energies
Article
Thermal Stability of Modified Insulation Paper
Cellulose Based on Molecular Dynamics Simulation
Chao Tang 1, *, Song Zhang 1 , Qian Wang 2 , Xiaobo Wang 1 and Jian Hao 3
1
2
3
*
College of Engineering and Technology, Southwest University, Chongqing 400715, China;
[email protected] (S.Z.); [email protected] (X.W.)
State Grid Chongqing Electric Power Co. Chongqing Electric Power Research Institute, Chongqing 401123,
China; [email protected]
Laboratory of Power Transmission Equipment & System Security and New Technology,
Chongqing University, Chongqing 400044, China; [email protected]
Correspondence: [email protected]; Tel.: +86-023-68251265; Fax: +86-023-68251265
Academic Editor: Issouf Fofana
Received: 26 January 2017; Accepted: 17 March 2017; Published: 20 March 2017
Abstract: In this paper, polysiloxane is used to modify insulation paper cellulose, and molecular
dynamics methods are used to evaluate the glass transition temperature and mechanical properties
of the paper before and after the modification. Analysis of the static mechanical performance of
the model shows that, with increasing temperature, the elastic modulus of both the modified and
unmodified cellulose models decreases gradually. However, the elastic modulus of the modified
model is greater than that of the unmodified model. Using the specific volume method and
calculation of the mean square displacement of the models, the glass transition temperature of
the modified cellulose model is found to be 48 K higher than that of the unmodified model. Finally,
the changes in the mechanical properties and glass transition temperature of the model are analyzed
by energy and free volume theory. The glass transition temperatures of the unmodified and modified
cellulose models are approximately 400 K and 450 K, respectively. These results are consistent
with the conclusions obtained from the specific volume method and the calculation of the mean
square displacement. It can be concluded that the modification of insulation paper cellulose with
polysiloxane will effectively improve its thermal stability.
Keywords: insulation paper; polysiloxane; glass transition temperature; mean square displacement;
molecular simulation
1. Introduction
The glass transition temperature of a polymer is very important, being the turning point at which
a polymer material transitions from a glass state to a highly elastic state [1]. The main component
of oil-immersed power transformer insulation paper is natural cellulose [2], which is composed of
crystalline and amorphous regions. The crystalline region is closely arranged and its structure is stable,
meaning that its performance is relatively stable under high temperatures. In contrast, the amorphous
region is loosely and irregularly arranged, and so thermal aging usually starts from the amorphous
region [3,4]. The initial mechanical properties of cellulose insulation paper can basically meet the
needs of its applications, but after its glass transition temperature changes, its mechanical properties
seriously decline [5], which causes the paper to fail to meet the needs of the application. The general
operating temperature of a transformer is below 100 ◦ C, but local winding temperatures can exceed
473 K under extreme conditions. At 473 K, the properties of many polymers change greatly [6]; thus,
the enhancement of the glass transition temperature of cellulose insulation paper is of important
practical significance.
Energies 2017, 10, 397; doi:10.3390/en10030397
www.mdpi.com/journal/energies
Energies 2017, 10, 397
2 of 11
Polysiloxane has many advantages, such as high temperature stability, oxidation resistance, and
water repellency, and so has been used to modify the toughness and thermal stability of polymer
in recent years [7–9]. Li et al. [10] used polysiloxane to graft and modify E-20 modified epoxy.
Spectral and differential thermal analyses showed that the thermal stability of the modified resin
was improved significantly. Su et al. [11] used dimethyl-diethoxy-silane to modify bisphenol A
epoxy resin, and found that the tensile strength, elongation at break, and mechanical properties
of the modified material were improved, and the glass transition temperature reached 166.07 ◦ C.
Zhang et al. [12] used polysiloxane to improve the thermal stability and mechanical properties
of resin. Guan et al. [13] used fluorine-containing silicone to modify resin, and reported that the
maximum thermo-gravimetric rate temperature of the modified composite material reached 375 ◦ C.
Regarding other polymers, Zong et al. [14] used polysiloxane to modify polyurethane, and found that
the water resistance properties of the modified material were improved. Obviously, modification with
polysiloxane has a good effect on the performance of polymers, and especially has great potential for
improving their thermal stability.
With the development of computer simulation technology, molecular simulation has been widely
applied to the study of material properties [15–17]. Moreover, the results of studies combining
molecular simulations and experiments have shown simulated and experimental values that were
highly consistent, indicating that molecular simulation of materials is feasible and effective [18,19].
Molecular simulation has been widely used in researching the properties of insulation paper cellulose
to enhance its thermal stability. Liao et al. [20] grafted dicyandiamide onto the C6 atoms of cellulose
chains, and showed through molecular simulation that the stability of the modified cellulose chain
in water and acid environment was improved. Cheng et al. [21] used inorganic particles to modify
cellulose, and reported that the thermal stability of the modified cellulose was improved. However,
the modification of insulation paper cellulose with polysiloxane has rarely been reported.
Therefore, this paper focuses on the two important indexes of the thermal stability of cellulose
insulation paper: glass transition temperature and mechanical properties. The effect of polysiloxane on
the thermal stability of insulation paper cellulose was evaluated by a molecular dynamics simulation
method. The internal mechanism of the effect of polysiloxane on the mechanical properties, exercise
intensity, and glass transition temperature of the cellulose was also studied.
2. Model Building and Parameter Setting
2.1. Model Building
Mazeau and Heux [22] used molecular simulation to study a model of the amorphous region of
cellulose chains with different degrees of polymerization (DP), of 10, 20, and 40. They found no obvious
difference in molecular conformation or physical and chemical properties. Chen et al. [23] suggested
that, for the amorphous models of cellulose with a DP of 20, the mechanic properties obtained by
molecular simulation were consistent with the experimental data. Meanwhile, our previous research
results [24,25] also verified the consistency of molecular simulation and experiment results when
DP is 20. Therefore, in this paper, a cellulose chain with a DP of 20 was chosen to build the model.
