Moderne Festkörper-NMR an Funktionspolymeren (2004)

In this work, three often used polymers and composite materials from different areas of polymer applications, i. e. natural rubber, poly(dimethylsiloxane) (PDMS) grafted onto hydrophilic silica, and clay-polyamide nanocomposites, were investigated with the use of modern solid-state NMR techniques regarding their microscopic structure. Besides the multinuclear spectroscopy for structure assignment that is well known from the use of NMR in chemistry, within the scope of this work it was possible to correlate the microscopic structure with the macroscopic properties of the polymers. For this, the direct interactions between the magnetic dipoles, that are not reduced to zero by the molecular motion of the polymer chains, were determined by the application of different techniques. This interactions, the so-called residual dipolar coupling, are a measure of the polymer dynamics. In the project on natural rubber, it was possible for the first time to determine the heteronuclear dipolar couplings between 1H and 13C nuclei. This was not possible so far, on the one hand because the heteronuclear dipolar couplings in natural rubber are very small due to the strong molecular motions and are therefore hard to determine, and on the other hand because the 13C nuclei are not very sensitive due to their low natural abundance. These problems were overcome by using a DSX500 spectrometer together mit modern techniques – HeDQ (heteronuclear double quantum), SEDOR (spin-echo double resonance), and REDOR (rotational echo double resonance) – experiments. The determined coupling constants show with all methods a linear dependence with the cross-link density of the elastomers. The extrapolation of the linear fits shows furthermore, that the direct dipolar interactions even without cross-linking are not reduced to zero. This quantitative analysis enabels the rubber industry to determine the cross-link density which is an important parameter for the properties of natural rubber. PDMS chains grafted onto silica, can be separated in two components – an interface and a mobile part. While the DMS units of the interface are bound relatively strongly to the silica, the mobile parts are further away from the silica surface and therefore contain a much higher mobility. These properties are directly visible in their different residual dipolar couplings. The problem in this project was the experimental separation of these different residual dipolar couplings. For this, MQ (multiple quantum) experiments were used as well, in the homonuclear (1H-1H) and the heteronuclear (1H-13C) case. By detecting the MQ-signals at different excitation times, so called MQ buildup curves were obtained, where the position of the maxima in the buildup curves are directly related with the strength of the residual dipolar couplings. Because grafted PDMS contain two components with strongly different residual dipolar couplings, two maxima are obtained at different excitation times. By using the simulation program SIMPSON and fitting with the program Mathematica it was possible for the first time to determine the residual dipolar couplings separately. By investigation of the homonuclear residual dipolar couplings as a function of temperature an additional phase transition of the grafted PDMS in the temperature range between 198 and 225 K was observed by a stepwise change in the coupling constants. Additionally, the effect of different solvents on the molecular mobility of the PDMS chains was observed by measuring the DQ buildup curves. The multinuclear spectroscopy is always a direct and descriptive methode to determine the structure of polymers. In the third system under investigation in this work – clay-polyamide nanocomposites –, it was possible to separate the crystalline and amorphous signals in the 13C spectra because of their different T1 relaxation times despite their similar chemical shifts. By the acquisition of 15N spectra and corresponding fitting of the signals a quantification of the ratio of the crystalline phases in the nanocomposites was possible. With this it could be proved that the addition of clay minerals increases the degree of crystallinity and therere changes the properties of these materials. The determination of the quadrupolar coupling constants of 27Al showed an additional species that probably is due to a structural change on the surface of the clay minerals and is induced by the polyamide.