A study on solid-state drawn fibers of polyethylene by confocal Raman microspectrometry: evaluation of the orientation profiles of amorphous and crystalline phases across the fiber section

Abstract

Polarized Raman macrospectrometry and confocal Raman microspectrometry have been used to evaluate the uniaxial draw of polyethylene (PE) at a draw ratio of ∼6. The draw was conducted by solid-state extrusion. The PE samples tested were synthesized to different molecular weights by slurry and gas phase processes at different temperatures. Raman spectra offer an important evaluation of draw, with bands identifiable for each: the amorphous, crystalline and interphase regions of the morphology. Such an interphase is preferentially located at the crystallite fold surface boundary and it has been associated with the presence of chain loops and entangled chain segments in this region. Orientations of the different phases are estimated by measuring the depolarization factors of the corresponding bands. Further, by confocal Raman microspectrometry, profiles of orientation across the fiber have been identified, by focusing the laser beam at different depths in the drawn PE. It was found that the crystalline component orients along the draw direction, while, across the fiber section, the interphase and the liquid-like amorphous component display preferred orientation normal to the draw direction. Correlations among orientation profiles, synthesis conditions and reactor powder ductility are discussed. Orientation profiles have been compared to the deformation profiles obtained by the displacement of pre-imprinted ink marks on the drawn fibers. A possible interpretation of the orientation profiles is attempted in terms of the relevant modes of deformation and alignment of chain segments in the different phases. The Raman results, in combination with wide angle X-ray analysis reveal steps in the drawn process that have implications for understanding the draw of PE and possibly for the general process of ‘necking’ on draw of semicrystalline thermoplastics.