The Trommsdorf or gel effect in free radical polymerization is due to the fact that the termination reaction becomes strongly diffusion controlled above a critical concentration associated with the onset of molecular entanglements. Therefore, an understanding of polymer self-diffusion in entangled systems becomes essential to understanding the Trommsdarf effect. Our group has previously proposed a molecular model for the gel effect which uses a specific theory for polymer diffusion (reptation). The present work represents an experimental attack on the same problem. Experimental studies of polymer self-diffusion in entangled systems are scarce. Quasielastic light scattering from ternary systems composed of solvent(1)-polymer(2)-polymer(3), in which species (3) is isorefractive to the solvent (i.e. partial derivative n/partial derivative c(3) = 0), offers an attractive way to study the tracer diffusion coefficient of species (2) in a binary mixture of composition c(3). In regimes of low momentum transfer (qR(G) <= 1,) where q is the scattered wave vector and R-G is the polymer radius of gyration, we have shown that the correlation function of the scattered electric field should decay with a single exponential decay time, given by (D(22)q(2))(-1) where D-22(c(2), c(3)) is the main ternary diffusion coefficient of component (2). Extrapolation to zero concentration of (2) at fixed concentration of (3) yields the tracer diffusion coefficient of (2) in the binary mixture of (1) and (3). The systems toluene(1)-polystyrene(2)-polymethylmethacrylate(3) (partial derivative n/partial derivative c approximate to 0.007 at 25 degrees C) and toluene(1)-polystyrene(2)-polyvinylmethylether (3) (partial derivative n/partial derivative c(3) = 0.012 at 60 degrees C) very nearly satisfy the above criteria. In both systems, we have found that the tracer diffusion coefficient of the polystyrene decreases with increasing concentration of the isorefractive polymer. Further studies have focused upon the dependence of the tracer diffusion coefficient upon polystyrene molecular weight, and upon the effect of incomplete index matching.
