Influence of ZrO2 nanoparticles and thermal treatment on the properties of PMMA/ZrO2 hybrid coatings

Article abstract of DOI:10.1016/j.jallcom.2014.10.040

In this work, ZrO₂ nanoparticles were synthesized by the sol-gel method, treated thermally at different temperatures (400, 600 and 800 °C), and added to a polymer matrix in two different weight percentages (0.5 and 1) by single screw extrusion in order to determine the influence of these parameters on the thermal stability and UV radiation resistance of PMMA/ZrO₂ composites. Fourier transform infrared spectroscopy (FT-IR), nuclear magnetic resonance (NMR), X-ray diffraction (XRD), transmission electron microscopy (TEM), confocal laser scanning microscopy (CLSM), ultraviolet-visible spectroscopy (UV-Vis), thermogravimetric analysis (TGA) and nanoindentation techniques were used to evaluate the structural, morphological, optical, thermal and mechanical properties of as-prepared composites. The average crystallite sizes for ZrO₂ sintered at 600 and 800 °C were about 17 and 26 nm, respectively. It was found that the incorporation of a low percentage of ZrO₂ nanoparticles increased the thermal properties of PMMA as well as its hardness and elastic modulus. The degradation temperature at 10 wt.% loss of the PMMA/ZrO₂ (0.5 wt.%, 400 °C) nanocomposite was approximately 48 °C higher than that of pure PMMA. The absorption in the UV region was increased according to the ZrO₂ heat treatment temperature and amount added to the polymer matrix.

Full text of DOI:10.1016/j.jallcom.2014.10.040

Journal of Alloys and Compounds xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
Influence of ZrO nanoparticles and thermal treatment on the properties
2
of PMMA/ZrO hybrid coatings
2
M.A. Reyes-Acosta a,b, A.M. Torres-Huerta , M.A. Domínguez-Crespo , A.I. Flores-Vela ,
b,
⇑
b
c
d
e
H.J. Dorantes-Rosales , E. Ramírez-Meneses
a
Instituto Politécnico Nacional, CICATA-Altamira, CIAMS, Mexico
b
Instituto Politécnico Nacional, CICATA-Altamira, Km 14.5 Carretera Tampico-Puerto Industrial Altamira, C.P. 89600 Altamira, Tamps., Mexico
Instituto Politécnico Nacional, CMP+L, Av. Acueducto s/n, Barrio La Laguna, Col. Ticomán, C.P. 07340 México D.F., Mexico
Instituto Politécnico Nacional, SEPI-ESIQIE, Departamento de Metalurgia, C.P. 07738 México D.F., Mexico
c
d
e
Departamento de Ingeniería y Ciencias Químicas, Universidad Iberoamericana, Prolongación Paseo de la Reforma 880, Lomas de Santa Fe, Distrito Federal C.P. 01219, Mexico
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online xxxx
In this work, ZrO2 nanoparticles were synthesized by the sol–gel method, treated thermally at different
temperatures (400, 600 and 800 °C), and added to a polymer matrix in two different weight percentages
(
0.5 and 1) by single screw extrusion in order to determine the influence of these parameters on the
Keywords:
PMMA
thermal stability and UV radiation resistance of PMMA/ZrO2 composites. Fourier transform infrared
spectroscopy (FT-IR), nuclear magnetic resonance (NMR), X-ray diffraction (XRD), transmission electron
microscopy (TEM), confocal laser scanning microscopy (CLSM), ultraviolet–visible spectroscopy (UV–Vis),
thermogravimetric analysis (TGA) and nanoindentation techniques were used to evaluate the structural,
morphological, optical, thermal and mechanical properties of as-prepared composites. The average crys-
tallite sizes for ZrO
the incorporation of a low percentage of ZrO
well as its hardness and elastic modulus. The degradation temperature at 10 wt.% loss of the PMMA/ZrO
ZrO
2
Nanoparticles
Polymer nanocomposites
UV radiation resistance
2
sintered at 600 and 800 °C were about 17 and 26 nm, respectively. It was found that
2
nanoparticles increased the thermal properties of PMMA as
2
(0.5 wt.%, 400 °C) nanocomposite was approximately 48 °C higher than that of pure PMMA. The absorp-
tion in the UV region was increased according to the ZrO
to the polymer matrix.
2
heat treatment temperature and amount added
Ó 2014 Elsevier B.V. All rights reserved.
