Studies on the biaxial stretching of polypropylene film. VII. Refractive index of film measured with abbé refractometer

The refractive index of a film measured with an Abbé refractometer differs often depending on which surface of the film faces the refracting prism of the refractometer. This phenomenon was elucidated by using well‐characterized films especially prepared for the purpose. The experimental results led to the conclusion that the measurement with an Abbé refractometer gives the refractive index of the film surface facing the refracting prism independently of the refractive index distribution within the film. The importance of this conclusion is stressed in relation to the structural studies of polymer films, especially of biaxially stretched films.