Mathematical models and experiments for swelling phenomena before dissolution of a polymer film

The initial dissolution characteristics of an organic soluble benzophenone-containing polyimide (BCPI) film were studied by using a microscopic method and laser interferometry, which could inspect the initial dissolution process at larger and smaller experimental times, respectively. Results from a dissolution cell showed that the rubbery layer thickness Λ(t), the position of the solvent—rubbery layer interface (front S), and the position of the rubbery layer—glassy polymer interface (front R) varied linearly with the square root of time at larger times of the initial dissolution process. As experimental time increased, dissolution became important that the linear relationship would no longer exist. The initial dissolution behaviour at t → 0 was investigated via a laser interferometer and it was found that Λ(t), plus the position of fronts S and R, were proportional to time. Explicit mathematical expressions to describe the swelling behavior before dissolution of BCPI film in N-methyl-2-pyrrolidone were proposed. Experimental data and theoretical values were in good agreement qualitatively.