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Influence of the carbon nanotube surface
modification on the microstructure of
thermoplastic binders
S. V. Larin,
a
A. D. Glova,
b
E. B. Serebryakov,
b
V. M. Nazarychev,
a
J. M. Kenny
acd
and S. V. Lyulin*
ab
The structural properties of polymer nanocomposites based on thermoplastic polyimides filled with
surface-modified carbon nanotubes (CNT) have been studied by means of fully-atomistic molecular-
dynamics simulations. The influence of the distribution of functional carboxyl groups over the CNT
surface on the polymer-matrix density distribution, and the orientational ordering of polymer chains
have been investigated. It was shown that the polymer shifts far away from the nanoparticle surface with
increase of the CNT modification degree. The orientational ordering of PI chains was not observed in the
case of nanocomposites filled with modified CNTs where carboxyl groups are distributed uniformly on
the surface. However, in case of the edge-modified CNTs the polymer can interact with the CNT
surface; such edge-modified nanoparticles induce orientational ordering of crystallisable polyimide
chains which can be considered as an initial stage of the polymer matrix crystallization.
Introduction
Reinforcement of a polymer matrix with nanollers is regarded
as one of the most promising ways to develop new high-
performance materials. This is due to the fact that even in the
case of a small volume fraction of the nanoller, the interface
area between the composite components can be large enough to
cause the required changes in the polymer binder properties.
The use of functional polymers with a rather complicated
structure combining high thermal properties (high glass tran-
sition and thermal destruction temperatures) with advanced
mechanical, electrical and optical properties as polymer
binders, is a very promising method to create novel composite
materials.
1–5
Among such polymers, thermoplastic aromatic
polyetherimides play a key role.
6–9
Their use allows the utiliza-
tion of ecologically safe melting technologies in order to create
composites and nanocomposites. This simplies signicantly
the processing compared to the loading of nanoller particles
into the polymer binder during polymerization.
10–13
Subject to the chemical composition of polyimides (PI), their
structural and physical properties may vary a lot. In particular,
some polyimides may crystallize.
14–19
The crystallization of poly-
imides is relevant in terms of technology, since the increase in
the crystallinity will lead to a general enhancement of the
mechanical performance of the crystallized polyimides, despite
possible side effects impairing the properties. Therefore, it is of
importance to understand the conditions of formation of crys-
talline domains in polyimides and polyimide-based composites
depending on the polymer chemical structure and type of
nanoparticles used as llers.
Carbon nanotubes (CNT) represent one of the most wide-
spread nanoller together with graphene and fullerenes.
4
It is
well-known that the loading of carbon nanollers in thermo-
plastic polyimides oen leads to enhancing their mechanical
and thermal properties.
20–22
However, the mechanism of these
changes is not completely clear since CNT loading into the
polyimide matrix as such does not necessarily mean signicant
improvement of the composite properties, compared to those of
the unlled polymer.
23
Furthermore, nanotubes may display a
rather small affinity to polyimides. In this case the mechanism
of improvement of the composite mechanical performance due
to the load transfer to the ller may be ineffective. Yet it is well-
known that both graphene and CNT can initiate the crystalli-
zation of various polymers, including polyethylene,
24–27
poly-
caprolactone,
28
polypropylene,
25,29–31
polyvinyl alcohol,
32
polylactic acid,
33,34
polyamides,
25
polyetheretherketone,
35,36
polyacrylonitrile,
37
poly(vinilidene uoride)
38
as well as hetero-
cyclic polyalkylthiophenes
39
and aromatic polyimides.
14–19
It is
interesting that nanotubes can even lead to the crystallization of
such polyimides as ODPA-P3 which normally do not crystallize
in bulk.
14
a
Institute of Macromolecular Compounds, Russian Academy of Sciences, Bol'shoi pr. 31
(V.O.), St. Petersburg, 199004, Russia. E-mail: s.v.lyulin@gmail.com; Fax: +7 812
328686; Tel: +7 812 3285601
b
Department of Physics, St. Petersburg State University, Ul'yanovskaya str. 1,
Petrodvorets, St. Petersburg, 198504, Russia
c
Materials Science and Technology Centre, University of Perugia, Loc. Pentima, 4,
Terni, 05100, Italy
d
Institute of Polymer Science and Technology, ICTP-CSIC, Juan de la Cierva, 3, Madrid,
28006, Spain
Cite this: RSC Adv.,2015,5,51621
Received 29th April 2015
Accepted 3rd June 2015
DOI: 10.1039/c5ra07851b
www.rsc.org/advances
This journal is © The Royal Society of Chemistry 2015 RSC Adv.,2015,5,51621–51630 | 51621
RSC Advances
PAPER
Therefore, the addition of a CNT to the crystallisable ther-
moplastic PI causes noticeable changes in the polymer binder
properties related to the changes in its structure,
14–19
which
modify the mechanical and thermal behaviour of the
composite, and should be taken into account when developing
new polymer materials.
