Untersuchungen von polymeren Netzwerken mit Methoden der Rasterkraftmikroskopie

Abstract

A suitable technique for scrutinizing polymer networks is given by atomic force microscopy AFM. This sophisticated method enables not only imaging of structures but also nanomanipulation and detection of material properties on the nanometer scale. Several operating modi are predestinated for nondestructive detections under different ambient conditions. One of the benefits of atomic force microscopy is the simple specimen preparation.The detection, measurement and nanomanipulation of polymer networks is the main topic of this PhD thesis. Within the scope of this work different polymers have been analyzed. The influence of different sample preparations on the structures of the polymers were discussed in detail. Absolutely essential hardware improvements for detection and nanomanipulation of the network structure of the different polymers are a further topic of this work. A lot of comparative measurements for the interpretation of the network-like structure were made with the custom built atomic force microscope. After swelling in toluene and subsequent spincoating the bulkpolymer shows a network-like structure on appropriate surfaces. Droplet-like structures of the swollen gel were found at the interconnects of the network strands. The swollen droplet structure consists of the same polyethylene. The only difference between the network strains and the droplets is the distribution of the molecular weight. The strands consist of long molecule strains with a higher molecular weight and the droplets consist of shorter molecule strains swollen with toluene. At room temperature the polymers show a nonstationary two-phase system. The dropletlike structures were built of shorter molecules embedded in highly swollen amorphous gel with a high viscosity. Organic swelling agents were mostly toluene, seldom hexane and heptane. The documented evidence of conformity of toluene in the droplet-like structures were shown with raman spectroscopy. The network-like structures consist of some parallel long single molecule strains with few y-branchings inside and outside the droplet-like structures. Furthermore some molecule strains pass the droplet-like structures undisturbed, in other droplets physical entanglements of the molecule strains were detected. After heating and evaporation of the swelling agent the polymer samples show a collapse of the droplet-like structures with a dramatic decreasing of the original droplet high. Simultaneously an increase of the droplet numbers and a shrinking of the average contact area to the substrate were detected. Network models for the interpretation of the prepared structures were constructed. Afterwards calculations about the number of single molecule strains in a network strain and the forces in the network were evaluated. Different pulling and nanomanipulation experiments were done under ambient conditions in air and in liquid. The visible changes after the nanomanipulations were detected with the custom built atomic force microscope and simultaneously with an improved optical microscope. Appropriate modes of operation as well as advantages and disadvantages for the measurements of the very soft samples were discussed in detail. Comparative measurements of the network-like structure were analyzed with different electron microscopes (SEM, TEM). Wide-angle X-ray scattering (WAXS) for the polymer characterisation was used, but the results of the different strechted polymers were unfortunately ambiguous. An outlook for further applications of the different operation modes and nanomanipulations, by a simultaneous force detection of the nanomanipulations and optical real time imaging with best possible resolution will be discussed.