Synthesis, Characterization, Thermomechanical and Rheological Properties of Long Chain Branched Metallocene Polyolefins

Abstract

<p>The objectives of this research were to prepare model long chain branched polyolefin samples with controlled molecular weights and chain structural properties, and to elucidate the effects of long chain branching on the rheological and mechanical properties of polyethylene and polypropylene using a variety of synthesis and characterization techniques. Significant long chain branch (LCB) formation was found by ¹³C NMR measurements in polyethylene (PE) samples produced using the homogeneous catalyst bis(cyclopentadienyl) zirconium dichloride in a semi-batch slurry polymerization. Enhanced levels of LCB were attributed to the in-situ reaction of ethylene macromonomer and the encapsulation of active centres by precipitated polymer chains. PE samples with controlled levels of LCB were synthesized using a two-stage polymerization process. Narrowly dispersed poly( ethylene-co-propylene) (EPR) macromonomer was first produced in a continuous stirred tank reactor (CSTR). The macromonomer was subsequently copolymerized with ethylene in a semi-batch reactor using the constrained geometry catalyst (CGC-Ti). The two-stage reaction process was also employed to synthesize polypropylene (PP) with EPR long chain branches using the catalyst system rac-dimethylsilylenebis(2-ethylbenz[e]indenyl)zirconium dichloride / MMAO. It was found that the stoichiometry of the macromonomer could be used to efficiently control the long chain branch frequency (LCBF) of the copolymers without greatly influencing the copolymer molecular weight. It was also possible to control the long chain branch length by varying the molecular weight of the macromonomer. The rheological properties of various long chain branched polyolefins were measured. These polymers exhibited significantly higher zero shear viscosities (110) and displayed greater shear thinning than linear polymers with similar molecular weights. It was observed that the branch MN had to be greater than 7000 g/mol in order to form sufficient entanglements to significantly influence the rheological responses. The 110 was found to be a sensitive indicator of branch frequency and branch length. Increasing the branch MN led to enhanced values of 110, improved shear-thinning, and elevated flow activation energies. Long chain branching played a significant role in the dynamic mechanical behaviour of polyolefins. Increasing the frequency of branching increased the stiffness of polyethylene, as reflected by the storage modulus. Long chain branching also served to enhance the damping or energy dissipation of PE, shown by increased values of the loss modulus. The dynamic mechanical behaviour of polypropylenes with EPR long chain branches was also characterized. There appeared to be a critical EPR branch length at a MN of approximately 6000 g/mol. When the branch length was below this critical MN, a homogenous system resulted. If this critical MN was exceeded, a two-phase system developed, with fine rubbery domains dispersed in a PP matrix. When a two-phase system developed, it enhanced the loss modulus of the copolymer in a manner similar to impact modified PP.</p>