25,40
The efficiency of the CNT loading into the polymer may be
reduced a lot as a result of their tendency to aggregation.
38,41,42
To prevent CNT aggregation, their surface is modied with
functional groups increasing the affinity of the nanoller and
the polymer matrix.
38,43–45
The CNT surface modication usually
starts with attaching hydroxyl (–OH) or carboxyl (–COOH)
groups to it. As a rule, they attach themselves to the nanotube
surface during the oxidation with oxygen, air, aqueous solution
of hydrogen peroxide, concentrated sulfuric acid, nitric acid or
mix of acids.
46
The number of –COOH and –OH groups on the
CNT surface increases with growing temperature during the
acid treatment as well as in the case of the extended treatment
duration.
38,47,48
The number of functional groups attached to the
surface also depends on the oxidation procedure and oxidizing
agents.
Preferable sites for the hydroxyl and carboxyl groups to
attach to the CNT are carbon atoms at the CNT edges
48
and CNT
surface defects which represent inclusions of ve or seven-
membered carbon rings into the hexagonal grid made of
carbon atoms constituting the CNT surface, and other parts
where CNT carbon atoms change sp
2
-hybridization to sp
3
-
hybridization. Such defects can be created intentionally during
the distorting functionalization of the CNT surface. In this case,
defective sites of the surface usually contain about 5–10% of the
CNT atoms.
49
The distorting modication does not change
electrical and mechanical properties of the tubes,
46
which is why
the CNT defects play an important part since they can be used
as a basis for the further CNT surface functionalization.
Attachment of carboxyl groups to the CNT surface offers a
number of advantages compared to that of other groups,
because they can be used for further nanotube functionaliza-
tion. Presence of –COOH groups on the CNT surface makes it
possible to attach organic or inorganic molecules,
47,48,50,51
including the binder oligomers, which is important for
changing the CNT solubility in the solutions of low-molecular
and high-molecular compounds and for improving the
dispersion in nanocomposites. Therefore, the functionaliza-
tion of the carbon nanotube surface with –COOH groups is an
important stage to prevent their aggregation in the polymer
matrix.
Since it is assumed that the nucleation ability of carbon
llers are related to the interaction of the polymer matrix chains
with the ller surface which are determined for heterocyclic
polymers by p–pinteractions, whereas in the case of the above
mentioned surface modication, it is possible that the crystal-
lization is not initiated, which can be critical for maintaining
the required composite properties. Indeed, the study by Liang
et al.
33
carried out for polylactic acid-based composites shows
that the functionalization of the nanotube surface leads to the
loss of CNT nucleation activity. The changes observed are
attributed to the decrease in the ller surface area available for
the contact with polymer chains, which is related to the steric
effect of functional groups coupled with the nanotube.
Molecular-dynamics (MD) atomistic simulations are the
most efficient methods to study the phenomena at the atomistic
level which take place during the nanoparticles loading into the
polymer matrix, and to establish the mechanism of the associ-
ated changes. Such simulation enables us to explore structural
properties of the systems under scrutiny on the atomistic scale,
and establish the mechanisms stipulating the changes in the
polymer properties aer nanoller loading.
51–56
Composites based on thermoplastic PIs R-BAPB and R-BAPS
lled with carbon nanotubes
52
or graphene
53
have been studied
in our previous papers using atomistic computer simulations
on a microsecond time scale. It was shown that the R-BAPB-
based composites featured the formation of ordered struc-
tures, both near the ller surface and at a some distance. The
ordering effect can be interpreted as the initial stage of the
polymer crystallization induced by the nanoller.
57
This process
is caused by the ordered alignment of subsequent at phenyl-
phthalimide (PPI) and diphenyl (DP) fragments of the R-BAPB
polyimide repeating unit (Fig. 1),
52,53
resulting from the inter-
action with the ller surface.