The DP of the polysiloxane used for grafting and modification was 3. The polysiloxane is grafted
onto the hydroxyl group of the C6 atom in the cellulose unit, which is shown in Figure 1. The model
for the modified cellulose was established using the method used to construct the amorphous region
model proposed by Theodorou and Wsuter [26]. The target density of the model was 1.5 g/cm3 .
The constructed model is shown in Figure 2.
Energies 2017, 10, 397
Energies2017,
2017,10,
10,397
397
Energies
3 of 11
of11
11
33of
O3
O3
O4
O4
C4
C4
O5
O5
C5
C5
C6
C6
C1
C1
O2
O2
C3
C3
C2
C2
O1
O1
Figure1.1.Unit
Unitof
ofinsulation
insulationpaper
papercellulose.
cellulose.
Figure
Cellulose,DP=20
DP=20
Cellulose,
Polysiloxane
Polysiloxane
DP=3
DP=3
CC
HH
OO
SiSi
Figure2.2.Amorphous
Amorphousmodel
modelof
ofcellulose
cellulosemodified
modifiedby
Figure
bypolysiloxane.
polysiloxane.
2.2.
Parameter
Setting
2.2.
2.2.Parameter
ParameterSetting
Setting
Structural
optimizationand
and
energy
optimizationof
ofthe
themodel
modelwas
wasrequired.
required.First,
First,
5000
steps
of
Structural
Structuraloptimization
optimization
andenergy
energyoptimization
optimization
of
the
model
was
required.
First,5000
5000steps
stepsof
of
energy
minimizationwere
were
carried
out.
The model
model
wasstructurally
then structurally
structurally
optimized
through
energy
minimization
were
carried
was
then
optimized
through
energy minimization
carried
out.out.
The The
model
was then
optimized
through molecular
molecular
dynamics simulation.
simulation.
The force
force
field
used
in the
the structural
structural optimization
optimization
and molecular
molecular
molecular
used
in
and
dynamics dynamics
simulation.
The force The
field
usedfield
in the
structural
optimization
and molecular
dynamics
dynamics
simulation
was
PCFF
(polymer
consistent
forcefield)
[27],
which
is
very
suitable
for the
the
dynamics
PCFF consistent
(polymer forcefield)
consistent [27],
forcefield)
which
is very
for
simulationsimulation
was PCFFwas
(polymer
which[27],
is very
suitable
for suitable
the treatment
of
treatment
of
carbohydrates
and
other
organic
molecules.
During
the
structural
optimization,
treatment
of
carbohydrates
and
other
organic
molecules.
During
the
structural
optimization,
carbohydrates and other organic molecules. During the structural optimization, dynamics simulations
dynamics
simulations
of50
50at
pstemperatures
werecarried
carriedout
out
attemperatures
temperatures
of1000
1000K.
K,Energy
800K,
K,and
and600
600K.
K.Energy
Energy
dynamics
simulations
ps
were
at
of
K,
800
of 50 ps were
carried of
out
of
1000
K, 800 K, and
600
minimization
was
minimization
was
carried
out after
after
every dynamics
dynamics
simulation,
and the
the obtained
obtained
minimized
minimization
was
carried
out
every
simulation,
and
carried out after
every
dynamics
simulation,
and
the obtained
minimized
structure
was minimized
used
as the
structure
was used
used as
as the
the
initial
conformation
for the
the
nextdynamics
dynamicssimulation
simulation.
Then,
dynamics
structure
was
conformation
for
next
dynamics
simulation.
dynamics
initial conformation
for
the initial
next
dynamics
simulation.
Then,
of Then,
50
ps and
energy
simulation
of
50
ps
and
energy
minimization
were
carried
out
at
400
K.
After
the
above
processing,
simulation
of
50
ps
and
energy
minimization
were
carried
out
at
400
K.
After
the
above
processing,
minimization were carried out at 400 K. After the above processing, the local unreasonable structure
the
local
unreasonable
structure
inthe
theamorphous
amorphous
model
was
basically
eliminated,
which
made
the
the
local
unreasonable
structure
in
basically
eliminated,
which
made
in the
amorphous
model
was basically
eliminated,model
whichwas
made
the model
stable and
closer
tothe
the
model
stable
and
closer
to
the
real
material,
and
provided
a
reasonable
equilibrium
geometry
model
stable
and
closer
to
the
real
material,
and
provided
a
reasonable
equilibrium
geometry
real material, and provided a reasonable equilibrium geometry conformation for the next molecular
conformation
for the
the[28].
nextOn
molecular
dynamics
simulation
[28]. simulation
On this
this basis,
basis,
thecarried
molecular
conformation
for
next
molecular
dynamics
simulation
[28].
On
the
molecular
dynamics simulation
this basis,
the molecular
dynamics
was the
out.
dynamics
simulation
was
the
carried
out.
To
determine
the
glass
transition
temperature
before
and
dynamics
simulation
was
the
carried
out.
To
determine
the
glass
transition
temperature
before
and
To determine the glass transition temperature before and after modification, a temperature range
after
modification,
temperature
range
of200–650
200–650
wasselected
selected
forthe
the
simulation,The
withmolecular
every50
50
after
modification,
aatemperature
of
was
for
simulation,
with
every
of 200–650
K was selected
for therange
simulation,
withKK
every
50
K a target
temperature.
K
a
target
temperature.
The
molecular
dynamics
simulation
of
each
target
temperature
was
based
on
Kdynamics
a target temperature.
molecular
dynamics simulation
each
was based
on
simulation ofThe
each
target temperature
was basedofon
thetarget
fully temperature
optimized structure.
First,
the
fully
optimized
structure.
First,
equilibrium
simulation
of
100
ps
was
carried
out,
after
which
the
fully
optimized
structure.
First,
equilibrium
simulation
of
100
ps
was
carried
out,
after
which
equilibrium simulation of 100 ps was carried out, after which molecular dynamics simulation of 200 ps
molecular
dynamics
simulation
of200
200ps
pswas
was
carried
out.
The
integral
steplength
lengthof
was
fs,and
andwas
the
molecular
dynamics
of
out.
integral
step
was
fs,
the
was
carried
out. Thesimulation
integral step
length
wascarried
1 fs, and
theThe
dynamic
information
the11system
dynamic
information
ofthe
thesystem
systemwas
wascollected
collectedevery
every5000
5000fs.
fs.
dynamic
of
collectedinformation
every 5000 fs.