1
. Introduction
polymer or blend, whereas the minor constituent is at least one
dimension below 100 nm [4]. Recently, polymer/inorganic particle
nanocomposites have garnered considerable interest since they
can improve properties such as optical, thermal, mechanical, and
chemical,, as a result of the inorganic contribution to the polymer
matrix [5–8]. Thus, the final properties of these nanocomposites
depend on several factors including: composition, particle size,
surfactant, dispersion of particles in the polymer matrix, thermal
properties of inorganic particles, polymer–particle interface, and
Poly (methyl methacrylate) (PMMA), an important thermoplas-
tic polymer, displays excellent properties such as transparency,
lightness, hardness and high elastic modulus [1,2]. Furthermore,
it is a common substitute for inorganic glass due to its higher
impact strength and fracture behavior (ductile rather than brittle
fracture prone) [3]. It is widely used in many optical, automotive
and consumer applications. Nevertheless, it is limited in many
applications including computer/electronics, aerospace industry
and architectonic materials due to its low thermal stability and
poor resistance to UV radiation. A possible alternative to improve
these properties is to combine it with different inorganic nanopar-
ticles to obtain hybrid polymers or polymer nanocomposites. These
materials, also known as inorganic–organic hybrids, constitute a
multicomponent system in which the major constituent is a
2
the fabrication method [9,10]. Among the fillers, SiO is one of
the most widely used in PMMA nanocomposites due to the fact
that it has demonstrated its ability to enhance the mechanical
properties of PMMA [11–13]. Other possible materials with excel-
lent properties like chemical resistance, good mechanical strength
and good thermal stability, which can be used to reinforce the
polymeric matrix, are the ZrO
are related to the addition of ZrO
2
nanostructures [14]. A few reports
to the PMMA matrix and, as a
2
consequence, the interaction between the organic and inorganic
parts is still far from being fully understood [15,16]. The
⇑
Corresponding author.
E-mail address: atorresh@ipn.mx (A.M. Torres-Huerta).
http://dx.doi.org/10.1016/j.jallcom.2014.10.040
925-8388/Ó 2014 Elsevier B.V. All rights reserved.
0
Please cite this article in press as: M.A. Reyes-Acosta et al., J. Alloys Comp. (2014), http://dx.doi.org/10.1016/j.jallcom.2014.10.040

2
M.A. Reyes-Acosta et al. / Journal of Alloys and Compounds xxx (2014) xxx–xxx
Crystalline phase identification of the ZrO
performed using a D8 Advance Bruker X-ray diffractometer with Cu K
matic radiation. The XRD patterns were collected in the 20–80° 2h range at room
2
powders and nanocomposites was
2
PMMA/ZrO system is commonly produced by two conventional
a
monochro-
methods: in situ polymerization or mixing solution. The melt mix-
ing method is commercially more attractive than the methods
mentioned above [17] due to the fact that it is inexpensive, envi-
ronmentally friendly, straightforward, and seems to be a valuable
method for industrial application [18,19]. Additionally, different
strategies such as the non-aqueous-sol–gel method have demon-
strated that they provide an easily controllable way to increase
temperature. The average crystallite size of ZrO
2
was estimated using the Scherrer
formula [22].
0
:9k
D ¼
ð2Þ
B cos h
B
where D is the average crystallite size, k is the X-ray wavelength (1.54056 Å), B is the
FWHM width of the diffraction peak, and h is the corresponding diffraction angle of
B
the diffraction peak.
reactivity, including ternary systems (PMMA/ZrO
potential applications in optical and mechanical fields [20]. In the
present work, different PMMA/ZrO polymer composites were suc-
2 2
/TiO ) with
2
ZrO nanoparticles were examined by TEM using a JEOL-2000 FX-II microscope
coupled with an energy dispersive X-ray spectrometer (EDS) operating at 200 kV.
The nanocomposites were also characterized by CLSM using a Carl ZEISS micro-
scope, model LSM 700. This tool is equipped with a spectral channel used to detect
autofluorescence signals, which come from sample components. The fluorescence
intensity measurements were performed using the built software ZEN of the LSM
710. The intensity peak, characteristic of a fluorescence emission signal, was
2
cessfully obtained by single screw extrusion to evaluate the influ-
ence of different sintering temperatures (400, 600 and 800 °C) and
amounts of ZrO
2
nanostructures (0.5 and 1 wt.%) on their thermal
stability and UV radiation resistance. ZrO
2
nanoparticles were syn-
thesized by the low-cost-sol–gel method and thereafter samples
were characterized by proper techniques to determine size, shape,
dispersion and hardness in the polymer matrix.
2
461 nm for PMMA and 598 nm for the ZrO nanoparticles. UV–vis absorption spec-
tra of the studied nanocomposites were measured in the 200–1100 nm wavelength
range using a Perkin–Elmer Lambda 40 UV–vis spectrometer. The thickness of the
analyzed samples was about 0.5 mm.
