PHOTOPHYSICAL STUDIES OF MOLECULAR MOBILITY IN POLYMER-FILMS AND BULK POLYMERS .3. DYNAMIC EXCIMER FORMATION OF PYRENE IN BULK PDMS

Molecular mobility in polymer films and bulk polymers has been determined by measuring the excimer formation of pyrene. Excimer formation of pyrene in polystyrene and epoxy film was not observed at pyrene concentrations ≤0.1 M. This indicates a very low mobility of the probe molecules in the above polymer films. However, excimers in high yield were formed dynamically in bulk poly(dimethylsiloxanes) (PDMS). The rate constants of excimer formation in PDMS (1-2.5 X 106 cP) are 7.1 X 109-9.2 X 108 M-1 s-1. The rate constants are larger than those in squalane, the room-temperature viscosity of which is only 35.4 cP. Viscosity changes induced by various molecular weights of PDMS and by various temperatures impose different effects on the mobility of pyrene molecules. The data are explained by concepts of macroviscosity, microviscosity, the basic structure of the polymer matrix and by free-volume theory. It is found that the natural logarithm of the diffusion rate constant is proportional to the density of polymer in studies with linear PDMS. © 1990, American Chemical Society. All rights reserved.