Positive birefringence

Craze initiation occurs in regions where chains were oriented transversely to direction of stress (birefringence positive)... 

Copolymers and blends of BPA-PC and Modified PS. Theoretically, a blend or copolymer of 60 parts BPA-PC (positively birefringent) and 40 parts PS (negatively birefringent) should yield a product free of birefringence (Fig. 25) (207). In spite of modifications to PC to improve the compatibiUty, no blend could be produced which would be optically isotropic and thus suitable as a substrate material. The same holds tme for PC/PS-copolymers (208) in which... 

Reduction or even complete compensation of birefringence by mixing polymers with positive birefringence (PC, PVC, PETP, PPE, PVDF, etc) with polymers with negative birefringence (PMMA, PS, PAN, etc) has been the consistent strategy. 

Relative to specimens examined under the microscope, the a or fast axis corresponds to the direction of the minimum refractive index, the minimum dielectric constant, and the maximum velocity. The y or slow axis corresponds to the maximum refractive index, the maximum dielectric constant, and the minimum velocity. Occasionally, a (3 axis is recognized with intermediate properties between a and y. When working with elongated bireffingent structures, birefringence usually is taken as positive when the y axis is parallel to the longitudinal axis. 

Dynamic behaviour. Time dependent curves of the birefringence An(t) induced by applying successively a positive and a negative electric... 

Physical properties of liquid crystals are generally anisotropic (see, for example, du Jeu, 1980). The anisotropic physical properties that are relevant to display devices are refractive index, dielectric permittivity and orientational elasticity (Raynes, 1983). A nematic LC has two principal refractive indices, Un and measured parallel and perpendicular to the nematic director respectively. The birefringence An = ny — rij is positive, typically around 0.25. The anisotropy in the dielectric permittivity which is given by As = II — Sj is the driving force for most electrooptic effects in LCs. The electric contribution to the free energy contains a term that depends on the angle between the director n and the electric field E and is given by... 

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