Anisotropic self-diffusion in block copolymer cylinders

Self-diffusion measurements have been performed on an ordered asymmetric poly(ethylene-alt-propylene-b-dimethylsiloxane) PEP-PDMS block copolymer (M-n = 30 000, f(PEP) = 0.79) using forced Rayleigh scattering (FRS). Samples were prepared with three different degrees of macroscopic alignment of the hexagonally packed PDMS cylinders. The diffusion coefficients parallel (D-par) and perpendicular (D-perp) to the cylinder axes were resolved. For a high degree of macroscopic alignment both D-par and D-perp were found to be significantly retarded compared to D-0, the diffusion coefficient in the absence of any interactions. D-perp follows a hindered diffusion mechanism, while D-par is consistent with a block retraction mechanism; both of these mechanisms have been described previously in the context of block copolymer spheres and lamellae, respectively. The extracted D-perp was dependent on the degree of alignment, with the apparent D-perp increasing as the degree of alignment deteriorated, whereas D-par was essentially independent of the degree of alignment. This behavior of D-perp is interpreted in terms of defects in the samples (i.e., both misoriented cylinders and dislocations).