Sharp diffusion front in diffusion problem with change of state

Direct visualization of pyrene diffusion in polyethylene and polyoxymethylene passive samplers

While passive sampling of ultra-low aqueous concentrations of hydrophobic organic compounds in environmental aqueous media has emerged as a promising analytical technique, there is a lack of good understanding of the fundamental diffusive processes. In this research, we used a fluorophore, pyrene, as a model compound to track diffusion in polymers through absorption and environmental media exchange processes. We directly tracked the penetration of pyrene into polyethylene (PE) and polyoxymethylene (POM) rods during absorption from water by sectioning the rod after different stages of absorption and observing the fluorescence signal through a microscope. Diffusion profiles of pyrene in polymers were simulated by numerical integration of Fickian diffusion. The results indicated that the uptake process in PE is governed by Fick's law and the absorption and desorption kinetics are similar in this polymer. However, the observed uptake profiles of pyrene in POM were non-Fickian and the release kinetics out of POM was slower compared to uptake into the polymer. We show that slower desorption from POM makes corrections for nonequilibrium using performance reference compounds (PRCs) problematic for deployments in water or sediment where there is significant advection. However, for static sediment deployments, the overall kinetics of exchange is controlled by slow transport through sediment and the hysteretic behavior of POM may not preclude the use of PRCs to interpret equilibrium status.

Glassy Polymers—Diffusion, Sorption, Ageing and Applications

For the past few decades, researchers have been intrigued by glassy polymers, which have applications ranging from gas separations to corrosion protection to drug delivery systems. The techniques employed to examine the sorption and diffusion of small molecules in glassy polymers are the subject of this review. Diffusion models in glassy polymers are regulated by Fickian and non-Fickian diffusion, with non-Fickian diffusion being more prevalent. The characteristics of glassy polymers are determined by sorption isotherms, and different models have been proposed in the literature to explain sorption in glassy polymers over the last few years. This review also includes the applications of glassy polymers. Despite having many applications, current researchers still have difficulty in implementing coating challenges due to issues such as physical ageing, brittleness, etc., which are briefly discussed in the review.

Light-regulated molecular diffusion in a liquid crystal network

Photo-responsive liquid crystal polymer networks offer promising means to generate useful functional devices, but many of them focus on their mechanical response so as to generate surface features or shape change. Here, we investigate the photomechanical effect of the polymer network for molecular transport purposes. Dual wavelength illumination of an azobenzene-functionalized cholesteric liquid crystal polymer film produces excess free volume within the film, which results in an accelerated molecular diffusion through the film. Moreover, the polarization of the UV light exposure on the cholesteric network plays an important role in a remarkable enhancement of molecular diffusion. When linearly polarized UV light rotates along with the twist of the helical axis of the cholesteric polymer, excess free volume forms sequentially from the diffusion network toward the dry network in the polymer. It works in concert with the concentration gradient of the diffusant and greatly improves the diffusion through the film.