Termodinamica di assorbimento di acqua in polimeri termoplastici vetrosi ad elevate prestazioni
Creators: name: Ragosta, Michele Maria
Publisher: name: Università degli Studi di Napoli Federico II
Year: 2015

description: Sorption thermodynamics of water in polymers is a subject of primary interest both from a fundamental point of view and for its practical implications. In fact, several polymer matrices, when exposed to a humid environment, absorb significant amounts of water which adversely affect most physical- mechanical properties. In this thesis, modelling of water sorption thermodynamics in high performance glassy thermoplastic matrices of polyetheretherketone (PEEK) and polyetherimide (PEI) and equilibrium water sorption in miscible PEEK/PEI blends as a function of blend ratio, has been investigated. Equation of state theories originally developed to interpret thermodynamics of mixtures at equilibrium, namely Sanchez-Lacombe Hydrogen Bonding (SLHB) and Non Random Hydrogen Bonding (NRHB), have been extended to the case of glassy polymer- low molecular weight penetrant mixtures, displaying possible hydrogen bonding interactions (HB). To this aim, the approach developed by Doghieri and Sarti in deriving the Non Equilibrium Theory for Glassy Polymers (NETGP), which is based on thermodynamics of internal state variables, has been adopted to extend the equilibrium models to out-of-equilibrium glassy systems, resulting in NETGP-SLHB and NETGP-NRHB theories. Making reasonable choices about the selected internal state variables and based on critical assumptions concerning the rate of evolution of these variables, it has been possible to obtain workable equations which have been used to interpret water sorption isotherms in PEEK and PEI, determined at temperatures ranging from 30 to 70 C°. For comparative purposes the Non-Equilibrium Lattice Fluid theory (NELF), which does not account for possible interactions between the components, has been also employed. The NELF model provided an unsatisfactory prediction of sorption isotherms, in view of its inability to reproduce the upward concavity exhibited at high vapor water activities, which is related to clustering of absorbed water molecules. Conversely, NETGP-SLHB and NETGP-NRHB models provided a good fitting of equilibrium sorption isotherms in the whole activity range investigated. In particular, differently from the NELF theory, the models displayed the same upward concavity as the experimental data. This improvement is to be ascribed to the HB contributions to the Gibbs energy. Once the best fitting parameters have been determined from experimental sorption isotherms, the developed models have been used to predict the amount of self-HB and cross-HB interactions occurring at equilibrium in the mixtures as a function of water mass fraction absorbed within polymers. At low water concentration cross-HB interactions prevailed while the water self-HB interactions increased their importance as total water concentration increased. For the PEI/water system the theoretical predictions were in good agreement with the experimental results obtained from FTIR spectroscopy. Differential scanning calorimetry measurements of PEEK/PEI mixtures showed a single glass transition temperature that increased with enhancing PEI content, confirming the complete miscibility in the amorphous state of components. The equilibrium water sorption in the blends decreased with increasing PEEK content and the sorption isotherms showed trends similar to those of neat components. At high activities the isotherms exhibited an upward concavity due to clustering of water molecules and not to plasticization effects in view of the low amount of water absorbed and of the very high glass transition temperature of the mixtures. On PEEK/PEI blends a study to evaluate their chemical resistance in Skydrol, Jet-fuel and methyl-ethyl-ketone (MEK) has been also performed. These solvents were chosen to represent a range of chemically aggressive environments that high performance polymers may encounter in aeronautic applications. Sorption kinetics showed modest absorptions of Skydrol and low absorptions of Jet-fuel for PEEK and PEI, while the blends exhibited a small weight lost, which decreased with enhancing the amount of PEEK. Conversely, higher absorptions were recorded with MEK, mainly for PEI and for blends rich in PEI.