Hydrogen-bonded layer-by-layer films of block copolymer micelles with pH-responsive cores

Abstract

We report on construction of hydrogen-bonded monolayers and multilayers of micelles of the poly(2-(diethylamino)ethyl methacrylate)- block-poly(N-isopropyl acrylamide) (PDEA- b-PNIPAM) with PNIPAM-corona and polybasic PDEA cores. Films were constructed at pH 7.5 and 25 °C to assure the deposition of PDEA- b-PNIPAM in the micellar form. When monolayers of block copolymer micelles (BCMs) were exposed to moderately acidic pH values, micellar cores dissolved, while PDEA- b-PNIPAM remained adsorbed at the surface as unimers. In contrast to reversible micellization of PDEA- b-PNIPAM in solution, micelle-to-unimer transition was irreversible at the surface. Adsorption of a layer of tannic acid (TA) or polyethacrylic acid (PEAA) on top of BCM monolayers inhibited pH-triggered morphological changes within adsorbed BCMs. By taking advantage of the high p K a values of TA and PEAA, we were also able to construct multilayers of PDEA- b-PNIPAM micelles through hydrogen bonding interactions between micellar PNIPAM coronas and TA or PEAA. Similar to BCM monolayers coated with TA or PEAA, dissolution of BCMs was also inhibited when incorporated within hydrogen-bonded multilayers. Such inhibition of dissolution is due to enhanced hydrogen bonding interactions between coronal PNIPAM chains and protonated TA molecules or PEAA chains at decreasing pH values restricting the pH-induced conformational changes of the micellar core chains within the adsorbed layer. Films of responsive BCMs are attractive coatings for controlled delivery of functional molecules from surfaces due to a combination of stimuli-response properties with the relatively high loading capacity for functional molecules.