Biological high-performance composites inspire to create new tough, strong, and stiff structural materials. We show a brittle-to-ductile transition in a self-assembled nacre-inspired poly(vinyl alcohol)/nanoclay composite based on a hydration-induced glass-to-rubber transition in the 2D-nanoconfined poly(vinyl alcohol) layers. The findings open routes to design dissipative toughening mechanisms to combine stiffness and strength in nanocomposites.

Even though nanocomposites have provided a plethora of routes to increase stiffness and strength, achieving increased toughness with suppressed catastrophic crack growth has remained more challenging. Inspired by the concepts of mechanically excellent natural nanomaterials, one-component nanocomposites were fabricated involving reinforcing colloidal nanorod cores with polymeric grafts containing supramolecular binding units. The concept is based on mechanically strong native cellulose nanocrystals (CNC) grafted with glassy polymethacrylate polymers, with side chains that contain 2-ureido-4[1H]-pyrimidone (UPy) pendant groups. The interdigitation of the grafts and the ensuing UPy hydrogen bonds bind the nanocomposite network together. Under stress, UPy groups act as sacrificial bonds: simultaneously providing adhesion between the CNCs while allowing them to first orient and then gradually slide past each other, thus dissipating fracture energy. We propose that this architecture involving supramolecular binding units within side chains of polymer grafts attached to colloidal reinforcements opens generic approaches for tough nanocomposites.

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Double-hydrophilic block copolymers of ethylene oxide and acrylic acid or methacrylic acid have been synthesized via radical polymerization using two different methods. Depending on the synthesis method used, branched or linear structures have been obtained. Solution properties of two different kinds of block copolymers have been investigated by dynamic light scattering and viscosimetry. The effects of the solution pH, ionic strength, and temperature on the complexation of the polymers have been studied. Branching affects strongly the flow properties of the polymer solutions. Rheologically the solutions of branched polymers are Newtonian. Fast diffusion of the polymer substructures could be detected by light scattering. Linear block copolymers show shear thinning behavior, as expected. The conformations of the polymer chains have been shown to depend on the solution pH, ionic strength and temperature. Also, the structure of polyelectrolyte complexes changes with temperature at low pH.

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