Effects of ultrasound time on the properties of methylcellulose-montmorillonite films
Effects of ultrasound time on the properties of methylcellulose- montmorillonite films
Akbar Jokar 0 1
Mohamad Hossyn Azizi 0 1
Zohre Hamidi Esfehani 0 1
Solyman Abbasi 0 1
Film X-ray 0 1
0 Department of Food Science, College of Agriculture, Tarbiat Modares University , Tehran , Iran
1 Food Science, Agriculture Engineering Research Department, Fars Agricultural and Natural Resources Research and Education Center , Shiraz , Iran
Methylcellulose-montmorillonite films were prepared via solvent casting method. The effects of different ultrasound times (0, 15, 30, 45, 60, and 75 min) on the properties of methylcellulose-montmorillonite films were evaluated. Fourier transform infrared and X-ray diffraction were applied to investigate and prove the effects of ultrasound time. The films were characterized by mechanical properties, opacity, water vapor permeability, yellowness index, and color. Ultrasound time significantly affected the characteristics of the films, except for elongation. Maximum tensile strength, opacity, YI, and b* as well as minimum L* and water vapor permeability were related to 60 min. The results from X-ray diffraction and Fourier transform infrared verified the effects of sonication time on the films properties, especially for 60 min. The Fourier transform infrared spectrum related to 60 min had more new and sharper peaks. The maximum compactness and strength of methylcellulose-montmorillonite films and the highest X-ray diffraction peak were also attributed to 60 min. Using ultrasound radiation for the production of such films is strongly recommended. To obtain the best quality and reach the required properties, considering the aim of the films, optimization of sonication time is mandatory.
Ultrasound time; Methyl cellulose; Montmorillonite; Fourier transform infrared diffraction
Recently, composites such as polymer-layered silicate have
become prevalent. Silicate layers should have at least one
dimension of less than 100 nm [1–3]. Montmorillonite
(MMT) is a type of silicate clay that has been widely used
in polymer composites. High intercalation chemistry,
strength, abundance in nature, low gas permeability, safety,
and economic properties of MMT have led to the
widespread use of this material [4–7].
Many investigations have shown that incorporating
nanoscaled silicate layers into polymers increases their mechanical
properties, heat, and moisture resistance, and decreases their
moisture adsorption, permeability, and flammability [6, 8].
Depending on silicate dispersion, two types of composite, i.e.,
intercalated and exfoliated, can be obtained. The latter is known
as delaminated silicate and is preferred to the former because of
having better barrier and mechanical properties [7, 9, 10].
Producing clay composites can be achieved by four
methods: in situ interactive polymerization, in situ
synthesis, melt intercalation, and solution intercalation.
Researchers have applied suitable blending methods, such
as shear, high pressure, centrifuge, and ultrasonication
mixing, for producing a high performance composite.
Ultrasonication is one of the most important methods for
increasing intergallery spacing between silicate layers and
dispersing them in the polymers. Ultrasound treatment can
help in terms of easily achieving exfoliated clay structure
and increasing d-spacing in comparison to non-sonicated
samples [11, 12]. Furthermore, ultrasonication widely
affects the polymer itself, and as a result, changes
composite properties. Therefore, power and time of sonication
are critical and should be seriously considered in composite
preparations [13–15]. Acoustic cavitations, bubbles, and
their collapses are the main reasons for chemical reactions
(sonochemistry) and physical changes in the substances
which are exposed to ultrasound radiation. Bubble violent
explosion generates extreme temperatures and pressures
inside the bubbles and solvent. So, the materials in the
solvent are disintegrated and several highly reactive
radicals would be generated. Several chemical reactions can
occur between these active radicals and substances in the
medium. Finally, more chemical bonds, like H and
covalent bonds will be generated [14, 16, 17].
Alshabanat et al. found that increasing sonication time in
polystyrene resulted in the creation of crystalline structures
in the amorphous region. The maximum peak intensity in
X-ray diffraction (XRD) was obtained after 1 h of sonicating
polystyrene-MMT composite, which showed higher
interaction and chemical bonds with MMT . Intergallery
spacing of epoxy-clay composite increased with increasing
sonication time at low clay loading (2%). Dispersion of
MMT at high sonication times was significantly better.
