Moderate Chain Branching in Waterborne Pressure-Sensitive Adhesives Combines Strain Hardening with Entanglements Formed during Film Formation: A Study Based on Förster Resonance Energy Transfer and Mechanical Testing

Förster resonance energy transfer (FRET) was employed to study polymer interdiffusion in drying latex dispersions designed for application as pressure-sensitive adhesives. The materials differed in the degree of chain branching, where branching was controlled by the addition of styrene as a retarder. The FRET efficiency, ΦET, quantifies the degree of interdiffusion between particles. A plot of log (ΦET) versus log (time) showed an upturn after a drying time of a day or more. This increased slope can be explained in the frame of the Rouse model and reptation. ΦET(t) showed a step at the time of first contact between particles, the height of which increased with increased branching. Branches delayed interdiffusion in the later stages of drying. Presumably, these two effects are linked to short-chain branching and long-chain branching, respectively. As adhesion tests on these samples show, their tackiness is the largest when they contain an intermediate amount of retarder.