Temperature-Induced, Reversible Swelling Transitions in Multilayers of a Cationic Triblock Copolymer and a Polyacid

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Abstract

In this article, we seek to enable large-scale, fully reversible, thermally induced volumetric changes in layer-by-layer (LbL) electrostatically self-assembled thin films through the incorporation of A-B-A triblock copolymers. Poly(N,N-dimethylaminoethyl methacrylate)-b-poly(propylene oxide)-b-poly(N,N-dimethylaminoethyl methacrylate) (abbreviated “PD-PPO-PD”) was used as a dual pH and temperature-responsive component in the electrostatic self-assembly of multilayer thin films. In solutions of this triblock copolymer with poly(N,N-dimethylaminoethyl methacrylate) (PD) weak polyelectrolyte end blocks, the dehydration temperature of the central poly(propylene oxide) (PPO) block was strongly dependent on solution pH, as shown by microdifferential scanning calorimetry (micro-DSC) and dye solubilization techniques. Multilayer films were then assembled with poly(acrylic acid) (PAA) or poly(4-styrene sulfonate) (PSS) as anionic binding partners at various pH values, where the triblock copolymer was incorporated within the film either as unimers or micelles. Using in situ ellipsometry, we showed that the polyanion type and the self-assembly pH were both critical parameters for constructing functional films, which change their swelling degree in response to temperature. In particular, strongly associated PD-PPO-PD/PSS multilayers lacked temperature sensitivity and maintained a constant swelling degree in a wide range of pH and temperature. In contrast, the temperature response of PD-PPO-PD/PAA films was strongly dependent on the self-assembly pH. Whereas swelling of PD-PPO-PD/PAA films constructed at pH ≤ 5 was independent of temperature, multilayers assembled at pH ≥ 6 showed fully reversible, three- to five-fold changes in film thickness in response to temperature cycling between 6 and 20 °C, enabled by the ability of PPO domains to transit reversibly between the swollen hydrated and collapsed dehydrated states. These nanocomposite coatings show highly responsive, reversible swelling transitions that can be useful for future biomedical and device applications.

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