Polymer/Boron Nitride Nanotube (BNNTs) Nanocomposites


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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.

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Polymer/Boron Nitride Nanotube (BNNTs) Nanocomposites

  1. 1. Polymer/Boron Nitride Nanotube (BNNTs) Nanocomposites METE 560 Ümit TAYFUN Middle East Technical University Polymer Science & Technology
  2. 2. Boron nitride nanotubesBoron nitride nanotubes, firstly synthesized in1995, are structural analogues of carbonnanotubes with boron and nitrogen atomsinstead of carbon atoms.BNNTs can be imagined as a rolled uphexagonal BN layer or as a carbon nanotube(CNTs) in which alternating B and N atomsentirely substitute for C atomsSimilar to CNTs, BNNTs have chiralities, animportant geometrical parameter, but for them,the chiralities do not play an important role indetermining electrical propertiesAtomic models of BNNT; (a)arm-chair (b)zig-zag (c)chiral
  3. 3. Properties of BNNTsBNNTs are chemically inert, oxidation resistant, andstructurally stable.BNNTs are electrically isolating materials with uniformelectronic properties independent of their size andchirality.Therefore, they are evaluated as suitable fillers for thefabrication of mechanically and thermally enhancedpolymer composites, while preserving the electrical TEM images of single to multi-wall BNNTs with six wallsisolation of the polymer matrixExcellent mechanical and thermal propertiesUnusually efficient electrical insulatorsStructurally stable and inert to most chemicalsUniform band gap (5.5 eV)High sensitivity for sensor materialsHigh resistance to oxidationTGA showed that the oxidation of BNNTs startsapproximately at 800 °C, which is much higherthan the oxidation temperature of CNTs, which isabout 400 °C. High oxidation resistance of BNNTsallows their applications in high temperatureenvironments.
  4. 4. 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 hasbeen 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 andwide band-gap of 5.5 eV. Therefore, they are electrically isolating independent of their size orchirality‟s. The electronic properties of BNNTs make them suitable nanofillers for theproduction of isolating polymeric materials.The obvious and most appealing difference between BNNTs and CNTs is their visibleappearance:BNNTs are pure white (sometimes slightly yellowish due to N vacancies) while CNTs are totallyblackComparison of properties of carbon nanotubes and boron nitride nanotubes Images of (a) CNTs and (b) BNNTs
  5. 5. Synthesis Methods of BNNTsThere are several methods used for synthesizing boron nitride nanotubes.Mainly used methods are:arc-discharge,laser ablation,ball milling,chemical vapor deposition
  6. 6. Polymer/BNNT compositesThe studies on the polymeric composites of BNNTs have been flourishedonly over the last years.The exciting properties of BNNTs, such as high elastic modulus and highthermal conductivity make them advantageous for novel nanofillers incomposite materials to obtain mechanical reinforcement, high thermalconductivity and a low coefficient of thermal expansion in a matrix.Polymer/BNNT composites that have been studied to date were prepared asthin films via solution–mixing, evaporation and melt-mixing techniques
  7. 7. Mechanical Properties C. Zhi et al. fabricated PS/BNNT composites using a solution method The mechanical properties of a polymer were improved It was found that the results were solvent- dependent, that is, when chloroform was used to disperse BNNTs, the elastic modulus 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 solvents Benefiting from the pure white appearance of BNNTs, the composite films retained good transparency (a) a blank PS film (b) BNNT/PS film (c) BNNT/PmPV/PS filmC. Zhi, Y. Bando, C. Tang, S. Honda and H. Kuwahara, J. Mater. Res., 2006, 21, 2794.
  8. 8. 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 3 wt%IPDI–BNNTs IPDI–BNNTs leads to a considerable 1 wt%IPDI–BNNTs 3 wt%IPDI–BNNTs increase in both Young’s modulus and Pure PVA 1 wt%IPDI–BNNTs 1 wt% BNNTs tensile strength. 3 wt% BNNTs Pure HPMC 1 wt% BNNTs 3 wt% BNNTs When the amount of ap-BNNTs was added, both tensile strength and Young’s BNNT/PVA BNNT/HPMC 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 BNNT/PVA BNNT/HPMC surface, pristine BNNTs exhibit limited dispersibility and poor interfacialS-J Zhou et al, Nanotechnology 23 (2012) 055708 interactions with PVA and HPMC.
  9. 9. 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 between BNNTs and polymer chains exists.C. Y. Zhi et al., Journal of Nanomaterials, 2008, 642036
  10. 10. Mechanical PropertiesFour kinds of polymeric composites with BNNTs were fabricated by ChunyiZhi et al.Vickers hardness of polymethyl methacrylate (PMMA), polystyrene (PS),polyvinyl butyral (PVB), and polyethylene vinyl alcohol (PEVA) was onlyslightly affected when they were loaded with the BN nanotubes.This indicates that there is no obvious negative effect on the mechanicalproperties of the composites.With the exception of PVB, the Vickers hardness did not notablydecrease after adding BNNTsChunyi Zhi et al., Adv. Funct. Mater. 2009, 19, 1857–1862