TGA studies were carried out using a Simultaneous Thermal AnalyserLabsysEvo
1
600 of Setaram. Samples were placed in aluminum crucibles. An empty aluminum
2
. Experimental procedure
crucible was used as reference. The heating ramp applied to the samples was from
2
5 to 450 °C at a constant heating rate of 5 °C/min under argon atmosphere.
Nanoindentation technique was employed to evaluate the mechanical proper-
2
.1. Materials and synthesis of ZrO nanoparticles
2
ties. Measurements were carried out with the CSM Instruments Nanoindentation
Tester (TTX-NHT) with a Berkovich triangular diamond pyramid indenter. A loading
ZrO
nium n-propoxide [Zr(OCH
acetylacetone (CH COCH COCH
3 2-
hydrolysis rate of the zirconia precursor and its reactivity) and isopropanol [(CH )
CHOH, Sigma–Aldrich 99.9%] as solvent, using the following procedure: zirconium
n-propoxide was mixed with n-propanol (0.6 M) applying vigorous magnetic stir-
ring for 30 min. The obtained solution was complexed by using acetylacetone at a
2
nanoparticles were synthesized by the sol–gel method starting from zirco-
CH CH 70 wt.% in 1-propanol, Aldrich] as precursor,
, Aldrich 99%) as chelating agent (to control the
2
2
3 4
)
ꢁ
1
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 in order to study the creep behavior
before the indenter was unloaded. The hardness (H) and elastic modulus (E) were
estimated from the initial gradient of the unloading curves using the Oliver and
Pharr model [23]. Five indents were performed on each sample on randomly
selected locations to obtain average values of hardness and elastic modulus of the
nanocomposites.
3
2
3
1
:1.5 M complexing agent/alkoxide ratio. The solution was quickly stirred at room
temperature for 120 min, maintaining pH fairly constant during the synthesis pro-
cess (pH ꢀ 7). The sols were found to be stable and transparent with no precipitate
2
or turbidity. The ZrO powders were obtained by gelification of the solutions and
drying of the gels at 120 °C for 24 h to remove organic material; afterwards, the
3
. Results and discussion
xerogel samples were ground in an agate mortar and then thermally treated at
3.1. FT-IR analysis
4
00, 600 and 800 °C for 2 h in air to study the effect on the structure and particle
size. Finally, the powders were crushed by a horizontal ball mill at 350 rpm for 10 h.
The FT-IR spectra of pure PMMA and different PMMA/ZrO
2
ꢁ1
2.2. Preparation of nanocomposites
nanocomposites acquired in the 4000–450 cm range are shown
in Fig. 1. For pure PMMA, the bands around 2996, 2952 (asymmet-
A commercial grade poly (methyl methacrylate) (PMMA, PlexiglasV825, Altu-
ꢁ1
ric) and 2844 cm (symmetric) are assigned to CAH stretching
glas International, Arkema) in pellet form was used; the reported melt flow rate
of this material at 230 °C/3.8 kg is 3.7 g/10 min. PMMA pellets were first ground
in a Thomas-Wiley laboratory mill using a 2-mm sieve to obtain fine PMMA
particles. Thereafter, polymeric particles were dried at 80 °C for 1 h before being
3
vibrations. The bending vibration bands of the methyl (ACH )
group appear at 1484 and 1436 cm in the FTIR spectra, whereas
ꢁ
1
the deformation mode of the methylene (ACH A) group appeared
2
processed. PMMA-ZrO
screwextruder (D = 19 mm, L/D = 30 mm) with three heating zones. Two different
amounts of ZrO nanostructures (0.5 and 1 wt.%) were added to the polymer matrix.
2
nanocomposites were prepared using a laboratory single-
ꢁ1
around 1387 cm . In addition, the sharp and intense band at
728 cm is attributed to the stretching vibrations of the carbonyl
ꢁ1
1
2
The blending temperature profiles on the extruder were 235, 235 and 240 °C, from
hopper to die zone, and the screw rotating speed was 80 rpm; in the last step, nano-
composite extrudates (filaments or narrow films) were cooled in a water bath at
room temperature.
2.3. Characterization and measurements
FT-IR studies were carried out to determine the functional groups and bonding
types in the samples. These studies were performed using a Spectrum One Perkin-
Elmer spectrometer equipped with an attenuated total reflectance probe (ATR). The
IR spectra of the nanocomposites were obtained in the transmittance mode, in the
ꢁ
1
ꢁ1
4
50–4000 cm spectral region with 15 scans and at a resolution of 4 cm
.
