Morphology and Viscoelastic Behavior of Styrene-Diene Block Copolymers in Pressure Sensitive Adhesives

Linear (SDS) and radial (SD)_x block copolymers of styrene (S) and dienes (D=butadiene or isoprene), varying in composition and molecular weight, were formulated as pressure sensitive adhesives. The morphology of these compositions was determined by electron microscopy of ultra-thin sections and dynamic viscoelastic measurements were made at 35 Hz between -90° and + 140°C or higher. Pressure sensitive tack and holding power were determined and interpreted in terms of morphological and rheological properties.

A high degree of tack resulted only when the tackifying resin was compatible with the polydiene segments of the block polymer and incompatible with the polystyrene segments, provided also that the polydiene-tackifier phase was the continuum with the polystyrene phase forming spherical domains. All effective tackifying resins raised the glass transition temperature (T_g) of the rubbery phase, but plasticized the polymer at temperatures well above T_g Polystyrene domain connectivity was found to lead to diminished tack in block polymers containing more than 30% styrene, a result of decreased creep compliance on the time scale of the bonding process and failure to achieve full contact with the substrate. For adhesives not limited by contact, tack increased with the loss modulus of the adhesive on the time scale of the debonding process. Holding power (shear resistance) increased with polymer styrene content and molecular weight, the polystyrene domain structure effectively inhibiting viscous flow at temperatures sufficiently below T_g of the styrene blocks.