Date: Monday 10th of June, 13:00.
Location: DCB1104 (David Chiddick Building, #19).

‘Standing up and peeling off of nanosized shish-kebab structures formed in melt-spun high density polyethylene and nylon 6/clay nanohybrid fibres’

by Wiebke F. C. Sager, Peter Grünberg Institute, Forschungszentrum Jülich, Germany.

Abstract:

Shish-kebab structures are known to develop after cessation of flow from semicrystalline polymer melts and/or upon uniaxial drawing in the processing of polymer films and fibres. The prenucleating shish is commonly assumed to align with the flow direction inducing orthogonal crystallisation of kebab-forming folded chain lamellae. Here we combine results from transmission electron microscopy (TEM), wide angle X-ray diffraction (WAXD) and/or small angle X-ray scattering (SAXS) to follow the microstructural evolution of the fibres upon drawing. For the high density polyethylene (HDPE) fibres we evidence for the first time a horizontal alignment of the main part of the shish-kebab nanostructures and therewith a meridional crystallographic a-axis alignment at a moderate extrusion rate. A crystalline a-axis orientation has so far only been reported for helically twisted kebabs, whereby the a- and c- (chain direction) axes rotate around the b-axis (chain fold direction). In our case flat shish-kebabs align perpendicular to the flow direction due to a diverging extensional flow at the end of the spinneret as predicted by Trebbin et al [1]. Increasing winding speeds induce, due to the unidirectional pulling on the surrounding polymer chains, a gradual azimuthal reorientation and finally an axial up-right standing of the shish-kebabs (axial and meridional c-axis alignment). Crystal dimensions and orientation distributions determined from an extended WAXD analysis are used to model the two-dimensional SAXS intensities. The structural evolution of the polyamide 6 (PA6)/clay nanohybrid fibres depends significantly on the amount of organically modified nanoplatelets (NCPs), that are exfoliated and axial aligned in the as-spun fibre. NCPs impede formation of shish-like γ PA6 crystallites occurring in the unmodified fibres, but induce crystallisation of folded chain γ PA6 lamellae on their aligning surface sites resulting in orthogonal growth perpendicular to the fibre axis and the formation of nanohyrid shish-kebab structures. Mechanical drawing causes delamination of the formed lamellae and shish nucleation in the bulk. This leads to an unprecedented microstructural transition from nanolamellar (kebab-like) to nanofibrillar (shish-like) structures, that is held up with increasing NCP loadings. The total crystallinity can be described within the Avrami model by a 2D decompostion of the nanolamellae followed by bulk crystallisation of the nanofibrils. The mor-phological transition observed precedes the strain induced structural transformation from the γ to the α PA6 polymorph that is retarded in the equatorial plane in the NCP presence. The present findings have a strong impact on establishing structure-property relationships and in designing advanced materials with polymeric or nanohybrid shish-kebab structures.

[1] M. Trebbin et al., PNAS 2013, 110, 6706.