THERMODYNAMICS OF PHASE-SEPARATION IN MIXTURES OF ASSOCIATING POLYMERS AND HOMOPOLYMERS IN SOLUTION

The phase behavior of mixtures of a hydrophobically modified polymer c apable of forming associations and an unmodified polymer of the same c hemical composition has been investigated. The associative thickener ( AT) consists of poly(ethylene glycol) (PEG) end-capped at both ends wi th hydrophobic alkane groups. It is found that mixtures of the AT solu tion and the unmodified PEG phase separate into two distinct solutions , provided AT molar fractions are <0.65. This is despite the fact that the overall polymer concentration throughout both phases remains the same. However, the viscosities of the two phases were found to be very different. For example, for a 50:50 mixture at an overall concentrati on of 5 wt %, the viscosity of one phase was found to be 170 Pa s, con sistent with the existence of an AT network, whereas the viscosity of the less viscous solution was measured to be 0.004 Pa s, indicating th at nearly all the polymer in this phase is of the unmodified type. Whe n the AT to total polymer ratio is >0.65 or when the overall polymer c oncentration is below 2%, no phase separation is observed. It is propo sed that the phase separation behavior in this type of system is an en tropically driven phenomenon. It is shown that by considering the entr opy changes due to the association of the end groups, within a Flory-H uggins framework, all of the above features can be accounted for theor etically.