Nanocellulose-polyelectrolyte complexes

2017-02-13T04:02:12Z (GMT) by Praveena Veronica Raj
Cellulose is a highly hierarchical biodegradable material that is abundant in nature. Cellulose is made up of nano scaled fibres that bundle together and form micron scaled fibres. Nanocellulose fibres have received significant attention due to their attractive properties, namely high strength, high work- of-fracture, low moisture adsorption, low thermal expansion, high thermal stability, high thermal conductivity, exceptional oxygen barrier properties, and high optical transparency. This has been applied in wide-spread applications such as reinforcement in bio-composites, electronic displays, strength additives in papermaking, production of hydrogels, optically transparent flexible electronics and anti-microbial films.
   The resultant properties of nanocellulose films, as well as the rheological properties and hydrodynamic behaviour of nanocellulose suspensions, are dictated by the structural parameters of nanocellulose fibres. However, since nano scaled fibres are too long to be observed in its entirety under high magnification, traditional microscopy techniques are only able to accurately measure the diameter of the fibres. In the past, a sedimentation technique has been used to obtain the aspect ratio of nanocellulose fibres using the connectivity threshold, also known as the gel point. However, there is still a lack of understanding with regards to the effect of different sources of nanocellulose and mechanical treatment on the aspect ratio of nanocellulose produced. In this thesis, three different feedstocks; bleached eucalypt kraft pulp (BEK), commercial microfibrillated cellulose (MFC) and spinifex grass fibres were processed at different homogenisation levels to produce nanocellulose. Using the sedimentation technique, gel point was shown to be an excellent tool to track the quality development of nanocellulose fibres through different levels of mechanical treatment. This data could then be used to compare the potential of different cellulose feedstocks for nanocellulose production.
   Furthermore, another critical issue for the commercialisation of nanocellulose films is the drainage time. The drainage time of nanocellulose films has been shown to decrease significantly with the addition of cationic polyelectrolytes. However, the colloidal mechanisms behind this improvement have not been explored extensively in literature. Thus, this thesis explored the fundamentals of nanocellulose-polyelectrolyte interaction. The effects of charge density, molecular weight, morphology and dosage on nanocellulose-polyelectrolyte flocculation mechanisms, floc size and strength, were quantified. Linear cationic polyacrylamide (CPAM), branched polyethylenimine (PEI), linear PolyDADMAC (PDADMAC) and a polysalt, PAC, were chosen as the cationic polyelectrolytes due to their extensive use in practical applications. Gel point from sedimentation experiments and focussed beam reflectance measurements (FBRM) were used to quantify dynamic and static flocculation behaviour of MFC-polyelectrolyte flocs. Reflocculation ability of flocs after breakage, adsorption isotherms and zeta potential measurements of MFC-polyelectrolytes suspensions were also measured. Linear CPAM was found to correspond to a bridging mechanism. This was proven by the minimum gel point occurring at half surface coverage and partial reflocculation ability. Addition of branched PEI showed total reflocculation ability and corresponded to a charge neutralisation mechanism. PAC formed small dense flocs with MFC, which was attributed to the reduction in the electrical double layer thickness. MFC-PDADMAC flocs had a high reflocculation ability over all the dosages investigated and showed characteristics corresponding primarily to a patching mechanism. A linear master curve showing that the maximum surface coverage is inversely proportional to the charge density of polyelectrolytes was obtained, with the exception of PAC. This was independent of polyelectrolyte morphology. The results obtained from this part of the thesis show that floc size, density, reflocculation ability and polyelectrolyte surface coverage, charge density and molecular weight, can be engineered to cater for specific industrial applications, depending on the desired outcome.
   With regards to industrial applications, the effect of polyelectrolyte characteristics on the process parameters of nanocellulose film production, nanocellulose-polyelectrolyte suspensions and final film properties were quantified in this thesis. Linear CPAM and branched PEI were used at varying dosages, charge densities and molecular weights to accelerate the drainage of nanocellulose suspensions into films. The dewatering force required to dewater nanocellulose-polyelectrolyte suspensions and properties of the film were analysed. It was found that a lower gel point reduced the dewatering force needed to drain water through the fibre network. The drainage time to form a wet film reduced by two-thirds when halving the gel point. The more open 3D floc structure was retained upon drying the 2D nanocellulose film, shown by the increase in porosity of the film for a lower gel point MFC-polyelectrolyte suspension. A master curve was developed, which proved that the independent variable controlling the structure of nanocellulose-polyelectrolyte suspensions and the structure of the film formed, was the gel point. This was independent of polyelectrolyte morphology. Specific properties of nanocellulose films can now be engineered by selecting the optimum charge density, molecular weight, dosage and morphology of cationic polyelectrolyte.