The Reinforcement of Nanocomposites by Boron Nitride Nanosheets and Nanotubes

Abstract

The project has studied the mechanical reinforcement mechanism in two-dimensional (2D) boron nitride nanosheets (BNNSs) and one-dimensional (1D) boron nitride nanotubes (BNNTs)-based nanocomposites. It has been found that despite the Raman scattering from insulating hexagonal boron nitride-based materials being relatively weak compared with that from their carbon analogues, Raman spectroscopy is still a powerful technique to study both the BNNSs/polymer and BNNTs/polymer nanocomposites. A detailed review of the relevant literature is presented first of all. Single BNNSs of different thicknesses of up to 100 nm have been deposited upon a polymer substrate and deformed in unixial tension. The in-plane E2g Raman mode (the G band) of the BNNSs has been used to evaluate the stress transfer both between the individual hBN layers in the nanosheets and between the nanosheets and the substrate. The efficiency of internal stress transfer between the different hBN layers has been quantified from the G band shift rate of BNNSs with different thicknesses. The stress transfer between the BNNSs and the polymer substrate has been monitored by mapping the strain along a BNNS flake using Raman spectroscopy. It has been shown that shear-lag theory can be used to evaluate the BNNSs/polymer interfacial stress transfer. Three types of BNNSs with different geometries have been prepared by liquid-phase exfoliation and used to reinforce poly(vinyl alcohol) (PVA) with different loadings. Raman spectroscopy has been used to both evaluate the distribution of the BNNSs in the nanocomposites and follow stress transfer from the polymer matrix to the BNNSs. The reinforcement of the polymer has then been modelled using a combination of the rule of mixtures and modified shear lag theory. It has been demonstrated that the BNNSs with the larger aspect ratio are needed for realizing high levels of reinforcement. A detailed study has also been undertaken of the mechanisms of stress transfer in a nanocomposite consisting of BNNTs in a PVA matrix. The structure of the BNNTs has been characterized using transmission electron microscopy (TEM). The dispersion of nanocomposites containing up to 1 wt% of both pristine and hydroxyl-functionalized nanotubes (OH-BNNTs) in PVA have been characterized using a combination of TEM and Raman mapping. Stress transfer from the PVA matrix to both the BNNTs and OH-BNNTs has been evaluated from stress-induced shifts of the hBN Raman G band and larger band shifts have been obtained for the latter. This is in agreement with the mechanical testing results where functionalized nanotubes exhibit better reinforcement. Moreover, it has been found that polarized Raman spectroscopy can be used to characterize of orientation of BNNTs in the nanocomposites.

Date of Award	1 Aug 2021
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Robert Young (Supervisor) & Mark Bissett (Supervisor)