Surrounding water vapor induced diffusion and physisorption of water within thermoplastic polyurethane TPU samples: A critical analysis of DVS data

In this paper, dynamic vapor sorption (DVS) experiments on thermoplastic polyurethane (TPU) samples are presented. Experiments were conducted at different constant temperatures – at 15, 25, and 35 degrees C – giving access to the sorption isotherm showing almost no temperature dependency. As the specimens were prepared with a defined geometry (disc’ with different thicknesses from 600 to 1600 μm and a diameter of 30 mm), the recorded mass history in response to a quasi-instantaneous change of the sample-enclosing humidity was employed to backcalculate diffusivity in narrow ranges for the prescribed externally assigned relative humidities, with a step-wise increase (or decrease) of ten percent. The backcalculated diffusion coefficients show dependency on (i) the temperature what is well described in Arrhenius plots and (ii) the relative humidity range. The influence of sample thickness is marginal, indicating that the surface resistance to mass transfer, as quantified by the Biot number, is negligible when interpreting DVS data presented in this paper. Furthermore, the order of magnitude of the backcalculated diffusion coefficients and their temperature dependency in terms of the associated activation energies is discussed, depending on whether the physi-sorbed water content within the polymer network or the (externally) assigned water vapor content is employed as potential/state variable for the mathematical description of the diffusion process (gradients of the state variable represent the driving force of the diffusion process). When water vapor content is employed, the slope of the sorption isotherm has to be considered in the diffusion analysis.