Reinforcement of Polymer-Based Nanocomposites by Thermally Conductive and Electrically Insulating Boron Nitride Nanotubes

A detailed study has been undertaken of the mechanisms of stress transfer in a nanocomposite consisting of hexagonal boron nitride (hBN) nanotubes (BNNTs) in a poly(vinyl alcohol) (PVA) matrix based upon the use of nonresonance Raman spectroscopy. The structure of the BNNTs was characterized by using transmission electron microscopy (TEM) where it was shown that the majority of the nanotubes had 2-5 walls with some having over 10 walls. The structure and mechanical properties of nanocomposites containing up to 1 wt % of both pristine and hydroxyl-functionalized nanotubes (OH-BNNTs) in PVA were investigated. The dispersion of the BNNTs in the nanocomposites was characterized by using a combination of transmission electron microscopy and Raman mapping. The mechanical properties of the nanocomposites were evaluated by tensile testing, and it was found that the Young's modulus, yield strength, and fracture stress all increased on the addition of the BNNTs. A further improvement in the mechanical properties was obtained for nanocomposites containing the OH-BNNTs. The variation of the Young's modulus of the nanocomposites with volume fraction of the BNNTs was evaluated by using the rule of mixtures, and it was shown that the effective Young's modulus (E-eff) of the BNNTs approached 825 +/- 100 GPa at low volume fractions. The value of E-eff was found to decrease with increasing BNNT volume fraction as the result of nanotube bundling. By use of nondestructive Raman spectroscopy, stress transfer from the PVA matrix to the BNNTs was evaluated from stress-induced shifts of the hBN Raman G band, enabling the analysis of interfacial adhesion in the nanocomposites. Larger band shifts were obtained for the OH-BNNTs indicating a stronger interface between the BNNTs and the PVA matrix and a better dispersion. A value of 1.34 +/- 0.72 was determined from the stress-induced Raman band shifts for the Griineisen parameter of the BNNTs. In consideration of their efficient reinforcement of a polymer at very low additions and unique electrically insulating and thermally conductive properties, BNNTs are shown to have great potential to be used as nanofillers for composites in a number of applications.