Nacre composition and synthesis to produce ultrastong and stiff materials

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Mother of pearl, also known as nacre, is one of the strongest mineralized biomaterials known. It is composed of sheets of planar calcium carbonate bound together with elastic biopolymers like proteins or polysaccharides. Researchers at University of Michigan have successfully mimicked the components of this material as well as the layer-by-layer (LBL) synthesis mechanism. The material shows favorable physical properties including: stress response similar to steel, light weight composition, and transparency. Instead of calcium carbonate, the researchers synthesized nanoplatelets made of a silicate based mineral (MTM clay). Polyvinyl alcohol (PVA) acts as the elastic mortar to the platelets brick. Layers of each (of only nanometer-scale thickness) were added in an alternating manner- 600 total layers produced material about as thick as plastic wrap. This LBL method yielded dense, planar nanoplatelets in linear uniform growth. Cooperative hydrogen bonds (i.e. Velcro effect) were found to form between the MTM and PVA. Moreover, covalent bonds between PVA and aluminum atoms in the MTM form to produce a strong, load-bearing, cross-linked matrix. A further cross-linking agent called glutaraldehyde (GA) was found to increase the physical properties to match those of Kevlar and carbon nanotube-based fibers. The researchers performed this synthesis at room temperature. Layers were added simply by programming a robotic arm to dip a glass stick into alternating solutions of PVA and MTM.

Key Differentiators

Ceramics must be produced at tremendous temperatures. In contrast, the described ceramic-like polymer can be synthesized at room temperature. The biomimetic approach (which involved adding nanometer thick layers one by one) results in efficient stress transfer between the strong (MTM) and elastic (PVA) sections. The resulting materials are much stronger than earlier iterations.

Challenges Solved

Although nano-scale materials are incredibly strong at that particular size, macroscopic materials made from them often fail to maintain that strength.

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Journal article
Can Nature’s Design be Improved Upon? High Strength, Transparent Nacre-Like Nanocomposites with Double Network of Sacrificial Cross LinksJ. Phys. Chem. BJuly 1, 2008
Podsiadlo P; Kaushik AK; Shim BS; Agarwal A; Tang Z; Waas AM; Arruda EM; Kotov NA

Journal article
Ultrastrong and Stiff Layered Polymer NanocompositesScienceOctober 5, 2007
Podsiadlo P; Kaushik AK; Arruda EM; Waas AM; Shim BS; Xu J; Nandivada H; Pumplin BJ; Lahann J; Ramamoorthy A; Kotov NA

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