Oct. 2008, Volume 2, No.10 (Serial No.11)
Journal of Materials Science and Engineering, ISSN1934-8959, USA
Interactions between cellulose and silica nanoparticles
studied by molecular dynamics simulations
MO Zun-li1,2, QIAO Li-jun1, CHEN Hong1, GUO Rui-bin1, SUN Ya-ling1, LI He-jun2
(1. College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou 730070, China;
2. College of Material Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, China)
Abstract: Molecular dynamics (MD) simulation have been used
to examine the structure and dynamics of a system containing
polymer filled with an inorganic nanoparticle. The aim for this
research is to probe and investigate the microstructure and the
interface interactions of amorphous cellulose and silica
nanocomposites. This research mainly examined the interactions
of amorphous cellulose and silica nanopaticles with different
radius. Geometry and energy optimization of amorphous
cellulose were performed at the beginning of simulation. Then,
under the COMPASS force field, 300ps runs were carried out on
the whole system of amorphous cellulose filled with sphere
silica nanopaticles. The result of simulation showed that the
interactions between cellulose and silica increased with the
radius of silica nanocomposites increasing. And, it was found
that cellulose chains were more attracted to the modified silica
surfaces due to the induction force and hydrogen bonds between
cellulose chains and silica surfaces.
Key words: molecular simulation; cellulose; silica
nanopaticles; interaction
1. Introduction
Cellulose is the most abundant renewable organic
substance on earth. It is one of the most popular and the
most used polysaccharide by industry in either its
native or regenerated forms. The cellulose microfibril
consists of crystalline area and amorphous area[1]. In
recent years, there are many reports which discuss
Acknowledgment:
The authors would like to thank the
financial support from the Environment-kindly of Gansu
Province (No. GH2005-10), the Natural Science Foundation of
Gansu Province (No. 0803RJZA009), Science and Technology
Tackle Key Problem Item of Gansu Province (No.
2GS064-A52-036-08) and Gansu Key Laboratory of Polymer
Materials (ZD-04-14).
Corresponding author: MO Zun-li, Ph.D., professor; research
field: nanocomposite. E-mail: mozl@163.com.
crystal cellulose including the microstructure of crystal
cellulose[2-4], the behavior of cellulose molecules in
aqueous environment[5], mechanical properties of
crystal cellulose[6], the interactions of cellulose and
water[7], urea, aromatic azo-dyes and aryl ammonium
compounds[8], et al.. However, attentions were seldom
paid to the amorphous cellulose and the interactions of
the
amorphous
cellulose
and
inorganic
nanocomposites. Actually the amorphous portion of
cellulose is much more open and accessible than the
crystalline portion, some chemical reactions as such
crosslinking reactions occur mainly in these
amorphous regions. Therefore, it is very important to
examine the microstructure and interactions of
amorphous cellulose and silica. But it is rather difficult
to precisely measure the interactions of polymer and
nanopaticles in the direct experiment.
Cellulose and silica nanocomposites have been
successfully prepared by sol-gel process[9] and in situ
synthesis[10]. Silica embedded into cellulosic material
considerably caused to diminish the hydrophilicity and
improve thermal stability of the nanocompoties. The
effectiveness of silica additives depends on many
factors, including surface chemistry. It affects or even
determines many important physical and chemical
properties, such as mechanics, electrics, and carrier
transportation of material[11]. Surface chemistry
affecting properties are considered to be the interaction
of chains with the filler-silica surfaces including
chemical reactions, van der Waals forces, and
hydrogen bond.
17
Interactions between cellulose and silica nanoparticles studied by molecular dynamics simulations
Molecular simulation is adapted as an effective
way to investigate the microstructure and interaction of
the amorphous cellulose and silica nanocomposites.
Our previous researches have probed the interaction
energy of polymer and inorganic hybrid[12-13].
