Denvid Lau

A molecular dynamics investigation on the cross-linking and physical properties of epoxy-based materials

Epoxy-based materials are extensively used in industry due to their excellent mechanical and thermal stability. As the epoxy products are getting smaller and smaller nowadays, there are difficulties arising from the material processing and measurement, which need to be carefully considered in the design stage. SU-8 photoresist, which is commonly used in micro-electro-mechanical systems (MEMS), is chosen as a representative of epoxy-based materials in this study. Here, we propose an effective dynamic cross-linking algorithm, which can be used to construct the SU-8 epoxy network with cross-linking degree higher than 80%. Using an equilibration process incorporating successive pressure controls, the density of the cross-linked structure can be accurately obtained. By performing the dynamic deformations in the molecular dynamics simulations, elastic properties of the equilibrated SU-8 photoresist can be determined, which are in good agreement with the experimental measurements. The good predictions of the physical properties demonstrate a strong mechanical stability of the SU-8 structure at the nano-scale. The dynamic cross-linking algorithm described in the present study can be applied in other polymeric material investigations involving the cross-linked network model through homopolymerization.

A robust nanoscale experimental quantification of fracture energy in a bilayer material system

Significance Accurate measurement of interfacial properties is critical any time two materials are bonded—in composites, tooth crowns, or when biomaterials are attached to the human body. Yet, in spite of this importance, reliable methods to measure interfacial properties between dissimilar materials remain elusive. Here we present an experimental method for robust characterization of organic–inorganic interfaces. Combining theory, experiment, and molecular simulation, we show that debonding between layers of materials can be controlled through residual stresses in a deposited metallic layer. We validate the method and show that moisture degrades the bonding between epoxy and silica, a technologically important interface. This may enable us to design more durable concrete composites that could be used to innovate our nations aging infrastructure. Accurate measurement of interfacial properties is critical any time two materials are bonded—in composites, tooth crowns, or when biomaterials are attached to the human body. Yet, in spite of this importance, reliable methods to measure interfacial properties between dissimilar materials remain elusive. Here we present an experimental approach to quantify the interfacial fracture energy Γi that also provides unique mechanistic insight into the interfacial debonding mechanism at the nanoscale. This approach involves deposition of an additional chromium layer (superlayer) onto a bonded system, where interface debonding is initiated by the residual tensile stress in the superlayer, and where the interface can be separated in a controlled manner and captured in situ. Contrary to earlier methods, our approach allows the entire bonded system to remain in an elastic range during the debonding process, such that Γi can be measured accurately. We validate the method by showing that moisture has a degrading effect on the bonding between epoxy and silica, a technologically important interface. Combining in situ through scanning electron microscope images with molecular simulation, we find that the interfacial debonding mechanism is hierarchical in nature, which is initiated by the detachment of polymer chains, and that the three-dimensional covalent network of the epoxy-based polymer may directly influence water accumulation, leading to the reduction of Γi under presence of moisture. The results may enable us to design more durable concrete composites that could be used to innovate transportation systems, create more durable buildings and bridges, and build resilient infrastructure.

Mesoscopic packing of disk-like building blocks in calcium silicate hydrate

At 100-nanometer length scale, the mesoscopic structure of calcium silicate hydrate (C-S-H) plays a critical role in determining the macroscopic material properties, such as porosity. In order to explore the mesoscopic structure of C-S-H, we employ two effective techniques, nanoindentation test and molecular dynamics simulation. Grid nanoindentation tests find different porosity of C-S-H in cement paste specimens prepared at varied water-to-cement (w/c) ratios. The w/c-ratio-induced porosity difference can be ascribed to the aspect ratio (diameter-to-thickness ratio) of disk-like C-S-H building blocks. The molecular dynamics simulation, with a mesoscopic C-S-H model, reveals 3 typical packing patterns and relates the packing density to the aspect ratio. Illustrated with disk-like C-S-H building blocks, this study provides a description of C-S-H structures in complement to spherical-particle C-S-H models at the sub-micron scale.

Molecular dynamics study on stiffness and ductility in chitin–protein composite

Chitin–protein composite is the structural material of many marine animals including lobster, squid, and sponge. The relationship between mechanical performance and hierarchical nanostructure in those composites attracts extensive research interests. In order to study the molecular mechanism behind, we construct atomistic models of chitin–protein composite and conduct computational tensile tests through molecular dynamics simulations. The effects of water content and chitin fiber length on the stiffness are examined. The result reveals the detrimental effect on the stiffness of chitin–protein composite due to the presence of water molecules. Meanwhile, it is found that the chitin–protein composite becomes stiffer as the embedded chitin fiber is longer. As the tensile deformation proceeds, the stress–strain curve features a saw-tooth appearance, which can be explained by the interlocked zigzag nanostructure between adjacent chitin fibers. These interlocked sites can sacrificially break for energy dissipation when the system undergoes large deformation, leading to an improvement of ductility.

