Synthesis and properties of anionically grafted thermoplastic elastomers
Through the use of a sec-butyllithium N,N,N',N'- tetramethylethylene-diamine chelate complex it was possible to anionically graft a conventional unvulcanised rubber, SBR (solprene 1204), with vinyl aromatic monomers such as styrene, o-methylstyrene and vinyltoluene. By choosing suitable solvents and reaction conditions high yields were obtained. Grafting efficiencies, determined from the amounts of homopolymer present in the gel permeation. chromatograms of the products, were found in the 60 to 100& range. Grafting efficiencies for the o-methylstyrene only systems were generally found at the lower end of this range whereas the grafting efficiencies for the other JBR graft copolymers were usually greater than 80%. Possible reasons for these differences are discussed. Characterisation of the graft copolymers, in terms of molecular weight and composition, was carried out using gel permeation chromatography, membrane osmometry and nuclear magnetic resonance techniques. Because it was found, via ozonolysis and gel permeation chromatography, that graft molecular weight was the same as that of the homopolymer produced as a result of inefficient grafting, the presence of this latter species in the products was used as a means for readily determining the molecular weight characteristics of the grafts. The number of grafts per polymer chain was calculated from the amount of 'active' initiator added. Dynamic mechanical studies' of moulded and solvent cast samples of the SBR graft copolymers have shown that a phase separated structure exists. The position and shape of the high temperature tan b damping peak did, however, suggest that the hard phase was a mixed phase containing a small amount of SBR. The ultimate properties of the graft copolymers were comparable with those of vulcanised SBR. Compression set values, measured under standard conditions, were poor compared to conventional vulcanisates.