A method for manufacturing a film or sheet comprises deforming a polymeric alloy at a temperature effective to impart to the deformed alloy a birefringence retardation of greater than or equal to about 350 nanometers. A method of substantially eliminating comets and veins in an optical film or sheet comprises annealing the film or sheet comprising a polymeric alloy to a temperature proximate to a glass transition temperature of a polymeric resin contained in the alloy. A composition comprises a first polymeric resin in an amount of about 1 to about 99 wt %; and a second polymeric resin in an amount of about 1 to about 99 wt %, wherein the polymeric resins are treated with a deforming force or with an energy effective to produce a polymeric alloy having a birefringence retardation of greater than or equal to about 350 nanometers or with a birefringence retardation of less than or equal to about 150 nanometers.