Use of the RAFT technique as an efficient method to synthesise well defined polymer-clay nanocomposites with improved properties
Thesis (PhD (Chemistry and Polymer Science))--University of Stellenbosch, 2009.
Synthesis and structural characterization of two novel cationic and three new neutral reversible addition–fragmentation chain transfer (RAFT) agents is described. The cationic RAFT agents bear a quaternary ammonium group: N,N-dimethyl-N-(4- (((phenylcarbonothionyl)thio)methyl)benzyl)ethanammonium bromide (PCDBAB) and N-(4-((((dodecylthio)carbonothioyl)thio)methyl)benzyl)-N,N-dimethylethanammonium bromide (DCTBAB). The three neutral RAFT agents synthesized are 1,4- phenylenebis(methylene)dibenzene carbodithioate (PCDBDCP), didodecyl-1,4- phenylenebis(methyllene)bistrithiocarbonate (DCTBTCD) and 11-(((benzylthio)carbonothioyl) thio)undecanoic acid (BCTUA). The self-assembly behaviour in diluted aqueous solutions of the cationic RAFT agents, PCDBAB and DCTBAB, is described. The self-assembly behaviour was promoted by the presence of the thiocarbonyl- thio group on the RAFT agents, in addition to the overall chemical structure of the surfactant that also influence self-assembly. The RAFT agents were used for the bulk or miniemulsion RAFT-mediated controlled free-radical polymerization in the presence of clay to yield polymer–clay nanocomposites (PCNs). Bulk polymerization resulted in PCNs with better control of molar mass and polydispersity index (PDI) values when compared to PCNs prepared by miniemulsion polymerization. In both bulk and miniemulsion polymerizations the molar masses and PDI values were dependent on the amount of clay and RAFT agent present in the system. Free-radical bulk neutral RAFT agent-mediated polymerization resulted in PCNs with predominantly intercalated morphology. This was attributed to radical–radical coupling of the initiator anchored onto the clay galleries on which polymerization took place. On the other hand, when the cationic RAFT agent anchored onto clay, i.e. RAFT-modified clay was used, bulk polymerization resulted in predominantly exfoliated PCNs. However, miniemulsion polymerization carried out in the presence of the RAFT-modified clays resulted in PCNs with a morphology that ranged from partially exfoliated to intercalated morphology, as the clay loading was increased. The changing morphology for miniemulsion-based PCNs was attributed to the decreasing molar mass as the clay loading was increased. The PCNs obtained had enhanced thermo-mechanical properties as a result of the presence of clay. The thermo-mechanical properties depended on the molar mass, PDI, clay loading, and the morphology of the PCNs.