EFFECT OF MOLECULAR WEIGHT AND MOLECULAR WEIGHT DISTRIBUTION ON THE PRESSURE SENSITIVE ADHESIVE PROPERTIES OF POLYISOBUTYLENE

The effect of molecular weight on pressure sensitive adhesive properties was studied in a model system of commercial polyisobutylenes having a range of number average molecular weight 25,000 to 750,000. High and low molecular weight samples were blended to artificially create two series of broad distribution samples with polydispersities of 4 to 11. Characterization was done by size exclusion chromatography and membrane osmometry.^ Adhesive performance was assessed for each homopolymer and blend by shear, peel and probe tack testing based on standardized test methods. In the homopolymer series, with an increase in molecular weight, 180(DEGREES) peel and probe tack decrease to a constant level. Resistance to shear (or creep) passes through a maximum at a number average of 300,000. While the broad distribution blends generally showed slight improvement over the homopolymer with similar number and weight average molecular weight, there was no conclusive evidence to suggest a relationship with increasing polydispersity.^ Rheological properties were determined for the homopolymers and blends as a function of frequency. Established trends for molecular weight and frequency were observed in the homopolymer series. Among the blends, little difference in rheological properties was found.^ The adhesive performance for the homopolymers and blends was linked with the rheological properties of the sample. Each adhesive test was modeled so that a rate of deformation could be calculated for comparison to rheological properties at similar frequencies. In probe tack testing, the ability of the sample to establish contact with the test surface is related to the tensile creep compliance at a time corresponding to probe dwell time. Tack values increase with increasing compliance. Once a sample has wet a surface, its ability to resist subsequent deformation was found to depend on the dissipation factor (G\u27\u27/G\u27) in the homopolymer series. With increasing dissipation factor, there is increased peel and probe tack. The magnitude of the increase is test rate dependent. Resistance to shear (creep) increases sharply with an increase in complex viscosity. The lack of variation in rheological properties and adhesive performance of the blends made it impossible to make any positive conclusions regarding the influence of molecular weight distribution.