Materials on the basis of polymers, e.g., plastics, are of enormous importance in everyday life. In many applications, polymers do not exist as pure materials, but they are part of a composite material. Such composites offer the possibility to adjust the material properties in a wide range by mixing of appropriate components. For a systematic improvement of the material properties, it is important to know the behavior of the components in the composite, in particular in the vicinity of interfaces. In many cases, interface and bulk behaviors differ substantially. We use NMR experiments and MD simulations to characterize structure and dynamics of polymers in neat and composite materials. In particular, we strive for a fundamental understanding of changes in the vicinity of interfaces.
NMR experiments on polymer electrolytes
For a preparation of polymer electrolytes, it is exploited that a number of polymers can dissolve salts. The resulting mixtures feature fast ion transport. Hence, when employing lithium salts, polymer electrolytes are promising candidates for application in lithium ion rechargeable batteries. We use 7Li and 2H NMR to investigate lithium and polymer dynamics, respectively. The results show that strong couplings of lithium and polymer motions exist. In particular, fast lithium ion transport requires a high flexibility of the polymer matrix. Thus, for a fundamental understanding of charge transport in polymer electrolytes, it is necessary to analyze changes of the polymer dynamics in the presence of ions. In 2H NMR experiments on polymer electrolytes on the basis of poly(propylene oxide), we find that presence of ions leads to a strong slowdown of the polymer dynamics. Furthermore, additional structural disorder, associated with the distribution of the ions in the polymer matrix, is accompanied by very broad distributions of correlation times for the polymer segmental motion, in particular for intermediate salt concentrations. Hence, pronounced dynamical heterogeneities exist.
NMR experiments and MD simulations on polymer-based nanocomposites
Addition of nanoparticles is often a very successful approach to improve the material properties of polymers. For example, the mechanical and electrical properties of polymer electrolytes can usually be enhanced in this way. We perform 2H NMR experiments to study nanocomposite polymer electrolytes on the basis of poly(ethylene oxide) and TiO2 nanoparticles. The results reveal that an enhanced ionic conductivity can be traced back, at least in part, to a reduced crystallinity of poly(ethylene oxide) in the nanocomposite. Our MD simulations on poly(ethylene oxide)-TiO2 nanocomposites show that, in the vicinity of nanoparticles, the polymer forms layers parallel to the interfaces. Thus, the reduced proneness toward crystallization in the nanocomposite may be a consequence of an interference of different ordering tendencies related to interface and crystallization, respectively. Moreover, the simulation data indicate that polymer dynamics is strongly slowed down in the vicinity of TiO2 nanoparticles.