To determine the interaction between the organic and inorganic phases, 1H
1
3
NMR and
C NMR spectra were carried out using a Bruker Ascend 750
(
750 MHz) NMR spectrometer for liquids at room temperature with deuterated
1
13
842 667
chloroform (CDCl
3
,
H: d = 7.26 ppm, C: d = 77.0 ppm) as a solvent and tetrameth-
989
065
1
4841387
1192
ylsilane (TMS, d = 0.00 ppm) as the internal reference. The syndiotactic percentage
(r) in each sample was obtained by using the following equation:
2
844
1
2
996
1
728 1436
1272
913
753
1
A
ðrrÞ
þ
A
ðmrÞ
2
2952
1150
1242
%
Syndiotacticity ¼ ðrÞ ¼
ꢂ 100
ð1Þ
A
T
4
000
3500
3000
2500
2000
1500
1
1000
500
where A(rr) and A(mr) are the areas of the signals corresponding to the syndiotactic
and heterotactic configuration, respectively. A is the sum of all the areas within
the PMMA tacticity (A(rr) + A(mr) + A(mm)) [21]. The relative peak intensities were cal-
-
Wavenumber (cm )
T
culated with an electronic integrator.
2
Fig. 1. FT-IR spectra of pure PMMA and PMMA/ZrO nanocomposites.
Please cite this article in press as: M.A. Reyes-Acosta et al., J. Alloys Comp. (2014), http://dx.doi.org/10.1016/j.jallcom.2014.10.040

M.A. Reyes-Acosta et al. / Journal of Alloys and Compounds xxx (2014) xxx–xxx
3
C@O) group. The strong band at 753 cm 1 is attributed to the out
ꢁ
3.3. Structural and morphological characterizations
(
of plane bending vibration of the C@O group. The PMMA/ZrO
2
nanocomposites show absorption bands that matched with pure
PMMA, indicating a weak interaction between PMMA and ZrO
nanoparticles. Only the intensity of the C@O group at 1728 and
Fig. 3a and b shows XRD patterns of the ZrO
2
nanoparticles trea-
2
ted thermally at three different temperatures 400, 600 and 800 °C,
pure PMMA and PMMA/ZrO nanocomposites. Fig. 3a shows the
evolution of the crystalline phase. It was found that at 400 °C,
the ZrO nanoparticles show low intensity broad peaks at around
2
ꢁ1
7
53 cm is reduced in the PMMA/ZrO
is characteristic of the weak interaction between the ZrO
ticles and PMMA with the adsorption of the polymer on the inor-
ganic surfaces via the methoxycarbonyl group (AC(O)OCH
24,25]. Bands corresponding to the vibration absorption of ZrAO
2
nanocomposites, which
2
nanopar-
2
2h = 30.2°, 50.3° and 60.2°, which are assigned to reflections from
planes (011), (112) and (121), respectively, corresponding to
tetragonal ZrO (t-ZrO ). It is clearly seen that these reflection
2 2
peaks become sharper and narrower with higher treating temper-
atures (600 °C), indicating the increase in the average crystallite
2
size and the improvement of the crystallinity of the ZrO nanopar-
ticles. In addition, the emergence of peaks at 34.8° (002), 35.2°
(110), 59.6° (013), 62.9° (202), 74.5° (220), 81.9° (123) and
84.9° (222) can be observed, confirming the presence of a tetrag-
3
)
[
ꢁ1
groups (between 450 and 750 cm ) are not observed owing to
the incorporation of a low percentage of nanoparticles. In agree-
ment with previous reports, in the range between 1050 and
ꢁ1
1
300 cm , there are five infrared absorption bands associated
with the ester group of syndiotactic poly(methyl methacrylate),
s-PMMA, which so arises from specific interactions [26]. Among
them, the
and 1242 cm
mation of the ester group, respectively, are found. The bands at
m
a
(CACAO) coupled with the
m
(CAO) vibration at 1272
onal phase. The diffraction pattern for the ZrO
is dominated by the more intense, characteristic peaks of the
monoclinic phase (m-ZrO
), ð1 1 1Þ, (111) and (220), centered at
28.1°, 31.4° and 50.1°, respectively. That is to say that the ZrO
2
2
treated at 800 °C
ꢁ1
1 2
(m and m ) associated with the trans and cis confor-
2
ꢁ1
1
192 and 1150 cm
(m
3
and
m
4
) are assigned to skeletal stretching
coupled with internal CAH deformation. Finally, the band at
nanoparticles present, basically, the monoclinic phase. However,
a peak centered at 30.2° (101) is observed, indicating the presence
of a small content of the tetragonal structure at 800 °C. Addition-
ꢁ
1
1
065 cm
(m
5
) is attributed to the planar zigzag arrangement of
synthe-
the backbone carbon atoms [13]. Thus, the PMMA/ZrO
2
sized in this work shows mainly a syndiotactic configuration. It is
well known that the thermal stability of PMMA changes according
to the molecular weight and tacticity: in an inert atmosphere, the
syndiotactic form of PMMA displays a higher degradation temper-
ature than the isotactic one, which is attributed to the slower chain
mobility of the first one [26,27].
