LIGHT-SCATTERING FROM RANDOM COILS DISPERSED IN SOLUTIONS OF RODLIKE POLYMERS

The thermodynamics and mobility of a random-coil polymer were studied by light scattering in toluene solutions containing a rodlike polymer. The random-coil polymer was polystyrene (PS). The rodlike polymer was helical poly(gamma-stearyl alpha,L-glutamate), or PSLG, which aggrega tes end-to-end in toluene to produce long filaments. As PSLG is almost isorefractive with toluene, the scattering of PS can be measured in P SLG/toluene mixtures with almost no interference from PSLG. The appare nt second virial coefficient of the PS component decreases rapidly wit h addition of the ''invisible'' rodlike polymer component, while the P S radius of gyration does not. A theta condition is reached at slightl y more than 1% added PSLG. Although PSLG and PS probably have a disfav orable interaction parameter, enthalpic interactions are of secondary importance. This point was underscored by experiments involving the ad dition of 5% dodecane to binary PS/toluene solutions; the small, aliph atic solvent had virtually no effect. The reduction of the virial coef ficient, but not the size, of the random-coil polymer in the presence of PSLG is due to the occupation of connected (and linearly correlated ) space. Parallel effects were observed in the mutual diffusion coeffi cient of the coil component, which increased with PS concentration at low rod content but did the opposite when enough PSLG was added. Extra polated to zero PS content, the mutual diffusion coefficient is expect ed to approach closely the self-diffusion of trace quantities of PS in the PSLG/toluene solution. So obtained, the self-diffusion coefficien t decreased with added rodlike PSLG, but not as fast as the viscosity increased; thus, the Stokes-Einstein relationship was not obeyed by PS probes in PSLG/toluene solutions. Scaling arguments are presented for the dependence of the size of a random coil in the presence of rods a nd for the crossover from Stokes-Einstein diffusion of the coil to a r eptative type of motion. The available data are not well suited to tes t these relationships, due to limitations in the matrix concentration imposed by polymer incompatibility.