Peel Tests on Thin Films of Segmented Poly(urethane ureas) and Dynamics of Interface Broadening by Neutron Reflection

The rate of self-adhesion and interface width development between optical smoothness thin films of segmented poly(urethane urea) (PUU) has been investigated. The advantage of smooth solvent cast films for peel tests is that measurements can be made at early times when the interface strengths are very low. Viscoelastic dissipation contributions to the peel forces are also reduced because of the nondissipative nature of PUU, relative to other adhesives. The interfaces between self-adhering PUU thin films were found to remain relatively narrow by neutron reflection even after long annealing times, because the interpenetration of the mobile soft segments is strongly limited by the hard segment physical cross-links, although very strong interfaces eventually developed after aging at room temperature for a day or more or at elevated temperatures after shorter times. The time dependence of the peel strength for self-adhesion between very thin films of PUU follows a t 1/2 dependence, as has been predicted and experimentally verified for interdiffusion of linear polymers, suggesting that diffusion of mobile soft segments is the limiting process, even though center of mass motion of entire PUU chains is prohibited. The interface width reaches a limiting value of about 70 Å after about 2 days at 22 °C, while the interface strength measured by peel tests continues to increase up to remarkably longer times of more than 40 days. A tentative molecular mechanism is proposed related to hydrogen bonding of interdiffused segments.