Structural changes in melts of butadiene-styrene and isoprene-styrene block polymer-based pressure-sensitive adhesives

Rheological and small angle x‐ray scattering (SAXS) measurements show that typical pressure sensitive adhesives prepared from block polymers of butadiene or isoprene with styrene maintain the domain structure of the block polymer well beyond the polystyrene domain Tg, but form homogeneous melts above a critical temperature, Tc. For the examples investigated, Tc lies some 20°C below the usual hot melt processing temperature. The extremely high resistance to creep of the adhesives at service temperatures is explained as being the result of viscous flow with the domain structure in a state of dynamic equilibrium, in which polystyrene blocks are detached from domains and reattached to others. A transition between nonequilibrium and equilibrium domain structures is revealed clearly by SAXS for one of the block polymers used in this work.