Advanced Method to Control Orientation Anisotropy

The negative in-plane birefringence in CTA suggests that the direction of the polarizability anisotropy associated with the acetyl group is perpendicular to the main chain. This result corresponds to the previous reports [36,43, 44, 53]. 

It has been revealed that orientation [in-plane] birefringences of a polymer film is modified by addition of a low-mass compound 

Yamaguchi and co-workers studied the effect of the small addition of poly(lactic acid] (PLA] on the orientation birefringence of CAP. They found that PLA is miscible with CAP on a molecular scale when the amount of PLA is less than 3 wt% [8]. Moreover, the orientation birefringence of CAP is greatly enhanced even with a small addition of PLA as shown in Fig. 9.24. Considering that 

Furthermore, the blends with more than 3 wt% of PLA exhibits large haze values, indicating the existence of phase-separated morphology as previously reported by Tatsushima and co-workers [56]. As a result, the blends lose transparency and show similar level of orientation birefringence to the blend with 1 wt% of PLA. This is reasonable because PLA chains in the dispersed phase have no nematic interaction with CAP chains, thus the orientation will relax immediately as compared to the CAP. As demonstrated, much attention has to be focused on the miscibility for this method. 

Although the orientation of the main chain does not directly affect the orientation birefringence for cellulose esters, it has an impact on the orientation of the ester groups as well as the hydroxyl group. Furthermore, the orientation of the main chain strongly affects the nematic interaction with low-mass compounds. 

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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. 

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