Rice University researchers who probed the interfacial interactions of polymer (blue) and cement (yellow) discovered that the right mix of hydrogen bonds is critical for making strong, tough and ductile composite materials for infrastructure. Using computer simulations like that shown in the illustration, the researchers were able to measure the strength of the bonds as hard cement slides past the soft polymer in a layered composite that mimics the structure of nacre, seen in the background. Image: Probhas Hundi/Multiscale Materials Laboratory.The right mix of hydrogen bonds in polymer and cement composites is critical to making strong, tough and ductile infrastructure material, according to researchers at Rice University who want to mimic the mechanics of mother-of-pearl and similar natural composites with synthetic materials.
Seashells made of mother-of-pearl, or nacre, get their remarkable properties from overlapping micron-sized, mineralized plates held together by a soft matrix. Reproducing this structure in cement and polymer composites could, for example, lead to better earthquake-resistant concrete, according to Rouzbeh Shahsavari, an assistant professor of civil and environmental engineering at Rice.
Shahsavari and Rice graduate student Navid Sakhavand ran more than 20 computer simulations of how polymers and cement molecules come together at the nanoscale and what drives their adhesion. These showed that the proximity of oxygen and hydrogen atoms is the critical factor in forming a network of weak hydrogen bonds that connects soft and hard layers. Common polyacrylic acid (PAA) proved best at binding overlapping layers of cement crystals, with an optimal overlap of about 15nm.
"This information is important to make the best synthetic composites," said Shahsavari. "A modern engineering approach to these materials will have a large impact on society, especially as we build new and replace aging infrastructure." Shahsavari and Sakhavand report their results in a paper in Applied Physics Letters.
While engineers understand that adding polymers improves cement by blocking the damaging effects of ‘aggressive’ ions that can invade its pores, details about how the materials interact at the molecular scale have remained unknown. To find out, the two researchers modeled composites made with PAA, as well as polyvinyl alcohol (PVA), both soft matrix materials that have been used to improve cement.
They discovered that the two 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. Oxygen in PAA had eight ways to bond with hydrogen (six for PVA) and could also participate in salt bridging between the polymer and cement, which makes the bonding network even more complex.
The researchers tested their simulated structures by sliding layers of polymer and cement against each other and found that the complexity allowed the bonds between PAA and cement to break and reconnect more frequently as the material was stressed. This significantly increased the composite’s toughness, its ability to deform without fracturing, which allowed the researchers to determine the optimum overlap between cement crystals.
"In contrast to the common intuition that hydrogen bonds are weak, when the right number of them – the optimum overlap – cooperate, they provide sufficient connectivity in the composite to confer high strength and high toughness," Shahsavari said. "From an experimental standpoint, this can be done by carefully tuning and controlling the addition of the polymers with the right molecular weight while controlling cement mineral formation. Indeed, a recent experimental paper by our colleagues showed a proof of concept toward this strategy."
This story is adapted from material from Rice University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.