Increasing sonication time enhanced tensile strength, while
hardness did not change . Chen et al. revealed that
15 min ultrasound time could break cross-links between
amylose and amylopectin in maize starch, which in turn
caused depletion in opacity, water vapor permeability
(WVP), and elongation in the films as well as increasing
tensile strength (TS) . Ultrasound and microwave
combination, especially lower than 20 min, can significantly
affect barrier and mechanical properties of the films from
methyl cellulose (MC), wheat bran cellulose and soy protein
[15, 19, 20]. To the best knowledge of the authors, there are
not any reports that show and prove the effects of ultrasound
time on biodegradable films like MC [21, 22].
The present research was designed to show and prove
the relationship between sonication time and properties of
methyl cellulose-montmorillonite (MC-MMT) films as
well as to illustrate how time of sonication will affect the
films properties. The research was also designed to show
the importance of sonication time optimization.
Materials and methods
MC was prepared from Sigma Aldrich, 274429 (average
molecular weight of 40000 and 2% solution viscosity of
400 cp) (PubChem CID: 44263857). MMT without any
modifications was supplied from Southern Clay Products
Inc., USA (Cloisite Na?). Glycerol (PubChem CID: 753)
was purchased from Merck Company.
MMT preparation is illustrated in Fig. 1.
Standard techniques used for MMT preparation
All the MMT preparations were carried out in 100-ml
laboratory beakers. Stirrers and magnet stirring sets were
Heidolf MR 3001 (Germany) and 2-cm magnet, respectively. In
MMT and also in film preparations, a water bath sonicator
was used (ULTRASONS-H, PSelectaCE95, 50 kHz,
1000 W). From the beginning to the end of the MMT
preparation, the top of the beakers was wrapped tightly with a
soft polyethylene sheet to avoid water vaporization.
Each polymer has individual characteristics and the method
for its film preparation is exclusive. The process of film
preparation is illustrated in Fig. 2.
Standard techniques used for film preparation
In the film preparation, stirrer and magnet stirring were
Heidolf MR 3001 and 6-cm magnet, respectively. In the
film preparation, a water bath sonicator was used
(ULTRASONS-H, PSelecta CE95, 50 kHz, 1000 W).
From the beginning to the end of the film preparation,
500-ml blue cap glass containers were used and tightly
caped to avoid the solvent evaporation.
Properties of MC-MMT films
The MC-MMT films were characterized by mechanical
properties (TS and elongation at break), opacity, WVP,
Fig. 1 MMT preparation for adding to MC solution
color, yellowness index (YI), and thickness. XRD was used to
investigate the dispersion of MMT in MC polymer. Fourier
transform infrared (FTIR) spectroscopy was applied to show
chemical bonds and the interaction between MMT and MC/
glycerol at different sonication times.
TS and elongation at break evaluations were performed
using a Universal Testing Machine (Instron, Hounsfield
H50KS, England) according to ASTM D882-12 .
Speed of upper Instron’s jaw was 50 mm/min. Rectangular
strips (8 9 1 cm) were cut from the film sheets and 2 cm
from the top and bottom of the strips was carefully
wrapped using adhesive paper scotch. The latter was done to
avoid compression, rupture, and slipping of the films
between the Instron’s jaws. The distance between the jaws
was 4 cm. The strips were equilibrated at 25 C and 50%
RH for 72 h before doing the test.
X-ray diffraction (XRD)
Exfoliate and intercalate structures of MC-MMT films were
evaluated using X’PertMPD, Philips, Holland XRD,
diffractometer equipped with CO Ka 1.79 A˚ (scanning rate
of 0.02 /S, scan step time of 2 s, voltage of 40 kv, and current
of 30 mA). Collections of the data were performed at 2h
angle from 1 to 12 . Using Bragg equation (Eq. 1), the XRD
software calculated the d-spacing between the MMT layers.
