AbstractsBiology & Animal Science

Pattern formation on single crystals of isotatic polystyrene in thin films

by Hui Zhang

Institution: Universität Freiburg
Department: Mathematik und Physik
Degree: PhD
Year: 2014
Record ID: 1107417
Full text PDF: http://www.freidok.uni-freiburg.de/volltexte/2014/9798/


It is important to study crystallization behavior of polymer thin films in order to understand and control the ultimate properties of materials. In addition, in such quasi-two dimensional systems, the film thickness characterizes the degree of confinement. Transport of molecules to the growing crystal is limited by a diffusion process, having a significant impact on the crystallization process. Experimentally, the crystallization behavior of polymers in thin films exhibits significant differences to that in bulk systems and thus provides new insights into the understanding of the physics of polymer crystallization. For that reason, polymer crystallization in thin films has been extensively investigated in recent years. However, a clear understanding of central fundamental aspects like nucleation or of certain morphological characteristics like correlated stacks of lamellae has still not yet been reached. Crystallizable polymers typically form quasi-two dimensional lamellar crystals of the order of 10 nanometers in thickness. Multiple stacks of uniquely oriented lamellae, which demonstrate excellent registry despite being separated by amorphous fold surfaces, are often observed. These correlations require multiple synchronized, yet unidentified, nucleation events. In search for such a mechanism providing correlations, we examined the physical parameters allowing a primary lamella of i-PS single crystal in a thin film to induce nucleation on the fold surface. A new general concept of “self-induced secondary lamella” was proposed to interpret the multiple stacks of lamellar crystals formed on the fold surface of the underlying crystal. All secondary lamellae were in registry with each other and the underlying lamella, i.e. all polymers within these stacks of crystalline lamellae exhibited an identical molecular orientation. These correlations provide a means for propagating the order and molecular orientation of the underlying crystal into three dimensions. Such crystal organization was shown to be strongly correlated with the morphology of both secondary lamellae and the underlying crystal. The number density (ns) of secondary lamellae was found to be controlled by the area (w2) of the side branch of the underlying primary crystalline lamella. This relation was found to be independent of molecular weight, crystallization temperature and film thickness. Based on these findings, we subsequently proposed a nucleation mechanism based on insertion of polymers into a branched morphology of the underlying primary lamellar crystal. The nucleation of secondary lamellae, which directs the growth of lamellar crystals in three dimensions, significantly influenced the kinetics of crystal growth, consequently also the resulting crystal structures. A non-linear crystal growth behavior, manifested by periodic variations of the depletion zone ahead of the crystal front, was observed and attributed to the nucleation of secondary lamellae. This periodic process led to periodically arranged hexagonal bands within single crystals of…