Abstract

Isothermal crystallization kinetics and properties related to the crystalline structure of four series of random propylene 1-alkene copolymers have been comparatively studied in this work. Comonomers studied include ethylene, 1-butene, 1-hexene and 1-octene in a concentration range up to 21 mol%. All copolymers were synthesized with the same metallocene catalyst to provide an equivalent random distribution and a similar content of stereo and regio defects within the series. This has ensured that differences in crystallization kinetics and in crystalline properties of copolymers with matched compositions reflect the affinity of the comonomer type for co-crystallization with the propene units, and the effect of content and type of co-unit in the development of the crystalline structure. In the nucleation-driven crystallization range, that is for Tcs > Tc max, the values of the rate follow the sequence PB > PE > PH = PO for comonomer contents <13 mol%, and PB > PE > PH > PO for >13 mol% comonomer. These trends in overall crystallization are guided by differences in undercooling due to a similar progression of the degree of participation of the comonomer in the crystalline lattice. The variation of the rates at Tcs < Tc max follows the melt segmental dynamics driven by differences in Tg, especially at the highest co-unit contents, resulting in a reverse rate sequence for PHs and POs >15 mol%, i.e., PB > PE ~ PO > PH. In addition to crystallization kinetics, a comparative polymorphic analysis and unit cell expansion, crystalline morphology, and melting behavior have been instrumental in resolving the partitioning of the four types of co-units between crystalline and non-crystalline regions. 1-Butene units participate at the highest level followed by the ethylene units, as demonstrated by solid-state NMR. However, both units are defects that hinder crystallization, as given by the decreasing rates, decreased levels of crystallinity and lowered melting temperatures with increasing co-unit content. All crystalline properties of PHs and POs conform to a rejection model of the 1-octene units from the crystals in the whole compositional range, and rejection of the 1-hexene units for PH <13 mol%, a conclusion also supported by NMR. The ability of PH >13 mol% to pack comonomer-rich sequences into a stable trigonal lattice leads at Tcs > Tc max to an increased number of crystallizable sequences, and to faster crystallization rates than for matched PO copolymers. © 2008 Elsevier Ltd. All rights reserved.