The most suitable, and popular method for calculating
dspacing of MMT in such films and proving the penetration of
polymer into the MMT layers (Intercalate structure) is using
Bragg equation. Disappearing MMT peak in the XRD
spectra of the films, shows exfoliate structure [24, 25].
Where, k is the wavelength of the X-ray beam (nm), d is
the spacing (nm) between the two layers, and h (A˚ ) is the
angle of incidence.
Fourier transform infrared spectroscopy (FTIR)
FTIR spectra were collected using FTIR spectrometer
(Nexus 6700, Thermo Nicolet, USA). Transmission
method was applied to the MC-MMT films. IR absorption
spectra of the MC-MMT films were obtained for the
purposes of measuring and scanning. The spectral collections
were performed in the wavenumber range of
400–4000 cm-1 with the resolution of 4 cm-1.
The WVP evaluations were based on the modified ASTM
E96/E96 M-14 method . Before running the test, the
circle samples (19.625 9 10-4 m2 in the surface area)
were cut and equilibrated at 25 C and 50% RH for 72 h.
Then, they were carefully placed and sealed using grease
oil on the top of the glass cells (7.065 9 10-4 m2 in
internal surface area, 19.625 9 10-4 m2 in edge surface
area, and 3.5 cm depth), which contained 8 ml of saturated
NaCl (74% RH). To prevent the leakage of moisture
through the seals, in addition to using grease oil, circle
rubber rings were put on the films as well as on the edges of
the cells and tightly gripped by four metal clamps. The
glass cells were put in a desiccator containing 800 g of
silica gel and kept at 25 C and 50% RH. The cells were
weighed every 5 h for 72 h. WVP was calculated by
Where WVP is water vapor permeability (g/m h KPa),
Dm is the total weight loss of the cells (g), X is the
thickness of the films (m), A is internal or exposed area of
the film (m2), Dt is the time of vapor penetration (h), and
Dp is the pressure gradient (dp = 2.368 Kpa was
calculated using a psychrometric chart from Universal Industrial
Gases, Inc, Pennsylvania, USA).
Table 1 Optimization conditions and selected sample
L*, a*, and b* (mode of CIE) parameters of the MC-MMT
films were measured in a tristimulus colorimeter
(ColorFlex EZ, Bench top Spectrophotometers, USA). Five
replicates were performed for each sample .
YI, which shows yellowish color of the films, was measured
according to ASTM standard E313-15 . Equation (3)
was used for YI calculation.
Where XYZ are tristimulus factors, measured by a
tristimulus colorimeter (ColorFlex EZ, Bench-top
Spectrophotometers, USA), and Cx and Cz are constants
obtained from ASTM standard E313-15.
Opacity of the MC-MMT films was measured using a Cary
60 UV/VIS spectrophotometer. Rectangular pieces
(8 9 1 cm) of the films were prepared and put in the
sample position of the spectrophotometer. Empty
measurement was used as the reference. The opacity of the
films was calculated using Eq. (4) [6, 27, 29].
Where Abs 600 is absorbance at 600 nm and X is the
thickness (mm) of the films.
Using a hand micrometer, the thickness of the MC-MMT
films was measured (Mitutoyo, Model 0052526, Japan).
Five locations of the films were evaluated and the average
value was used for each sample.
Selected sample with
The MC-MMT films were produced in a completely
randomized design (CRD). The treatments were: 0 min time
of sonication as control (MC 0), 15 min (MC 15), 30 min
(MC 30), 45 min (MC 45), 60 min (MC 60), and 75 min
(MC 75). Design Expert ver. 7.0 was used for optimization.
Using numerical optimization, the goals for each response
were set then the software generated optimal conditions
(Table 1). Data were analyzed mainly by SPSS20 software
and where needed other statistical softwares such as
MSTATC and Design Expert ver. 7.0 were also used.
Multiple range Duncan test was performed for mean
comparisons (a = 0.05). All the experiments and
measurements were done with at least three replications.
Results and discussion
Time of sonication remarkably affected tensile strength
(p \ 0.01), while it did not have any significant effect on
elongation (p [ 0.05). After 60 min, tensile strength and
elongation reached 66.92 MPa and 37%, respectively.