Nano- and mesoscale modeling of cement matrix

AbstractAtomistic simulations of cementitious material can enrich our understanding of its structural and mechanical properties, whereas current computational capacities restrict the investigation length scale within 10 nm. In this context, coarse-grained simulations can translate the information from nanoscale to mesoscale, thus bridging the multi-scale investigations. Here, we develop a coarse-grained model of cement matrix using the concept of disk-like building block. The objective is to introduce a new method to construct a coarse-grained model of cement, which could contribute to the scale-bridging issue from nanoscale to mesoscale. PAC codes: 07.05.Tp, 62.25.-g, 82.70.Dd

One Dimensional Predictive Model of Interfacial Stiffness in FRP- bonded Concrete System using Acoustic-Laser Technique

Fiber-reinforced polymer (FRP) has become increasingly popular in the application of strengthening and retrofitting existing concrete elements, such as beams, columns, slabs, and bridge decks. In view of the maintenance and the safety issues against these retrofitted systems, development of a robust and reliable nondestructive testing (NDT) technique that provides an accurate and remote assessment of interfacial properties in the FRP-bonded concrete system is required. In this paper, a one dimensional predictive model of interfacial stiffness in the FRP-bonded concrete system based on an acoustic-laser technique is proposed. This model is constructed based on a traditional beam theory in which an infinitely long beam sits on top of a series of springs, which can be regarded as a soft foundation. It is noticed that the resonance frequency can indicate the interfacial stiffness when the size of detected defect is small enough. The result from this predictive model is compared with the existing literature on the interfacial stiffness in epoxy-silica system and a good agreement is observed.

Atomistic Prediction of Nanomaterials: Introduction to Molecular Dynamics Simulation and a Case Study of Graphene Wettability.

This article discusses about case study of molecular dynamics (MD) simulation to predict the free energy of a graphene-water system based on the nanoscale perspective.This work can also be considered as a demonstration on the applications of MD in nanomaterials and nanostructure research.

Moisture-induced Debonding in Concrete-epoxy Interface

Concrete-epoxy interface can be found in many strengthened or retrofitted structures, especially in Hong Kong where concrete structures are the majority. In practice, epoxy is usually used in repairing concrete structures because of its excellent adhesion, chemical and heat resistance, good mechanical properties and electrical insulating properties. The bond between concrete and epoxy has been found to be strong enough such that failure does not occur at the interface under dry condition. However, prior research studies have indicated that moisture-affected debonding in a concrete-epoxy bonded system is a complex phenomenon that may often involve a distinctive dry-to-wet debonding mode shift from material decohesion (concrete delamination) to interface separation in which concrete-epoxy interface becomes the critical region of failure. Such premature failures may occur regardless of the durability of the individual constituent materials forming the bonded material systems. In this paper, a review on moisture degradation of concrete-epoxy interface is presented, followed by an explanation of such phenomenon using an atomistic approach which can fundamentally describe the interaction between the bonded system and the surrounding water molecules. It is highly recommended that the durability of concrete-epoxy interface should be carefully considered in the design stage due to the detrimental moisture effect.

Moisture effect on interfacial integrity of epoxy-bonded system: A hierarchical approach

The epoxy-bonded system has been widely used in various applications across different scale lengths. Prior investigations have indicated that the moisture-affected interfacial debonding is the major failure mode of such a system, but the fundamental mechanism remains unknown, such as the basis for the invasion of water molecules in the cross-linked epoxy and the epoxy-bonded interface. This prevents us from predicting the long-term performance of the epoxy-related applications under the effect of the moisture. Here, we use full atomistic models to investigate the response of the epoxy-bonded system towards the adhesion test, and provide a detailed analysis of the interfacial integrity under the moisture effect and the associated debonding mechanism. Molecular dynamics simulations show that water molecules affect the hierarchical structure of the epoxy-bonded system at the nanoscale by disrupting the film-substrate interaction and the molecular interaction within the epoxy, which leads to the detachment of the epoxy thin film, and the final interfacial debonding. The simulation results show good agreement with the experimental results of the epoxy-bonded system. Through identifying the relationship between the epoxy structure and the debonding mechanism at multiple scales, it is shown that the hierarchical structure of the epoxy-bonded system is crucial for the interfacial integrity. In particular, the available space of the epoxy-bonded system, which consists of various sizes ranging from the atomistic scale to the macroscale and is close to the interface facilitates the moisture accumulation, leading to a distinct interfacial debonding when compared to the dry scenario.

A review of nondestructive testing approaches using mechanical and electromagnetic waves

Mechanical and electromagnetic waves are commonly used in nondestructive testing (NDT) techniques for evaluating the materials and structures in civil engineering industry, due to their good examination of defects inside the matter. However, the individual use of mechanical wave or electromagnetic wave in NDT methods sometimes does not fulfill the satisfactory detection in practice because of the operational inconvenience and low sensitivity. It has been demonstrated that the combination of using both types of waves can achieve a better performance for NDT application and would be the future direction for defect detection, as the advantages of each physical wave are picked out whereas the weaknesses are mitigated. This paper discusses the fundamental mechanisms and the current applications of using mechanical and electromagnetic waves for defect detection, with the goal of providing the physical knowledge and the perspectives of developing the NDT applications with these two types of waves. Typical mechanical-wave-based NDT methods such as acoustic emission, ultrasonic technique, and impact-echo method are reviewed. In addition, NDT methods using electromagnetic wave, which include optical fiber sensing technique, laser speckle interferometry and laser reflection technique are discussed. Advantages and disadvantages of these methods are outlined. In particular, we focus on a recent NDT method called acoustic-laser technique, which utilizes both the mechanical and electromagnetic waves. The basic principles and some important experimental data recorded by the acoustic-laser technique are described and its future development in the field of defect detection in civil infrastructure is presented.