However, to our knowledge, the study on interactions
of the cellulose and silica nanocomposites by
molecular simulation has never been reported. In this
paper, cellulose/silica nanocomposites were selected as
the model system to investigate the interactions
between them. The models of amorphous cellulose and
sphere silica nanopaticles of different radius have been
generated and simulated under the COMPASS
(condensed-phase optimized molecular potentials for
atomistic simulation studies) force field. We have
compared the energies of the composite systems filled
silica with different radius.
includes molecular mechanics and molecular dynamics
calculations using the discover and amorphous cell
module[14-15]. The simulation was performed under the
COMPASS force-field[16], which is one of the first ab
initio force field approaches that has been
parameterized
and
validated
using
the
condensed-phase properties. In this study, Van Der
Waals (VDW) and coulomb non-bond interactions
were treated using Ewald summation method.
Non-bond cutoff distance of 0.950 nm (with a spline
width of 0.100 nm and a buffer width of 0.050 nm) was
employed. The tail correction was applied to
non-bonded interactions during the molecular
dynamics simulation run. Temperature in all the
simulations was controlled by Andersen thermostat[17].
H
H
HO
2. Methods
H
Molecular dynamics simulation was executed
using Material Studio 4.0, which was developed by
Accelrys Software Inc. The simulation methodology
Fig. 2
18
H
O
H2C
O
H
OH
H
The repeat unit cellobiose is made up
of two β-D-glucopyranose rings
(b)
O
HO
H
OH
Fig. 1
OH
H2C
O
O
(a)
H
OH
(c)
(d)
(e)
(f)
The silica nanopaticles of different radius and models of the composite system
Interactions between cellulose and silica nanoparticles studied by molecular dynamics simulations
Cellobiose, which can be considered as the repeat
unit for cellulose[18], was used to build the cellulose
chain with degree of polymerization of 10. The
chemical structure of cellobiose is shown in Fig. 1.
Two chains, each with 10 cellobiose repeat units, were
built and then were packed into a 21.39×21.39×23.65
Å3 cubic cell. Periodic boundary conditions were used
in the amorphous cellulose model. A 5000-step energy
minimization was performed at the initial stage to
stabilize the conformational equilibrium.
The silica particles, which were cut from the
structural model of a α-quartz crystal supplied by the
software, were nearly spherical in shape respectively
with approximate diameter of 8Å, 12Å, 16Å[19]. The
unsaturated sites on silica particles were saturated by
hydrogen atoms (see Fig. 2(a)-(c)). Then the silica
particles were embedded into the cellulose model and
the models of the composite systems were formed (see
Fig. 2(d)-(f)). Every composite system was minimized
by smart minimizer method in order to eliminate local
non-equilibrium, and then a molecular dynamics run
(NVT, 500ps) was performed for each model in order
to further equilibrate the models before using them for
data production.
3. Results and discussion
Table 1
R silica
0.4nm
0.6nm
0.8nm
R silica
0.4nm
0.6nm
0.8nm
The interaction energies of different composite system (kJ/mol)
⊿E
-304.6785
1293.0649
13485.0080
Table 2
3.1 Interactions between cellulose and silica
When small size particles are well dispersed in the
polymer matrix, the interactions between polymer and
nanopaticles becomes very important in determining
the properties of the hybrid material[20]. In the
composite system, the weak interaction between
cellulose and silica mainly consists of three parts: van
der Waal, electrostatic interaction and hydrogen bond.
The interaction energy (⊿E) was calculated according
to equation (1). At first, the potential energy (E total) for
the model containing both cellulose and silica
nanocomposite was calculated and then, the potential
energy of cellulose (E cellulose) was calculated without
any contribution from the silica nanocomposite.
Finally, the silica nanocomposite was kept and
cellulose was removed to calculate the potential energy
of the silica nanocomposte (E silica). The interaction (⊿
E) between cellulose and silica was computed as:
⊿E ＝E total－(E cellulose＋E silica)
(1)
The potential energy E total consists of three parts,
as follows:
E total ＝E valence＋E crossterms＋E non-bond
(2)
In the equation (2), E valence , E crossterms, E non-bond are
valence interaction, valence across terms and non-bond
energy, respectively.
