Glasses and amorphous solids in general, are widely used in various fields of life and material science. Amorphous drugs, for instance, usually possess a higher bioavailability than their crystalline polymorphs. Thus, when stored below the glass transition temperature (Tg), a crystallisation would be economically harmful, but occurs occasionally. Otherwise the selective and exclusive crystallisation of specific polymorphs with special properties is difficult to handle due to the effect of metastability. Directing the nucleation process, and with it, further crystallisation below Tg is the key issue to tackle such problems. Furthermore we are interested in understanding how molecular additives influence the crystallisation process of semicrystalline polymers.
Using special kinds of additives, the optical or mechanical properties of the polymer can be enhanced dramatically. The aggregation mechanisms of polymers on the crystalline surface of the additive are still unknown. Nevertheless, they are essential for developing polymers with specific properties. By using selected model compounds we study all these phenomena, namely diffusionless crystal growth in glasses, polymorphism of molecular crystals and interactions of polymers with self-assembled crystalline nucleation agents. For all these systems the crucial part of unravelling the structural properties and dynamical processes of subcritical nuclei is addressed with the powerful approach of combining the synthesis of isotopically enriched materials with classical chemical analytics like X-ray powder diffraction or DSC/DTA as well as ambitious multi-dimensional solid-state NMR techniques. This approach is supported by quantum chemical calculations extracting, for example, anisotropic chemical shift properties needed for simulations of one- and more-dimensional NMR spectra.
Right: Static 13C spin-diffusion spectra of two crystalline compounds with different correlations of the CSA-tensor.
Middle: Numerous red, orange and yellow polymorphs of the organic compound named ROY (molecule on top).
Bottom: X-ray powder pattern of a highly ordered nucleation agent for semicrystalline polymers.