Influence of comonomer content and short branch length on the physical properties of metallocene propylene copolymers

Abstract

In this work, new copolymers of propylene/1-hexene (PHC) and propylene/1-octene (POC) were synthesized by using a highly isospecific metallocene catalyst system based on rac-Me 2Si(2-ethyl,4-phenyl,1-indenyl) 2ZrCl 2, in the homogeneous and heterogeneous forms, activated by methylaluminoxane (MAO). An investigation about the copolymerization of propylene with 1-hexene and 1-octene employing this catalyst system illustrates the potential for the tailoring of propylene/higher α-olefin copolymers with controlled thermal and mechanical properties by varying the comonomer concentration in the polymerization feed. Both catalyst systems showed high activity and produced random copolymers with very low or no detectable crystallinity. It was observed that properties such as enthalpy of crystallization (Δ H c), crystallization temperature ( T c), melting temperature ( T m), glass transition temperature ( T g), storage modulus ( E′) and density decreased in a linear pattern with increasing comonomer content in the copolymer. That might allow to suggest differential scanning calorimetry (DSC) and dynamic thermal mechanical analysis (DMTA) as fast and low cost analytical methods to determine comonomer content in propylene copolymers, in the range of concentration studied, as a less expensive alternative to Solution State Carbon 13 Nuclear Magnetic Resonance ( 13C NMR). The influence of the short chain branch length was also investigated and it was observed that, compared to 1-hexene, much less 1-octene was necessary to disrupt the crystalline structure and impart rubbery behavior to the copolymers. Solid state 13C NMR analysis presented excellent correlation with the results obtained with DMTA. A lowering of T 1 ρ H values was observed for copolymers with higher comonomer content and longer alkyl branch, which parallels the decline of storage modulus ( E′) and glass transition temperature ( T g), indicating an increase in materials' flexibility.