Non-Gaussian Conformational Behaviour in Diblock Copolymer Melts: Is the RPA Valid?

By Monte Carlo simulation of a three-dimensional lattice model for a symmetric diblock copolymer melt, we show that significant deviations from Gaussian copolymer conformational behaviour set in far above the order-disorder transition. This gradual stretching of the copolymers with increasing interaction strength, accompanied by the emergence of a collective scale characterizing compositional fluctuations, implies a failure of the random phase approximation (RPA) throughout the disordered phase.