This study aims to investigate the inuence of the most wide-
spread type of the CNT surface modication with carboxyl
groups on a CNT ability to act as a crystallization nucleant. This
possibility is related to both the position of the modifying
carboxyl groups on the CNT surface, and to the CNT surface
area remaining available for the interaction with the polymer
chains. It can be assumed that in the case of a certain graing
density of functional groups onto the CNT surface or in the case
of their certain distribution on the surface, the inuence of
steric limits on the polymer interaction with the CNT will be
reduced, and the initiation of crystallization with modied
nanotubes can be similar to the case of a non-modied ller
loading. These effects will be studied in the present paper in the
nanocomposites based on thermoplastic polyimides lled with
Fig. 1 Chemical structures of polyimides R-BAPB, R-BAPS, EXTEM
and ULTEM (top down). Arrows indicate flat phenylphthalimide (PPI)
and diphenyl (DP) fragments of the polyimide monomer units.
51622 |RSC Adv.,2015,5,51621–51630 This journal is © The Royal Society of Chemistry 2015
RSC Advances Paper
CNTs with a surface variously modied by carboxyl groups.
First, the CNTs modied with carboxyl groups uniformly
distributed on the nanotube surface were taken into account.
However, as the preferential direction of carboxylic groups
attachment to CNT is terminal atoms,
48
the more realistic
model with carboxyl groups attached to the CNT ends was
investigated also.
The simulation of the systems based on several various
polymer binders enables us to determine the inuence of the
polymer chemical composition on their structural behaviour in
the nanocomposites. Obtained results may help to establish the
interrelation between composite properties and the chemical
structure of its components, which is of relevance for the
developing of new types of composite materials and polymer
binders for them.
The second section of this paper deals with the model and
simulation approaches used in the study. In the third section we
discuss the inuence of the modied carbon nanotubes loading
on the structural properties of polyimides near the ller surface,
subject to the polyimide chemical composition, modication
degree of the carbon nanotube and the distribution type of
functional groups on the CNT surface. Particular emphasis is
laid on the ability of modied CNTs to initiate the ordering of
polyimide chains near the nanoparticle surface. The nal
section contains major conclusions made on the basis of the
obtained results.
Model and simulation method
The study addresses nanocomposites based on heat-resistant
thermoplastic polymers. Two main types of polyimides for
matrices are considered which are synthesized and actively
explored in the Institute of Macromolecular Compounds of the
Russian Academy of Sciences:
11,13,15–19,58,59
crystallisable R-BAPB
based on 1,3-bis-(30,4-dicarboxyphenoxy)-benzene (dianhydride
R) and 4,40-bis-(400-aminophenoxy)-diphenyl (diamine BAPB)
and amorphous R-BAPS based on dianhidride R and 4,40-bis-
(400-aminophenoxy)-diphenylsulfone (diamine BAPS), whose
chemical structures are given in Fig. 1. These polyimides have
identical dianhydride fragments in repeating units and differ in
the diamine fragment composition. The R-BAPS diamine has an
additional hinge group SO
2
between benzene rings. Such
modication of the chemical structure enhances the exibility
of polymer chains of R-BAPS compared to that of R-BAPB; at the
same time it leads to larger glass transition temperature of R-
BAPS
60–62
due to the increased contribution of dipole–dipole
interactions into the total system energy, related to the presence
of the polar sulfone group in the R-BAPS repeating unit, as was
demonstrated in our previous papers.
58,63–65
The increased ex-
ibility of the chain, and enhanced dipole–dipole interactions in
the PI R-BAPS, as opposed to the R-BAPB, prevent crystallization
of this polymer even in the presence of carbon nanoparticles
which can act as good nucleants. Notably, the loading of
modied CNTs with polarized carboxyl groups on the surface
into the R-BAPS may have a different inuence on the polymer
structure, compared to that of the R-BAPB polyimide where
electrostatic interactions do not play such a signicant role as in
the case of the R-BAPS. Therefore, the comparison of the
structure of composites based on the polyimides containing
polar groups in the repeating unit with composites based on PIs
without polar groups allows to evaluate the signicance of
electrostatic interactions for the ordering processes occurring
in the polymer matrix aer the nanoller loading.
For reference, we additionally simulated composites which
are based on commercial heat-resistant polyimides ULTEM and
EXTEM produced by SABIC Innovative Plastics.
