Summary: Highly oriented high‐pressure injection‐molded (HPIM) rods from polyethylene (PE) were heated until the discrete small‐angle X‐ray scattering (SAXS) had vanished. Thereafter, non‐isothermal and isothermal crystallization was investigated in situ by means of ultra small‐angle X‐ray scattering (USAXS). The orientation of the crystallites could be controlled by choice of the melt annealing temperature (shish‐kebab model: memory or self‐nucleation effect caused by stable shishs). Both the scattering patterns and the multidimensional chord distribution function (CDF) were interpreted. A three‐stage model of crystallization was also developed. This model comprises row structure nucleation, the almost statistical insertion of extended lamellae and finally the insertion of blocky crystallites. It was found that the nanostructure evolution in the isotropic fraction of the material was the same as in the highly oriented one. The lateral extension of the lamellae was largest during isothermal crystallization. The correlation among domains was increased by non‐isothermal crystallization. The shishs in the core of the HPIM rod appeared less stable than those in the shell. Lobe‐shaped reflections observed during and after quenching were not due to an orientation distribution of layer stacks, but reflected a correlation between long period and lateral extension of crystallites. During quenching, a lateral modulation of the layer peaks in the CDF grew stronger and showed the arrangement of block‐shaped crystals proposed by Strobl to be the precursors of lamellae. The thin crystals formed during rapid cooling were built from a central block surrounded by one or two rings of satellites. The long period observed in the scattering pattern during quenching is due to correlations among crystalline blocks in a chain, and not from correlations among lamellae.USAXS scattering patterns from isothermal, oriented crystallization of HPIM‐PE material (bottom row: during final quenching after 30 min at 127 °C).magnified imageUSAXS scattering patterns from isothermal, oriented crystallization of HPIM‐PE material (bottom row: during final quenching after 30 min at 127 °C).

Summary: Small-angle X-ray scattering (SAXS) microtomography (micro-CT) resolves structure variation in an anisotropic polyethylene (PE) gradient material with fiber symmetry. 4 900 reconstructed SAXS patterns describe the nanostructure as a function of volume element position in the scanned fiber cross-section. Reconstruction errors were observed. Their first-order effect was eliminated by transformation of the SAXS into a multidimensional chord distribution function (CDF). 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.Original SAXS patterns (pseudocolor and 3D plot) from different voxels obtained by micro-CT reconstruction from measured SAXS projection patterns of a PE rod.

The microhardness of a series of melt crystallized samples of linear polyehtylene was investigated in a wide range of molecular weights. The x-ray long period was analyzed to study the variation of the hardness-derived constant b as a function of molecular weight (Mq). It is pointed out that b offers a measure of the hardness depression due to the finite thickness of the lamellar crystals. The data obtained show that the increase and final leveling-off (for Mq 200 000) of b with M, parallels the concurrent increase of the surface free energy, as derived from DSC experiments. Results are discussed using the concept og chain folded lamellae as thermodynamically stable non-homogeneous microphases. Comparison of experimental and calculated data supports the view that the number of molecular entanglements, segregated onto the defective surface boundary of the heterogeneous crystals influence the shearing mechanism within the "mesocrystals" and thereby control the yield behavior of the material.

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