DISPOSICION FINAL

Araki and Kuga (2001) claimed that they are the only team to have observed nematic phase behaviour in static suspensions of cellulose nano-fibrils. In their work only bacterial cellulose with its high aspect ratio resulted in a nematic phase, under very specific conditions and only after allowing several days for the suspension to stabilise. This on the surface suggests that it may be possible to spin a continuous fibre in which all the cellulose fibrils are aligned to the main axis of a fibre. However, the work by Orts et al. (1995 and 1998), Yoshiharu et al. (1997), Ebeling et al. (1999) and Qizhou et

al. (2005 and 2006) has shown that high shear can lead to alignment of the fibrils. This

raises the question as to the most practical mechanism to induce shear. Appaw (2004) offers a good overall summary of the rheology of cellulosic liquid crystalline solutions.

It was reported by Appaw (2004) that under high rate flow, the chiral nematic mesophase aligns along the flow direction and uncoils to form a nematic structure (sometimes referred to as a pseudo-nematic phase). Upon cessation of flow, the chiral nematic phase will reform and the molecular orientation will decrease. This is due to the driving force for the liquid crystalline solution to form the more thermodynamically stable chiral nematic structure. Orts et al. (1998) demonstrated this with cellulose nano- fibril suspensions and showed that the rate of relaxation is dependent upon the aspect ratio of the fibres and the shear rate used in the first instance.

Appaw (2004) gave a useful summary of the concentration dependence of viscosity. Isotropic solutions and suspensions give a monotonic increase in shear viscosity with increasing concentration. The viscosity increases to a maximum when the biphasic region is approached. Upon formation of an anisotropic phase the viscosity begins to decrease. After which viscosity increases exponentially as the concentration continues to increase. However, Zugenmaier (1994) found that the correlation of the viscosity maximum with the formation of the anisotropic phase was only valid when the shear rate was low. At a higher shear rate, high viscosity solutions shift to a lower value. When the shear rate is high enough to cause shear-induced orientation (pseudo-nematic

43 phase) the viscosity maximum disappears and a monotonic increase of viscosity vs. concentration is observed. This indicates that under these conditions, the viscosity of the nematic or pseudo-nematic mesophase is less sensitive to concentration than that of the chiral nematic phase.

Anisotropic suspensions of cellulose nano-fibres demonstrate a similar viscosity- concentration relationship. The viscosity vs. concentration curve has a maximum, which disappears at high shear rates, Bercea and Navard (2000).

Onogi and Asada (1980) proposed the universal existence of three shear flow regimes to describe the viscosity of polymer liquid crystals: a shear thinning regime at low shear rates (I), a Newtonian plateau at intermediate shear rates (II) and another shear thinning regime at high shear rates (III) (Fig. 2.7). The commonly used method of describing how flow affects rheology is to use a logarithmic plot of viscosity against flow rate, viscosity against flow stress, or flow stress against shear rate.

In practice region (II) is seldom distinct for thermotropic systems, whereas region I is sometimes not seen in lyotropic systems. In the latter case, this has been attributed to the inability of instruments to make accurate measurements at sufficiently low shear rates.

Figure 2.7: Plot of viscosity against shear rate for a liquid crystal polymer.

It is generally considered that the behaviour in region I is related to the presence of defects in the liquid crystalline structure which have a large effect upon viscosity at low

III

II

I

Log

η

44 shear rates. The behaviour in region I can thus be attributed to the re-orientation of the directors associated with the multiple domains. At low shear rates, the director undergoes a continuous chaotic rotation (tumbling). This distorts the director field, which responds to this motion by creating defects to decrease the elastic free energy (Keates et al., 1996).

However, it was proposed that some polymer liquid crystal systems do not show all regimes because not every regime lies in the accessible shear rate range. It is important to determine the shear flow properties of cellulose nano-fibril suspensions in identifying optimal conditions for fibril alignment.

Li et al. (1996) noted the existence of the electroviscous effect, which is observed, with suspensions of charged rods. This is influenced by: (i) the ability of the diffuse layer of ions surrounding the charged rods to resist distortion, (ii) the electrostatic repulsion between neighbouring rods. Such effects are more pronounced in concentrated suspensions. Adding salt suppresses the electrical double layer and reduces viscosity of the suspension.

Lyotropic solutions exhibit elasticity as well as viscosity. The elasticity of lyotropic solutions arises because the stress field increases the alignment of molecules relative to the director, as discussed above. Release of the applied stress allows the system to relax to the equilibrium state. If the strain of the system is coupled to molecular orientation, then the strain will also relax. This is thought to be related to the existence of pleated structures found in many fibres spun from lyotropic solution. This was first observed by using transmission electron microscopy of Kevlar™ fibres (Dobb et al., 1977). In rheological experiments, it is nearly always associated with strain relaxation. In Kevlar fibres (Fig. 2.8) it has been shown that the bands correspond to a ‘pleated sheet’ structure with radial symmetry. These banded structures are characteristic of liquid crystalline polymers. Romo-Uribe and Windle (1999) have shown that there is a threshold molecular weight below which banded structures are not seen. Whether such a phenomenon would be observed in a fibre made of nano-crystals is not known.

45

Figure 2.8: Pleated structures visible in etched Kevlar fibres (Shahin, 2003).

The understanding of rheology under conditions of flow is clearly important in the processing of lyotropic solutions. According to Donald et al. (2002) there is not, as yet, a full theoretical understanding of the response of liquid crystals to flow fields. In a theoretical description it is necessary to consider not only the orientation of molecules (or nano-particles) within a domain, but also the polydomain nature of the liquid crystal polymer. It is also important to understand the viscoelastic behaviour of the system.