In order to avoid dehydration, plants utilize a thin composite membrane, the cuticle, as a protective barrier. These membranes consist primarily of three components: a non-polar cutin matrix (a cross-linked, fatty acid-based polyester), hydrophilic polysaccharides, which usually ramify from the epidermal cell wall to the outside, where their concentration is gradually reduced, and hydrophobic waxes, which dominate the surface. Thus, cutiles are compositionally graded membranes in which the hydrophilicity drops from the inside to the outside. Inspired by this architecture, we prepared structurally similar, dense nanocomposite membranes consisting of cellulose nanocrystals (CNCs) – which provide the hydrophilic features – and a hydrophobic poly(styrene)-b-poly(butadiene)-b-poly(styrene) (SBS) matrix. We combined solvent casting and sedimentation processes to create compositionally graded membranes, in which the CNC content varied in the direction orthogonal to the membranes, leading to CNC-rich and CNC-free sides. This architecture was confirmed by attenuated total reflectance infrared spectroscopy as well as Raman and scanning electron microscopy. We investigated the impact of this asymmetry on the water-transport properties by testing water permeation from both sides through the membranes. Indeed, direction-dependent permeation properties were observed, with asymmetry factors as high as 2.5. Interestingly, it appears that in spite of the long-known structural asymmetry of plant cuticles, asymmetric water transport properties through such membranes has not been previously reported. Thus, we also investigated the water transport through olive cuticles (Olea europaea) in a similar manner. Gratifyingly, we were able to confirm that also in the biological model, water transport is directional and reflects the graded structure of these biological membranes. In summary, graded architectures are omnipresent in nature and lead, as we show here, to asymmetric transport properties. We introduce a straightforward method to achieve such graded structures with artificial nanomaterials and their directional transport characteristics could potentially be interesting for dense membrane applications such as “smart” directional packaging, gas separations, and pervaporation processes.

Nanocomposite graded membranes acting as water permeation valves inspired by plant cuticles

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.