Thermodynamics of crew-cut micelle formation of polystyrene-b-poly(acrylic acid) diblock copolymers in DMF/H2O mixtures

The thermodynamics of formation of ''crew-cut'' micelles of polystyrene-block-poly(acrylic acid) (PS-b-PAA) diblock copolymers were investigated. The PAA/PS ratios were in the range 0.08-0.33, and N,N dimethylformamide/water (DMF/H2O) mixtures were used as the selective solvent. For the block ratios investigated here, the copolymers form spherical crew-cut micelles with a large PS core and a thin PAA corona. The temperature dependence of the critical micelle concentration (cmc) was determined under different conditions from scattered light intensity measurements as a function of temperature. The closed association model was found to describe the micellization process and was used to calculate the standard Gibbs free energies (Delta G degrees), the standard enthalpies (Delta H degrees), and the standard entropies (Delta S degrees). The effects of solvent composition, copolymer composition, and added NaCl content on the thermodynamic functions were studied. From the experimental data, a single set of combined equations for the standard thermodynamic functions was obtained. These are the following: Delta G similar to(298K) (kJ mol(-1)) = 20.4 - 0.00859N(PS) +/- 0.00938N(PAA) - 10.2W(H2O) -3.65mM(NaCl); Delta H degrees (kJ mol(-1)) = -151 + 0.0906N(PS) - 0.0692N(PAA) + 738W(H2O) - 229mM(NaCl); Delta S degrees (J mol(-1) K-1) = -574 +/- 0.333N(PS) - 0.264N(PAA) + 590W(H2O) - 645mMNaa. NPS and NPAA are the block lengths of the two copolymer components; W-H2O is the weight percent of water in DMF; mM(NaCl) is the millimolar concentration of NaCl Delta G degrees, Delta H degrees, and Delta S degrees values were all found to be negative under experimental conditions, which indicates that the standard entropy is unfavorable and that the standard enthalpy is solely responsible for micelle formation. However, it was also found that the enthalpy-driven process could be transformed to an entropy-driven process by increasing the water content or increasing the PS block length. The effect of homopolystyrene on the light scattering behavior during micellization was also studied. It was found that the addition of homopolystyrene increased the critical micelle temperatures (cmts) significantly.