
between
260
and
300
nm
[43].
light;
specifically,
the
photodegradation
ofPMMA
PMMAtook
tookplace
light;specifically,
specifically,the
thephotodegradation
photodegradationof
placeby
byirradiation
irradiation
between260
260and
and300
300nm
nm[43].
[43].
The
transmitting
efficiency
of
the
PMMA/CeO
hybrid
systems
in
the
UV
band
up
to
350
nm
isis
The
The transmitting
transmitting efficiency
efficiency of
of the
the PMMA/CeO
PMMA/CeO222 hybrid
hybrid systems
systems in
in the
the UV
UV band
band up
up to
to 350
350 nm
nm is
approximately
zero,
demonstrating
that
such
hybrid
systems
are
good
option
for
exterior
applications.
approximately
approximatelyzero,
zero,demonstrating
demonstratingthat
thatsuch
suchhybrid
hybridsystems
systemsare
areaaagood
goodoption
optionfor
forexterior
exteriorapplications.
applications.
Figure 10.
10. UV-Vis spectra
of pure
PMMA and
PMMA/CeO22 hybrid
systems.
Figure
Figure 10.UV-Vis
UV-Visspectra
spectraof
ofpure
purePMMA
PMMAand
andPMMA/CeO
PMMA/CeO2hybrid
hybridsystems.
systems.
Polymers 2015, 7
Polymers 2015, 7
1652
15
Figure 11.
Photographs of
of the
the pure
pure PMMA
PMMA and
and PMMA/CeO
PMMA/CeO22 hybrid
hybrid systems.
systems.
Figure
11. Photographs
3.2.3.
3.2.3. Thermal
Thermal Studies
Studies
Different
been performed
performed on
in the
the thermal
thermal degradation
of
Different studies
studies have
have been
on the
the different
different steps
steps involved
involved in
degradation of
PMMA.
According to
to Kashiwagi
Kashiwagietetal.
al.[44],
[44],the
thebond
bonddissociation
dissociationenergy
energyofof
H–H
linkages
is estimated
PMMA. According
H–H
linkages
is estimated
to
to
of C–C
the C–C
backbone
the steric
large steric
hindrance
and inductive
of
be be
lessless
thanthan
thatthat
of the
backbone
bondbond
due todue
thetolarge
hindrance
and inductive
effect ofeffect
vicinal
˝
vicinal
ester groups.
first degradation
step (155–220
°C) is initiated
by scissions
of head-to-head
ester groups.
The firstThe
degradation
step (155–220
C) is initiated
by scissions
of head-to-head
linkages
linkages
(H–H).
Likewise,
PMMA
chains
having
unsaturated
end
group
are
less
thermally
stable
than
(H–H). Likewise, PMMA chains having unsaturated end group are less thermally stable than those with
those
withend
saturated
groups. Therefore,
step˝ C)
(230–300
°C) by
is initiated
at
saturated
groups.end
Therefore,
the second the
stepsecond
(230–300
is initiated
scissionsby
at scissions
unsaturated
˝
unsaturated
ends
involvingscission
a hemolytic
scission
β to and
the vinyl
and theC)last
step (>300
°C) is
ends involving
a hemolytic
β to the
vinyl group
the lastgroup
step (>300
is initiated
by random
initiated
random
scission
withinchain.
the main
polymer
chain.
Typical TGA
and derivative thermogravimetry
scission by
within
the main
polymer
Typical
TGA
and derivative
thermogravimetry
(DTG) results on
(DTG)
results
on
PMMA
and
PMMA/CeO
hybrid
materials
degraded
under
argon
atmosphere
are
2
PMMA and PMMA/CeO2 hybrid materials degraded
under argon atmosphere are shown
in Figure 12.
˝
shown
in 12a,
Figure
12. be
In observed
Figure 12a,
canpure
be observed
that the
pure PMMA
shows
weight
In Figure
it can
thatit the
PMMA shows
appreciable
weight
lossappreciable
at around 240
C,
loss
around
240 °C, 2while
the PMMA/CeO
nanocomposites
loss at 320
whileatthe
PMMA/CeO
nanocomposites
show 2detectable
weight show
loss atdetectable
320 ˝ C. It weight
is also evident
that °C.
the
It
is
also
evident
that
the
stability
of
the
nanocomposites
is
enhanced
with
the
thermal
treatment
applied
stability of the nanocomposites is enhanced with the thermal treatment applied to CeO2 nanopowders.
