(290b) Measurement of Diffusion and Permeation Coefficients of Oxygen in Polymers Based on Luminescence Quenching

Motivation/Background

The emission intensity of some fluorophores is quenched in the presence of oxygen. Based on this luminescence quenching phenomenon, a new method for determining diffusion and permeation coefficients of oxygen in polymers was recently developed. In most cases, the dye was directly dispersed in the polymer and average oxygen concentration change in the polymer film was monitored by studying the average intensity change of dye using a spectrofluorometer. The luminescence quenching that occurs in the sensor film in response to O₂ concentration can be modeled by the linear Stern-Volmer (SV) equation. However, for many fluorophores, average intensity change with oxygen concentration does not follow the linearity of Stern-Volmer equation. It is reported in the literature¹ that the nonlinearity of response can be due to heterogeneity of dye dispersed in the polymer matrix. Our present work focuses on the measurement of permeability and diffusivity of oxygen in polymers using such fluorescence sensing using a fluorescence microscope. Use of inverted fluorescence microscopy allowed us to study the heterogeneity of fluorophores in polymers and to analyze it's response at micrometer spatial resolution.

Methods

Two simple diffusion experiments were performed which utilize luminescence quenching. In the first, termed film-on-sensor technique, polymer was directly cast on a luminescence sensor film, allowing for measurement of the concentration at the polymer/sensor boundary. In the second, termed accumulation in volume technique, the specimen polymer has been separated from the fluorescent sensor by a small volume. In both cases, change in luminescence was monitored using an inverted fluorescence microscope to measure oxygen concentration. Based on the SV analysis of different regions of the sensor film, the intensity change of regions which follow the linearity of SV equation was examined for evaluation of oxygen diffusion parameters. Diffusion coefficients of oxygen in PDMS, Teflon and 3-6265 HP polymer (silicone elastomer) have been measured. The oxygen diffusion characteristics are determined for PDMS containing different weight percentages of zeolite, using film on sensor technique to evaluate the effect of zeolite content on the oxygen diffusion coefficient. Unsteady-state diffusion of oxygen in the polymer was modeled using Fick's law and the SV equation for luminescence quenching, to extract diffusion and permeation coefficients.

Results

It was found from the SV analysis that, though the dye was not homogeneously dispersed, the data obtained from all the different regions (from low intensity to high intensity) followed the linearity of SV equation. However, the SV constants were different for the different regions. SV constant was lower and the photobleaching effect was relatively higher in the high-intensity region where the dye was aggregated, than in other regions. For Teflon, the accumulation in volume technique was used for diffusion and permeation coefficient measurements, and for all other polymers, the film on sensor technique was used (Figures 1 to 4). Results are summarized in Table 1. Data for Teflon and PDMS compare well with literature values. No effect on the oxygen diffusion coefficient in PDMS was observed regardless of the amount of zeolite loading. This may be due to high oxygen diffusion coefficient for this PDMS system.

Table1.  Diffusion data for Polymers

References

[1] Bedlek-Anslow, J. M.; Hubner, J. P.; Carroll, B. F.; Schanze, K. S. Langmuir, 16, 9137(2000).

[2]Paul, D. R. & Dibendetto, A. T.. Journal of Polymer Science.C, 10, 17 (1965).

[3]Rharbi, Y., Yekta, A. & Winnik, M. A.  Anal. Chem. 71, 5045 (1999).

[4] Shiku, H. et al., Chemistry Letters, 35, 234 (2006).

[5] W. Koros, J.Wang, R. Felder, J .App. Polym. Sci. 26, 2805(1981).