Surface segregation in polymer mixtures

One may wonder why then the system does not simply have a very thin layer of the material of lower surface energy at the surface, to get the free energy 

Experimental techniques might be sensitive to 0i or z, whereas our aim in a theory would be to be able to calculate the whole profile 0(z). 

The simplest such theory, due originally to Cahn (1977) and adapted for polymer blends by Schmidt and Binder (1985), uses the square gradient approach of section 4.2 one writes down the free energy as a functional of the volume fraction profile, including a term proportional to the square of the 

Here is the volume of the Flory-Huggins lattice cell (usually taken as the monomer volume - note that b should not be confused with the statistical step length a) for convenience we have absorbed a factor of kT into the definitions of Ml and g. 

Of course this argument neglects any entropic factors arising from changes of correlations near the surface and from local packing effects and cannot be expected to be more than a very crude approximation. 

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