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.
sured relative to the cis and corrected for the back reaction43 and for the amount of isomerization produced by light absorbed by the piperylene ( 0i« -> trans = 0.09).6a Light intensities were measured by uranyl oxalate actinometry and were typically 1.2 X 1017 photons/sec. Optimum data were obtained with piperylene isomerization of 5-15 %. The >¡0 values are shown in the Results.Emission Spectra. All solutions for fluorescence studies were degassed by bubbling with argon for 12 min; such solutions were found to be equivalent to four freeze-pump-thaw cycles at vacuum of less than 1 µ. Studies were made with «-hexane as solvent at 25°. Phosphorescence spectra were determined in ethanol glasses at 77°K. The cell was a 3 or 4 mm (i.d.) quartz tube attached by epoxy resin to an O-ring seal; a second O-ring seal was attached to a Kel-F high-vacuum stopcock and the two pieces joined by a Teflon (43) A
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