Journal of Polymer Science Part A: Polymer Chemistry, Vol.44, No.5, 1600-1612, 2006
Thermal and mechanical analysis of metallocene-catalyzed ethene-alpha-olefin copolymers: The influence of the length and number of the crystallizing side chains
Copolymers of ethene and 1-octene, 1-dodecene, 1-octadecene, and 1-hexacosene were carried out with [Ph2C(2,7-di-(tert)BuFlu)(Cp)]ZrCl2/methylalumoxane as a catalyst to obtain short-chain branched polyethylenes with branch lengths of 6-26 carbon atoms. This catalyst provided high activity and a very good comonomer and hydrogen response. In this study, the influence of the length and number of the side chains on the mechanical properties of the materials was investigated. The crystalline methylene sequence lengths of the copolymers and lamellar thicknesses were calculated after the application of a differential scanning calorimetry/successive self-annealing separation technique. By dynamic mechanical analysis, the storage modulus as an indicator of the stiffness and the loss modulus as a measure of the effect of branching on the alpha and beta relaxations were studied. The results were related to the measurements of the polymer density and tensile strength to determine the effect of longer side chains on the material properties. The hexacosene copolymers had side chains of 24 carbons and remarkable material properties very different from those of conventional linear low-density polyethylenes. The side chains of these copolymers crystallized with one another and not only parallel to the backbone lamellar layer, depending on the hexacosene concentration in the copolymer. The side chains crystallized even at low hexacosene concentrations in the copolymer. A transfer of these results to 16 carbons side chains in ethene-octadecene copolymers was also possible. (c) 2006 Wiley Periodicals, Inc.
Keywords:differential scanning calorimetry (DSC);fractionation of polymers;mechanical properties;metallocene catalysts;microstructure;polyethylene (PE)