Thermodynamics of aggregation of poly(2-ethyl-2-oxazoline-b-4-vinylbenzoic acid) in water and colloidal stability of their aggregates in water/DMF mixtures

Abstract

Block copolymer aggregates have attracted considerable interest due to their potential applications which vary from biomedicine to catalysis. Herein, the synthesis and the self-assembly behavior of poly(2-ethyl-2-oxazoline-b-4-vinylbenzoic acid) - P(EtOx-b-4VBA) - in aqueous medium were reported. A series of P(EtOx-b-4VBA) with different compositions were prepared by the sequential cationic ring-opening polymerization of EtOx and atom transfer radical polymerization of 4VBA, achieving molar dispersity indices below 1.3. Their critical aggregation concentration (c.a.c.) and the thermodynamics of their aggregation in water were studied by fluorescence spectroscopy using pyrene as a probe, revealing a strong dependence on temperature and on the 4VBA/EtOx molar ratio. P(EtOx-b-4VBA) presented a c.a.c. vs. temperature behavior different from that usually observed for low molar mass surfactants. The thermal properties were studied, and the relationship of the glassy P4VBA block (Tg > 143 °C) with the self-assembly in solution was discussed in terms of the stability of P(EtOx-b-4VBA) aggregates and the possible protocols that can be used to prepare them. The self-assembly promoted by solvent switch from a DMF solution with water and dialysis of this solution against water resulted in aggregates with different sizes and colloidal stability. The rate of water addition was determinant towards formation of kinetically stable aggregates as well as on their properties, as revealed by DLS, Zeta-potential measurements, and cryo-TEM images. Finally, the loading of a model hydrophobic molecule, Nile red, was studied, indicating that the aggregates display capacity for the encapsulation of hydrophobic compounds.