Uniaxially oriented polypropylene (PP) is molten in the synchrotron beam and crystallized from the quiescent melt, keeping its orientation in order to study the mechanisms of its crystallization. We document the different nanostructures observed as a function of melt-annealing temperature, undercooling, and time. In order to obtain a melt that crystallizes with high preferential orientation again, a melt-annealing temperature between 170 and 176 °C is chosen. Isothermal crystallization at 155 °C results in slow formation of (primary) lamellae placed at random. As the crystallization temperature is decreased (150, 145, and 140 °C), more and more secondary crystallites are observed which develop from a block mesostructure according to Strobl's mechanism. During the isothermal phase the blocks are fusing more or less to form imperfect lamellae. The structure evolution observed in the time-resolved small-angle X-ray scattering (SAXS) data during crystallization and remelting facilitates discrimination between the block structure and another frequently discussed morphology, Keller's cross-hatched structure. While after all our quiescent crystallization experiments most of the crystallites are blocks or incompletely fused lamellae, the hard-elastic precursor material which has been made under extreme gradients of temperature and pressure exhibits the melting of homogeneous and extended lamellae. As we apply a steep temperature gradient (−100 °C/min) to our melt in a nonisothermal crystallization experiment, we initially observe the formation of homogeneous and extended lamellae as well.

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.

Summary: In a series of papers the crystallization of a commercial polyethylene (PE) material has been studied by in situ ultra small‐angle X‐ray scattering (USAXS) and it has been found that the placement of crystallites is predominantly a random car parking process, dealt with in the field of random sequential adsorption. Here we investigate the secondary effect, the generation of order from the chaos, and characterize how lamellae agglomerate to form clusters. Owing to the chaotic principle, clusters are of different size and thus the nanostructure is, in general, heterogeneous. During isothermal crystallization, we only find order inside sparse clusters of lamellae. Globally the nanostructure resembles a random blend of these clusters. A geometric relation couples the number fraction of lamellae to the size of the cluster they belong to. More longitudinal order is achieved if the material is subsequently quenched. Amorphous gaps of favorable width act as nests in which thin crystals are preferentially generated (harmonic completion). Together with their neighboring parents they imprint excess order. During continuous non‐isothermal crystallization we have observed that clusters with several members are selectively built from thinner crystals. Order is growing in the late state of crystallization. This violation of the common assumption of homogeneity makes the classic notion of a distorted lattice (convolution polynomial, paracrystal) become restricted to a subset of the global crystallite thickness distribution.A weak (a) and a strong (b) order generating mechanism found during PE crystallization.magnified imageA weak (a) and a strong (b) order generating mechanism found during PE crystallization.

During cooling from the quiescent melt of a highly oriented polyethylene rod, highly oriented proto-lamellae are formed first, which are not crystalline. This is shown in scattering data which are recorded on two-dimensional detectors with a cycle time of 1 s and an exposure of 0.1 s. In the experiments small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) are registered simultaneously during the first 3 min after quenching to a crystallization temperature. A non-uniform thickness between 20 and 100 nm is characteristic for the ensemble of proto-lamellae. During the first minute of isothermal treatment the number of proto-lamellae slowly increases without a change of the thickness distribution. As crystallization starts, the crystallites are not oriented in contrast to the proto-lamellae. During crystallization the layer thickness distribution narrows. The number of lamellae rapidly increases during the following 2 min of isothermal treatment (at 128 degrees C and 124 degrees C). The results are obtained by interpretation of the WAXS and of the multidimensional chord distribution function (CDF), a model-free real-space visualization of the nanostructure information contained in the SAXS data.