Thermodynamics of Crew-Cut Micelle Formation of Polystyrene-b-Poly(acrylic acid) Diblock Copolymers in DMF/H₂O Mixtures

Abstract

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,Ndimethylformamide/water (DMF/H 2 O) 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 (∆G°), the standard enthalpies (∆H°), and the standard entropies (∆S°). 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: ∆G°2 98K (kJ mol -1 ) ) 20.4 -0.00859N PS + 0.00938N PAA -10.2W H2O -3.65mM NaCl ; ∆H°(kJ mol -1 ) ) -151 + 0.0906N PS -0.0692N PAA + 7.38W H2O -22.9mM NaCl ; ∆S°(J mol -1 K -1 ) ) -574 + 0.333N PS -0.264N PAA + 59.0W H2O -64.5mM NaCl . N PS and N PAA 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. ∆G°, ∆H°, and ∆S°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.

Poly[2,7‐(9,9‐dihexylfluorene)]‐block‐Poly(2‐vinylpyridine) Rod–Coil Star‐block Copolymers: Synthesis, Micellar Structures, and Photophysical Properties

“…Because of the interplay between the interfacial tension and the shell repulsion, the system tends to decrease its free energy through increasing the lengths of aggregates to form the rods or vesicles. [14] This morphological phenomenon of PF 8 -b-P2VP 11 is quite different from that in our previous report (PF , di-PFPVP), which maintained the spherical micelles with increase of methanol content. [5c] This is due to the relative small fraction of coil P2VP of PF 8 -b-P2VP 11 that prefers to form vesicle due to the lower interfacial curvature in coilselective solvent.…”

Morphological Phase Diagram for a Ternary System of Block Copolymer PS310-b-PAA52/Dioxane/H2O

“…The above process was repeated several times until no homopolystyrene was detected in the reverse micelle phase by GPC. 50 The sodium acrylate blocks were then converted back to the acrylic acid form. The copolymers PS 310 -b-PAA 52 and PS 310 -b-PAA 75 were used in the present study; the subscript labels indicate the number of repeat units in the blocks.…”

Block‐Selective Solvent Influence on Morphology of the Micelles Self‐Assembled by PS38‐b‐P(AA190‐co‐MA20)

“…[5,9] One of the noteworthy phenomena associated with crew-cut aggregates is the accessibility of a wide range of morphologies such as spheres, rods, vesicles, lamellae, large compound vesicles, a hexagonally packed hollow hoop structure, large compound micelles and nanotubes. [10][11][12][13][14][15][16][17][18] The morphology of the aggregates is a function of several variables, such as Summary: The influence of block-selective solvent on the self-assembly of polystyrene-block-poly[(acrylic acid)-co-(methyl acrylate)] was studied. The nature of the blockselective solvent, which is a binary solvent mixture with different composition, exerts remarkable influence on the morphology of the resulting micelles.…”