Analysisof
ofSimulation
SimulationResults
Results
3.3.Analysis
3.1.Mechanical
MechanicalProperties
Properties
3.1.
relationship between
between stress
stress and
and
strain
can
be
obtained
by
the
generalized
Hooke’s
law:
Therelationship
andstrain
straincan
canbe
beobtained
obtainedby
bythe
thegeneralized
generalizedHooke’s
Hooke’slaw:
law:
The
Energies 2017, 10, 397
4 of 11
Energies 2017, 10, 397
4 of 11
σ i  Cij ε j
(1)
σi = Cij ε j
(1)
ε is the stress matrix, σ is the strain matrix.
where C represents a 6 × 6 elastic coefficient matrix,
First, the inverse matrix S of the C matrix is obtained, and then the effective bulk modulus
where C represents a 6 × 6 elastic coefficient matrix, ε is the stress matrix, σ is the strain matrix.
and shear modulus are calculated by Reuss’ average method [29]:
First, the inverse matrix S of the C matrix is obtained, and then the effective bulk modulus and
1
shear modulus are calculated by Reuss’ average
method
Κ  3(
a  2b)[29]:
(2)
K = [3( a +52b)]−1
G
G=
In the formula:
In the formula:
(2)
(3)
4a  45b  3c
(3)
4a − 4b + 3c
1
a 1 S  S 22  S 33  ,
+ S33 ),
a = 3(S1111+ S22
3
1
b 1 (SS1212 +
S
b=
S23
+SS3131 ), ,
23 
33
11
c=
+ S ). .
c  (SS4444+ S55
55  S 6666
33
Through the relationship between the modulus of the same-phase material:
Through the relationship between the modulus of the same-phase material:
EE=
 2G
2G(11+ν) =
 33K
K(11−
 2ν
2), ,
(4)
(4)
where EErepresents
thethe
volume
modulus,
is the
shear
modulus,
and
ν
representsthe
theelastic
elasticmodulus,
modulus,K Kis is
volume
modulus,G G
is the
shear
modulus,
and
represents
Poisson’s
ratio.
v represents
Poisson’s
ratio.
The ratio
ratio of
of strain
strainand
andstress
stressis iselastic
elastic
modulus
E, which
reflects
rigidity
a material.
modulus
E, which
reflects
the the
rigidity
of a of
material.
The
The
greater
the value
is,greater
the greater
the rigidity
andstronger
the stronger
the ability
the material
to
greater
the value
of E of
is, E
the
the rigidity
and the
the ability
of theof
material
to resist
resist
deformation
willThus,
be. Thus,
E can
reflect
macro-mechanicalstrength
strengthofoftransformer
transformer insulation
deformation
will be.
E can
reflect
thethe
macro-mechanical
paper [30,31]. The
The elastic
elastic modulus
modulus calculated
calculated for
for the
the modified model and unmodified model at
different temperatures
temperatures is
is shown
shown in
in Figure
Figure 3.
3.
10
Unmodified
Modified

12
8
10
4
6
E (GPa)
E (GPa)
6
8
2
4
2
200
300
400
500
600
0
700
T (K)
Figure
3. Elastic
and unmodified
unmodified models.
models.
Figure 3.
Elastic modulus
modulus of
of modified
modified and
With increasing temperature, the elastic modulus of both models gradually decreases. However,
With increasing temperature, the elastic modulus of both models gradually decreases. However,
the elastic modulus of modified model is larger than that of the unmodified model in the whole
the elastic modulus of modified model is larger than that of the unmodified model in the whole
simulation process. In Figure 3, E is the difference between the elastic modulus of the modified
simulation process. In Figure 3, ∆E is the difference between the elastic modulus of the modified
model and the elastic modulus of the unmodified model. E is positive in the whole temperature
model and the elastic modulus of the unmodified model. ∆E is positive in the whole temperature
range, which shows that the elastic modulus of the modified model is improved over that of the
range, which shows that the elastic modulus of the modified model is improved over that of the
unmodified model. This is because, when modifying cellulose by polysiloxane grafting, the bond
Energies 2017, 10, 397
Energies 2017, 10, 397
5 of 11
5 of 11
energy
of themodel.
flexibleThis
Si–O
introduced
on the cellulose
chain
is 451 kJ/mol,grafting,
which isthe
greater
unmodified
is bond
because,
when modifying
cellulose
by polysiloxane
bond
than
theof356
the C–C
The greater
bond energy,
it which
is to break
the bond,
energy
thekJ/mol
flexibleofSi–O
bondbond.
introduced
on thethe
cellulose
chain isthe
451harder
kJ/mol,
is greater
than
so
the
elastic
modulus
of
the
model
modified
with
polysiloxane
is
always
higher
than
that
of
the 356 kJ/mol of the C–C bond. The greater the bond energy, the harder it is to break the bond, so the
the
unmodified
model
increasing
Thisis means
the
deformation
resistance,
elastic modulus
of thewith
model
modified temperature.
with polysiloxane
always that
higher
than
that of the unmodified
mechanical
properties,temperature.
and thermal This
stability
of that
the model
modified with
polysiloxane
are enhanced.
model with increasing
means
the deformation
resistance,
mechanical
properties,
and thermal stability of the model modified with polysiloxane are enhanced.
3.2. Glass Transition Temperature
3.2. Glass Transition Temperature
As its temperature is gradually increased, an amorphous material will undergo glass transition.
As its temperature
is gradually
increased,
ana amorphous
undergo
glass
transition.
The transition
temperature
[1] of a material
from
glass state tomaterial
a highlywill
elastic
state is
known
as the
The transition
material
from acontinues
glass state
a highly
elastic
state is
known
as
glass
transitiontemperature
temperature[1]
Tg.of
If a
the
temperature
to to
rise,
a polymer
material
will
change
the glass
transition
Tg . Ifflow
the state
temperature
continues
a polymer
will
from
a highly
elastictemperature
state to a viscous
at its melting
point,toTrise,
f. Generally,
the material
properties
of
change from
highlybefore
elastic glass
state to
a viscousAtflow
state at its higher
meltingthan
point,the
Tf .glass
Generally,
the
polymers
area stable
transition.
temperatures
transition
properties of the
polymers
are stable
before of
glass
transition.
