1 Preparation of rigid Bentonite/PAM nanocomposites by an adiabatic process


Preparation of rigid Bentonite/PAM nanocomposites by an adiabatic
process: Influence of load content and nano-structure on mechanical
properties and glass transition temperature
Lahcene Tennouga*1, Souhila Khobzaoui1, Ismet Benabadji1, Asma Mansri1, Bouras Brahim1

1 Laboratoire d’Application des Electrolytes et des Polyélectrolytes Organiques (LAEPO).
Université de Tlemcen. Département de Chimie. B.P. 119 Tlemcen, 13000, Algeria.

* Corresponding Author: Email: l 1 4 _ t e n @ ya h o o . f r

Polyacrylamide bentonite nanocomposites were prepared in an aqueous suspension. The
suspension was obtained by mixing the dispersion of both bentonite and acrylamide in
bidistilled water, and followed by adiabatic processes of radical polymerization with
ammonium persulfate as initiator. The percentage by weight of bleaching clay (BC) was fixed
at 1%, 3% and 5%. Thin films were obtained using an evaporation solution (thickness of
films were in the range 100- 300 micrometer).
X-ray diffraction (XRD), scanning electron microscopy (SEM), differential scanning
calorimetry (DSC), micro-indentation technique was used to characterize the obtained films.
SEM micrographs show an exfoliated structure in polymer composites that originates from
the nature and method of polymerization used (a radical adiabatic polymerization under
neutral condition). DSC measurements reveal that the glass transition
temperature increases with percentage in weight of BC.


The mechanical tests confirm that the obtained materials have high values of hardness.
We conclude that our materials have a special nano-structure that determines the good
mechanical properties. It is also shown that average micro-hardness decrease with increasing
amount of BC which implies some changes from the initial structure.

Key words: Polyacrylamide (PAM), bleaching clay BC, adiabatic processes, micro-
indentation, glass temperature.

1. Introduction
Bentonite is one of the clay minerals, hydrated aluminum silicate. Its dominant constituent is
montmorillonite (MMT), which belongs to the family of silicate minerals known as
dioctahedral smectites. This kind of mineral clay is structured by two tetrahedral sheets with
Si(IV) as a central atom and one octahedral sheet containing Al(III), which can be substituted
by Fe(III) or/and Mg(II). This type of arrangement exhibits cation exchange properties,
swelling ability, plasticity, cohesion, compressibility, adsorptive properties and catalytic
activity 1.
Bentonite is widely used in different fields because of its properties 2. When this
clay is combined with a polymer, it is expected that the final product, the polymer composite,
will present some enhanced properties. Polymer composites have been regarded as smart
materials for many applications due to their unique properties: high particle dispersion, gas
permeability, structural ?exibility, ?re retardance and thermal and mechanical stability 3.
Therefore, a continuous research has been focused to improve general properties of
polymer composites. Profound understanding of the multiple dependences between molecular
structure, morphology, polymerization, processing methods and ultimate mechanical
properties of the polymers is necessary to discover higher quality materials. Moreover, new


classes of materials have appeared the so-called nano-structured polymers, nano-polymers or
nano-composites, which have structural size below 100 nm. Nowadays, we can investigate
such small systems by means of sophisticated techniques. This in turn promises to open up
ways to improve their properties such as sti?ness, strength or toughness that might result in
better quality materials 4.
In fact, the micromechanical behavior of polymers and the correlation with
morphology and microstructure have been extensively investigated in several applications
belonging to medical, mechanical or aeronautic fields 5. In the last few years a number of
studies concerning the mechanical characterization of polymer composites using micro-
indentation techniques have been reported 5.
We cite below a few examples of works that studied different factors to improve the
mechanical and general properties of polymer nano-composites:
Chun-Ki et al. 6 presented the influence of cluster size effect on hardness of
nanoclay/epoxy composites. It was found that the hardness of the nanocomposites increased
with increasing nanoclay content. However, it was also seen that there was an optimal limit.
This could be due to the size of the clusters reaching a crucial limit and thus the strengthening
function of the nanoclays decreased beyond it. Li et al. 7 investigated the mechanical
properties of polyimide composites filled with SiO2 nano-particles at room temperature and
cryogenic temperatures.
Long et al. 8 used an electrophoretic deposition process and in-situ polymerization of
MMT/PAM, the process inolves preparation of water-based suspension, electrophoretic
deposition and ultraviolet initiated polymerization. This process can produce nano-laminated
composite films, similar to nacre in both structure and organic content. The composite films
display significant enhancement in Young’s modulus and hardness, compared with that of
pure montmorillonite film, the values of Young’s modulus and hardness obtained were


