Polymer/Boron Nitride Nanotube (BNNTs) Nanocomposites

METE 560METE 560

Ümit TAYFUNÜmit TAYFUN

Middle East Technical University

Polymer Science & Technology

Boron nitride nanotubes

Boron nitride nanotubes, firstly synthesized in 1995, are structural analogues of carbon nanotubes with boron and nitrogen atoms instead of carbon atoms.

BNNTs can be imagined as a rolled up hexagonal BN layer or as a carbon nanotube (CNTs) in which alternating B and N atoms entirely substitute for C atoms

Similar to CNTs, BNNTs have chiralities, an important geometrical parameter, but for them, the chiralities do not play an important role in determining electrical properties

Atomic models of BNNT;

(a)arm-chair (b)zig-zag (c)chiral

Properties of Properties of BNNTsBNNTs are chemically inert, oxidation resistant, and structurally stable.

BNNTs are electrically isolating materials with uniform electronic properties independent of their size and chirality.

Therefore, they are evaluated as suitable fillers for the fabrication of mechanically and thermally enhanced polymer composites, while preserving the electrical isolation of the polymer matrix

Excellent mechanical and thermal properties Excellent mechanical and thermal properties Unusually efficient electrical insulators Unusually efficient electrical insulators Structurally stable and inert to most chemicals Structurally stable and inert to most chemicals Uniform band gap (5.5 eV) Uniform band gap (5.5 eV) High sensitivity for sensor materials High sensitivity for sensor materials High resistance to oxidation High resistance to oxidation

TGA showed that the oxidation of BNNTs starts approximately at 800 °C, which is much higher than the oxidation temperature of CNTs, which is about 400 °C. High oxidation resistance of BNNTs allows their applications in high temperature environments.

TEM images of single to multi-wall BNNTs with six walls

BNNT vs CNTBesides their structure, mechanical and thermal properties of BNNTs are very similar to CNTs.

Both BNNTs and CNTs have superb mechanical properties: the Young’s modulus of them has been predicted to reach a TPa level.

However, BNNTs have better resistance to thermal oxidation than CNTs.

The electronic properties of BNNTs are also different from CNTs. BNNTs have a constant and wide band-gap of 5.5 eV. Therefore, they are electrically isolating independent of their size or chirality‟s. The electronic properties of BNNTs make them suitable nanofillers for the production of isolating polymeric materials.

The obvious and most appealing difference between BNNTs and CNTs is their visible appearance:BNNTs are pure white (sometimes slightly yellowish due to N vacancies) while CNTs are totally black

Comparison of properties of carbon nanotubes and boron nitride nanotubes Images of (a) CNTs and (b) BNNTs

Synthesis Methods of BNNTs

There are several methods used for synthesizing boron nitride nanotubes. Mainly used methods are:

arc-discharge, laser ablation, ball milling, chemical vapor deposition

Polymer/BNNT composites

The studies on the polymeric composites of BNNTs have been flourished only over the last years.

The exciting properties of BNNTs, such as high elastic modulus and high thermal conductivity make them advantageous for novel nanofillers in composite materials to obtain mechanical reinforcement, high thermal conductivity and a low coefficient of thermal expansion in a matrix.

Polymer/BNNT composites that have been studied to date were prepared as thin films via solution–mixing, evaporation and melt-mixing techniques

Mechanical Properties

(a) a blank PS film (b) BNNT/PS film (c) BNNT/PmPV/PS film

C. Zhi et al. fabricated PS/BNNT composites using a solution method

The mechanical properties of a polymer were improvedIt was found that the results were solvent-dependent, that is, when chloroform was used to disperse BNNTs, the elasticmodulus of the composite film was decreased.However, improvements can be obtained by using DMF as a solvent. This is attributed to different BNNT dispersions in different organic solventsBenefiting from the pure white appearance of BNNTs,the composite films retained good transparency

C. Zhi, Y. Bando, C. Tang, S. Honda and H. Kuwahara, J. Mater. Res., 2006, 21, 2794.

Mechanical Properties

Zhou et al. used isophorone diisocyanate (IPDI) activated BNNTs to synthesize BNNT/polyvinyl alcohol (PVA) and hydroxypropyl methylcellulose (HPMC) composites

Addition of a small fraction of activated IPDI–BNNTs leads to a considerable increase in both Young’s modulus and

tensile strength.

When the amount of ap-BNNTs was added, both tensile strength and Young’s modulus were decreased

Activated IPDI–BNNTs exhibit good dispersibility and chemical activity.

Adding IPDI–BNNTs into the solution of PVA or HPMC, the strong interfacial interactions between BNNTs and polymers were achieved

In contrast, due to the well-crystallized surface, pristine BNNTs exhibit limited dispersibility and poor interfacial interactions with PVA and HPMC.