At temperatures
higher than
glass
temperature,
mechanical
properties
polymers,
namely
mechanical strength
andthe
thermal
transition will
temperature,
the mechanical
polymers,
namely of
mechanical
and
stability,
be significantly
reduced.properties
Therefore,of the
improvement
the glassstrength
conversion
thermal stability,
will bepaper
significantly
Therefore, the
improvement
of the glass conversion
properties
of insulation
cellulosereduced.
at high temperature
is significantly
important.
properties
of the
insulation
paper
cellulose
at high
is significantly
important.
Among
methods
used
to evaluate
thetemperature
glass transition
temperature
of polymers, the most
Among
methods
to evaluate the glass
temperature
polymers,
the(fixed
most
common
andthe
reliable
is theused
volume-temperature
curvetransition
method [32,33].
In theofNPT
ensemble
common
reliable
is theN,volume-temperature
curve method
[32,33]. In
the NPT ensemble
(fixed
values
of and
particle
number
pressure P, and temperature
T), molecular
simulation
was performed
values
particle
N, pressure
P, and temperature
molecular
simulation was
on
the of
models
atnumber
each selected
temperature
to calculateT),
the
density parameter.
The performed
reciprocal on
of
the models
at each
selected
temperature
to calculate
densityvolume
parameter.
The temperature
reciprocal of density
density
is the
specific
volume,
thus, the
figure ofthe
specific
versus
can be
is the specific
volume,
thus, the of
figure
specific
be obtained.
Next,
obtained.
Next,
the intersection
linesoffitting
thevolume
regionsversus
before temperature
and after thecan
inflection
point of
the
the intersection
lines fitting the
before
and afterturning
the inflection
volume
specific
volumeoftemperature
is regions
the glass
transition
point. point
Thus,of the specific
corresponding
temperature is the
point. The
Thus,glass
the corresponding
temperature
is the
glass
temperature
the glass
glasstransition
transitionturning
temperature.
transition temperature
fitting
curve
is
transition
temperature.
The glass transition temperature fitting curve is shown in Figure 4.
shown
in Figure
4.
Unmodified
Modified
Unmodified curve fitting
Modified curve fitting
Specific volume (cm3•g-1)
0.80
0.78
0.76
0.74
450K
0.72
402K
0.70
200
300
400
500
600
700
T (K)
Figure 4. Specific
Specific volume–temperature curve.
The
The glass
glass transition
transition temperature
temperature of
of the
the unmodified
unmodified model
model is
is 402
402 K,
K, while
while that
that of
of the
the model
model
modified
with
polysiloxane
is
450
K,
which
means
that
the
modification
improves
the
glass
modified with polysiloxane is 450 K, which means that the modification improves the glass transition
transition
temperature
of
the
cellulose
chain
by
48
K.
In
the
Handbook
of
Polymers
[34],
the
glass
temperature of the cellulose chain by 48 K. In the Handbook of Polymers [34], the glass transition
transition
temperature
of
variety
of
industrial
cellulose
is
reported
to
be
between
250
and
580
K,
but
temperature of variety of industrial cellulose is reported to be between 250 and 580 K, but this is only
this
is only Thus,
a reference.
the glass
transitionof temperatures
the simulated
cellulose
and
a reference.
the glassThus,
transition
temperatures
the simulated of
cellulose
and modified
cellulose
modified
cellulose
models
in
this
paper
are
credible,
and
the
comparative
analysis
of
the
glass
models in this paper are credible, and the comparative analysis of the glass transition temperature of
transition
temperature
of the modified
unmodified model is valid.
the modified
and unmodified
model is and
valid.
The glass transition temperature of the modified model is higher than that of the unmodified
model, which means that the process from the glassy state to highly elastic state of the modified
Energies 2017, 10, 397
6 of 11
The
glass
transition
Energies
2017,
10, 397
temperature of the modified model is higher than that of the unmodified
6 of 11
model, which means that the process from the glassy state to highly elastic state of the modified model
model
is slower
than
thatunmodified
of the unmodified
model. Therefore,
the modification
of cellulose
should
is slower
than that
of the
model. Therefore,
the modification
of cellulose
should result
in
result
in excellent
performance
and enhanced
thermal stability.
excellent
performance
and enhanced
thermal stability.
3.3. Glass
Glass Transition
Transition Temperature
3.3.
Temperature Analysis
Analysis
The more
more intense
The
intense the
the chain
chain movement
movement in
in insulation
insulation paper
paper cellulose,
cellulose, the
the worse
worse its
its mechanical
mechanical
performance,
which
leads
to
a
poor
thermal
stability.
The
intensity
of
the
chain
movement
can be
be
performance, which leads to a poor thermal stability. The intensity of the chain movement can
analyzed by
by the
the mean
mean square
square displacement
displacement (MSD).
(MSD). The
The MSD
MSD describes
describes the
the overall
overall movement
movement of
of the
the
analyzed
molecular chain
chain centroid
centroid [35],
molecular
[35], and
and can
can be
be calculated
calculated as:
as:
*
*
 (0)22 ,
MSD
=  rrii ((t)) −
(5)
MSD
 rri (i 0
) ,
(5)
*

*
and rrii ((00)) represent
representmolecular
molecularor
oratomic
atomic position
position vectors
vectors at
at time t and initial time
(t) and
where rrii(t
time i,i,
respectively, and
represents the
the ensemble
The MSD
MSD of
of the
the modified
modified model
model and
and the
the
respectively,
and <
<>
> represents
ensemble average.
average. The
unmodified
model
are
shown
in
Figure
5.
unmodified model are shown in Figure 5.
200K
450K
250K
500K
300K
550K
350K
600K
400K
650K
10
(a)
MSD (Å2)
8
6
4
2
0
0
50
100
150
200
t (ps)
200K
450K
10
250K
500K
300K
550K
350K
600K
400K
650K
(b)
8
MSD (Å2)
6
4
2
0
0
50
100
150
200
t (ps)
Figure 5.