66–70
Similar to
the case of R-BAPB and R-BAPS polyimides, the major difference
in their chemical structure (shown in Fig. 1 as well) is the
diamine modication with the presence of a sulfone group in
the EXTEM polyimide. Thus, in the case of EXTEM, dipole–
dipole interactions between the polar groups of polymer chains
and of the CNT surface may impact the polymer matrix struc-
ture in the nanocomposites under the study. Generally,
comparison of the simulation results for polyimides R-BAPB
and R-BAPS with the results obtained for ULTEM and EXTEM
will enable us to establish the inuence of electrostatic inter-
action on the structural performance of nanocomposites with
modied CNTs.
Simulation of the composites based on crystallisable poly-
imide R-BAPB with CNT which differ in the degree of the surface
modication by functional groups u
M
and their location make
it possible to determine the inuence of the CNT surface
modication on the orientation ordering processes of the
polymer chains near the ller surface which can be regarded as
the initial stage of the polymer crystallization.
52,53
Similar to our previous paper
52
carbon nanotubes of 4.7 nm
long with chirality (5, 5) were used as llers with carboxyl groups
attached to the CNTs. To determine the inuence of the degree
of the CNT surface modication on the microstructure of the
polymer matrix in the nanocomposite, nanotubes were explored
with various number of attached carboxyl groups corresponding
to different surface modication degrees u
M
¼5% and 10%. In
the rst case, 20 carboxyl groups were attached to the CNT
surface that have 400 carbon atoms, and 40 carboxyl groups
were attached in the last one (Fig. 2). The positions of the
groups were chosen so that they were distributed more or less
uniformly on the carbon nanotube surface, that is, with similar
intervals between them both along the nanotube axis and in the
tangential direction. It was found that the simplest location
variant with regard to these requirements was the position of
carboxyl groups in rings equally spaced along the CNT axis, with
each ring containing 5 carboxyl groups, which may be consid-
ered as a rst approximation to the uniform distribution.
The fully-atomistic model and simulation approach used for
this research were proved in our previous studies exploring the
structural and thermal properties of the heat-resistant poly-
imides
58,60–65
and composites based on them.
52,53
The computer simulation of the systems under the study was
performed with the Gromacs computational package
71
using
the Gromos53a5 force eld.
72
The Gromacs package allows for
computer simulation using atomistic models on a microsecond
time scale, which is required for the investigation of initial
stages of the structural ordering of crystallisable polyimides
near the carbon nanoparticle surface.
This journal is © The Royal Society of Chemistry 2015 RSC Adv.,2015,5,51621–51630 | 51623
Paper RSC Advances
Similar to our previous studies
52,58,60–65
the systems consid-
ered contain one CNT and 27 polyimide chains with a poly-
merization degree of n¼8 (for R-BAPB and R-BAPS) or n¼9 (for
ULTEM and EXTEM), which corresponds to the beginning of so-
called “polymer regime”in thermal properties with molecular
mass around 6–6.5 kg mol
1
.
52,59,60,64
The simulation was performed in the NpT ensemble at a
temperature of 600 K under a pressure of 1 bar. The pressure
and temperature were maintained at the required level using
the Berendsen thermostat and barostat;
73
the time constants for
which were taken as s
T
¼0.1 ps for the thermostat, and s
P
¼0.5
ps for the barostat. Test simulations showed that the use of the
Nose–Hoover thermostat and the Parhinello–Rahman barostat
do not lead to any signicant changes in the discussed struc-
tural properties. To maintain the set lengths of the bonds
between the atoms, the LINCS algorithm was used.
74
The values of the partial charges for the atoms of carboxyl
groups attached to the CNT were taken as standard values for
the Gromos53a5 force eld
72
which was parameterized for
simulating systems in the media with low permittivity. Param-
etrization of the charges in this force eld was done using the
HF/6-31G*method.
72,75
The same method was used to calculate
the partial atomic charges in the considered polyimides. The
charge distribution was performed using the Mulliken method.
Such parametrization of electrostatic interactions enables us to
reproduce experimental values of the thermal properties of the
heat-resistant PIs with a high degree of accuracy for the set of
PIs.
58,62,65
The calculated partial charges were published in our
previous studies.
52,64
Ewald summation method (PME)
76
was
used to account for the electrostatic interactions.
As was mentioned above, the simulation was performed
within the microsecond time scale similar to our previous
studies.
