Crystallization Phenomena and Local Structure in Oligomer Thin Films and Polymer Fibers

This thesis explores the structure formation of two different soft material systems: naphthyl end-capped oligothiophenes in thin film configuration and ultra-high molecular weight polyethylene (UHMWPE) fibers. The molecular packing of organic semiconductors in thin films determines their optoelectronic properties.

Although the final state of these films is often well known, the fundamental dynamic processes governing the self-organization of the molecules are not yet fully understood. In the first part of the thesis, by using in situ grazing incidence X-ray scattering, we study the crystallization of naphthyl end-capped oligothiophene thin films in realtime during vacuum deposition.

We find that the thin film packing deviates from the one found in single crystals, and that surface-molecule interactions play a dominant role in the formation of the first few molecular layers of the film. On silicon substrates, these early layers grow in a layer-by-layer fashion with a strained unit cell.

Upon reaching a critical thickness, subsequent layers follow three-dimensional growth and the unit cell is relaxed. On strongly interacting substrates, such as graphene, we observe a change in the molecular orientation, which may be used to control and enhance the optical absorption. These results are complemented by scattering experiments with microfocused X-ray beams, which reveal lateral variations in the film’s microstructure, for example in the proximity of the bottom-contact electrodes in organic field-effect transistors.

Fibers made from ultra-high molecular weight polyethylene have remarkable tensile strength and modulus, owing to their high degree of crystallinity and near perfect alignment of the polyethylene chains along the fiber direction. While these fibers are known to exhibit skin-core structures, in which the structure near the surface deviates from the structure in the core, few attempts have been made to quantify this phenomenon.

In the second part of the thesis, we present a framework based on micro- and nanofocused X-ray scattering techniques supported by polarized Raman spectroscopy for mapping the spatially resolved microstructure and morphology in polymer fibers with 1 µm resolution. We demonstrate its usefulness by showing mapping results from individual gel-spun UHMWPE fibers, whose diameter falls within limits of tens to hundreds of micrometers.

All samples display clear skin-core structure, with an increased degree of orientation of the PE crystallites and increased interlamellar spacing in the skin region. Our findings have importance in both fundamental science and engineering applications.