respectively equal to 16.92 GPa and 0.95 GPa for the system PAM 1/MMT 99% in weight
Hua et al. 9 presented nano-indentation studies on Nylon 11/clay nanocomposites
where the modulus and hardness improved by about 17%, as compared with their neat
counterparts. It was also found that the pace of increment in modulus and hardness slowed
down when further increasing the clay content from 2 to 5 wt%. This is probably related to
the clay morphology within the matrix 9. Tsai et al. 10 studied the effects of modified clay
on the morphology and thermal stability of PMMA/clay nanocomposites. Exfoliated
PMMA/clay nanocomposites with different weight percent loadings were successfully
prepared by in situ free-radical polymerization. Three types of modi?ed clays were used to
prepare the PMMA/clay nanocomposites, and were discussed in terms of their thermal,
optical, gas barrier, and scratch resistance properties. It was observed that the CL88 and
CL120 samples with suitable cation exchange capacities CEC values displayed excellent
thermal properties, effective gas barrier characteristics, and scratch resistance properties with
5 wt% of clay incorporated in the PMMA matrices due to their exfoliated morphology. The
incorporation of a small amount of clay into the PMMA matrix resulted in a clear
enhancement in the thermal properties of PMMA. Optical clarity was also excellent in these
PMMA/clay nanocomposites.
The pencil hardness of the pure PMMA ?lm was H, but that of the PMMA/clay
nanocomposite ?lms was at least 2H. Once the clay was exfoliated and dispersed into the
PMMA, the pencil hardness and storage modulus of the PMMA/clay nanocomposites were
signi?cantly improved with respect to pristine PMMA 10.
To the best of our knowledge, we believe that the exfoliation of the system PAM/BC
where the content of bleaching clay is under the amount of 1, 3, 5 wt% has not been


investigated before. However, there are less well-known process and studies for this system
In the present study, a new adiabatic process for the polymerization of acrylamide
under neutral state is used. This technique can produce polymers under ultra-rapid kinetics
and without exchanging energy with the external area. In this way, we expect to obtain
exfoliated bentonite with new and unknown properties. It must be mentioned, however, that
several theoretical and experimental studies have been devoted to study the kinetics of this
reaction (adiabatic conditions) 12. Ferdinard et al. studied the adiabatic photo
polymerization of AM, a method used to produce high molecular weight materials.
We describe in this paper the polymerization method of PAM/BC, synthesized by
adiabatic radical polymerization in aqueous solution 13.
Furthermore and in order to examine the correlation between the morphology and the
nano-structure with the mechanical properties of materials, indentation tests at the micro-scale
have been performed. Accordingly, we investigated the hardness variation of the
nanocomposites with bentonite content. The prepared materials contain three different ratios
in weight percent: PAM 99/BC 1, PAM 97/BC 3 and PAM95/BC 5.

2. Experimental Section
2.1. Materials
A sample of bentonite (BC) was supplied by a local company (ENOF), it was obtained from
industrial treatment of natural clay issued from the field of Hammam Bougherara-Maghnia,
Algeria, and it is composed essentially of montomorillonite. The chemical composition of
bentonite is shown in table 1 14.
Table 1: Chemical composition of bentonite BC
Species SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O TiO2 LOI


%(w/w) 65.2 17.25 2.10 1.20 3.10 2.15 0.60 0.20 8.20

2.1.1. Reagents
The acrylamide monomer compound was provided by Merck; ammonium persulfate (APS)
was provided by Aldrich and was used without further purification.
2.1.2. Synthesis of polyacrylamide bentonite nanocomposites
Bentonite/acrylamide nanocomposites were obtained using an adiabatic process of radical
polymerization (in-situ) in aqueous solution. The samples were prepared in the ratio of (1-
5%:99-95%) in weight of bentonite/acrylamide and ammonium persulfate (APS) was used to
propagate the polymerization. The diameter of spherical bentonite particles was in the range
between the micrometer and the nanometer scale.
At first x% of the bentonite suspension was stirred under a nitrogen atmosphere for 30
min, APS (0.2%) was stirred for 5 min, y% of AM dissolved in bi-distilled water was also
stirred for 30-35 min after a fast heating. The AM and APS suspensions were added to the
bentonite suspension and stirred for 20 min.
The polymer obtained was dispersed in ethanol, filtered and dried under vacuum for

2.2. Instruments and methods
A JSM 6610 LA SEM instrument (Japan) was used to examine the morphology of our
materials prepared with different percentage.
A differential scanning calorimetry (DSC) Perkin-Elmer DSC-7 was employed to
measure the glass transition temperature Tg of the samples (calibration with indium, heating
speed in the range of 10°C/ min to 20° C/min, samples mass fixed at 1 mg) 15.