˝
to
CeO2 nanopowders.
Particularly,
PMMA/CeO2 hybrid systems with 0.5–1.0 wt % CeO2 thermally
treated
Particularly,
PMMA/CeO
2 hybrid systems with 0.5–1.0 wt % CeO2 thermally treated at 120 C began to
˝
at
120a°C
beganloss
to show
weight
below 250
°C. ˝Underneath
200is°C
the weightreferred
loss is commonly
show
weight
belowa250
C. loss
Underneath
to 200
C the weighttoloss
commonly
to removal
˝
referred
to
removal
of
water
and
organic
components
from
the
structure,
whereas
between
200 and
of water and organic components from the structure, whereas between 200 and 300 C corresponds
300
°C corresponds
to nitrates
andorganic
carbonization
organic
parts [45].
of the Furthermore,
precursor [45].a
to nitrates
decomposition
and decomposition
carbonization of
parts ofofthe
precursor
˝
Furthermore,
single
broad loss,
extending
to about
°C, is observed
for all nanocomposites.
single broad aloss,
extending
from
320 to from
about320
450
C, is450
observed
for all nanocomposites.
From
) and) degradation
50% (T0.5)
From
12a,
datataken
werefortaken
for the
onset temperature
at which
(T0.1(T
FigureFigure
12a, data
were
the onset
temperature
at which 10%
(T 0.1 ) 10%
and 50%
0.5
degradation
occurs
and the corresponding
difference
(ΔT)the
between
the onset temperatures
occurs and the
corresponding
temperaturetemperature
difference (∆T)
between
onset temperatures
of hybrid
of
hybrid
systems
and
PMMA
(Table
2).
Evidently,
the
onset
degradation
temperatures
of
PMMA/CeO
systems and PMMA (Table 2). Evidently, the onset degradation temperatures of PMMA/CeO2 hybrid2
hybrid
to higher
temperatures
in comparison
with of
thatpure
of pure
PMMA,
independently
of
systemssystems
shift toshift
higher
temperatures
in comparison
with that
PMMA,
independently
of the
the
2 thermal-treatment temperature and content, indicating that the CeO2 nanocrystals can significantly
CeOCeO
2 thermal-treatment temperature and content, indicating that the CeO2 nanocrystals can significantly
improve
the thermal
thermal stability
stability of
of the
the polymer.
that the
improve the
polymer. As
As seen
seen in
in Figure
Figure 12b,
12b, it
it is
is confirmed
confirmed that
the temperature
temperature
at
maximum
degradation
rate
increases
largely
from
273
°C
for
pure
PMMA
to
365
°C
for all
all the
the
˝
˝
at maximum degradation rate increases largely from 273 C for pure PMMA to 365 C for
PMMA/CeO
PMMA/CeO22 nanocomposites.
nanocomposites. Although
Although the
the CeO
CeO22 nanoparticles
nanoparticles are
are agglomerated
agglomerated within
within the
the PMMA,
PMMA,
their
homogeneous
presence
play
an
important
role
enhancing
the
thermal
stability
of
PMMA
as
their homogeneous presence play an important role enhancing the thermal stability of PMMA as aa result
result
of
the
mobility
restriction
of
polymer
chains
near
their
interfaces
due
to
the
steric
hindrance,
hindering
of the mobility restriction of polymer chains near their interfaces due to the steric hindrance, hindering
the out-diffusion of the volatile decomposition products [46,47].
the out-diffusion of the volatile decomposition products [46,47].
Polymers
Polymers2015,
2015,77
1653
16
3.5
(a)
100
PMMA
PMMA/CeO2, 0.5wt.%, 120 °C
PMMA/CeO2, 1wt.%, 120 °C
dWt. (mg/°C)
Weight (%)
80
PMMA/CeO2, 0.5wt.%, 235 °C
60
PMMA/CeO2, 1wt.%, 235 °C
PMMA/CeO2, 0.5wt.%, 400 °C
40
PMMA/CeO2, 1wt.%, 400 °C
PMMA/CeO2, 0.5wt.%, 600 °C
20
PMMA/CeO2, 1wt.%, 600 °C
PMMA/CeO2, 1wt.%, 800 °C
150
200
PMMA/CeO2, 1wt.%, 120 °C
PMMA/CeO2, 0.5wt.%, 235 °C
2.5
PMMA/CeO2, 1wt.%, 235 °C
2.0
PMMA/CeO2, 0.5wt.%, 400 °C
PMMA/CeO2, 1wt.%, 400 °C
1.5
PMMA/CeO2, 0.5wt.%, 600 °C
PMMA/CeO2, 1wt.%, 600 °C
1.0
PMMA/CeO2, 0.5wt.%, 800 °C
PMMA/CeO2, 1wt.%, 800 °C
0.0
250
300
350
400
450
50
100
150
Temperature ( C)
200
250
300
º
100
º
50
(b)
PMMA/CeO2, 0.5wt.%, 120 °C
0.5
PMMA/CeO2, 0.5wt.%, 800 °C
0
PMMA
3.0
350
400
450
Temperature ( C)
Figure 12. (a) TGA and (b) DTG curves of pure PMMA and PMMA/CeO2 hybrid systems.
