Water Sorption Thermodynamics and Mass Transport in Poly (e-Caprolactone): Interactional Issues Emerging from Vibrational Spectroscopy

In this work FTIR spectroscopy in the transmission mode has been used to study the water sorption and transport in poly-?-caprolactone (PCL). The characteristic spectrum of absorbed water was isolated by difference spectroscopy to obtain a description of the water sorption mechanism at a molecular level. Moreover, the spectroscopic data were coupled with gravimetric sorption measurements to correlate the equilibrium absorbance with water concentration in PCL. The transient sorption stage was also analysed, allowing an estimation of the water diffusion coefficient in PCL. The analysis of the ?(OH) spectral region revealed the presence of two species of absorbed water, i.e. water molecules directly interacting with carbonyl group of the polymer backbone and self-associated water corresponding to a second shell hydration layer. Finally, the experimental sorption isotherms were successfully interpreted using an equation of state thermodynamic approach grounded on a compressible lattice fluid framework accounting for self- and cross-hydrogen bond formation. Based on the evidences of the FTIR spectroscopy, in the implementation of the model, only one proton acceptor group (i.e. carbonyl) has been assumed to be present on the polymer backbone.