A x-ray diffractometer, Cu k? radiation (? = 1,541 Å), was used in the continuous scan
mode with scan step size of 0,0167 degrees and time per step 4,84 s 8.
Micro-indentation tests were performed with a Leica VMHT MOT tester with a square
diamond base, indenter speed 45 µm/s, indentation time fixed at 6s and load 500 mN. Ten
measurements were taken for each sample in order to calculate the micro-hardness.
3. Results and discussion
3.1. SEM of polymer nanocomposites at different ratios of BC/AM

Figure 1: SEM micrographs of (A): pure bentonite (magnification x5000)


(B): PAM/BC 1% (x1000) (C): PAM/BC 3% (x1000) and (D): PAM/BC 5% (x1000)

Figure 1 (A) shows the SEM micrograph of pure BC without polymer (5 µm scale bar) and
using a voltage of 20kV.
Figure 1 (B) corresponds to the SEM micrograph of PAM99/BC1 (10 µm scale bar)
and the same voltage as before. In the latter figure, it is noteworthy the existence of the clay
aggregates and intercalated layers. Therefore, the morphology is heterogeneous, which reveals
an intercalated/exfoliated structure (BC was partially exfoliated in this system).This kind of
nano-structure has been confirmed by XRD analysis. The heterogeneity of this arrangement
was related to the amount of BC and to a worse stirring.
Figure 1 (C) represents the SEM image of PAM97/BC3 (10 µm scale bar), taken at a
voltage of 20kV. In this figure, we clearly observed that the polymer is cross-linked with BC
aggregates, which proves that the polyacrylamide PAM was not intercalated in BC gallery.
Figure 1 (D) shows a SEM image of PAM95/BC 5 (10µm scale bar, voltage fixed at
20kV). A stracted BC layers on the matrix was obsereved in this structure.
3.2. XRD analyses for polymer composites with different ratio of BC/PAM


Figure 2: XRD spectra of BC; 2 theta from 0, 70; Imax for the first peak 400cps, for the
second peak is about 1000cps.

Figure 2 examines the structure of the pure bentonite BC. We observe the quartz peak
at 26.7° 16 and the distance between the crystal layers is calculated from the peak at the
lowest angle 6.04° that corresponds to a distance of 14.6Å. The maximum intensity for the
first peak was 400cps and for the second one the maximum was about 1000cps.


Figure 3: XRD scan of composite prepared with 1% of BC; the quartz peak appears at 2 theta
= 27° with Imax = 1000cps.

Figure 3 shows the XRD spectra of a composite prepared with 1% of BC. This data reveals a
special structure of this system (PAM 99/BC 1wt%), therefore, a quartz peak appeared in the
spectrum at the angle equal to 27°, which means that the bleachy clay was partially exfoliated
in the polymer matrix. This result was already interpreted by SEM of the polymer composite
with one percent in weight of BC.


Figure 4: XRD spectra of composite prepared at 3% of BC

Figure 4 represents the XRD spectra of composite prepared at 3% of BC. An amorphous
structure was obtained in the product prepared with ratio of (3% wt BC/97%wt P AM).
This analysis confirmed that the polymer was not penetrated in the gallery clay.


Figure 5: XRD spectra of composite prepared at 5% of BC.

Figure 5 corresponds to the XRD spectra of composite prepared at 5% of BC. As shown in
this analysis, the structure of polymer composite obtained by ratio of (5% wt BC/ 95%wt
PAM) is amorphous. No intercalation was achieved in this product.

3.3. DSC thermal analysis of the different samples prepared
A second scanning heating was carried out between 30° and 250°C.

Figure 6: DSC scan obtained for the composite prepared with 1 percent in weight of BC,
Heating speed =5°C/min, Delta Cp = 0.478 J/g*°C, Delta Cp = 0.478 J/g*°C


Figure 7: DSC scan obtained for the composite PAM/ BC 3% Heating speed =20°C/min,
Delta Cp = 0.519 J/g*°C, Tg= 198.77 °C.

Figure 8: DSC scan obtained for the composite PAM/BC 5%, Heating speed =20°C/min,
Delta Cp = 0.587 J/g*°C, Tg= 199,77 °C

DSC heating scans of PAM 99/1 BC, PAM 97/3, PAM 95/5 are reported in Figures 6, 7, 8
17. Heating temperature was between 30° and 250°. Heating speed was fixed at 5 °C/min


for the first specimen prepared with 1 percent in weight of BC and it was 20°C/min for PAM
97/BC3, and PAM 95/BC5.
Increasing of delta Cp, with increasing of bentonite content, reveals that the crystallinity index
is inversely proportional to BC content. In fact, the exfoliation degree increases with rising
weight fraction of bleachy clay.

