Translational diffusion of small molecules in polymers. Laser flash experiments with polydimethylsiloxanes

The quenching of triplet-excited acetophenone by naphthalene served as a probe reaction for studying the mobility of small molecules in polymeric matrices. The rate constants of the quenching process in polydimethylsiloxanes having weight-average molecular weights between 3600 and 86000 were found to depend only slightly on the macroscopic viscosity η at constant temperature: k_q ∝η ^−0,1. In the temperature range from 190 to 298 K (T_g = 152 K), the activation energy of the quenching reaction was determined as E_q = 15,1 kJ/mol, i.e. it is practically equal to the value of the activation energy of viscous flow. The high translational mobility of small molecules in very viscous polymeric matrices is explained on the basis of the concept of microviscosity according to which the trnaslational motion of small molecules in polydimethylsiloxanes at T > T_g is determined by the mobility of free volume elements due to segmental motions rather than by motions of the centers of mass of the macromolecules.