  Using the language of the Flory chi parameter, we develop a theory that
unifies the treatment of the single-chain structure and the solution
thermodynamics of polymers in poor solvents. The structure of a globule and its
melting thermodynamics is examined using the self-consistent filed theory. Our
results show that the chain conformation involves three states prior to the
globule-to-coil transition: the fully-collapsed globule, the swollen globule
and the molten globule, which are distinguished by the core density and the
interfacial thickness. By examining the chain-length dependence of the melting
of the swollen globule, we find universal scaling behavior in the chain
properties near the Theta point. The information of density profile and free
energy of the globule is used in the dilute solution thermodynamics to study
the phase equilibrium of polymer solution. Our results show different scaling
behavior of the solubility of polymers in the dilute solution compared to the
F-H theory, both in the chi dependence and the chain-length dependence. From
the perspectives of single chain structure and solution thermodynamics, our
results verifies the consistency of the Theta point defined by different
criteria in the limit of infinite chain length: the disappearance of the second
viral coefficient, the abrupt change in chain size and the critical point in
the phase diagram of the polymer solution. Our results show the value of chi at
the Theta point is 0.5 (for the case of equal monomer and solvent volume),
which coincides with the value predicted from the F-H theory.
