DYNAMIC MEASUREMENTS OF HUMIDITY ATTACK ON POLYMER GLASS INTERFACES UNDER STRESS/

A simple, compact apparatus has been constructed for simultaneous tens ile testing and optical microscopy in a time-resolved fashion of small adhesive droplets subjected to rapid variations in humidity. We utili ze this system to study the 'attack' of humidity on the bonding of a p olybutadiene rubber (PER) and an isoprene-based pressure-sensitive adh esive (PSA) to soda lime glass. In the PER, interfacial failure in hum id air begins at a well-defined threshold stress; nevertheless, the ra te of subsequent interfacial crack growth is a complex function of the sample history (stress and humidity). In PSA, interfacial failure in humid air begins soon after fibril formation (accompanying meniscus in stabilities around the circumference of the adhesive drop). When humid air is introduced after fibril formation, interfacial failure begins promptly. Fibril formation produces stress concentrations which render the interface very susceptible to water attack - even at relatively l ow applied stresses. Although fibril formation enhances energy dissipa tion in PSA tested in dry air (due to fibril elongation), it limits en ergy dissipation in humid air (by inducing premature interfacial failu re).