Thermodynamic Analysis of Scanning Calorimetric Transitions Observed For Dilute Aqueous-Solutions of ABA Block Copolymers

Dilute aqueous solutions of a series of poly(oxyethylene)-poly(oxypropylene)-poly(oxyethylene) block polymers have been shown to undergo phase transitions. A high-sensitivity differential scanning calorimetric (HSDSC) study of these block copolymers bas enabled the thermodynamic parameters for the phase transitions to be obtained. The thermodynamic parameters are all dependent on poly(oxypropylene) content of the polymer and riot on poly(oxyethylene) content or total polymer molecular mass. The importance of the poly(oxypropylene) in determining the aqueous solution phase properties of these polymers is further emphasized by the positive value for the heat capacity change from pre- to postphase transition. A positive heat capacity change is characteristic of "melting" of water ordered by exposed nonpolar groups. The work reported in this paper shows that the thermodynamic parameters describing the polymer phase transitions can be derived from basic thermodynamic principles and from consideration of the poly(oxypropylene) content. The theoretical analysis described in this paper also predicts that these polymers should undergo a second phase transition at high temperature and indeed these transitions have been detected by HSDSC.