Conformations of single polymer chains on surfaces

Frontali et al. [137] showed in 1979 that it is possible to characterize the structure of polymers by analyzing high resolution microscopy images. The authors adsorbed DNA molecules onto mica and imaged them using electron microscopy. From the vectorized contours of single chains, they were able to determine both contour and persistence lengths so that the polymers were fully described in the framework of the wormlike-chain (WLC) model. In contrast to the conventional characterization methods for polymer solutions this approach has two important advantages: 1) single molecules instead of an ensemble are investigated, and 2) the results can be compared to the model in a much more direct way. As a disadvantage, high resolution microscopy such as transmission electron microscopy (TEM) or scanning force microscopy (SFM) require the polymers to be adsorbed onto a solid substrate surface which influences the chain conformations. Until now, the method is not well established. In particular, basic knowledge about single molecule conformations on surfaces is missing. Investigations in literature are mainly concerned with adsorption under equilibrium conditions. Often however, the interaction between molecule and substrate is strong so that the equilibration is inhibited and chains remain trapped in the initial conformation determined by the kinetics of the adsorption process. The goal of this thesis was to develop and establish methods for the characterization of single chains of polymers using scanning force microscopy. Therefore, methods for the analysis and procession of SFM-images were developed. This includes a method for the determination of the persistence length which is very efficient and features an analytic expression for the statistical error. Furthermore, a new algorithm for background removal was found which replaces the commonly used flattening. It avoids the misleading “shadows” characteristic for SFM-images and therefore enables elaborate quantitative analysis of height values, e.g. along the contour of an adsorbed polymer molecule. In addition an automated vectorization method has been developed which avoids errors due to the discreteness of the image pixels. Models were developed which allow for the first time a detailed prediction of conformational characteristics in the non-equilibrium. It is shown that two kinds of very regular conformations can appear on strong interacting substrates: sine-like undulations and spiral shaped “tron”-conformations. Undulations have been observed by other groups before but were probably misinterpreted, e.g. as helices. Strongly physisorbed polymers were investigated experimentally for two model systems: charged dendronized polymers on mica and DNA on poly-ornithine layers. For the dendronized polymers conformations were found to be undulated with a period of 13±3 nm. The adsorbed DNA molecules showed “tron” features with a dependence on the ion concentration in agreement to the proposed model. The persistence length of poly(isocyanodipeptides) (PIC) has been determined for the first time. The obtained value of 76±6 nm shows that these polymer molecules are extraordinarily rigid, more rigid than double stranded DNA and probably the most rigid synthetic polymers yet. This behavior is attributed to the helical backbone which is additionally reinforced by a network of side groups connected by hydrogen bonds. The persistence length has also been determined for PICs where the polymerization was catalyzed using acid instead of Ni, leading to a contour length of up to 5.3 μm. Those long chains were not able to equilibrate on the surface, nevertheless the same value for the persistence length was obtained confirming the idea that chains can equilibrate on the local scale of up to 100 nm. The method of single chain nanomanipulation has been developed in our group during the last years. It allows to move molecules in a precise way on the substrate surface. The resulting lateral forces have been used in this work to investigate the mechanical properties of adsorbed molecules. Of special interest was the investigation of dendronized polymers because special mechanical properties can be expected for this new class of molecules. Dendronized polymers carry a regularly branched side group (dendron) at each of the repeat units. The mass of a dendron grows exponentially with the number of branching generations. For heigh generation numbers it can be expected, that the mechanical bending behavior of the chain is strongly influenced by the dendrons and therefore deviates from the usual chain flexibility. Our experiments indicate that a glassy state exists for molecules of generation 3 and 4, in which the molecule no longer behaves as a flexible chain but instead plastically keeps the shape in which it is frozen-in, similar to a macroscopic body. Further experiments show that also a liquid state exists for elevated temperatures and good solvents in which molecules are flexible. The glassy state of a single molecule is a new and unusual property