pH-induced reversible complexation of poly(ethylene glycol) and poly(ε-caprolactone)-b-poly(methacrylic acid) copolymer micelles

The pH-induced reversible complexation between poly(ethylene glycol) (PEG) and the polymer micelles of a pyrene-labeled poly(epsilon-caprolactone)-b-poly(methacrylic acid) copolymer (Py-PCL-b-PMAA) was investigated in an aqueous phase. At pH 7.4-4.0, the polymer micelles and PEG existed separately. The complexation began at pH 3.9 through hydrogen-bonding to produce the colloidal hybrid featuring PEG-mediated long-range interconnected micelles. Further pH decrease resulted in the precipitation of the white complex hybrid in water. This pH-dependent complexation was a reversible process. The polymer micelles in the solid complex hybrid could be quantified by fluorescence spectroscopy. The pyrene fluorescence intensity of the aqueous mixture was constant in the pH range 7.4-4.0, indicating that the solid hybrids were not formed, whereas the pH decrease to 3.9 resulted in the decrease of fluorescence intensity at a level of 58.5%, indicating complexation of 58.5% micelles into the solid hybrid. The pyrene fluorescence intensity at pH below 3.5 corresponded to about 0.1% of that of the initial micelle solution prepared at pH 7.4, reflecting that about 100% of the micelles are complexed with PEG to form the solid hybrid. The micelles assembled in the hybrid could be reversibly dispersed as micelles at pH > 4.0. The pH-controlled release experiments showed that the micelles could be redispersed from the hybrid at physiological pH (pH 7.4), whereas at an acidic pH (pH 2.0) the micelle release was completely prohibited.