Optical stack having birefringent layer of optically symmetrical crystallites

摘要

An optical stack (400) having a plurality of alternating polymeric layers (401, 402) is described. The alternating layers may be alternating birefringent (syndiotactic polystyrene, sPS) and isotropic (CoPENa) layers, or alternating positively and negatively birefringent layers. Birefringent layers are made using polymers which form optically symmetrical crystallites upon stretching of the polymer. The optical stack has a large refractive index difference in the x-direction (the stretching direction) and small refractive index differences in the y- and z-directions (the non-stretching directions). The optical stack can be made using standard film tentering methods and may be a multilayer reflective polarizer.

主权项

1. An optical stack comprising:
a plurality of alternating birefringent and isotropic layers, the birefringent layers comprising semi-crystalline polymer having optically symmetrical crystallites, wherein:
x- and y-directions define a plane parallel to the layers and a z-direction normal to the plane, the x-, y- and z-directions being orthogonal to each other;the birefringent layers have refractive indices n1x, n1y and n1z in the x-, y- and z-directions, respectively; andthe isotropic layers have refractive indices n2x, n2y and n2z in the x-, y- and z-directions, respectively; the optically symmetrical crystallites being substantially aligned such that:
the difference between n1x and n1y is greater than about 0.10; andthe difference between any two of n1y, n1z, n2y and n2z is less than about 0.03.