Interfaces in Modified Cellulose Nanocrystals Reinforced Polyethylene Composites

Abstract

Composites, comprising high density polyethylene (HDPE) reinforced with cellulose nanocrystals (CNCs), with difference sources of CNCs (cotton and tunicate) and different types of compatibilisers, have been prepared by melt compounding. The weight fraction of CNCs was varied from 0.5 to 5 wt.%. CNCs are rod shaped nano-sized crystalline fractions of cellulose fibres, with high aspect ratios, that can be obtained via acid hydrolysis of cellulosic material. The Young’s modulus of CNCs around 57-143 GPa depending on the origin of CNCs and this value is comparable to man-made glass fibres (~70 GPa), considering that CNCs has a lower density (1.3-1.6 g cm-3) than glass fibres (2.5 g cm-3). All these properties are highly favourable for using CNCs as a nanofiller/reinforcements in polymer matrix composite materials. Better understanding of cellulose nanocomposites motivate the study to look deeply on interaction between polymer matrix and filler (nanocellulose) hence lead to the advancement of nanocellulose values so that it could be used universally into various biomaterials. In this study, a comparison between two different sources of CNCs demonstrated that the origin of the starting raw cellulosic governs the resulting reinforcing effect via aspect ratio, surface charge and crystallinity index. Unfortunately, the tendency of CNCs to form agglomerates led to difficulties in achieving effective reinforcement especially when simply mixing into a matrix material in the solid state. As a result, it has become clear that new approaches to composite construction will be required if effective composite reinforcement using CNCs is to be achieved. Enhanced mechanical properties of CNCs reinforced HDPE composites with the addition of low loadings of CNCs were reported, compared to neat HDPE samples. Tunicate CNCs reinforced HDPE composites led to higher strength and modulus than cotton CNCs reinforced HDPE composites at the same CNCs concentration. This is thought to be due to the enhanced reinforcing effect of tunicate, due to their much larger aspect ratio (60.7 ± 30.7) compared to cotton (16.3 ± 5.7). The use of maleic anhydride polyethylene and polyethylene oxide as a compatibiliser was found to increase the tensile strength and Young`s modulus of the CNCs reinforced HDPE composites. The mechanical properties of these composites were found to mainly depend on the aspect ratio of CNCs and the interaction of the HDPE matrix and the reinforcement phase. Further studies are conducted to investigate the stress transfer mechanism in CNCs reinforced with HDPE composites using Raman spectroscopy. The peak position of a Raman band located initially at the ~1095 cm-1 position is reported to shift towards a lower wavenumber under the application of tensile deformation. These shifts correspond to the direct deformation of the molecular backbone of cellulose, which is dominated by a C-O stretching mode. Higher Raman band shift rates with respect to tensile strain of this band are observed for the nanocomposites produced using the MAPE and PEO as compatibilisers. This demonstrates that stress is transferred from the matrix to the fillers more effectively with the presence of the compatibiliser, supporting the enhancement of the mechanical properties of the composite. Nanocomposites made from tunicate CNCs shows higher gradient of shift compares to nanocomposites made from cotton CNCs due to higher aspect ratio of tunicate CNCs. Finally, the combination of Raman mapping with chemical images and image analysis has been used to study the morphology of the CNCs in the nanocomposites. The cross-sectional areas of nanocomposite samples were investigated using confocal Raman microscopy. Raman spectroscopy is shown to provide and collect an accurate `fingerprint` of the composition of the cross-sections of the nanocomposites. The conversion of these set of Raman spectra to chemical images provides high contrast and reliability for the image analysis. The image analysis approach allows a quantitative assessment of the degree of mixing and degree of aggregation of CNCs in the HDPE matrix. This analysis showed that CNCs were mixed to varying degrees in the HDPE matrix. These results provide a further step in understanding and inspecting the mechanism of CNCs enhanced polymer matrices.