Compared with the control sample, there was 43.7%
enhancement for tensile strength (Fig. 3). Rimdusit et al.
 reported that tensile strength and elongation of
MCMMT films (10% MMT) were 92 MPa and 19%,
respectively. Dobrovol et al.  announced tensile strength and
elongation of MC-MMT films (10% MMT) around
137 MPa and 22%, respectively [22, 30]. Due to the
presence of glycerol in this research, tensile strength and
elongation were lower and higher than the other researches,
Tensile strength decreased after 15 min of sonication
(Fig. 3). The viscosity of the MC polymer solution was
high; so, low time of sonication not only could increase the
film’s strength, but also could diminish it. In addition,
15 min of sonication was not sufficient for generating
radicals and their interaction with polymer and other
Fig. 3 Comparing tensile strengths of MC-MMT films at different
times of sonication. Different letters show significant differences
(p \ 0.05)
materials in the solution. On the other hand, probably some
of the bonds of the polymer were decomposed at the 1st
min of sonication; as a result, tensile strength was
decreased. The results proved that time of sonication is
critical and should be optimized.
Increasing time of sonication improved tensile strength
up to 60 min, while it decreased at 75 min (Fig. 3). If
sonication time and intensity had more increase, the
polymer and its chemical bonds would be probably
decomposed and adversely affect mechanical properties,
like what was seen at 75 min of sonication time. Several
researchers have evaluated the effect of ultrasound
radiation on different film properties and observed that tensile
strength is improved, while different results are obtained
about elongation [14, 15, 31].
If the solvent is water, active H and OH radicals and
hydrogen peroxide will be usually generated. Several
chemical reactions occur between these active radicals and
substances in the medium, like MC, MMT, and glycerol in
this study. Finally, more chemical bonds, like H and
covalent bonds, will be generated. Furthermore, sonication
causes the exit of air bubble from the solution, which in
turn results in more compact films with higher strength and
resistance toward elongation . All these mechanisms
could be the reasons for improving tensile strength at
higher sonication time in MC-MMT films. We can
conclude that the compactness of the films, chemical bonds,
and their intensity at 60 min were higher than that of the
The effect of sonication time on WVP of MC-MMT films
was significant (p \ 0.05). Minimum WVP was related to
60 min of sonication time (0.56 9 10-3 g/m h KPa). The
data showed that 60 min of sonication could decrease
21.35% of WVP in comparison to the control. Figure 4
shows the decreasing trend and mean comparisons of
Fig. 4 Comparing WVP of the MC-MMT films at different
sonication times. Different letters show significant differences (p \ 0.05)
WVP. As was completely described in the previous
sections, the most compact film was obtained at 60 min. So,
water vapor could not easily pass through such a film.
Other researchers have reported similar results [14, 19].
There was not any direct report about WVP of MC-MMT
films in other papers, while Turhan and Sahbaz 
reported WVP of MC films (without MMT) around
0.4 9 10-3 g/m h KPa, which was nearly like the obtained
result in present research . Yu et al.  announced
that WVP of carboxymethylcellulose films with 4% MMT
was 0.8 9 10-5 g/m h KPa. The difference of the basic
polymers and the presence of glycerol are the main reasons
of higher WVP in present study.
Optimization analysis by Design Expert showed that
60 min of sonication time was the optimum treatment (with
90.5% desirability) for using the films in food packaging.
According to the results of mechanical and WVP properties
and as the 60 min treatment was optimum, other properties
(XRD, FTIR, and Optical) were evaluated at 45, 60, and
75 min. Optimization conditions are depicted in Table 1.
At the first glance on XRD graphs of MC-MMT films
(Fig. 5), it can be found that the films had an intercalated
structure, since the peak angle of MMT decreased from 8.7
(pure MMT) to nearly 2 A˚ . If MMT structure in the films
was exfoliated, we could not see any peak in XRD
spectrum [6, 34].
