# Mechanical and Thermal Properties of Polymethyl Methacrylate-BN Nanotube Composites

Authors: C. Y. Zhi; Yoshio Bando; Wenlong L. Wang; Chengchun C. Tang; Hiroaki Kuwahara; Dmitri Golberg
Journal: Journal of Nanomaterials
 (2008)

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## Abstract

Polymethyl methacrylate (PMMA)-BN nanotube (BNNT) composites were fabricated and their mechanical and thermal properties were analyzed. Using a 1 wt.% BNNTs fraction in a polymer, the elastic modulus of PMMA was increased up to 19%. In addition, thermal stability and glass transition temperature of PMMA were also positively affected. The thermal conductivity of PMMA with BNNT additions increased three times. The resultant BNNT-PMMA composites possess the high electrical breakover voltages. Thus our studies clearly indicate that BNNTs are promising nanofillers for improvement of mechanical and thermal conductivity of dielectric polymers under preservation of their electrical insulation.


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## Body

## 1. Introduction

The discovery  of carbon nanotubes (CNTs) and observations
of their unique, mechanical, thermal, and electrical properties have soon stimulated
the studies on their usage in composites [1–3]. The composites
with a matrix made of metal or ceramic have in fact been intensively studied, but
the polymeric composite researches have become more widespread due to many comprehensive applications. For
example, CNTs have been used to improve mechanical properties, thermal
conductivity, and electrical conductance of polystyrene, polyacrylonitrile, polymethyl
methacrylate (PMMA), and so forth [4–14]. However,
there have been some unsolved problems with respect to real
applications of CNTs-polymer composites, for example, nanotube dispersion at
high concentrations. In addition, the  mechanical performance of a CNT-polymer
composite strongly depends on which kind of tubes, that is, arc-discharge-grown
or catalytically produced, is
in use. The arc-discharge CNTs possess excellent mechanical properties due to a
high-temperature growth, but their production yield is typically low.
Oppositely, a large amount of CNTs can be produced through a catalytic growth,
but such tubes have plenty of structural defects, which is harmful for
applications [15, 16].Boron nitride nanotubes
(BNNTs), which have very similar atomic structure to CNTs, exhibit a number of
physical properties which could be advantageous (compared to CNTs) in polymeric
composites. For example, a BNNT is a constant wide band gap material
[17–19]. BNNTs could
emit ultraviolet light under excitation by an electron beam. The theoretical
estimates of the elastic modulus of a BNNT gave a value of ~850 GPa,
[20], that
is, approximately 0.8 times of CNT. Moreover, BNNTs are chemically inert and
structurally stable; they withstand heating in air up to 900°C [21].
It is worth noting that the thermal conductivity of BNNTs has been predicted to
be even higher than that of CNTs [22, 23]. All these factors make a BNNT useful
as a filler in polymeric
composites. We envisage that BNNTs can improve mechanical properties and
thermal conductivity of a polymer while preserving its dielectric properties. Moreover,
since BNNTs are pure white in color, the BNNT-loaded polymers should keep good
transparency
at low fraction of BNNTs. However, the BNNT-polymer composite-related
studies have been overlooked in the literature. Only recently, the present
authors have reported that BNNTs improve the tensile modulus of a polystyrene
[24]. The detailed studies on other polymers and their properties have still been
lacking.In this work, utilizing large quantities of highly pure
BNNTs synthesized via a
chemical vapor deposition method using boron powder and metal oxide as
reactants (BOCVD), PMMA-BNNTs composites were fabricated using a solution
method. Then the expected mechanical reinforcement and thermal conductivity
improvement of the polymers were indeed been verified [25].


## 2. Experimental Details

Tens of grams quantities of highly pure BNNTs were
synthesized using the BOCVD method [25–28]. The as-grown BNNTs were heated up to 1900°C under
protection of Ar gas in order to remove catalysts and other impurities.
Scanning electron microscopy (SEM) was used to investigate the
tube morphology, and electron energy loss (EELS) and energy  dispersive X-ray (EDX)
spectroscopes
were used to confirm the absence of foreign species.In order to fabricate
PMMA-BNNT composites, 10 mg BNNTs were mixed with 10 mLN,N-dimethylformamide. The
mixture was magnetically stirred for 5 hours and sonicated over 100 minutes and
a BNNT solution was obtained. Then, 990 mg PMMA was dissolved in the solution, followed by
stirring and sonication. The obtained mixture was cast onto a glass plate. The doctor blades with 200 μm and 400 μm slit widths were
used for casting of the
resultant composite
films. After
heating to 60°C and solvent evaporation, the composite
films were peeled off from the glass plates, and mounted on a frame for the
complete removal of solvent. The dried films were cut into 9×1cm slices
for the ex-situ tensile tests on an RTC 1225A apparatus (Orientec Inc., Tokyo,
Japan). The
slices were stretched until they broke. All experiments were performed at room
temperature at a deformation rate of 5 mm/min.Thermogravimetry (TGA) experiments were run
on a Thermoplus TG8120 machine (Rigaku Inc., Tokyo, Japan).
For differential scanning calorimetry (DSC), an SII Exstar6000 DSC6220 apparatus (Seiko
Instruments. Inc., Tokyo, Japan) was utilized. The curves were
recorded for the second heating cycle. Thermal mechanical analysis (TMA) was performed
on a universal V2.5H TA instrument using the sample slices with a
length of around 1 cm.


