Enthalpy of micellisation of a diblock copoly(oxyethylene/oxypropylene) by isothermal titration calorimetry. Comparison with the van't Hoff value

Block copolymer P94E316 (P = oxypropylene, E = oxyethylene, and the subscripts denote number-average block lengths) has been studied by isothermal titration calorimetry to determine critical micelle concentrations (c.m.c.) and enthalpies of micellisation at temperatures in the range 20-35°C. Comparison with values of the c.m.c. determined previously by other methods shows good agreement. Comparison of the enthalpies of micellisation with standard values determined using the van't Hoff equation indicates a marked difference in values derived from the two methods. We speculate that the effect is caused by the P-block-length distribution in the copolymer.