Increasing ultrasound time caused the enhancement of
d-spacing between MMT layers in the MC-MMT films,
and this shows the penetration of MC into the d-spacing of
MMT layers and intercalated structure. Data calculation by
Eq. 1 showed that increasing time of sonication from 45 to
Fig. 5 XRD of MC-MMT films
at different sonication times
75 min increased d-spacing from 3.88 to 4.83 nm, while
peak angles decreased from 2.64 to 2.12 A˚ . Therefore,
incorporation of MC and glycerol into the d-spacing of
MMT layers was increased by increasing time of
sonication [6, 11, 34].
There are some relatively small (sodium) ions in the
interlayers of MMT and these ions can be exchanged with
onium cations in the polymer (MC)/glycerol. This
ion-exchange reaction results in widening the gap between the
single sheets, enabling polymer chains to move in between
them and the surface properties of each single sheet are
changed widely [1, 35]. As explained in FTIR section MC
does not have high compatibility with MMT because of the
presence of low hydrophilic ions. Therefore, MC showed
low interaction with MMT, while glycerol had high
interaction with MMT and created high new bonds (Fig. 6).
These facts provide evidences for the penetration of MC
into the intergallery space of MMT sheets and increasing
Intensity/height of the XRD peaks shows the dispersity
of MMT layers in the polymers. The lower intensity of the
peaks had more scattering of MMT and the height of XRD
peaks was noticeably different (Fig. 5). Since the viscosity
of the MC solution was high, ultrasound and probably new
chemical bonds played significant roles in MMT
dispersing. The height of the peak in the 75 min treatment was
considerably lower than that of the others, which showed
that 75 min of sonication could decompose polymer chains
along with decreasing consistency, and as a result, MMT
layers were widely dispersed in the medium. As can be
observed in the mechanical properties, the generated bonds
at the 60 min of sonication were higher than those of the
other treatments. So, the medium (matrix) became more
consistent and MMT scattering in this medium grew less
than the other treatments, even lower than 45 min.
Alshabanat et al.  reported nearly the same results. Results of
this study represented less consistency and polymer
crosslinking for 45 min of ultrasound treatment. Tensile
strength, WVP, and FTIR spectra verified the XRD results.
The ultrasound treatments had remarkable and wide effects
on the MC-MMT films. The FTIR spectra for MC-MMT
films at different sonication times are shown in Fig. 6 and
Table 2. At high wave numbers, control MC-MMT films
(MC 0) had absorption bands related to O–H stretching at
3450, 3251 and 3138 cm-1. By applying 45 min of
sonication and increasing time to 60 and 75 min, the O–H
stretching bands, 3450 and 3251 shifted to lower wave
numbers, while 3138 shifted to higher wave numbers
(Fig. 6). These values were nearly consistent with those
reported by Rimdusit et al.  and Pandey . In
addition to shifting and emerging new peaks, the intensity
of peaks, especially in 60 and 75 min increased and got
In low wave numbers, control MC-MMT films (MC 0)
had absorption peaks related to C–C stretching, C–H rock
and C–O stretching at 1431, 1356 and 1159, respectively.
These bands shifted to lower wave numbers by applying
45 min sonication, while these bands disappeared at 60 and
75 min and new peaks emerged (Fig. 6). These values are
again reported by Rimdusit et al.  and Pandey .
Control MC-MMT films had a band at 1099 related to
Si–O Stretching. Applying 45 min sonication and
increasing it to 60 and 75 min shifted this peak to lower wave
numbers. According to the reports of Tzavalas and
Gregoriou , we can attribute these peaks to the alteration of
environment of the Si atoms (in MMT) due to the presence
and penetration of polymer and alkyls between MMT
layers. Higher intensities of these peaks show the progress
of intercalation in films.
Fig. 6 FTIR spectra of MC-MMT films
Range of high wave number
Assignments Range of low wave number
O omitted peaks, N new peaks
As can be seen in Fig. 6 control MC-MMT film had 648,
613 and 519 bands related to C-H bends. These bands
changed widely by applying ultrasound and increasing its
time (Table 2). These bands are called fingerprint region
and assigning them are difficult, for example, Alshabanat
et al.  assigned 528 and 466 as Si–O asymmetric
bending in MMT.
