Local Monomer Activation Model for Phase Behavior and Calorimetric Properties of LCST Gel-Forming Polymers

Abstract

We investigate the phase diagrams and calorimetric signatures of gel-forming polymers with a lower critical solution temperature (LCST). We construct a model that captures many of the observed properties of this type of polymer. The model assumes the existence of a sharp internal conversion, at a characteristic temperature T*, in which monomers are activated from effectively hydrophilic to associative-hydrophobic states. The location of the gel transition is determined by Flory's model of gelation, while phase separation of polymer and solvent is determined by assuming that the effective immiscibility χ parameter is generated solely from gel-forming contacts. The resulting phase diagrams exhibit an immiscibility loop and a gel−sol boundary. At low temperatures both the phase and the sol−gel boundaries are almost independent of concentration. The gel transition does not produce a signature in the calorimetric properties (heat capacities at constant concentration), but the internal transition produces a clear signal near the gel transition point. The phase separation produces a finite positive jump in the excess heat capacity but is immediately followed by an exothermal feature. We discuss examples of experimentally studied polymeric systems with properties consistent with those of the model.