Deutsche Tagung f ¨ur Forschung mit ... - SNI-Portal

Weiche Materie Poster: Do., 13:00–15:30 D-P355
Nanostructure Evolution and Nanostructure Gradients in Oriented Polymer
Parts
Norbert Stribeck 1
1 Universität Hamburg, Institut TMC, Bundesstr. 45, 20146 Hamburg
The semicrystalline nanostructure of anisotropic samples made from commercial polymers
is visualised from small-angle X-ray scattering (SAXS) data and compared to
the information gathered by simultaneous two-dimensional wide-angle X-ray scattering
(WAXS). The method is based on a combination of advanced experimental technique
and novel methods of data analysis based on Fourier transformation theory.
To-date manageable is the experimental and analytical effort for samples with fibre
symmetry. From time-resolved experiments at a synchrotron source conclusions on
the mechanisms of structure evolution can be drawn. Spatial resolution is achieved by
tomographic reconstruction starting from a set of 7000 scattering patterns recorded
while scanning a thick sample. As a result information is retrieved on the variation
of nanostructure inside a fibre. Experiments have been performed using commercial
grades of various polymers (PE, UHMWPE, PP, PVDF). Adequately evaluated and
published are studies of PE and UHMWPE materials, which are presented.
Concerning the mechanisms of structure evolution during quiescent crystallisation of
polyethylene we have always found that the formation of crystallites is preceded by
the formation of nanostructure (nanoforming) in the melt. the corresponding domains
exhibit a very broad size distribution. We imagine regions that are discriminated by
their entanglement density. Crystallisation is forcing the corresponding nanodomains to
settle down. The places of settlement are random. Thus the population mechanism is
described by Rényi’s random car parking process. Even if the nanoformed domains
are highly oriented lamellae, the crystallite orientation is isotropic. During secondary
crystallisation the first lamellae can grow perfect and reorientation of crystallites is
observed during this period. This principle is not valid for the nanodomains formed
late. They populate the centres of the remnant white spots and, by doing so, are
responsible for the ordered arrangement and the ultimate long period of the crystallised
material, but their crystallites do not show preferential orientation.
The SAXS microtomography of an injection moulded polyethylene rod demonstrates
the method using a “fibre” which is thick enough for the spatial resolution that
presently is accessible at HASYLAB, Hamburg. It is well-known that many fibres
exhibit a core-shell structure and that the shell exhibits a regular and highly oriented
nanostructure that is completely different from that found in the core. By means of
filtered backprojection the studied material is virtually sliced into many small cubes
(voxels) resulting in set of SAXS patterns related each to one of the voxels. Its analysis
shows oriented lamellae stacks in a shell layer and extended chains in the central core of
the fiber. We document zones of uni- and bimodal structure, variation of long periods,
stack heights and lateral domain extension.