Figure 12. (a) TGA and (b) DTG curves of pure PMMA and PMMA/CeO2 hybrid systems.
Table 2. Onset and difference temperatures (˝ C) at 10 and 50 wt % loss of pure PMMA and
Table 2. Onset and difference temperatures (°C) at 10 and 50 wt % loss of pure PMMA and
PMMA/CeO2 hybrid systems.
PMMA/CeO2 hybrid systems.
Sample
Sample
T0.1 T 0.1
PMMA PMMA
261
˝
PMMA/CeO
,
0.5
wt
%,
120
C
PMMA/CeO2, 0.5 wt %,2 120 °C
222
˝
PMMA/CeO
, 1.0 wt %, 120 178
C
PMMA/CeO
2, 1.0 wt %,2 120 °C
˝
PMMA/CeO
C
2 , 0.5 wt %, 235 277
PMMA/CeO
2, 0.5 wt %, 235 °C
PMMA/CeO2 , 1.0 wt %, 235 ˝ C
328
PMMA/CeO2, 1.0 wt %, 235 °C
PMMA/CeO2 , 0.5 wt %, 400 ˝ C
291
PMMA/CeO2, 0.5 wt %, 400 °C
PMMA/CeO2 , 1.0 wt %, 400 ˝ C
316
PMMA/CeO2, 1.0 wt %, 400 °C
PMMA/CeO2 , 0.5 wt %, 600 ˝ C
337
PMMA/CeO
˝
2, 0.5 wt %, 600 °C
PMMA/CeO
2 , 1.0 wt %, 600 C
˝
°C
339
PMMA/CeO
2, 1.0 wt %, ,600
PMMA/CeO
2 0.5 wt %, 800 C
PMMA/CeO
2, 0.5 wt %,2 ,800
PMMA/CeO
1.0 °C
wt %, 800 ˝338
C
PMMA/CeO2, 1.0 wt %, 800 °C
342
261
222
178
277
328
291
316
337
339
338
342
∆T
ΔT0.1
0.1
T 0.5
∆T
T0.50.5
ΔT0.5
-´39
−39
´83
−83
16
16
67
67
30
30
55
55
76
76
78
78
77
77
81
287
360
361
362
365
364
362
369
366
367
369
287
73
360
74
361
75
362
78
365
77
364
75
362
82
369
79
366
80
367
82
369
-
81
73
74
75
78
77
75
82
79
80
82
The glass transition temperature is generally taken as the inflection point of the specific heat
The glass transition temperature is generally taken as the inflection point of the specific heat
increment at the glass-rubber transition in DSC experiments. Figure 13 shows the effect of CeO2
increment at the glass-rubber transition in DSC experiments. Figure 13 shows the effect of CeO2
nanoparticles on the T g of the PMMA/CeO2 hybrid systems measured by DSC. It can be seen that
nanoparticles on the Tg of the PMMA/CeO2 hybrid systems measured by DSC. It can be seen that the
the incorporation of low contents of CeO2 nanocrystals (0.5 and 1.0 wt %) thermally treated at different
incorporation of low contents of CeO2 nanocrystals (0.5 and 1.0 wt %) thermally treated at different
temperatures slightly increased the glass transition temperature up to ~125 ˝ C in comparison with pure
temperatures slightly increased the glass transition temperature up to ~125 °C in comparison with pure
PMMA ~121 ˝ C. In agreement with previous reports with other composites [33,48], the trend of the
PMMA ~121 °C. In agreement with previous reports with other composites [33,48], the trend of the
glass transition shifting to a slightly higher temperature is attributed to the electrostatic interactions
glass transition shifting to a slightly higher temperature is attributed to the electrostatic interactions
between
between the
the PMMA
PMMA matrix
matrixside
sidechain
chainand
andCeO
CeO22,, which
which hinder
hinder the
the rotation
rotation of
of polymeric
polymeric chains,
chains, leading
leading
to
increased
T
.
g
to increased T .
g
Polymers 2015, 7
Polymers 2015, 7
1654
17
Figure 13. DSC thermograms of the pure PMMA and PMMA/CeO2 hybrid systems.