Figure 9: Glass transition temperature as a function of weight percentage of BC.

Figure 9 shows the variation of glass transition temperature with percentage in weight of BC.
As shown in this curve, the glass temperature increases with percentage in weight of Bleachy
clay. This means that percentage in weight of composition influences the thermal properties.
Indeed, Tg depends on the structure of our material and the increase in the glass temperature
was associated with constraints on molecular mobility of the polymer chains imposed 18 by
the presence of BC sheets and agglomerations. 012345
vitreous temperature ° C
percentage in weight of BC (%)


3.4. Micro-indentation results
A Vickers square-based diamond indenter was employed to measure the microhardness (H)
from the residual impression on the sample surface after an indentation time of 6s loads of
250, 500 mN were used to derive a load indenting value of H (MPa) that was estimated by the
following equation:
Where d(µm) is the indentation diagonal, P(N) the applied load, and k a geometric factor
equal to 1.85.
For all samples, 10 imprints were taken for each load and the error for the H-values could be
estimated by means of:
;#55349;;#57087;;#55349;;#56379;=2 ?;#55349;;#56401;
;#55349;;#56401; 19, 20 (2)

? parallel H values (H//) and perpendicular (H?) to the indented surface sample were measured
using equation 1. The indentation anisotropy () was calculated from the relation (3):
?;#55349;;#56379;=1? ;#55349;;#56379;//

? H?


Figure 10: The residual indentation on samples prepared with 1, 3 and 5 percent in weight of
Figure 10 represents the residual indentation on samples prepared with 1, 3 and 5 percent in
weight of BC.

Figure 11: Micro-hardness variation in x (parallel) and y (perpendicular) direction with
percentage in weight. Average micro hardness variation with percentage in weight of BC.

Figure 11 represents the micro-hardness variation in x direction and y direction, average value
with the percentage in weight of BC.
Decreasing of micro hardness values in x direction from 538,6 MPa to 304,91 MPa
with increasing of bentonite content varied from 1 wt% to 5 wt% in composite polymer. 12345
microhardness in x direction
microhardness in y direction
average microhardness
microhardness (Mpa)
percentage in weight of BC (%)


Micro hardness variation in y direction show the same behavior as that for the x axis, reducing
from ~375MPa to ~270MPa with a change in BC percentage in weight from 1% wt to
5%wt.Additionally, average micro hardness was calculated and represented in figure 11. They
varied from 457 MPa to 288 MPa with variation of BC percent in weight (1 wt % – 3 wt % –
5 wt %).
In y direction, the micro hardness values show some discrepancy with the
measurements obtained in the x one. This explains why the response of the polymer matrix to
the stress field is not the same in both directions. This anomaly is due to the structure of the
crystalline phase BC, which has a hexagonal shape. Moreover, the anisotropy between x and y
data probably occur owing to disorder orientation between bentonite sheets and
macromolecular chains direction.
The lessening of micro hardness in both ways is necessarily due to the composition of
agglomerations and dispersion of BC sheets in the polymer matrix.
Undeniably, exfoliation degrees and morphology influence the micro-hardness of PAM/BC.

4. Conclusion
A novel process was used to prepare the PAM/BC nanocomposites with different percentage
in weight of BC, PAM/BC (PAM99/BC1, PAM97/BC3, PAM95/BC5); they were
synthesized by a radical adiabatic process in aqueous solution with different percentage in
weight of BC.
From the data discussion, it can be concluded that:
The morphology and the structure obtained with polymer prepared with one percent in weight
of BC present the most important mechanical thermal and structural properties.
Micro indentation measurements and DSC analysis reveal somewhat strong contacts between
BC sheet and PAM, this conclusion interprets the increasing of Tg with augmentation of BC


% in weight. However, the Van Der Waals bonds and the structure of the polymer composite
are the dominant factors in decreasing of micro hardness with content of BC% wt. The
storage modulus decreases with the percentage weight of BC in the composite. The load
tangent increase with %BC in the polymer, the nano-hardness decreases for the two starting
values of the percentage of weight in BC (1% and 3%), then it comes a little increase from 3%
to 5% percent of BC. These variations depend on the structure and surface roughness of
samples. The effect of amount of BC and the level of reticulation PAM on the structure and
physical properties of PAM/BC will be studied in a separate work.

The authors thank the National Agency for the Development of University Research
(ANDRU) in Algeria for financial support. S. K. is also indebted to IEM-CSIC (Madrid,
Spain) and Dr. Fernando Ania for hosting her at the Macromolecular Physics Department
during the performance of the surface mechanical experiments.

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