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 polystyren e-block-poly(acrylic acid) (PS-b-PAA) diblock copolymers were investig ated. The PAA/PS ratios were in the range 0.08-0.33, and N,N dimethylf ormamide/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 temp erature dependence of the critical micelle concentration (cmc) was det ermined under different conditions from scattered light intensity meas urements as a function of temperature. The closed association model wa s found to describe the micellization process and was used to calculat e the standard Gibbs free energies (Delta G degrees), the standard ent halpies (Delta H degrees), and the standard entropies (Delta S degrees ). The effects of solvent composition, copolymer composition, and adde d NaCl content on the thermodynamic functions were studied. From the e xperimental 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 + 0.0 906N(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) - 645mMNa a. 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 millimol ar concentration of NaCl Delta G degrees, Delta H degrees, and Delta S degrees values were all found to be negative under experimental condi tions, which indicates that the standard entropy is unfavorable and th at the standard enthalpy is solely responsible for micelle formation. However, it was also found that the enthalpy-driven process could be t ransformed to an entropy-driven process by increasing the water conten t or increasing the PS block length. The effect of homopolystyrene on the light scattering behavior during micellization was also studied. I t was found that the addition of homopolystyrene increased the critica l micelle temperatures (cmts) significantly.