Thermal and Structural Behavior of Metallocene-Type Random Propylene Copolymers.

Introduction Fast advances in the development of metallocene-type catalysts allow polymerization and co-polymerization of olefins with a well-defined homogeneous molecular structure (1,2). These materials present narrow molecular weight and narrow comonomer composition distribution and, thus, present different properties compared to the classical copolymers polymerized with Ziegler-Natta type catalysts which are known to be very polydisperse (3). Metallocene propylene copolymers are commercially attractive as a means to improve consistency in the material and to expand into new areas of applications. From a more a academic point of view, the homogeneous copolymers permit the study of thermodynamic properties and the details of the lamellar structure as a function of increasing comonomer content and the type of comonomer or molecular weight as independent variables, without the need of lengthy fractionations. We are interested in studying the effect of incorporating low concentrations of comonomer (up to 10 mol %) in the polypropylene chain on physico-chemical and structural properties of the copolymers. The properties of copolymers with four different types of comonomers, ethylene, butene, hexene and octene will be compared and discussed. These studies are relevant as a means to predict product performance since similar copolymers are produced industrially when highly crystalline polypropylene with somewhat lowered melting points or improved fracture toughness and transparency, are requested for special applications. Previous studies have been carried out in samples characterized by a broad polydispersity in comonomer composition and molecular weight (4, 5) or in fractions obtained from heterogeneous propylene co-ethylene copolymers (3, 6, 7). While the melting temperatures are invariably reported to decrease with increasing ethylene content, however, the degrees of crystallinity obtained either by DSC or WAXS are found to decrease (4, 5) or to remain constant (6, 7) with increasing co-monomer content. Thus, there is no consensus on issues of fundamental basis such as the co-crystallizability of the comonomer unit. From x-ray diffraction and DSC data, Zimmermann (5) concluded that the ethylene units are completely rejected from the crystal phase. However, Avella et al. (8), Busico et al. (4) and Gedde et al. (6, 7) on the basis of unit cell volume, crystallinity and crystal thickness concluded that the ethylene units are partially incorporated in the crystal lattice. In a recent work (9) the melting temperatures (Tm) of propylenebutene copolymers were found to be significantly higher than those of propylene/alpha-olefin copolymers with a comonomer length changing from 6 to 16 carbons. Small differences were found in the Tm of the propylene/higher alpha-olefin copolymers. The issue of comonomer crystallizability is addressed in the present work from two different perspectives. First, the presence of comonomer units in the crystal is investigated by well-developed 13 C NMR methods (10). In a second approach, the properties of different types of propylene copolymers polymerized with the same catalyst, are compared at a fixed comonomer content and molecular weight. Both approaches require an initial detailed characterization by high resolution 13C NMR spectroscopy of the stereo, regio and comonomer fractions present in the polypropylene molecule. The properties studied include thermal behavior, phase structure, crystallite thickness, relative contents of the alpha and gamma polymorphs and an analysis of the linear growth rates. The kinetics of the development of the gamma polymorph are also studied and discussed on the basis of the requirements for the formation of the same polymorph in homopolymers (11).