Figure 13. DSC thermograms of the pure PMMA and PMMA/CeO2 hybrid systems.
3.2.4. Hardness Tests
Tests
Figure 14 shows typical load-displacement
load-displacement curves
curves obtained
obtained in
in the
the nanoindentation
nanoindentation tests
tests for
for pure
pure
PMMA and PMMA/CeO
PMMA/CeO22 hybrid
hybrid systems.
systems. It is clear that the addition of CeO22 nanoparticles within
PMMA increases the mechanical
mechanical properties
properties with
with decreasing
decreasing penetration
penetration depths.
depths. For example, for pure
PMMA,
PMMA, the
the maximum
maximum indentation
indentation depth
depth at
at maximum
maximum load
load (5
(5 mN)
mN) was
was 1049
1049 nm,
nm, whereas
whereas with
with 0.5
0.5 and
and
˝
1.0 wt
wt %
% CeO
CeO22 thermally
at 800
1.0
thermally treated
treated at
800 °C,
C, the
the maximum
maximum depth
depth was
was about
about 874
874 and
and 832
832 nm,
nm, respectively.
respectively.
), Vickers
Vickers hardness
hardness (HV
(HVIT
), elastic
elastic modulus
modulus (E
(EIT
and creep
creep (C
(CIT
results for
for pure
pure
The hardness
hardness (H
(HIT
The
IT),
IT),
IT)) and
IT)) results
polymer and
presented
in Table
3. As
the Hthe
IT and
IT values
polymer
and PMMA/CeO
PMMA/CeO22hybrid
hybridsystems
systemsare
are
presented
in Table
3. expected,
As expected,
H ITHV
and
HV IT
were increased
with nanoceria
powderspowders
content content
from 0.5from
to 1.00.5wtto%1.0
as wt
well%asaswith
values
were increased
with nanoceria
wellthermal
as withtreatment
thermal
temperature,
due to average
size crystal
and crystallinity
CeO2 nanoparticles.
Elastic modulusElastic
in the
treatment
temperature,
due crystal
to average
size and of
crystallinity
of CeO2 nanoparticles.
˝
best case in
(treated
samples
at 235 and
400 °C)
up to C)
24%,
although
was observed.
modulus
the best
case (treated
samples
at enhanced
235 and 400
enhanced
upnotoclear
24%,trend
although
no clear
In general,
a decreaseIningeneral,
creep displacement
been
observed inhas
PMMA/CeO
systems with2
2 hybrid
trend
was observed.
a decrease inhas
creep
displacement
been observed
in PMMA/CeO
increasing
CeOwith
showing
the incorporation
ofthe
rigid
CeO2 nanocrystals
the creep
2 content,
hybrid
systems
increasing
CeOthat
showing that
incorporation
of rigid improves
CeO2 nanocrystals
2 content,
resistance.the
Finally,
time-dependent
of the composites
under
load was
in the
the
improves
creep the
resistance.
Finally,deformation
the time-dependent
deformation
of the
theapplied
composites
under
range ofload
2.46%–7.48%.
In a previous
study, similar
increments
in PMMA
IT and EITstudy,
applied
was in the range
of 2.46%–7.48%.
In aHprevious
similarwere
H IT observed
and EIT increments
nanocomposites
ZrO2 nanoparticles
prepared
by melt
blending andprepared
ZnO nanoparticles
synthesized
were
observed inwith
PMMA
nanocomposites
with ZrO
by melt blending
and
2 nanoparticles
by spin
coating [49,50].
This mechanical
enhancement
is a mechanical
result of a enhancement
homogenous dispersion
of
ZnO
nanoparticles
synthesized
by spin coating
[49,50]. This
is a result of
combined
withofanaggregates
increase in
crystallinity
nanoparticles.
