Studies on the biaxial stretching of polypropylene film. V. Orientation mechanism of the one‐step biaxially stretched film

The molecular orientation behavior during one‐step biaxial stretching in air of a quenched, low crystalline film differs from that of a slightly crystallized, highly crystalline film. In the case of a quenched film a plot of −Δn_ss versus \documentclass{article}\pagestyle{empty}$ 1/\sqrt {va} $ is composed of three intersecting straight lines with different slopes, −Δn_ss and v_A being the birefringence with respect to the normal to the film surface and the degree of stretching in area, respectively. On the other hand, −Δn_ss of crystalline film changes somewhat differently. It does not increase up to fairly high stretching ratio. When a film with surface layers of different crystallinity was stretched biaxially, the relations between −Δn_ss and \documentclass{article}\pagestyle{empty}$ 1/\sqrt {va} $ for the two surface layers differ, that is, a surface layer of low crystallinity shows a change of −Δn_ss for the two surface layers differ, that is, a surface layer of low crystallinity shows a change of −Δn_ss like that of a quenched film type and the surface layer of higher crystallinity, follows the behavior typical of highly crystalline film. However, a commercial film having different crystallinites on the two surface layers exhibits another type of −Δn_ss change, which may be ascribed to the stress remaining frozen in the film in the manufacturing process. In addition, it is found that an Abbé refractometer can give the refractive indices of both the upper and lower surface layers of a film when the film structure is not the same on both surface layers.