Investigation of Polymer-Filler Interactions Using Functionalized Nanoparticles

Abstract

Silica and carbon black (CB) particles were successfully functionalized by exploiting the characteristic features of the reversible addition--fragmentation chain transfer (RAFT) polymerization. This enabled both the contribution to a better understanding of the interactions within filled rubber compounds and the improvement of their mechanical properties. Silica nanoparticles were functionalized with styrene butadiene rubber (SBR) via grafting-from and grafting-to approaches using various RAFT agents for the polymerization from the particle surface as well as two different strategies for the grafting of polymer to the surface. The grafting density of surface bound polymer was determined via thermogravimetric analysis (TGA) and elemental analysis (EA), revealing it's depended on the used approach and RAFT agent, respectively. The so-obtained hybrid silica nanoparticles were used as fillers in typical tire compounds and physical tests were conducted to investigate the influence of the functionalization on the interactions within the filled compounds. It was demonstrated that the mechanical properties were improved compared to the use of unfunctionalized or even silanized silica nanoparticles. This was mainly attributed to an increase of polymer-filler interactions through the entanglement of the surface bound polymer with the polymeric matrix. The results showed that the use of functionalized silica nanoparticles is a powerful tool for the adjustment and tuning of the mechanical properties of tire compounds. A different strategy was adopted for the functionalization of CB particles applying a grafting-to approach using copolymers with anchor groups within the polymer side chain. Pyridine, furan and epoxide units were chosen as anchor groups, due to their different binding mechanisms with the CB surface. As material design requires the ability to precisely tune a materials properties, further investigations into the grafting conditions were conducted using methacrylate copolymers and thereby new insights into the binding mechanisms were revealed. It was found that the Diels-Alder reaction of the furan anchor group can only occur up to a certain amount of functional groups at the CB surface. The grafting of the epoxide anchor group via a ring opening reaction is favored by external energy input, i.e. by elevated temperatures. In contrast, the non-covalent grafting of the pyridine unit to the CB surface occurs fast, even at room temperature. Additionally, the grafting-to approach was transferable to butadiene-based copolymers. The grafting density of the respective pyridine anchor group-containing copolymer was comparable to that of the methacrylate copolymer, confirming the applicability of the grafting-to approach with the chosen anchor groups for a variety of monomer classes and materials. Another part of this thesis was the investigation into the kinetics of n-pentyl methacrylate (PnMA) radical polymerization and the determination of the Mark-Houwink coefficients (MHCs) of poly(pentyl methacrylate) (PPnMA). Both have not been reported before, but the precise knowledge of rate coefficients is of key importance for the understanding and the application of radical polymerization processes and the MHCs are essential for the analysis of the molecular weight distribution (MWD) obtained from size-exclusion chromatography (SEC). For the first time, the MHCs were determined based on the principle of universal calibration from a double-log plot of SEC data obtained with poly(methyl methacrylate) calibration in combination with the calculated molecular weights of the PPnMA samples from RAFT polymerization. Propagation rate coefficients kp of PnMA radical polymerization were measured in bulk and in toluene solution over an extended temperature range via pulsed laser polymerization in conjunction with SEC (PLP-SEC). The so-obtained data allows for modeling PnMA polymerization kinetics and product properties at moderate degrees of monomer conversion. The comparison with reported data for several other alkyl methacrylates showed a distinct family behavior of this group of monomers.