Polymer Plus Poor Solvent

We now consider a set of chains A (degree of polymerizationiV a) in a solvent of small molecules B Nb = 1). The chain-solvent interactions are always characterized by a Flory parameter x eq. (IV.6)] which depends on temperature T. In most (but not all) systems x(7) is a decreasing function of temperature. Experimental phase diagrams are always given in terms of a concentration c and a temperature T. For our general discussion it is more convenient to use more the fundamental quantities l = ctP (volume fraction) and x(7)- The phase diagram then has an universal structure, shown in Fig. IV,8, 

The following points are essential in connection with this phase diagram. 

Phase diagram for a polymer-solvent system, x is the Flory interaction parameter, and is the volume fraction occupied by the polymer. The condition x = I /2 defines the Flory 6 temperature, in usual cases such as polystyrene-cyclohexane, 

X is a decreasing function of the temperature T high temperatures correspond to the lower part of the diagram. 

This defines approximately the position of line L. Of course, L is not a sharp boundary it defines a region of crossover between ideal and swollen chains. 

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