Rotator phases and twist solitons in polyethylene and n-alkanes
- Event time: 1:00pm
- Event date: 5th May 2011
- Speaker: Scott Milner (The Pennsylvania State University)
- Location: Room 2511, James Clerk Maxwell Building (JCMB) James Clerk Maxwell Building Peter Guthrie Tait Road Edinburgh EH9 3FD GB
Polyethylene (PE) and normal alkanes display multiple ordered phases --- a conventional crystal, and two ''rotator'' phases RI and RII, in which chains are rotationally disordered about their axes. Experimental evidence suggests that the RII phase serves as a metastable intermediate in the nucleation of PE. We have computed the nucleation barrier for RII and direct crystal nuclei, combining theory and data for the bulk and surface free energies of the two structures, to see which barrier is lower. To deepen our understanding of the structure and dynamics of these multiple ordered phases, we performed atomistic MD simulations on C23 n-alkanes. This led us to focus on how such molecules rotate, by diffusive transport of twist solitons along the molecule. These motions occur even in crystalline PE, and play a key role in plastic deformation. Combining simulations and analytical theory, we determined the formation energy and mobility of twist solitons in the crystal and rotator phases, with results in good agreement with NMR experiments and MD observations.
This is a weekly series of informal talks given primarily by members of the soft condensed matter and statistical mechanics groups, but is also open to members of other groups and external visitors. The aim of the series is to promote discussion and learning of various topics at a level suitable to the broad background of the group. Everyone is welcome to attend..