Amit K. Kaushik

LBL Assembled Laminates with Hierarchical Organization from Nano- to Microscale: High-Toughness Nanomaterials and Deformation Imaging

Layer-by-layer assembly (LBL) can generate unique materials with high degrees of nanoscale organization and excellent mechanical, electrical, and optical properties. The typical nanometer scale thicknesses restrict their utility to thin films and coatings. Preparation of macroscale nanocomposites will indicate a paradigm change in the practice of LBL, materials manufacturing, and multiscale organization of nanocomponents. Such materials were made in this study via consolidation of individual LBL sheets from polyurethane. Substantial enhancement of mechanical properties after consolidation was observed. The resulting laminates are homogeneous, transparent, and highly ductile and display nearly 3x higher strength and toughness than their components. Hierarchically organized composites combining structural features from 1 to 1 000 000 nm at six different levels of dimensionality with a high degree of structural control at every level can be obtained. The functionality of the resulting fluorescent sandwiches of different colors makes possible mechanical deformation imaging with submicrometer resolution in real time and 3D capabilities.

The Role of Interface and Reinforcement in the Finite Deformation Response of Polyurethane-Montmorillonite Nanocomposites

Nanoscale control over the structures is critical in designing nanocomposites with advanced properties. Structural parameters such as interface and volume fraction of reinforcement have tremendous effects on the mechanical properties of the nanocomposites, yet have proven difficult to control uniformly and consistently. Here we investigate the effect of reinforcement and interface on the finite deformation response of polyurethane (PU-) montmorillonite (MTM) nanocomposites at low and high strain rates. The multilayered PU-MTM nanocomposites, with alternate layers of PU and MTM, were manufactured using an exponential layer-by-layer (e-LBL) manufacturing technique. The systematic variation in MTM nanoparticle was obtained by either replacing several layers of MTM by polyacrylic acid (PAA) or by varying the thickness of PU layer. The interface was altered by replacing MTM layers by PAA. The deposition of PAA resulted in the formation of a complex polymer layer due to its diffusion through PU layer. The nanocomposites demonstrated an increasing yield strength and stiffness with increased strain rate and MTM volume fraction. The weaker interface interaction between the polymer and MTM nanoparticles resulted in a decreased yield strength and stiffness. The design parameters that will result in structures with optimum mechanical properties will be demonstrated.

High Strain-Rate Response of Polyurethane-Clay Nanocomposites and Their Use for Blast Applications

The dispersion of strong nanoscale building blocks into polymers may result in nanocomposites that can mimic the structural and mechanical properties of advanced materials found in nature. In this study, exceptionally high strength and stiffness (in-plane modulus: 270 GPa) clay nanoparticles are used to synthesize polyurethane-clay nanocomposites with enhanced mechanical properties using a layer-by-layer (LBL) technique. The LBL technique allows spatial and orientational control of these clay nanoparticles within the polymer matrix at the nano-scale. Moreover, the structure of LBL manufactured nanocomposites resembles the structure of naturally occurring tough biocomposite Nacre. A series of nanocomposite films with a wide range of volume fractions of clay nanoparticles was manufactured and investigated at low and high strain rates in uniaxial tension and compression deformation states respectively. The growth of these films in the thickness direction was enhanced by replacing alternate layers of MTM nanoparticles with (poly) acrylic acid. Thick samples for the uniaxial compression tests were made by hot-pressing several of these films together. The nanocomposites demonstrated an increasing yield strength and stiffness with volume fractions of MTM nanoparticles. The nanocomposites, at high-strain rate compression, showed a rapid strain hardening with true stresses as high as 0.45 GPa at a strain of 0.8. The incorporation of clay nanoparticles resulted in a plastic deformation leading to large energy dissipation which makes these materials suitable for applications in increasing the survivability of structures under blast loadings.© 2009 ASME