Simulation of Polymer Nanocomposites

Polymers at solid surfaces are of general technological interest, be it lacquers, or adhesives, or surface modifiers which improve water repellence and corrosion protection, to mention a few applications. A closely related task that received increasing interest in recent years is the modification of inorganic particles with organic polymer layers leading to well defined hybrid nanoparticles with tunable properties. Polymers are either attached to the particle (`grafting to` approach) or grown from the surface (`grafting from` techniques) the latter allowing for higher grafting densities. Among other techniques a rather new and most promising one is the RAFT (reversible addition fragmentation chain transfer) polymerization where the propagating species (polymer radicals) are in equilibrium with the dormant polymeric RAFT agents which may be fixed at the particle. Most recently, methods have been developed which allow not only the preparation of surface-anchored chains (`tails`, `trails`) but also surface-confined loops opening a new class of hybrid material. The proposed project aims at a systematic investigation of such hybrid polymer nanoparticles, i.e., at properties like shape, size and orientation of chains and loops attached to solid surfaces, as well as on the investigation of shielding effects appearing during the polymerization process, at least for stabilization reactions at the surface. The use of computer simulation methods offers a highly efficient route for both, the investigation of the properties of the final products (material) as well as for the investigation of kinetic aspects of the preceeding polymerization process: not only global chain properties may be studied on a molecular scale but also details of interaction between segments located at distinct positions within the molecules depending on the chemical environment. For the simulation of the products we decided on Dissipative Particle Dynamics, an off-lattice molecular dynamics method which proved to be most promising for the evaluation of static and dynamic properties. In addition, shielding of reactive segments is accessible in a fully dynamic way. This latter results should be supported by an independent technique based on a combination of lattice based Monte Carlo simulations and exact enumeration as already successfully applied to investigate shielding effects in Z-RAFT star polymerization.

The application of polymers layers upon solid surfaces is of great technological interest to modify surface properties. A detailed understanding of polymer size, shape and orientation is necessary to predict properties of such composite materials. Polymer layers may be either attached in their full length to the surface grafting to-approach or grown from the surface grafting from-approach. Choosing the second approach, usually allowing higher grafting densities, it is necessary to understand the influence of the surface on the polymerization process. Computer simulations are an excellent tool to study polymer layers close to solid surfaces on a molecular level, providing insights inaccessible to the experimental approach. By using two very different simulation approaches, Monte Carlo and Dissipative Particle Dynamics (DPD), a large variety of systems could be simulated, gaining for example insight into polymer length dependencies, concentration dependencies and solvent quality dependencies. Following an earlier FWF-project (P20124 Eigenschaften sternförmig verknüpfter Polymere) properties of star-shaped polymers close to surfaces have been evaluated, especially understanding the difference in size and shape between anchored polymers and non- anchored ones. In analogy to the excluded volume between any two particles, an excluded distance between a particles and a surface has been introduced and its properties for polymers have been calculated. It has been shown, that the excluded distance scales with the length of the polymer chain with the same scaling exponent as measures of size of the polymer itself, e.g. radius of gyration. Reactions of polymers close to surfaces are especially important if any grafting from approach is chosen to create the polymer layer. Reactive sides may be shielded by the surrounding polymer itself, but also by the surface. In the special case of surface initiated Reversible Addition-Fragmentation chain Transfer (si-RAFT-) polymerisation, a controlled form of radical polymerisation, contact probabilities between segments on an anchoring polymer and segments of free chains are of interest. The dependency of these probabilities on chain length, solvent quality, surface coverage and bulk density has been calculated. In the diluted case, they follow a scaling law regarding the chain length with an exponent depending on relative position along both polymer chains. An increasing bulk density lowers the shielding of reactive sites, while an increasing surface coverage effectively hides reactive sides on anchored polymers.