Polymer/silica composite films as luminescent oxygen sensors

In this paper, we examine the influence of 10 nm diameter silica nanospheres on oxygen diffusion in films of two different amorphous polymers characterized by a low glass-transition temperature and a high oxygen permeability. The two polymers, poly(dimethylsiloxane) (PDMS) and poly(n-butylamino thionylphosphazene) (C(4)PATP), are useful matrixes for oxygen sensors based upon luminescence quenching. In these applications, the dye platinum octaethylporphine (PtOEP), with a long-lived excited state, is incorporated into the polymer, and the presence of oxygen is registered through a quenching of the dye luminescence. For some sensor applications, these linear polymers themselves are too soft and tacky. Silica as a filler improves the mechanical properties of the matrix but perturbs the measurement of oxygen diffusion and permeation. We show that PtOEP adsorbs to the silica particles in PDMS but remains in the polymer matrix iri C4PATP. The quenching kinetics of dye fluorescence is complex in PDMS because of contributions of oxygen adsorbed to the silica surface and that dissolved in the polymer matrix. In contrast, the quenching kinetics in C(4)PATP remains almost unaffected by the presence of silica. Time-scan experiments on PDMS films show good accord with Fick's laws of diffusion for films containing up to 30 wt % (16 vol %) silica. The diffusion constant, D-02, is reduced but only by a factor of about two. In C(4)PATP, the time-scan experiments give more complex results because the oxygen adsorbed to the surface of the silica particles serves as an additional reservoir for oxygen in the system. Because the PtOEP remains in the polymer matrix in C(4)PATP, the oxygen adsorbed to the silica does not participate in quenching until it diffuses away from the particles and into the matrix.