Hydrogen-bonded multilayers of micelles of a dually responsive dicationic block copolymer

Abstract

We report the fabrication of hydrogen-bonded multilayers of micelles of a dually responsive, dicationic block copolymer, poly[2-(N-morpholino)ethyl methacrylate-block-2-(diisopropylamino)ethyl methacrylate] (PMEMA-b-PDPA). By taking advantage of the difference in the hydrophilicity of PMEMA and PDPA blocks, micelles with a PMEMA-corona and a PDPA-core were obtained above pH 6.5 and were assembled layer-by-layer at the surface with tannic acid (TA) at pH 7.4 through hydrogen bonding interactions between morpholino units of PMEMA and hydroxyl groups of TA. Destruction of PMEMA-b-PDPA micelles/TA films could be controlled at both acidic and basic conditions. At basic pH (pH = 8.75), multilayers disintegrated due to ionization of TA and disruption of hydrogen bonding interactions between layers of micelles and TA. At moderately acidic pH values, partially dissolved PMEMA-b-PDPA micelles and monomers underwent a restructuring with TA molecules and remained adsorbed at the surface. Complete dissolution of the multilayers occurred at around pH 3.6 due to further protonation of the tertiary amino groups on both blocks of PMEMA-b-PDPA, resulting in a charge imbalance between PMEMA-b-PDPA and TA layers followed by disintegration of the films. We have also encapsulated pyrene in the micellar cores and found that pyrene released from PMEMA-b-PDPA micelles/TA films increased 1.5- and 2.5-fold when the pH was decreased from 7.5 to 6 and 5, respectively. Such an increase in the amount of pyrene released was due to pH-controlled dissolution of the micellar cores. We have also found that at pH 7.5, increasing the temperature to 40 degrees C enhanced the release of pyrene by approximately 2-fold. Such an increase is due to lower critical solution temperature (LCST) behaviour of coronal PMEMA chains leading to temperature-induced conformational changes on the coronal chains, facilitating the release of pyrene through the coronal chains into the solution. Hydrogen bonded multilayers of micelles of a dicationic block copolymer are interesting due to the response of both multilayers and micellar cores at different pH paving the way for multiple pH-controlled delivery of functional molecules from surfaces.