INHOMOGENEOUS NANOFIBER SWELLING MEASURED WITH ENVIRONMENTAL AFM

Abstract

This work presents a study of the swelling of polymeric nanofibers as a function of relative humidity. The nanofibers were synthesized using the electrospinning technique, for which a homemade electrospinning chamber was implemented. Using the aforementioned technique, nanofibers were deposited on silicon substrates using different polymeric solutions. In particular, pure PVA (Polyvinyl Alcohol) dissolved in water and mixtures of PVA with Chitosan and Salmon Gelatin are studied. Furthoremore, we identified the main parameters which control the morphology of the nanofibers. The main techniques used to characterize the morphology of nanofibers are from Scanning Electron Microscopy (SEM), from which we can extract a quick reading of the coating density of nanofibers on silicon, and Atomic Force Microscopy (AFM), which allows us to study the three-dimensional morphology of individual nanofibers, i.e. height, width and volume, as well as their mechanical properties. Morphogical changes of individual nanofibers with humidity were measured with an Atomic Force Microscope with environmental control. To vary the humidity within the environmental AFM, two methods were implemented: using saturated salt solutions and using a gas flow system that purges the chamber with a mixture of nitrogen and water vapor. To compare the results obtained and estimate the volume of dry solid, Dynamic Vapor Sorption (DVS) measurements were performed for pure PVA nanofibers and mixtures of PVA with Chitosan and Salmon Gelatin. The results show in-homogeneous swelling of individual nanofibers, i.e. the nanofiber does not swell isotropically and its volume grows non-linearly with humidity. We model this swelling behaviour with isothermal sorption equations. As future work, we propose to study the mechanical properties of individual nanofibers as a function of relative humidity