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

Abstract

Apparent molar volumes (V Φ) of aqueous solutions of some copolymers, based on ethylene oxide (EO) and propylene oxide (PO) units, were determined as functions of concentration at three temperatures. Viscosity measurements were also carried out on some of these systems. The effects studied include how the molecular architecture and the molecular weight affect the aggregation of the copolymer, keeping constant the EO/PO ratio. Modeling of 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 micellization and the aggregation number. Analysis of the viscosity data supported the insights obtained by modeling of the volumetric data. At a given temperature, both V° and V M, normalized for the number of the EO and the PO units, are linearly related to the fraction of the EO in the copolymer, regardless of the copolymer nature. These correlations are powerful tools for predicting values of both V° and V M for copolymers not yet investigated. For macromolecules having the same molecular architecture, the standard Gibbs free energies of micellization ( $$\Delta G^\circ _{\rm m}$$ ) are slightly negative within the errors of their determination, and are hardly affected by temperature changes. Also, their aggregation numbers are small. From the quantitative analysis of the viscosity data, insights were obtained that corroborated the thermodynamic findings. Finally, values of $$\Delta G^\circ _{\rm m}$$ , normalized for the EO and the PO units, show that the same driving forces control the self-assembling processes for copolymers having different molecular weight but the same EO/PO ratio.