52,53,60–65
The equilibration procedure included stages of
the system compression from the initial low-density
conguration (polymer gas), cyclic annealing within the
temperature range of 600 to 290 K, equilibration at a high
temperature for 1.5 ms, and further production run during 1 ms.
The given stages were carried out without accounting for elec-
trostatic interactions (EI). In this case, the simulation times of
about 1 ms are close to the typical time of the polymer chain
centre of mass displacement to the distance comparable to the
chain size, which enables us to obtain the well-equilibrated
congurations.
52,62
Since the presence of EI slows down the diffusion of the
polyimide chains in the melt approximately by two orders of
magnitude,
52,61,62
the simulation with switched-on electrostatics
was performed using the equilibrated congurations of the
systems with switched-offelectrostatics. To this end, the system
conguration was chosen which were obtained aer the
described above simulation for 2.5 ms without electrostatics.
Then these systems were simulated with switched-on partial
charges for at least 100 ns. The specied period of the simula-
tion with electrostatic interactions is characterized by the
relaxation of density and local structure of the systems under
the study. The observable changes in nanocomposite structure,
i.e. the change of density distribution, occur during rst 10 ns of
simulation aer switching on electrostatic interactions. At the
same time the chain size and shape do not change sufficiently
during simulation with partial charges switched-on. Thus, we
suppose that 100 ns simulation with accounting for EI aer
long-time equilibration without EI is enough to obtain reliable
data on structural properties of the nanocomposites studied.
The previous research demonstrated that the account of elec-
trostatic interactions has a rather little inuence on the global
structural order on the composites under scrutiny.
52
Final
congurations were subsequently used for analysis.
To characterize structural properties of composites based on
modied nanotubes we calculated density distributions of
Fig. 2 CNT configurations with (a) non-modified surface and with different degree of modification with carboxylic groups u
M
¼5% (b) and 10%
(c); (d) configuration of CNT with carboxylic groups attached to the edge carbon atoms (edge-modified CNT).
51624 |RSC Adv.,2015,5,51621–51630 This journal is © The Royal Society of Chemistry 2015
RSC Advances Paper
polyimides as a function of distance from the nanotube axis r(r)
and pair polyimide-nanotube distribution functions g
PI-CNT
(r)
similar to the composites with pristine CNTs.
52
We also calcu-
lated the orientation parameters of at PPI fragments of the
polyimide monomer units (Fig. 1): distribution of the angle q
between the PPI fragment of the monomer unit and the nano-
tube axis subject to the distance between them, and the relevant
order parameter S(r):
S(r)¼3/2hcos
2
q(r)i1/2, (1)
where hcos
2
q(r)iis the mean square cosine of the angle between
the nanotube axis and the polyimide monomer unit fragment
located at a distance rfrom the nanotube axis. The angle qis
determined as shown in Fig. 3.
The polyimide density distribution relative to nanotube axis
r(r) is calculated as following
rðrÞ¼ mðrÞ
phCNT Drð2rþDrÞ;(2)
where m(r) is the mass of the polymer in the cylindrical layer
with thickness Drat a distance rfrom the nanotube axis. The
height of the cylindrical layer is determined by the nanotube
length: h
CNT
z4.7 nm.
The pair correlation functions g
AB
(r) describes the local
distribution density of the component B near the component A
in relation to the mean density of the component B in the
system:
gABðrÞ¼rBðrÞ
hrBi¼1
NAhrBiX
NA
i˛A
X
NB
j˛B
drij r
4pr2:(3)
In this equation, N
A
and N
B
is the number of atoms of the
components A and B in the system, correspondingly, hr
B
iis
average density of B component in the system, r
ij
is the distance
between the two considered atoms of the components A and B,
and dis the delta function.
Fig. 4 Density distribution for polyimides (a) R-BAPB, (b) R-BAPS, (c) ULTEM, and (d) EXTEM in the composites based on them, filled with
nanotubes with various surface modification degrees (with switched-on electrostatics). u
M
¼0% corresponds to pristine CNT.
Fig. 3 Determination of the angle qbetween the flat PPI fragment of
the polyimide repeat unit and the carbon nanotube axis.
This journal is © The Royal Society of Chemistry 2015 RSC Adv.,2015,5,51621–51630 | 51625
Paper RSC Advances
Resulted pair correlation functions make help to understand
the distribution of various components in the composite. Such
analysis is widely used for the systems in a condensed state,
including composite materials.
