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Swelling Transitions in Layer-by-Layer Assemblies of UCST Block Copolymer Micelles

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journal contribution
posted on 26.04.2018, 00:00 by Anbazhagan Palanisamy, Svetlana A. Sukhishvili
An upper critical solution temperature (UCST) block copolymer, poly­(acrylamide-co-acrylonitrile)-b-polyvinyl­pyrrolidone (P­(AAm-co-AN)-b-PVP), was synthesized by reversible addition–fragmentation chain transfer (RAFT) polymerization, assembled in solution, and deposited within functional layer-by-layer (LbL) temperature-responsive films. In aqueous solutions, the polymer formed well-defined block copolymer micelles (BCMs) at ambient temperature and exhibited a reversible micelle–unimer transition within an ∼40–50 °C temperature range in a wide range of pH and ionic strengths. Temperature-induced dissociation of BCMs to unimers was completely suppressed, however, when BCMs were assembled with tannic acid (TA) within LbL films. Instead, reversible changes in micellar sizes and film swelling occurred as a result of UCST-driven uptake/release of water within/from the micellar cores. The solution pH modulated strength of hydrogen bonding between TA and PVP in the micellar corona, thus strongly affecting film growth, micelle morphology, and film swelling. Spherical micellar morphology and high film swelling degrees were observed with films at neutral pH values, where hydrogen bonding was counteracted by the negative charge of partially ionized TA. In contrast, strong hydrogen bonding and absence of charge in TA caused crumpling of BCMs and reduced the film swelling degree in acidic solutions. Film swelling was also dependent on number of assembled micellar layers, with thicker films exhibiting larger swelling amplitudes and sharper temperature transitions. The LbL assemblies were stable in phosphate buffer saline up to pH 7.5 at 50 °C and preserved their response after 55 heating/cooling cycles. The robustness of the temperature transitions in these films taken together with their occurrence in an aqueous environment in a wide range of pH and ionic strengths makes these films potentially useful for controlling responses of soft interfaces in biological environments.