## 3. Results and Discussion

The tensile
mechanical properties of BNNT/PMMA composite films are summarized in Table1. With a 1 wt.% BNNTs
fraction, the elastic modulus of PMMA was improved by 19%, but the tensile
strength slightly decreased. No behavioral variations were observed for the
different film
thicknesses, as shown in Table 1. The slit widths of the doctor blades used
were 200 μm and 400 μm, thus
much larger than the
BNNTs
lengths, and it is thought that a change in slit width should
not affect the
arrangement of BNNTs in a polymer matrix. It is worth noting that the film’s thicknesses significantly
decrease after solvent
evaporation. These
results show that
the external force can be transferred to BNNTs in some degree, which results in
an increase in
elastic modulus; while the defects induced by BNNTs are still a problem, which leads
to a decrease of
strength. In all cases, the addition of fillers induces a dramatic reduction in
elongation, which also indicates that the interaction between BNNTs and polymer
chains exists.Table 1
Elastic modulus, strength and elongation of a blank PMMA, and its BNNTs’ composites.
MaterialsThickness (μm)Modulus (GPa)Strength (MPa)Elongation
(%)PMMA252.0758.319.91PMMA;  1 wt.% BNNT272.4754.353.93PMMA; 1 wt.% BNNT522.5154.734.21For
comparison, it
is worth mentioning some related results on CNT/PMMA composites. W.-J. Lee et al. fabricated
an MWCNT/PMMA nanocomposite by using both the
injection molding and film casting processes [29]. The tensile strength of
an MWCNT/PMMA nanocomposite increased by more
than 15% and the tensile toughness increased by ~17.5%, compared to the blank PMMA. It is well known that surface
modification can improve the
nanotube dispersion and filler/matrix interfacial
bonding. Recently, Liu and Wagner have used MWCNTs functionalized through covalent
linking of long alkyl chains and prepared PMMA/CNT composites [30]. A 32% rise in
tensile modulus and a 28% increase in tensile strength were observed for a
composite loaded with a 1.0 wt% fraction of CNT fillers. Keeping in mind that no surface functionalization
and/or surfactants were used in our experiments, our results are considered to
rival this literature data.In most of previous experiments
related to CNT/polymer composites catalytically grown CNTs were utilized. However,
the catalytically produced CNTs are not structurally perfect and are prone to
defects. By contrast, the present BNNTs grown via the BOCVD method are well structured and possess a high
elastic modulus of500~700 GPa, as was
documented using the in situ TEM techniques [31]. We note that it is rather
difficult to directly compare the pre-existing CNTs and the present BNNTs
impacts since very different numbers have been reported for the CNT-loaded
composites.Since
we did not use any special pretreatments of the BNNTs and special
techniques for composite film fabrication it is envisaged here that there is
still a plenty of room for further modulus improvement. In this respect, an estimate was performed to elucidate the theoretical
mechanical
properties of BNNTs/PMMA composites. Ideally, if BNNTs strongly
bond to PMMA, the external
tensile load will be transmitted from a matrix to BNNTs through the interfacial
shear stress. Thus, a BNNTs/PMMA
composite film can be considered as a randomly oriented
discontinuous fiber lamina and the composite modulus,Ec, can be
estimated  using the following
equations [32]: (1)EC=[381+2(lNT/dNT)ηLVNT1−ηLVNT+581+2ηTVNT1−ηTVNT]Epmma,ηL=(ENT/Epmma)−1(ENT/Epmma)+2(lNT/dNT),ηT=(ENT/Epmma)−1(ENT/Epmma)+2, where E  is the modulus, lNT is the
length, and dNT is the
outer diameter of BNNTs. The volume fraction, VNT, is estimated to be ~0.70 vol.% for a 1 wt.% BNNT
fraction. For the sonicated BNNTs used in our experiments, the average diameter
and length are 50 nm and ~6 μm,
respectively. Thus Ec    was calculated
to be 1.35 EPMMA  for a
composite with a 1 wt.% BNNTs loading fraction. By taking a 3 wt.% BNNTs
fraction
the modulus of 1.98  EPMMA was obtained. These estimates confirm
that BNNTs are indeed
very promising nanomaterials for the mechanical
enhancement of polymeric composites. It  is also predicted that,
experimentally,  notably better reinforcement can be achieved for covalently modified
BNNTs.The coefficient of
thermal expansion (CTE) of composite films is shown in
Figure1. After
adding BNNTs, the CTE of PMMA dramatically decreases from 215 to 115ppm/°C, that indicates that BNNTs significantly
restrict the
mobility of polymer chains. This is consistent with the
above-discussed variations in the reinforced PMMA film elasticity
and toughness.Figure 1
TMA of a blank PMMA and BNNTs/PMMA composites; the CTE is significantly
