Polydispersity Correction to the Order-Disorder Transition of Symmetric Diblock Copolymers Melts*

Simulations of symmetric diblock copolymer melts have shown that the order-disorder transition (ODT) is given by (χN)_ODT = 10.495 + 41Ñ^-1/3 + 123Ñ^-0.56, where Ñ is the invariant polymerization index. This result, however, is specific to monodisperse molecules, whereas there is always some degree of polydispersity in experiments even when the molecules are synthesized by anionic polymerization. Using simulations, the polydispersity correction to the ODT is evaluated for the small degrees of polydispersity characteristic of anionic polymerization. We find that polydispersity reduces (χN)_ODT for large Ñ, but increases (χN)_ODT for small Ñ. These corrections should allow experimentalists to estimate the Flory-Huggins χ parameter with greater accuracy.