ally, the average D crystallite sizes of ZrO
culated on the basis of Eq. (1) and were found to be 17.1 ± 0.4 and
26.4 ± 0.9 nm for ZrO nanoparticles treated thermally at 600 and
800 °C, respectively. Fig. 3b shows XRD patterns of the PMMA/
ZrO nanocomposites. The diffraction pattern of pure PMMA shows
2
nanoparticles were cal-
2
2
a broad diffraction peak at 2h = 14.5, typical of an amorphous
material, together with two bands of lower intensities centered
at 30.7° and 42.1°. The diffraction patterns of all PMMA/ZrO nano-
2
3
.2. NMR studies
composites show the same three bands observed in pure PMMA,
indicating that neither the presence of the nanoparticles nor the
preparation process change the orientation of the PMMA chains.
1H NMR spectra of pure PMMA and PMMA/ZrO
(0.5 and 1 wt.%,
2
8
00 °C) nanocomposites are reported in Fig. 2a. The methoxy pro-
) of PMMA show a peak at 3.60 ppm, whereas methy-
lene protons (ACH ) show peaks at the 2.2–1.5 ppm range. In
Polymer nanocomposites synthesized from annealed ZrO
2
tons (AOCH
3
nanoparticles at 400 and 600 °C display a sharp tendency to show
an amorphous structure with narrow peaks at about ꢀ30.2°, which
seems to correspond to the tetragonal structure (011), whereas
hybrid materials using sintered nanostructures at 800 °C show
two low intensity peaks at 28.1° and 31.4°, corresponding to the
2
addition, the peaks at 1.22, 1.01 and 0.83 ppm are related to the
isotactic (mm), heterotactic (mr) and syndiotactic (rr) configura-
tions of
all the spectra have the same number of peaks without any
modification in their shape. However, with the addition of ZrO
3
a-methyl protons (ACH ), respectively. It can be seen that
ð1 1 1Þ and (111) planes of the monoclinic ZrO
2
. It is important
to highlight that the structural phase of ZrO nanoparticles does
2
2
nanoparticles (0.5 and 1 wt.%) thermally treated at 800 °C, the peak
at 1.59 ppm related to the methylene group of PMMA is shifted
towards lower values. These small shifts suggest the existence of
not modify the amorphous nature of PMMA, only observing ZrO
crystalline domains embedded in the PMMA matrix.
Fig. 4a–c shows TEM images in bright field mode with their cor-
responding selected area electron diffraction pattern (SAEDP). The
2
electrostatic interactions between the PMMA structure and ZrO
nanoparticles via methoxycarbonyl groups (AC(O)OCH ), which
is proposed in the inset scheme. The content of the different triads
were determined by the electronic integration of the -methyl sig-
nals, thereby obtaining a percentage of syndiotactic PMMA of 69%.
2
3
2
TEM micrographs show that the ZrO nanopowders consist of crys-
tal-faced-shaped nanocrystals with variable size distributions
along with a high agglomeration degree. The size of crystals seems
to be decreased with the treatment temperature, forming bigger
agglomerates. As it is well known, nanoparticles are aggregated
due to the large specific surface area and high surface energy; then
it is expected to enhance the agglomerate size with the treatment
a
1
3
Fig. 2b shows the C NMR spectra of pure PMMA and PMMA/ZrO
0.5 and 1 wt.%, 800 °C) nanocomposites. The peaks between 22
2
(
and 16 ppm are assigned to the methyl group; peaks at 44 ppm
are related to the quaternary carbon of the polymeric chain; the
peak at 52 ppm is related to the methoxy group; the peak at
temperature as in our case. The ZrO
mally at 400 °C, are constituted of aggregates with sizes between
0.2 and 0.3 m and composed by crystals of about 50–100 nm in
diameter. ZrO , thermally treated at 600 °C, shows aggregates
between 0.35 and 4 m with irregular smaller crystals of about
20 nm, while ZrO , thermally treated at 800 °C, consists of aggre-
gates with sizes ranging from 0.33 to 0.45 m with even smaller
2
nano powders, treated ther-
5
1
4 ppm is associated with the methylene group and peaks between
77 and 178 ppm are related to the carbonyl carbon. No modifica-
l
2
tion in the chemical shift is observed in the nanocomposites, indi-
cating that no chemical bond occurred. From the analysis of the
l
2
1
3
carbonyl, quaternary carbon and
a-methyl signals of the
C
l
NMR spectra, an average percentage of the syndiotactic configura-
tion of about 70–71% was obtained. It has been reported that the
possible measurement error by NMR is about 3–4% [28]; therefore
it can be considered that these values are similar to those obtained
by H NMR analysis. Finally, these results confirm the high degree
of syndiotacticity observed by FT-IR.
