Viscoelasticity and macromolecular topology in single-site catalyzed polyethylene

Abstract

The relationship between polymer chain topology, i.e. long chain branching (LCB), and rheological properties in polyolefins obtained using single-site catalysts has been the topic of major scientific activity since the late 1990s [1–14]. Indeed, melt-state rheological measurements are being considered as one of the most sensitive methods for detecting sparse LCB in polyethylene (PE) [1, 15]. For example the flow activation energy, Ea, related to the temperature dependence of viscoelastic properties, is altered by the presence of LCB, and the cross-point frequency between storage and loss moduli, xx, and the crosspoint modulus, Gx, are also very sensitive features to the molecular architecture [16–18]. In this context, the study of these properties can give us information about the relationship between polymer chain topology and catalyst structure. In this work, a set of PE samples was synthesized using different metallocene catalysts. The main objective is to find relationships between chemical structure of the catalysts and polymer chain architecture by rheological testing. This is an interesting task as there is still a lack of information in the literature concerning how the catalyst structure affects the creation of LCB during polymerization [15]. The PE samples were synthesized using the catalysts shown in Table 1, together with MAO as co-catalyst. The synthesis of the samples was carried out under the same experimental conditions: a reactor temperature of 20 C, an ethylene pressure of 2 bar, 3 9 10 mol L of metallocene concentration, and a molar ratio MAO (methyl aluminoxane)/metallocene of 3,000 [17–21]. The metallocene catalysts were prepared using synthetic protocols that we have previously reported [20, 21]. The molecular properties of the polymer samples obtained by size exclusion chromatography analysis are listed in Table 1. The details about the experimental procedures are described elsewhere [18, 19]. It has been very difficult to estimate the molecular features in the case of C4 sample, probably due to extremely high molecular weight. In this case we have estimated molecular weight and polydispersity index by rheological methods. The materials were stabilized with 1.0 wt.% Irganox 1010 to avoid degradation at high temperatures. The rheological measurements were carried out using parallel plate geometry (15 mm diameter). Frequency sweeps in the linear viscoelastic regime were performed with a Bohlin CVO rheometer in the angular frequency range between 0.006 and 60 rad s. The linear viscoelastic regime has been located by previous strain sweeps. The measurements have been performed in the temperature range between 145 and 190 C and the time-temperature superposition principle (TTSP) applied to a reference temperature of T0 = 190 C. From these results, the characteristic features described above (Ea, xx, and Gx) have been obtained for the materials under study (see Table 1). The material labeled as A0 has been synthesized by using the [Zr(g-C5H5)2Cl2] catalyst (A), which is the simplest structural fragment used in this study. This catalyst has been previously reported to produce LCB [2, 14]. The sample shows a very high Ea value and a significant elastic character, in spite of the very narrow molecular J. F. Vega (&) J. Martinez-Salazar Departamento de Fisica Macromolecular, Instituto de Estructura de la Materia, CSIC, Serrano 113 bis, 28006 Madrid, Spain e-mail: imtv477@iem.cfmac.csic.es