NSLS-II Seminar

Abstract:

Scanning transmission x-ray microscopy (STXM) has been used to establish a correlation of Near Edge X-ray Absorption Fine Structure (NEXAFS) spectra of crystalline n-tetracontane (C40H82) with its structural anisotropy in orthorhombic (001) plane. Two subcomponent peaks of the spectral doublet near 287.5eV reverse their intensity ratio when the orientation of polarization vector of incident light swaps from along a axis to b axis [1]. Such an intra-plane linear dichroism complements prior known linear dichroism along c axis [2]; together they can be used to determine three dimensional orientations for linear alkane chains or ordered C-C backbone chains in semi-crystalline polyethylene (PE).

This new information allows us to understand details of the growth orientation of polyethylene (PE) in ultrathin films. In linear low density PE (LLDPE) films, STXM shows large dichroic signals between various sample features for photon energies of ~287.4 eV and ~294 eV, irrespective of film thickness. This suggests that in these thin films, the average C-C bonds are in the plane of the thin film with predominantly edge-on lamellae orientation. In very thin films, the averaged NEXAFS spectrum switches from a slight dominance of b-axis signal to one of slight a-axis signal dominance. This suggests that the interfacial constraints alter the average orientations of crystallites. Linear medium density PE (LMDPE) thin films showed a transition from edge-on to flat-on lamellae (C-C backbone parallel to the surface normal) in films thinner than 30 nm [3].


References:

1. Y. Zou, T. Araki, G. Appel, A.L.D. Kilcoyne, H. Ade, Chem. Phys. Lett. 430, 287 (2006)
2. J.X. Fu, S.G. Urquhart, J. Phys. Chem. A, 109, 11724 (2005); T. Ohta, K. Seki, R. Yokoyama, I. Morisada, and K. Edamatsu, Physica Scripta 41, 150 (1990)
3. Y. Wang, M. Rafailovich, J. Sokolov, D. Gersappe, T. Araki, Y. Zou, A.D.L. Kilcoyne, H. Ade, G. Marom, A. Lustiger, Phys. Rev. Lett. 96, 028303 (2006)