BNNT/PVA BNNT/HPMC

BNNT/PVA BNNT/HPMC

3 wt%IPDI–BNNTs1 wt%IPDI–BNNTs

Pure PVA1 wt% BNNTs3 wt% BNNTs

1 wt%IPDI–BNNTs

Pure HPMC

1 wt% BNNTs

3 wt%IPDI–BNNTs

3 wt% BNNTs

S-J Zhou et al, Nanotechnology 23 (2012) 055708

Mechanical Properties

PMMA/BNNTs composites were fabricated using a solution method by C. Y. Zhi et al.

The elastic modulus of PMMA was improved up to 19% while using only a 1wt.% BNNTs loading fraction. These results show that the external force can be transferred to BNNTs in some degree

Tensile strength decreased

The elongation also decreased, indicates that the interaction betweenBNNTs and polymer chains exists.

C. Y. Zhi et al., Journal of Nanomaterials, 2008, 642036

Mechanical Properties

Four kinds of polymeric composites with BNNTs were fabricated by Chunyi Zhi et al.

Vickers hardness of polymethyl methacrylate (PMMA), polystyrene (PS), polyvinyl butyral (PVB), and polyethylene vinyl alcohol (PEVA) was only slightly affected when they were loaded with the BN nanotubes.

This indicates that there is no obvious negative effect on the mechanical properties of the composites.

With the exception of PVB, the Vickers hardness did not notably decrease after adding BNNTsChunyi Zhi et al., Adv. Funct. Mater. 2009, 19, 1857–1862

Mechanical Properties

NASA have developed new materials with greater anti-penetration characteristics. By using BNNT polymer composites, researchers have successfully fabricated the new materials to demonstrate enhanced material toughness and hardness.

Nonwoven mats of BNNTs are used as toughening layers to maximize energy absorption and/or high hardness layers to rebound or deform penetrators.

They can also be used as reinforcing inclusions, combining with other polymer matrices to create reinforcing composite layers to maximize anti-penetrator protection

NASA Langley, Jefferson Lab, www.nianet.org

Microindentation test of BNNT composite

Thermal Properties

After adding BNNTs, the coefficient of thermal expansion (CTE) of PMMA dramatically decreases, This indicates that BNNTs significantly restrict the mobility of polymer chains

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.

C. Y. Zhi et al., Journal of Nanomaterials, 2008, 642036

Thermal Properties

Low CTE is a thermal parameter in polymeric composites used in packaging materials.

Chunyi Zhi et al. fabricated polymethyl methacrylate (PMMA), polystyrene (PS), polyvinyl butyral (PVB), and polyethylene vinyl alcohol (PEVA) composites filled with BNNTs by solution mixing.

All composites exhibit much lower CTE compared with the corresponding neat polymers. This implies the appearance of constraints to the polymer chain movements due to their interactions with BNNTs.Due to the different affinity of BNNTs for various polymers, the BNNT absorb different fractions of

polymer. The weight fractions of BNNTs in the composites range from 18 to 37 wt%. It was found that the weight fraction of BNNTs in a composite can be controlled by the concentration of the polymer solution.

Chunyi Zhi et al., Adv. Funct. Mater. 2009, 19, 1857–1862

Thermal Conductivity

Chunyi Zhi et al. also performed Thermal conductivity measurements;

Neat polymers possess low thermal conductivity. After embedding BNNTs, this property was improved. Thermal conductivity of PMMA sample drastically increases up to a 21-fold gain after adding BNNT. The thermal conductivity improvements of the composites are roughly related to the BNNTs fractions in them. In the case of a PVB composite loaded with BNNTs, a 7-fold increase was documented.

It is also assumed that an interfacial (BNNT–polymer) thermal transfer varies from one polymer to another, inducing the observed discrepancy in thermal conductivity values for almost the same BNNT loading fractions in different matrices.

Chunyi Zhi et al., Adv. Funct. Mater. 2009, 19, 1857–1862

Thermal Conductivity

Composite films with 5wt.% and 10wt.% BNNTs fractions of PMMA nanocomposites were chosen by Zhi et al. for the thermal conductivity measurements.

Thermal conductivity of PMMA loaded with a 10wt.% BNNT fraction was improved 3 timescompared to blank PMMA

It should be emphasized that this gain is likely to display the lower estimate for the observed improvement since the BNNTtexture within the film is generally misaligned with the direction used for the heat flow measurements

C. Y. Zhi et al., Journal of Nanomaterials, 2008, 642036

Thermal Conductivity

Huang et al. demonstrated that POSS modified BNNTs are very effective nanofillers for making dielectric epoxy composites with high thermal conductivity.

The room temperature thermal conductivity of the pure epoxy is about 0.2. The highest measured room-temperature thermal conductivity is 2.77 at 30.0 wt% BNNT fraction, which is 13.6 times higher than that of the pure epoxy resin.

