Pijush Ghosh

The grain-grain interface of cement, which is composed of the complex needle-shaped microstructure of calcium silicate hydrate (C-S-H), is crucial in the development of the mechanical properties of cementitious composites. These C-S-H needles grow radially outward from the grain surface. This work proposes a combined experimental and modeling approach to incorporate the finer details of these needle geometries and the distribution of mechanical properties in an interface-based multiscale mechanical model for hydrating tricalcium silicate (C3S). At micrometer and sub-micrometer length scales (<5 μm), electron microscopy images revealed that the geometrical nature of these needles at the grain interface varies with days of hydration. The mechanical properties of C-S-H at the nanoscale were observed to be higher at the inner core and reduced toward the outer product. The model developed can incorporate the details of these needle microstructures and their mechanical properties at the microscale and can predict the bulk properties of hydrated C3S at higher scales.

Water at interfaces and under nano-confinement is part of many natural processes. The behavior of this water is greatly influenced by the nature of the surfaces it is in contact with and the confinement distance. The objective of this study is to understand the behaviour of confined water between dissimilar (X-Y) surfaces under varying confinement spacing. The surfaces considered were hydrophilic in nature and the combinations were considered based on crystal structure and surface energy. The critical distance of influence of mineral substrates on the water molecules was determined by applying time-averaged static properties such as interfacial layer density and orientation and dynamic properties such as diffusion. It was observed that dynamic properties provide a higher value of critical distance compared to static properties for dissimilar surface combination. The reason for this disparity is probed in terms of mineral-water and water-water interactions. The disproportion of strong and weak H-bonds was observed to be significant in determining the dynamic behaviour of interfacial layer. We applied hydrophilic surface combinations of tricalcium silicate (C3S) and dicalcium silicate (C2S) for our investigations.

High performance thermoresponsive soft, controllable and reversible actuators are highly desirable for diverse applications. The practical implementation of the existing poly(N-isopropylacrylamide) (pNipam) based soft thermoresponsive actuators faces serious limitations due to their functional requirement of proximal bulk solvent medium. In this work, addressing this issue, we report the development of a bilayer based actuator composed of a solvent responsive biodegradable polymer and temperature responsive pNipam. The designed bilayer is capable of achieving reversible and irreversible actuation as needed when exposed to a physiological range of body temperature, without any solvent bath around. The solvent or water supplied by the pNipam layer at its lower critical solution temperature (LCST) builds a concentration gradient across the thickness of the polymer layer. The concentration gradient results in a strain gradient, causing an out-of-plane folding of the bilayer. The underlying coupled diffusion-deformation interaction during folding and unfolding is incorporated in the reported finite element model, capable of predicting actuation characteristics under different initial conditions. The combined experimental and modelling effort in this work highlights the possibility of engineering 2-dimensional films into complex 3-dimensional shapes, which could have potential applications in soft machines and robotics.

We report an interplay between the desorption of intrinsic water and relaxation of polymer chains resulting in an unusual thermomechanical response of a hydrogel, wherein the elastic modulus increases in a certain temperature range followed by a sharp decrease with a further increase in temperature. We establish that, in a hydrogel, the desorption of disparate water types having distinct binding energy affects the consolidation and relaxation behaviour of the matrix, which in turn affects the mechanical properties at different temperature ranges. Using temperature-dependent dielectric relaxation spectroscopy and nanoindentation techniques, the chain dynamics and mechanical properties are investigated.

Polymer and hydrogel thin films on a compliant substrate are gaining significant attention these days in the fields of soft electronics, digital display, biomedical devices and sensors to name a few. The functional characteristics of these thin films depend on the performance of its interface with the substrate. The interface response is a non-linear function of surface energy and stiffness of a substrate beside other factors. The deviation of thin film properties under confinement from its bulk counterparts and its ultra-low physical dimension makes characterization of an interface extremely challenging. Probing and shearing of these interfaces with appropriate load value and up to the critical load rate can provide insight into the integrity of these interfaces under different service conditions. We have studied the response of PMMA thin film (150 nm) on a softer epoxy substrate and a stiffer glass substrate of increasing interfacial strength applying nanoindentation (NI) and nanoscratching. Pile up behaviour of the film material at confined state, the critical load at the interface proximity and the plastic work done during indentation as determined in this work can be successfully applied to characterize interface behaviours. These parameters also help in developing interface models and build qualitative correlations between thermodynamic work of adhesion and true mechanical work of adhesion at the interface. We hypothesize that the elasto-plastic region around load tip plays a significant role in determining pile up above film surface. We have discussed the possible molecular mechanisms in this region under stress leading to pile up.

