Self-diffusion of an asymmetric diblock copolymer above and below the order-to-disorder transition temperature

The self-diffusivity of an asymmetric poly(ethylenepropylene)-poly(dimethyl siloxane) diblock copolymer (PEP-PDMS) was investigated using pulsed field gradient NMR both in the ordered and the disordered state. In a temperature range below the order-to-disorder transition temperature, the diblock copo lymer forms micelles which order in a simple cubic structure, and at lower temperatures, a noncubic structure is formed. We have found a broad distrib ution of self-diffusivities. The distribution width increases with decreasi ng temperature. The self-diffusivity as well as its distribution width chan ges only slightly at the order-to-disorder transition. In contrast to self- diffusion in symmetric diblock copolymers where single chains diffuse indep endently, our results are interpreted as a collective diffusion of chains c onfined to micelles which further cluster to larger aggregates. This behavi or cannot be described by existing models of chain dynamics in the melt. Th e disordered state is characterized by a large temperature range above the order-to-disorder transition where concentration fluctuations dominate. Thi s is revealed in the temperature dependence of the self-diffusivity and its distribution width, in the viscosity and in the inverse intensity of the f irst-order Bragg reflection in small angle neutron scattering (SANS). (C) 1 999 American Institute of Physics. [S0021-9606(99)50229-8].