Lamellar phases shear behaviour

For a quenched lamellar phase it has been observed that G = G"scales as a>m for tv < tvQ. where tvc is defined operationally as being approximately equal to 0.1t and r is a single-chain relaxation time defined as the frequency where G and G" cross (Bates et al. 1990 Rosedale and Bates 1990). This behaviour has been accounted for theoretically by Kawasaki and Onuki (1990). For a PEP-PEE diblock that was presheared to create two distinct orientations (see Fig. 2.7(c)), Koppi et al. (1992) observed a substantial difference in G for quenched samples and parallel and perpendicular lamellae. In particular, G[ and the viscosity rjj for a perpendicular lamellar phase sheared in the plane of the lamellae were observed to exhibit near-terminal behaviour (G tv2, tj a/), which is consistent with the behaviour of an oriented lamellar phase which flows in two dimensions. These results highlight the fact that the linear viscoelastic behaviour of the lamellar phase is sensitive to the state of sample orientation. 

Van der Linden and co-workers have developed a theory for the rheological behaviour of multilamellar vesicles on the basis of their bending and dynamic properties, which could be suitable for an understanding of the phase transitions of lamellar phases under shear. These authors calculated the energy required for the deformation of the vesicles, which is determined both by the bending energy of the bilayer and by the interactions between such layers. With this model, they could obtain relaxation times and elastic frequencies for the different modes of deformation of the multilamellar vesicles that were of the same order of magnitude, like the shear rates at which the transformation of the lamellar phases into monodisperse multilamellar vesicles takes place. 

---