5. MSD
MSD of
of (a)
(a) unmodified
unmodified model
model and
and (b)
(b) modified
modified model
model at
at different
different temperatures.
Figure
temperatures.
According to
degree
of chain
movement
increases
gradually
with temperature,
which
According
toFigure
Figure5,5,thethe
degree
of chain
movement
increases
gradually
with temperature,
indicates
that insulation
paper cellulose
affected byistemperature
transformer
operation.
In the
which
indicates
that insulation
paperis cellulose
affected byduring
temperature
during
transformer
unmodifiedInmodel,
between 350
and 400
K, the350
MSD
theK,
chain
appears
to jump,
is basically
operation.
the unmodified
model,
between
andof400
the MSD
of the
chainwhich
appears
to jump,
which is basically consistent with the glass transition temperature of the unmodified model of 402 K.
Between 500 and 550 K, the MSD of the unmodified model appears to jump again, which may
correspond to the transition temperature of cellulose chain from the highly elastic state to the
viscous flow state. In the modified model, the first jump in the MSD of the chain between 400 and
Energies 2017, 10, 397
7 of 11
consistent with the glass transition temperature of the unmodified model of 402 K. Between 500
and 550 K, the MSD of the unmodified model appears to jump again, which may correspond to the
transition temperature of cellulose chain from the highly elastic state to the viscous flow state. In the
modified model, the first jump in the MSD of the chain between 400 and 450 K also corresponds to
the glass transition temperature (450 K) of the modified model. The MSD of the modified model does
not appear to jump again obviously within the studied temperature range. This may be because the
modification of the cellulose chain not only increased the glass transition temperature of the chain,
but also increased the temperature of the highly elastic state to the viscous flow state transition. It can
also be seen that, before the glass transition, the intensity of the movement of the chain was relatively
small, and the MSD value of the two models is less than 1 Å2 . After the glass transition, the intensity
of the movement of the chain is obviously enhanced; the maximum MSD of the unmodified model is
about 10 Å2 , while that of the modified model is about 9 Å2 .
The chain movement is the direct embodiment of the thermal movement ability of the insulation
paper cellulose chain. The more intense the chain movement is, the worse the mechanical performance
will be, which leads to a poor thermal stability. The thermal stability of insulation paper cellulose is
improved by grafting with polysiloxane. In oil-immersed transformers, cellulose insulation paper
is wound around the copper conductor as electrical insulation. If the mechanical properties of the
insulation paper are affected by temperature and become worse, the insulation paper wrapped outside
the copper wire will be easily damaged when the conductor coil is subject to mechanical stress,
which will damage the insulation of the transformer. Therefore, the improvement of the mechanical
properties of insulation paper also indirectly improves the insulation of the transformer, thus, ensuring
its safe operation.
4. Analysis and Discussion
4.1. Enhancement Mechanism
Polysiloxane is a cross-linked polymer with Si–O–Si as the main chain, and silicon atoms connected
to other organic groups. The organic groups can be methyl or phenyl, so polysiloxane can have organic
and inorganic characteristics. Polysiloxane has many excellent properties, such as heat resistance and
chemical corrosion resistance. The Si–O–Si bond is the basic bond form of polysiloxane, essentially
the same as quartz, but its side groups are connected to organic groups. The thermal stability of
inorganic compounds composed of Si–O bonds can be as high as 2073 K. When organic substituents
are introduced into the polymer, its thermal stability will decrease by 623–873 K, but its heat resistance
will still be better than that of common organic compounds. Additionally, because of its organic and
inorganic characteristics, it can solve compatibility problems well. The bond energy of the Si–O bond
is 451 kJ/mol, much higher than the energy of the C–C bond of about 356 kJ/mol. Thus, the thermal
stability of the Si–O bond is better than that of the C–C bond. The length of the Si–O–Si bond is also
longer (0.164 nm), and its bond angle is larger (140–180◦ ) [36]. The chain structure of polysiloxane
is very soft, the interaction between chains is weak, and its surface tension is low, which means that
polysiloxane can still maintain its original performance in a wide range of temperatures. Therefore,
the mechanical properties of cellulose modified with a certain amount of polysiloxane change little
with increasing temperature, resulting in a very good thermal stability.
Energy is a parameter that can best reflect the stability of the system. The energy of the system
under a PCFF force field [6] can be expressed as:
Etotal = ( Einternal + Ecross ) + Enobond ,
(6)
where Einternal + Ecross represents the bond energy and Enobond represents non-bond energy. Figure 6
shows the overall potential energy and non-bond energy of the unmodified model and modified model
at each temperature studied in this paper.
Energies 2017, 10, 397
Energies 2017, 10, 397
8 of 11
8 of 11
1000
Total energy
Non bond energy
900
(a)
Energy (kcal•mol-1)
800
700
390K
600
500
400
300
200
300
400
500
600
700
T (K)
800
700
Total energy
Non bond energy
(b)
Energy (kcal•mol-1)
600
500
400
452K
300
200
100
0
200
300
400
500
600
700
T (K)
Figure 6.
6. Energies
Energies of
of the
the two
two models:
models: (a)
(a) the
the unmodified
unmodified model;
model; and
and (b)
(b) the
the modified
modified model.
model.
Figure
The overall potential
potential energy
energy and
and the
the non-bond
non-bond energy
energy of
of both
both models
models increase
increase linearly,
linearly, but the
growth rate
rate of
of the
thenon-bond
non-bondenergy
energyisisobviously
obviouslylower
lower
than
that
overall
potential
energy.
In
than
that
of of
thethe
overall
potential
energy.
