In the production of most textile fibres or plastics parts polymer melts are crystallised, and the developing nanostructure from crystalline domains in an amorphous matrix determines the materials properties. Engineers control temperature, composition and other parameters of the melt in order to tailor the product, but the mechanisms behind polymer crystallisation are not yet understood. We have developed a method of real-space visualisation of the domain arrangement from small-angle X-ray scattering (SAXS) patterns. In situ experiments were carried out at beamline ID02. Based on results of the new method we expect to elucidate the mechanisms which govern crystallisation and melting of polymers and their variation upon change of processing parameters.

The method combines modules from various fields of science such as beamline engineering, digital image processing, computer tomography, scattering theory and animation rendering. It extracts the information on the samples nanostructure from two-dimensional (2D) SAXS patterns with fibre symmetry using a topology (r) [crys,ramorph] of phases with distinct densities. The SAXS patterns are collected by a low-noise CCD camera with fast readout that is coupled to an X-ray image intensifier (XRII­FReLoN, cycle time 7 s, exposure time between 0.1 s and 3 s). The exposure is continuously readjusted in order to keep the signal-to-noise ratio constant and high throughout the monitored process. As a result, the collected data in each image exploit the full dynamic range of the SAXS detector. 2D wide-angle X-ray scattering (WAXS) data are collected simultaneously with a second CCD camera that is coupled to a multi-channel plate (MCP-Sensicam CCD detector). Movies resulting from automatic data evaluation show an "edge-enhanced autocorrelation function" z(r) - the autocorrelation of the gradient (r). This "chord distribution function" (CDF) shows peaks where ever there are domain surface contacts between domains in (r) and its displaced ghost as a function of ghost displacement. The movies exhibit the evolution of nanostructure and demonstrate the mechanisms of polymer crystallisation.



Fig. 53: Crystallisation of an oriented polyethylene melt, one minute after quenching to 130°C. Top: Movie frame with four panels: (left column) SAXS, WAXS; (right column) z(r), -z(r). Bottom: Sketch of the nanostructure. Off-meridional peaks (n) identified as entanglement strands and transverse shift (o) in twin-layers. Layer shape of crystals (p) from meridional peak in z(r) and self-correlation triangle in -z(r).


Figure 53 shows one movie frame representing the state 1 minute after quenching of an oriented polyethylene (PE) melt to 130°C. We observe primary, extended lamellae (stage "d" from Figure 54, which shows sketches of the observed phases of nanostructure evolution).



Fig. 54: Principal steps of crystallisation from a highly oriented, quiescent PE melt via a mesophase (a,b,c) coupled to primary (p) and secondary (s) crystal nuclei, extended crystalline layers (d), blocky crystals with longitudinal (e) and lateral (f) correlation.


In Figure 53 the layer character of the crystallites is demonstrated by the fact that the peaks in the CDF are extending in the direction transverse to fibre direction. The small number of peaks shows that only two layers (blue disks in the sketch) are correlated to each other. A transverse offset (p) between the members of the pair of correlated layers is induced by entities of a mesophase.

In conclusion, we found that PE crystallisation was always preceded by a mesophase structure. Based on the observed structure evolution it was suggested that its entities are entanglement-rich and disentangled regions in the melt, respectively. Extended lamellae prevail only at high crystallisation temperatures and become more perfect with time. Nevertheless, the majority of the crystallites formed during the final stages of crystallisation are small, imperfect and unoriented (i.e. "blocky" crystallites, Figure 54e,f). They are situated in the centre of free gaps so that correlations among crystallites are increased.

Principal Publication and Authors
N. Stribeck (a), P. Bösecke (b), R. Bayer (c), and A. Almendarez Camarillo (a) Progr. Colloid Polym. Sci., (2005) in print
(a) Institute TMC, University of Hamburg (Germany)
(b) ESRF
(c) Institute of Mater. Science, GH Kassel (Germany)