We employ molecular dynamics simulations to understand the influence of non-affine deformation and recovery of free volume on the diffusion behavior of water molecules in polymers, as a function of tensile strain. This study is analogous to strain dependent diffusion of water in polymer thin films which undergo folding in response to water, which is not completely explored. Results reveal that diffusion coefficient of water molecules increases upto 20% strain followed by gradual reduction at higher strain. Non-affine deformation analysis indicates the coupled behavior of relaxation of polymer chains and recovery of free volume. Relaxation process at lower strain coalesces the free volume regions, enhancing the diffusion coefficient. Meanwhile, larger strain dissociates the highly deformed free volume regions. Interestingly, the dissociated regions undergo negligible changes in orientation and shape resulting in reduced diffusion coefficient. The above mechanisms coupled with hydrogen bonds are primarily responsible for shape change in polymer films.

Cluster-assembled solids (CASs) formed by the self-assembly of monodispersed atomically precise monolayer-protected noble metal clusters are attractive due to their collective properties. The physical stability and mechanical response of these materials remain largely unexplored. We have investigated the mechanical response of single crystals of atomically precise dithiol-protected Ag29 polymorphs, monothiol-protected Ag46, and a cocrystal of the latter with Ag40 (formulas of the clusters have been simplified merely with the number of metal atoms). The Ag29 polymorphs crystallize in cubic and trigonal lattices (Ag29 C and Ag29 T, respectively), and Ag46 and its cocrystal with Ag40 crystallize in trigonal and monoclinic lattices (Ag46 T and Ag40/46 M, respectively). The time and loading-rate-dependent mechanical properties of the CASs are elucidated by measuring nanoindentation creep and stress relaxation. The obtained Young's modulus (Er) values of the CASs were similar to those of zeolitic imidazolate frameworks (ZIFs) and show the trend Ag29 T > Ag29 C > Ag40/46 M > Ag46 T. We have also studied the viscoelastic properties of all of the four CASs and found that the value of tan damping factor of monothiol-protected Ag46 T was higher than that of other CASs. The unusual mechanical response of CASs was attributed to the supramolecular interactions at the surface of nanoclusters. This observation implies that the stiffness and damping characteristics of the materials can be modulated by ligand and surface engineering. These studies suggest the possibility of distinguishing between the crystal structures using mechanical properties. This work provides an understanding that is critical for designing nanocluster devices capable of withstanding mechanical deformations.

We demonstrate the formation of a versatile luminescent organo-inorganic layered hybrid material, composed of bovine serum albumin (BSA)-protected Au30 clusters and aminoclay sheets. X-ray diffraction revealed the intercalation of Au30@BSA in the layered superstructure of aminoclay sheets. Coulombic attraction of the clusters and the clay initiates the interaction, and the appropriate size of the clusters allowed them to intercalate within the lamellar aminoclay galleries. Electron microscopy measurements confirmed the hierarchical structure of the material and also showed the cluster-attached clay sheets. Zeta potential measurement and dynamic light scattering probed the gradual formation of the ordered aggregates in solution. The hybrid material could be stretched up to 300% without fracture. The emergence of a new peak in the luminescence spectrum was observed during the course of mechanical stretching. This peak increased in intensity gradually with the degree of elongation or strain of the material. A mechanochromic luminescence response was further demonstrated with a writing experiment on a luminescent mat of the material, made by electrospinning.

The molecular mechanism behind the process of biodegradation and consequently the loss in mechanical properties of polylactic acid (PLA) requires detailed understanding for the successful designing of various technological devices. In this study, we examine the role of free water and chain scission in this degradation process and quantify the mechanical properties of pristine and nanoparticle-reinforced PLA as it degrades over time. The in situ mechanical response of the degrading polymer is determined experimentally using nano-dynamic mechanical analysis (nanoDMA). Water present in the polymer matrix contributes to hydrolysis and subsequent scission of polymer chains. Water in excess of hydrolysis, however, alters the load transfer mechanism within the polymer chains. Molecular mechanism study applied in this work provides detailed insights into the relative role of these two mechanisms, (i) chain scission and (ii) solvation, in the reduction of mechanical properties during degradation. Functional groups such as ester (-COO-) and terminal acid (-COOH) interact with water molecules leading to the formation of water bridges and solvation shells, respectively. These are found to hinder the load transfer between polymer chains. It is observed that, compared to scission, solvation plays a more active role in the reduction of mechanical properties of degrading PLA.

In this study, variation in the thermodynamic and kinetic properties of the casting solution achieved by tailoring initial composition (IC) of the polymer–solvent-nonsolvent ternary mixture is investigated to engineer pores in a polymer film prepared by liquid–liquid phase separation (LLPS). The driving force for liquid–liquid phase separation, identified as thermodynamic enhancement factor (TE), is observed to influence the LLPS rate. At an IC closer to the phase boundary, the LLPS rate is higher. The kinetic restraint (KR), which is dictated by the viscosity of the casting solution, also alters the LLPS rate. The interplay of these two opposing factors determines the final film morphology. The higher LLPS rate obtained from a larger TE value leads to finger-like pore structure, while lowering LLPS rate by considering a casting solution with lower TE results in polymer film with spherical sponge-like pores irrespective of casting solution viscosity. On the other hand, when the concentration of both the polymer and nonsolvent is high, i.e., for high KR value, polymer film with interconnected pores and higher pore number density are obtained.