reduced after adding BNNTs.TGA data is presented in Figure2(a). The oxidation of the composites was characterized by the
parameter T0.1,
which is the temperature corresponding to a 10% weight loss rate. T0.1 is
slightly changed when a 1 wt.% BNNTs fraction is in use. In line
with our previous
studies, it is suggested that BNNTs can in fact improve stability
to oxidation. However, the effect is much smaller than in case of conventional
phenolic antioxidants [33]. Such results are reasonable: on one hand, there are
some localized states, such as vacancies, dangling bonds, and functional groups
on BNNT
surfaces which may be
responsible for radical termination during polymer degradation. On the other hand, the
effect only takes place on the interfaces between BNNTs and polymers, and the
concentration of such lattice defects in BNNTs is low. These
factors
result in only
marginal improvement of the oxidation resistance of PMMA and
thus make doubtful an expected design of the fireproofing
polymers out of such composites.(a) DSC and (b) TGA of a blank PMMA and BNNTs/PMMA composites,
which indicate that the thermal stability and glass transition
temperature are only slightly
modified.
(a)(b)Blank PMMA
has a glass transition
temperatureTg  of 82.9°C, as
shown on a DSC curve in Figure 2(b). The Tg is estimated from the midpoint of a heat capacity peak. The Tg of
a
PMMA/BNNT composite becomes ~85.2°C. In case of organic-inorganic nanocomposites, the mobility of polymer
chains is significantly
affected by the confinement and strength of polymer-surface interactions. This
applies to the interactions between BNNTs and
PMMA chains. The
chains which
are in contact with
BNNTs would exhibit slower dynamics than those within a polymer matrix. Actually, even in
the absence of particularly favorable interactions, the contact with a
hard BNNTs’ surface would tend to increase the local segment density profile. This, in turn,
leads to an increase in the configuration
entropy, and thus an increase in Tg looks reasonable. It is
worth mentioning that the similar phenomenon was observed in many inorganic-organic
composite systems [34].Composite films with 5 wt.% and 10 wt.% BNNTs
fractions were
chosen for the thermal conductivity
measurements. The results are shown in Figure3. With a 10 wt.% BNNTs
fraction, the thermal conductivity of PMMA increases from 0.17 to 0.50 W/mK
(nearly 3-fold). It should be emphasized that this gain is
likely to display the lower estimate for the observed improvement since the BNNT texture
within the film is generally misaligned with the direction used for the heat flow
measurements (because of experimental constraints) [35, 36].Figure 3
Thermal conductivity of a blank PMMA and BNNTs/PMMA composites,
almost 3-fold improvement was obtained with a 10 wt.% BNNT
loading fraction.The appealing point of BNNT usage
in polymeric
composites is that
the original
dielectric nature of
a polymer is kept in the
resultant composite. This fact is crucial in many cases, such as packing
materials for electrical circuits, power modules, and
so forth. Needless
to say, carbon nanotube-filled polymers are
not suitable for these purposes due to high electrical conductivity.
In Table2, the
electrical breakover voltages of a blank PMMA and its composites are compared. This reveals that both blank PMMA and its
BNNTs composites have a similar breakover electric field of ~10 V/μm. Therefore,
the presently developed BNNTs/polymer composites are surely suitable materials for heat-releasing parts due to unique combination of decent thermal conductivity
and perfect electrical
insulation.Table 2
Electrical
breakover voltages of a blank PMMA and its BNNTs’ composites.
MaterialsThickness (μm)Volts applied (V)Breakover
electric field(V/μm)PMMA2526310.52PMMA;
5 wt.% BNNT3032410.80PMMA;
10 wt.% BNNT343129.17


## 4. Conclusion

In
summary, BNNTs/PMMA composites were fabricated and their properties were elucidated. The elastic
modulus of PMMA was improved up to 19% while using only a 1 wt.% BNNTs loading
fraction. The
coefficient of thermal expansion of a PMMA film dramatically reduced due to profound interactions between BNNTs and
PMMA chains. This
fact was consistent with the
corresponding variations in glass transition
temperatures. Thermal conductivity of PMMA loaded with a 10 wt.% BNNT fraction
was improved 3 times compared to blank PMMA. The developed BNNT/PMMA composite
films simultaneously possessing (i) improved thermal conductivity, (ii)
enhanced mechanical properties, and (iii) perfect electrical insulation are
envisaged to find diverse applications in many polymeric fields.



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*Source: 642036-2008-12-21.xml*