Volumes of Aqueous Block Copolymers Based on Poly(Propylene Oxides) and Poly(Ethylene Oxides) in a Large Temperature Range: A Quantitative Description

The focus of this paper was on a quantitative comprehension of temperature effect on the volumes of aqueous di-block and triblock copolymers, based on propylene oxide (PO) and ethylene oxide (EO) units. To this purpose, literature data dealing with (EO316PO94 + water) and (EO13PO30EO13 + water) mixtures were analyzed. The volume vs. temperature trends were rationalized on the basis of the (unimers + aggregate) equilibrium by taking into account the temperature effect on both the partial molar volumes of the unimeric and the aggregated copolymer as well as the equilibrium constant of micellization. The analysis extended to the expansibility allowed to quantify the contribution for the shift of the (unimers + aggregate) equilibrium induced by temperature changes which is responsible for the sharp and large maximum exhibited by the expansibility vs. temperature trend. The simulated curves agree with the experimental points supporting the reliability of the approach and the used parameters. From this treatment one may deduce that the volume vs. temperature trend can be divided into three regions: (1) the domain below the critical micellar temperature (cmt), where the property increases linearly, characterized by the presence of the unimeric copolymer; (2) the region above cmt, where the property changes monotonically, reflecting the coexistence of the copolymer in both the unimeric and aggregate states and (3) the domain in the high temperature region, where the property once more increases linearly, featured by the presence of the copolymer in the aggregate state. Such a description may be considered valid also for other similar copolymer systems which exhibit the same profile of the volume vs. temperature curve. From this analysis one concludes that caution must be exercised when the bulk volumes are examined to acquire information on the state of the aggregated copolymer. For instance, to achieve reliable insights on the hydration of the micellar core, in the absence of data modeling or acquaintance of the fraction of the aggregated copolymer, one may compare the volume of pure PO with the volume of PO, obtained from copolymers in water, in the highest temperature region. (c) 2006 Elsevier Ltd. All rights reserved.