Mapping structural gradients in isotactic polypropylene using scanning wide-angle X-ray scattering

The internal microstructure of injection-molded dogbone specimens of talc-filled isotactic polypropylene has been investigated using a home laboratory scanning wide-angle X-ray scattering setup, yielding two-dimensional maps of molecular orientation and ordering for cross-sections parallel and perpendicular to the flow direction with unprecedented details. In addition to demonstrating a high degree of alignment of both the iPP b-axis and the talc c-axis parallel to the sample wall normals, thus corroborating earlier reports, the synthesized maps also yield new insights into the molecular organization. We find an asymmetry in the orientation maps, which we ascribe to a lateral temperature gradient across the mold during solidification. This interpretation is supported by numerical simulations of heat transport during the solidification following material flow, leading to orientation maps in full qualitative agreement with the experimental data. Recent developments in X-ray scattering equipment, in particular fast-readout low-noise area detectors, have facilitated the present experiments, which demonstrate the usability of raster scanning for advanced polymer characterization.