  Simulations of five different coarse-grained models of symmetric diblock
copolymer melts are compared to demonstrate a universal (i.e.,
model-independent) dependence of the free energy on the invariant degree of
polymerization $\overline{N}$, and to study universal properties of the
order-disorder transition (ODT). The ODT appears to exhibit two regimes:
Systems of very long chains ($\overline{N} \gtrsim 10^{4}$) are well described
by the Fredrickson-Helfand theory, which assumes weak segregation near the ODT.
Systems of smaller but experimentally relevant values, $\overline{N} \lesssim
10^4$, undergo a transition between strongly segregated disordered and lamellar
phases that, though universal, is not adequately described by any existing
theory.
