As you describe your system, I feel that water uptake in the film is related both with physical features of the film affecting permeability an other possible, rheologic properties (the structure of the material can be more "open", less restrictive to water molecules as is chemically treated and the surfacial groups are chemically changed affecting in addition the main polarity of the environment). As water can interact "quasi-chemically" with the film, providing, for example, hydrated domains within it, you could develop some theoretical formulae taking into account the differents steps that a water molecule can follow between an initial free state in the liquid in contact with the anhydrous film, and the final state as water "bound" within the film. Each step may eventually needs to be energetically (and entropically) characterised, and the chemical potential would be the thermodynamic parameter to be used for that characterisation ( F = E - TS ) using this one (and not G) because of the condensed nature of the system, without (to my knowledge) gaseous/vapour phases involved. Do not forget that the intuitive concept of chemical potential involves "the evaluation of the tendency to escape of a chemical species from a phase". This conceptual view can be useful in this context. To find tables with useful chemical potentials to be used in this study can be, by far, the hardest task to face in a proper, theoretical approach of this interesting system. Greetings from Mendoza, Argentina. Gustavo.

I assume that the increase in water uptake is due to the fact that you get more polar bonding sites for water in case that you go over the 1.7. This bonding sites will lower the chemical potential of water via the energetic contribution to this potential. Given a certain fixed chemical potential (density) of water in the environment, the density of water in the PU film will be higher.

I am curious about the physical picture of your system. For example, is the water uptake in to a film?

In any case, I wonder also about the chemical and physical properties of your systems at various NCO:OH ratios. For example, to what extent are you changing the pKA of the surfaces versus changing the physical surface area as you change the NCO:OH ratio?

Changing the isocyanate group concentration changes the amount of cross linking in the system. When you hit a certain point there is no more crosslinking making the material more hydrophilic. The added hydrophilic nature increases the water pickup. More water more hydrolysis less thermal stability. If you are looking to increase the thermal stability in the presence of water you need to reduce the hydrolysis reaction sites. So the problem is related to chemical potential since all reactions are driven by their energy state.

As you describe your system, I feel that water uptake in the film is related both with physical features of the film affecting permeability an other possible, rheologic properties (the structure of the material can be more "open", less restrictive to water molecules as is chemically treated and the surfacial groups are chemically changed affecting in addition the main polarity of the environment). As water can interact "quasi-chemically" with the film, providing, for example, hydrated domains within it, you could develop some theoretical formulae taking into account the differents steps that a water molecule can follow between an initial free state in the liquid in contact with the anhydrous film, and the final state as water "bound" within the film. Each step may eventually needs to be energetically (and entropically) characterised, and the chemical potential would be the thermodynamic parameter to be used for that characterisation ( F = E - TS ) using this one (and not G) because of the condensed nature of the system, without (to my knowledge) gaseous/vapour phases involved. Do not forget that the intuitive concept of chemical potential involves "the evaluation of the tendency to escape of a chemical species from a phase". This conceptual view can be useful in this context. To find tables with useful chemical potentials to be used in this study can be, by far, the hardest task to face in a proper, theoretical approach of this interesting system. Greetings from Mendoza, Argentina. Gustavo.

Since NCO groups react readily with water and amines, it is generally preferred to formulate systems at NCO:OH ratio less than 1, but exceptions exist, particularly in waterborne systems. Beyond the stoichiometric proportion of 1:1, the excess NCO will react with moisture and/or water. Thus, water uptake will be a function of the unreacted NCO content. Do you notice any bubbling in your films that could indicate CO2 formation? You may need to check the equivalent weights of your polyol and isocyanate if you are consistently observing better performance beyond the stoichiometric ratio. Depending on the reactivity of your system, you may have unreacted moieties even in a stoichiometric system.

The chemical potential depends on the molar composition, i. e. the molar volume of structural units. In polyurethanes, the water uptake strongly depends on the amount of hydrophilic structural units, mainly ethylene oxide in polyethers or ethylene glycol or diethylene glycol units in polyesters. You have to calculate the molar volume of such structual units and relate them to the water uptake to get usually at lower concentrations a linear, at higher concentrations a logarithmic correlation. If the isocyanate index is increased to the extent described you'll receive on hydrolysis amino groups which lead to crosslink break and add to the hydrophilic character as hydrophilic structural units.

Dear ALL,

Thank you for your opinions and ideas. Do you have any references or research articles that relate to your explanations?

You may check: K.-F. Arndt, F. Krahl, S. Richter, G. Steiner, Swelling-Related Processes in Hydrogels, in: G. gerlach, K.-F. Arndt (eds.), Hydrogel Sensors and Actuators, Springer-Verlag, Berlin 2009, pages 69 to 135 where you'll find a full introduction into swelling of polymers