Thermodynamic Behavior of Non-Ionic Tri-block Copolymers in Water at Three Temperatures

Apparent molar volumes (V ) of some copolymers, based on ethylene oxide(EO) and propylene oxide (PO) units, in aqueous solutions were determined as functions ofconcentration at three temperatures. Viscosity measurements were also carried out on some ofthese systems. The effects studied include how the molecular architecture and the molecularweight, keeping constant the EO/PO ratio, affect the aggregation of the copolymer. Modelingof the volumetric data yielded the partial molar volume of the copolymer in the standard(V ◦ ) and the aggregated (V M ) states, as well as the equilibrium constant for micellizationand the aggregation number. Analysis of the viscosity data supported the insights obtainedby modeling the volumetric data. At a given temperature, both V ◦ and V M , normalized forthe number of the EO and the PO units, are linearly related to the fraction of the EO inthe copolymer, regardless of the copolymer nature. These correlations are powerful tools forpredicting values of both V ◦ and V M for copolymers not yet investigated. For macromoleculeshaving the same molecular architecture, the standard Gibbs energies of micellization ( G ◦ m )are slightly negative within the errors of their determination and are hardly affected bytemperature changes. Also, their aggregation numbers are small. From the quantitativeanalysis of the viscosity data, insights were obtained that corroborated the thermodynamicfindings. Finally, values of G ◦ m , normalized for the EO and the PO units, show that thesame driving forces control the self-assembling processes for copolymers having differentmolecular weight but the same EO/PO ratio