BLOCK-COPOLYMER SELF-DIFFUSION IN THE GYROID AND CYLINDER MORPHOLOGIES

Forced Rayleigh scattering (FRS) and pulsed-field-gradient NMR have be en used to measure the self-diffusion coefficient, D, of a poly(ethyle ne oxide)-poly(ethylethylene) diblock copolymer in the molten state. T he copolymer contains 42% PEO by volume and has a total molecular weig ht of 4100 g/mol. Upon heating from room temperature the sample transf orms from crystalline lamellae to hexagonal cylinders, and then to a b icontinuous cubic ''gyroid'' phase (with Ia (3) over bar d space group symmetry), before finally disordering at 175 degrees C. FRS measureme nts were performed in the gyroid and cylinder phases, and NMR measurem ents in the gyroid and disordered states. Cylinder samples both with a nd without shear alignment were employed. A hysteresis loop permitted measurements of D in both cylinder and gyroid phases at the same tempe rature (60 degrees C). FRS decays from cylindrical samples were descri bed by a sum of two exponentials. For the aligned samples, values of t he diffusivity along (D-par) and across (D-perp) the cylinders were ex tracted; the mobility along the cylinders was approximately 2 orders o f magnitude larger. This is consistent with the estimated enthalpic pe nalty for withdrawing the minor block from the cylindrical microdomain . FRS decays from the gyroid phase were consistently single exponentia l and gave a diffusivity consistent with D-par in the cylinders, reduc ed by the tortuosity of the gyroid network. The FRS and NMR results ag reed very well, and the mobility varied smoothly with temperature thro ugh the order-disorder transition. However, the magnitude of the copol ymer mobility was significantly lower than that of either constituent homopolymer or of two other disordered PEO-PEE diblocks, even after ac counting for differences in molecular weight. This is tentatively attr ibuted to the onset of entanglement effects.