Connectors in Adhesion and Contact Angle Hysteresis

Stability and dewetting behavior of thin polymer films on model substrates consisting of "bi-modal brushes" of identical polymers were investigated by optical microscopy. Due to entropic losses at high grafting density of the short polymers autophobic behavior was observed. The longer grafted molecules acted as "connectors" and re-established wettability once their areal density was larger than an extremely low threshold value. At this point the force necessary to pull out the connectors from the melt of identical molecules was just balancing capillary forces. This allowed us to determine the force necessary to pull a single polymer out of its own melt. Increasing the areal density of connectors increased also the difference between advancing and receding contact angle. Although connectors generated a positive spreading coefficient, thin films could still be unstable due to unfavorable long range interactions across the film.