In the
the
unmodified
model,
the
overall
potential
energy
starts
to
become
larger
than
the
non-bond
unmodified model, the overall potential energy starts to become larger than the non-bond energy at
energy
at the
390modified
K. In the model,
modified
overallenergy
potential
energy
starts to
become
than the
390 K. In
themodel,
overallthe
potential
starts
to become
larger
thanlarger
the non-bond
non-bond
at about
452 K, a difference
62 K. Equation
(6) that
shows
thatthe
when
the of
value
energy at energy
about 452
K, a difference
of about of
62about
K. Equation
(6) shows
when
value
the
of
the
overall
potential
energy
starts
increase
beyond
that
of
the
non-bond
energy,
the
value
changes
overall potential energy starts increase beyond that of the non-bond energy, the value changes from
from
negative
to positive.
This indicates
the repulsive
force between
greater
than the
negative
to positive.
This indicates
that thethat
repulsive
force between
atoms is atoms
greateristhan
the attraction
attraction
forceatoms
between
in of
the
of the
andbond
the becomes
chemicalunstable.
bond becomes
force between
in theatoms
interior
theinterior
molecule,
and molecule,
the chemical
As the
unstable.
As is
thegradually
temperature
is gradually
increases,
bonds
and become
glycosidic
bonds
become
temperature
increases,
some active
bondssome
and active
glycosidic
bonds
easy
to break
and,
easy
break and,
thus, thermal
of the
cellulose
occurs.
Upon thermal
degradation,
the
thus,to
thermal
degradation
of the degradation
cellulose occurs.
Upon
thermal
degradation,
the DP
of the cellulose
DP
of
the
cellulose
will
decrease,
resulting
in
decreased
mechanical
properties
of
the
cellulose
will decrease, resulting in decreased mechanical properties of the cellulose insulation paper.
insulation paper.
4.2. Mechanism of Effect of Polysiloxane on Chain Movement Intensity and Glass Transition Temperature
of Cellulose
4.2.
Mechanism of Effect of Polysiloxane on Chain Movement Intensity and Glass Transition Temperature of
Cellulose
The free volume of a material has a great influence on its glass transition temperature. According to
the free
theory proposed
by Fox has
and Flory
[37],
the volume
is composed
of the free
Thevolume
free volume
of a material
a great
influence
on ofitsa polymer
glass transition
temperature.
volume occupied
by the
polymer.
theFlory
difference
in the
expansion
coefficient,
According
to the and
free unoccupied
volume theory
proposed
byDue
Fox to
and
[37], the
volume
of a polymer
is
when
the
temperature
increases,
the
volume
of
the
polymer
increases
linearly,
and
the
free
volume
composed of the free volume occupied and unoccupied by the polymer. Due to the difference
in the
will jump near
the glass
transition
temperature
[30]. In this
the change
in the
expansion
coefficient,
when
the temperature
increases,
the paper,
volumethe
ofmechanism
the polymerofincreases
linearly,
glass
transition
temperature
is
analyzed
using
the
free
volume
theory
from
a
microscale
perspective.
and the free volume will jump near the glass transition temperature [30]. In this paper, the
The hard sphere
method
used
to calculate
the sizeisdistribution
of thethe
free
volume
in
mechanism
of theprobe
change
in thewas
glass
transition
temperature
analyzed using
free
volume
theory from a microscale perspective. The hard sphere probe method was used to calculate the size
distribution of the free volume in the model, and the probe radius was set to 1 Å . Figure 7 shows the
Energies 2017,10,
10, 397
Energies
Energies2017,
2017, 10,397
397
of 11
999of
of11
11
free
volume
distribution
of the
model and
model, in
which
blue
free
volumeand
distribution
theunmodified
unmodified
andmodified
modified
whichthe
the
bluepart
partis
isthe
the
the model,
the probeofradius
was set tomodel
1 Å. Figure
7 shows model,
the freeinvolume
distribution
of
the
distribution
of
the
free
volume.
distribution
of
the
free
volume.
unmodified model and modified model, in which the blue part is the distribution of the free volume.
(a)
(a)
(b)
(b)
Figure7.7.
7.Free
Freevolume
volumeof
ofthe
thetwo
twomodels:
models:(a)
(a)the
theunmodified
unmodifiedmodel;
model;and
and(b)
(b)the
themodified
modifiedmodel.
model.
Figure
Figure
Free
volume
of
the
two
models:
(a)
the
unmodified
model;
and
(b)
the
modified
model.
Figure
the
free
volume
fitting
curve
the
unmodified
model
and
modified
model.
In
Figure 88 shows
shows the
thefree
freevolume
volumefitting
fittingcurve
curveofof
of
the
unmodified
model
and
modified
model.
In
shows
the
unmodified
model
and
modified
model.
In the
the
unmodified
model,
the
free
volume
changes
little
below
a
temperature
of
392
K.
At
392
K,
the
the
unmodified
model,
volume
changes
below
a temperature
of 392
K.392
At K,
392the
K, free
the
unmodified
model,
the the
freefree
volume
changes
littlelittle
below
a temperature
of 392
K. At
free
volume
jumps
gradually
increases.
In
the
model,
the
free
changes
free
volume
jumps
and then
then
gradually
increases.
Inmodified
the modified
modified
model,
the volume
free volume
volume
changes
volume
jumps
and and
then
gradually
increases.
In the
model,
the free
changes
little
little
below
440
K,
and
increases
440
An
in
free
increases
the
little
below
440
K,then
andthen
thengradually
gradually
increases
overK.
440
K.
Anincrease
increase
involume
freevolume
volume
increases
the
below
440 K,
and
gradually
increases
over over
440
AnK.
increase
in free
increases
the space
space
for
the
movement
of
the
cellulose
chain.
When
the
molecular
chain
starts
to
move,
the
space
the movement
of thechain.
cellulose
chain.
When the
molecular
chain the
starts
to move,
the
for the for
movement
of the cellulose
When
the molecular
chain
starts to move,
cellulose
changes
cellulose
changes
from
the
glass
state
to
the
highly
elastic
state.
After
conversion
to
the
high
elastic
cellulose
changes
from
the
glass
state
to
the
highly
elastic
state.
After
conversion
to
the
high
elastic
from the glass state to the highly elastic state. After conversion to the high elastic state, the mechanical
state,
the
properties
the
chain
and
its
stability
state,
themechanical
mechanical
properties
of
thecellulose
cellulose
chaindecrease
decrease
dramatically,
andalso
itsthermal
thermal
stability
properties
of the cellulose
chainof
decrease
dramatically,
and its dramatically,
thermal
stability
decreases.
