Smart Nanohybrids of RAFT Polymers and Inorganic Particles
Within the work presented here, the special features of reversible addition fragmentation chain transfer (RAFT) polymerization were exploited to explore pathways for the production of novel nanohybrids composed of polymers and inorganic particles. Surface confined radical polymerizations were carried out employing RAFT agents tethered to silica surfaces via methoxysilyl moieties located at the end of their stabilizing Z groups. The RAFT agents were adaptively designed to meet the special demands imposed by different types of silica substrates. A doubly anchored RAFT agent led to polymer loops on the surface of fumed silica through a polymerization mechanism in which the propagating radicals as well as the mediating groups remain attached to the surface. By the use of a RAFT agent with an anchor group only capable of forming a single covalent bond (rather than three as for the first RAFT agent) the crosslinking of colloidally dispersed carrier particles of silica 50nm) during the immobilization process could successfully be prevented. Poly(N isopropylacrylamide) (pNIPAm) samples with distinct molar masses were prepared in the presence of a trithiocarbonate RAFT agent. By analyzing aqueous solutions of these multiresponsive polymers in an optical autoclave cell up to pressures of 3 000bar and down to temperatures of 20 degC, a completely new aspect of their phase behavior was found. It was demonstrated that the addition of several organic cononsolvents in low concentrations leads to an increasing solubility of the polymer in the low temperature/high pressure region, while, on the contrary, the solvents reduce the solubility of the polymer and therefore its cloud temperature at atmospheric pressure. It was found that the phase separation behavior is gradually and completely inverted, regarding both its pressureand temperature dependence, when the mole fraction of added ethanol is increased over the whole cononsolvency range. As a special case, at a mole fraction of x 0.23, the system clears up independent of temperature at a constant pressure of about 1 000 bar. In terms of solvent solvent or solvent polymer interactions, these findings support the assumption of substantially different cloudingmechanisms for the high temperature and the low temperature regions. The pNIPAm samples were grafted to gold nanoparticles (AuNPs, 14 nm) obtained from the reduction of gold ions with sodium citrate. In a survey of different liquid chemical AuNP synthesis methods, these particles had been found