54,55,77–80
The simulations performed with various starting points give
similar results on the properties investigated. We estimate run-
to-run variations of the results to be less than 5%.
Results
Density distribution and pair distribution functions
The PIs density distribution, eqn (2), in nanocomposites with
CNTs with various degrees of the surface modication by
carboxyl groups is shown in Fig. 4. In the case of composites
with pristine nanotubes (u
M
¼0%) a dense polymer layer
appears around the ller at a distance of r0.7 nm away from
the CNT axis. For composites containing a CNT with u
M
¼10%,
this subsurface polymer layer shis to a distance of r0.9 nm.
This shiis related to the impossibility for the polyimide to
reach the nanotube surface due to the steric limits, related to
the presence of high-volume carboxyl groups on the CNT
surface. Meanwhile, in the case of composites containing CNTs
with intermediate value of u
M
¼5%, the r(r) dependence
demonstrates that the polyimide chain fragments may be
located either at the same distance as in the case of pristine
CNTs, or far from it corresponding to the st peak of r(r) in case
of u
M
¼10%. Consequently, in the case of a CNT with u
M
¼5%,
the polymer density distribution displays a typical bimodal
behavior. These curves are characterized, rst of all, by the
maximum at r0.7 nm, and, secondly, a shoulder or second
maximum at r0.9 nm. The shoulder corresponds to the
shiing of the polyimide far away from the CNT surface, which
is facilitated by carboxyl groups on the surface.
Fig. 5 Pair distribution functions g
PI-CNT
(r) of the R-BAPB polyimide
atoms in relation to the CNT atoms in nanocomposites with switched-
on electrostatics filled with nanotubes with various surface modifica-
tion degrees u
M
.
Fig. 6 The order parameter S(r) of the PPI flat fragment in composites based on polyimide (a) R-BAPB, (b) R-BAPS, (c) ULTEM, and (d) EXTEM
filled with CNTs with different surface modification degrees u
M
.
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RSC Advances Paper
Notably, the general patterns of the curves describing the
density distribution of various polyimides in nanocomposites
with modied CNTs do not differ from each other, similar to the
case of composites containing pristine CNTs.
52
This is due to
the fact that the atomic composition of the polymers under
study is almost identical. The only signicant distinction of the
density distribution is that in the case of polyimides containing
polar sulfone groups (R-BAPS and EXTEM), the density distri-
bution maxima are narrower and are located at a smaller
distance from each other, which is associated with additional
compaction of these polymers due to the dipole–dipole
interactions.
The pair distribution functions, see eqn (3), for the PI atoms
relative to the carbon nanotube atoms g
PI-CNT
(r) were calculated
for all considered nanocomposites. All the polyimides feature
the identical general pattern of the pair distribution functions
and the nature of the changes in composites with various CNTs
(see Fig. 5, where the pair distribution functions g
PI-CNT
(r) for
nanocomposites based on the polyimide R-BAPB are shown as
an example). The increase of the CNT surface modication
degree leads to the disappearance of the rst maximum or
shoulder in the pair distribution function g
PI-CNT
(r) corre-
sponding to the subsurface polyimide layer, and to the general
shiof g
PI-CNT
(r) towards larger values of distance rdue to the
growing effective radius of the CNT with a modied surface.
These results conrm the previous conclusion that the
increasing of modication degree of the nanotube surface leads
to the polymer shiing far away from the CNT axes.
Orientation of at fragments
Similar to the case of nanocomposites lled with pristine
CNTs,
52
the calculation of the orientation characteristics (order
parameters and distribution of the orientation angles relative to
the nanotube axis) of the various PI chain fragments for systems
with switched-on EI showed that they are qualitatively the same
as the results obtained without electrostatics. Also the values of
the orientation characteristics for these systems display rather
strong uctuations
52
due to the mobility reduction of polymer
chains observed in the simulation with EI switched-on.
52,61,62
Therefore, all orientation characteristics of polyimide chains
shown further in this section have been obtained for the
systems simulated without electrostatic interactions, for the
clarity.
