The Flory–Fox equation serves the purpose of providing a model for how glass transition temperature changes over a given molecular weight range.  Another method to modify the glass transition temperature is to add a small amount of low molecular weight diluent, commonly known as a plasticizer, to the polymer.  The presence of a low molecular weight additive increases the free volume of the system and subsequently lowers T , thus allowing for rubbery properties at lower temperatures.  This effect is described by the Fox equation:
Where w  and w  are weight fractions of components 1 and 2, respectively.  In general, the accuracy of the Fox equation is very good and it is commonly also applied to predict the glass transition temperature in (miscible) polymer blends and statistical copolymers.