Mass Transport and Stress Field in Polymer-Solute Systems: Analysis and Solutions of Coupled Problems

This work focuses on the analysis of transport phenomena in solid polymer-solute systems and it specifically addresses problems in which a mutual influence exists between mass transport and stress development in polymers. The first problem refers to the design of an apparatus which allows to measure in-plane stresses that arises and relaxes in polymer coating of commercial polystyrene, around room conditions, below the glass transition temperature (Tg) as result of vapor sorption/desorption processes. The in-plane stress is measured using bending beam cantilever technique for which an apparatus was specifically designed and set up in this work. In-plane stress in PS coating was measured according to prescribed laws. From the stress state, knowing the material proprieties, the estimation was finally attempted of change in the system volume during the sorption/desorption process and results have been compared with data registered under the same conditions for mass uptake in the system from gravimetric measurements. The second problem selected refers to the analysis of the effect of elastic mechanical characteristics on separation performances for dense polymeric membrane for organic solvent nanofiltration. In this view, two different commercial PDMS-based membranes were characterized and based on the experimental results a model was developed to provide a thermodynamic and mechanical frame for the representation of flux and rejection for the case of different solutes and solvent mixtures. Permeation rates and solute rejection were thus investigated in details for two different PDMS-based membranes, through an extensive experimental campaign. Because of the relevance of this problem, emphasis is given in this work to the elaboration of equation-of-state in the field of lattice fluid or tangent-hard-spheres-chain theories which could be implemented with terms suitable for the representation of the effect of mechanical constraint to the system as well as of the constitutive properties for stress-strain relationships.