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The deformation response of nanoporous gold nanopillars under compression, with varying nanopillar and ligament diameters, is analyzed from a series of molecular dynamics simulations. We have measured the ligament size-dependent elastic modulus of nanoporous nanopillars from the elastic region of stress–strain curves, showing that the elastic modulus at a given solid fraction depends on both the geometrical characteristics of the nanopillars and topology of the ligament structure. We modified the Gibson–Ashby scaling law for nanopillars, to predict the size-dependency found in the simulations. To compare the modified model and the simulation results, we have developed a method to estimate the average number of load-bearing ligaments by employing a unique post-processing technique. The comparison between the scaling law and the simulation results lead us to the conclusion that the elastic response of the nanopillars is predominantly due to the compression of ligaments. The size effect due to the new dimensionality vanishes at larger nanopillar diameter and eventually, the scaling laws resemble the classical laws, which is a function of solid fraction only. Besides the size effect, the need to generate nanoporous nanopillars with different topologies and to ensemble average over the various topologies is discussed.

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.

We report the controlled reversible and irreversible folding behavior of a biopolymer film simply by tuning the solvent characteristics. Generally, solvent triggered folding of soft membranes or film is achieved by unfolding. Here, we show that this unfolding behavior can be suppressed/delayed or even completely eliminated by altering the intrinsic nature of the solvent. A reversible folding of biopolymer film is observed in response to water, whereas, an irreversible folding is observed in the presence of an aromatic alcohol (AA) solution of different molar concentrations. The folding and unfolding behavior originates from the coupled deformation-diffusion phenomena. Our study indicates that the presence of an AA influences the relaxation behavior of polymer chains, which in turn affects the release of stored strain energy during folding. Controlling the reversibility as well as the actuation time of the biopolymer film by tuning the solvent is explained in detail at the bulk scale by applying appropriate experimental techniques. The underlying mechanism for the observed phenomena is complemented by performing a simulation study for a single polymer chain at the molecular length scale. Due to the solvent-triggered hygromorphic response, biopolymer films exhibit huge potential as sensors, soft robots, drug delivery agents, morphing medical devices and in biomedical applications. We provide experimental evidence for the weight lifting capacity of permanently folded membranes, amounting to ∼200 times their own weight.

We report on the plastic deformation characteristics of nanoporous gold nanopillars under compression in molecular dynamics simulations. The stress-strain curves demonstrate an initial non-linear regime up to compressive true strains of about 5%, followed by a stress plateau and a strong hardening stage. The plateau stress is increasing with the ligament size. In addition, a power-law is fitted to the stress variation during the plateau and hardening stages. In order to relate the mechanical response to the dislocation activity, an in-house post-processing technique was developed to skeletonize the structure and to identify the yielded ligaments. We found that the elastic-plastic transition starts at a strain substantially lower than that of the plateau. The load-bearing capacity is further retained by the unyielded ligaments within the three-dimensionally connected network, allowing further increase in the stress and at the onset of the plateau stress 40% of the ligaments have already yielded plastically. Calculation of the genus of the subnetwork of unyielded ligaments shows that it drops to zero at the onset of the plateau stage. Additionally, the rate of dislocation nucleation is maximum at the onset of the plateau, allowing to continue the plastic flow without increasing the stress. The stress starts increasing again as the ligaments coalesce and the nanoporous structure densifies. Based on the analysis of the genus of the whole structure during compression, we propose a correlation between the hardening and the topology of the structure, suggesting that hardening is dominated by the coalescence of ligaments. During coalescence, grain boundaries were formed between coalesced ligaments, some are either removed or retained. As a result, at very large compressive strains the single crystal nanoporous nanopillar turns into a polycrystalline specimen with some disconnected pores.

The self-folding behavior in response to external stimuli observed in hydrogels is potentially used in biomedical applications. However, the use of hydrogels is limited because of its reduced mechanical properties. These properties are enhanced when the hydrogels are cross-linked and reinforced with nanoparticles. In this work, molecular dynamics (MD) simulation is applied to perform uniaxial tension and pull out tests to understand the mechanism contributing towards the enhanced mechanical properties. Also, nanomechanical characterization is performed using quasi static nanoindentation experiments to determine the Young's modulus of hydrogels in the presence of nanoparticles. The stress-strain responses for chitosan (CS), chitosan reinforced with hydroxyapatite (HAP) and cross-linked chitosan are obtained from uniaxial tension test. It is observed that the Young's modulus and maximum stress increase as the HAP content increases and also with cross-linking process. Load displacement plot from pullout test is compared for uncross-linked and cross-linked chitosan chains on hydroxyapatite surface. MD simulation reveals that the variation in the dihedral conformation of chitosan chains and the evolution of internal structural variables are associated with mechanical properties. Additional results reveal that the formation of hydrogen bonds and electrostatic interactions is responsible for the above variations in different systems.

