Lamellar block copolymers: Diffusion and reduction of entanglement effects

Motion of symmetric diblock copolymer chains in the macroscopically oriented lamellar phase is studied by molecular-dynamics simulation. For diblocks of length N = 40 (1.1N(e)) and 100 (3N(e)), where N-e is the entanglement length of the homopolymer melt, the diffusion constant parallel to the interface D-parallel to is nearly independent of chi, while the perpendicular component D-perpendicular to is strongly suppressed as chi, the Flory-Huggins parameter, is increased in agreement with theoretical predictions based on the Rouse model. For even longer chains (N up to 400) a strong reduction of entanglement effects is observed. We argue that due to the reduced chain overlap in the lamellar phase in this intermediate regime the chain motion is similar to an ellipsoidal molecule in a smectic phase.