Peel tests on thin films of segmented poly(urethane ureas) and dynamics ofinterface broadening by neutron reflection

The rate of self-adhesion and interface width development between optical s moothness thin films of segmented poly(urethane urea) (PUU) has been invest igated. The advantage of smooth solvent cast films for peel tests is that m easurements can be made at early times when the interface strengths are ver y low. Viscoelastic dissipation contributions to the peel forces are also r educed because of the nondissipative nature of PW, relative to other adhesi ves. The interfaces between self-adhering PUU thin films were found to rema in relatively narrow by neutron reflection even after long annealing times, because the interpenetration of the mobile soft segments is strongly limit ed by the hard segment physical cross-links, although very strong interface s eventually developed after aging at room temperature for a day or more or at elevated temperatures after shorter times. The time dependence of the p eel strength for self-adhesion between very thin films of PUU follows a t(1 /2) dependence, as has been predicted and experimentally verified for inter diffusion of linear polymers, suggesting that diffusion of mobile soft segm ents is the limiting process, even though center of mass motion of entire P UU chains is prohibited. The interface width reaches a limiting value of ab out 70 Angstrom after about 2 days at 22 degrees C, while the interface str ength measured by peel tests continues to increase up to remarkably longer times of more than 40 days. A tentative molecular mechanism is proposed rel ated to hydrogen bonding of interdiffused segments.