Liquid Water Transport in Polymeric Films and Membranes

Abstract

Accurately measuring and understanding transport mechanisms of liquids in polymers is important for the development of materials used in applications such as packaging, water purification, and corrosion protection.

In this work, an in situ pressure-contact Fourier transform infrared attenuated total reflectance (FTIR-ATR) spectroscopy apparatus was designed and developed for accurately measuring liquid water transport in free-standing polymer films. This new technique allows time-resolved infrared data of liquid transport in free-standing films to be collected accurately without solution casting the film directly onto the ATR element. Liquid transport in a rubbery polymer, glassy polymer, and a crosslinked polymer was measured and compared using this technique. The in situ pressure-contact FTIR-ATR spectroscopy apparatus showed similar results to those obtained using the conventional solution-cast FTIR-ATR apparatus for the rubbery polymer. Liquid water diffusion in the rubbery polymer exhibited non-Fickian behavior. Non-Fickian behavior was observed for the glassy polymer which was attributed to diffusion-relaxation and difference between the conventional solution-cast FTIR-ATR apparatus and the pressure-contact apparatus. Liquid water diffusion in the crosslinked polymer exhibited Fickian behavior and was attributed to the suppressed polymer strain response to the diffusant.

Diffusion in glassy polymers can be difficult to measure because of their nonequilibrium nature (i.e., Tg, of the polymer is much higher than the experimental temperature. In this work, the effect of glass transition temperature, Tg, on liquid water diffusion behavior in polymer-ionic liquid (IL) mixtures was investigated. Liquid water transport in glassy polymer-ionic liquid mixtures of varying IL content (0 to 50 w/w % IL) was studied to determine the effect of changing the glass transition temperature, Tg, on diffusion behavior. The results of this study showed that as IL content increased and Tg decreased, the liquid water diffusion behavior became more Fickian. This is because as Tg decreases, the free volume of the polymer increases, and the polymer approaches equilibrium.

Liquid water transport in polymers is also important for separations applications such as water purification. Recent research has focused on a promising hybrid technology, reactive electrochemical membranes (REMs) which act both as a membrane and a reactive electrochemical surface for the electrochemical degradation of pollutants However, they are limited by morphology, and therefore conductive REMs with tailored porosities are needed. In this study porous, flexible reactive electrochemical membranes (REMs) for water purification were synthesized by a novel simultaneous electrospinning/electrospraying (E/E) technique. The fabricated E/E REMs were durable, highly porous, and conductive membranes, performing comparably to carbon nanotube flow-through reactors, which are among the highest performing in literature. Further studies exploring the use of FTIR-ATR spectroscopy for CO2 separations studies, and for studying surface interactions of membranes fabricated via simultaneous E/E will be discussed.