Polyester copolymers for use in optical films

An optical film comprising a plurality of alternating polymeric first and second interference layers numbering greater than 25 in total and disposed between, and integrally formed with, first and second skin layers, each of the first and second interference layers having an average thickness less than about 250 nm,

• the first skin layer comprising polyester,
• the first interference layers comprising a substituted naphthalate, hexanediol and ethylenediol copolymer, wherein at least 30% by weight of diol moieties in the first interference layer is hexanediol,
• the second skin layer having an average thickness greater than about 100 micrometers and comprising at least 70% by weight of polyethylene terephthalate, and,
• for substantially normally incident light and for each wavelength in a predetermined wavelength range extending at least from about 400 nm to at least about 600 nm, the optical film having an optical reflectance of at least 50% for each of orthogonal first and second polarization states,
• adjacent first and second interference layers having respective in plane indices of refraction:
• n1x and n2x along the first polarization state,
• n1y and n2y along the second polarization state,
• and n1z and n2z along a z-axis orthogonal to the first and second polarization states, such that for at least one wavelength in the predetermined wavelength range:
• each of n1x and n1y is greater than n1z by at least 0.18;
• a maximum difference between n2x, n2y and n2z is less than about 0.01;
• and
• a difference between n1x and n2x is greater than about 0.18.

An optical film comprising a plurality of alternating first and second polymeric layers numbering greater than 25 in total and disposed between, and co-extruded and then co-stretched at one or more temperatures between about 190° F. to about 220° F., with first and second skin layers, each of the first and second polymeric layers having an average thickness less than about 250 nm, the first and second polymeric layers and the first and second skin layers having glass transition temperatures less than about 95° C., the first polymeric layers having in-plane birefringences greater than about 0.21, the second skin layer comprising at least 70% by weight of polyethylene terephthalate, wherein for substantially normally incident light and for at least one wavelength between about 400 nm to about 1500 nm, the optical film reflects at least 20% of the incident light having a first polarization state,

• wherein the first polymeric layers comprise a copolyester polymeric material comprising:
• 40 to 51 mol % substituted naphthalate units;
• 10 to 40 mol % ethylene units;
• and
• 10 to 40 mol % hexane units.

An optical film comprising a plurality of alternating first and second polymeric layers numbering greater than 25 in total and disposed between, and co-extruded and then co-stretched at one or more temperatures between about 190° F. to about 220° F., with first and second skin layers, each of the first and second polymeric layers having an average thickness less than about 250 nm, the first and second polymeric layers and the first and second skin layers having glass transition temperatures within about 5°° C. of each other, the first polymeric layers having in-plane birefringences greater than about 0.21, the second skin layer comprising at least 70% by weight of polyethylene terephthalate, wherein for substantially normally incident light and for at least one wavelength between about 400 nm to about 1500 nm, the optical film reflects at least 20% of the incident light having a first polarization state, wherein the first polymeric layers comprise a copolyester polymeric material comprising:

• 40 to 51 mol % substituted naphthalate units;
• 10 to 40 mol % ethylene units;
• and
• 10 to 40 mol % hexane units.