crystallite sizes. The large zirconia agglomerates are built up of
aggregates of smaller units. The observed reduction in the crystal
size with temperature seems to resemble the statement regarding
the decrease in the crystallite size by Zener, which pins the effect
in combination with the polymorphic transformation from the
t-phase to the m-phase or both.
1
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4
M.A. Reyes-Acosta et al. / Journal of Alloys and Compounds xxx (2014) xxx–xxx
Fig. 2. NMR spectra of pure PMMA and PMMA/ZrO
2
nanocomposites by (a) 1H NMR and (b) 13C NMR.
(a)
(b)
Δ
Δ
Δ
Δ
Δ
Δ
Δ
Δ
Δ
Δ
Δ
Δ
Δ ΔΔ Δ Δ
Δ
Δ
Δ
ΔΔ
Δ
Δ
2
0
30
40
50
60
70
80
90
2
0
30
40
50
60
70
80
90
2θ (degrees)
2θ (degrees)
2 2
Fig. 3. XRD patterns of (a) nanocrystalline ZrO powders thermally treated at different temperatures and (b) pure PMMA and PMMA/ZrO nanocomposites.
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M.A. Reyes-Acosta et al. / Journal of Alloys and Compounds xxx (2014) xxx–xxx
5
2
Fig. 4. TEM images and SAEDPs of ZrO thermally treated at different temperatures (a) 400 °C, (b) 600 °C and (c) 800 °C.
The SAEDPs of ZrO
2
thermally treated at 600 and 800 °C show
technique allows scans at various x–y planes corresponding to dif-
ferent depths of the sample, and, thus to reconstruct the 3-D region
of the specimen [29]. In this case, the three-dimensional structure
reveals the spatial position of the particles in all directions, which
is helpful to evaluate the effects on the optical, thermal and
mechanical properties. It is well recognized that the distribution
of the nanoparticles play an important role in the final properties.
Fig. 5 shows the 3D-CLSM images of the PMMA/ZrO nanocompos-
2
ites with 0.5 and 1 wt.% of particles thermally treated at different
temperatures. From the measurements, it can be seen that the
concentric rings, which also indicate that the crystallite size is on
nanoscale. The SAEDPs were indexed according to the ICDD cards
of tetragonal and monoclinic phases; thus, the results are consis-
tent with XRD patterns, corroborating that as the temperatures is
increased an increase in crystallite size is obtained.
To corroborate the TEM results, dynamic light scattering (DLS)
also known as photon correlation spectroscopy was used to mea-
sure the particle size distribution (PSD) in anA Malvern Zetasizer-
Nano ZSP, model ZEN5600. Different particle diameter moments
(
number-average diameter D
n
; weight-average diameter D
w
and
ZrO
matrix, but tend to form agglomerates with irregular morphology.
In addition, it is clearly observed that the ZrO particles tend to
2
nanoparticles show a uniform dispersion within the PMMA
z-average diameter) were calculated using Eqs. (3)–(5), and the
polydispersity index (PDI) was determined using Eq. (6):
2
form bigger agglomerates as the heating treatment temperature
is increased and only a small quantity of nanoparticles can be seen.
This result is due to crystal size increments, which consequently
decreases their fluorescence emission intensity.
P
i i
n D
D
D
D
n
w
z
¼ P
ð3Þ
n
i
P
n
i
Di
4
3
¼ P
ð4Þ
ð5Þ
n
i
Di
P
n
i
D
i6
3.4. Optical properties
¼ P
n
i
Di
5
D
D
w
The transmission spectra in the UV–Vis region (200–1100 nm)
PDI ¼
ð6Þ
for pure PMMA and PMMA samples with 0.5 and 1 wt.% of ZrO
at different heating treatment temperatures are shown in Fig. 6a.
