AbstractsBiology & Animal Science

Upon crystallization, long polymers fold and form various meta-stable states, differing e.g. in melting temperature. The degree of meta-stability and its variance depend on growth rate but can also measurably change with time after growth, causing older crystalline regions to be more stable than younger ones. Thus, variations in meta-stability can be interpreted as a measure of age. Here, in analogy to archaeology, we “excavated” the older parts by a controlled and patient annealing procedure, and thereby unveiled the initial stages of growth of a polymer crystal. Doing so, geometrically simple, faceted single crystals transformed into periodically branched crystals. This annealing induced change in morphology demonstrates that faceted polymer single crystals consist of a branched framework, which was subsequently filled-up by inserting polymers from the surrounding solution. The characteristic length-scale of the branches could be derived from the general concept of the Mullins-Sekerka instability and varied accordingly with crystallization conditions. Furthermore, we present an approach to precisely control nucleation and crystallization of polyethylene through dewetting-induced molecular orientation. The network of oriented crystalline structures, which we call “circular shish-kebabs”, was induced by dewetting of a thin hot solution film on a solid substrate In contrast to a bulk system, the here formed dewetting-induced shish-kebab structures allowed to calculate the force which was needed to stretch a single shish consisting of 20~100 polymer chains. In order to follow in real time the thickening and melting behavior of extended-chain and folded-chain crystalline structures and to understand the corresponding structural evolution within the individual crystalline shish-kebab structures, quantitative studies were carried out using in-situ and ex-situ atomic force microscopy. In-situ observations showed that the lamellar thickness of kebabs increased with increasing annealing temperature. Interestingly, ex-situ observations showed that shish-kebab structures could re-appear even after annealing the sample for 4h at 200°C followed by a quench to room temperature. The lifetime of a shish depended not only on annealing temperature but also on the cross-sectional area and the corresponding stretching of the shish. The high thermal stability is attributed to either the high degree of alignment of the extended chains or to the high capillary pressure acting on the extended polymer chains linking two molten droplets. The relaxations of the extended polymer chains was slow and thus the chains remember their orientation for a much longer time than the reputation time, the longest relaxation of these polymers in an equilibrated melt. Bei der Kristallisation, falten sich lange Polymere und bilden verschiedene metastabile Zustände, die sich z.B. in der Schmelztemperatur unterscheiden. Der Grad der Meta-Stabilität und seine Varianz hängen von der Wachstumsrate ab, können sich aber auch messbar mit der Zeit nach dem…