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

in this work, new copolymers of propylene/1-hexene (PHC) and propylene/1-oc tene (POC) were synthesized by using a highly isospecific metallocene catal yst system based on rac-Me2Si(2-ethyl,4-phenyl,1-indenyl)(2)ZrCl2, in the h omogeneous and heterogeneous forms, activated by methylaluminoxane (MAO). A n investigation about the copolymerization of propylene with 1-hexene and 1 -octene employing this catalyst system illustrates the potential for the ta iloring of propylene/higher alpha -olefin copolymers with controlled therma l and mechanical properties by varying the comonomer concentration in the p olymerization feed. Both catalyst systems showed high activity and produced random copolymers with very low or no detectable crystallinity. It was obs erved that properties such as enthalpy of crystallization (DeltaH(c)), crys tallization temperature (T-c), melting temperature (T-m), glass transition temperature (T-g), storage modulus (E') and density decreased in a linear p attern with increasing comonomer content in the copolymer. That might allow to suggest differential scanning calorimetry (DSC) and dynamic thermal mec hanical analysis (DMTA) as fast and low cost analytical methods to determin e comonomer content in propylene copolymers, in the range of concentration studied, as a less expensive alternative to Solution State Carbon 13 Nuclea r Magnetic Resonance (C-13 NMR). The influence of the short chain branch le ngth was also investigated and it was observed that, compared to 1-hexene, much less 1-octene was necessary to disrupt the crystalline structure and i mpart rubbery behavior to the copolymers. Solid state C-13 NMR analysis pre sented excellent correlation with the results obtained with DMTA. A lowerin g of T-1 rho(H) values was observed for copolymers with higher comonomer co ntent and longer alkyl branch, which parallels the decline of storage modul us (E') and glass transition temperature (T-g), indicating an increase in m aterials' flexibility. (C) 2001 Elsevier Science Ltd. All rights reserved.