Thus, of
thethe
overall
hardness
aaggregates
homogenous
dispersion
combined
with of
anthe
increase
in crystallinity
nanoparticles.
resultsthe
of overall
as-prepared
samples
indicate
that the PMMA/CeO
systems
provide2greater
2 hybrid
Thus,
hardness
results
of as-prepared
samples indicate
that the
PMMA/CeO
hybrid stiffness
systems
than
pure
PMMA
and
therefore
are
less
ductile.
provide greater stiffness than pure PMMA and therefore are less ductile.
Polymers 2015, 7
Polymers 2015, 7
1655
18
Figure 14. Load-displacement curves of PMMA and PMMA/CeO hybrid systems.
Figure 14. Load-displacement curves of PMMA and PMMA/CeO22 hybrid systems.
Table 3.
Hardness (H
(H IT),), Vickers
Vickers hardness
hardness (HV
(HV IT),), elastic
elastic modulus
modulus (E
(EIT)) and
and creep
creep (C
IT))
Table
3. Hardness
(CIT
IT
IT
IT
values for
for pure
pure PMMA
PMMA and
and PMMA/CeO
PMMA/CeO2 hybrid
hybrid systems.
systems.
values
2
Sample
Sample
IT (MPa)
HITH(MPa)
HVIT
(Vickers)
IT (Vickers)
HV
PMMAPMMA
˝
PMMA/CeO
%, 120 °C
2, 0.5 wt
PMMA/CeO
2 , 0.5 wt %, 120 C
PMMA/CeO
%, 120
°C120 ˝ C
2, 1.0 wt
PMMA/CeO
wt %,
2 , 1.0
PMMA/CeO
%, 235
°C235 ˝ C
PMMA/CeO
wt %,
2, 0.5 wt
2 , 0.5
PMMA/CeO
wt %,
PMMA/CeO
%, 235
°C235 ˝ C
2, 1.0 wt
2 , 1.0
PMMA/CeO
wt %,
PMMA/CeO
%, 400
°C400 ˝ C
2 , 0.5
2, 0.5 wt
˝
PMMA/CeO
wt %,
2 , 1.0
%, 400
°C400 C
PMMA/CeO
2, 1.0 wt
˝
PMMA/CeO
wt %,
2 , 0.5
PMMA/CeO
%, 600
°C600 C
2, 0.5 wt
˝
PMMA/CeO
wt %,
2 , 1.0
PMMA/CeO
%, 600
°C600 C
2, 1.0 wt
˝
PMMA/CeO
2 , 0.5 wt %, 800 C
PMMA/CeO
2, 0.5 wt %, 800 °C
˝
PMMA/CeO
wt %,
2 , 1.0
, 1.0 wt
%, 800
°C800 C
PMMA/CeO
254254
± 4˘ 4
246246
± 4˘ 4
277277
± 5˘ 5
288288
± 3˘ 3
307307
± 6˘ 6
332
332 ± 5˘ 5
334334
± 7˘ 7
350
350 ± 3˘ 3
439439
± 9˘ 9
395395
± 3˘ 3
466
466 ± 8˘ 8
23.5 ˘
± 0.4
23.5
0.4
22.8 ˘
± 0.6
22.8
0.6
2
25.7 ˘
± 0.3
25.7
0.3
26.7 ˘
± 0.5
26.7
0.5
28.4 ˘
0.8
± 0.8
30.7 ˘
0.6
± 0.6
30.9 ˘
0.9
± 0.9
32.4
˘
0.7
32.4 ± 0.7
40.7
0.6
40.7 ˘
± 0.6
36.6
0.3
36.6 ˘
± 0.3
43.2
˘
0.4
43.2 ± 0.4
EITE(GPa)
IT (GPa)
CIT (%)
CIT (%)
4.10
± 0.04 6.97 ˘6.97
4.10
˘ 0.04
0.06± 0.06
4.70
± 0.05 5.46 ˘5.46
4.70
˘ 0.05
0.08± 0.08
5.02
±
0.03
7.48
5.02 ˘ 0.03 7.48 ˘ 0.09± 0.09
4.87
± 0.06 6.65 ˘6.65
4.87
˘ 0.06
0.06± 0.06
5.09
˘ 0.06
0.05± 0.05
5.09
± 0.06 6.78 ˘6.78
5.08
˘
0.04
5.64
˘
0.08± 0.08
5.08 ± 0.04
5.64
5.04
˘ 0.08
0.07± 0.07
5.04
± 0.08 6.31 ˘6.31
4.99
˘
0.09
5.92
˘
0.04± 0.04
4.99 ± 0.09
5.92
5.44
˘ 0.05
0.09± 0.09
5.44
± 0.05 4.97 ˘4.97
5.73
˘ 0.09
0.05± 0.05
5.73
± 0.09 5.82 ˘5.82
4.21
˘
0.07
2.46
˘
0.09± 0.09
4.21 ± 0.07
2.46
4.
