Synthesis and Characterisation of Styrene Butadiene Styrene Based Grafted Copolymers for Use in Potential Biomedical Applications

In the annals of history the evolution of the synthetic rubber industry can be traced to the early 1930s where the first emulsion polymerised styrene butadiene rubber known as Buna S was prepared by I. G. Farbenindustrie in Germany. But it was not until the US Government in 1940 established the Rubber Reserve Company, a stockpile of natural rubber and the development of a synthetic rubber program came into full fruition. However, when the United States entered World War II, the synthetic rubber plants owned by the US Government were either closed or sold to private industry between the years 1946 and 1955, and from this the development of this formidable technology began. In the early 1960’s one primary objective prevailed and that was the economical polymerisation of polyisoprene with a high cis–1,4 structure, which is the synthetic version of natural rubber(Holden & Hansen, 2004). Around this time, workers at Shell investigated lithium metal initiators for isoprene polymerisation and found that alkyllithiums yielded some interesting results. In particular, there was no chain termination or chain transfer steps present. Thus, when all of the original monomer was consumed, the polymer chain still remained active and could initiate further polymerisation if more monomer, either of the same or different species, were added (Holden & Hansen, 2004). Parallel with these developments, tri-block copolymers using difunctional initiators were also reported in the literature (Szwarc et al., 1956; Szwarc, 1956). These block copolymers were produced under conditions that gave polydiene segments a relatively low 1,4 content(Holden & Hansen, 2002). However, poor elastomeric properties were acknowledged whereby the rheological properties of both polybutadiene (PB)(Gruver, 1964) and isoprene(Holden, 1965) resulted in the materials exhibiting Newtonian behaviour and the viscosities of the pure polymers approach constant values as the shear rate approaches zero. This behaviour resulted in bales of these elastomers appearing to be solid but in fact behaved as viscous liquids which hindered both their storage and commercial attractiveness. In light of this, Shell chemical research polymerised polydiene elastomers with various molecular weights to combat this problem (Holden & Hansen, 2004). Later studies included work on block copolymers resulting in the formation of a material which contained short blocks of polystyrene on either end of the elastomeric chain to form a styrene butadiene styrene (SBS), as illustrated in Figure 1. In contrast to the diene homopolymer, these block copolymers demonstrated, non-Newtonian