Chain and Structural Dynamics in Melts of Sphere-Forming Diblock Copolymers

Melts of asymmetric sphere-forming diblock copolymers form a dense liquid of micelles at temperatures above the order-disorder transition (ODT) temperature and below a critical micelle temperature (CMT), above which micelles dissolve. Molecular dynamics simulations were used to study how tracer diffusivity, the rate of chain exchange between micelles, and the rate of structural relaxation change with increasing degree of segregation or decreasing temperature in this state. Results for tracer diffusion and chain exchange are consistent with experimental results and confirm that diffusion in well-segregated systems occurs via hops of chains between neighboring micelles. A structural relaxation time τ_s is obtained from decay of the dynamic structure factor S(q*,t) at the peak wavenumber q*. The time τ_s increases dramatically with increasing degree of segregation, reaching values near the ODT of order 10² times the homopolymer Rouse time, with larger increases for systems with lower invariant degrees of polymerization. A theoretical prediction of the micelle lifetime, based on a model of stepwise micelle dissociation, yields an estimated lifetime of order 10³τ_s near the ODT, indicating that micelle birth and death may occur too slowly to be observed in these simulations. The relationship to experimental evidence is discussed.