Optical coherence tomography enables direct monitoring of diffusion and swelling in water-soluble PVA films

Liquid absorption in polymeric films is both an open problem in materials science, due to the complex coupling between solvent diffusion and matrix deformation, and a crucial process in numerous industrial applications, including healing, solvent-sealing, degradation, and the reprocessing of thermosetting polymers. Here, we experimentally investigate solvent diffusion and swelling in polymer films by exploiting, for the first time, optical coherence tomography (OCT), a technique previously used mainly in biomedical imaging. Differently from other imaging techniques, OCT allows monitoring diffusion and swelling in a geometry that closely reproduces industrial process conditions. We perform OCT measurements on different poly(vinyl alcohol)/water mixtures, including films of varying age and thickness, and compare the results with standard optical microscopy (OM). We demonstrate that OCT enables simultaneous measurements of both liquid and swelling front positions over time, with performance far exceeding that of OM, in terms of both accuracy and reliability of long-time measurements. The experimental results are interpreted using a recently introduced continuum-mechanics model. Our OCT-based approach paves the way for optimizing widespread industrial processes and for characterizing functional polymeric materials with targeted properties such as high solubility and self-healing capability.