Complexation of poly(2-ethyl-2-oxazoline)-block-poly(ε-caprolactone) micelles with multifunctional carboxylic acids

Polymeric micelles derived from amphiphilic block copolymers of poly(2-ethyl-2-oxazoline) (PEtOz) and poly(ε-caprolactone) (PCL) were prepared in an aqueous phase. Dynamic light scattering and TEM experiments showed PEtOz-PCL micelles have a single core-shell structure with a mean hydrodynamic diameter range of 18-25 nm. The complex formation behavior of PEtOz-PCL micelles with low molecular weight multifunctional carboxylic acids such as malonic acid (MA), tricarballylic acid (TCA), and 1,2,3,4-butanetetracarboxylic acid (BTCA) was investigated using dynamic light scattering in an aqueous media. As the molar concentration ratio of COOH group to EtOz repeating unit ([COOH]/ [EtOz]) in the block copolymer increased, MA and TCA began to induce intermicellar aggregation at [COOH]/[EtOz] = 160, which resulted in the formation of large aggregates with a diameter range 96-146 nm. As [COOH]/[EtOz] continued to increase, the aggregate size became larger. On the other hand, the mixture of micellar solution and BTCA began to undergo intermicellar aggregation even at [COOH]/ [EtOz] = 4. The complex formation behavior of PEtOz-PCL micelles with poly(carboxylic acid)s was also investigated. Addition of poly(acrylic acid) (PAA) with M_w = 2000 into the PEtOz-PCL micellar solutions induced the formation of complex precipitates at [COOH]/[EtOz] = 2.5. The block copolymer micelles formed complex precipitates with PAA2000 (M_w = 2000) and PAA5000 (M_w = 5000) at pH < 3.3. The higher the molecular weight of PAA utilized for the complex formation, the higher the yield of complex precipitates. On the other hand, the block copolymer micelles formed complex precipitates with poly-(methacrylic acid) (PMAA) with M_w = 6400 at pH < 4.9. The complex formed from micelles and PAA or micelles and PMAA could be reversibly redispersed as micelles at pH > 3.6 or pH > 5.2, respectively. The reversible micelle releasing behavior from the complex was investigated in aqueous media. The micelle release rate was highly dependent on the pH of release medium, molecular weight, and chemical structure of poly(carboxylic acid)s.