Wavelength Dispersion of Orientation Birefringence

In order to produce the optical films with high performance, various techniques such as blending with other polymers [29-32] or small molecules [33-36] and copolymerization [37-40] have been studied. 

In principle, the extraordinary wavelength dispersion of birefringence produced by the blend and copolymerization techniques is achieved by using a similar concept to that of the piling method [Fig. 9.2] as Illustrated in Fig. 9.3. 

Illustration of the miscible blend and random copolymerization methods to produce extraordinary wavelength dispersion of orientation birefringence. Reproduced with permission from M. Yamaguchi, M. E. A Manaf, K. Songsurang, and S. Nobukawa, Cellulose, 2012,19,601. 2012, Springer [41]. 

Among these techniques, the addition of small molecules is relatively easier than the others due to good miscibility with polymers. In contrast, the combination of species is limited for the polymer blend and copolymerization technique. Therefore, addition of small molecules is becoming a good candidate these days. Addition of needle-shaped solid particles having strong optical anisotropy also modifies the orientation birefringence and its 

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The wavelength dispersion of orientation birefringence must be controlled precisely for color display. Based on the Kuhn and Griin model proposed for the stress-optical behavior of cross-linked rubbers, the orientation birefringence Anin(l] of an oriented polymer is expressed as follows [20, 23-27] ... 

Figure 9.2 Combination of two films having different wavelength dispersion to produce extraordinary wavelength dispersion of orientation birefringence.

Wavelength dispersion of orientation birefringence for cellulose esters containing tricresyl phosphate, 44,3942-3949. 

Material design of retardation films with extraordinary wavelength dispersion of orientation birefringence A review, 19,... 

Figure 9.25 Wavelength dependence of orientation birefringence for (open circles) CTA, (closed circles) CTA/TCP (95/5) and (closed diamonds) CTA/TCP (90/10), stretched at a draw ratio of 1.5. In the figure, the straight dotted line represents the ideal wavelength dispersion for a multi-band quarter-wave plate with a thickness of 400 pm. Reproduced with permission from M. E. A. Manaf, M. Tsuji, Y. Shiro3 ma, and M. Yamaguchi, Macromolecules, 2011, 44, 10, 3942. 2011, ACS Publications [36].

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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