Total
4808.3334
1185.8955
-8162.9539
Vdw
-38.0717
1636.5603
5797.5586
Induction
9479.1153
13454.0346
23093.9248
Dispersive
-9517.1870
-11817.4744
-17296.3662
The total potential energies of different composite system (kJ/mol)
Total
4808.3334
1185.8955
-8162.9539
Valence
1533.2135
4892.1850
9467.8675
The interaction energies between cellulose and
silica are listed in Table 1 and Table 2. As shown in
Table 1, with the radius increasing, the interaction
energy of cellulose and silica increases, while the total
Crossterms
-555.0342
-576.4735
-988.7714
Non-bond
3840.1541
-2829.7160
-16642.0598
potential energy decreases. It shows that the binding
probability between cellulose and silica increase, and
the composite system became more stable. From Table
2, it can be found that the total potential was affected
19
Interactions between cellulose and silica nanoparticles studied by molecular dynamics simulations
by the non-bond interaction including van der Waal
and hydrogen bond.
24000
binding energy
repulsive energy
Energy/kJ mol-1
20000
16000
12000
8000
4000
0
4
5
6
7
8
Radius/Angstrom
hydrogen bond owing to surface hydroxyls of cellulose
and silica (see Fig. 4). Hydrogen bonds were identified
based on geometrical considerations[21]. A hydrogen
bond was considered formed when the distance
between a hydrogen atom and an oxygen atom was less
than 3.0 Å and the angle between the proton acceptor,
the proton and the proton donor was greater than 90˚. In
the composite system hydrogen bonds are easily
formed between the hydroxyls on silica surface and
cellulose chains, as the silica nanocomposites are
coated by the cellulose chains. It can be concluded that
cellulose can well interact with silica nanopaticlees by
van der Waal and hydrogen bonds.
(a)
total potential energy
non-bond energy
4000
Energy/kJ mol-1
0
-4000
-8000
-12000
-16000
4
5
6
7
8
(a)
Radius/Angstrom
(b)
Fig. 3 (a) Varying trend of interaction energy and
induction force of the composite system; (b) Varying
trend of the total potential energy and non-bond energy
The reason why polymer and inorganic
nanocomposites can interact together is that
intermolecular interactions exist. The interactions
between polymer and inorganic nanocomposites
mainly include van der Waal, while the different ratio
between induction and dispersive force has a big effect
on Van Der Waal. As shown in Fig. 3, the interaction
between cellulose and silica is mainly influenced by
induction force. Because surfaces of cellulose and
silica nanocomposites both consist of polar hydroxyls,
it results in an induction dipole, and induction force can
be formed. On the other hand, it is easy to form
20
Fig. 4
(b)
(a) The hydrogen bonds(the dashed) of the
composite system,
(b) magnified figure of (a)
3.2 Radial distribution functions
The radial distribution function[22] g(r), which can
directly reflect the microstructure of composite system,
is helpful to investigate the intermolecular hydrogen
Interactions between cellulose and silica nanoparticles studied by molecular dynamics simulations
bond between cellulose and silica. Under the
COMPASS force-field, the radial distribution
functions of amorphous cellulose and silica composite
system were analyzed (see Fig. 5). The RDF for each
model has four large peaks around 1.0-1.5 Å, which
correspond to the C-H, O-H, C-C and C-O bond
lengths, in addition to many smaller peaks between 2.0
and 3.0 Å which include the hydrogen bond atom
distances[15]. The fact that peaks of the composite
system is obviously lower than the pure cellulose
shows us that the changes of microstructure. It is weak
interactions between cellulose and silica that lead to the
changes of microstructure, which is consistent to the
above analysis of the interaction energy.
Radial Distribution Function g(r)
25
amorphous cellulose
cellulose-silica
20
15
10
5
0
0
2
4
6
8
r /Angstrom
Fig. 5
The radial distribution function of pure amorphous
cellulose and cellulose/silica composite system
4. Conclusions
Molecular simulation should be a useful tool to
probe the interaction of cellulose composites with
modified silica surfaces. In the composite system, it is
found that cellulose chains were more attracted to the
modified silica surfaces due to the induction force and
hydrogen bonds between cellulose chains and silica
surfaces. Besides, the greater the radius of silica
particles varies, the stronger the interactions change. It
can be suggested that the microstructure of cellulose
composites filled with silica nanopaticles changes, thus
resulting in the different thermal and mechanics
properties. In the next work, we will investigate the
physical properties of the composite system, like the
density, the glass transition temperature and so on.
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(to be continued on Page 25)
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