In polymer chemistry and polymer physics, the Flory–Fox equation is a simple empirical formula that relates molecular weight to the glass transition temperature of a polymer system.  The equation was first proposed in 1950 by Paul J. Flory and Thomas G. Fox while at Cornell University.  Their work on the subject overturned the previously held theory that the glass transition temperature was the temperature at which viscosity reached a maximum.  Instead, they demonstrated that the glass transition temperature is the temperature at which the free space available for molecular motions achieved a minimum value.  While its accuracy is usually limited to samples of narrow range molecular weight distributions, it serves as a good starting point for more complex structure-property relationships.
Recent molecular simulations have demonstrated that while the functional form of the Flory-Fox relation holds for a wide range of molecular architectures (linear chain, bottlebrush, star, and ring polymers), however, the central free-volume argument of the Flory-Fox relation does not hold since branched polymers, despite having more free ends, form materials of higher density and glass transition temperature increases.