Anisotropy, cellulose orientation

The variation in the relative intensities of the 1095 and 1123 cm"l bands between the 0 and 45" spectra suggests anisotropy in the cellulose orientation. Table 1 shows that these peaks are skeletal stretching modes that are most incense when the electric vector of the incident light is parallel to the chain axis. Since the 1095 cm"l peak is very sensitive to the orientation of the incident electric vector relative to the chain axis, the intensity variation suggests that Che plane of sectioning was not exactly perpendicular to Che cellulose chain axes so chat Che chains are tilted relative to the plane of sectioning. 

Thin films of the solutions between microscope slides were sheared by applying even pressure on a coverslip while sliding it approximately one cm. The anisotropy appeared to increase as measured by increases in the birefiingence. Solutions containing 10-16% (w/w) cellulose developed a threaded texture and the mesophases were stable with time and oriented in the direction of shear. These observations, while not definitive, suggested a cholesteric to nematic transition occurred on shearing. 

However, some stretched films of cellulose esters are known to show extraordinary wavelength dispersion. In this chapter, wavelength dispersion of in-plane and out-of-plane birefringences for cellulose esters is mentioned in detail after a brief introduction on the concept of controlling the optical anisotropy. The molecular orientation and the birefringence of a solution-cast film for CTA are explained. Finally, an advanced method to obtain the extraordinary dispersion of the in-plane and out-of-plane birefringences is proposed. 

Nobukawa, S., Hayashi, H., Shimada, H., Kiyama, A., Yoshimura, H., Tachikawa, Y, and Yamaguchi, M. (2014). Strong orientation correlation and optical anisotropy in blend of cellulose ester and poly(ethylene 2,6-naphthalate) oligomer, I. Annl. Polvm. Sci.. 131, 40570. 

Table 15.4 clearly shows that anisotropic composite films present better mechanical properties. It is worth noting that anisotropic composites also have higher Young s modulus, yield stress and ultimate tensile strength than cross-linked isotropic homo-logues [13]. This, along with results presented in Table 15.4, seems to indicate that the mechanical properties of these all-cellulosic based composites depend on matrix anisotropy and fiber orientation rather than on cross-linking. 

This order parameter is a macroscopic measure of the anisotropy of the composites but it, nevertheless, reflects the microstrutural organization of the all-cellulosic based composites. The variation of Se with HPC content is similar to that observed for5(Fig. 15.3) and A (Fig. 15.4). Table 15.3 (and also Fig. 15.2c)shows that HPC matrix (0% w/w HPC of fibers) is clearly anisotropic and therefore the anisotropy in these composites arises from the synergy between the Uquid crystalline character of the matrix and the fiber orientation. 

Under certain conditions, cellulose derivatives possessing the characteristics of cholesteric liquid crystals present cholesteric helical structures dissolution and transition from the cholesteric to the nematic phase [98]. When shear is over, the system is relaxed over a determined time and intense, shifting to a transition state, where the energy of deformation is minimal and the orientation ordering is maintained, causing the appearance of band structures. When the external field is removed, the shear-induced anisotropy is affected by the inevitable relaxation of the macromolecular chains. Structural relaxation after removal of the external field depends on the shear history and relaxation mechanism [99,100]. Moreover, literature suggests a possible competition between the order induced by shear and thermodynamically, and also a correlation between the viscosity peak and the appearance of the anisotropic phase at low shear rates [ 101,102]. 

Extensive studies on the optical anisotropy of hydrated cellulosic gel in relation to deformation have been reported by Kratky " and by Hermanns. "" Kratky studied the deformation mechanism of fibrous materials by means of X-ray diffraction and birefringence, and postulated two kinds of deformation models for the swollen gel, in order to explain the orientation mechanism of the structural units in terms of the orientation distribution (but not in terms of the average degree of orientation of the structural units). Hermanns carried out studies on the same lines as Kratky, and introduced one of the orientation factors as a measure of the average degree of orientation. ... 

Compliances of the crystalline phase, such as sf and s p, where i= 1,2,3 i,j= 1,2,3 and p=4,5,6 respectively, can be written as equations (88)-<90), where the mechanical anisotropy of the cellulose crystal is assumed to be transversely isotropic with respect to the crystal b axis (the molecular chain direction). Similarly, stiffnesses of the crystalline phase, such as c%, clj and c p are given by equations (91)-(93). The compliances and stiffnesses of the noncrystalline phase can be described similarly in terms of the degrees of biaxial orientation of the noncrystalline chain segments and stiffnesses. 

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