Bimodal polyethylenes form one pot synthesis : effect of flow induced crystallization on physical properties

The ultimate properties of a polymer are governed not only by the chemical structure of the polymer chains but also by the processing conditions applied during fabrication of the final product, in particular as a result of orientation of long-chain molecules. The intrinsic properties of a polymer are governed mainly by its molecular weight; the higher the weight-average molar mass (Mw), the better are the physical properties of a polymer. High molecular weight is essential for better mechanical properties such as tensile strength, toughness and wear properties. However, high molecular weight polymers, due to a prohibitively high melt viscosity, become intractable and difficult to process using conventional processing techniques. Up to a critical molecular weight (Mc, typically twice the average molecular weight between entanglements) of a polymer, the viscosity increases linearly with molecular weight (MwMc ; h0 ~ Mw 3.4). Intractability of a high molecular weight polymer limits the means of its processing and its applications. Therefore the finished products become too expensive due to the high cost involved in specially designed processing techniques. Thus, for a successful product design, chemistry, physics and engineering have to be addressed in parallel with equal stress. In practice, polymer (plastics) processing involves a compromise between properties (for which high molecular weight is desirable) and (fast) processability, which normally requires lower molecular weight. Often, a polymer with low molecular weight is mixed with a high molecular weight polymer. However, melt-phase mixing (blending) of two polymers is difficult, particularly when one of the components has high molecular weight, and therefore a very high melt viscosity. Solution blending is not a realistic option on an industrial scale due to the high costs. In industry, bimodal polymers are produced by sequential polymerization using two or more reactors in series; low molecular polymer is produced in the first reactor and high molecular weight polymer in the second. In this thesis bimodal polyethylenes are synthesized by immobilizing two different catalysts on a single support, in order to overcome the problem of miscibility of the high and low molecular weight components. This route overcomes the usage of cascade reactors and provides the required homogeneous distribution of high molecular weight polymer (Mw > 1 million g/mol) in a matrix of low molecular weight polyethylene. In addition to the synthesis, significant attention has been paid to the morphology and rheology (flow properties) of the materials produced. Under defined flow conditions, different relaxation times of the low and high molecular weight chains lead to crystallization of the high molecular weight material, forming a so-called shish structure. This means that the long chains are pulled/oriented in the flow direction and can act as nucleation sites for crystallization of the low(er) molecular weight chains present in the blend, resulting in the known shish-kebab morphology; a fibrous extended-chain nucleus comprising oriented (high molecular weight) chains, decorated with folded-chain crystals, whereby the low molecular weight polymer can crystallize at relatively high temperatures. The desired ultimate properties, such as the Emodulus and tensile strength, are linked to this structure development during flow. Several catalysts were immobilized on supports of type MgCl2/AlRn(OR’)3-n to synthesize polyethylenes differing in molecular weight and molecular weight distribution. Melt rheometry was effectively used to differentiate between so-called single-centre and multi-centre catalysts. Investigation of the melt rheological properties of polyethylenes prepared with chromium, titanium, vanadium and zirconium catalysts immobilized on MgCl2 supports confirmed narrow (Schulz-Flory) molecular weight distribution, indicative of single-centre catalysis, for Cr-, Vand Zr-based systems, but not for Ti. In the case of polymers prepared with MgCl2-immobilized Ti complexes and having narrow MWD (Mw/Mn 2-3) according to GPC, deviation from a Schulz-Flory distribution is evident from a decrease in storage modulus with decreasing angular frequencies, whereas polyethylenes prepared with analogous vanadium complexes exhibit a constant (plateau) modulus over a wide frequency range. The presence of a plateau modulus is characteristic for narrow MWD polyethylene, so that melt rheology provides a valuable tool to prove, or disprove, the presence of a Schulz-Flory distribution in cases where GPC does not provide a definitive answer. Co-immobilization of 1-(8-quinolyl)indenyl CrCl2 and [2,6-{(2-chloro-4,6- dimethylphenyl)N=C(Me)}2C5H3N]FeCl2 on a MgCl2/AlEtn(OEt)3-n support was used to produce bimodal polyethylene in a single reactor, termed a one pot process. In this way, intimate miscible blends of high (Mw = 106 g/mol, Mw/Mn = 2.0) and low (Mw = 1 × 105 g/mol, Mw/Mn = 5.5) molecular weight PE were synthesized. The relative proportions of the high- and low-molecular weight components are controlled by varying the loadings of the two precatalysts. Rheological studies of bimodal blends suggest homogeneous mixing of the high and low molecular weight components within the time frame of the experiments under low strain. Homogeneously mixed very high and low molecular weight polyethylenes synthesized in this way are model systems to study the influence of molecular weight on crystallization under shear conditions. Due to distinct differences in the relaxation times, the influence of molar mass on crystallization and the resultant crystalline morphology during shear flow was investigated by time-resolved X-ray techniques. The observations are that highly stable shish structures are generated from high molecular weight chains, providing nucleation sites for further crystallization. Shish structures thus formed are found to be stable above the equilibrium melting temperature of unconstrained extended chain crystals of polyethylene, in the absence of flow. Structure development during flow is linked to the mechanical properties of the bimodal polyethylene. Several of the bimodal polyethylenes synthesized, differing in the proportions of the high and low molecular weight components, were used for solid state drawing. During solid state drawing at 137 °C, orientation of the high molecular weight chains was found to be influenced by the presence of the low molecular weight matrix. This was confirmed by a strong dependence of the mechanical properties of bimodal PE on the amount of high molecular weight component. Finally, as part of the investigations on polymerization catalysis, several experiments were carried out involving the co-immobilization of [PhC(NSiMe3)2]VCl2(THF)2 and [2,6-{(2-chloro- 4,6-dimethylphenyl)-N=C(Me)}2C5H3N]FeCl2 on a MgCl2/AlEtn(OEt)3-n support, which indicated a synergistic effect on the catalyst activity. This effect was not evident in the case of co-immobilized chromium and iron precatalysts. Several possible reasons for this synergistic effect were investigated; however no definitive reason could be found. Nevertheless, two new vanadium complexes, namely [2,6-{(2-chloro-4,6-dimethylphenyl)-N=C(Me)}2C5H3N]VCl3 and [2,6-{(2,4,6-trimethylphenyl)-N=C(Me)}2C5H3N]VCl3, were synthesized during the course of this investigation. These complexes are not active for ethylene polymerization under homogeneous conditions, but were active after immobilization on a MgCl2/AlEtn(OEt)3-n support. [2,6-{(2-chloro-4,6-dimethylphenyl)-N=C(Me)}2C5H3N]VCl3 gave high molecular weight polyethylene with narrow (Schulz-Flory) molecular weight distribution