The analysis of the order parameters S(r) describing the
orientation of the PPI at fragments in nanocomposites with
various CNT surface modication degrees (Fig. 6) demonstrates
that the increased u
M
results in the reduction of the ordering
degree of the PPI fragments relative to the nanotube axis. For
Fig. 7 Distributions of orientation angles of the PPI flat fragment in nanocomposites based on crystallisable polyimide R-BAPB filled with a CNT
with a different surface modification degree (a) u
M
¼0%, (b) u
M
¼5%, and (c) u
M
¼10%, obtained after the simulation for 2.5 ms without
electrostatic interactions.
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Paper RSC Advances
polyimides R-BAPB and R-BAPS, the maxima tend to disappear
on the curve S(r) at the any u
M
, which is related to the lack of PI
chains ordering along the nanotube axis even in immediate
proximity to the CNT surface (Fig. 7). Fig. 7 also shows that
composites containing CNTs with u
M
¼10% have no domi-
nating orientation direction, even for the chain fragments
located closest to the nanotube axis. Similar results were
obtained for composites based on all the polyimides under
study as well. Notable, polyimides ULTEM and EXTEM do not
show any signicant ordering of the chain at fragments near
the nanotube even in the case of composites with a pristine
CNT.
Therefore, we can conclude that the modication of the CNT
surface with carboxyl groups leads to the decrease in the
nucleation ability of carbon nanoller. This may be attributed
to the fact that the carboxyl groups located on the CNT surface
prevent the PI chains from interacting with unmodied sites of
the ller surface, which reduces the CNT nucleating effect.
A possible solution to avoid this modied CNT effect is
increasing the unmodied CNT surface area available for the
interaction with PI chains. In the model used, it corresponds to
the increase of distance along the nanotube axis between the
rings of modifying groups. To verify this assumption we have
simulated additionally nanocomposites based on the crystal-
lisable polyimide R-BAPB containing modied CNTs with
functional groups attached only to the carbon atoms at the CNT
edges (Fig. 2d). The surface modication degree of such a CNT
equals 5% as well, yet the surface area available for the inter-
action with PI chains is substantially higher than that of
modied CNTs with functional groups uniformly distributed
over the surface. Structural properties of such composites are
shown in Fig. 8.
In the case of carboxyl groups attached to the terminal
carbon atoms of CNT, the PI density distributions practically
coincide in composites with modied and with pristine CNTs.
Furthermore, the edge-modied CNT loading into the matrix
does not signicantly change the pair distribution functions,
testifying the negligible inuence of such modication on the
polymer interaction with the nanotube surface and suggesting
the possible formation of ordered structures in composites
based on the R-BAPB polyimide lled with such CNTs.
Indeed, the analysis of the orientation angle distributions of
the PPI at fragments of the R-BAPB polyimide relative to the
nanotube axis and the corresponding order parameter, showed
that composites lled with edge-modied CNTs display the
formation of a structure with the PI chain at fragments
Fig. 8 (a) Orientation angle distribution of the PPI flat fragment in the composites based on the R-BAPB polyimide filled with CNTs modified with
carboxyl groups at the terminal carbon atoms, with a surface modification degree of 5%, and (b) order parameters for R-BAPB based nano-
composites. (c) Polyimide density distribution in composites relative to the CNT axis. (d) PI-CNT pair distribution functions.
51628 |RSC Adv.,2015,5,51621–51630 This journal is © The Royal Society of Chemistry 2015
RSC Advances Paper
oriented along the CNT, both near the ller surface and at a
distance from it, similar to composites with pristine CNTs.
52
Conclusions
We have simulated the structural properties of various heat-
resistant polyimides in composites lled with carbon nano-
tubes with a surface modied by carboxyl groups. The depen-
dence of the structural organization of polyimide chains on the
CNT surface modication degree and the location of the func-
tional groups over the surface have been studied.
It is shown that the CNT surface area available for contacts
with PI matrix inuences signicantly the polymer matrix
structure. Increase of CNT surface modication degree leads to
reduced surface area of the ller available for the interaction
with polymer chains. Thus, in the case of uniform distribution
of functional groups on the CNT surface no orientation
ordering of the at fragments of polyimide chains have
observed even near the ller surface.
On the other hand, in the case of edge-modied CNT the
most of the CNT surface can interact with crystallisable R-BAPB
matrix providing its ordering and further crystallization.
Acknowledgements
The study was carried out with the nancial support from the
Ministry of Education and Science of the Russian Federation
under the Contract no. 14.Z50.31.0002 (megagrant of the
Government of the Russian Federation according to the Reso-
lution no. 220 of April 9, 2010).
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