Improvement of thermal conductivity in the present epoxy/BNNT nanocomposites is nonlinear: at a high fraction of BNNTs, a more effective improvement was observed. This implies that efficient thermal transfer pathways start to form at a high fraction of BNNTs due to tube-to-tube connections

Xingyi Huang et al., Adv. Funct. Mater. 2012, 201201824

Dielectric Properties

The dielectric loss tangent is closely associated with the electrical conductivity in the epoxy composites, which is determined by a charge carrier density at the certain temperature. Therefore, the decreased dielectric loss tangent of epoxy/BNNT nanocomposites should be attributed to a reduction of the electrical conductivity, which is confirmed by the conductivity spectra of the composites

One of the possible reasons for lower dielectric constant obtained in the epoxy/BNNT composites is the relatively low intrinsic dielectric constant of hexagonal BNNTs

Besides this factor, the other contribution may come from the restriction of bulk polarization in epoxy resin due to the immobility of polymer chains hinderedby BNNTs.

Xingyi Huang et al., Adv. Funct. Mater. 2012, 201201824

Dielectric Properties

Chunyi Zhi et al. examined the breakdown electric fields of neat polymers with that of their BNNTs composites.

Only in the case of PS does the breakdown electric field decrease, while in the other three cases, it marginally increases.

In any case, all the materials remain insulating and possess a high breakdown electric field, which is fully suitable for dielectric packages.

Chunyi Zhi et al., Adv. Funct. Mater. 2009, 19, 1857–1862

Dielectric Properties

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 and power modules.

C. Y. Zhi et al., Journal of Nanomaterials, 2008, 642036

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

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.

Radiation Shielding Properties

NASA have developed a neutron shielding material using boron-containing polymer nanocomposites, which include boron nanoparticles (BNPs) (0D), boron nitride nanotubes (BNNTs) (1D), and boron nitride nano-platelets (2D).

The large neutron absorption cross section, along with the light weight and large surface area of BNNT, enable effective shielding with much less volume and weight.

NASA Langley, Jefferson Lab, www.nianet.org

Morphology

Huang et al. performed the SEM observations of the fractured surface of the BNNT-POSS based epoxy composites It is seen that BNNTs are uniformly dispersed in the epoxy matrix.

In addition, interface-debonding between BNNTs and the epoxy resin is not observed, suggesting the strong interfacial adhesion.

Such uniform dispersion of BNNTs and strong interface are beneficial to the thermal conductivity enhancement

Xingyi Huang et al., Adv. Funct. Mater. 2012, 201201824

10 wt% BNNT-POSS 10 wt% BNNT-POSS

20 wt% BNNT-POSS 30 wt% BNNT-POSS

Morphology

D. Lahiri et al reinforced b biodegradable polylactide–polycaprolactone copolymer (PLC) with 0, 2 and 5 wt.% BNNTs

Figures show the BNNTs bridges

within PLC matrix.

Dangling BNNTs with the other end fully

embedded in the polymer matrix are also observed.

BNNTs behave as rigid reinforcements and provide benefits of short fiber strengthening.

D. Lahiri et al., Acta Biomaterialia, 2010

ConclusionMechanical properties of CNTs and BNNTs are similar. They are both ideal for mechanical applications.

High oxidation resistance of BNNTs allows their applications in high temperature environments.

The electronic properties of BNNTs are different from CNTs. BNNTs have a constant and wide band-gap of 5.5 eV. The electronic properties of BNNTs make them suitable nanofillers for the production of isolating polymeric materials

The exciting properties of BNNTs, such as high elastic modulus and high thermal conductivity make them advantageous for novel nanofillers in polymer composites to obtain mechanical reinforcement, high thermal conductivity and a low coefficient of thermal expansion in a matrix.

Chemical modification of inert BNNTs results dispersibility improvement in polymeric matrices.

Future research efforts are needed to demonstrate the performance of functionalized BNNTs in mechanical, electronic, chemical, and biological applications.

ReferencesReferencesSheng-Jun Zhou et al., 2012 Nanotechnology, 23, 055708

C. Zhi, Y. Bando, C. Tang, S. Honda and H. Kuwahara, J. Mater. Res., 2006, 21, 2794.

C. Y. Zhi, Y. Bando, C. Tang, H. Kuwahara, and D. Golberg, Journal of Nanomaterials, 2008, 642036

Chunyi Zhi, Yoshio Bando, Takeshi Terao, Chengchun Tang, Hiroaki Kuwahara, and Dimitri Golberg, Adv. Funct. Mater. 2009, 19, 1857–1862

Xingyi Huang , Chunyi Zhi , Pingkai Jiang , Dmitri Golberg , Yoshio Bando, Toshikatsu Tanaka, Adv. Funct. Mater. 2012, 201201824

NASA Langley, Jefferson Lab, www.nianet.org

D. Lahiri et al., Acta Biomaterialia, 2010