DYNAMIC LIGHT-SCATTERING FROM DILUTE, SEMIDILUTE, AND CONCENTRATED BLOCK-COPOLYMER SOLUTIONS

The dynamic light scattering (DLS) properties of four symmetric polyst yrene-polyisoprene diblock copolymers have been examined in the neutra l good solvent toluene. Concentrations (c) ranged from the dilute, thr ough the semidilute, and into the concentrated regimes. Pulsed-field-g radient NMR has been used to determine the translational diffusivities (D-s) of the same copolymers, at selected concentrations. The results are in substantial agreement with models developed by Benmouna and co workers and by Semenov. Three modes are predicted: cooperative diffusi on (D-C), corresponding to relaxation of fluctuations in polymer conce ntration; an internal mode (Gamma(I)), reflecting relative motion of t he two blocks within one molecule; a heterogeneity mode (D-H), due to chain-to-chain fluctuations in composition and which relaxes by transl ational diffusion. In dilute solutions, a single, diffusive mode is se en and is attributed to a superposition of D-C and D-H. In semidilute and concentrated solutions, D-C and D-H are cleanly resolved. D-C show s the expected scaling with c and molecular weight (M), i.e., D-C simi lar to c(0.7)M(0). Comparison with the NMR results confirms that the v alues of the diffusion coefficients D-H and D-s are very similar; this result extends the previous work of Balsara and co-workers, who first noted the appearance of a block copolymer DLS mode resembling transla tional diffusion. Selected measurements in two other good solvents, TH F and chloroform, establish that D-C and D-H are independent of solven t refractive index (n(s)), as expected. The relative amplitudes of the cooperative and heterogeneity modes also scale with c, M, and n(s) in agreement with theory. The predicted internal mode is generally diffi cult to resolve but is seen for the highest molecular weight sample ex amined. The concentration dependences of D-H and D-s are insensitive t o the order-disorder transition, in agreement with previous measuremen ts in solutions and melts. Preliminary measurements of D-H with a nove l Fourier transform heterodyne DLS apparatus were also in good agreeme nt with the other techniques. Finally, the results are compared with p revious DLS measurements of symmetric diblock. copolymers in solution. It is proposed that the apparent discrepancies among the previous stu dies can be resolved, by appropriate assignment of the heterogeneity m ode.