ANISOTROPY OF DIFFUSION IN A LAMELLAR STYRENE-ISOPRENE BLOCK-COPOLYMER

The self-diffusion properties of a poly(styrene-b-isoprene) diblock co polymer (M-PS = 10000, M-PI = 13000) in a lamellar microstructure have been determined by forced Rayleigh scattering. Diffusion coefficients along the lamellar interface (D-par) and through the layers (D-perp) were resolved. The lamellae were macroscopically oriented using large amplitude oscillatory shear; small-angle X-ray scattering and transmis sion electron microscopy confirmed a high degree of alignment. The mea sured anisotropy of mobility was surprisingly large at 90 degrees C: D -par/D-perp approximate to 40. However, at 110 degrees C the anisotrop y decreased by an order of magnitude. This reduction is only partly at tributable to the approaching order-disorder transition (at 160 degree s C); the strong temperature dependence of the styrene monomeric frict ion factor in styrene-rich domains is also implicated. Further measure ments were made on a copolymer with half the total molecular weight (M -PS = 5000, M-PI = 6500), which was in the disordered state over the t emperature range of interest. These data confirm the importance of the composition and temperature dependences of the monomeric friction fac tors, coupled with the spatial homogeneity/heterogeneity of the averag e composition. The results are discussed in terms of proposed mechanis ms of diffusion in block copolymer microstructures.