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 s
tudy 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.
We determine the statistics of the local tube width in F-actin solutions, beyond the usually reported mean value. Our experimental observations are explained by a segment fluid theory based on the binary collision approximation (BCA). In this systema
tic generalization of the standard mean-field approach effective polymer segments interact via a potential representing the topological constraints. The analytically predicted universal tube width distribution with a stretched tail is in good agreement with the data.
