We report measurements of chain diffusion in macroscopically oriented,
well-entangled lamellar block copolymers. Diffusion in the lamellar p
lane is proposed to occur by an ''activated reptation'' mechanism when
the composition profile is roughly sinusoidal. At higher degrees of s
egregation a crossover to an entropically limited ''block retraction''
mechanism is apparent. Diffusion through the layers reflects a thermo
dynamic barrier determined by the combination of temperature and molec
ular weight.