Fabrication of Multilayers via Layer-by-Layer for Mimicking the Mechanical Performance of Nacre

Abstract

Nacre inspired mechanically strong, robust, colorful and flame retardant multilayered films were successfully prepared using Layer-by-Layer (LbL) self-assembly. The nanofabrication of multilayered thin films was carried out on pre-activated silicon wafers, quartz slides and flexible polyurethane foam (FPUF) through dipping under ambient conditions. The components including poly (ethylene imine) (PEI), cresol novolac epoxy resin (CNER Mn ~ 870, CNER Mn ~ 1020), modified montmorillonite (NH2-MMT), triphenyl phosphate, oligomeric siloxane (PDMSs), graphene oxide (GO), laponite (LAP), poly (vinyl alcohol) (PVA) and phosphorylated chitin (P.Chitin) were employed. The selected components, owing to their functionality and intrinsic reactivity, were assembled into thin films with architectures; PEI(CNERh/NH2-MMT/PEI) n, PEI(CNERl/NH2- MMT/PEI) n; PEI(CNERl/NH2-MMT/tPP) n ; (PEI/MMT/PDMSs) n ; (PEI/P.Chitin/MMT/GO) n; and (PVA/P.Chitin/LAP/GO)n where number of layer pairs (n) ranges 6-12, 6-25, 7-15, 4-20 and 2-50, respectively. The optimization for the buildup was done by testing various solvents, concentration, adsorption/dipping time, ratio of concentration and ratio of dipping time as a function of a layer pairs. The film growth was monitored through UV-vis spectrophotometry and ellipsometer after deposition of each layer pair. Afterwards, characterization using AFM, SEM, water contact angle, XPS and testing through Vicker’s indenter, cone calorimetry, nanoindentation, universal testing machine, UL-94 and resistance to corrosive solvents was studied. The PEI(CNER/NH2- MMT/PEI) n multilayer thin films with a driving force hydrogen bonding and covalent linkage showed remarkable hardness (H ~ 1.996 GPa). Due to the barrier properties of clay, PEI(CNER/NH2-MMT/PEI) n and PEI(CNER/NH2-MMT/tPP) n multilayered thin films provided 20 and 25%, reduction in heat release rate (HRR) exhibiting significant protection for flexible polyurethane foam (FPUF) from burning. However, in (PEI/MMT/PDMSs) n thin films, mechanical properties such as reduced modulus Er ~ 65 GPa and hardness H ~ 9.40 GPa with good chemical resistance due to layered structure was obtained. Whereas, hybrid (PVA/P.Chitin/LAP/GO)n multilayered thin films resulted in relatively thicker, robust, and colorful coatings on quartz slides with Er ~ 25.53 GPa and H ~ 1.45 GPa. Free standing (PEI/P.Chitin/Chi-MMT/GO) n multilayered thin films showed the Young’s Modulus ~ 384.17 MPa with outstanding flame retardance. Vacuum xix assisted filtration (VAF) assembly was also used to prepare hierarchical nacre-like structures using PVA, P.Chitin, GO, LAP, Chi-MMT, and PEI. Henceforth, multilayered structure possessing comparable mechanical performance to nacre using environmental friendly technique and benign materials are obtained. These multilayered thin films are found to have potential to be applied as barrier against corrosive solvents, fire, as well as in packaging, ballistic and other protective coatings for advanced applications.