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 (Mn = 30 000, fPEP = 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 (Dpar) and perpendicular (Dperp) to the cylinder axes were resolved. For a high degree of macroscopic alignment both Dpar and Dperp were found to be significantly retarded compared to D0, the diffusion coefficient in the absence of any interactions. Dperp follows a hindered diffusion mechanism, while Dpar 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 Dperp was dependent on the degree of alignment, with the apparent Dperp increasing as the degree of alignment deteriorated, whereas Dpar was essentially independent of the degree of alignment. This behavior of Dperp is interpreted in terms of defects in the samples (i.e., both misoriented cylinders and dislocations).