Studies on biaxial stretching of polypropylene film. IV. Overall orientation during one‐step biaxial stretching and its application to the estimation of the principal refractive indices of isotactic polypropylene

Polypropylene film was biaxially stretched in one step in air at 140°C or 152°C, and the deformation was studied optically. A linear relation held between Δ n ss and v A −½ for v A > 10, at both temperatures, where Δ n ss is the birefringence with respect to the normal to the film and v A is the degree of stretching expressed as the factor by which the area of the film is increased. Extrapolation of data in this linear region yielded a value of 20 × 10 −3 for −Δ n ss at infinite v A . Since it is presumed that the polypropylene molecules lie completely parallel to the film surface when the film is stretched infinitely, −Δ n ss at v A −½ = 0 must be just half Δ n °, the intrinsic birefringence in the case of completely parallel orientation. Thus, Δ n ° must be 40 × 10 −3 . This value was obtained experimentally in uniaxial stretching when the birefringence with respect to the direction of drawing was extrapolated to infinite extension. Similar relations held between n p , the average of the refractive indices in the two stretching directions, and v A , and between n ss , the index normal to the film, and v A . By similar extrapolations, (1/2)( n ′ γ + n ′ β ) and n ′ β = n * α ′ were estimated, and thence n α′ was obtained. Here, n ′ α and n ′ β are the refractive indices along the c axis (molecular chain axis) and b axis. All these optical parameters refer to a density of 0.900 g/cm 3 . Hence by applying a density correction to those values, the principal refractive indices and the intrinsic birefringence of polypropylene crystal were evaluated as follows: n α = 1.5522, n β = n * α = 1.5106 and Δ n c ° = 4.16 × 10 −3 , where n * α is the refractive index prependicular to the b and c axes of the crystal.