A study of the morphology and physical properties of polypropylene films^†

Data are presented showing that rapidly quenched polypropylene films contain a paracrystalline phase in addition to crystalline and amorphous phases. The density of the paracrystalline phase was found to vary between 0.8890 and 0.9080 g./cm.³. Other data show that the density of quenched polypropylene films increases during aging, the increase being a linear function of the logarithm of the film age. The physical properties of the films, secant modulus, impact strength, and coefficient of friction, were found to vary not only with crystallinity, i.e., composition-dependent density, but also to vary during aging when the density was increasing with little change in crystallinity. An example of this dependence is the increase in secant modulus of 10.9 × 10⁶ psi/g./cm.³ during aging and only 3.7 × 10⁶ psi/g./cm.³ as the crystallinity increased. Because of the three phases and the density–time change, it is possible to have films with the same density and different physical properties and, conversely, films with the same physical properties and different densities.