Following mass transport through barrier membranes with confocal Raman microscopy

Abstract

This work describes a setup to characterize the effective diffusivity of barrier membranes to mass transport. Membranes with controlled and well‐defined barrier microstructures are produced from an elastomeric polydimethylsiloxane (PDMS) matrix. Physical obstacles to mass transport are generated by carving in the matrix microscopic holes with well‐defined shapes and in specific positions. Lateral Raman mapping is used to follow mass transport of a penetrant molecule, hexadecanol (HDOL), in the carved matrix. The influence of circular and elongated holes over mass transport is examined by monitoring the HDOL Raman intensity over time, after it passes the barrier microstructure. Results are compared with computer simulations of mass transport and excellent agreement is found. It is shown that holes act as bidimensional barriers to HDOL and that the effective diffusion coefficient of HDOL in the barrier membrane is reduced with respect to that of the homogenous material. Overall, Raman measurements provide a valuable base of data generation to contrast theoretical models of mass transport in heterogeneous barrier membranes.

Permeability in barrier membranes: Comparison between experiments, direct calculations, and analytical models

“…Figure 1A illustrates the experimental platform for diffusion experiments, see further details elsewhere. 28 Experiments are based on promoting diffusion of a penetrant species, HDOL in a polymeric matrix made of crosslinked PDMS. PDMS films, about 200 μm thick, were prepared onto glass microscope slides.…”

“…Obstacles have dimensions comparable with those of the whole sample in such a way that the bi-dimensionality of the diffusion process is readily apparent in the diffusion maps. FEM simulations were carried out using a value of diffusion coefficient D = 2.2 Â 10 À6 cm 2 /seg obtained from independent experiments of HDOL diffusion in homogeneous PDMS, 28 and the geometrical details shown in the figures. Predicted two-dimensional diffusion maps are shown in Figures 2A and 3A for several diffusion times.…”

“…The normalization factor corresponds to the saturation value of HDOL in PDMS, measured independently from sorption experiments, which was found to be about 5 wt%. 28 The HDOL reservoir is placed at the bottom of each sample and the diffusion takes place starting from the reservoir finishing at the top of it. In the color code used, red represents the maximum HDOL concentration (normalized to 1) and blue none of it.…”

“…Raman measurements are based on the spectral differences between the penetrant species, HDOL, and the PDMS matrix. 28 Localized spectra are resolved to obtain individual contributions of HDOL and PDMS by linear spectral decomposition. 29 HDOL concentration was determined as the ratio of the spectral contribution of HDOL and the sum of HDOL and PDMS contributions.…”

“…This ratio was about 0.05 for the saturated sample that contains about 5 wt% of ODOL. 28 Raman results were normalized with respect to this value (0.05) so 1 correspond to the saturated sample and 0 to pure PDMS, the same scale used in diffusion simulations.…”