Tailoring of the α-, β-, and γ-Modification in Isotactic Polypropene and Propene/Ethene Random Copolymers

An isotactic polypropene (i-PP) prepared with a metallocene catalyst system and two random copolymers of propene and ethene with 2.8 and 5.7 wt % ethene, respectively (PP-co-E-2.8, PP-co-E-5.7), were isothermally crystallized during simultaneous small- and wide-angle X-ray scattering (SAXS, WAXS) in synchrotron measurements. Identical measurements were carried out after adding 0.3 wt % N,N‘-dicyclohexyl-2,6-naphthalene dicarboxamide, a β nucleating agent (β-NA). All samples form the γ-modification to a certain extent. WAXS data show that the α-modification is predominant in i-PP, PP-co-E-2.8, and PP-co-E-5.7 isothermally crystallized without β-NA at temperatures below 125 °C. Adding β-NA leads to significant amounts of the β-modification at all crystallization temperatures in all three samples. Thus, the three modifications can be tailored by the crystallization regime of the nucleated samples. The simultaneous occurrence of the three modifications has also implications on the SAXS data. The formation of the γ-modification in addition to the α-modification does not lead to a one-dimensional correlation function K(z) with two distinct long periods or lamella thicknesses. In contrast, the formation of the β-modification results in a long period which can be distinguished from that of the other modifications after generating K(z).