Patent ID: 8226860

Claim:
A method for controlling positive birefringence in a compensation film for liquid crystal display comprising: (a) determining a value of R/D for a polymer, wherein the polymer comprises a polymer segment having a polymer backbone, a light-stable optically anisotropic sub-unit (OASU) attached directly to the polymer backbone via at least one covalent bond, wherein R represents the maximum dimension of the OASU in the direction perpendicular to the direction of the vector sum of the at least one covalent bond and D represents the distance along the polymer backbone between the attaching points of two neighboring OASUs, and wherein OASU affects the rigidity and long-range linear corkscrew shape of the polymer backbone such that the average orientation of the OASU is perpendicular to the polymer backbone, and the higher the perpendicularity of the OASUs, the larger the value of the negative segment birefringence of the polymer segment; (b) selecting the polymer as having a controlled negative segment birefringence if said value of R/D>about 2.7; and (c) processing the polymer having a controlled negative segment birefringence (Δn s ) by solution casting onto a substrate, followed by uniaxial stretching or biaxial stretching or a combination thereof, wherein the polymer has a negative segment order parameter (O s ) and the polymer film has a positive birefringence (Δn) that satisfies the relation Δn=Δn s ×O s >0.