  11. 11. Mechanical PropertiesNASA have developed new materials with greater anti-penetration characteristics.By using BNNT polymer composites, researchers have successfully fabricated thenew materials to demonstrate enhanced material toughness and hardness.Nonwoven mats of BNNTs are used as toughening layers to maximize energyabsorption and/or high hardness layers to rebound or deform penetrators.They can also be used as reinforcing inclusions, combining with other polymermatrices to create reinforcing composite layers to maximize anti-penetratorprotection Microindentation test of BNNT compositeNASA Langley, Jefferson Lab, www.nianet.org
  12. 12. 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
  13. 13. Thermal PropertiesLow 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), andpolyethylene vinyl alcohol (PEVA) composites filled withBNNTs by solution mixing.All composites exhibit much lower CTE comparedwith the corresponding neat polymers. This implies theappearance of constraints to the polymer chainmovements due to their interactions with BNNTs.Due to the different affinity of BNNTs for variouspolymers, the BNNT absorb different fractions ofpolymer.The weight fractions of BNNTs in the compositesrange from 18 to 37 wt%. It was found that the weightfraction of BNNTs in a composite can be controlled bythe concentration of the polymer solution. Chunyi Zhi et al., Adv. Funct. Mater. 2009, 19, 1857–1862
  14. 14. Thermal ConductivityChunyi 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 a21-fold gain after adding BNNT. The thermal conductivity improvements of the composites areroughly related to the BNNTs fractions in them.In the case of a PVB composite loaded with BNNTs, a 7-fold increasewas documented. It is also assumed that an interfacial (BNNT–polymer) thermal transfervaries from one polymer to another, inducing the observed discrepancyin thermal conductivity values for almost the same BNNT loadingfractions in different matrices. Chunyi Zhi et al., Adv. Funct. Mater. 2009, 19, 1857–1862
  15. 15. 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 times compared to blank PMMA 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 measurementsC. Y. Zhi et al., Journal of Nanomaterials, 2008, 642036
  16. 16. 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 connectionsXingyi Huang et al., Adv. Funct. Mater. 2012, 201201824
  17. 17. 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 hindered by BNNTs.Xingyi Huang et al., Adv. Funct. Mater. 2012, 201201824
  18. 18. Dielectric PropertiesChunyi Zhi et al. examined the breakdown electric fields of neat polymerswith that of their BNNTs composites.Only in the case of PS does the breakdown electric field decrease, while inthe other three cases, it marginally increases. In any case, all the materials remain insulating and possess a highbreakdown electric field, which is fully suitable for dielectric packages.Chunyi Zhi et al., Adv. Funct. Mater. 2009, 19, 1857–1862
  19. 19. Dielectric PropertiesThe appealing point of BNNT usage in polymeric composites is that the originaldielectric nature of a polymer is kept in the resultant composite. This fact is crucial inmany cases, such as packing materials for electrical circuits and power modules. 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. C. Y. Zhi et al., Journal of Nanomaterials, 2008, 642036
  20. 20. Radiation Shielding PropertiesNASA have developed a neutron shielding material using boron-containingpolymer 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 andlarge surface area of BNNT, enable effective shielding with much less volumeand weight.NASA Langley, Jefferson Lab, www.nianet.org
  21. 21. Morphology 10 wt% BNNT-POSS 10 wt% BNNT-POSS 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 20 wt% BNNT-POSS 30 wt% BNNT-POSS 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 enhancementXingyi Huang et al., Adv. Funct. Mater. 2012, 201201824
  22. 22. MorphologyD. Lahiri et al reinforced biodegradable bpolylactide–polycaprolactone copolymer(PLC) with 0, 2 and 5 wt.% BNNTsFigures show the BNNTs bridgeswithin PLC matrix.Dangling BNNTs with the other end fullyembedded in the polymer matrix are alsoobserved.BNNTs behave as rigid reinforcements andprovide benefits of short fiber strengthening.D. Lahiri et al., Acta Biomaterialia, 2010
  23. 23. ConclusionMechanical properties of CNTs and BNNTs are similar. They are both ideal formechanical applications.High oxidation resistance of BNNTs allows their applications in high temperatureenvironments.The electronic properties of BNNTs are different from CNTs. BNNTs have a constantand wide band-gap of 5.5 eV. The electronic properties of BNNTs make them suitablenanofillers for the production of isolating polymeric materialsThe exciting properties of BNNTs, such as high elastic modulus and high thermalconductivity make them advantageous for novel nanofillers in polymer composites toobtain mechanical reinforcement, high thermal conductivity and a low coefficient ofthermal expansion in a matrix.Chemical modification of inert BNNTs results dispersibility improvement in polymericmatrices.Future research efforts are needed to demonstrate the performance of functionalizedBNNTs in mechanical, electronic, chemical, and biological applications.
  24. 24. ReferencesSheng-Jun Zhou et al., 2012 Nanotechnology, 23, 055708C. 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 ofNanomaterials, 2008, 642036Chunyi Zhi, Yoshio Bando, Takeshi Terao, Chengchun Tang, Hiroaki Kuwahara, andDimitri Golberg, Adv. Funct. Mater. 2009, 19, 1857–1862Xingyi Huang , Chunyi Zhi , Pingkai Jiang , Dmitri Golberg , Yoshio Bando,Toshikatsu Tanaka, Adv. Funct. Mater. 2012, 201201824NASA Langley, Jefferson Lab, www.nianet.orgD. Lahiri et al., Acta Biomaterialia, 2010