It can be seen that the transmittance response of PMMA/ZrO
2
n
where n
i
is the number of ZrO
2
NPs with diameter D
i
. The particle
2
size distributions (PSD) as well as average particle size and polydis-
persity index for the evaluated samples are shown in Table 1. From
these data, it can be observed that the ZrO particle size was
2
nanocomposites is attenuated in comparison with pure PMMA in
the visible region. The transmittance of the nanocomposites is
strongly decreased as the content of the inorganic moiety and
increased with temperature and it can be considered as a polydis-
persed system (PDI > 1). The tendency to form bigger agglomerates
with temperature, observed in the TEM micrographs, was
confirmed with dynamic light scattering measurements.
the heating treatment temperature of the ZrO
increased. Then, as the ZrO content is decreased, the transmit-
tance percentage is increased up to the highest value obtained in
the PMMA sample with 0.5 wt.% of ZrO thermally treated at
2
nanoparticles are
2
2
To evaluate the dispersion degree of ZrO
2
nanostructures, the
400 °C. Even in the previous case, the transmittance is reduced
PMMA/ZrO
2
nanocomposites were characterized by CLSM. This
about 25% compared to pure PMMA, whereas PMMA with 1 wt.%
Table 1
Particle size distribution of the ZrO
2
nanopowders heat treated at 400 °C, 600 °C and 800 °C.
ZrO 600 °C
ZrO
2
400 °C
2
2
ZrO 800 °C
Size d (nm)
Mean number (%)
Size d (nm)
Mean number (%)
Size d (nm)
Mean number (%)
7
9
1
1
–
–
–
8.82
1.28
05.7
22.4
7
396.1
458.7
531.2
615.1
–
12.5
37.5
37.5
12.5
–
458.7
531.2
615.1
712.4
825
0.8
8.9
24.5
31.7
22.9
9.5
32
43
18
–
–
–
–
–
–
–
955.4
1106
1.7
D
z
= 109.4 nm
D
z
= 536.9 nm
D
z
= 845 nm
Polydispersity index = 1.0436
Polydispersity index = 1.0483
Polydispersity index = 1.0977
Please cite this article in press as: M.A. Reyes-Acosta et al., J. Alloys Comp. (2014), http://dx.doi.org/10.1016/j.jallcom.2014.10.040

6
M.A. Reyes-Acosta et al. / Journal of Alloys and Compounds xxx (2014) xxx–xxx
2
Fig. 5. 3D CLSM representations of pure PMMA and PMMA/ZrO nanocomposites.
100
80
60
40
20
0
(
b)
(a)
200 300 400 500 600 700 800 900 1000 1100
Wavelength (nm)
2 2
Fig. 6. (a) UV–vis spectra of pure PMMA and PMMA/ZrO nanocomposites, and (b) optical images of pure PMMA and PMMA/ZrO nanocomposites.
of ZrO
2
nanocomposites, treated at 800 °C, showed the lowest
inorganic particles have critical importance for transparency of
polymer/inorganic particle composites [31,32]. Besides, the forma-
tions of small crystalline domains embedded in the amorphous
PMMA also contribute to the percentage reduction of transmit-
tance in the visible range. Synthetic polymers are susceptible to
degradation by UV and visible light, however, it has been reported
that photodegradation of PMMA has taken place by irradiation at
260–300 nm, but not under irradiation at k > 320 nm [33], which
value. From these results and optical images (Fig. 6b), it is evident
that the presence of even a small percentage of ZrO leads to a dra-
matic transparency loss, which can be attributed either to the
obtained structural phase or to the presence of agglomerated
2
ZrO
30], and therefore, causes haze or even turbidity; in fact, previous
studies have shown that size, structural phase and distribution of
2
nanoparticles since these can act as a strong scattering center
[
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M.A. Reyes-Acosta et al. / Journal of Alloys and Compounds xxx (2014) xxx–xxx
7
furthers the photostability of aliphatic acrylic and methacrylic
polymers, which is generally very high in comparison with poly-
olefins [34]. Additionally, spectra of pure PMMA and PMMA/ZrO
nanocomposites show two absorption bands at about 272 and
15 nm. These bands may be attributed to the n– * transition of
the C@O group in the PMMA [35,36].
PMMA
5
4
3
2
1
0
PMMA/ZrO , 0.5 wt.%, 800 °C
2
2
PMMA/ZrO , 1 wt.%, 800 °C
2
3
p
5
mN max load
0s pause
3.5. Thermal studies
1
It is widely accepted that the degradation of PMMA is initiated
by the scission of head-to-head linkages (155–220 °C), followed by
b-scission at the vinylidene chain ends (230–300 °C), and finally by
random main chain scission (>300 °C) [37]. TGA curves of pure
PMMA and PMMA/ZrO hybrid materials under argon atmosphere
2
are presented in Fig. 7a. It can be observed that the thermal stabil-
ity of the hybrid materials was greatly improved by the addition of
0
200
400
600
800
1000
Displacement, h (nm)
ZrO
stability of the nanocomposites is almost independent of the ZrO
content, but strongly dependent on the heating treatment temper-
ature. This may explain the behavior of the PMMA/ZrO (0.5 wt.%,
00 °C) nanocomposite, where low thermal stability compared
2
moieties. It can be seen that the improvement of the thermal
2
Fig. 8. Load–displacement curves of pure PMMA and PMMA/ZrO nanocomposites.
