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    Do seashells hold the secret to earthquake-proof structures?

    Research by scientists at the Rice University has revealed how the right mix of hydrogen bonds in polymer and cement composites can help create strong, tough and ductile infrastructure material.

    The researchers say they aim to mimic the mechanics of mother-of-pearl and similar natural composites using synthetic materials.

    Seashells made of mother-of-pearl get their remarkable properties from overlapping micron-sized, mineralised plates held together by a soft matrix.

    Similarly, cement and polymer composites can make better earthquake-resistant concrete, says Rouzbeh Shahsavari, an assistant professor of civil and environmental engineering at Rice.

    The scientists at the Rice lab ran more than 20 computer simulations of how polymers and cement molecules come together at the nanoscale and what drove their adhesion.

    The proximity of oxygen and hydrogen atoms was found to be the critical factor in forming a network of weak hydrogen bonds that connected soft and hard layers.

    Common polyacrylic acid (PAA) proved best at binding the overlapping layers of cement crystals with an optimal overlap of about 15 nanometres.

    According to Shahsavari, who ran the project with Rice graduate student Navid Sakhavand, the information was important to make the best synthetic composites.

    He also believes a modern engineering approach to these materials will have a significant impact on society, as new infrastructure replaces the old.

    To find out how the materials interacted at the molecular scale, the researchers modelled composites with PAA as well as polyvinyl alcohol (PVA), both soft matrix materials that have been used to improve cement.

    They discovered that the two different oxygen atoms in PAA (as opposed to one in PVA) allowed it to receive and donate ions as it bonded with hydrogen in the crystals of tobermorite cement.

    The simulated structures were tested by sliding layers of polymer and cement against each other; the complexity allowed the bonds between PAA and cement to break and reconnect more frequently as the material was stressed, which significantly increased its toughness and the ability to deform without fracturing.

    This allowed the researchers to determine the optimum overlap between cement crystals.

    Shahsavari added that the right number of hydrogen bonds cooperating can provide sufficient connectivity in the composite to confer high strength and toughness.

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