Molecular dynamics modeling of the structure, dynamics, energetics and mechanical properties of cement-polymer nanocomposite

Efforts to tune the performance of organic/inorganic composites are hindered owing to a lack of knowledge related to the interfacial interaction mechanisms. Here we investigated the interfacial structure, dynamics, energetics and mechanical properties between calcium silicate hydrates (C-S-H) and polymers by molecular dynamics (MD) simulation. In this work, polyethylene glycol (PEG), polyvinyl alcohol (PVA) and polyacrylic acid (PAA) are intercalated into nanometer channel of C-S-H sheets to construct the model of polymer/C-S-H composite. In the interfacial region, the calcium ions near the surface of C-S-H play mediating role in bridging the functional groups in the polymers and oxygen in the silicate chains by forming O-s-Ca-O-p bond. In addition to ionic bonding, the bridging oxygen (C-O-C) in the PEG, hydroxyl (C-OH) in the PVA and carboxyl groups (-COOH) in the PAA provide plenty oxygen sites to form H-bonds with silicate hydroxyl, interlayer water and calcium hydroxyl in C-S-H substrate. The interfacial binding energy is dependent on polarity of functional groups in the polymers, the stability of the H-bond and Ca-O bond, ranking in the following order: E(PAA) > E(PVA) > E(PEG). The PVA with small number of H-bonds formed between oxygen in PVA and water molecules, resulting in increasing the mobility of confined water in the interlayer region. On the other hand, PAA and PVA, with strong polarity, can provide more number of non-bridging oxygen sites that widely distributed along the polymer chains to associate with more calcium ions and H-bonds. Furthermore, uniaxial tensile test is utilized to study the mechanical behavior of the composites. The incorporation of polymers, strengthening the H-bonds in the interfacial region and healing the defective silicate chains, can inhibit the crack growth during the loading process, which both enhance the cohesive strength and ductility of the C-S-H gel. In particular, the intercalated PAA increases the Young's modulus, tensile strength and fracture strain of C-S-H gel to 22.27%, 19.2% and 66.7%, respectively. The toughening mechanism in this organic/inorganic system can provide useful guidelines for polymer selection, design, and fabrication of C-S-H/polymer nano composites, and help eliminate the brittleness of cement-based materials from the genetic level.