2
2
8
2 2
PMMA/ZrO nanocomposites. The incorporation of ZrO nanoparti-
with all nanocomposites is seen, perhaps because ZrO
2
nanoparti-
cles into the PMMA matrix resulted in a higher penetration resis-
tance compared to that of pure PMMA. For example, for PMMA,
the maximum indentation depth at the maximum load was
1049 nm, whereas with 0.5 and 1 wt.% of ZrO2 thermally treated
at 800 °C, the maximum depth was about 1003 and 985 nm,
respectively. Similar values and trends were observed for as-
prepared composites from ZrO2 nanoparticles sintered at 400 °C.
The results of the nanoindentation experiments are in good agree-
ment with the mechanical properties of nanocomposites obtained
by standard macroscopic methods [38]. Hardness is important
since it is related in many cases to the strength or fracture tough-
ness of the specimen. A high hardness generally corresponds to a
high abrasive wear resistance [39]. The hardness, elastic modulus
cles are aggregated in the PMMA matrix. Fig. 7b indicates the effect
of ZrO content and thermal treatment on the decomposition
2
temperature at 5 and 10 wt.% loss of the nanocomposites. In
comparison with PMMA, the degradation temperature of PMMA/
ZrO (1 wt.%) nanocomposites at 5 and 10 wt.% loss was increased
2
in about 33–47 °C and 45–50 °C, respectively. These results imply
that the nanocomposites are more thermally stable than pure
PMMA due to the electrostatic interaction between the ZrO
2
nanoparticles and PMMA matrix. It is well recognized that the
effective dispersion of inorganic nanoparticles in the polymer
matrix enhances the thermal resistance of the nanocomposites
because the increasing number of active sites increases the decom-
position temperature, which is in agreement with CLSM observa-
tions [8]. The improvement of thermal stability is explained by
the restriction of mobility of polymer chains resulting from the
steric hindrance and radical trapping effect of inorganic fillers.
and creep for PMMA and PMMA/ZrO nanocomposites are summa-
2
rized in Table 2. It is evident that the hardness and elastic modulus
are increased with the addition of stiff ZrO nanoparticles into the
2
matrix [40,41]. However, a significant change in these values when
2
the ZrO content is increased from 0.5 to 1 wt.% is not observed.
3.6. Hardness tests
The creep percentage at the maximum hold segment was
decreased (i.e., increasing the creep resistance) with the addition
Nanoindentation tests were used to study the ZrO
2
load effect
of a low content of nanoparticles, indicating that the PMMA/ZrO
2
on the mechanical properties of PMMA/ZrO
higher thermal treatment. Fig. 8 displays the typical load–displace-
ment curves at a maximum indentation load of 5 mN on the
2
nanocomposites at
nanocomposites provide greater stiffness than pure PMMA, further
suggesting that the nanocomposites are less ductile than pure
PMMA.
4
3
3
3
3
00
75
50
25
00
5
1
% weight loss
0 % weight loss
1
00
(a)
(b)
8
6
4
2
0
0
0
0
0
PMMA
PMMA/ZrO , 0.5 wt.%, 400 °C
PMMA/ZrO , 1 wt.%, 400 °C
PMMA/ZrO , 0.5 wt.%, 600 °C
PMMA/ZrO , 1 wt.%, 600 °C
PMMA/ZrO , 0.5 wt.%, 800 °C
PMMA/ZrO , 1 wt.%, 800 °C
2
2
275
250
225
200
2
2
2
2
50
100 150 200 250 300 350 400 450
(a)
(b)
(c)
(d)
(e)
(f)
(g)
Temperature (°C)
2 2
Fig. 7. (a) TGA curves of pure PMMA and PMMA/ZrO nanocomposites, and (b) effect of ZrO on the decomposition temperature of nanocomposites.
Please cite this article in press as: M.A. Reyes-Acosta et al., J. Alloys Comp. (2014), http://dx.doi.org/10.1016/j.jallcom.2014.10.040

8
M.A. Reyes-Acosta et al. / Journal of Alloys and Compounds xxx (2014) xxx–xxx
Table 2
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PMMA/ZrO
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IT (MPa)
E
IT (GPa)
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