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

Abstract

Linear (SDS) and radial (SD), 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 scctions and dynamic viscoelastic measurements were made at 35 Hz between -90" and +14O"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 sphcrical domains. All effective tackifying resins raised the glass transition temperature (T,) of the rubbery phase, but plasticized the polymer at temperatures well above Tg. 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 adhcsives 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 Tg of the styrene blocks.

Rheology, composition, and peel‐mechanism of block copolymer–tackifier‐based pressure sensitive adhesives

“…( 4 ) . G,9, = V;(p/M,)RT(l + 2.5C + 14.1Cz) ( 5 ) In eq. (5), C is the volume fraction of polystyrene domain in the entire composition and V2 is the volume fraction of polymer in the tackified polyisoprene phase.…”

Pressure sensitive adhesives based on interpolymer complexes

“…This was explained as the highest modulus that still allowed the adhesive to be sufficiently compliant to come into molecular contact with a substrate and form dispersive interfacial bonds. Investigators who followed Dahlquist have accepted this requirement [12][13][14][15][16], termed the "Dahlquist Criterion of Tack", even though the true criterion is defined as a dimensionless quantity.…”

Adhesion and tack of polymers: Influence of mechanical properties and surface tensions

“…Various authors [18][19][20][21] have found that one effect of a tackifier in pressure-sensitive adhesives of natural rubber, polyacrylates and styrene-diene blockcopolymers is the reduction of the modulus in the plateau zone. This is achieved by an increase in the entanglement spacing in full agreement with the results presented here.…”