  Recent experiments in some polymer melts quenched below the melting
temperature have reported spinodal kinetics in small-angle X ray scattering
before the emergence of crystalline structure. To explain these observations we
propose that the coupling between density and chain conformation induces a
liquid-liquid binodal within the equilibrium liquid--crystalline solid
coexistence region. A simple phenomenological theory is developed to illustrate
this idea, and several experimentally testable consequences are discussed.
Shear is shown to enhance the kinetic role of the hidden binodal.
