Solvent-Induced Stresses during Sorption in Glassy Polycarbonate: Experimental Analysis and Model Simulation for a Novel Bending Cantilever Apparatus

A new experimental apparatus has been set up to analyze the polymer dilation as well as the stress profile induced by vapour sorption in constrained films. The device couples the bending cantilever technique with a new digital micrometer which allows to measure both deflection and polymer swelling at the same time. The apparatus has been applied to study the effects of acetonitrile mass uptake in polycarbonate at 40°C, obtaining the time evolution of swelling and of cantilever deflection during integral sorption tests, in the activity range from zero up to 0.35. A simulation model is also proposed, which couples Fick’s law for diffusion to the study of strain and stress distributions in the cantilever; the mechanical problem has been formulated following the classical laminate theory. The relevant values of the material properties entering the model, as solubility, diffusivity and penetrant induced dilation, have all been determined through separate independent measurements, and the results obtained are in good agreement with the corresponding values reported in the literature. The simulation model is able to predict with good precision both the kinetics of deflection during sorption and its final equilibrium value, as well as the overall swelling kinetics, with no need of any adjustable parameters. The model allows also to calculate the time evolution of the stress profile in the polymer film during sorption/desorption steps.