Stresses associated with diffusion in polyimide and polyacrylics films

When a constrained polymeric thin film is immersed in a liquid medium, its state of stress changes because the diffusion of the liquid into the film causes it to swell. Polyimide and polyacrylics films are used in the laminated structures of inkjet printheads. Swelling behavior of the films in various ink component solutions are of great interest because they are closely related to the delamination problem. Swelling stress at constant strain was measured using an environmental tensile tester at both ambient and elevated temperatures. Two kinds of information were extracted. One is the extent of stress relaxation, which is the amount of stress decay between initial stress and equilibrium stress. The difference in the extent of stress relaxation could result in delamination if two different materials with different swelling characteristics are bonded together and exposed to a swelling agent. The other piece of information was the diffusion coefficients of the liquid agents into the polymer films. The magnitude of the diffusion coefficient indicates the rate of proceeding by the penetrant, i.e., the rate of swelling of the films. The results show that the extent of stress relaxation and diffusivity are different for two films. In addition, the diffusion in polyimide film is Fickian and the diffusion in polyacrylics film is pseudo-Fickian. The changes in mechanical properties after swelling also differ. Among all the ink solutions tested, one component was identified as the most influential and detrimental agent. (c) 2005 Wiley Periodicals, lnc. J