1998-12-11
15:00 at CAB B9

Computer Simulation of Structure and Dynamics of Ethylene-alpha-olefin

Eduard F. Oleynik

Institute of Chemical Physics, Russian Academy of Sciences

Copolymers of ethylene with alpha-olefins recently have become very popular materials. The length of branches and their distribution along the chain seriously influence product properties. Recent success in the field of metallocene catalysts permits to regulate amount and distribution of the branches in copolymer macromolecules, and the properties as a result. We model structure and properties of ethylene/alpha-olefin copolymers. We started by using the following general approach. The first step - to model the ideal crystal of polymethylene chains, to compare the structural features of the computer crystal with existing experimental data, and, if they agree, to make the second step. The second step - to replace H-atoms in the chains of the perfect crystal by side chains of desirable length and chemical structure. During this step, one can regulate the fraction of the substituted alpha- olefin, its location in the chain (iso-, syndio-, head-to head, tail-to-tail, etc.) and trace the changes in structure, chain conformations, dynamics, and also check an "amorphization" process in the initial perfect crystal with the degree of substitution and location of substituents in the chains. In this presentation, I will concentrate on: The ideal crystal of C-50 and E/P copolymers with up to 40% CH3 groups as substituents. We have performed molecular dynamic calculations. Structural changes of the C-50 crystal with temperature will be described. Prior to melting, the originally orthorhombic structure of the C-50 crystal is transformed to a hexagonal one. The molecular mobility in the hexagonal structure will be discussed. Then, structural changes in the crystal as a function of the concentration of CH3 groups will be analyzed at a constant temperature. A comparison of the behavior of chains containing methylene and statistically CH3-substituted chains will be presented.