From
also
decreases.
From
the
present
free
volume
theory
analysis,
the
difference
of
48
K
between
the
also
decreases.
the
present
free volume
theoryofanalysis,
the difference
between
the
the present
free From
volume
theory
analysis,
the difference
48 K between
the jumpof
in 48
theKfree
volume
of
jump
in
the
free
volume
of
the
unmodified
model
and
modified
model
is
basically
consistent
with
jump
in
the
free
volume
of
the
unmodified
model
and
modified
model
is
basically
consistent
with
the unmodified model and modified model is basically consistent with the conclusions obtained from
the
conclusions
obtained
from
the
previous
specific
method
Therefore,
itit isis
the previous
conclusions
obtained
from
theanalysis.
previousTherefore,
specific volume
method
analysis.
Therefore,
specific
volume
method
itvolume
is confirmed
thatanalysis.
polysiloxane
modification
confirmed
that
polysiloxane
modification
will
confirmed
that
polysiloxane
modification
willenhance
enhancethe
thethermal
thermalstability
stabilityof
ofcellulose.
cellulose.
will enhance
the
thermal stability
of cellulose.
900
900
Unmodified
Unmodified
Modified
Modified
Unmodified
Unmodifiedcurve
curvefitting
fitting
Modified
Modifiedcurve
curvefitting
fitting
800
800
Freevolume
(Å33))
volume(Å
Free
700
700
600
600
392K
392K
500
500
400
400
440K
440K
300
300
200
200
300
300
400
400
500
500
600
600
700
700
TT(K)
(K)
Figure
Figure8.8.Fitting
Fitting
curveof
offree
freevolume.
volume.
Fittingcurve
5.
5.Conclusions
Conclusions
In
In this
this paper,
paper, the
the molecular
molecular dynamics
dynamics method
method isis applied
applied to
to evaluate
evaluate the
the glass
glass transition
transition
temperature,
temperature, mechanical
mechanical properties,
properties, and
and chain
chain movement
movement intensity
intensity of
of insulation
insulation paper
paper cellulose
cellulose
Energies 2017, 10, 397
10 of 11
5. Conclusions
In this paper, the molecular dynamics method is applied to evaluate the glass transition
temperature, mechanical properties, and chain movement intensity of insulation paper cellulose
before and after modification with polysiloxane. Through comparison of the model data, the following
conclusions are obtained:
(1)
(2)
(3)
For the modified cellulose insulation paper, the anti-deformation energy of the cellulose is
increased, and its mechanical properties are improved.
Based on the specific volume method, the glass transition temperature of the modified cellulose
model increased by 48 K than that of the unmodified model. When the temperature increases
near the glass transition temperature, the mean square displacement of the two models appears
to jump, and the chain movement of the insulation paper cellulose becomes intense.
The mechanism of the change in the mechanical properties of the model is discussed based
on energy. The glass transition temperature range obtained from the free volume theory is
basically the same as that obtained by the specific volume method. Therefore, the modification of
insulation paper cellulose by polysiloxane grafting will greatly enhance the thermal stability of
insulation paper.
Acknowledgments: The authors wish to thank the scientific project funded by the State Grid Chongqing Electric
Power Cooperation Chongqing Electric Power Research Institute, and the Fundamental Research Funds for the
Central Universities (Grant No. XDJK2014B031) for their financial support.
Author Contributions: Chao Tang, Song Zhang conceived and designed the simulations; Song Zhang and
Xiaobo Wang performed the simulations; all authors analyzed the data; Chao Tang and Song Zhang wrote the
paper, while other authors offered their modification suggestions for the manuscript.
Conflicts of Interest: The authors declare no conflict of interest.
References
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
Ke, Y.C.; He, P.S. Macromolecule Physical Tutorial; Chemistry Industry Press: Beijing, China, 2006; pp. 157–161.
Liu, R.Q. Basis of Cellulose Chemistry; Science Press: Beijing, China, 1985; pp. 115–123.
Zhu, M.Z. Molecular Dynamics Study of Thermal Aging of Oil-impregnated Insulation Paper. Ph.D. Thesis,
Chongqing Univeristy, Chongqing, China, April 2012.
Yan, J.Y.; Wang, X.L.; Li, Q.M.; Zhou, Y.; Wang, Z.D.; Li, C.R. Molecular dynamics simulation on the pyrolysis
of insulation paper. Proc. CSEE 2015, 35, 5941–5949.
Yang, T. Molecular Dynamics Study on the Impact of Temperature and Electric Field on the Microscopic
Properties of Oil-Immersed. Ph.D. Thesis, Chongqing Univeristy, Chongqing, China, May 2013.
Wang, Y.Y.; Yang, T.; Liao, R.J.; Zhang, D.W.; Liu, Q.; Tian, M. Glass Transition in amorphous region of
transformer insulation paper by molecular dynamics. High Volt. Eng. 2012, 31, 1199–1206.
Li, X.M.; Pan, Q.; Lin, Y.D.; You, B.; Sun, Y.J. Synthesis and Characterization of epoxy/poly siloxane hybrid
materials. J. Chongqing Univ. 2011, 34, 112–122.
Zhao, S. Study on Preparation and Properties of Polysiloxane Modified Epoxy Encapsulating Material.
Ph.D. Thesis, Harbin University of Science and Technology, Harbin, China, March 2014.
Zhang, H.Z. Study on Polysiloxane Copolymerization Modified Epoxy Resin Capability. Ph.D. Thesis,
Chongqing University, Chongqing, China, May 2008.
Li, Y.W.; Shen, M.M.; Huang, H.Y.; Ma, Y.J.; Ha, C.Y. Synthesis and properties of polyphenylsilicone and
polymethylphenylsilicone modified epoxy resins. Acta Polym. Sin. 2009, 11, 1086–1090. [CrossRef]
Su, Q.Q.; Liu, W.Q.; Wang, W.R.; Liu, Y.F. Investigation on epoxy resin modified by diethoxydimethylsilane.
