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).