Rheological and mechanical properties of systems containing nanocellulose
Doctoral thesis, 2016
Nanocellulose is a cellulose entity at the nanoscale. There are several different types of nanocellulose such as cellulose nanofibrils (CNF), which are long, somewhat flexible fibrils and cellulose nanocrystals (CNC), which are shorter and more rod-like particles. These entities are considered to have good mechanical properties; this combined with a low density makes nanocellulose an interesting candidate for several applications, e.g. transparent high strength paper, barrier films, reinforcing elements in composites and rheological modifiers. In the manufacturing of such products the rheological properties of the material are of great importance. A substantial part of this thesis has been devoted to assessment of the rheological behaviour of aqueous suspensions of CNF/CNC as well as of polymer melts containing the same nanomaterials. In general, both systems exhibited, quite a complex behaviour; the suspensions were shear-thinning and also exhibited a viscoelastic character (in agreement with other studies). An increase in the concentration of CNF/CNC generates a corresponding increase in shear viscosity and of the magnitude of the storage and loss moduli, characterizing the viscoelastic behaviour. For the first time, it was also shown that aqueous CNF suspension exhibited an extensional thinning viscosity during contraction flow, which can be of consequence in processing operations. It may be remarked that this complex rheological behaviour was noted already at low concentrations of CNF/CNC and could be attributed to a low percolation threshold for network formation. It is not only the concentration of the nanoelements that is of importance in this context. Here, different types of CNF/CNF were characterized with regard to their dimensions using atomic force microscopy and transmission electron microscopy and relations between the rheological properties and the dimensions were noted and discussed. . In a similar way, presence of charged surface groups and grafted polymer segments on CNF/CNC were found to have a strong effect on the rheology of these suspensions. Another area focussed on in this thesis was the use of CNF as reinforcing elements in polymer matrices. Two polymer matrices were used; poly(ethylene glycol) and an ethylene-acrylic acid copolymer. With the latter matrix, the amount of CNF used was high, up to 70 volume-%. In both systems, the addition of CNF substantially improved the modulus of the materials (and in some cases also the strength). Even at temperatures higher than the melting point of the matrices, the material could carry load indicating the forming of a coherent CNF-network. An analysis of the tensile modulus using the Cox-Krenchel model, pointed to that the fibrils formed aggregates in the composite resulting in stiffness values less than possibly could be expected.