4. Conclusions
Conclusions
Fluorite
Fluorite phase
phase CeO
CeO22 nanoparticles
nanoparticles were
were successfully
successfully synthesized
synthesized by
by the
the sol–gel
sol–gel method
method and
and their
their
structure
and nanocrystallite
nanocrystallite size
sizewere
wereconfirmed
confirmedby by
XRD
TEM
measurements.
Similarly,
structure and
XRD
andand
TEM
measurements.
Similarly,
the
the
PMMA/CeO
systems
were
preparedusing
usingconventional
conventionalsingle-screw
single-screwmelt
melt compounding
compounding and
2 hybrid
PMMA/CeO
systems
were
prepared
and
2 hybrid
were
were systematically
systematically investigated
investigated as
as aa function
function of
of the
the amount
amount and
and thermal
thermal treatment
treatment temperature
temperature of
of
11
inorganic
inorganic nanoparticles.
nanoparticles. H
HNMR
NMRspectra
spectra revealed
revealed only
only the
the presence
presence of
of electrostatic
electrostatic interactions
interactions between
between
the
PMMA
and
CeO
nanoparticles.
The
XRD
analysis
showed
that
the
incorporation
of
CeO
the
2 into
the PMMA and CeO22 nanoparticles. The XRD analysis showed that the incorporation of CeO
2 into
polymer
did did
not not
change
thethe
amorphous
structure
the polymer
change
amorphous
structureofofthe
thePMMA.
PMMA.The
ThePMMA/CeO
PMMA/CeO22 hybrid
hybrid systems
systems
showed
showed substantially
substantially lower
lower UV
UV transmission
transmission than
than pure
pure PMMA.
PMMA. Transparency
Transparency of
of the
the hybrid
hybrid systems
systems
decreased
with
the
amount
of
nanoceria
loading
in
the
PMMA
because
of
the
structural
phase
decreased with the amount of nanoceria loading in the PMMA because of the structural phase and
and
agglomeration
of
CeO
nanoparticles.
UV
transmission
in
PMMA/CeO
hybrid
systems
with
0.5
wt
agglomeration of CeO22 nanoparticles. UV transmission in PMMA/CeO22hybrid systems with 0.5 wt % %
of
Polymers 2015, 7
1656
nanoparticles varies from 10% to 19% in the 250–350 nm region, whereas it was close to zero with
1.0 wt %. Thus, only samples PMMA/CeO2 with 0.5 wt % of nanoparticles sintered at 400 ˝ C showed a
balance between UV absorption, color of the material and transparency, which is a necessary condition
for building applications. Nevertheless, the other systems that displayed excellent UV-shielding
properties are expected to be applied in multifunctional materials industry.
The addition of nanoceria powders increases the degradation and glass transition temperatures of
PMMA, independently of the thermal treatment temperature and CeO2 content, which was the result
of a homogeneous dispersion of the aggregates and the restriction of mobility of polymer chains.
Nanoindentation revealed that the hardness and elastic modulus increase with raising the particle contents
and thermal treatment temperature. Finally, these results demonstrate that the crystalline CeO2 particles
are useful fillers to increase the mechanical properties of PMMA.
Acknowledgments
María A. Reyes-Acosta is grateful for her postgraduate scholarship from SIP-IPN. The authors are also
grateful for the financial support provided by CONACYT through the CB2009-132660 and CB2009-133618
projects and to IPN through the SIP 2015-0227 and 2015-0202 projects and SNI-CONACYT. Thanks to
Adela E. Rodríguez-Salazar for her technical support during the revision of the manuscript.
Author Contributions
María A. Reyes-Acosta, Aidé M. Torres-Huerta and Aberlardo I. Flores-Vela designed the experiments.
Maria A. Reyes-Acosta performed the experiments and analyzed the data. Héctor J. Dorantes-Rosales
and José A. Andraca-Adame performed the morphological characterization. María A. Reyes-Acosta
and Miguel A. Domínguez-Crespo wrote the manuscript. Aidé M. Torres-Huerta directed the research,
provided the funds and revised the manuscript.
Conflicts of Interest
The authors declare no conflict of interest.
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