Water-responsive biopolymer thin films with engineered matrix characteristics can accomplish desirable shape changing properties such as self-folding. Self-folding response of chitosan film is experimentally characterized by its total folding time and rate of folding. Here, atomistic simulation is employed to investigate the molecular mechanism responsible for modified self-folding behavior observed in nanoparticle reinforced chitosan films. The nanocomposite system is solvated with water content varying from 10% to 100% of total mass of the system. The free volume available for diffusion of water molecules is affected by the flexibility of glycosidic linkages present in chitosan chains. The increase in mobility of water molecules with increase in water content decides the rate of folding. A separate molecular system is modeled with confined region between nanoparticles densified with chitosan chains and water molecules. The thickness of confined region is determined from the critical distance of influence of nanoparticles on water molecules. The adsorption of water on nanoparticle surface and relaxation of chitosan chains are responsible for increased total folding time with nanoparticle concentration. This simulation study, complemented with experimental observations provides a useful insight into the designing of actuators and sensors based on the phenomenon of hygromorphism.

Water responsive biopolymers are gaining enormous attention in the different areas of research and applications related to self-folding. In this work, we report that cross-linking is an efficient means of modifying a single layer biopolymer film for a controlled and predictable pathway of folding. The initiation of the folding of a film is caused by the diffusion of water molecules along the film thickness. However, this folding is observed to take place in an unpredictable and random fashion with a pristine biopolymer film and a nano-particle reinforced film. The mechanical properties and the diffusion characteristics of the film are strongly interrelated and affect the overall folding behavior. The underlying mechanism behind this relation is appropriately substantiated by an in depth molecular dynamic study. The detailed characterization of the folding shape and material behavior is performed applying suitable experimental techniques. The potential application of the controlled folding of the cross-linked film as a sensor and as a soft crane is demonstrated in this report.

Chitosan (CS) is a biomaterial that offers many sophisticated and innovative applications in the biomedical field owing to its excellent characteristics of biodegradability, biocompatibility and non-toxicity. However, very low mechanical properties of chitosan polymer impose restriction on its further development. Cross-linking and nanoparticle reinforcement are the two possible methods to improve the mechanical properties of chitosan films. In this research, these two methods are adopted individually by using tripolyphosphate as cross-linker and nano-hydroxyapatite as particle reinforcement. The nanomechanical characterizations under static loading conditions are performed on these modified chitosan films. It is observed that nanoparticle reinforcement provided necessary mechanical properties such as ductility and modulus. The mechanisms involved in improvement of mechanical properties due to particle reinforcement are studied by molecular dynamics (MD). Further, improvement in mechanical properties due to combination of particle reinforcement and cross-linking agent with chitosan is investigated. The stress relaxation behavior for all these types of films is characterized under dynamic loading conditions using dynamic mechanical analysis (nanoDMA) experiment. A viscoelastic solid like response is observed for all types of film with modulus relaxing by 3-6% of its initial value. A suitable generalized Maxwell model is fitted with the obtained viscoelastic response of these films. The response to nano-scratch behavior is also studied for particle reinforced composite films. � 2015 Elsevier Ltd.

Biopolymers are new generation polymers which find applications in biomedical field, in food packaging, as edible films etc., due to its unique property of biodegradability and biocompatibility, which is a major concern in case of fossil derived polymers. The application of biopolymers gets limited due to its low mechanical properties. The mechanical properties of these biopolymers however can be enhanced by reinforcing with suitable fillers of nanometer size range, thereby forming nanocomposites. Chitosan (CS) is a polysaccharide which is one of the most extensively used biopolymer in drug delivery, bone tissue engineering etc. Chitosan/Hydroxyapatite (HAP) nanocomposite can be formed by dispersing HAP nanoparticles in chitosan matrix. The mechanical properties of nanocomposites are dependent on the interactions between nanoparticles and polymer matrix. It is thus essential to understand the mechanism between matrix and nanoparticles in order to tailor the mechanical properties for suitable applications. Molecular Dynamics (MD) is one of the possible tools to study the interactions at atomic level. It can also contribute significantly in the prediction of macro level properties. In this work, MD has been applied to study the underlying mechanisms at atomic length scale during the uniaxial deformation process of CS/HAP nanocomposites. The interaction between HAP and CS has been analyzed using radial distribution function, evolution of hydrogen bonding and electrostatic interactions during the deformation process. The initial results indicate an increase in the modulus of elasticity of CS/HAP nanocomposite when compared to pure chitosan for a given strain rate. It is observed that the primary interaction between nanoparticle and polymer matrix is in the form of an electrostatic attraction between the calcium present in HAP and the oxygen in chitosan chains.