Overall, by applying ultrasound to the MC solution and
increasing time, chemical bonds changed outstandingly
(Table 2). Some of the peaks shifted positively and some
negatively and most of them got sharper with higher
intensity, showing that the polymer/glycerol was
incorporated into the intergallery space of the MMT layers
and new chemical bonds were formed [1, 37, 38]. By
increasing time of sonication up to 60 min, fewer changes
occurred at low wave numbers, while at high wave
numbers, peaks got wider and sharper (Fig. 6). In addition,
75 min of ultrasound made more changes in the peaks at
both low and high wave numbers, representing that high
ultrasound times caused changes in the polymer bonds and
as a result in the film properties. Depending to the aim of
using the films, these changes might be useful or not. So, to
get the best quality, preparation of the films should be
optimized in terms of sonication time.
Ultrasound times (min)
Different letters show significant differences (p \ 0.05)
Values are mean ± standard deviation
Opacity (9 10-4)
Optical properties of MC-MMT films
Opacity is one of the most important features of films,
especially for food packaging. Ultrasound time had a
significant effect on the opacity of the MC-MMT films
(p \ 0.05). In addition, 60 min of sonication had the
highest opacity (0.0017) in the films (Table 3).
Analysis of variance showed that time of sonication had
no significant effects on a* (redness) factor of the
MCMMT films, while it significantly affected L* (lightness)
and b* (yellowness) factors (p \ 0.01) as well as YI
(p \ 0.05). Maximum opacity, YI, and b* of the films and
minimum L* were attributed to 60 min of sonication
(Table 3). Color and YI results verified opacity feature of
the films, which was very low at 60 min. Formation of
more chemical bonds and more exit of air bubbles from the
polymer solution at 60 min resulted in more compactness
of the film . Therefore, transmittance would be
decreased in such a film. Tensile strength, FTIR, and WVP
results were in accordance with the optical properties.
Rodriguez et al.  reported that b* of cellulose acetate
with 7.5% MMT was 1.6 and Yu et al.  announced that the
b* of carboxymethylcellulose with 8% MMT was 0.09, while
in present research the maximum b* (3.475; 60 min) was
outstandingly higher than the other reported values. Tunc and
Duman  reported the opacity of MC-MMT films (with 20%
MMT) around 6.7. The reason of lower opacity in this study
(1.7, when thickness is in micron) is applying lower MMT
content (13%) in the films. Rodriguez et al.  and Yu et al.
 reported the opacity of cellulose acetate and
carboxymethylcellulose films 2.81 and 2.18, respectively
[6, 36, 39]. The kind of polymer, MMT content, and the method
of film preparation are the reasons of these differences.
The average thickness of the MC-MMT films was
64.48 l. Times of sonication did not significantly affect the
thickness of the films (p \ 0.05).
Sonication time remarkably affected the properties of the
MC-MMT films. Emerging new bonds between polymers
and nanomaterials, increasing intensity of them, and more
dispersion of nanomaterial in the polymer are the effects of
sonication time. Because of these effects, properties of the
films will change widely. With an increase in sonication
time tensile strength (46.57–66.92 MPa), opacity
(12.05–17.07), b* factor (3.33–3.475), and YI (5.3–5.8)
increased outstandingly up to 60 min, while continuing
sonication resulted in the depletion of these traits. On the
contrary, L* factor (90.81–90.66) and WVP
(0.66–0.56 9 10-3 g/m h KPa) decreased by increasing
time of sonication up to 60 min. The results of XRD and
FTIR verified the effects of sonication time on the film
properties. Our results presented in this report suggest that
for achieving desirable quality of the films, time of
sonication must be optimized. Showing, proving, and verifying
the importance of sonication time by FTIR and XRD were
illustrated in this paper.
Acknowledgements This work was done by the support of Tarbiate
Modares University, as a part of a PhD dissertation. So, we gratefully
appreciate their kind cooperation. In addition, we sincerely
acknowledge the technicians of Food and Central Laboratories in
College of Agriculture.
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