Computer simulations are used to study composition fluctuations in disordered diblock copolymer melts over a range of values of the chain length $N$, and test several theories for the structure factor $S(q)$. Specifically, we test the random-phase approximation (RPA), which is based on a self-consistent field treatment of fluctuations, the Fredrickson-Helfand theory, which was designed to describe fluctuations near the order-disorder transition, and the relatively new renormalized one-loop (ROL) theory. The results confirm claims that the RPA is exact in the limit $N\to \infty$ and that the ROL theory yields the dominant corrections to the RPA within a systematic expansion in powers of $N^{-1/2}$, and show that the ROL theory is much more accurate than either older theory.

• Received 8 June 2011

DOI:https://doi.org/10.1103/PhysRevLett.108.238301