Flory Huggins Theory
The assumptions made in the Flory-Huggins theory are


Quasi-solid lattice in the liquid
Inter-changeability of segments (not necessarily the same as the polymer structure
units) of polymer and solvent molecules in the lattice
Independence of lattice constants on composition (artificial)
Polymer molecules are of same size
Average concentration of polymer segments in cells adjacent to cells unoccupied
by the polymeric solute is taken to be equal to the overall average concentration
The expected number of available positions for each successive segment is
overestimated in the formulation as the formula includes double counting of
segments, separated by 2 or more segments in the same chain, that will fall on the
same position twice


In particular the assumption that polymer molecules are of the same size is a very unrealistic one, which was later removed in the Flory-Krigbaum theory