Styrenic thermoplastic elastomers have a basic unit structure in which polystyrene components (hard segments) and polyethylene-polybutylene components (soft segments) are block-copolymerized (Figure 1). These materials are generally referred to as SEBS. The terminal polystyrene blocks and the intermediate polyethylene-polybutylene blocks are chemically bonded, but they are thermodynamically incompatible at room temperature. The terminal polystyrene blocks therefore aggregate into ultrafine particles (microdomains) that are uniformly dispersed (Figure 2). These polystyrene microdomains act as physical cross-linking points equivalent to the cross-links in cross-linked rubber, giving SEBS its elastomeric properties. At temperatures of 140–230°C, however, the polystyrene and polybutadiene or polyethylene-polybutylene components melt together, enabling injection and extrusion molding and exhibiting the flow characteristics of a thermoplastic resin.