Enhancing the thermal and mechanical properties of polyvinyl alcohol (PVA) with boron nitride nanosheets and cellulose nanocrystals
Graphical abstract
Introduction
The rapidly increased thermal load as a result of the continuous down-scaling of electronic devices and increase in power densities requires faster and more effective heat dissipation. The reliability of an electronic device shows exponential dependence on the operating temperature and therefore a small increase in temperature can cause significant reduction in lifespan. Thermal management is more and more critical to the performance, reliability and lifetime of the next generation miniaturized, integrated electronics [1,2]. For example, thin electronic devices such as solar cells, printed-circuit boards, wearable devices and flexible displays require superior thermal dissipation with the development of new generation high-power-density electronics [2,3]. Polymer films are usually used as the base materials for flexible devices, however, since polymers contain amorphous regions, voids, chain ends, and entanglements that act as defects or interfaces and hinder thermal conduction, they have low thermal conductivity (approximately 0.1 W/m K) that prohibits their application in high-power-density electronics [4,5].
Monolayer hexagonal boron nitride (h-BN) is composed of alternating boron and nitrogen atoms in a honeycomb arrangement, instead of only carbon atoms as in graphene [6]. With a similar atomic structure to graphene, the atomic layered h-BN is also called “white graphene”. As two-dimensional layered nanomaterials, hexagonal boron nitride nanosheets (BNNSs) have raised great research interests due to their distinct properties such as electrical insulator with a wide energy bandgap of ∼5.5 eV, high thermal conductivity (∼2000 W/m K by theoretical calculation [7]), excellent adsorption ability [8,9], high mechanical strength [10], superb thermal and chemical stability and high oxidation resistance [[11], [12], [13], [14]]. Introducing highly thermally conductive BNNSs into polymer matrices can improve the overall thermal conductivity as well as the mechanical properties of the polymer nanocomposites.
Cellulose is an organic compound that is the structural component of the primary cell wall of green plants, many forms of algae and the oomycetes. It constitutes the most abundant renewable polymer resource on Earth. Cellulose nanocrystals (CNCs) have gained interest from both academia and industries as ‘sustainable nanomaterials’, as they have high-axial mechanical properties, high reinforcing capability, low density, as well as abundancy, renewability and biodegradability [15,16]. As the main components of the cellulose fibres, the crystalline and amorphous parts can be generally separated by mechanical, chemical or enzymatic processes. Cellulose nanocrystals, prepared from fibres or fibrils by acid hydrolysis, have a small aspect ratio and have received great research attention due to their unique physical and chemical properties and the inherent abundance, renewability and sustainability. Nanocellulose therefore has been widely used as reinforcing agents, rheology modifiers, coating components, stabilizers of multiphase systems [17]. Since the cellulose nanocrystals possess a close to ideal structure with minimal defects that is beneficial for thermal conduction, the incorporation of cellulose nanocrystals into polymers would enhance the thermal conductivity [18,19].
Nanocellulose has been recently used together with boron nitride nanomaterials to improve the thermal conductivity of vacuum-filtered papers due to the stronger interaction between the nanofillers [20,21]. Zeng et al. reported a nanocomposite consisting of boron nitride nanotubes and cellulose nanofibres that exhibits high thermal conductivity (21.39 W/m K) at 25.0 wt% boron nitride nanotubes, attributed to the high intrinsic thermal conductivity of boron nitride nanotubes and cellulose nanofibres, the one-dimensional structure of boron nitride nanotubes, and the reduced interfacial thermal resistance due to the strong interaction between the boron nitride nanotubes and cellulose nanofibres. Zhu et al. fabricated nanocomposite papers from filtration of BNNSs and nanofibrillated cellulose, in which the BNNSs form a thermally conductive network and the nanocellulose provides mechanical strength. In addition, the great demand for transparent and deformable electronics such as next-generation displays, makes high light transmittance and fast heat dissipation very desirable properties. Nanocellulose has been successfully fabricated into transparent films or papers that opens up a wide range of applications in optoelectronics such as displays, touch screens and interactive paper [22,23].
In this context, our present work has used highly water-dispersible few layer h-BNs and cellulose nanocrystals (CNCs) to modify the mechanical and thermal properties of a water-soluble polyvinyl alcohol (PVA) matrix. The tensile property, fractography, out-of-plane thermal conductivity, UV-Visible absorption, light transmittance, X-ray diffraction and thermogravimetric analysis (TGA) were characterised on the prepared nanocomposites.
Section snippets
Synthesis of BNNSs and CNCs
PVA (weight-average molecular weight = 89,000–98,000, 99+% hydrolyzed) was purchased from Aldrich and used as received. BNNS dispersion was exfoliated from commercial hexagonal boron nitride (h-BN) powder by using the ball milling (Pulverisette 5, Fritsch) method with urea as agents at 400 rpm for 20 h at room temperature [12]. The simple, one-step mechano-chemical process functionalizes h-BN into highly water-dispersible, few-layer h-BN containing amino groups. For synthesis of the CNCs, 3 g
Morphology of the synthesized BNNS and CNCs
Fig. 1 presents the SEM images of the synthesized BNNSs and the CNCs. The BNNSs have a lateral size of 60–80 nm and thickness of less than 10 nm. The synthesized cellulose nanomaterials show fibrous morphology and have mainly crystalline cellulosic regions after acid hydrolysis [24]. The diameter of the CNC materials is in the range of 20–40 nm.
Mechanical properties of PVA and PVA nanocomposites
As it can be seen in Fig. 2, the tensile tests showed that both BNNSs and CNCs can increase the tensile strength and the Young's modulus of the PVA
Conclusions
The influence of boron nitride nanosheets (BNNSs) and cellulose nanocrystals (CNCs) on the thermal, mechanical properties, UV absorption and optical transparency of these nanofiller reinforced PVA composites was studied. The tensile tests showed that both BNNSs and CNCs can increase the tensile strength and the Young's modulus of the PVA matrix. The use of both BNNSs and CNCs led to the highest tensile strength, i.e. 109 MPa, with the filler content of 1.6 wt% for each type of nanofillers,
Acknowledgement
The authors would like to thank Ms Yulei Tai for her assistance with thermal imaging.
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