Self-diffusion and tracer diffusion in sphere-forming block copolymers

The self-diffusion and tracer diffusion of asymmetric poly(ethylene-alt-propylene-b-dimethylsiloxane), PEP-PDMS, copolymers were measured using forced Rayleigh scattering. Diffusion measurements were made in the disordered state, in disordered micelles, and in the body-centered-cubic (bcc) sphere phase. The FRS intensity decays were fit with a single-exponential function in the disordered state and a stretched exponential function in disordered micelles and bcc spheres. The decays are interpreted as the diffusion of single chains with a broadened distribution of relaxation times when micelles are present, due to the polydispersity of chain lengths. The diffusion coefficients in disordered micelles and bcc spheres are retarded compared to the disordered state. This retardation is consistent with a hindered diffusion process, D ∼ D₀ exp(-AχN_A), where χ is the Flory-Huggins interaction parameter, D₀ is the diffusion coefficient as χ approaches zero, and N_A is the length of the core block. The parameter A is found to increase as χN_A increases. These results are compared with recent experimental and computational work.