Hydrated behavior of multilayer polyelectrolyte-nanoclay coatings on porous materials and demonstration of shape memory effect

Layer-by-layer (LbL) assembly is a powerful technique for fabricating nanocomposite thin-film coatings with a diverse range of constituents, properties, and functionalities. Templated deposition of these coatings has enabled the translation of mechanical properties from the microscale of thin-films to the macro-scale of nanocomposite-coated porous materials and has been used to tailor the elastic modulus and porosity of coated open-cell foams for potential applications including lightweight structures and engineered tissue scaffolds. However, the presence of moisture in these application environments is expected to affect the physico-mechanical behavior of the nanocomposite coating. In this work, open-cell foams coated with nanocomposites consisting of poly(ethyleneimine), poly(acrylic acid), and Na⁺-montmorillonite were characterised under high relative humidity and upon complete submersion in water. The nanocomposite coating imparted a substantial increase in compressive elastic modulus when tested under ambient conditions, from 0.08 ± 0.00 MPa to 4.90 ± 0.46 MPa, but had little to no mechanical effect when hydrated, and upon drying the mechanical properties of coated foams recovered to pre-hydrated levels. Chemical crosslinking of amine groups within the polymers resulted in the retention of significant compressive elastic modulus of 2.91 ± 0.49 MPa when hydrated. Initial trials showed that un-crosslinked coated foams exhibit a hydration induced shape memory effect that could be used to enable the actuation or expansion of a previously passive open-cell foam.