Chem. Mater. Constr. 2008, 24, 23–25.
Zhang, S.; Xie, J.L.; Deng, L.J. Study on organosilicon modified epoxy resin adhesive for heat-resistance.
Mater. Rev. 2006, 20, 23–25.
Guan, D.B.; Cai, Z.Y.; Fang, C.; Qiu, X.M.; Dou, Y.L. Preparation and properties of fluorinated silicone
modified epoxy resin composite. Polym. Mater. Sci. Eng. 2015, 31, 120–125.
Energies 2017, 10, 397
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
26.
27.
28.
29.
30.
31.
32.
33.
34.
35.
36.
37.
11 of 11
Zong, J.P.; Zhang, Q.S.; Li, J.S.; Sun, H.F.; Yu, Y.T.; Liu, S.J.; Liu, Y.H. Comparative of polydimethylsiloxane
modified graft and block polyurethane dispersions. Polym. Mater. Sci. Eng. 2011, 27, 62–65.
Sun, W.F.; Wang, X. Molecular dynamics simulation study of polyimide/copper-nanoparticle composites.
Acta Phys. Sin. 2013, 62, 186202.
Lin, C.P.; Liu, X.J.; Rao, Z.H. Molecular dynamics simulation of the thermophysical properties and phase
change behaviors of aluminum nanoparticles. Acta Phys. Sin. 2015, 64, 083601.
Kopper, K.P.; Küpper, D.; Reeve, R.; Mitrelias, T.; Gunn, D.S.D.; Jenkins, S.J. Chemically selective modification
of spin polarization in ultrathin ferromagnetic films: Microscopic theory and macroscopic experiment.
Phys. Rev. B 2009, 80, 1956–1960. [CrossRef]
Yu, S.; Yang, S.; Cho, M. Molecular dynamics study to identify mold geometry effect on the pattern transfer
in thermal nanoimprint lithography. Jpn. J. Appl. Phys. 2009, 48. [CrossRef]
Zhu, M.Z.; Chen, Y.F.; Gu, C.; Liao, R.J.; Zhu, W.B.; Du, X.M. Simulation on thermodynamic properties of
amorphous cellulose based on molecular dynamics. High Volt. Eng. 2015, 41, 432–439.
Liao, R.J.; Nie, S.J.; Zhou, X.; Wang, K.; Yuan, L.; Yang, L.J.; Cheng, H.C. Molecular dynamics simulation on
the hydrophilicity of physical modification cellulose insulating materials. Proc. CSEE. 2013, 39, 1–7.
Cheng, W.; Wang, K.; Fu, Q. Preparation of inorganic particle grafted cellulose fibre and its thermostability.
Plast. Ind. 2014, 42, 113–117.
Mazeau, K.; Heux, L. Molecular dynamics simulations of bulk native crystalline and amorphous structures
of cellulose. J. Phys. Chem. B 2003, 107, 2394–2403. [CrossRef]
Chen, W.; Lickfield, C.G.; Yang, Q.C. Molecular modeling of cellulose in amorphous state. Part 1: Model
building and plastic deformation study. Polymer 2004, 45, 1063–1071. [CrossRef]
Tang, C.; Zhang, S.; Zhang, F.Z.; Li, X.; Zhou, Q. Simulation and experimental about the thermal aging
performance improvement of cellulose insulation paper. Trans. China Electrotech. Soc. 2016, 31, 68–76.
Tang, C.; Zhang, S.; Li, X.; Xiong, B.F.; Xie, J.T. Experimental analyses and molecular simulation of the thermal
aging of transformer insulation paper. IEEE Trans. Dielectr. Electr. Insul. 2015, 22, 1131–1138. [CrossRef]
Theodorou, D.N.; Wsuter, U. Detailed molecular structure of a vinyl polymer glass. Macromolecules 1985, 18,
1467–1478. [CrossRef]
Sun, H. Ab-initio calculations and force field development for computer simulation of polysilanes.
Macromolecules 1995, 28, 701–712. [CrossRef]
Tao, C.G.; Feng, H.J.; Zhou, J.; Lv, L.H.; Lu, X.H. Molecular simulation of oxygen adsorption and diffusion in
polypropylene. Acta Phys. Chim. Sin. 2009, 25, 1373–1378.
Watt, J.P.; Davies, G.F.; O’Connell, R.J. The elastic properties of composite materials. Rev. Geophys. Space Phys.
1976, 14, 541–563. [CrossRef]
Tanaka, F.; Iwata, T. Estimation of the elastic modulus of cellulose crystal by molecular mechanics simulation.
Cellulose 2006, 13, 509–517. [CrossRef]
Zhang, S.; Tang, C.; Chen, G.; Zhou, Q.; Lv, C.; Li, X. The influence and mechanism of nano Al2 O3 to the
thermal stability of cellulose insulation paper. Sci. Sin. Techol. 2015, 45, 1167–1179.
He, P.S. The Mechanical Properties of High Polymer; Press of University of Science and Technology of China:
Hefei, China, 2008; pp. 185–196.
Fu, Y.Z.; Liu, Y.Q.; Lan, Y.H. Molecular simulation on the glass transition of polypropylene. Polym. Mater.
Sci. Eng. 2009, 25, 53–56.
Brandrup, J.; Immergut, E.H.; Grulke, E.A. Polymer Handbook; Wiley-Interscience Publication: New York, NY,
USA, 1999; pp. 476–479.
Mazur, P.; Maradudin, A.A. Mean-square displacements of atoms in thin crystal films. Phys. Rev. B 1981, 24,
2996–3007. [CrossRef]
Hao, Z.F.; Zhang, J.; Wu, Y.H.; Yu, J.; Yu, L. Synthesis and thermal stability properties of boron-doped
silicone resin. J. Appl. Polym. Sci. 2014, 131, 1366–1373. [CrossRef]
Fox, T.G.; Flory, P.J. Second-order transition temperatures and related properties of polystyrene. I. influence
of molecular weight. J. Appl. Phys. 1950, 21, 581–591. [CrossRef]
© 2017 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC BY) license (http://creativecommons.org/licenses/by/4.0/).